Microstructural evolution and mechanical properties of a WA-DED Mg-8Gd-1Er-0.5Zr alloy under different post-solution cooling paths

Post-solution cooling governs solute redistribution and defect evolution in Mg-rare-earth alloys, but its influence on the intrinsically heterogeneous microstructure of wire-arc directed energy deposition (WA-DED) alloys remains poorly understood. Here, air cooling (AC), furnace cooling (FC), and deep cryogenic treatment (DCT) were applied after solution treatment to a WA-DED Mg-8Gd-1Er-0.5Zr (wt.%) (GE81) alloy, followed by peak aging. Grain structure, dislocation state, β′ precipitation, and mechanical response were correlated across melt-pool regions. DCT retained the refined heterogeneous grain structure, with grain sizes of 19.93, 10.39, and 13.27 μm in heat-affected zones, melt-pool boundaries, and melt-pool centres, respectively, and the highest geometrically necessary dislocation density of 0.52–0.61 × 10¹⁴ m−². FC promoted grain-boundary precipitation and reached peak hardness after 57 h, whereas DCT delayed the peak to 69 h and produced the largest hardness increment. The average β′ length/width was 13.1/6.3 nm for AC, 22.7/9.1 nm for FC, and 18.8/8.5 nm for DCT. Peak-aged DCT specimens achieved an ultimate tensile strength of 296 MPa, yield strength of 186 MPa, and elongation of 14.6%. Quantitative analysis identified β′ precipitation as the dominant strengthening mechanism. Thus, post-solution cooling effectively regulates inherited microstructural heterogeneity, precipitation kinetics, and the strength-ductility balance of WA-DED Mg-rare-earth alloys.