Development of a kinetic phase diagram for Al-Si alloys to enable phase constituents to be determined across a broad range of cooling rates and manufacturing technologies

The microstructures observed in Al-Si alloys are very different depending on the manufacturing technologies from coarse divorced eutectics in slowly cooled alloys to highly interconnected Si with extended solubility of Si in the α-Al in additive manufacturing. In this study, a ‘kinetic’ or effective Al-Si phase diagram is proposed by analysing the phase constituents of an Al-10Si alloy fabricated using high-pressure die casting (HPDC) and powder bed fusion-laser beam (PBF-LB) processes and comparing it with known phase equilibria. It was shown that the high cooling rates, in the order of 106 K/s in PBF-LB, resulted in approximately 415 K eutectic undercooling that in turn increased the eutectic composition from 12.6 wt.% to approximately 70 wt.%. Even the moderately high cooling rates in HPDC (102-103 K/s) have some effect on the effective eutectic composition and temperature. The detailed microstructure characterisation, and solidification path modelling using the Scheil-Gulliver equation, leads to a series of empirical relationships to describe the effect of cooling rate on the effective Al-Si phase diagram. The eutectic suppression with cooling rate can explain microstructure observations in additive manufacturing including increased primary α-Al, eutectic Si morphology changes and the elevated solute Si content in the eutectic α-Al. It is apparent that the elevated solute Si is due to the eutectic undercooling rather than diffusion related solute trapping phenomena. The relationships developed will benefit microstructure modelling, process design, and alloy development for processes with very high solidification rates.