Unforeseen precipitation sequences in ex-situ/in-situ TiB₂-reinforced Al2618 composites manufactured by PBF-LB/M

Wrought aluminum alloys are of particular interest for powder-based additive manufacturing (AM) such as laser powder-bed fusion (LPBF), but many of them suffer from poor manufacturability due to their hot-cracking susceptibility. One strategy to address this challenge is slightly modifying the alloy composition of the powders by introducing alloying elements such as Ti and B to form TiB₂ particles. These particles not only reduce hot-cracking susceptibility but also enhance elastic modulus and strength of the material. However, the amount of TiB₂ that can be introduced in situ into the powder is limited by the reaction between B and Ti in the molten metal. An alternative is the ex-situ addition of TiB₂ to the powder. In this work, a hybrid approach has been developed to produce a novel TiB2 reinforced Al2618 composite powder by combining the beneficial effects of both the composite production methods. More specifically, it offers the opportunity to refine grains via the in-situ addition of Ti and B in gas-atomized powder and the possibility to add high percentage of TiB2 by the ex-situ methods to target high elastic modulus/strength. Subsequently, the composite powder was processed by LPBF, followed by T5 and T6 heat-treatments. Microstructural analysis revealed deviations from the typical precipitation sequence of Al2618, attributed to the excess of Ti in the pre-alloyed powder. T5 heat-treatment promoted the formation of Al₃Ti and S' precipitates at cell boundaries. T6 heat-treatment resulted in Ti–Si-rich phases forming needle structures arranged in a corona-like pattern and Al-Mg-Ti particles that inhibit the precipitation of the S' phase. The T5 condition exhibited high strength, while the T6 condition demonstrated the lowest coefficient of thermal expansion and highest thermal conductivity. The combined in-situ/ex-situ strategy led to a notable increase in elastic modulus and strength compared to conventional Al2618, while maintaining satisfactory ductility and fracture toughness.