Microstructural evolution and mechanical properties of Laser Metal Deposition Al–Mg-Sc-Zr alloy using recycled powders in an open environment

Abstract

Al–Mg-Sc-Zr alloys with flat surface, low porosity and excellent mechanical properties were prepared by Laser Metal Deposition (LMD) and annealed (A) using secondary recycled Al–Mg-Sc-Zr powders in an open environment. The optimal LMD parameters are laser power 1800 W, scanning speed 5 mm/s and scanning spacing 3 mm The Al–Mg-Sc-Zr alloy deposited by multi-layer multi-track under optimal parameters and subsequent annealing treatment displays a flat surface, porosity of 2.08%, hardness of 134 HV, ultimate tensile strengthen (UTS) of 348 MPa, yield strength (YS) of 235 MPa and uniform elongation (εu) of 16.71%. The porosity mainly concentrates in the molten pool overlap due to the influence of Marangoni convection. Solute redistribution and compositional subcooling lead to periodic segregation of Sc elements, finally contributing to the appearance of bright and dark spiral stacking features centered in the molten pool. Due to the small G/R value of the molten pool, coarse and fine equiaxed grains are alternately developed during single-track deposition. The in-situ heat treatment induced by the subsequent multi-layer multi-track deposition transformed the alternately distributed coarse and fine equiaxed grain structure into a homogeneous grain structure. This results from the combination influence of the difference in the proportion of large-angle grain boundaries occupied by the coarse/fine grains and the pinning of the Al3(Sc,Zr) grains. The strength of the present alloy is mainly provided by solid solution strengthening, fine grain strengthening and second phase strengthening. The stress concentration is reduced for fine grain size and the intergranular strain transfer is favored because of small internal orientation difference. Meanwhile, the interlayer retention and annealing treatment timely release the thermal stresses during the LMD process, which results in the Al–Mg-Sc-Zr alloys keep good plasticity.

Keywords

• Laser metal deposition
• Al–Mg-Sc-Zr alloys
• Process optimization
• Microstructure
• Mechanical property