A Preliminary Evaluation on the Fracture Toughness of Heat Treated Aluminium High Pressure Diecastings

Abstract

Until recently, the solution heat treatment of conventional aluminum high pressure die cast (HPDC) parts has been considered impractical because the high temperatures involved cause surface blistering and dimensional instability. Now, a new heat treatment procedure has been developed by the CSIRO Light Metals Flagship in Australia which avoids these problems and, in many cases, allows tensile properties such as 0.2% proof stress to be doubled with little change to ductility. This development has the potential to reduce costs by allowing existing HPDC parts to be re-designed to use less metal and still achieve performance requirements. One issue, however, is the possibility that heat treating die castings to increase tensile properties may have an adverse effect on fracture toughness. This paper reports preliminary results of Kahntype tear tests conducted to assess the fracture resistance of as-cast and heat treated HPDCs. Studies of the alloys A360, A380 and C380 have shown that T4 and underaged (UA) T6 tempers produce an optimal combination of fracture resistance and tear strength. Furthermore, the fracture properties compare well with permanent mold and sand cast aluminium alloys that have similar tensile properties.