Materials processing 1

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Advantages of Sand Casting

Part scalability

Versatile material selection

Cost-effective (for low-volume production)

Design freedom

Disadvantages of Sand Casting

Poor surface finish

Reduced precision

Slow production yield

Defect risk

Slower cooling

Shell-Mold Casting

Form Shell

Combine Shells

Permanent mold Pros

reusable, better finish, faster cooling, finer grains, stronger parts.

Permanent mold cons

Permanent molds require:

• Machined steel/iron molds

• Precision gating and cooling channels

• Heat‑resistant materials

expensive

Limited Geometric Complexity

Higher Risk of Mold Wear and Thermal Fatigue

Slower Setup for Low‑Volume Production

Smaller sizes

Potential for Higher Internal Stresses

Because permanent molds cool metal quickly, they can create:

• Higher thermal gradients

• Residual stresses

• Increased risk of cracking in some alloys

Re

Laminar Flow Required

• Laminar: re < 2000

• Turbulent: re > 4000

  • Turbulence → erosion, porosity, oxide inclusions

Aspiration

casting defect where air is sucked into the molten metal as it flows down the sprue when fast‑moving liquid metal creates a low‑pressure zone, similar to how a vacuum forms.

Tapering

Prevents aspiration

tapered sprue gets narrower toward the bottom so Velocity ↑, Cross‑sectional area ↓

Sprue

avoids defects like aspiration, turbulence, and oxide entrapment when metal touches walls

Bernoulli’s Equation → Velocity Increases with Height

Area Must Decrease as Velocity Increases to keep the sprue completely full of metal and touch walls

Chill Zone Grains

metal first touches the cold wall, heat is extracted extremely fast = many nucleation sites → very small grains

• Random orientation

• High strength (Hall–Petch)

• High hardness

• Lower ductility

outer surface of castings is usually stronger and harder than the interior.

Fine grains resist crack initiation → better fatigue resistance.

Random grain orientation

isotropic properties = same mechanical behavior in all directions

Grain Size & Strength — Hall–Petch Relationship

Smaller d → higher yield strength

• Grain boundaries block dislocation motion

• More boundaries = harder for dislocations to move = stronger metal

Grain Structure Before Forging

• Random, cast grains

• Possible porosity

• Columnar grains from solidification

Forged parts

• Have superior mechanical properties

• Show directional grain flow that improves fatigue resistance

• Are stronger than cast parts

During forging

• Grains elongate and align with the direction of deformation

• Porosity collapses

• Defects weld shut

• Grain flow improves mechanical properties

After hot forging

• Recrystallization forms new equiaxed grains

• Grain refinement → higher strength + toughness

Grain structure in rolling

• Breaks up cast grains

• Produces elongated grains

• Cold rolling increases strength via strain hardening

Grain structure in heat treatment

• Can grow or refine grains

• Controls strength, hardness, ductility

Open‑Die Forging

• Upsetting (making thicker and shorter) between flat dies.

• Friction → barreling.

• Reduce barreling with lubrication or heated dies.

barreling

thicker in the middle and thinner near the die surfaces.

Cold Forging

higher forces, strain hardening, better finish.

Hot forging

Hot forging: lower forces, recrystallization, grain refinement.

Slab Method

A forging analysis technique that accounts for friction and geometry to estimate forging pressure and force

The slab method relies on:

• Plane strain (no deformation in the length direction)

• Constant volume

• Rigid‑perfectly‑plastic material

• Coulomb friction:

τf=μσ

• Small or negligible barreling

• Symmetry

Slab Method Key steps:

1. Compute new height from reduction

2. Use volume constancy to find new width

3. Compute average pressure

4. Multiply by contact area to get force

Impression‑Die Forging

A forging process where heated metal is squeezed between two dies that contain shaped cavities. Excess metal flows out into a thin gap, forming flash.

Impression Pros

• Good for moderate complexity parts.

• Dies are simpler and cheaper than precision forging dies.

• Dimensional accuracy is good, but not perfect.

• good for high‑volume production.

• Strong parts due to controlled grain flow

• Lower tooling cost than precision forging

Impression Disadvantages

• Flash removal required

• More material waste

• Less dimensional accuracy

• More machining needed afterward (Requires trimming after forging.)

Precision‑Die Forging

A forging process where the dies fully enclose the metal, producing a near‑net‑shape part with little or no flash.

Dies are heated and controlled carefully (sometimes isothermal).

Precision Pros

• Extremely accurate dimensions

• Very smooth surface finish

• Minimal material waste

• Excellent mechanical properties

• Reduced machining and post‑processing (no flash)

  • near‑net‑shape parts → minimal machining

Precision Cons

• Requires very precise die design and tight tolerances.

• Requires higher pressure

  • Requires advanced presses and controlled environments

• Not economical for low‑volume production

• Dies wear faster due to pressure

Stress

how much force per unit area the material is experiencing.

Strain

how much a material changes shape under stress.

Machining

The controlled removal of stock

material to into a precise final part.

Powder Metallurgy/Sintering

the process of turning loose powder into a solid uniform body

Elastic Modulus

E : Measure of stiffness of a

material

Modulus of

Toughness

Measure of energy absorption required to cause fracture.

Modulus of

Resilience:

Measure of allowable energy absorption to return to original shape.

stress-strain curves

Tensile behavior for materials of varying mechanical

behavior

Temperature and Strain Rate Effects

Effect of temperature on mechanical properties of a carbon steel.