UNDERSTANDING QUALITY IN ALUMINUM CASTINGS G. Sigworth


Foseco, Cleveland, OH, USA Copyright © 2011 American Foundry Society Abstract


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


Introduction


When one talks with buyers of castings, it is common to hear the following story. A cast component is obtained from foundry ‘A’ with good results. However, after some time the purchase agent puts the part out for bid and found- ry ‘X’ comes in with a lower price. The order is awarded to ‘X’, but the mechanical properties are found to be signifi- cantly lower when castings are delivered. What has caused this change? We are using the same alloy. We might even be using the same tooling. Why are the material properties different? This story illustrates why net-shaped castings


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.


Keywords: aluminum castings, casting quality, degassing, defects, melt treatment, oxides, porosity, fatigue life


sometimes have a poor reputation compared to wrought products. It also illustrates why most handbook values for mechanical properties should be taken with a ‘grain of salt’, since little is said about the casting process or melt treatment used.


Considering this problem, how can we develop better engi- neering information for casting users? And how do we get the best quality? One approach is to use a test casting. This may be a production casting with a great deal of past history. It may also be a standardized casting produced using a speci- fied mold design.


Figure 1. AA permanent mold test casting (location of tensile test samples is shown at the right). International Journal of Metalcasting/Winter 11 7


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