The effect of Liquid Hot Isostatic Pressing on fatigue properties of al based castings

Abstract

Because of weight and cost savings automotive manufactures are making more extensive use of cast aluminium alloys to replace forged and cast iron or steel components. The presence of pores in certain locations may reduce the fatigue life and it is cause of a large scatter in the mechanical properties of cast parts. This is of special concern for safety critical components.

In this work A356 T6 heat-treated specimens were drawn from a suspension arm, that was manufactured with two different low pressure foundry processes, permanent mould and Disamatic process, and they were subjected or not to liquid hot isostatic pressing in a salt bath at 120 MPa and 540 °C for 30 s. Then high-cycle axial fatigue tests were performed on smooth plane samples at room temperature with a tension compression ratio of –1, in order to obtain fatigue S-N curves in the different conditions.
After LHIP it was found that there is a large increase of fatigue resistance and a remarkable decrease of the scatter of fatigue data, which is due to the presence of internal discontinuities. Optical microscopy and scanning electron microscopy (SEM) were used to document the elimination of porosity after LHIP and to show the different nucleation sites of fatigue cracks on the fracture surfaces in no-LHIP and LHIP conditions.