52593094-casting

Casting“Net Shape” or “Near-Net Shape” Process Advantages:• Product is ~finished right out of mold.• High complexity with few steps (usually)• No machining waste

General Casting Disadvantages:• Expensive and time-consuming patterns/molds/dies • Solidification issues: shrinkage, porosity, ~low strength, brittleness• Some methods require many steps (e.g., Investment casting)

Expendable vs. Non-Expendable:• Patterns• Molds

Casting: Solidification

• Grains perpendicular to wall shut-off other grains, so columnar structure naturally develops perpendicular to mold wall.• Grain boundaries tend to be weak columnar castings tend to be brittle (unless loaded parallel to the column direction, as in turbine blades). • Equiaxed structure usually preferred for strength, can be achieved with innoculating agents and/or fast cool.

Kalpakjian

Kalpakjian

Casting Impurities

Slag/dross:• Metal oxides that form brittle inclusions within casting• Slag floats, so skim off top and/or pour from bottom of ladle

Porosity: trapped gas. Minimize by these methods:• Design part and mold to minimize turbulence of molten metal as it enters mold • Don’t overheat the molten metal (dissolves more gas)• Melt in a vacuum ($$$)• Melt in a protective atmosphere ($$)• Use scavenging agents to collect gas bubbles• Pour smoothly (sand casting, permanent mold casting, )• Pressurize the “pour” (die casting)

Casting: Design Practices• Draft angle (1-3 deg) is needed to allow removal of pattern from mold (sand casting), or removal of part from mold (e.g, die casting)• Upon solidification, thicker sections tend to form cavities inside unless fed by riser or directionally solidified.• Aim for the same wall thickness everywhere or plan solidification direction carefully.• Offset intersection of ribs to achieve uniform thickness.

Kalpakjian

Kalpakjian

Kalpakjian

Casting: Directional Solidification

• Porosity and cavities form when melt cannot reach solidifying/contracting regions.• Chills used to initiate local solidification and achieve directional solidification away from the chill.• Risers feed melt opposite to solidification direction.

Schey

Kalpakjian

Sand Casting: Parts of a Sand Mold (expendable mold)Key terms: Flask, Cope, Drag, Sprue, Runner, Gate, Riser, Mold Cavity, Core, Parting Line, Draft (not shown).

Kalpakjian

Casting: Riser Design

Chvorinov’s Rule• Solidification time = B * (V/A)n

• B = mold constant• n = 1.5 – 2.0• V = volume of casting• A = surface area of casting

Riser and mold cavity:• Want riser to supply molten metal to mold cavity as casting solidifies• Riser must solidify after casting: T-riser = 1.25 * T-casting• Mold constant is the same for riser and casting

Schey

DeGarmo

Sand Casting: Patterns

DeGarmo

Shell-Molding Process

Kalpakjian

Investment Casting

• aka “Lost-wax” casting• Unlimited design freedom since draft angles, cores, parting lines, etc., are ~irrelevant• Accurate parts with good surfaces • Many steps• Patterns and molds are expendable• Expensive

Kalpakjian/Howmet Corp

Wax patternof turbine rotor

Cut-away ofceramic moldapplied overover wax pattern

Cut-away showingwax meltedout of mold.(Metal then poured into mold.)

Finished turbine rotor, near-net shape

Investment Casting a Turbine Rotor

Turbine Blade Casting

Directionalsolidification

Kalpakjian

Directional solidification for single-crystal blade

Single-crystal blade with a spiral attached

Single Crystal Silicon “Boule”

Directionally solidified from bottom to top as a single crystal (no grain boundaries anywhere).

Silicon wafers cut from the boule, made into semiconductor devices (microchips, solar cells, etc.)

Kalpakjian

Permanent Mold Casting

• No pattern is needed, saving time and cost• Mold is machined directly out of cast-iron (adding time and cost)• Mold complexity is limited, 2-3 deg draft angles needed• Molten metal is gravity fed into mold• Good dimensional accuracy and surface finish• Castings cool quickly so strength tends to be good• Molds last 10,000 – 100,000 parts if casting a soft metal (aluminum, zinc)• Special graphite molds ($$) may be made for casting steel parts (unusual)

www.aurorametals.com

www.offshoresolutions.com

Permanent Mold Casting: Aluminum piston

Kalpakjian

Risers

As cast After machining

Die Casting

• Molten metal is injected under pressure (2000-30000psi) into mold• Mold machined from tool steel ($$$ and time)

• Molds last ~100,000 parts• Difficult to modify once made

• Very accurate dimensions, excellent surface finish, intricate details• Aluminum and zinc most commonly cast (steel would erode mold)

• Aluminum melts at ~1050F, Zinc at ~700F• Both are ~brittle when diecast

• Part size is limited by injection cylinder size (20 lb max) and clamping force (P*A)• No risers needed (hi-pressure runners feed metal)• Slides/cores used to make holes parallel to parting line• Air is vented along parting line, but porosity is often a problem • Very fast production rates possible, fastest of any casting method• Expensive dies/molds and machines: only suitable for mass production

Die Casting Advantages

• High volume at high speed

• Duplicates intricate design details

• No pattern

• Long mold life: ~100,000 cycles

www.kurt.com

www.incastinc.com

www.aluminum.org

Die Casting Limitations• Complex and large machinery: expensive

• Molds (dies) machined from hardened tool steel: expensive

• Molds cannot take extreme heat so “melt” limited to low-melting point alloys: zinc, copper, aluminum, and zinc-aluminum alloys.

• Effects of high pressure limited part size

www.atplonline.com

samkwangprecision.en.ec21.com

Die Casting: Hot-Chamber

Process:zinc alloys

Kalpakjian

Die Casting: Cold-Chamber Process: aluminum alloys

Kalpakjian

Toggle mechanism- Separating force = Pressure * Area = 400 to 4000 tons (800,000 – 8,000,000 lb)- As in Vise-grip, toggle multiplies clamp force many times. Double Toggle.- Keeps die-halves from separating, minimizing “flash”

Kalpakjian

NADCA

(a)

Die (Mold) Design- alignment pins- “slides” make holes perpendicular to die-separation direction.

www.toolingtec.com

www.toolingtec.com

www.toolingtec.com

Die Casting: part and runners

NADCA

Porosity in Castings

- Turbulent injection entraps air- Many solutions but still a common problem

NADCA

www.vidisco.com www.eng.ysu.edu

Explosion Risk

Water trapped under hot metal

Water expands to steam (1500x volume)

Explosion and spray out of the furnace

Possible secondary explosion

Avoid water near a casting operation

NADCA

Costs Comparison for Different Casting Processes

Kalpakjian

References

DeGarmo: E.P. DeGarmo et al, Materials and Processes in Manufacturing, Wiley, 2003.

Schey: J.A. Schey, Introduction to Manufacturing Processes, McGraw-Hill, 2000.

Kalpakjian: http://www.nd.edu/~manufact/index3.htm

NADCA: North American Die Casting Association Introduction to Die Casting CD