LASER MELTING TREATMENT BY OVERLAPPING PASSES OF PREHEATED NICKEL ELECTRODEPOSITED COATINGS ON AL-SI ALLOY

Microstructure improvements of a nickel electrodeposited Al-Si alloy were studied after high-power laser melting treatment through a single pass or partially overlapping successive adjacent passes. In some cases, laser melting treatment was preceded by a 5-hour heating of the specimens at 500-degrees-C in an argon atmosphere furnace. Microstructure observations and microhardness measurements were carried out on the specimens before and after laser melting treatment with and without preheating. Best results concerning microhardness, microstructural homogeneity, and porosity elimination, as well as adhesion of the nickel coating on the Al-Si alloy, were achieved when the specimens were first subjected to heating at 500-degrees-C in an argon atmosphere furnace for 5 hours and then submitted to a laser melting treatment through successive adjacent laser passes with an overlapping rate of 70 pct. Microstructure studies were carried out employing X-ray diffraction analysis (XRD) and energy-dispersive spectrometry (EDS). AlNi and Al3Ni phases were detected in the diffusion area which resulted from the 5-hour heating. AlNi, Al3Ni, and Al3Ni2 phases were identified in the laser melted zones (LMZs). Each one of the above phases was found to be the main phase under different conditions.