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Chapter 1:
Three basic categories of engineering materials: Metals, Ceramics, and Polymers.
Composites are a mixture of at least two of the basic materials.

Metals
1)Metals: Ferrous, based on iron has 75% metal tonnage in the world.
            Mostly Steels (.02 to 2.11% C)
            And Cast iron (2% to 4% C)

2) Nonferrous metals – all other metallic elements and alloys: aluminum, copper, etc.

Ceramics
Compounds that contain metallic and non metallic elements
For processing, ceramics are divided into

  1. Traditional ceramics (clay), and Modern ceramics (alumina - Al2O3).
  2. Glasses (silica - SiO2)

Polymers
Formed of repeating structural units called mers.

  1. thermoplastics – can be heated multiple times without altering molecular structure
  2. thermosetting – cannot be reheated, molecules cure into a rigid structure
  3. elastomers – have significant elastic behavior.

Composites
A material consisting of two or more phases that are processed separately and then bonded together.
Phase – a homogeneous mass of material, like grains of an identical unit cell structure in a solid metal.

Processing Operations

  1. shaping operations – alter the geometry of the starting material
  2. property-enhancing operations – improve physical properties without changing the shape
  3. surface processing operations – clean, treat, coat, or deposit material on the exterior surface of the material

Net shape processes – when most of the starting material is used, and no subsequent machining is required.
Near shape processes – minimum machining is required.

Chapter 2

Bonding between atoms and molecules
Primary bonds – attractive forces between atoms

  1. ionic – transfer of valence electron (properties: low ductility, low electrical conductivity)
  2. covalent – sharing of electrons (properties: high hardness, low electrical conductivity)
  3. metallic – electron cloud. (properties: high electrical conductivity, good conduction and good ductility)

Secondary bonds – attractive forces between molecules

  1. dipole forces – unequal sharing of electrons. In HCl, the H is slightly positive,  The Cl of an adjacent HCl molecule will be attracted to the H.
  2. hydrogen bonding – (special case of dipole forces) A hydrogen attached to an electronegative atom of one molecule and an electronegative atom of a different molecule.
  3. London forces – temporary dipoles caused by rapid motion of electrons are created when electrons are on one side of the molecule rather than the other.

 

Primary bonds are much stronger than secondary bonds

Crystalline – atoms are located at regular and recurring positions in 3D.
Imperfections

  1. Point defect – vacancy or interstitial, missing pair of ions of opposite charge (Schottky defect)
  2. Line defect – a connected group of point defects that form a line in the lattice.
    1. Edge Dislocations and line dislocations.
  3. surface defects – grain boundaries: interface between two grains in a polycrystalline material.

Slip – key in plastic deformation. Slip is the relative movement of atoms on opposite sides of a plane in the lattice, called a slip plane. Order of increasing slip directions:
HCP<FCC<BCC.
Dislocations are important in slip for metals. A structure with an edge dislocation deforms much easier than a perfect structure.
Why is it easier to move a dislocation through the lattice than to deform the lattice itself?
Atoms at the edge dislocation need a smaller displacement within the distorted lattice structure in order to reach a new equilibrium position. For this reason, a lower energy level is required to realign the atoms into the new positions.
Dislocations are good because the metal is more ductile and easier to form during manufacturing
Dislocations are bad because the metal is weaker than it would be without dislocations from a design standpoint.

Polycrystalline – collection of many randomly oriented grains in a metal.
When a metal is cooled from a molten state and starts to solidify, nucleation of individual crystals occurs at random positions and orientations. As the crystals interfere with each other they form a grain boundary, which is a few atoms thick.
Size of an atom = 1 angstrom = 1 x 10-10m
Smaller grain size means higher strength and hardness, because there are more grain boundaries, which interfere with dislocation movement. Faster cooling promotes smaller grain size.

Noncrystalline (amorphous) – no long range order in molecular structure, differences in melting and thermal expansion characteristics.

Metals become noncrystalline when melted.

 

Chapter 3
Mechanical properties like high strength that are desirable to the designer make manufacturing more difficult.

Engineering stress
Engineering stress

Tensile strength -  maximum stress of a stress-strain curve

Ductility is the ability of a material to plastically strain without fracture.
Elongation - , where Lf is length at fracture

True Stress – using instantaneous area

True strain -


Just use dotted curve for true stress and strain curve!

Thus true stress increases continuously in the plastic region. And this also means that the metal is becoming stronger as strain increases.

Flow curve: , K = strength coefficient, n= strain hardening exponent.

Perfectly Elastic:
            Ceramics, cast irons, and thermosetting polymers

Elastic and perfectly plastic:

Metals behave like this when heated to sufficiently high temperatures above recrystallization

Elastic and Strain Hardening:

 

Compression:
Engineering Stress-Strain curve

Bending test – for ceramics and hard brittle materials
3 point bendin test – gives transverse rupture strength.

Failure happens when the tensile strength of the outer fiber is exceeded.
Cleavage is the failure type for ceramics, where separation rather than slip occurs along certain crystallographic planes.

At higher temperature, metals are more ductile and have less strength.
Hot hardness – ability of a material to retain hardness at elevated temperatures.

Recrystallization:
If a metal is heated above .5Tm (recrystallization temperature in Kelvin), and then deformed, instead of strain hardening, new grains are formed that are free of strain. The metal behaves as a perfectly plastic material, with a strain-hardening exponent of n=0.
Recrystallization time is time in which new grains are formed, and it is about one hour.
Forming metals at temperatures above recrystallization temperatures is called Hot Working.

Chapter 4

Strength to weight ratio = tensile strength divided by density
Density is a function of temperature. T goes up, density goes down. Volume per unit weight increases with increasing temperature.

 

 

Melting point Tm is the temperature at which a pure element transitions from solid to liquid state.
Heat of fusion – energy required at the melting temperature to transform the metal from solid to liquid state.
Supercooled – when a liquid remains in the liquid state below its freezing point (if nucleation of its crystals did not begin immediately.
Metal alloys do not have a single melting point but rather are between a pure metal and an amorphous structure. Melting starts at the solidus, and finishes at the liquidus.
Eutectic alloys melt and freeze at a single temperature.

 

Flow of electrical current involves movement of charge carriers. In solids the charge carriers are electrons, and in liquids, charge carriers are positive and negative ions.
Electrical discharge machining uses electrical energy in the form of sparks to remove material from metals
Arc welding uses electrical energy to melt joint metal.

A Semiconductor has resistance that lies between an insulator and a conductor

Electrolyte – the ionized solution
Electrodes – where the current enters and leaves the solution
Anode – the positive electrode
Cathode – the negative electrode
At each electrode, some chemical reaction occurs, such as the deposition or dissolution of material, or the decomposition of gas from the solution.

Electroplating adds a thin coating of one metal to the surface of another metal. The workpart is the cathode so positive ions of the coating metal are attracted to the negatively charged part.

Electrochemical machining removes material from the surface of a metal part.
The workpart is the anode, and a tool with the desired shape is the cathode

Chapter 6

An alloy is a mixture of two or more elements, where at least one of which is metallic.

Phase diagrams:
Overall composition of the alloy is given by its position along the horizontal axis. Composition of liquid and solid phases are not the same, and can be found by drawing a horizontal line at the temperature of interest.
The compositions of solid and liquid phases are given by the intersection of the line with the solidus and liquidus.
Use lever rule for proportions

The eutectic temperature is always the lowest melting point for an alloy system.
Melting temperatures

Aluminum – 660 oC
Tin – 200 oC
Lead - 300 oC
Iron – 1500 oC
Tungsten - 3500 oC
Silicon - 1400 oC
Nickel – 1700 oC
Titanium - 1600 oC

Iron metals make up 85% of US metal tonnage.
Electrolytic iron is the most pure, at 99.99%, used in research
Ingot iron – contains .1% impurities, used for high ductility and corrosion resistance.
Wrought iron – contains 3% slag, easily shaped in forging.

Ferrite (alpha α) up to .022%
Austenite (gamma γ) up to 2.1%
The difference in solubility between alpha and gamma provides opportunities for strengthening by heat treatment.

FeC3 is cementite

Steel – an iron carbon alloy containing .02% to 2.1% carbon
Cast iron = an iron carbon alloy containing between 2.1% to 4 or 5%.

Eutectic composition of Iron Carbon is 4.3%.

Blast furnace –used to make iron. Diameter is 3 stories wide by 12 stories tall
Iron ore (Fe2O3) + coke (high carbon fuel)+limestone (CaCO3  reacts with molten iron and removes impurities as slag). Hot gasses are forced in at the bottom to achieve combustion and reduction of the iron.
7 tons of raw materials are required to produce one ton of iron!

Iron tapped from the base of the blast furnace (pig iron) has 4% C and other impurities.
A furnace called a cupola is used to convert pig iron into gray cast iron.
Steel needs compositions of impurities brought down to lower levels.

Steel making

  1. Basic Oxygen Furnace – accounts for 70% of US Steel production. 5 m inside diameter. Can process 200 tons of steel in 45 minutes.

 

  1. Electric Arc furnace – produces higher quality steel than the basic oxygen furnace, but at a higher cost. Process takes 4 hours total.

Molten steel is then cast as an ingot, or continuously cast, which is better since solidification time is less than ingot casting.

Steel

  1. plain carbon steels – Carbon is the main alloying element
  2. Low alloy steels – F-C alloys that contain additional elements in amounts less than 5 wt%.
  3. Stainless steels – main alloying element is Cr (>15%), and Ni also to reduce corrosion. Stainless steels are not supposed to corrode or oxidize.
  4. Tool steels – highly alloyed, heat treated steels for use as industrial cutting tools, dies, and molds.

Cast Irons
Contains 2.1 - 4% carbon, and 1-3% silicon.

Aluminum

Aluminum can be extracted by 3 steps

  1. washing and crushing the ore into fine powders
  2. converting powders to alumina (Al2O3) by precipitation
  3. electrolysis – separation of alumina into aluminum and oxygen gas (O2)

Properties of aluminum

  1. lightweight
  2. high electrical and thermal conductivity
  3. excellent corrosion resistance
  4. very ductile
  5. pure aluminum is not strong, but can be alloyed to increase strength.

Magnesium
Extracted from seawater. Seawater mixed with milk of lime, which precipitates a slurry, which is made more concentrated by adding acid. Then electrolysis is used to decompose salt into Mg and Cl2.

Mg is the lightest of the structural metals. It is easy to machine, but Mg can oxidize rapidly, so it poses a fire hazard.
Mg is relatively soft, but can be alloyed to increase strength. Its strength to weight ratio is good for aircraft and missile components.

Copper
Excellent electrical conductor, and a good thermal conductor
One of the noble metals (gold and silver are also noble metals), meaning it is corrosion resistant.

Copper ore is crushed, concentrated by flotation, and then smelted (melted or fused with a chemical reaction to separate the metal from the ore).
            Resulting copper is 98-99% pure
            Electrolysis can be used to obtain higher purity levels

Copper is alloyed to improve strength. Copper and Zinc (65% Cu and 35% Zn) is brass.

Nickel
Similar to iron because of magnetic properties, and similar modulus of elasticity E
Different from iron because Ni has good corrosion resistance (used in stainless steel)

To extract Nickel, ore is crushed and ground with water. Flotation is used to separate sulfides from other minerals. Then the Nickel sulfide is heated to burn off the sulfur, and then smelted to remove iron and silicon, and the end product is NiS. Then electrolysis is used to separate Ni from S.

Ni alloys are corrosion resistant and have high temperature performance.

Titanium
Lightweight, and has good strength to weight ratio.

To extract Ti, react TiO2 with chlorine gas. Then cast ingots.

Titanium has a low coefficient of thermal expansion, has good strength at high temperatures, but is reactive in the molten state. At room temp., it has good corrosion resistance, so it is used for marine components and prosthetic implants.

Zinc
Has a low melting point, so good for die casting
Corrosion protection when coated onto steel – galvanized steel refers to steel coated with zinc.
Also used as an alloy with copper to make brass.

Ore is Zinc blende (ZnS). Crushing, then grinding with water to create a slurry. Mineral particles float to the top and are skimmer off. The ZnS is roasted so ZnO is formed, and then electrolysis separates the Zinc from the oxygen.

Lead and Tin
Low melting point, and used for soldering alloys

Refractory Metals – metals capable of enduring high temperatures and maintaining high strength and hardness.
Molybdenum – high melting point, good high temperature strength. Used in jet engines

Tungsten – highest melting point among metals, one of the densest and stiffest and hardest of pure metals. Used in jet engines, filament wire in incandescent light bulbs.

Superalloys – high performance alloys that meet requirements for strength and resistance to surface degradation at high service temperatures. Very expensive. Room temperature strength properties are good, but not amazing. Key thing is high temperature performance for tensile strength, and corrosion resistance. Operating temperature is around 1100 oC
For all superalloys, strengthening is done by precipitation hardening.

 

Chapter 7
Ceramics – an inorganic compound consisting of a metal and one or more nonmetals.
Silica – SiO2
Alumina - Al2O3

Most common elements in the earth’s crust – Oxygen (50%), Silicon (26%), Aluminum (7%), Iron (5%).
Properties of ceramics – hard, brittle, electrical and thermal insulators, chemical stability, and high melting temperature. Also, some ceramics are translucent like window glass.

Ceramic products
1. Clay construction – bricks, clay piping, building tiles
2. Refractory ceramics – capable of high temperature applications like furnace walls, crucibles, and molds.
3. Whiteware products – pottery, stoneware, fine china, porcelain, based on mixtures of clay and other minerals.
4. glass, glass fibers, abrasives (oxides and carbides for grinding wheels), cutting tools.

 

Ceramic compounds are covalently and ionically bonded, which are stronger than metallic bonds. Metals have the advantage of slip, which allows for plastic deformation. Because ceramics do not have slip, they cannot absorb stresses easily, and can easily experience brittle fracture, since they have the same imperfections in crystal structures as metals (vacancies, interstitials, microscopic cracks). Ceramics have low tensile strength and low toughness.
Ceramics can handle compression much better than tension
Ceramics are less dense than metals but denser than polymers
Ceramics have higher melting temperatures than most metals.
Ceramics have less thermal expansion than metals, but effects are more damaging because of brittleness.

Three Basic types of ceramics – melting temperature of ceramics is about 3000 oC
1)Traditional Ceramics – silicates used for clay products like pottery and bricks and cement. Silica, such as quartz, is the most important raw material for traditional ceramics. Glazing involves the application of a surface coating usually silica or alumina to make the product less pervious to moisture and more attractive to the eyes.
2) New Ceramics – more recently developed ceramics based on nonsillicates such as oxides and carbides. New ceramics have properties that are superior or unique. compared to traditional ceramics.
            a) oxides – most important oxide ceramic is Alumina Al2O3. Alumina has good hot hardness, low thermal conductivity, and good corrosion resistance
            b) carbide ceramics – Silicon carbide (SiC) is considered a traditional ceramic since it was discovered a century ago. WC and TiC are new ceramics though, and have high hardness and wear resistance (good for cutting tools)
            c) nitrides – BN and TiN – hard brittle, high melting temperatures
3) Glasses – silica structure which is noncrystalline. Glasses can be considered as traditional ceramics. As a state of matter, glass refers to an amorphous, or noncrystalline structure of a solid material. The glassy state occurs when insufficient time is allowed during cooling from the molten state to form a crystalline structure. Silica is the main component (50-75%) of glass. It is used in glass because silica naturally transforms into a glassy state upon cooling, whereas most ceramics crystallize upon solidification.

Glass-ceramics – a ceramic material produced by converting glass into a polycrystalline structure through heat treatment. Crystalline phase – 90-98%. Grain size is about a micro meter.
Glass ceramics are much stronger than the glass from which they are derived. Glass ceramics and also opaque due to their crystal structure.
Advantages of glass-ceramics: efficiency of processing in the glassy state, close dimensional control over final shape, high strength, low thermal expansion, high resistance to thermal shock, absence of porosity. Glass-ceramics are used in cooking ware, heat exchangers, and missile radomes.

Graphite – carbon in the form of layers of covalently bonded carbon. Layers are bonded by weak Van der Waals forces. Structure makes graphite very anisotropic (properties vary significantly with direction). Graphite can be used as a lubricant, or as a fiber in fiber reinforced plastics.

Diamond – carbon with a cubic crystalline structure with covalent bonding. Covalent bonding makes diamond have a high hardness. Diamond is used as a cutting tool and grinding wheel for machining hard and brittle materials. It can be fabricated by heating to 3000 oC and high pressure.

Chapter 8
Polymer – a compound consisting of long chain molecules made up of repeating units connected together. Most polymers are based on carbon, so are organic chemicals.

Thermoplastic polymers and thermosetting polymers are plastics, and elastomers are rubbers.

Thermoplastic polymers
Solid at room temperature, but when heated to temperatures of only a few hundred degrees, become viscous liquids. This allows for easy and economical shaping.
Thermoplastics can be heated and cooled multiple times without degradation
Mod of elasticity is low, 2 to 3 orders of magnitude lower than metals and ceramics.
Low tensile strength, about 10% of a metal
Less dense than metals or ceramics, higher coefficient of thermal expansion, much lower melting temperatures, higher specific heats.
Much less hard than metals or ceramics, but more ductile on average.
Example: polyethylene, PVC, polystyrene, nylon.

Thermosetting Polymers
When initially heated, they soften and flow for molding. Elevated temperatures also produce a chemical reaction that hardens the material into an infusible solid. After reheating, thermosets degrade and char rather than soften.
Rigid, E is twice that of thermoplastics, brittle, no ductility,
Example: phenols, epoxies

TS is not used as much as TP

Elastomers
Have very elastic behavior when subjected to relatively low mechanical stress.
Some can be stretched by a factor of 10 and completely recover their original shape when the load is released.
Example: natural rubber
Rubber is derived from latex, a milky substance produced by various plants, including the rubber tree that grows in tropical climates.
Vulcanization is the curing to cross-link elastomers.
Carbon black is used in tires as an additive. It reinforces the rubber and increases tensile strength and resistance to tear.
Tonnage of synthetic rubbers is more than 3 times that of natural rubbers. Main raw material for synthetic rubbers is petroleum.

 

Thermoplastics are the most commercially important, and have about 70% of the tonnage of all synthetic polymers. On a volume basis, current annual usage of polymers exceeds that of metals.

Polymers are important because plastics can be molded to intricate and complicated part shapes, molded to net shape, cost competitive with metals, less energy required than for metals, and some plastics have translucent or transparent properties, which make them competitive with glass.

Polymers have low density relative to metals and ceramics, good strength to weight ratios, high corrosion resistance, and low electrical and thermal conductivity.
Polymers have low strength relative to metals and ceramics, low modulus of elasticity, low service temperatures, viscoelastic properties which can be bad in load bearing applications, and some polymers degrade due to sunlight and other radiation.

Ethylene is C2H4

A macromolecule has n repeating mers. n is an average which is called the Degree of Polymerization. Higher DP means higher strength, but also increases viscosity in the fluid state, which makes processing more difficult.

Molecular weight = n times molecular weight of each repeating unit.

 

Thermoplastics are linear, and branched. Elastomers are loosely cross-linked, and thermosets are tightly cross linked.

Below the glass transition temperature, an amorphous polymer is hard and brittle
As degree of crystallinity is increased, a polymer has increased density, stiffness, and melting temperature increase.

Chapter 9 Composites
Composite – a material composed of two or more distinct phases whose combination produces aggregate properties that are different from those of its constituents.
Composites can have strength to weight ratios several times that of steel or aluminum. Fatigue properties are better than for other metals. Toughness is greater, and it’s possible to achieve combinations of properties that are impossible with the three basic materials alone.

 

Composites are anisotropic (properties differ in the direction in which they are measured)

Traditional composites have been used for decades or centuries, some from nature, like wood.
Synthetic composites are manufactured.

Many polymer based composites are subject to attack by chemicals or solvents.
Composites are expensive, and manufacturing is slow and costly

Primary phase is called the matrix. It provides the bulk form of the composite, and when a load is applied the matrix shares the load with the secondary phase. In some cases the matrix deforms so the stress is essentially born by the reinforcing agent.

Secondary phase is embedded in the primary phase as the reinforcing agent because it strengthens the composite material.

Glass is the most widely used fiber in polymers

Interface – between the matrix and the fiber
Interphase – an additional ingredient used to bond the primary and secondary phases.

 


The fiber is a brittle material (glass)
The matrix is a soft and ductile material such as a polymer.

Chapter 10
Casting – a molten metal flows by gravity or pressure into a mold where it solidifies into the shape of the mold cavity.

Advantages of casting – can create complex geometries, can create both external and internal shapes, can produce very large parts, suited for mass production.
Disadvantages of casting – limitations on mechanical properties, poor dimensional accuracy and surface finish, safety hazards to workers due to hot molten metals, environmental problems

Foundry- factory for making molds, melting and handling the molten metal, and performing the casting process.

Actual size and shape of mold cavity must be slightly oversized to allow for the shrinkage of the metal during solidification.

Expendable mold processes – the mold must be destroyed to remove casting. Usually made of sand. More intricate geometries are possible

Permanent mold processes – uses a permanent mold which can be reused multiple times to produce many castings. Usually made of metal. Part shapes in permanent mold processes are limited by the need to open the mold. More economic in high production operations.

In sand casting, cores are generally made of sand.
Riser – the reservoir in the mold which is a source of liquid metal to compensate for shrinkage of the part during solidification. The riser must be designed to freeze after the main casting.

In order to pour the molten metal successfully, the metal must flow into all regions of the mold before solidifying, most importantly the main cavity.

 for a pure metal
 grain structure in a casting for a pure metal

Bottom trapezoid should be a different color to show segregation in alloy solidification, in which the regions where the casting freezes first (at the outside near the mold walls) are richer in one component than the other, so the remaining molten alloy is deprived of that component by the time freezing occurs at the interior.

Chvorinov’s rule:
A is surface area, n is typically 2, and Cm is the mold constant. Mold constant of riser and casting will be equal

To achieve directional solidification, use Chvorinov’s rule by locating the sections of casting with lower V/A ratios away from the riser, so freezing occurs first in these regions.

Chills are internal or external heat sinks that cause rapid freezing in certain regions of the casting.

Chapter 11
Expandable mold processes have the advantage of more complex shapes possible, but the disadvantage of production rates being determined by the time required to make the mold rather than the casting itself.

Permanent mold processes have higher production rates, but have geometries that are limited by the need to open the mold.

Sand Casting – most widely used casting process, steel, nickel, and titanium can be cast.
Sand casting requires a pattern, or a full-sized model of the part which is enlarged to allow for shrinkage. The pattern is commonly made out of wood because it is easy to work, but it warps. Metal is more expensive for the pattern, but lasts much longer. Plastic is compromise between wood and metal.

Core – a full-scale model of interior surfaces of the part. Usually made of sand.
Chaplets – made of a metal with a higher melting temperature than the casting metal. Used to support the core.

Desirable mold properties

    1. strength – to maintain shape and resist erosion
    2. permeability – to allow hot air and gasses to pass through voids in the sand
    3. thermal stability – to resist cracking on contact with molten metal
    4. Collapsibility – ability to give way and allow the casting to shrink without cracking the casting.
    5. reusability of sand

Sand Molds: Silica or Silica mixed with other materials. Small grain size yields better surface finish on the cast part. Large grain size is more permeable, which allows gases to escape during pouring. Sand is held together by water and clay.

Green sand molds – mixture of sand, clay, and water. Green means mold contains moisture at time of pouring

Dry sand mold – organic binders rather than clay, and mold is baked to improve strength.

Skin dried mold – drying mold cavity surface of a green sand mold to an inch.


Buoyant force = weight of metal displaced – weight of core
Density of sand core is 1.6 g/cm3

Shell Molding – mold is a thin shell of sand held together by a thermosetting resin binder. Box containing sand is inverted so sand falls on a hot pattern. Shell is heated in an oven and then stripped from the pattern.
            Advantages and disadvantages – smoother cavity allows easier flow of molten metal and better surface finish. Good dimensional accuracy, but more expensive

Vacuum Molding – uses a sand mold held together by vacuum pressure. Vacuum refers to the making of the mold rather than the casting operation itself.
Advantages and disadvantages: the sand can easily be recovered since there are no binders, moisture-related defects are absent. Disadvantages: slow process, not easily automated

Expanded polystyrene process
Uses a mold of sand packed around polystyrene foam that vaporizes when the molten metal is poured into the mold. Advantages: pattern need not be removed from the mold. Simplifies and speeds up the mold making process since two mold halves are not required. Disadvantages: a new pattern is needed for every casting.

Investment casting (lost wax process)
A pattern made of wax is coated with a refractory material to make the mold, after which the wax is melted away prior to pouring the molten metal.
1) wax patterns are produced
2) several patterns are attached to a sprue to form a pattern tree.
3)the pattern tree is coated with a thin layer of refractory material
4) the full mold is formed by covering the coated tree with refractory material to make it rigid
5) the mold is inverted and heated so wax melts and drips out of the cavity.
6) molten metal is poured
Advantages: can cast complex parts, good dimensional control, net shape process
Disadvantages: many processing steps, and expensive

Permanent mold casting processes
Includes basic permanent mold casting, die casting, and centrifugal casting.

Basic permanent mold casting – uses a metal mold constructed of two sections
Molds used

Advantages: good dimensional control and surface finish, rapid solidification, high volume production.
Disadvantages: generally limited to metals of lower melting point. Need to open mold only allows for simpler part geometries.

Die Casting
A permanent mold casting in which molten metal is injected into the mold cavity under high pressure. Pressure is maintained during solidification, then the mold is opened and the part is removed. Molds are called dies. 500 parts per hour!

Hot-chamber die casting – metal is forced into mold by a plunger.

Cold-chamber die casting – metal is poured into an unheated chamber and THEN a ram forces the metal to flow into a die.

Advantages: good for large quantities, good accuracy and surface finish
Disadvantages: limited to metals with low melting points

Centrifugal casting
Mold is rotated at high speeds so centrifugal force distributes molten metal to outer regions of die cavity.
True centrifugal casting – molten metal is poured into a rotating mold to produce a tubular part.

Semicentrifugal casting – mold is designed with part cavities located away from axis of rotation, used for smaller parts, which do not have to be tubular.

Casting defects

Misrun:
Cold shut  two portions of metal flow together but there is a lack of fusion due to premature freezing.

Cold Shots
 metal splatters during pouring

Shrinkage cavity

 

Now SAND CASTING DEFECTS
Sand blowballoon shaped gas cavity caused by the release of mold gases during pouring

Pin Holes formation of many small gas cavities below surface of casting

Penetration when fluidity of liquid metal is high, it penetrates into sand mold or core

Mold Shift cope and drag are displaced sideways.

 

Metals for casting
            Most commercial castings are alloys instead of pure metals because alloys are easier to cast.
Cast iron is the most important of casting alloys. Typical pouring temperatures are about 1400 oC.

Steel is harder to cast because its pouring temperature is at 1650 oC.

 

Superheat is the temperature difference above the melting point at which the metal is poured.

Typical machining allowances for sand casting is a few millimeters.

 

Chapter 13
Total volume of polymers exceeds that of metals.
Plastic molding is a net shape process.
Less energy is required than for metals due to lower processing temperatures

To shape a thermoplastic polymer, it must be heated so it softens to the consistency of a liquid. In this form it is called a polymer melt.

Viscosity is a property that relates shear stress to the shear rate.
Due to its high molecular weight, a polymer melt is a thick fluid with high viscosity.

Viscosity decreases with increasing shear rate.
Viscosity also decreases with increasing temperature
In both cases, the fluid becomes thinner.

Viscoelasticity is a combination of viscosity and elasticity, and is possessed by both polymer solids and polymer melts.

Die swell  - extruded polymer remembers its previous shape when in the larger cross section of the extruder, and tries to return to it after leaving the die orifice.

Extrusion:
Material is forced to flow through a die orifice to provide a long continuous product. Used for both thermoplastics and elastomers to mass produce items such as tubing, pipes, sheets and films. Continuous process, so extrudate is then cut into desired lengths.

The screen pack (includes the breaker plate) acts to:
            1. filter contaminants and hard lumps from the melt
2. build pressure in the metering section
3. straighten the flow and remove its memory of the circular motion imposed by the screws.

 

Blown film process
 for the high production of thin tubular film

Fiber and Filament production – spinning

    1. for synthetic fibers, spinning is the extrusion of polymer melt through a spinneret, then drawing and winding onto a bobbin.
    2. Spinneret = die with multiple small holes
    3. Similar to drawing and twisting natural fibers into yarn or thread.


Injection molding
Polymer is heated to a highly plastic state and forced to flow under high pressure into a mold cavity where it solidifies. Produces net shape. Cycle time is less than a minute
Complex shapes are possible
Shape is limited by allowing for part removal from mold.
Part size is 2 oz to 50 lb. Injection molding is only economical for large production quantities.
Thermoplastics are most widely used.

Thermoforming
A flat thermoplastic sheet or film is heated and deformed using a mold.
Widely used in packaging of products

After heating, a vacuum draws the sheet into the cavity.
Negative mold has a concave cavity
Positive mold has a convex cavity

 

Chapter 18 – Metal Forming
Plastic deformation is used to change the shape of metal workpieces. The tool is called a die, and it applies stresses that exceed the yield strength of the metal.

It is desirable to have materials with low yield strength, and high ductility
At higher temperatures, strength decreases and ductility increases.

Bulk deformation – significant deformations and massive shape changes. Bulk refers to relatively low surface area to volume ratios

 

  

Sheet Metalworking – forming operations on metal sheets, strips, and coils.
High surface area to volume ratio
Parts are called stampings
Usual tools: punch and die.

 

In the plastic region, a metal’s behavior is expressed by the flow curve:

 Flow curve uses true stress and strain
Yf is the flow stress

Average flow stress:

For any metal, K and n in the flow curve depend on temperature
Both K and n are reduced at higher temperatures.
Cold working – metal forming performed at room temperature or slightly above.
Advantages over hot working

  1. better accuracy
  2. better surface finish
  3. strain hardening increases strength and hardness of the part
  4. grain flow during deformation provides for good directional properties
  5. no heating of work, so lower furnace costs and higher production rates

Disadvantages

  1. higher forces and power required
  2. surfaces must be cleaned of dirt
  3. metal may not be ductile enough to be cord worked

Warm working – temperatures above .3 Tm, where Tm is the melting point in absolute temperature.
Advantages:
            1. lower forces and power
            2. more intricate work geometries possible
            3. need for annealing reduced

Hot working – deformation at temperatures above the recrystallization temperature (.5Tm). Strength coefficient K is less than at room temperature. N is zero (theoretically)
Advantages over cold working

  1. shape of the workpart can be significantly altered
  2. lower forces and power are required to deform the metal
  3. metals that usually fracture in cold working can be hot formed
  4. strength properties are generally isotropic

disadvantages
            lower dimensional accuracy, higher energy required, worse surface finish.

Strain rate sensitivity: as strain rate increases, resistance to deformation increases.
As temperature increases, C decreases, and m increases.
Log log axes, where m is the slope.

Friction in metal forming is undesirable because the metal flow is retarded, forces and power are increased, and tools wear out faster.
Chapter 19

Rolling – the slab or plate is squeezed between opposing rolls
Hot rolling – rolling hot worked metals
Cold rolling – produces finished sheet and plate stock
Flat rolling – used to reduce the thickness of a rectangular cross section
Shape rolling – square cross section is formed into a shape such as an I-beam.

Bloom – square cross section
Slab – rectangular cross section
Billet – square cross section but smaller than a bloom – used to make bars and rods


Draft d= to – tf  (original thickness – final thickness)
Reduction
Friction at the entrance side is greater so the net force pulls the work through the rolls.

 

Shape rolling – work is deformed into a contoured cross section rather than flat (rectangular cross sections)
Can make I-beams, L-beams, and railroad tracks.

Rolling mills
Basic rolling mill is the two-high rolling mill. In non reversing, rolls always rotate in the same direction.
Three-high rolling mill has 3 rolls in a vertical column

uses two smaller diameter rolls to contact the work


Smaller roll radius reduces roll work contact length, which reduces the forces, torque and power required for rolling.

Thread rolling is a bulk deformation process that can make threads on screws.
Advantages over thread cutting – higher production rates, better material utilization, and stronger threads due to work hardening.

Ring rolling – deformation process in which a thick-walled ring of small diameter is rolled into a thin-walled ring of larger diameter.

Forging

In Open Die Forging, the work is compressed between two flat dies, allowing metal to flow laterally with minimum constraint.

Impression Die Forging – die contains a cavity impression that is imparted to workpart. The metal flow is constrained so that flash is created.

Flashless forging – workpart is completely constrained in the die. No excess flash.

Open Die Forging – compression of workpart between two flat dies. Similar to compression test where workpart has a cylindrical cross section.
The height is reduced while diameter is increased.
If there is no friction, true strain is given by
With friction, there is a barreling effect.

Impression Die Forging (closed die forging)

Flash is formed by metal that flows beyond die cavity into small gap between die plates.
Flash needs to be trimmed later, but it serves to constrain material to fill the die cavity because of friction. Also in hot forging the metal flow is further restricted by cooling against the die plates.

Advantages compared to machining from solid stock

    1. higher production rates, less waste, greater strength,

Disadvantages

    1. not capable of close tolerances, additional machining is required to achieve accuracies and features.

Flashless forging

Compression of work material in punch and die tooling, with no flash.
Starting workpart volume must equal die cavity volume within very close tolerance.
Good for part geometries that are simple and symmetrical.
Flassless forging is often called precision forging.

Forging hammers (drop hammers) – apply impact load against the workpart.
Two types – gravity drop hammers (impact energy from falling weight of a heavy ram

    1. power drop hammers – accelerate the ram by pressurizing air or steam.

Upsetting and Heading – creates the head of a screw

    1. deformation process in which a cylindrical workpart is increased in diameter and reduced in length.
    2. Trimming is a cutting operation to remove the flash from a workpart in impression-die forging.

Extrusion
Compression forming process in which work metal is forced to flow through a die opening to produce a desired cross-sectional shape. Two types: direct and indirect extrusion
Direct Extrusion

As the ram approaches the die opening a small portion of billet remains that cannot be forced through. This extra portion is called the butt, and must be separated by cutting it just beyond the die exit.

Indirect Extrusion

Die is mounted to the ram. No friction at container walls, and therefore ram force is less than in direct extrusion.

Advantages of extrusion – variety of shapes possible

Extrusion of reduction ratio: for both direct and indirect

Wire and Bar Drawing
Cross section of a bar, rod, or wire is reduced by pulling it through a die opening.
Similar to extrusion except the work is pulled through the die in drawing instead of pushed as in extrusion.

Difference between bar and wire drawing is the stock size. Bar drawing is used for large diameter bar and rock stock, while wire drawing applies to smaller diameter stock.

Area reduction is drawing r =

Chapter 21

Machining is a manufacturing process in which a sharp cutting tool is used to cut away material to leave the desired part shape.
Shear deformation of the work material forms a chip which is removed and a new surface is exposed.
Machining has good dimensional accuracy, but is wasteful of material.
Machining is performed after the other manufacturing processes such as casting, forging, and bar drawing.

Turning – a cutting tool with a single cutting edge is used to remove material from a rotating workpiece to generate a cylindrical shape.


Single point tool has one cutting edge, and is used for turning
Multiple cutting edge tools have more than one cutting edge and are used for drilling and milling.

Roughing cuts are made before finishing cuts. Roughing removes large amounts of material from starting workpart, while finishing cuts complete the part geometry.
Cutting speed = v (primary motion)

Feed f is speed of moving tool (secondary motion)
Depth of cut d – penetration of tool below original work surface.

Material removal rate:

RMR is volume/time

 

Orthogonal Cutting model – Simplified 2-D model of machining that describes the mechanics of machining fairly accurately.

Rake angle = alpha
chip thickness ratio r = to/tc, where to thickness of the chip prior to chip formation. tc is thickness after the cut.
Chip thickness after the cut is always greater than before so chip ratio is always less than 1.

Shear plane angle phi
 
Shear strain gamma

Discontinuous chip – when hard brittle work materials are cut at low speeds, the chips form into separate segments.

Continuous chip – when ductile work materials are cut at high speeds and small feeds and depths, long continuous chips are formed.

Continuous with built up edge – when machining ductile materials at low to medium speeds, but friction between the tool and chip causes portions of the work material to adhere to the rake face of the tool.

Serrated chip – semi-continuous, have saw tooth appearance.
Coefficient of friction between tool and chip:
 
beta = friction angle
Fs = shear force
Shear stress S = , where As is area of shear plane

Merchant equation

Of all the possible angles at which shear deformation can occur, the work material will select a shear plane angle phi that minimizes energy.

Merchant equations tells us that…
            To increase the shear plane angle, either increase the rake angle, or reduce the friction angle (or coefficient of friction)

Higher shear plane angle means smaller shear plane, which means lower shear force, cutting forces, power, and temperature.

Power to perform machining:
 


 

Chapter 26 – nontraditional machining and thermal processes

Nontraditional – a group of processes that remove excess material by various techniques involving mechanical, thermal, electrical, or chemical energy. These processes do not use a sharp cutting tool in the conventional sense.

Mechanical – uses mechanical action to erode work material by a high velocity steam of abrasives or fluid.

Electrical – electrochemical energy to remove material

Thermal – thermal energy usually applied to small portion of work surface, causing that portion to be used and/or vaporized.

Chemical – chemical etchants selectively remove material from portions of a workpart, while other portions are protected by a mask.

Mechanical energy processes:
            Ultrasonic machining
            Water jet cutting
            Abrasive water jet cutting
            Abrasive jet machining

Ultrasonic machining – abrasives in a slurry are driven at high velocity against the work by a tool vibrating at low amplitude and high frequency. The tool oscillation is perpendicular to the work surface. The tool is slowly fed into the part.
Ultrasonic machining is used on hard brittle work materials such as ceramics, glass, and carbides. Can be used to create holes and coining operations where a pattern is imparted to a flat work surface.

Water Jet Cutting – uses high pressure, high velocity stream of water directed at work surface for cutting.
 
Standoff distance is the separation between the nozzle opening and the work surface.
Water Jet cutting can be used to cut narrow slits in flat stock such as plastic, textiles, composites, floor tile, carpet, leather, and cardboard.
Water Jet cutting is not suitable for brittle materials like glass because there is a tendency to crack during cutting
            Advantages – no crushing or burning of the work surface, minimal material loss, easily automated, no environmental pollution.

Abrasive Water Jet Cutting
When water jet cutting is used on metals, abrasive particles must be added to facilitate cutting.
Abrasives include aluminum oxide and silicon dioxide.
Grit sizes range between 60 and 120. Grits added to water stream at about .25 kg/min or .5 lb/min after it exits nozzle.

Abrasive Jet Machining – high velocity stream of gas containing small abrasive particles

Usually performed manually by an operator who directs nozzle
Normally used as a finishing process rather than cutting process
Applications – deburring, trimming and deflashing, cleaning, and polishing.
Work materials: thin flat stock of hard, brittle materials (glass, silicon, ceramics.

Electrochemical Machining – electrical energy used in combination with chemical reactions to remove material. (reverse of electroplating)
The work material must be a conductor

Processes include
            Electrochemical machining (ECM)
            Electrochemical deburring (ECD)
            Electrochemical grinding (ECG)

 

Electrochemical machining (ECM)
Material is removed by anodic dissolution, using electrode (tool) in close proximity to work but separated by a rapidly flowing electrolyte.
Material is deplated from anode workpiece (positive pole) and transported to a cathode tool (negative pole) in an electrolyte bath. The electrolyte flows rapidly between two poles to carry off depleted material so it does not plate onto the tool.
The electrode is made out of Cu, or brass.
Tool has the inverse shape of the part.


ECM applications:
            Die sinking – machining of irregular shapes and contours into forging dies, plastic molds, and other tools
            Multiple hole drilling – many holes can be drilled simultaneously with ECM
            Holes that are not round, since rotating drill is not used in ECM
            Deburring.

Electrochemical deburring (ECD)
Adaptation of ECM to remove burrs or sharp corners on holes in metal parts produced by conventional through-hole drilling.

Electrochemical Grinding (ECG)
Special form of ECM in which grinding wheel with conductive bond material augments anodic dissolution of metal part surface

Applications – sharpening of cemented carbide tools, grinding of surgical needles, other thin wall tubes, and fragile parts.

Advantages – deplating responsible for 95% of metal removal. Also because machining is mostly by electrochemical action, grinding wheel lasts much longer.


Thermal Energy Processes – very high local temperatures, material is removed by fusion or vaporization. Physical and metallurgical damage to the new work surface.

Thermal Energy Processes:
            Electrical Discharge machining
            Electric discharge wire cutting
            Electron beam machining
            Laser beam machining
            Plasma arc machining
            Conventional thermal cutting processes

Electrical Discharge Processes:
Metal is removed by a series of discrete electrical discharges (sparks) causing localized temperatures high enough to melt or vaporize the metal. Can only be used on electrically conducting work materials.
Two main processes are:
            Electric discharge machining
            Wire electric discharge machining
           
EDM is one of the most widely used nontraditional processes. The shape of the finished work surface produced by the shape of electrode tool.
Sparks occur across a small gap between tool and work
Requires dielectric fluid, which creates a path for each discharge as fluid becomes ionized in the gap.
Work materials must be electrically conducting
Hardness and strength of work materials are not factors in EDM
Material removal rate depends on melting point of work material.
Applications:
            tooling for many mechanical processes: molds for plastic injection molding, extrusion dies, wire drawing dies, forging and heading dies, and sheetmetal stamping dies.
            Production parts: delicate parts not rigid enough to withstand conventional cutting forces, hole drilling where hole axis is at an acute angle to the surface, and machining of hard and exotic metals.

Wire EDM
Special form of EDM which uses a small diameter wire as an electrode to cut a narrow kerf in work.

Kerf is the full diameter of the cut.
Work is fed slowly past wire along desired cutting path, like a bandsaw operation
CNC used for motion control.
While cutting, wire is continuously advanced between supply spool and take-up spool to maintain a constant diameter.
Dielectric required, using nozzles directed at tool-work interface or submerging workpart.

Applications:
Ideal for stamping die components
            Since kerf is so narrow, it is often possible to fabricate punch and die in a single cut.
            Other tools and parts with intricate outline shapes, such as lathe form tools, extrusion dies, and flat templates.

 

Electron Beam Machining (EBM)
Uses a high velocity stream of electrons focused on workpiece surface to remove material by melting and vaporization.
 
Electron beam gun accelerates a continuous stream of electrons to about 75% of the speed of light.
Beam is focused through electromagnetic lens, reducing diameter to as small as .001 in
On impinging work surface, kinetic energy of electrons is converted to thermal energy of extremely high density, which melts or vaporizes material in a very localized area.

Applications: works on any material
Ideal for micromachining
            Drilling small diameter holes, cutting small slots
Drilling holes with high depth to diameter ratios

Laser Beam Machining (LBM) – uses light energy from a laser to remove material by vaporization and ablation.
 
Laser = Light amplification by stimulated emission of radiation
Laser converts electrical energy into a highly coherent light beam that is both monochromatic (single wave length), and highly collimated (light rays are almost perfectly parallel).
These properties allow laser lights to be focused, using optical lenses, onto a very small spot with resulting high power densities.

Applications: drilling, slitting, slotting, scribing, and marking operations.
Work materials: metals with high hardness and strength, soft metals, ceramics, glass and glass, plastics, rubber

Chemical Machining
Material removal through contact with a strong chemical etchant.

Processes include:
            Chemical milling
Chemical blanking
            Chemical engraving
            Photochemical machining
All have the same mechanism of material removal

Cleaning – to insure uniform etching
Masking – a maskant (resist, chemically resistant to etchant) is applied to portions of work surface not to be etched.
Etching – part is immersed in etchant which chemically attacks those portions of work surface that are not masked
Demasking – maskant is removed.

Maskant usually polymer like polyvinylchloride
Masking is accomplished by any of three methods:
Cut and peel – maskant is applied over entire part by dipping, painting, or spraying. After maskant hardens, it is cut by hand using a scribing knife and peeled away in areas of work surface to be etched. Used for large workparts, low production quantities, and where accuracy is not a critical factor.
Photographic resist – Masking materials contain photosensitive chemicals. Maskant is applied to work surface and exposed to light through a negative image of areas to be etched. These areas are then removed using photographic developing techniques. Desired surfaces are protected by the maskant, and the remaining areas are vulnerable to etching. Applications are small parts produced in high quantities, integrated circuits and printed circuit cards.
Screen resist – Maskant applied by silk screening methods. Maskant is painted through a silk or stainless steel mesh containing stencil onto surface areas that are not to be etched. Applications: between other two masking methods, fabrication of printed circuit boards.

Etchant – factors in selection of etchant: work material, depth and rate of material removal, and surface finish requirements.

Etching occurs downwards and sideways under the maskant also. This is called Undercut:

Chemical Milling:

Applications of Chemical Milling:
            Remove material from aircraft wing and fuselage panels for weight reduction
            Applications to large parts where substantial amounts of metal are removed
            Cut and peel maskant method is used.

Chemical Blanking
Uses chemical erosion to cut very thin sheetmetal parts, and for intricate cutting patterns.
Conventional punch and die does not work because stamping forces damage the thin sheetmetal or tooling cost is prohibitive.
Maskant methods are either photoresist or screen resist.

Photochemical Machining (PCM)
Uses photoresist masking method
Applies to chemical blanking and chemical engraving when photographic resist method is used
Used extensively in the electronics industry to produce intricate circuit designs on semiconductor wafers.
Also used in printed circuit board fabrication.
PCM is used in metalworking when close tolerances or intricate patterns are required on flat parts.

Chapter 34
Rapid prototyping – A family of fabrication processes developed to make engineering prototypes in minimum lead time based on a CAD model of the item.

Traditional method is machining, and can require significant lead-times (several weeks)

Rapid prototyping allows a part to be made in hours or days given a CAD model.

Why is rapid prototyping important?
            Product designers want to have a physical model of a new part or a product design rather than just a computer model or line drawing.
Virtual Prototype – CAD model
Using RP to make the prototype, the designer can see and feel the part and assess its merits and shortcomings.

Steps to prepare control instructions –
            Geometric modeling – model the component using CAD to define its enclosed volume
            Tessellation of the geometric model, the CAD model is converted into a computerized format that approximates its surfaces by triangles or polygons.
            Slicing of the model into layers – computerized model is sliced into closely-spaced parallel horizontal layers.

Three different types of RP are
            Liquid based
            Solid based
            Powder based

 

Liquid Based Rapid Prototyping – starting material is a liquid
            Included are: stereolithography, solid ground curing, and droplet deposition manufacturing
           
Stereolithography (STL) – RP process for fabricating a solid plastic part out of a photosensitive liquid polymer using a directed laser beam to solidify the polymer.
Stereolithography is the most common RP technique

Part fabrication is accomplished as a series of layers, where each layer is added onto the previous layer to gradually build the 3D geometry.
Introduced in 1988 by 3D systems based on work by Charles Hull.

Each layer is .003 in to .02 inches thick. Thinner layers provide for better resolution and more intricate shapes, but processing time is longer.
Starting materials are liquid monomers.
Polymerization occurs on exposure to UV light produced by laser scanning beam.
Scanning speeds ~ 500 to 2500 mm/s

Solid Ground Curing – like stereolithography, solid ground curing works by curing a photosensitive polymer layer by layer to create a solid model based on CAD geometric data.
Instead of using a scanning laser beam to cure a given layer, the entire layer is exposed to a UV source through a mask above the liquid polymer. Hardening takes 2 to 3 seconds for each layer.

Sequence for each layer takes about 90 seconds.
Time to produce a part by solid ground curing is 8 times faster than for other RP systems.
The solid cubic form created in solid ground curing consists of solid polymer and wax.
The wax provides support for fragile and overhanging features of the part during fabrication, but can be melted away later to leave the free standing part.

Droplet Deposition Manufacturing – starting material is melted and small droplets are shot by a nozzle onto a previously formed layer.
Droplets cold weld to surface to form a new layer
Deposition for each layer controlled by a moving x-y nozzle whose path is based on a cross section of a CAD geometric model that is sliced into layers
Work materials include wax and thermoplastics.

Solid Based Rapid Prototyping Systems:
Starting material is a solid
Solid based RP systems include: Laminated object manufacturing, and Fused Deposition modeling.

Laminated Object Manufacturing (LOM)
Solid physical model made by stacking layers of sheet stock, each an outline of the cross-sectional shape of a CAD model that is sliced into layers.
Starting sheet stock includes paper, plastic, metals, or fiber reinforced materials.
The sheet is usually supplied with adhesive backing as rolls that are spooled between two reels.
After cutting, excess material in the layer remains in place to support the part during building.
 

 

Fused Deposition Modeling (FDM)
Rapid prototyping process in which a long filament of wax or polymer is extruded onto existing part surface from a workhead to complete each new layer.
Workhead is controlled in the xy plan during each layer and then moves up by a distance equal to one layer in the z-direction
Extrudate is solidified and cold welded to the cooler part surface in about .1 s
Part is fabricated from the base up, using a layer by layer procedure.

Powder Based RP systems:
Starting material is a powder.
Powder-based RP systems include: selective laser sintering and three dimensional printing.

Selective Laser Sintering –
Moving laser beams sinters heat-fusible powders in areas corresponding to the CAD geometry model one layer at a time to build the solid part.
After each layer is completed, a new layer of loose powders is spread across the surface.
Layer by layer, the powders are gradually bonded by the laser beam into a solid mass that forms the 3D part geometry.
In areas not sintered, the powders are loose and can be poured out of the completed part.

Three Dimensional Printing
Part is built layer by layer using an ink-jet printer to eject adhesive bonding material onto successive layers of powders.
Binder is deposited in areas corresponding to the cross sections of part, as determined by slicing the CAD geometric model into layers.
The binder holds the powders together to form the solid part, while the unbonded powders remain loose to be removed later.
To further strengthen the part, a sintering step can be applied to bond the individual powders.

Manufacturing applications:
Small batches of plastic parts that could not be economically molding by injection molding because of the high mold cost
            Parts with intricate internal geometries that could not be made using conventional technologies without assembly
            One of a kind parts such as bone replacements that must be made to correct size for each user.

Problems with rapid prototyping:
            Part accuracy – staircase appearance for a sloping part surface due to layering, and shrinkage and distortion of RP parts
            Limited variety of materials in RP – mechanical performance of the fabricated parts is limited by the materials that must be used in the RP process.

 

Chapter 35
Processing of Integrated Circuits
An integrated circuit is a collection of electronic devices (transistors, diodes, and resistors) fabricated and electrically intraconnected (integrated) onto a small flat chip of semiconductor material.
Silicon is the most widely used semiconductor material for integrated circuits.
Less common is gallium arsenide (GaAs)
Since circuits are fabricated into one solid piece of material, the term solid state electronics is used for IC technology.

The levels of integration started small, and are very big with many devices on a chip.
An integrated circuit consists of hundreds, thousands, or millions of microscopic electronic devices that have been fabricated and electrically intraconnected on the surface of a silicon chip.
A chip, also called a die, is a square or rectangular flat plate that is about .5 mm thick and typically 5 to 25 mm on a side.
Integrated circuit transistor:


Packaging of ICs:
To connect the IC to the outside world, and to protect it from damage, the chip is attached to a lead frame and encapsulated inside a suitable package. The package is an enclosure made of plastic or ceramic, that provides mechanical and environmental protection for the chip. The package includes leads by which the IC can be electrically connected to external circuits.

 

Processing sequence for Silicon ICs:
Silicon processing – sand is reduced to very pure silicon and then shaped into wafers.
IC fabrication – processing steps that add, alter, and remove thin layers in selected regions to form electronic devices. Lithography is used to define the regions to be processed on wafer surface
IC packaging – wafer is tested, cut into individual chips, and the chips are encapsulated in an appropriate package.

Boule – 3-10 feet with 12 inch diameter. Boules are cut up into thin wafers of thickness .5 mm

Clean rooms – most of the IC processing takes place in a clean room. A clean room is more like a hospital operating room than like a production factory.
A number in increments of ten is used to indicate the quantity of particles of size .5 micrometers or greator in one cubic foot of air.
Class 100 clean room must maintain a count of particles of size .5 micrometers of greater at less than 100/ft3
Class 10 clean room must maintain a count of particles of size .5 micrometers of greater at less than 10/ft3
A clean room is air conditioned to 70 degrees F and 45% relative humidity.

Silicon Processing –
            Microelectronic chips are fabricated on a substrate of semiconductor material. More than 95% of all semiconductor devices produced in the world are made of silicon.
Preparation of silicon substrate can be done in 3 steps. 1) production of electronic grade silicon. 2) crystal growing. 3) shaping of Si into wafers.

Silicon is one of the most abundant materials in the earth’s crust, and occurs naturally as silica in sand and silicates in clay.

Electronic grade silicon is a polycrystalline silicon of ultra high purity, such high purity that impurities are measured in parts per billion.

The silicon substrate for microelectronic chips must be made of a single crystal whose unit cell is oriented in a certain direction. Silicon used in semiconductor device fabrication must be of ultra high purity. Substrate wafers must be cut in a direction that achieves the desired planar orientation.
Most widely used crystal growing method is the Czochralski process, in which a single crystal boule is pulled from a pool of molten silicon.

Shaping of Silicon into wafers:

  1. Ingot (boule) preparation
    1. the ends are cut off
    2. cylindrical grinding is used to shape the boule into a more perfect cylinder
    3. one or more flats are ground along the length of the boule, whose functions, after the boule is cut into wafers, are for identification, orientation of ICs relative to crystal structure, and mechanical location during processing.
  2. Wafer slicing
    1. cutting edge is a very thin ring-shaped saw blade with diamond grit on internal diameter.
    2. ID used for slicing rather than the OD for better control over flatness, thickness, parallelism, and surface characteristics of the wafer.
    3. Wafers are cut .5-.7 mm thick for greater thicknesses for larger wafer diameters.
    4. To minimize kerf loss, blades are very thin: thickness ~ .33 mm
  3. Wafer preparation
    1. Wafer rims are rounded by contour-grinding wheel to reduce chipping during handling
    2. Wafers are chemically etched to remove surface damage from slicing
    3. A flat polishing operation is performed to provide surfaces of high smoothness for photolithography processes to follow
    4. Finally wafer is chemically cleaned to remove residues and organic films.

Lithography
- In the planar process, regions are fabricated by steps that add, alter, or remove layers in selected areas of the wafer surface.
- Each layer is determined by a geometric pattern representing circuit design information that is transferred to the wafer surface by lithography.

Lithographic Technologies: Photolithography, electron lithography, x-ray lithography, ion lithography
The differences are in the type of radiation used to transfer the mask pattern to the wafer surface.

Photolithography – Uses light radiation to expose a coating of photoresist on the surface of the silicon wafer. A mask containing the required geometric pattern for each layer separates the light source from the wafer, so that only the portions of the photoresist not blocked by the mask are exposed.

The mask consists of a flat plate of transparent glass onto which a thin film of an opaque substance has been deposited in certain areas to form the desired pattern. Thickness of glass plate is around 2 mm, while deposited film is a micrometer thick. The mask itself is fabricated by lithography, the pattern being based on circuit design data, usually in the form of digital output from the CAD system used by a circuit designer.

Photoresist – an organic polymer that is sensitive to light radiation in a certain wavelength range. Sensitivity causes either increase or decrease in solubility of the polymer to certain chemicals. Typical practice in semiconductor processing is to use photoresists sensitive to UV light (UV light has a short wavelength compared to visible light, permitting sharper imaging of circuit details on wafer surface).

Contact Printing – the mask is pressed against the resist coating during exposure to UV light. Advantages : better resolution. Disadvantages: wear on mask

Proximity printing – the mask from the resist coating by a distance of 10-25 micrometers
Advantages: no mask wear. Disadvantages: worse image resolution.

Projection Printing – high quality lenses project image through mask onto wafer. Preferred technique because non-contact (thus, no mask wear) and optical projection can obtain high resolution.

Photolithography processing sequence:

  1. surface of the silicon wafer has been oxidized to form a thin film of SiO2.
  2. It is desired to remove the SiO2 film in certain regions as defined by mask pattern

Processing of a negative resist as follows:
     

Other lithography techniques – as feature size in integrated circuits continues to decrease and UV photolithography becomes increasingly inadequate, other lithography techniques that offer higher resolution are growing in importance:
Extreme ultraviolet lithography
Electron beam lithography
X-ray lithography
Ion lithography

Layering Processes in IC Fabrication:           

  1. thermal oxidation – adds SiO2 layer on Si substrate
  2. Chemical vapor deposition – adds various layers
  3. Diffusion and ion implantation – alter chemistry of an existing layer or substrate
  4. Metallization processes – add metal layers for electrical conduction
  5. Etching processes – remove portions of layers to achieve desired IC details

Input Output (I/O) Terminals – Basic problem is to connect many internal circuits on the chip to I/O terminals so that the appropriate electrical signals can be communicated to the outside world
As the number of devices in the IC increases, the required number of I/O terminals also increases
The problem is aggravated by IC trends – decreases in device size, and increases in the number of devices in integrated circuits.

IC Packages are usually made of Ceramics and Plastics.
Ceramics (Al2O3 – Alumina)
            Advantages: hermetic sealing of IC chip and highly complex packages can be produced
            Disadvantages: poor dimensional control due to shrinkage during firing
Plastics (epoxies, polyimides, and silicones)
            Not hermetically sealed, but cost is lower
            Generally used for mass produced ICs, where very high reliability is not required.

For assembling IC package to a printed circuit board (PCB), two basic types called through-hole mounting, also called pin-in-hole technology, and surface mount technology

For through hole / pin in hole, the IC package and other components have leads inserted through holes in the PCB and soldered on underside.

For Surface mount technology, components are attached to surface of board.

 

IC package styles: dual in line package (DIP), square package, pin grid array

Dual In-Line Package – the most common form of IC package, and available in both through hole and surface mount configurations.

Square package – leads are arranged around the outside so that number of terminals on a side is nio/4

Pin Grid Array (PGA) – two dimensional array of pin terminals on underside of a chip enclosure to maximize the number of leads on a package. Center region has no pins because this region contains the IC chip.

Wafer testing – using a multiprobe which probes using a needle. Many of these tests are performed while ICs are still on wafer (before separation) When probes contact pads, tests are carried out to indicate short circuits and other faults. Chips that fail are marked with an ink dot.

Chip separation – wafer is cut into individual chips using a thin diamond saw blade.

Die Bonding – automated handling systems pick separated chips from tape frame and place them for die bonding. Techniques used to bond the chip to the packaging substrate include Eutectic die bonding (for ceramic packages), and epoxy die bonding (for plastic packages).

Wire bonding – After die is bonded to package, electrical connections are made between contact pads on chip surface and package lead frame using small diameter wires.
 
Yields of Major Processing Steps:

 

Chapter 36 – Electronics Assembly and Packaging
Level 0 is IC chip (die)
Level 4 is wiring and cabling connections in cabinet.

Printed Circuit Board (PCB)
- One or more thin sheets of insulating material with copper conducting paths on one or both surfaces that interconnect the components attached to the board.
 - PCBs are used in packaging electronic systems to hold and electrically interconnect components, and make connections to external circuits.
- PCBs are standard building blocks I virtually all electronic systems that contain packaged ICs and other components.

Materials for Printed Circuit Boards:
- insulation materials for PCBs are usually polymer composites reinforced with glass fabrics or paper (polymers include epoxy, phenolic, and polyimide, and E-glass is the usually fiber in glass reinforcing fabrics, especially in epoxy PCBs).
- substrate layer thickness = .8 to 3.2 mm
- copper foil thickness is .04 mm.

Principal types of printed circuit board

    1. single sided board – copper foil is only on one side of the insulation substrate
    2. double sided board – copper foil is on both sides of the substrate
    3. multilayer board – consists of alternating layers of conductive foil and insulation. Multilayer board is used when a large number of components must be interconnected with many track routings. 4 layers is the most common configuration

 

Production of copper foil: Produced by continuous electroforming, in which a rotating smooth metal drum is partially submersed in an electrolytic bath containing copper ions.

    1. the drum is the cathode in the circuit, causing the copper to plate onto its surface
    2. As the drum rotates out of the bath, the thin copper foil is peeled from its surface
    3. The process is ideal for producing very thin copper foil needed for PCBs.

Hole-making processes for PCBs:
- holes in the board are either drilled or punched, using the tooling holes for location
-drilled holes are cleaner, but punching is faster
- most holes in PCB fabrication are drilled
- A stack of three or four panels may be drilled together, using a CNC drill
For high production jobs, multiple spindle drills are sometimes used

    1. drilling uses standard twist drills

Circuit Pattern Imaging

    1. two basic methods to transfer circuit pattern to copper surface board
    2. screen printing
    3. photolithography
    4. both methods use a resist coating on the board surface to determine where etching of the copper will occur to create the tracks and lands of the circuit.

Plating:
- plating is needed on hole surfaces for conductive paths from one side to the other in double sided boards, or between layers in multilayer boards
- two plating processes used in PCB fabrication:
                                    1) Electroplating – higher deposition rate but coated surface must be metallic (conductive)
                                    2) Electroless plating – slower but does not require a conductive surface

 

Subtractive method- open portions of the copper cladding on the starting board are etched away from the surface, so that the tracks and lands of the desired circuit remain

Printed circuit board assembly: based either on pin in hole or SMT

Pin in Hole Assembly Technology:
processing of PCB assemblies with leaded components consists of:
1) component insertion – insertion of leads into through-holes in circuit board
            a) A single board may be populated with hundreds of separate components, all of which need to be inserted into the board
            b) most component insertions are done automatically

(a) is an axial lead, (b) is a radial lead
2) Soldering – leads are soldered into place in the holes
            a) important techniques are wave soldering, and hand soldering
                        Wave soldering: mechanized technique in which PCBs containing inserted components are moved by conveyor over a standing wave of molten solder. Only the underside of the board, with leads projecting through the holes, contracts solder. Combination of capillary action and upward force of wave causes the liquid solder to flow into clearances between leads and through holds to obtain a good solder joint. All solder joints are made in a single pass.

            b) Hand soldering – skilled operator using soldering iron makes circuit connections. Slow since each solder joint is made one at a time.
3) cleaning
            a) hand cleaning with appropriate solvents and vapor degreasing to remove flux, oil, grease, and dirt.
4) testing – tests to insure proper circuit operation
5) Rework – manual repair of defects found in testing

Surface Mount technology – Assembly method in which component leads are soldered to lands on the PCB surface rather than into holes running through the board.

Pin in hole has inherent limitations in packing density since components can only be mounted on one side of the board.
Advantages of SMT:
- smaller components with leads closer together, so increased packing density.
Components can be mounted on both sides of the board
Drilling of the many through holes during board fabrication is eliminated (but via holes to interconnect layers are still required)

Component placement in SMT
- Correct positioning of component on PCB and affixing it sufficiently until soldering provides a permanent mechanical and electrical connection.

    1. two placement and soldering methods: adhesive bonding of components and wave soldering, solder paste and reflow soldering.

Adhesive Bonding and Wave Soldering:

    1. adhesive is applied by one of three methods
      1. brushing liquid adhesive through a screen stencil
      2. automatic dispensing machine
      3. pin transfer method

Curing and wave soldering

    1. after component placement, adhesive is cured by heat, ultraviolet light
    2. with components now bonded to surface, PCB is wave soldered

Solder Paste and reflow soldering:
- solder paste = suspension of solder powders in a flux binder
After solder paste application, components are placed on the board by onsertion machines.
Low temperature baking operation is performed
Solder reflow process melts the solder particles to form a high quality mechanical and electrical join between component leads and circuit lands.

 

Chapter 37

Aspect ratio = height to width ratio is much greater than in IC fabrication
Bulk Micromachining – any process that removes the substrate material
Surface micromachining – planar structuring of the substrate surface, using much more shallow etching
 
Advantages of LIGA

    1. high aspect ratios are possible (large height to width ratios)
    2. wide range of part sizes
    3. close tolerances

Disadvantages of LIGA

    1. very expensive

Chapter 38

Scanning Tunneling Microscope – a voltage is applied to the probe, causing electrons on the surface to tunnel up to the probe tip.
Atomic force microscope – probe is attached to a delicate cantilever that deflects due to forces exerted by surface atoms as probe traverses the specimen surface.

Buckyballs – C60
 Carbon nanotubes – carbon atoms bonded together in the shape of a long tube. Has superior conductivity to Cu, and tensile strength greater than steel.

 

 

 

 

 

 

          

2006 Philosophy Paradise