Corrosion characteristics of a tungsten alloy die-casting mould material in molten aluminium alloy

To understand the corrosion characteristics of W90 tungsten alloy and compare them with those of conventional hot-worked steel (SKD61), immersion tests were carried out in molten aluminium alloy (ADC12). Severe corrosion occurred in SKD61, whereas the corrosion resistance of W90 was about 40 times greater, with little occurring even after 300 h immersion. However, significant microstructural changes occurred in W90 resulting from penetration of aluminium and silicon through grain boundaries to create WAl₅ and WSi₂ phases, detectable even after a few hours’ immersion. Relatively soft (hardness 1.5 GPa) WAl₅ formed between the W based grains, and hard (10 GPa) WSi₂ outside the W based grains. Tensile strength decreased slowly with increasing immersion time, while a significant reduction in fracture strain occurred. These changes in tensile properties were caused by the microstructural changes, e.g. soft and brittle intermetallic compounds. Crack propagated along the WAl₅ phases (intergranular fracture), whereas transgranular fracture was dominant for uncorroded or less corroded W90.