Failure modes in field-tested brass die casting dies

Tools for die casting of, for example, brass and aluminium alloys are exposed to severe thermal, mechanical, and chemical conditions. The performance and service life of the die components are limited because of reasons such as thermal fatigue cracking, erosion, corrosion, soldering, and gross fracture. To minimise these damages, the dies are normally made of hot work tool steel. This study aims at elucidating the life-limiting failure mechanisms in dies aimed for brass die casting. Two cavity inserts and eight cores of two hot work tool steels, quenched and tempered to different conditions, were examined and evaluated with respect to failure mechanisms after use in actual brass die casting. It was found that the dominating failure mechanism in the investigated tools was thermal fatigue cracking. The thermal fatigue crack initiation is associated to accumulation of the local plastic strain that occurs during each casting cycle, typical of a low-cycle fatigue process. The initial growth of the thermal cracks is facilitated by oxidation of the crack surfaces, and proceeded growth is facilitated by additional oxidation in combination with crack filling of cast material, and by softening of the tool material. The most striking observation was a mechanism of crack growth promotion that involves a local enrichment of Pb from the brass alloy melt at the crack front. Consequently, in addition to increasing the overall tool steel yield strength and/or oxidation resistance, there is a potential to improve the tool life either by removing or substituting the Pb in the brass casting alloy with any non-harmful element, or using a tool material not susceptible to Pb embrittlement.