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<p>In this study, a steel threaded rotating tool was used for friction stir welding of 5-mm-thick LZ91 Mg-Li alloy, and the microstructure and mechanical properties of single-pass and reciprocating welding with various parameters were studied. The single-pass welding resulted in a stir zone with two distinct parts, namely, a fine grain part (FGP) with uniformly mixed refined <i>α</i> and <i>β</i> phases, and a coarse grain part (CGP) showing <i>α</i> phase agglomeration and increased grain size. The reciprocating welding successfully eliminated the CGP and achieved a more uniform microstructure in the stir zone. At a rotation speed of 600&#xa0;rpm, reciprocating welding expanded the thermally mechanically affected zone and increased the grain size, while finer grains with high fraction of low-angle grain boundaries were formed at 60&#xa0;rpm. The stir zone of the reciprocating welded joints at 60&#xa0;rpm exhibited superior strength and elongation compared to the base metal, mainly due to the fine grain strengthening effect. Additionally, reciprocating welding significantly reduced local strain, allowing for simultaneous deformation along both sides of the stir zone during tensile testing. The results indicate that reciprocating welding at low rotation speed could effectively improve the microstructural uniformity and mechanical properties of LZ91 Mg-Li alloys.</p>

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Improved Mechanical Properties of Reciprocating Friction Stir Welded LZ91 Mg-Li Alloy at Low Rotation Speed

  • 期刊论文

  • 发表日期：

    2025年6月9日

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  • Shiquan Cui,
  • Weilong Liu,
  • Zhipeng Liu,
  • Li Ye,
  • Wenbo Cao,
  • Yufeng Sun,
  • Shaokang Guan

摘要

In this study, a steel threaded rotating tool was used for friction stir welding of 5-mm-thick LZ91 Mg-Li alloy, and the microstructure and mechanical properties of single-pass and reciprocating welding with various parameters were studied. The single-pass welding resulted in a stir zone with two distinct parts, namely, a fine grain part (FGP) with uniformly mixed refined α and β phases, and a coarse grain part (CGP) showing α phase agglomeration and increased grain size. The reciprocating welding successfully eliminated the CGP and achieved a more uniform microstructure in the stir zone. At a rotation speed of 600 rpm, reciprocating welding expanded the thermally mechanically affected zone and increased the grain size, while finer grains with high fraction of low-angle grain boundaries were formed at 60 rpm. The stir zone of the reciprocating welded joints at 60 rpm exhibited superior strength and elongation compared to the base metal, mainly due to the fine grain strengthening effect. Additionally, reciprocating welding significantly reduced local strain, allowing for simultaneous deformation along both sides of the stir zone during tensile testing. The results indicate that reciprocating welding at low rotation speed could effectively improve the microstructural uniformity and mechanical properties of LZ91 Mg-Li alloys.