Heritage precipitation-strengthened aluminium alloys (2xxx, 6xxx and 7xxx series) exhibit poor microstructural stability and mechanical properties above 200°C and are very difficult to process by additive manufacturing due to their high susceptibility to hot cracking. In this context, Constellium C-TEC develops new compositions of Al alloys designed specifically for high-temperature applications by leveraging the out-of-equilibrium solidification conditions of laser beam powder bed fusion (LB-PBF).
The high-temperature mechanical properties of the Al-4Mn-3Ni-2Cu-1Zr (in wt.%) HT1 alloy have been evaluated in various ageing conditions. The decrease in yield strength with temperature is larger for the peak-aged condition (presence of thermally stable Zr-rich nanoprecipitates) compared to its stress-relieved counterpart (no Zr-rich nanoprecipitates). The underlying reasons have been investigated and the supersaturated solid solution in Mn and the morphology of the intermetallic network have been found to have a strong influence on the strength above 200°C while the Zr-rich thermally stable nanoprecipitates have a minor effect. In addition, a ductility drop is observed at low strain rates above 200°C. This ductility drop has been linked to the microstructural heterogeneities, in particular to the presence of a high volume fraction of submicron equiaxed grains at the melt pool boundaries.
In light of the conclusions drawn from the study on the HT1 alloy, a new generation of Al alloys was introduced based on the Al-Fe-Cr-Mn-Zr system, with the aim of overcoming the limitations identified in the HT1 alloy. A novel prototype alloy, namely the Al-2Fe-2Cr-1Mn-0.7Zr (in wt.%) HT2 alloy has been studied. The influence of ageing at 400°C on its mechanical properties at both room and elevated temperatures have been investigated in light of the microstructural evolutions occurring upon ageing. A systematic comparison with the HT1 alloy is presented to highlight and explain the main improvements and remaining challenges of this novel generation of high-temperature Al alloys.