The effect of solidification time and heat treatment on the fatigue properties of a cast 319 aluminum alloy
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11/8/03
11:19 AM
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The Effect of Solidification Time and Heat Treatment on the Fatigue Properties of a Cast 319 Aluminum Alloy JAMES M. BOILEAU and JOHN E. ALLISON Solidification time and heat treatment are known to have a large effect on the microstructure of cast aluminum alloys. This study was conducted to quantify how the fatigue properties of a 319-type aluminum alloy are affected by solidification time and heat treatment. Both porosity-containing (non–hot isostatically pressed (HIP)) and porosity-free (HIP) samples in the T6 (“peak aged”) or T7 (“overaged”) heattreated conditions were tested. As the solidification time increased, the average initiating pore diameter increased and stress-controlled fatigue life decreased. Heat treatment was observed to have a large effect on fatigue properties of the HIP samples. However, in the non-HIP fatigue samples, heat treatment did not significantly change the fatigue life or fatigue strength of the cast 319-type alloy. The absence of an influence of heat treatment on fatigue response is attributed to the predominance of the microporosity in fatigue crack initiation in cast aluminum.
I. INTRODUCTION
IN order to reduce vehicle weight, many automotive components have been redesigned to take advantage of lightweight materials such as cast aluminum. To this end, aluminum casting alloys such as the 319 series (Al-Si-Cu) are being widely used for many automotive components. These alloys have good casting characteristics, reasonable mechanical properties, and are heat treatable. Because of the increasing use of the 319-series alloys in demanding applications, there is a need for detailed information on how the mechanical properties of these cast aluminum alloys are affected by the casting processing parameters. The majority of cast components have complex geometries with a wide range of solidification conditions. Thus, a variety of microstructures can be present within a single casting. The mechanical properties, being a function of the microstructure, will change as the microstructure changes. Any study of the properties of cast aluminum must therefore take into account the many factors that can influence the range of microstructures that can arise. One important factor is the solidification time, which is known to have a strong influence on the secondary dendrite arm spacing, intermetallic content, and the porosity size and distribution. Another important factor is the heat treatment, which is known to have a strong influence on the formation of non equilibrium precipitates in the microstructure. Thus, heat treatment can significantly influence the tensile properties in the 319-type alloys. However, the full effects of solidification time and heat treatment on the fatigue properties of the 319-type alloys are not well defined. Therefore, a study was undertaken to (1) quantify the fatigue behavior of a 319type aluminum alloy for three different solidification times and two specific heat treatments; and (2) to identify the key microstructural feature(s) influencin
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