Improvement of 319 aluminum alloy casting durability by high temperature solution treatment

Abstract

The purpose of this paper was to investigate the effect of one and two stage solution treatment on the mechanical properties of the 319 aluminum alloy, and to determine the effect of strontium addition on the characteristic temperatures of copper-rich eutectic phases. For an alloy containing 18 ppm strontium, it was found that traditional single-step solution treatment (495°C/8 h) did not produce optimum mechanical properties because it did not maximize the dissolution of copper rich phases, nor was it able to sufficiently modify the silicon particle morphology. A two-step solution treatment process, however, was capable of accomplishing these goals and is therefore recommended. The optimal two-step solution treatment, for the alloy investigated, involved a first step at 495°C for 2 h followed by a second step at 515°C for 4 h. This solution treatment was followed by water quenching to 74°C and artificial aging at a temperature of 250°C (for 3 h). Strontium addition appears to raise the characteristic temperatures of copper-rich eutectic reactions. Therefore, it is likely that in higher strontium content aluminum alloys, a higher temperature two-step solution treatment would be required to optimize casting mechanical properties. This hypothesis will be investigated in a forthcoming paper.