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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