High Performance Aluminium Alloys for Laser Powder Bed Fusion: Alloy Design and Development

Licentiatavhandling, 2021

Additive Manufacturing (AM), particularly Laser Based Powder Bed Fusion (LB-PBF) has proven to be a notable technology to produce novel microstructures and expand the ceiling of product design. However, the development of novel alloys in metal LB-PBF has not followed the same pace as development of product design. The commonly available Fe-, Al- or Ni- alloys are developed from their conventional cast / wrought counterparts with limited knowledge of their processability and adaptability to the LB-PBF process. There is limited research into developing new alloys by the means of pushing the boundaries of conventional metallurgy with help from computational tools available.
This research work focuses on addressing these limitations by working within aluminium alloy systems. It was seen that the prominence of printability issues such as solidification cracking becomes a major hinderance to developing fully dense materials. Alloys processed using LB-PBF were more susceptible to these issues due to the processing conditions, even though the same conditions could make them more desirable in some cases. Thus, simple ways to screen the alloy systems before printing could be used as a quick method to save time. Additionally, the major benefit of LB-PBF comes from rapid solidification wherein solidification rates of few orders of magnitude higher than cast/ wrought alloys are achieved. This can be leveraged to develop new families of Al-alloys, by utilising higher supersaturation of the transition series elements. A few examples are Manganese (Mn), Chromium (Cr) and Zirconium (Zr), which are shown to successfully create printable alloys with solubilities beyond equilibrium.
Printability and high solubility in solid solution of the alloys present a partial solution to the problem. The objective of this study was to showcase high performance Al-alloys, in this case high temperature performance. Mn, Cr and Zr were thus chosen due to their slow bulk diffusivities in aluminium. This means that the nucleation and growth kinetics of the precipitates generated by these elements could be slower than precipitates rich in elements such as Mg and Si. Such an occurrence is lucrative while developing precipitation hardened alloys as the optimum conditions for peak hardness can be controlled relatively easily and improved mechanical properties at high temperature could be expected. Interestingly, it was observed that Al-Mn family of precipitates preferentially grow at grain boundaries in the beginning of heat treatments and later shift to bulk growth. The growth of Al3Zr precipitates, which occurs in the bulk, could be controlled by optimising the kinetics of growth of these Al-Mn family of precipitates. This phenomenon has experimentally shown to provide a secondary hardening in the alloys and reach hardness values corresponding to that of high strength Al-alloys. This phenomenon is novel for Al-alloys as it would be difficult to recreate the same conditions via other processing routes due to significantly lower solubility of transition elements.