The beneficial effect of minor iron additions on the crack susceptibility of rapidly solidified aluminum alloy 6060 toward additive manufacturing applications

Solidification cracking presents a challenge for additive manufacturing of many materials, such as heat treatable aluminum alloys. The objectives of this article are to demonstrate the beneficial effect of minor Fe additions on crack elimination in rapidly solidified Al alloy 6060 and to elucidate the underlying mechanism. Blocks of 6060 + xFe (0 <= x <= 0.5) were cast and single-track laser melting trials were performed on the surface of the blocks to emulate solidification rates characteristic of additive manufacturing. Analysis of the melt pool cross sections revealed that cracks were present in all processing conditions for the baseline 6060 alloy, but a monotonic decrease in cracking was observed with increasing Fe content with no cracks observed in any conditions for the 6060 + 0.5Fe sample. Serial SEM imaging and FIB milling were used to reveal the 3D morphology of nanoscale particles in the melt pools, while TEM analysis was used to identify the phases. Discrete and nodular delta-AlFeSi particles were observed in samples containing 0.1-0.2 wt% Fe, while elongated and branched alpha-AlFeSi particles were observed in the 6060 + 0.5Fe sample. The crack mitigation mechanism is proposed to be due to bridging of the Al matrix by the AlFeSi particles, through the formation of a strong interface and mechanical interlocking, thereby resisting the thermally induced loads. It is concluded that minor Fe additions can effectively eliminate cracking in Al 6060, and that this cannot be explained by conventional mechanisms such as grain refinement or an increase in eutectic liquid.