ME 215 – Section 7: Casting, Powder Metallurgy, and Joining

What is materials processing?

Converting materials into useful shapes with structures and properties suited to their intended applications.

What if a mold restrains shrinking metal too much?

Excessive restraint during cooling can produce hot tears, cracks, distortion and residual stresses.

Define pattern, flask, core, cavity and riser.

Pattern forms the desired mold shape; flask supports molding material; core creates internal features; cavity receives liquid metal; riser feeds solidification shrinkage.

How is a casting process selected?

Consider metal, part size/complexity, surface finish, tolerances, quantity, tooling, production cost and required properties.

Why are shell molds permeable and collapsible?

Thin shell walls and low moisture allow gas venting; resin binder degradation at casting heat lets the shell yield as the metal cools.

Drawbacks of expendable-mold casting?

A new mold is required for every casting, raising material/labor and slowing production; finish and tolerance may also be less precise.

Permanent-mold materials and cast metals?

Molds often use cast iron, steel, bronze or graphite; common cast metals include aluminum, magnesium, zinc and other relatively low-melting alloys.

Permanent-mold advantages?

Reusable tooling, consistent parts, good finish/accuracy, faster cooling and generally finer-grained cast structures.

Risers or sand cores in high-pressure die casting?

Risers generally are not used because injection pressure feeds the mold; conventional sand cores are unsuitable, so retractable metal cores are used.

Best prospects for powder metallurgy?

Small, relatively intricate, high-volume near-net-shape parts; also controlled-porosity products and materials difficult to cast.

Some earliest P/M products?

Early examples include coins/medallions, platinum parts, tungsten lamp filaments, bearings, filters and cutting-tool materials.

Main P/M market areas?

Especially automotive components; also tools, hardware, appliances, machinery, aerospace, electronics and medical products.

Which powder methods mainly make elemental powder?

Electrolytic deposition and several chemical reduction/decomposition routes usually make individual-metal powder; atomization can make prealloyed particles.

What is green strength and why important?

Strength of the compact before sintering; enough is needed to eject, transport and handle parts without cracks or loss of detail.

Define welding.

Permanent joining by coalescence of materials using suitable heat, pressure, or both, with or without added filler.

Three essentials for a quality weld?

Appropriate heat/pressure; clean and protected joining surfaces; control of harmful metallurgical/thermal effects.

Why two regions in an oxyfuel flame?

Primary combustion occurs near the inner cone and remaining fuel gases burn with atmospheric oxygen in the outer envelope.

What is filler metal?

Added metal, commonly a rod or wire, that supplies material for forming/filling a weld joint.

Downsides of low heat-input rate in gas welding?

Slow welding, heat spreading over a wide area, larger HAZ, more distortion and poorer efficiency for thick parts.

Reduce resistance at electrode/workpiece contact?

Clean the contact surfaces, use conductive electrodes, maintain proper shape/contact area and apply suitable contact pressure.

What heats electroslag welding (ESW)?

Electrical resistance heating as high current passes through an electrically conductive pool of molten slag.

Special fusion-zone shape in EBW?

A deep, narrow weld with high depth-to-width ratio and relatively small heat-affected region.

Advantages of LBW over EBW?

No vacuum chamber, no electron-beam X-rays, easy beam steering/fiber delivery, and no electron-beam magnetic-field deflection.

Typical surfacing materials?

Steels and stainless steels, cobalt/nickel/copper alloys, plus hard ceramics and refractory compounds such as carbides or oxides.

Define brazing.

Filler metal liquidus exceeds 450°C yet is below the base metal melting point; the base stays solid and capillary action fills the joint.

Casting

Making a part by pouring molten material into a mold cavity and letting it solidify.

Mold cavity

Empty shaped space within a mold that receives liquid material.

Pattern

Model used to create the desired external mold cavity, usually with specified allowances.

Flask

Rigid frame supporting molding material in a sand mold.

Cope

Upper section of a horizontally parted sand mold.

Drag

Lower section of a horizontally parted sand mold.

Core

Insert used to create internal holes, passageways or recesses in a casting.

Parting line

Surface where mold sections meet.

Draft

Small taper added to pattern/part walls to help removal from the mold.

Gating system

Passage network directing molten metal from the pouring cup/sprue to the cavity.

Sprue

Mainly vertical passage conducting metal downward into a casting mold.

Runner

Channel moving molten metal toward the gates.

Gate

Opening that introduces molten metal into the mold cavity.

Riser

Reservoir of molten metal that feeds the casting as it solidifies and contracts.

Chill

High-conductivity material inserted to increase local cooling rate and promote directional solidification.

Directional solidification

Designing the casting so freezing progresses toward feeding risers, limiting shrinkage voids.

Chvorinov's rule

Solidification time is proportional to (casting volume/surface area)^n; commonly n about 2.

Solidification shrinkage

Volume decrease accompanying the liquid-to-solid transformation that can cause cavities.

Hot tear

Crack formed while a casting is hot and weak, usually because shrinkage is restrained.

Gas porosity

Voids formed by trapped or dissolved gases as a casting solidifies.

Cold shut

Defect formed when metal streams meet but do not fuse properly.

Misrun

Incomplete casting when metal solidifies before fully filling the mold.

Fluidity

Ability of molten metal to flow and fill a mold before freezing.

Sand casting

Expendable mold process using compacted sand around a pattern and usually separate mold halves.

Shell molding

Expendable sand mold made as a thin resin-bonded shell around a heated pattern.

Investment casting

Lost-wax casting method using an expendable pattern and ceramic shell for intricate shapes.

Expendable mold

Mold destroyed or broken to remove each completed casting.

Permanent-mold casting

Reusable metal-mold process, commonly gravity-poured, for repeated production.

Die casting

Process forcing molten metal into reusable dies under substantial pressure.

Hot-chamber die casting

Die casting with an injection mechanism immersed in molten metal; typically for suitable low-melting alloys.

Cold-chamber die casting

Die casting in which molten metal is ladled into a separate shot chamber before injection.

Centrifugal casting

Casting in rotating molds using centrifugal force to distribute molten material.

Continuous casting

Continuous solidification of molten metal into long stock such as billets or slabs.

Powder metallurgy (P/M)

Manufacture from metal particles by powder preparation, compaction and sintering.

Powder atomization

Breaking a liquid-metal stream into droplets that solidify as metal powder.

Powder blending

Combining powders or additives to obtain uniform composition and processing response.

Compaction

Pressing loose powder into a shaped part with sufficient green strength.

Green compact

Pressed but unsintered powder metal part.

Green strength

Strength of a compact before sintering, necessary for handling and die ejection.

Sintering

Heating a powder compact below the bulk melting point to bond particles and improve strength.

Porosity in P/M

Voids between particles, sometimes deliberately retained for filters or self-lubricating bearings.

Hot-isostatic pressing (HIP)

Applying elevated temperature and gas pressure around a powder or part to improve density.

Metal injection molding (MIM)

Mixing fine powder with binder, injection molding, removing binder, then sintering.

Near-net-shape processing

Producing parts close to final dimensions, limiting machining and scrap.

Consolidation process

Process used to create a larger part or assembly from separate materials or components.

Welding

Coalescence-based joining by heat, pressure, or both.

Fusion welding

Joining by locally melting base metal, with or without added filler.

Solid-state welding

Joining metals without melting their base material, generally using force and/or temperature.

Autogenous weld

Weld produced without adding filler metal.

Filler metal

Additional metal supplied to create/fill a welded or brazed joint.

Weld pool

Local molten metal region created during fusion welding.

Heat-affected zone (HAZ)

Base metal near a weld whose structure/properties change due to heating without melting.

Residual welding stress

Internal stress caused by constrained expansion, contraction and solidification of a welded assembly.

Weldability

Relative ease of producing an acceptable weld with a specified material and process.

Flux (welding)

Material that assists oxide removal and protects molten weld metal from contamination.

Shielding gas

Gas supplied around an arc/weld to reduce harmful atmospheric contamination.

Oxyacetylene welding

Oxyfuel welding using acetylene burned with oxygen as the heat source.

Primary oxyacetylene flame cone

Inner flame zone where fuel and supplied oxygen react first.

Secondary flame envelope

Outer flame zone where remaining combustible gases react with atmospheric oxygen.

Shielded metal arc welding (SMAW)

Arc welding using a consumable flux-coated stick electrode.

Gas metal arc welding (GMAW/MIG)

Arc welding with continuously fed consumable wire and external shielding gas.

Gas tungsten arc welding (GTAW/TIG)

Arc welding using a nonconsumable tungsten electrode and shielding gas.

Flux-cored arc welding (FCAW)

Consumable-wire arc welding using an electrode containing flux.

Submerged arc welding (SAW)

Arc welding conducted beneath a protective granular flux layer.

Resistance welding

Joining by electric resistance heating and pressure at contacting workpieces.

Resistance heating equation

H = I²Rt: heat in joules equals current squared × resistance × time.

Resistance spot welding

Resistance process creating local weld nuggets between overlapped sheets.

Faying surfaces

Workpiece surfaces that face/contact one another in a joint.

Friction welding

Solid-state joining using relative movement, frictional heat and pressure.

Friction-stir welding (FSW)

Rotating nonconsumable tool stirs heated, softened metal along a joint without bulk melting.

Electroslag welding (ESW)

Vertical welding process heated mainly by current flowing through molten slag.

Electron-beam welding (EBW)

Fusion welding using focused high-energy electrons, usually in vacuum.

Laser-beam welding (LBW)

Fusion welding using a concentrated laser beam as the heat source.

Surfacing/hardfacing

Adding a surface layer to improve properties or restore dimensions.

Thermal spraying

Depositing heated/accelerated particles to build a surface coating.