Design strategy for eliminating cracking and improving mechanical properties of Al-Mg-Si alloys fabricated by laser melting deposition

Laser-based additively manufactured Al-Mg-Si (6xxx) aluminum alloys have long suffered from coarse columnar grains and continuous hot cracks, which greatly limit their application. In this work, a novel and easy solution of adopting interlayer pause (IP) strategy is demonstrated to eliminate hot cracks and thus improve mechanical properties of the Al-Mg-Si alloys fabricated by laser melting deposition (LMD). It is found that increasing IP time not only facilitates the refinement of coarse columnar grains, but also inhibits the favored grain growth tendency along building direction and induces the columnar to equiaxed transition. Additionally, an appropriate IP time is demonstrated to eliminate hot cracks along the columnar grain boundaries and reduce the number density of porosities; but further prolonging IP time results in the formation of a large number of long horizontal lath-like cracks. On the other hand, the extremely-rapid solidification during LMD inhibits the formation of primary intermetallic particles and further facilitates the precipitation of age hardening precipitates. With the optimal IP strategy of 8 s per layer, the LMDed AA6061 alloy exhibits a yield strength (YS) of 281 ± 1 MPa and an elongation of 18% after T6 heat treatment, showing even better mechanical properties than their cast and wrought counterparts. This IP strategy provides a guideline for future LMD manufacturing of high-strength wrought Al alloys (such as 2xxx and 7xxx alloys).