UNDERSTANDING QUALITY IN ALUMINUM CASTINGS

The concept of metal quality has been pervasive, yet elusive. Everyone uses the term, but few are able to offer a precise definition for it. One approach has been to use a 'standard' mold to evaluate metal quality. The two most commonly used in North America are the ASTM B108 test bar, and a 'step' casting proposed by the Aluminum Association (AA). Some results with these molds are given for A356-T6 alloy. It is seen that better degassing practices have resulted in significant improvements in casting quality over the last thirty years. Iron is shown to be detrimental to quality. A numerical index is proposed to define the quality of castings, which describes the combinations of strength and elongation possible in heat treated castings. This quality index is considered in detail, and equations are presented which give the mechanical properties of castings as a function of defect concentration. An estimate is also given of quality in 'defect free' castings. When considering tensile properties in commercial aerospace castings, solidification rate is found to be especially important, because it determines the size and amount of microporosity (and brittle phases) in the casting. Mechanical property data are presented and rationalized in the form of casting quality plots for different casting and heat treatment conditions. The role of porosity on fatigue properties and the use of Weibull statistics to evaluate quality are also considered.