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UNDERSTANDING


LIMITATIONS AND THE POWER OF KNOWLEDGE


The power of advanced casting technology coupled with effi cient and effective design gives the metalcasting industry a bright future.


W RICHARD B. GUNDLACH, ELEMENT MATERIALS TECHNOLOGY, WIXOM, MICHIGAN


hat matters most to management is the ability to stay in business and stay cost competitive. A good knowledge of casting technology can be a powerful asset to produce better, more robust products. It is imperative that foundries educate their customers on the design of castings, and possibly provide de-


sign services to customers. Design improvements take advantage of the proper- ties of quality metal and the advantages of design only aff orded in the casting process. Good casting design can reduce defect sizes and quantities by using the many new technologies available today. By using the power of advanced casting technology coupled with effi cient and


eff ective design the metalcasting industry will have a bright future. T e kinetics of the solidifi cation process produces microshrinkage porosity, coarse solidifi cation structures and oftentimes undesirable inclusions and sec- ond phases that develop as a result of alloy segregation during solidifi cation. T e decrease in the solubility of gases during solidifi cation also can produce internal porosity. T e casting process also can introduce inclusions due to reoxidation of the molten metal during pouring, as well as cause the entrainment of foreign debris that further adds discontinuities to castings. T e various voids, inclusions and microstructural features reduce strength and ductility and cause castings to suff er reduced monotonic and cyclic properties compared to wrought and forged metals. T e microstructures in castings are also much coarser than the grain size and


34 | METAL CASTING DESIGN & PURCHASING | Nov/Dec 2016


second phase particles in wrought metals, further separating the proper- ties of castings from wrought and forged parts. Heat treatments can be applied to castings to refi ne the grain structure and improve the mechanical properties in castings, causing them to approach those of wrought metals. However, the sizes of the various dis- continuities and second phase particles in castings are not generally changed by heat treatment. T e size and shape of inclusions,


porosity voids and second phase particles have a major infl uence on the properties of castings. T ese damaging features have a limited eff ect on yield strength, but they have a major eff ect on tensile strength and elongation. T e ways in which these features infl uence the fracture mode are discussed. For many applications the cast- ing is subjected to cyclic loading, and fatigue strength is the most important mechanical property of a casting.


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