Mechanism of Grain Refinement Induced by Mn Element in Wire Arc Addition Manufacturing Al-Mg Alloy

Abstract

Two different Mn contents of Al-Mg-Mn alloy wires were employed for preparing wire arc addition manufacturing (WAAM) samples. The compositions of the as-deposited samples were Al-8.55Mg-0.39Mn and Al-8.67Mg-1.26Mn. During the wire additive manufacturing process, both Mg and Mn elements underwent significant volatilization. With the addition of Mn, the primary phases present included Al₈(FeMn)₅, Al₉(FeMn)₄, Al₁₁(FeMn)₄, and Mn atoms dissolved in the matrix. The addition of Mn significantly inhibited pore formation during the additive manufacturing process and refined the grain structure of the alloy. With an increase in Mn content, the average grain size in the deposited intermediate zone decreased from 72 μm to 37 μm. This grain refinement occurred through the process of repetitive melting and deposition in additive manufacturing. In the initial stages of solidification, the Al₈(FeMn)₅ phase preferentially precipitated from the liquid phase, forming irregular clustered structures but not serving as nucleation sites. The initially coarse columnar grains in the topmost layer after a single deposition transformed into fine equiaxed grain structures after multiple overlays. Additionally, the addition of Mn significantly improved the mechanical properties of the aged structure. The optimum mechanical properties were achieved after aging treatment at 470 °C for 6 h, with tensile strength, yield strength, and fracture elongation of 377 MPa, 202 MPa, and 29.2%, respectively. The improvement in mechanical properties after aging treatment primarily resulted from the precipitation strengthening effect of the Al₆Mn precipitate phase and the elimination of the intergranular Al₃Mg₂ segregation phase. The sizes of the Al₈(FeMn)₅, Al₉(FeMn)₄, and Al₁₁(FeMn)₄ phases did not undergo significant changes during the aging process.

Graphical Abstract