Effect of Alloying Modification on the Processability and Mechanical Properties of Al7075 by Selective Laser MeltingPart of:
High strength aluminium alloys are gaining increasingly worldwide interest in aerospace and automobile manufacturing, due to their high strength, lightweight, reasonable corrosion resistance and acceptable cost. For lightweight applications, precipitation hardening alloys, namely, 2xxx, 6xxx and 7xxx series alloys, are very attractive because of their high specific strength and stiffness. Selective Laser Melting (SLM) of high strength aluminium alloys, however, has proven to be challenging due to their high solidification cracking susceptibility and evaporation of volatile elements such as Zn and Mg. Solidification cracking, also known as hot tearing, usually occurs within the interdendritic regions of solidified dendritic/columnar grains as a result of micro segregation induced by the high thermal gradients and high cooling rates during SLM. In addition, a large solidification range of the alloy and a low fluidity of the interdendritic liquid, in combination with a local tensile stress, also contributes to hot tearing.
In this study, high density and crack-free Al-Zn-Mg-Cu-Si alloys were fabricated by Selective Laser Melting (SLM). Gas atomized powders with high Zn and Mg content were used to compensate for Zn and Mg losses during SLM. Solidification cracking was successfully mitigated by incorporating 4.58 wt% of pre-alloyed Si. The mechanism of mitigation of solidification cracking by the alloy modification will be discussed. The evaporation of Zn and Mg at different processing parameters was evaluated. Additionally, the influence of Si addition on the precipitation behaviour during post heat treatment was highlighted. This study provides new insights into the development of high strength Aluminium alloys for Additive Manufacturing (AM).