Hot isostatic pressing for the enhancement of copper dies incorporating a thermally sprayed protective layer

Abstract

Recent efforts have considered the feasibility of using copper dies protected with a thermally sprayed coating layer in pressure die casting processes. Although such dies offer considerable advantage over traditional designs, particularly improved processing times, a particular drawback is the poor quality of the deposited layer. The coating layer of wearresistant material, which can be deposited on to a die-cavity surface using an arc-wire thermalspray process, can exhibit undesirable porosity, cracks, and inclusions. This in turn can result in poor in-service performance with castings of poor surface quality or dies with a reduced life. As a route to identifying a suitable and effective coating treatment strategy to enhance coating performance on the die, the current paper presents work carried out to investigate the effectiveness of the hot isostatic pressing (HIP) process towards the healing of porosity (i.e. coating densification) in chrome-steel coating and the enhancement of its hardness and bond strength on chrome-copper substrates. The results of the investigation reveal significant improvement in coating densification and bond strength at all process conditions. Also investigated is the relationship between bond strength, densification, and HIP temperatures. The HIP process is also shown to reduce coating hardness to levels of hardness equivalent to that of traditional tool steel. On the whole, the investigation suggests that optimum performance could be achieved from chrome-steel coating on chrome-copper substrates if HIPped at a temperature of 925°C and a pressure of 100 MPa.