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Purpose: This paper aims to evaluate the corrosion resistance of fusion zones in different heat treated condition from laser welded Mg-Gd-Y alloys and analyze their intrinsic corrosion mechanism. Design/methodology/approach: The corrosion behavior of fusion zone from laser-welded Mg-Gd-Y alloy joints in the as-welded, ageing and solution + ageing conditions was investigated in the 3.5 wt.% NaCl solution by immersion tests. Findings: The corroded surface of as-welded fusion zone consists of dark and bright regions, and the bright regions were composed of high-density needle-like products, whereas some different extended direction of the cracks with lots of long ultrafine needle-shaped corrosion products appeared in the dark regions. The corrosion resistance of the fusion zone is slightly decreased after the ageing treatment. Originality/value: The solution + ageing treated fusion zone exhibits the highest corrosion resistance than that of the as-welded and ageing treated state because of the full dissolution of the cathodic coarse eutectic compound, grains growth and relatively compact protective films. The inhomogeneous distribution of the β′ can somewhat improve the corrosion rate of solution + ageing treated fusion zone when compared with base metal. [ABSTRACT FROM AUTHOR]

The precipitates evolution in the heat affected zone of laser-welded Mg-Gd-Y-Zr alloy in T6 condition was investigated based on the TEM observation. Some oval and long-rod shape precipitates along [ 2 1 ¯ 1 ¯ 0 ] α were observed, which were not involved in precipitation sequence. Compared with isothermal aged precipitates, the overaged β′ phase presented same morphology and size, while size of needle-shape β became much smaller. The width of complete dissolution zone of the β′ is smaller than that of partial dissolution and transformation zone. • Some oval and long-rod shape precipitates along [ 2 1 ¯ 1 ¯ 0 ] α were observed in the HAZ. • Some overaged β' phase can be precipitated in the HAZ. • The size of needle-shape β in the HAZ became much smaller, compared with isothermal aged precipitates. [ABSTRACT FROM AUTHOR]

The microstructure evolution in the fusion zone of laser-welded Mg-Gd-Y-Zr alloy during solution and aging treatment is investigated. The morphology of the Mg 24 (Gd,Y) 5 in the divorced eutectic at the grain boundary transforms from a continuous network to disconnected and fragmentized islands and then to spheroidal particles before complete dissolution during the solution treatment at 430 °C. During the subsequent aging treatment at 225 °C, the precipitation sequence in the fusion zone follows the order of supersaturated solid solution (SSSS) → βʺ(D0 19 ) → βʹ(cbco) → β 1 (fcc) → β(fcc). High-density precipitates are present at the original grain boundaries of the fusion zone from the welded structure but there are less precipitates in the interior of the original grains. The grain growth during the solution treatment at 430 °C comprises the slowly increasing stage, rapidly increasing stage, and stable stage. The network-distributed Mg 24 (Gd,Y) 5 impedes migration of the grain boundaries, restricts grain growth in the first slowly increasing stage, and segregation of zirconium near the grain boundaries also affects migration of the grain boundaries. [ABSTRACT FROM AUTHOR]