Microstructure and hardness of laser remelted surfaces of Al-5%Cu and Al-4%Cu-1%Ni alloys

Abstract

Al-based alloys are potential candidates for advanced rapid solidification processes, such as Laser Surface Remelting (LSR) and Additive Manufacturing (AM). However, currently few alloy candidates have emerged for use such as AlSi12 eutectic and AlSi10Mg. This occurs because other alloys are more susceptible to the formation of defects such as porosity, cracks and distortions, which impair high-performance applications. The addition of Ni in Al-Cu alloys is expected to improve the mechanical properties at high temperatures and favors the reduction of the solidification interval, which may decrease the fraction of solidification cracking and porosity. Given the context, the present work investigates the microstructural changes and hardness of Al-5 wt%Cu and Al-4 wt%Cu-1 wt%Ni alloys processed either by centrifugal casting or LSR. By varying laser beam speed and laser average power, a total of 4 combinations for each alloy was accomplished. The CALPHAD computations with 1%Ni alloying revealed a solidification interval reduction of approximately 30%, without reducing the total intermetallics fraction if compared to the binary Al-5%Cu alloy. The microstructure of the rapidly solidified Al-Cu-Ni samples was characterized by a dendritic α-Al matrix, surrounded by a α-Al+Al2Cu+Al7Cu4Ni constituent. A growth transition from epitaxial (bottom) to elliptical/globular (center) was observed in the molten pools with a significant cell spacing reduction from 7.5 µm (as-cast) to 0.6 µm (LSR), which favors an increase in Vickers hardness of approximately 85% if compared to the as-cast samples. The molten pool of the ternary alloy reached a higher hardness of 120 HV as a result of both AlCu and AlCuNi intermetallics acting mutually to strengthen the α-Al matrix.