Microstructural analysis and defect characterization of additively manufactured AA6061 aluminum alloy via laser powder bed fusion

Abstract

AA6061 is a widely used aluminum alloy with significant applications in the aerospace and automotive industries. Despite its popularity, the utilization of additively manufactured AA6061 through the laser powder bed fusion (LPBF) process has been hindered by the pronounced formation of pores and cracks during rapid solidification. This study quantitatively investigated defects, including pores and cracks, and microstructures, including texture, grain size, subgrain structure, and precipitates, of LPBF-manufactured AA6061 across a broad spectrum of laser power and speed combinations. A high relative density of more than 99 % was achieved with a low-power and low-speed condition, specifically 200 W and 100 mm/s, with minimal cracks. Large pores, akin to or exceeding melt pool dimensions, emerged under either low or high energy densities, driven by the lack of fusion and vaporization/denudation mechanisms, respectively. Solidification cracks, confirmed by the fractography, were propagated along grain boundaries and are highly dependent on laser scanning speed. Elevated power and speed exhibited finer grain size with refined subgrain cellular structures and increased precipitates at interdendritic regions. The cooling rate and thermal gradient estimated from thermal analytical solutions explain the microstructures’ characteristics. Nano-sized Si-Fe-Mg enriched precipitates are confirmed in both as-built and heat-treated conditions, whereas T6 heat treatment promotes a uniform distribution with coarsening of those precipitates. The low-power and low-speed conditions demonstrated the highest yield strength, consistent with defect levels. A minimum of 102.3 % increase in yield strength with reduced ductility was observed after heat treatment for all examined conditions. This work sheds light on printing parameters to mitigate the formation of pores and cracks in additively manufactured AA6061, proposing a process window for optimized fabrication and highlighting the potential for enhanced material properties and reduced defects through process control.