Strengthening and fracturing mechanisms of laser-directed energy deposited Al-7075 alloy

Abstract

Here, the strengthening and fracturing mechanisms of Al-7075 alloy fabricated by laser-directed energy deposition (L-DED) in the as-printed and heat-treated specimens are comprehensively analyzed. The defects in the as-printed specimen are mainly pores gathered in the interlayer region. These pores coalesce and grow during the tensile deformation, and may eventually become the failure source. The as-printed specimen consists mainly of columnar grains along the building direction. Within grains of the as-printed specimen, numerous dislocations and few MgZn2 precipitates. The strength and elongation in the vertical direction of the as-printed specimen reach 320 MPa and 5%, respectively. An optimized T6 heat treatment process is explored to improve the performance of the as-printed Al-7075 alloy. After heat treatment, more MgZn2 is precipitated and the dislocation disappears. The strength and elongation of the heat-treated specimen achieve 400 MPa and 8%, respectively. Due to the difference in microstructure, the strengthening mechanism of the as-printed specimen is mainly associated with dislocation strengthening and Orowan strengthening while that of the heat-treated specimen is dominated by Orowan strengthening. The study provides a guideline on the performance improvement of high-strength aluminum alloys processed by L-DED.