Papers by Author: Zhan W. Chen

Abstract: A strong research effort has continued to determine how selective laser melting (SLM) process parameters relate to the amount of defects present in SLM products. However, how these parameters affect the track profiles which should geometrically affect the amount of lack of fusion (LOF) during SLM has not been studied. This study is needed as severe LOF should be a severe quality and property issue. In this work, how laser power (P) and thus energy, as other build rate related parameters are kept unchanged, affects the size of SLM tracks and the formation of LOF during SLM of CoCrMo alloy has been studied. Through examining the track profiles, track sizes have been geometrically demonstrated to insufficiently overlay and overlap for LOF free when the recommended condition (P = 180W) was used. A further contributing factor for LOF is the irregularity of track shape. Increasing P increased the sizes and improved the stability of the melt and the shape regularity of the tracks, reducing the amount of LOF. It will be shown that there is a rapid decay in the amount of LOF as P increases from 180 W, predominantly as the result of geometrical effect of the track profile on track coverage.

Abstract: Knowledge on the contact condition at the tool/workpiece interface is essential for understanding many aspects of FSW. In the present study, FSW experiments were conducted using aluminium alloys followed by metallographic examination focusing on the tool shoulder-workpiece interface region. It was observed that an interfacial intermetallic layer and hence metallurgical sticking/soldering readily formed. Temperature measurements have suggested the presence of interface liquid, hence suggesting a mechanical sliding contact condition dominant. This has been supported by the observation on material flow within the shear layer.

Abstract: Mechanical properties of AZ91 cast alloy depend strongly on the morphology (size and distribution) of the second (β-Mg17Al12) phase. It was observed that low ductility of AZ91 alloy was attributed to the brittle nature of the β phase particles at which microcracks initiated. These microcracks then coalesced contributing to the fracture of alloy. Quantitative study on microcracking progress revealed that cast samples with coarse microstructures fractured at low strain due to the non-uniform distribution of bulk blocky β particles at interdendrite region. These fracture surfaces exhibited clear cleavage mode. Fine cast microstructure presented quasicleavage fracture mode with clear dimple and tear ridges. The partial melting (and resolidification) heat treatment improved tensile properties, which was in disagreement with the available data from literature.