ME251 Final Exam
Additional Effects of Cold Working
"Intermediate anneal" - resets material - Annealing heat treatments may be required ~ Allows additional cold working and deformation - Cold working often produces an anisotropic (stronger in one direction vs others) structure
Casting Metal
(compared to other fabrication processes) Advantages: - Complex shapes - Parts can have hollow sections or cavities - Very large parts - Use to form parts from metals that are difficult to machine (high strength) Disadvantages: - Surface finish :( (pretty bad, add polishing, etc) - Material waste Types: - Different mold materials - Different pouring methods
Forming Operations - Sheet
- Bending - Drawing - pressing sheet into mold - Stretching - tension to sheet, pull down over some geometry (form die)
Advantages/Disadvantages of NTM
- Capable of making complex, delicate, and very small (<<1mm features) - Tight tolerances - Often very good surface finish - Little or no burring or residual stresses - Materials with high strength or hardness can be machined * relatively high precision * Apply little to no force to workplace - NTM processes typically have low feed rates and high power consumption ($$$) ~ Some NTM processes can machine the entire surface at once (chemical)
Molten Metal Problems
- Chemical reactions can occur between molten metal and its surroundings ~ Metal oxides created due to reactions between oxygen and the molten metal (dross or slag aka garbage metal) - When remelted, metal absorbs gas in atmosphere. After pouring, metal solidifies and gas becomes less soluble, forms pores in solid metal
Isothermal Forming
- Deformation occurs under constant temperature - Dies, tooling, and workpiece are heated (everything heated to same temperature - eliminates heat loss to tooling) - Eliminates cracking - Inert atmospheres (minimize reactions) may be required - Very slows deformation - reduce heat generated by deformation - $$$ - Steep gradient
Deformation Processes
- Deformation processes exploit the ability of engineering materials to flow as a solid without deterioration of properties (plasticity) Pros: + No need to handle molten metal or deal with solidification problems + Often very low waste Cons: - Required forces can be very high - Machinery and tooling are often expensive
Gating Systems
- Delivers the molten metal to the mold cavity; proper design minimizes turbulence (can degrade mold and can mix air into metal) - Sprue well, runner well, and runner extension: designed to trap the first metal to enter the mold (dross and slag) - should just be good metal in those regions
Steps required to produce a product
- Design - Material selection - Process selection - Manufacture - Inspection and evaluation - Feedback
Grain Growth and Structure
- Direction, rate, and type of grain growth is determined by direction and rate of heat removal Three distinct regions or zones: 1. Chill Zone: - Rapid nucleation at cold walls of the mold - Strongest metal, fastest cooling region - Small grains 2. Columnar Zone: - Rapid growth perpendicular to the casting surface - Long and thin, highly directional - Weakest, cool slow - Huge grains 3. Equiaxed Zone: - Crystals in the interior of the casting - Spherical, randomly oriented crystals - Tiny crystals, sometimes get voids
Chip Formation
- Discontinuous - Good (best option, lowest amount of friction heat generated) - Continuous - Meh - Continuous with build ip edge (BUE) - Bad ** Brittle tend to form discontinuous ** Ductile tend to form continuous Free machining steels: 1. lubricants (lead, bismuth) reduce friction 2. particles
Thermal Machining: EDM
- Electrical Discharge machining (EDM) removes metals by discharging electric current from a pulsating DC power supply across a thin inter-electrode gap - The gap is filled by a dielectric fluid (globally insulator but in small gaps, sparks will jump) which becomes locally ionized * Thermal process (vaporizing material)
Electrochemical Machining Processes
- Electrochemical machining (ECM) removes material by anodic dissolution with a rapidly flowing electrolyte - The tool is the cathode - Work piece is the anode - Separated by electrolyte, which helps sweep away by-products of reaction (chemical reaction that dissolves the workpiece)
Thermal Machining: Electron, Ion, and Laser Machining
- Electron beam machining (EBM) uses a beam of high energy electrons focused on the workpiece to melt and vaporize a metal - Ion beam machining (IBM) or Focused ion beam (FIB) is a similar method that uses a focused beam of charged ions (usually gallium) to remove material on the surface of a work piece - Laser beam machining (LBM) uses an intensely focused coherent stream of light to vaporize or chemically ablate materials
Luders Bands (Stretcher Strains)
- Form when material is stretched to an amount less than the yield point run-out
Basic Components for Machining
- Four basic physical components of any machining process: 1. Machine tool or machining center 2. Workpiece 3. Work-holding device 4. Cutting tool - These components + cutting parameters determine final quality
Process Input Parameters
- General guidelines are available in many types of reference handbooks (Machinists Handbook) - Factors that must be considered to machine a given material: ~ Process ~ Material type and hardness ~ Cutting tool material and geometry * three above - feeds, speeds - These facts help determine cutting speed, depth of cut, feed rate
Structure from Hot Working
- Grain size not uniform - Plastic material flow *** Grain structure parallel to features, increases fracture resistant
Prediction of Solidification Time
- Heat removal is related to the surface area and volume of casting - Chvorinov's Rule: ~ ts = B(V/A)^n where n = 1.5 to 2.0 - B is the mold constant (mold material) - V is the volume of the casting - A is the surface area of the casting - n is a constant - B, V, A, and n are found experimental
Force and Friction
- High forces/pressures are required to deform a material (up to 50% of energy goes into overcoming friction) - Resistance to sliding contact between two surfaces. One of two cases: 1. Friction is proportional to the applied pressure (relatively low normal force) (linear portion) 2. Friction is proportional to the strength of the weaker material and the contact area (constant) (at high normal forces) (flat portion, surface deterioration bigger than in linear portion)
Fluidity
- How easily the liquid metal will pour * Determined by superheat
Surface Deterioration
- Lubricants used to reduce friction and wear (reducing coefficient of friction). Also can serve as a coolant, thermal barrier and/or flame retardant - Surface wear is related to friction ~ Wear of the workpiece is ok (shiny) ~ Wear of the tooling is not ok (lose tolerances, surface finish deteriorates, $$$)
Free Abrasives (mechanical NTM)
- Many mechanical NTM methods use fine grains of abrasive to chip away at the surface; especially good for machining hard, brittle materials - Typical abrasives: aluminum oxide, diamond, cubic boron nitride, silicon carbide (all ceramics) We want: * High hardness * High friability - breaks into new, smaller, sharp particles (opposite of toughness)
General Material Parameters
- Material being deformed must be well characterized - Important Parameters: ~ Resistance for deformation - Sy (yield strength) (how much force is needed for permanent deformation) ~ Formability limits (%EL) ~ Reactivity - oxidation? (how does it react at high temperatures) - Condition at different temperatures (important parameters) - Speed of deformation and its effects (brittle at high E (strain) rate?)
Chemical Machining Processes
- Material is removed form a workpiece by exposing it to a chemical etchant - Chemical reaction occurs everywhere the etchant contacts the workpiece in order to create patterns: ~ Gel milling (etchant is a gel) ~ Masking material ("stencil") - Few material limitations (metals, ceramics, glass..)
Nontraditional Machining (NTM) Processes
- Material removal using something other than mechanical cutting/scraping - Four basic groups of material removal using NTM processes: 1. Chemical (spontaneous chemical reaction) 2. Electrochemical (chemical reactions that requires electricity) 3. Thermal (heat melts or vaporizes material) 4. Mechanical ("non contact" physical removal)
Molten Metal Fluidity
- Metal should flow into all regions of the mold cavity and then solidify - Fluidity is the "runniness" of the liquid (inverse of viscosity) - Fluidity determines: minimum section thickness, maximum length of a thin section, fineness of detail, ability to fill mold extremities - Fluidity is determined by superheat and material - Increase superheat, increase fluidity, increase $$$, increase oxidation of material, can degrade mold
Sand Casting
- Molds made out of sand 1. Starts with a pattern (typically cut in two halves) looks like what the final product will look like 2. Put pattern into "flasks" (boxes) (top half is the cope, bottom half is the drag) 3. Add sand and binder to top and bottom halves, then let solidify 4. After solidification, assemble mold, and add pouring cup (hole in the top half of the sand mold that allows us to pour the metal in) 5. Pour molten metal into mold cavity (through pouring cup) pouring cup leaves some access metal, solidify 6. Break part out of mold 7. Cut off access material if needed, sanding, polishing, etc (secondary operations)
The Solidification Process
- Molten material cools (determines material properties!) to solidify into the final shape. Critical step for the final quality of the product. Two Stages: - Nucleation - Crystal growth (grain growth) * both determine grain size! - Casting defects occur during solidification: ~ Gas porosity ~ Oxidation reactions ~ Thermal expansion/contraction
Methods to Prevent Gas Porosity
- Prevent the gas from dissolving in the liquid: ~ Melting in a vacuum or covered with flux ~ Melting in environment with low solubility gases ~ Minimize turbulence - Gas Flushing - passing inert gases or reactive gases through the liquid metal
Introduction to Machining
- Process of removing unwanted material from workpiece in the from of chips to obtain a finished product of the desired shape, size, and finish - Capable of producing a wide variety of precision, surface finishes and speeds - Most products require machining at some stage of production
Risers and Riser Design
- Risers are reservoirs of liquid metal that feed extra metal to the mold to compensate for shrinkage - Designed to counteract metal contraction problem Live: receives last metal to enter mold (side riser) Dead: metal flows through mold cavity before filling the riser (top riser) Open riser: can see the metal from the outside Blind riser: cannot seen from outside
Forming Operations - Bulk
- Rolling: pass bars/sheets between two rotating wheels - Extrusion and Drawing: push/pull material through die - Forging - squeezing material between two mold halves - Sawging/Rotary forging
Chemical Machining
- Several different methods to make a masking layer: ~ Cut-and-peel ~ Scribe-and-peel ~ Screen printing ~ Photo-patterning <- (high precision >1mm) (exposing protractive polymer to light (UV, ultraviolet) through a glass/chrome structure) - Etch rates are slow (depth/time) in comparison to other NTM
Cooling Curves
- Start and end of solidification are indicated by change in slope. - Superheat: ~ higher - more fluidity ~ to high - can damage mold - Single solidification temperature means pure metal - For an alloy, the plateau region in the middle is slightly sloped, and this is the "freezing range"
Metal Properties and Cold Working
- Stress strain curves are useful in predicting results - Key Properties: ~ Magnitude of the yield-point stress (Sy) - Extent of plastic strain region from yield stress to fracture - Springback should also be considered when selecting a material
Patterns
- Two basic categories for casting processes: ~ Expendable mold processes (sand <- get more complex geometry) ~ Permanent mold processes (die) - Depending on process, patterns can be made from wood, metal, foam, plastic, etc - Patterns are generally larger than final part dimensions, to allow for: ~ Material contraction ~ Finishing operations ~ Draft (more common in reusable molds)
Thermal Machining: Thermal Deburring
- Used to remove burrs and fins by exposing the workpiece to hot corrosive gases for a short period of time - Technically a "thermo-chemical" process - Thermo deburring can remove burrs or fins from almost any material but is especially effective with materials of low thermal conductivity * takes a layer off of everything, clean up surface finish, remove burrs *
Thermal Machining: Plasma Arc Cutting (PAC)
- Uses a super heated stream of electrically ionized gas (plasma) to melt and remove material - PAC can be used on exotic materials at high machining rates (has to be electrically conductive)
Nucleation
- When it first starts to go from liquid to solid - Undercooling is the difference between the melting point and the temperature of nucleation. (ex. melting temp 400C, goes from liquid to sold at 390C, undercooling -> 10C) - Each nucleation event produces a grain ~ more grains -> better mechanical properties - "inoculation" or "grain refinement" is the introducing of solid particles to promote nucleation
Temperature Concerns
- Workpiece temperature often the most important of the process variables - Typically an increase in temperature is related to: ~ Decrease in strength (Sy) ~ Increase in ductility (%EL) ~ Decrease in rate of strain hardening *** Three things make material easier to form
Casting Process
1. Create mold cavity (single use or multiple use mold, (high temperature and/or complex geometry)) 2. Melt material 3. Pour material (slag and air pockets) 4. Solidify material (cracking, void formation) 5. Remove mold 6. Finishing and inspection (secondary operations (milling, cutting, polishing))
Cutting Parameters - Turning Processes
1. Cutting Speed (V) <- as V increases, surface finish is better - Primary cutting motion - Velocity of workpiece relative to the cutting tool (how fast workpiece is moving) 2. Feed Rate (fr) <- as fr increases, surface finish gets worse (bumpy) - Amount of material removed per revolution or per pass of tool over workpiece 3. Depth of Cut (d) - Distance tool engages work piece ~ as d increases, force required increases ~ as d decreases, force required decreases
Material Removal
1.Mechanical Machining: - Turning - Milling - Drilling - Boring - Sawing 2. Nontraditional Machining - Etching - Electroplating - Electrodishcharge machining - Water jet - Laser beam
Forces and Power in Machining
3D force in cutting can be thought of as three forces in three orthogonal directions: 1. Direction of the cut (Fc) - primary cutting force 2. Direction of the tool speed (Ff) 3. Direction perpendicular to the surface (Fr) % of Total Force Power Required Fc 58% 99% (Fc twice as Ff) Ff 28% <1% (Ff twice as Fr) Fr 14% <1% P = F x V
Hot Working
Above recrystallization temperature Pros: - Recrystallization eliminates the effects of strain hardening - Shape can be drastically altered without fear of fracture (almost always reason to choose hot over cold working) - Achieve deformation without using excessively high forces - Temperature promotes diffusion to improve homogeneity Cons: - Undesirable reactions (oxidation) between metal and surroundings - Tolerances are poorer - Structure may be nonuniform - Nonuniform temperatures can cause residual stresses
Advantages and Disadvantages of EDM
Advantages: - Applicable to all materials that are fairly good electrical conductors - Hardness, toughness, or brittleness of the material imposes no limitations - Fragile and delicate parts, complex geometries, tight tolerances (non contact! no forces!) Disadvantages: - Slow ($$$) compared to conventional machining - Produces a hard recast surface - Surface may contain fine cracks caused by thermal stress - Fumes can be toxic - Tool wear ** Often used in making tools and dies **
Electrochemical Machining Advantages and Disadvantages
Advantages: - ECM is well suited for the machining of complex two-dimensional shapes - Delicate parts may be made - Poorly machinable materials may be processed (even very strong, brittle or tough materials) - Little or no tool wear * last three no force!, just chemistry Disadvantages: - Initial tooling can be expensive - Environmentally harmful by-products - Control of electrolyte flow can be difficult - Current densities tend to concentrate at sharp edges or features (causing extra material removal near corners) *nonuniform
Advantages and Disadvantages of Chemical Machining
Advantages: - Induces no stress or cold working in the work piece - Can be applied to almost any material - Large areas can be etched simultaneously - Can be applied to virtually unlimited shapes - Possible to make thin sections and very small (<1 um) features Disadvantages: - Requires the handling of dangerous chemicals - Disposal of potentially harmful byproducts - Material removal rate is slow
Design Factors in Chemical Machining
Areas that are exposed longer will have more metal removed from them - Etch factor (E): E = U/d - Anisotropy (A): A= d/U =1/E ~ U: undercut ~ d: depth ~ Wf: final width of trench ~ Wm: width of mask ~ Wf = Wm - 2U
Filters
Can be used in gating systems to trap foreign material - Removes contaminants + - Decreases liquid flow rate - (not necessarily good) - Can also cause turbulence -
Warm Forming
Deformation at temperature between cold (useful for small volume parts and high precision) and hot (larger volume, large amounts of deformation) working Advantages compared to Cold Working: - Reduced loads on the tooling and equipment - Increased material ductility - Possible reduction in the number of anneals - Expanded range of materials and geometries Advantages compared to Hot Working - Less scaling and decarburization - Better dimensional precision and smoother surfaces than hot working - Less energy required - Finish machining reduced - Finer structure with strain hardening often eliminates heat treatments
Design Considerations
Draft Cores: - $$$ - Avoid undercuts -> usually helps get rid of cores Section Thickness: - Cooling is slower in thick sections (problematic) 1. Sunk-in regions 2. Different metal properties 3. Warping/ cracking
Considerations for EDM
Electrode (tool) material: - The choice of electrode material depends on its machinability and cost as well as the desired MRR, surface finish, and tool wear (especially sinker/plunge - EDM) (easy to cut, high melting temperature, graphite) The dielectric fluid has four main functions: - Electrical insulation - Spark conductor - Flushing medium - Coolant * first two de-ionized water
Free Abrasives (mechanical NTM) - Processes
Four major forms of mechanical nontraditional machining: 1. Ultrasonic (non contact) - Abrasives mixed in a slurry, ultrasonic transducers provide mechanical agitation to remove the material - Cuts glass 2. Water jet cutting (WJC) <- paper, fabric, leather, etc - Water at 60,000 psi and 3000 ft/s erode the material - 2D geometry 3. Abrasive waterjet cutting (AWC) - Abrasives are added to a waterjet to improve the efficiency - 2D geometry 4. Abrasive Jet Cutting (AJC) - Abrasives are mixed in a high velocity air stream at 1000 ft/s - 2D geometry * 2, 3, 4 grouped together * 1, 3, 4 grouped by abrasives
Mechanics of Machining (Dynamics)
Free Vibration: - Some (singular) input causes vibration - Dampens out over time - Least bad Forced Vibration: - Repeated input - Amplitude usually stays the same over time - Worse than free vibration Self Excited Vibration: - Input cause vibration at resonant frequency - Amplitude increase over time - Worst vibration
Mold Geometry
Gating system: - Path from outside to mold cavity - Made up of runner, sprue, and pouring cup Cope and Drag: - Top and bottom Riser: - Only in metal casting, essentially a reservoir for excess metal Core: - Make hallow geometry Mold cavity: - Geometry we actually want
Effects of Workpiece Material Properties
High strength and hardness materials require larger cutting forces, which increase: - Deflection - Friction (generates heat! -> increasing temperature) (avoid) - Power required (forces increase) - Tool wear Ductility (indirectly influences friction) is important in determining type of chips produced
Turning Process vs Milling Operations
In turning process, the work piece rotates about axis and tool fed into it. In milling operation tool rotates about axis and work piece is fed linearly into it
Etchant
Liquid or gas that will react with a solid in a way that will dissolve the solid
Material Removal Principle in EDM Processes
MRR = (C*I)/Tm MRR = material removal rate C = constant I = current Tm = melting temperature (determines materials properties)
Traditional Machining Limitations
Machining processes that involve chip formation (traditional machining) have a number of limitations: - Residual stresses and/or unwanted distortion of final part - Burrs - Delicate or complex geometries may be difficult or impossible - Many materials are difficult to machine (high strength, high hardness)
Casting Plastic vs Metal
Main differences in metal: - Much higher temperatures (risk of oxidation, molds must be made of high temperature compatible material) - Stronger adhesion between mold and material (difficult to remove parts) - Lower pressure than injection molding These differences make it necessary to use different geometry and materials when making molds to be used with metal.
Etch Rates
Many of these etchants are strong acids or highly caustic - need to make sure the workpiece material etches faster than the mask material (etch selectivity)
Solidification Shrinkage
Material contracts in 3 stages: 1. Contraction of liquid (2nd biggest cooling problem) 2. Solidification shrinkage (Biggest cooling problem) 3. Solid metal contraction * First two step case voids
Heat and Temperature in Metal Cutting
Power put into cutting is largely converted to heat, which elevates the temperature of all components -Three main locations of heat generation: 1. Percent of total going to work (small amount of energy becomes actual work(cutting)) 2. Percent of total going to tool (waste heat! mostly come from friction) 3. Percent of total going to chip (waste heat! mostly come from friction)
Temperature Concerns 2
Process classification based on working temperature and material being formed: - Plastic deformation below the re-crystallization temperature: Cold forming/working T < 30% of melting temp 2. Plastic deformation above the re-crystallization temp: Hot working/forming, T > 60% of melting temp 3. Plastic deformation under conditions of transition: Warm working/ forming, 30% Tm < T < 60% Tm
Cold Working
Pros: - No heating required (oxidation risk lower) - Good surface finish - Superior dimensional control and reproducibility - Strain hardening improves strength, fatigue, and wear properties - Directional properties can be imparted Cons: - Higher forces required; requires heavier and more powerful equipment - Ductility of material is lower - Surfaces must be clean and scale free - Strain hardening may require intermediate anneals to restore ductility - Direction properties may be detrimental - Undesirable residual stresses may be produced
Sand Cast vs Permanent Mold vs Die Cast
Sand Cast: - Slowest cooling rate (lowest strength) - Thermal insulator Permanent Mold: - In the middle Die Cast: - Fastest cooling rate (highest strength) - Thermally conductive **Faster cooling rates > smaller grain structure > better mechanical properties *** higher strength ***
Fundamentals
Seven(ish) basic chip formation processes 1. Turning*** 2. Sawing 3. Broaching (least common) 4. Grinding 5. Milling*** (most broadly applied and flexible) 6. Drilling 7. Shaping or Planing (depends whether the tool or work piece is held stationary)
EDM Processes
Two different types of EDM exist based on the shape of the tool electrode 1. Sinker EDM - Tool is lowered down and vaporizes material away as it comes close to workpiece - remove material from tool and workpiece - **noncontanct** - 3D geometry (usually traditional machining) 2. Wire EDM - Tool is a bit of wire, charge between wire and surrounding workpiece, wire vaporizes nearby material as it moves along a path - Creates very unique surface finish - 2D geometry
Defects in Etching
Workpiece + etchant -> soluble by-product (ends up in etching fluid!) - By products builds up or first on surface -> cause uneven etching (to get rid of use agitation)
