e-Tips Casting vs Forging Nr. 16 Casting

1 Casting vs ForgingThere are a variety of methods toproduce a given part. Each has itsown set of advantages anddisadvantages. Some of theimportant factors affecting theselection of a process include thefollowing:- Quantity of the materialrequired- Design of the part- Tolerances required- Metal specification- Surface finish required- Tooling costs- Economics of machiningversus process costs- Delivery requirementsThe aim of this e- Tip is to compareforging with 16e-TipsForgingForging is a manufacturing processwhere metal is shaped by plasticdeformation under great pressure intohigh strength of forgingForging or cold forming aremetalforming processes. There is nomelting and consequent solidificationinvolved. Plastic deformation producesan increase in the number of dislocationsresulting in a higher state of internalstress.

2 Indeed, strain hardening isattributed to the interaction ofdislocations with other dislocations andother barriers (such as grain boundaries).Simultaneously, the shape of primarycrystals (dendrites) changes after plasticworking of the metal. Dendrites arestretched in the direction of metal flowand thus form fibers of increasedstrength along the direction of may distinguish hot working fromcold working. Hot working is performedabove the recrystallization temperature;cold-working is performed below it. Inhot working strain hardening anddistorted grain structure are very rapidlyeliminated by the formation of newstrain-free grains as the result ofrecrystallization. Rapid diffusion at hotworking temperatures aids inhomogenizing the preform. Initialporosity can also be significantlyreduced, eventually completely phenomena such as strainhardening and recrystallization areimportant because these changes instructure result in an increase in ductilityand toughness over the cast Casting process consists of pouringor injecting molten metal into a moldcontaining a cavity with the desiredshape of the Casting .

3 Metal castingprocesses can be classified either by thetype of mold or by the pressure used tofill the mold with liquid of CastingCasting is a solidification , the microstructure can befinely tuned, such as grain structure,phase transformations and , defects such as shrinkageporosity, cracks and segregation are alsointimately linked to solidification. Thesedefects can lead to lower mechanicalproperties. A subsequent heat treatmentis often required to reduce residualstresses and optimize 1: Expanded view of a castwheel with mold and core presentingthe temperature realized with SAParc Scientifique EPFL, CH-1015 Lausanne, Switzerlandphone: +41 21 693 2918 / fax: +41 21 693 / ExampleMost modern performance wheels aremade from aluminum by Casting orforging. Forged wheels are manufacturedin multiple steps compared to the onestep in the Casting wheelCasting has the advantage of allowingthe designer more styling freedombecause the process is a more flexiblemethod.

4 Until recently, most wheels havebeen gravity cast (heavier and thicker).Today, low pressure die castingtechniques are used to substantiallyreduce porosity. Indeed, castings tend tocontain porosity which stronglyinfluences the mechanical integrity of thecomponent. Thus, cast wheels aregenerally designed larger and heavier inorder to achieve an acceptable structuralstrength for a given wheelThe forged wheel, because of theenormous pressures involved, compactsthe metal, eliminating porosity and thevoids that can be a source for cracks orcorrosion. The result is that less metal isrequired to achieve a given strength,meaning lighter wheels can be , due to the density of thegrain structure, the polished forgedwheel will maintain its luster for muchlonger than a polished cast wheel whichis very summarize, Forging yield wheels withhigher strength to weight ratio but thetooling due to the multiple steps processand the based alloy are comparativelymore expensive than in castingprocesses.

5 Furthermore, with lighterweight wheels, you will benefit fromincreased fuel savings, and betteracceleration due to less amount ofinertial weight at the rotational axis. Forthose reasons, usually forged wheels areonly used for high performanceapplications. Strength of Forging : - Good Mechanical properties(yield strength, ductility,toughness) - Reliability (used for critical parts) - No liquid metal treatmentWeakness of Forging : - Defects - Laps - Die unfill - Die failure - Piping - Shape limited when undercuts orcored sections are required - Overall cost usually higher thancasting - Multiple steps often requiredStrength of Casting : - Large and complex parts - High production rate - Design flexibilityWeakness of Casting : - Defects - Shrinkage porosity - Metallic projections - Cracks, hot tearing, coldshuts - Laps, oxides - Misruns, insufficientvolume - Inclusions - Requires close process controland inspections (porosity mayoccur)An important thing to keep in mind isthat quality of materials and heat treatingmay be a more important factor thancasting vs.

6 Forging in some 2: Simplified description of a one piece Forging wheel operation from billet,forged preform, piercing and rim spinning respectively.