An understanding of duplex microstructures encountered during high strength aluminium alloy laser beam melting processing

Abstract

Many developments on the laser beam melting (LBM) process of hot cracking prone Al alloys are focusing on the addition of Zr or Sc elements. The aim is to promote Al3Zr or Al3Sc precipitation, providing ideal low-energy heterogeneous sites for αAl phase nucleation. A duplex grain microstructure is often induced. Inside one melt pool, the solidification starts at the bottom with an equiaxed structure. Then, at a location around the centre of the pool, a resumption of columnar growth is noticed. The accepted reason in the literature is a variation of nucleant density with melting pool depth. The aim of the current paper is to explain such a behaviour using standard nucleation theory along with the presentation of further experimental data. To this purpose, ZrO2 and Y2O3 particles are added to an aluminium 6061 powder to confirm the assumptions made in a previous work on Yttria-Stabilized Zirconium addition, regarding the effect of zirconium addition on 6061 LBM processing. These experiments are found to provide useful informations for the theoretical nucleation framework. A good correlation has been found between the adapted nucleation model and the variation of the nucleant sites density with encountered thermal conditions. The critical cooling rate necessary to prevent Al3Zr precipitation increases with the concentration of zirconium, which allows an extension of the equiaxed band. The sensitivity of the adapted model parameters is finally discussed to rationalize its applicability.