The Role of Ti Addition in Grain Refinement of Aa6061 Aluminum Alloy Fabricated by Laser Powder Bed Fusion and its Influence on Mechanical and Thermal Properties

Abstract

Grain refinement is one of the most efficient solutions to resolve the hot cracking issue in additively manufactured aluminum alloys, which can be achieved by adding potent nucleates or solutes. Despite several successful attempts to eliminate hot cracks in 6xxx series aluminum alloys (AAs) fabricated by laser powder bed fusion (LPBF) via the incorporation of either nucleates or solutes, the rationale for selecting specific grain refiners has not been fully elucidated. Moreover, the influence of grain refiner and heat treatment on microstructures, mechanical properties, and thermal properties has not been extensively investigated. This work employs peritectic-forming solute to harness the benefits of both nucleates and solutes on grain refining efficiency, deliberately selecting Ti element due to its highest segregating power in Al and the low lattice misfit of the Al3Ti phase with the Al matrix. The formation of primary pro-peritectic Al3Ti particles at the melt-pool boundaries acted as heterogeneous nucleation sites, effectively refining the grains of AA6061. The grain size decreased with increasing Ti content and growth restriction factor (Q), although the grain size and 1/Q followed a non-linear relationship. T6 treatment resulted in partial recrystallization, the increment of thermal conductivity, and the elimination of Lüders bands and Portevin–Le Chatelier (PLC)  bands in the stress-strain curves of Ti-modified AA6061. Furthermore, the widely used Kou’s criterion and freezing range criterion was found ineffective in interpreting the hot cracking susceptibility of Ti-modified AA6061, indicating the need for a modified RDG criterion or a new approach to explicitly describe the effect of grain refinement on hot cracking susceptibility.

Keywords: Laser powder bed fusion, Aluminum alloys, Solidification cracking, Grain refinement, Heterogeneous nucleation, thermal conductivity