Development of Al-Ti-based alloys for laser powder bed fusion

In the development of high-strength aluminium alloys tailored specifically to additive manufacturing (AM), L1(2)-Al3X-forming elements have been proven to be particularly effective alloying additions, reducing the susceptibility of aluminium alloys to solidification cracking by grain refinement and increasing the strength by forming secondary nanoscale precipitates. In the present work, we employ laser powder bed fusion (L-PBF) to examine the feasibility of using Ti as the main strengthening, L1(2)-forming element for the design of a well-processable, precipitation-strengthened model alloy. Al-1.76Ti and Al-2.51Ti (wt%) alloys with and without ternary additions of Ni and Si are fabricated during processing from powder mixtures. Crack-free microstructures are produced, which display alternating fine-and coarse-grained regions. During heat treatment, one of the investigated alloys, Al-2.51Ti-0.7Si, exhibits an ageing response due to the formation of nanoscale, metastable L1(2)-(Al,Si)(3)Ti precipitates, reaching a peak hardness of 97 HV. This demonstrates that L-PBF-processed Ti-containing aluminium alloys can exhibit a precipitation hardening response similar to alloys containing Sc or Zr. In the other Al-Ti-based materials tested, no significant hardness increase was found. This discrepancy is ascribed to the effects Si exerts on the precipitation process. First, the addition of Si increases the thermodynamic driving force for homogeneous nucleation of L1(2)-Al3Ti precipitates. Secondly, Si-Ti co-clustering is observed by atom probe tomography, which is expected to enhance the otherwise sluggish diffusion of Ti. The Al-2.51Ti-0.7Si alloy developed in this work can in the future be further strengthened by adding additional alloying elements and by optimising the heat treatment.