manufacturing 231 exam 1

What are the four categories of engineering materials?

Metals, ceramics, polymers, composites.

Natural material vs. engineering material?

Natural materials are found in nature (raw ores, oil, rocks, clay). Engineering materials are made from natural ones and have enhanced characteristics such as strength and conductivity.

Metals: key traits?

Huge range of behavior (ductile or brittle, stable or volatile). • Usually a good balance of strength and density. • Very versatile and widely used. • Ex: carbon steel, stainless steel, Al, Cu, Mg, Mo, Ag, Au.

Ceramics: key traits?

Very brittle, barely elongate; fail catastrophically without warning (shatter). • Excellent surface-to-surface wear resistance. • Withstand very high temperatures. • Ex: aluminum oxide, boron nitride, tungsten carbide, silicon carbide.

Ceramics are brittle. Why use them at all?

They're very hard (resist wear) and usually very thermally stable (resist melting or softening).

Polymers: key traits?

Range from very ductile to brittle. • Good impact absorption. • Bad conductors of heat and electricity. • Melt at relatively low temperatures. • Ex: HDPE, polystyrene, PTFE (Teflon), PVC.

Composites: key traits?

Combination of 2+ dissimilar materials; inherits the benefits of each. Generally harder to manufacture. Ex: carbon fiber, fiberglass.

What are the five major topics of the course, and which do the labs cover?

Machining, casting, sheet metal forming, bulk deformation, joining. • Labs: casting, bulk deformation, machining. • The course deals almost exclusively with metal alloys.

Fundamental principle of all metal manufacturing operations?

Every operation (forging, machining, etc.) alters the workpiece's microstructure or macrostructure by imparting energy: mechanical, electrical, chemical, or thermal. Mechanical energy transfer is most common in this class.

What does transferring mechanical energy to a part cause, and how is it quantified?

The part deforms, quantified as strain (tension, compression, shear).

Engineering strain vs. true strain: what is each useful for?

Engineering strain → determining the failure point of a material. • True strain → determining work (strain) hardening.

Formulas: engineering stress, true stress, engineering strain, shear strain

Eng. stress S = F/A₀ (original area) • True stress σ = F/A (instantaneous area) • Eng. strain e = (l − l₀)/l₀ • Shear strain γ = a/b

Yield stress (S_y) vs. ultimate tensile stress (S_ut)?

Past S_y the specimen is permanently deformed even if the load is removed. Past S_ut the material necks and quickly fails.

Label the regions of an engineering stress-strain curve.

Elastic (linear, slope = E) → yield point S_y → plastic region. • Uniform elongation: from 0 to the peak (S_ut). • Necking: from the peak to fracture.

Brittle vs. ductile materials?

Brittle: barely deform before breaking, 'explode' without warning. • Ductile: deform a lot before failure, a good early-warning system.

What is toughness?

The ability to absorb energy before fracture. Defined as the area under the stress-strain curve. Ductile materials are 'tough'.

Explain strain hardening.

As strain ↑, grains are crushed into each other and friction ↑. Higher friction makes the material resist further deformation, so S_y ↑ (strain ↑ → S_y ↑). This is why the true stress-strain curve doesn't drop after S_ut: materials get stronger as strain increases.

Why does true stress-strain keep rising after S_ut while the engineering curve falls?

True stress uses the instantaneous area and reflects strain hardening, so it keeps rising. The engineering curve (F/A₀) turns down once necking begins.

What does PSPP stand for, and what's the logic?

Processing → Structure → Properties → Performance. • Cause and effect run left to right; goal/means run right to left. • Performance requirements come from the customer, so you must process the material to get the micro/macrostructure that gives the needed properties.

How does temperature affect mechanical properties?

It influences S_y, S_ut, E, and elongation. In general hot materials are weaker than cold ones: strength and modulus fall, elongation rises.

Name the common hardness tests and their indenters.

Brinell: 10 mm steel or tungsten carbide ball. • Vickers: diamond pyramid. • Knoop: elongated diamond pyramid. • Rockwell (A, C, D): diamond cone. Rockwell (B, F, G): 1/16 in. ball. E: 1/8 in. steel ball.

Why does thermal expansion/contraction matter in manufacturing? Give three examples.

It affects final part geometry (macrostructure). • Casting: molten metal shrinks significantly. • Forging: part heated before pressing, shrinks when cool. • Machining: friction heats the part → too much material removed. • It can also exert huge forces and rip parts (e.g., skinny regions on castings).

Define casting.

Molten metal is poured into a mold cavity that has the desired part shape, cooled and solidified, then removed and cleaned or machined to final dimensions. Done in a foundry. Ingots and complex shapes are cast products.

When is casting the right process?

When ALL are true: • 1) complicated geometry is desired, • 2) machining would waste time and material, • 3) bulk deformation can't reasonably make the geometry (e.g., radial cooling fins; bulk deformation suits symmetrical parts like connecting rods).

Why is machining from a block often wasteful compared to casting?

You start with a block much bigger than the part and turn most of it into scrap.

Casting history in brief?

One of the oldest manufacturing processes (~4000 B.C.). Gold, copper, bronze in stone molds. First for sculpture, then tools and weapons. Church advances ~1000 A.D. (bells, cannons). American and Industrial revolutions made casting what it is today.

Casting terminology: pouring cup, gating system, sprue, runner, riser

Pouring cup: metal is poured through it. • Gating system: controls flow of molten metal. • Sprue: tapered vertical channel; metal flows downward. • Runners: channels carrying metal into the mold cavity. • Risers: supply extra molten metal to prevent shrinkage defects.

Other mold terms: cope, drag, parting line, flask, core, well, gate, choke

Cope: upper mold half. Drag: lower half. Parting line: horizontal split between them. Flask: holds the mold. Core: insert that forms internal cavities. Well: base of the sprue. Gate: where metal enters the cavity/runner. Choke: narrowing at the sprue bottom that slows flow.

What should a successful gating system do?

Keep contaminants out of the mold cavity (they stick to gating walls), and prevent premature cooling, turbulence, and gas entrapment.

What does a riser do?

Feeds molten metal to the casting to compensate for shrinkage. It should solidify AFTER the part (use Chvorinov's rule to check).

Steps of investment (lost-wax) casting?

1 Inject wax into a metal die → pattern • 2 Remove pattern • 3 Assemble patterns onto a wax sprue ('tree') • 4 Dip in ceramic slurry • 5 Apply stucco (fine refractory particles); repeat layers • 6 Dry → hard ceramic shell • 7 Melt out wax (autoclave/furnace) → hollow cavity • 8 Pour metal • 9 Shakeout: break away shell • 10 Cut castings from the tree; minimal finishing

Investment casting: why multiple slurry/stucco layers?

To build enough mold strength and heat resistance to survive pouring molten metal.

Why is investment casting called near-net-shape?

It makes highly accurate parts that need minimal additional machining.

Investment casting: advantages and limitations?

Pros: excellent surface finish, high dimensional accuracy, complex geometries, works with high-temperature alloys, many parts per tree. • Cons: expensive, time-consuming, many process stages.

Pure metal vs. alloy melting behavior?

Pure metals melt at an exact temperature. Alloys melt over a range that depends on composition.

Why must you 'overshoot' the melting temperature (superheat) when casting?

Once at the melting point you must keep adding heat to fully liquefy the metal, and you want to pour above it to avoid early freezing.

How do density and volume change with melting and heating?

Melting usually decreases density. Heating increases volume.

Describe the temperature-vs-time heating curve of a PURE metal.

Heating phase (T rises) → melting phase: temperature is CONSTANT until all solid is melted (solid/liquid mixture) → superheating phase (all liquid, T rises again).

How does the ALLOY heating curve differ from a pure metal's?

During the melting phase the temperature slowly rises as alloyed solids melt, instead of staying flat. The solid/liquid region is the 'mushy zone'.

On a binary phase diagram, what do the liquidus and solidus mean?

Above the liquidus: all liquid. Below the solidus: all solid. Between them: liquid + solid (mushy zone). Alloys melt/freeze across this range, which sets their melting points.

Cu-Ni phase diagram: what does the first solid to form look like for a 50/50 alloy?

It's richer in the higher-melting element (Ni): about 36% Cu-64% Ni, with the remaining liquid ~50% Cu-50% Ni. Composition shifts as cooling continues, ending as a uniform solid solution.

Name the three stages of shrinkage in a casting.

1) Liquid contraction (level drops after pouring) • 2) Solidification shrinkage (most severe; can leave a shrinkage cavity) • 3) Solid thermal contraction as the solid cools

Formulas for thermal contraction of a solid?

ΔL = α L₀ ΔT (length) • ΔV = 3 α V₀ ΔT (volume) • α = coefficient of thermal expansion (1/K), L₀ = starting length, ΔT = temperature change.

Which metals shrink most / expand on solidification?

Aluminum contracts most (~7.1%), then zinc (~6.5%), Al-4.5% Cu (~6.3%), gold (~5.5%). • Bismuth, silicon, and gray iron EXPAND on solidification (3.3%, 2.9%, 2.5%).

What determines cast microstructure?

Material selection and heat transfer out of the mold.

Solidification structure of pure metals vs. alloys?

Pure metals: planar solidification fronts. • Alloys: begin planar, then develop an equiaxed core. • Both: a chill zone at the outer shell where the metal freezes very rapidly.

What does 'equiaxed' mean, and how do you promote it?

Approximately equal dimensions in all directions. Formed by using an inoculant. Dendrites ('like trees') are the branching structure that forms in columnar growth.

Cooling rate R vs. grain size?

R ≈ 10⁰-10³ °C/s → large grains • R ≈ 10³-10⁵ °C/s → small grains • R > 10⁵ °C/s → amorphous (no grain structure)

What is alloy segregation? Name the three types.

Cast alloys are rarely homogeneous (fixing it needs very low cooling rate, often impractical). • Normal: planar fronts push lower-melting elements to the casting center. • Inverse: dendritic fronts grab lower-melting elements and hold them toward the outer edges. • Gravity: heavier elements sink in the melt before solidifying. • These are macrosegregation; micro versions exist too.

What does fluid flow control in a casting?

Microstructure, freezing speed, freezing locations, gas entrapment, contaminant inclusion.

What's an 'ideal pour'?

One at a rate that minimizes gas entrapment, contaminant inclusion, early freezing, and turbulence, and so minimizes defects.

Laminar vs. turbulent flow?

Laminar: smooth, orderly layers ('laminated'); completely reversible. • Turbulent: chaotic, unpredictable.

Reynolds number formula and terms?

Re = inertial forces / viscous forces = vLρ/η • v: velocity (m/s), L: characteristic length (m), ρ: density (kg/m³), η: dynamic viscosity (Pa·s). • Round pipe: L = d.

Reynolds number thresholds for laminar/transition/turbulent?

Re < 2000 laminar • 2000 < Re < 20,000 transition • Re > 20,000 turbulent • Most casting literature allows Re up to 20,000; gating systems typically run 2000-20,000.

Why is turbulence bad in casting?

It traps air and forms dross (oxidized metal) and slag (nonmetallic impurities) inside the part, weakening it. Re above 20,000 means severe turbulence and significant air entrapment. Dross/slag filters can be built into the mold.

How do you find the velocity of metal entering the mold cavity, with friction?

v_exit = c√(2gh) • c = friction factor (0 to 1), g = 9.81 m/s², h = height of the downsprue. • c depends on mold material, runner layout, and channel size (and can include turbulence/viscous losses); usually found experimentally.

Why is high exit velocity a problem?

It can erode the mold (mold wash) and induce turbulent flow.

PRACTICE. Molten Al poured from h = 0.5 m into a 1 cm round runner. ρ = 2.4 g/cm³, η = 1 mPa·s. Laminar, transition, or turbulent at the runner ENTRANCE?

SI units first: ρ = 2400 kg/m³, d = 0.01 m, η = 0.001 Pa·s. • v = √(2gh) = √(2·9.81·0.5) ≈ 3.13 m/s • Re = vLρ/η = (3.13)(0.01)(2400)/0.001 ≈ 75,170 • Re ≫ 20,000 → very turbulent.

PRACTICE. Same problem, but friction factor c = 0.2. Flow type at the runner EXIT?

v_exit = c√(2gh) = 0.2 × 3.13 ≈ 0.63 m/s • Re = 0.2 × 75,170 ≈ 15,000 (slide prints 15,170; 0.2 × 75,170 = 15,034) • 2000 < Re < 20,000 → transitional. • Friction lowers exit velocity, and Re scales with v.

Mass continuity equation for a sprue?

Q = A₁v₁ = A₂v₂ • Q: volumetric flow rate (m³/s), A: inlet/outlet area (m²), v: velocity (m/s).

Derive A₁/A₂ = √(h₂/h₁) from mass continuity.

Q = A₁v₁ = A₂v₂ → A₁/A₂ = v₂/v₁. • Free-fall velocities: v = √(2gh). • A₁/A₂ = √(2gh₂)/√(2gh₁) = √(h₂/h₁).

Formula for the optimal downsprue outlet area?

A₂ = A₁ √(h₁/h₂) • h measured downward from the pouring height (h = 0 at the pouring surface); h₁ = depth of sprue inlet, h₂ = depth of sprue outlet (runner). For round sprues: d₂ = d₁ (h₁/h₂)^¼.

Why are downsprues conical (tapered)?

Metal in free fall speeds up, and Q is constant, so the stream's cross-sectional area DEcreases as it falls. A tapered sprue matches the stream shape so the metal stays in contact with the walls.

What happens in a straight-walled sprue?

The falling stream pulls away from the walls, letting air in: aspiration (air trapped in the liquid). A choke at the bottom slows flow into the runner and helps prevent it.

If h₂ > h₁, is the optimal outlet smaller or larger than the inlet?

Smaller: A₂ = A₁√(h₁/h₂) < A₁ since h₁ < h₂. That's why the sprue narrows as it goes down.

PRACTICE. Mg poured from 0.2 m above the cope into a conical downsprue: 5 cm upper diameter, 10 cm tall. Optimal outlet diameter to minimize air entrapment?

h₁ = 0.2 m; h₂ = 0.2 + 0.1 = 0.3 m. • A₂ = A₁√(h₁/h₂) = (π/4)(0.05 m)²·√(0.2/0.3) ≈ 0.0016 m² • A₂ = (π/4)d₂² → d₂ ≈ 0.045 m = 4.5 cm. • (Quick check: d₂ = 5 cm × (2/3)^¼ ≈ 4.5 cm.)

EXTENDED RESPONSE outline: explain how mass continuity is used to design a downsprue.

1) Continuity: Q = A₁v₁ = A₂v₂ (Q constant). • 2) Metal falls freely, so v = √(2gh) and speed rises with depth. • 3) Substitute → A₁/A₂ = √(h₂/h₁), so A₂ = A₁√(h₁/h₂). • 4) Pick h₂ from the runner depth below the pouring height and compute A₂ so the sprue matches the stream's shape. • 5) Result: contact with walls is kept, aspiration/air entrapment is reduced. A choke can help on straight sprues.

What is fluidity?

The ability of molten metal to fill mold cavities ('pourability'); influenced by metal characteristics and process parameters.

Metal characteristics that affect fluidity?

Viscosity (resistance to flow; varies with temperature). • Surface tension (high tension ↑ flow resistance; affected by oxide films). • Inclusions (contaminants ↑ flow resistance). • Solidification pattern: a big gap between solidus and liquidus means an earlier mushy zone → more flow resistance.

Process parameters that affect fluidity?

Mold design: should be easy for a liquid to traverse. • Mold material: high heat conduction → faster cooling → more flow resistance (same for rough surfaces). • Pouring temperature: higher → lower viscosity. • Pouring rate: lower Q → faster cooling → more flow resistance.

How does pouring time relate to solidification time?

Pouring time should be a small fraction of the solidification time.

Chvorinov's rule?

t_solid = C (V/S)ⁿ • V: mold (casting) volume, S: surface area, C: mold-dependent constant (s/m²), n: experimentally derived exponent, usually 2. • Best for simple castings; breaks down for complex molds. (The formula for C is NOT on the exam.)

What is Chvorinov's rule especially useful for?

Determining riser solidification time. The riser must solidify later than the part.

Higher V/S ratio: what does it do to solidification time?

Longer (t ∝ (V/S)ⁿ). Bulky, compact shapes freeze slowly; thin, high-surface-area shapes freeze fast.

PRACTICE. Aluminum cube, side s = 50 cm, C = 720,000 s/m², n = 2. Solidification time?

V/S = s³/6s² = s/6. • t = C(s/6)² = 720,000 × (0.5/6)² ≈ 5,000 s. • ⚠ Slide 17 prints 50 s, but that plugs in 0.05 m (a 5 cm cube). Watch your units, and ask your prof which value they expect.

Solid 'skin' growth and slush casting?

Solidification starts at the mold wall, and the solid skin thickens with time. Slush casting pours out the still-molten interior to leave hollow objects (ornamental/decorative).

Shrinkage porosity: what/why?

Jagged, angular cavities when metal shrinks and solidifies non-uniformly. The cavity is uneven and distorts the component's shape. Poor runner and gating design also cause it.

Gas porosity: what/why?

Small holes, voids, or air pockets in the metal. Air is trapped during casting, and solid metal can't hold as much gas as liquid metal.

Cold shut: causes?

Metal flows into the mold at more than one point (streams meet without fusing); low liquid-metal temperature; thin casting sections.

Hot porosity: what is it?

A heat-related casting abnormality: thermal connections on the metal surface, from improper solidification and gating system placement.

Thermal fatigue in dies: causes?

Dies see high thermal and mechanical loads; repeated cycles between very low and very high temperatures cause structural weakness and cracks. Especially prevalent in aluminum casting. Contributors: continuous high temperatures; small radii on edges and corners.

How do liquid metals and gases interact during casting?

Liquid metals absorb lots of ambient gas (gases are 'soluble'), often without reaction. Freezing forces the gas out; rejected gas can get trapped in the microstructure as pores. Fix: add elements that react with the gas, then skim the slag.

Which defects are tied to flow/gating design vs. heat?

Flow/gating: gas porosity (trapped air), shrinkage porosity (poor runner/gating), cold shut (multiple entry points). • Heat: cold shut (low temperature/thin sections), hot porosity, thermal fatigue.

Discrete vs. continuous ingot casting?

Discrete: simple, common for transporting material or making raw stock; still needs good pour quality, and porosity can be a problem given exposure to air. • Continuous: casts strips rather than ingots; may be coiled.

How does strand casting work?

Material freezes onto a bar, which is pulled between rollers to produce a continuous casting. Variants: horizontal strand casting and strip casting.

Sand casting: strengths and sand requirements?

Most versatile technique, lowest investment. • Sand should be collapsible (follows the shrinking metal, carries heat away) and permeable.

Shell mold casting process?

Hot pattern is coated in sand mixed with resin. Resin melts and binds the shell, which is removed from the pattern. Shells are adhered together and put in a flask for pouring. Smoother mold than conventional sand casting.

Plaster and ceramic mold casting?

Mold material is poured over a positive that is then removed (destructively or not) once dry. Ceramics must be dried and burnt off to withstand molten metal.

Vacuum casting process and best use?

Shell-like mold on a robotic arm with a gate at the bottom, plunged into the furnace. A vacuum pulled on the mold sucks metal up and forces it through. Good for thin-walled parts.

Lost foam casting?

Styrofoam pattern placed in a sand-filled cask; molten metal is poured directly onto the foam, which vaporizes.

Classify: sand, shell, plaster/ceramic, vacuum, lost foam, lost wax.

Disposable mold, permanent pattern: sand, shell, plaster/ceramic, vacuum. • Disposable mold, disposable pattern: lost foam, lost wax. • Permanent mold: die casting (hot/cold chamber), centrifugal, and single-crystal (as grouped in the lecture).

Hot chamber die casting?

Metal is pushed from a molten reservoir into the die cavity at 5-40 MPa. Pressure is held until the metal solidifies. Die is usually cooled to speed solidification.

Cold chamber die casting? How does it compare to hot chamber?

Hot metal is deposited into an unheated shot chamber, then a hydraulic ram pushes it into the cavity at 20-150 MPa (vs. 5-40 MPa for hot chamber). No large vat of molten metal needed, but the machines are huge to exert the pressure.

Three types of centrifugal casting?

True centrifugal: poured into a rotating cylinder to coat the inside walls. • Semicentrifugal: poured into a rotating non-cylindrical mold. • Centrifuging: like lost-wax trees, but rotation forces metal into the casting.

Single-crystal casting: how and why?

Mold is dunked into molten metal incredibly slowly for unidirectional solidification; a constriction at the mold base lets only one crystal grow upward. The part is a single crystal, highly resistant to thermal creep.

Casting economics: how does cost per piece change with quantity?

Cost per piece falls as quantity rises, at different rates. High-throughput processes (die casting) get cheaper faster than low-throughput ones (sand, plaster). Choose the process to match forecasted production volume.

General casting guidelines (list).

1 Uniform thickness → easier heat treatment • 2 Otherwise smooth transitions with fillets • 3 Sharp inside corners concentrate stress • 4 Sharp outside corners dump heat • 5 Sharper corner = worse • 6 Complex shapes/features crack more • 7 Large thin surfaces warp ('potato chipification') • 8 Recheck every precise feature after heating

Why are chills used, and how do they work?

On cooling, metal contracts and pulls surrounding material with it. At elbows and intersections, the still-hot interior can rip material off the frozen skin, leaving internal pores. Chills solidify the internal regions sooner to prevent this.

Design-for-casting takeaways?

Easy to cast ↔ easy to heat treat. • Use ribs/webbing to prevent warpage. • Fillet 'everything'; keep wall sections uniform. • Consider core placement difficulty (side cores can sometimes be eliminated by redesign).