Surface Defects CONVENTIONAL DIE CASTING DEFECTS

11.2 CONVENTIONAL DIE CASTING DEFECTS

159 Laminations are a type of cold shut that occurs at the surface of the component. Due to the turbulent and complex flow patterns within a die during metal injection, a portion of the die may re- ceive a small quantity of metal that freezes quickly to the die surface, forming a thin layer of skin. As the die continues to fill, a bond does not form between this thin layer of solidified skin and the subsequent metal filling the cavity. The result is a thin, partially attached lamination. A short shot is the incomplete filling of the die cavity caused by an undersized volume of metal being metered into the metal injection system. Such defects are easy to detect by examining the ejected components and the runner system. Without enough metal in the injection system, the biscuit at the start of the runner system may be absent or is very small. In most cases, the com- ponent is not fully formed. Sinks are surface defects caused by localized solidification shrinkage beneath the surface of the casting. Although this defect is common in conventional and vacuum die casting, this problem may be alleviated by using squeeze casting and semi-solid metal- working. Repetitive thermal cycling of dies from the injection of metal followed by the application of lubricants results in fine cracking of the die face. This phenomenon is referred to as heat checking. Once these cracks are present, metal will fill these fine cracks creating veins or fins on the manufactured component. Although die maintenance may delay the onset of heat checks, die cavities must be replaced to correct this issue. If aesthetics is not a con- cern, veins from heat checking may not render a component unfit for use. Lubricants are typically applied to the surface of a die to avoid interaction between the die surface and the metal injected into the die. However, interactions may still occur resulting in a metallur- gical bond between the die and the injected metal. This phenom- enon is known as soldering and occurs most often when injecting aluminum into ferrous dies.

11.2.2 Internal Defects

Several defects can occur below the surface of a component re- sulting in less than ideal mechanical properties. Such defects are not visible to the manufacturer, making their detection difficult. 160 DEFECTS IN HIGH PRESSURE CASTING PROCESSES Contamination occurs when unwanted debris is mixed with the metal during injection. Common sources of contamination include degrading refractories, manufacturing lubricants, fractured equip- ment, and unclean remelted scrap. In most cases, contamination can be controlled by general good housekeeping practices, pre- ventative maintenance, and proper metal melting methods. Several techniques may be utilized to clean and prepare metal for injection into the die, including the use of fluxes. Excessive amounts of flux may contaminate the metal. Components manu- factured with flux-contaminated metal have less than ideal me- chanical properties and higher susceptibility to corrosion. When heating metal for injection into the die, surface oxidation may occur. If the metal is not cleaned, the oxide may scatter throughout the component during injection, forming inclusions. Since most metal oxides are abrasive, inclusions often cause ma- chining problems and excessive wear on cutting tools. Porosity is a potential defect commonly found in conventional die castings. In many cases, the functionality of a component is not affected. However, porosity is a serious problem in pressure vessels and structural members. Porosity may be attributed to two main sources: solidification shrinkage and gas entrapment. Most alloys have a higher density in the solid state as compared to the liquid state. This results in shrinkage during solidification. Centerline porosity cavities can occur in alloys that freeze over a narrow temperature range, as in eutectic alloys and pure metals. Interdendritic porosity can occur in alloys that freeze over a wide temperature range. Inadequate feeding of metal to the die cavity during solidification will result in porosity due to shrinkage. Entrapped gas can originate from several sources and cause porosity. Air can become physically entrapped in the metal during injection. Gases soluble in the liquid alloy may exceed their sol- ubility limit during solidification and evolve as a gas, resulting in porosity. Reactions can occur between the metal and slags, pro- ducing a gas, which causes porosity in the final component. De- composition of lubricants and chemicals used during manufacture can result in gas formation and entrapment in the metal. In most cases, porosity from entrapped gas is caused by multiple sources, making its elimination difficult. When dealing with liquid alloys, sludging may occur if inter- metallic compounds are allowed to precipitate in the metal. In