Relationship between cooling rate, microstructure features and wear behavior in end-chill cast Zn–27%Al alloys containing more than 2% Cu

An end-chill cast was conducted on Zn–27%Al–2%Cu and Zn–27%Al–4%Cu alloys to produce a variety of microstructural features. In order to study the relationship between microstructure features and wear behavior of these alloys, microstructural characteristics of three sections at distances of 1, 10 and 90 mm from the chill were investigated by optical and scanning electron microscopy and their wear behaviors were examined using a pin-on disk machine. It was found that the grain size, secondary dendrite arm spacing, density of ɛ precipitates and amount of eutectoid α + η increased with decreasing cooling rates. In addition, it was observed that the morphology of ɛ precipitates varied from semi-plate-like precipitates to fairly spherical precipitates along the height of the casting, i.e. with decreasing cooling rate. Microscope observations showed that the wear process proceeded by subsurface deformation, crack nucleation and crack growth, i.e. principle mode of wear was delamination. Also, wear tests showed that microstructures which solidified in the highest cooling rate displayed the maximum wear loss and rate in both alloys and also, the wear rate increased with increasing copper content. As a result of these investigations, the differences in wear behaviors of microstructures of as-cast Zn–27%Al–(more than 2%)Cu alloys were explained based on the relative contribution of each microstructural characteristic on fracture properties, crack nucleation mechanism and crack growth rate.