Development of high performance aluminium alloys tailored for powder bed fusion-laser beam

Doctoral thesis, 2023

The powder bed fusion-laser beam (PBF-LB) process has lately been regarded as a top choice for creating complicated structures which are not possible via conventional manufacturing. Nevertheless, the pace of alloys for PBF-LB has been slower. Commercially available alloys are derived from cast/ wrought counterparts with limited knowledge of their suitability to PBF-LB. To fully exploit the inherent advantages of PBF-LB process, there is thus a growing need to develop alloy compositions with help from computational tools. This research work focused on the development of aluminium alloy systems tailored for the PBF-LB process. Leveraging the possibilities and limitations of PBF-LB process and with the help of CALPHAD tools, two types of alloying approaches were investigated namely in-situ alloying and ex-situ mixing. The key alloy design objectives were to avoid solidification cracking while attaining higher solid solubilities combined with a refined microstructure. The mechanical property objective was >450 MPa strength and high-temperature strength up to 573 K combined with general corrosion resistance.
Al-Mn-Cr-Zr based alloy system resulting from this thesis study include several variants with different amounts of alloying elements. Gas-atomised powder was used, and fully dense samples were processed using optimised PBF-LB process. This was followed by post-processing heat treatments to optimise mechanical properties. This created an alloy system with mechanical properties including yield strengths 250-500 MPa, elongation to failure 5-25% and bending fatigue 140-200 MPa. In as-printed state, strengthening was caused by a combination of solid solution strengthening and grain size effect. The strengthening from precipitates was observed after direct ageing heat treatments. The microstructure was characterised by SEM, TEM and in-situ synchrotron measurements. Long-term isothermal testing at 623 K for >1000 h showed a superior performance (-17 HV or 12% drop). High-temperature tensile testing at 573 K showed yield strengths >150 MPa, surpassing most commercially available Al-alloys.
These novel high performance alloys expand the available material performance envelope and create an edge over currently available systems while completely avoiding critical or rare earth elements. Such tailored alloy systems are shown to better utilise PBF-LB processing conditions to enhance material properties thus increasing the potential applications.