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<p>Friction-stir welding of A356 aluminum alloy is an important process in the new-energy vehicle industry. However, the abnormal grain growth (AGG) that occurs in the stir zone (SZ) during post-weld heat treatment (PWHT) hinders further improvement in the mechanical performance of the joints. The aim of this study is to clarify the fundamentals and find appropriate methods to inhibit AGG during PWHT. Accordingly, Humphreys’ model was modified by incorporating the strain-induced grain boundary migration mechanism that was characterized by electron backscatter diffraction technology. The percentage of the embryonic AGG grains (<i>N</i>) and their mean relative growth rate (<i>V</i>) were proposed and used to predict the AGG grain sizes. The results demonstrated that the AGG grain sizes increased as the magnitude of <i>V</i>/<i>N</i> increased. As the pinning force (<i>P</i><sub><i>Z</i></sub>) increased, the AGG grain sizes increased in the SZ, resulting in fewer coarse AGG grains in the SZ. When <i>P</i><sub><i>Z</i></sub> exceeded the grain boundary migration driving force of all the grains, grain growth was eliminated. An increase in the difference in the stored strain energy among grains by pre-deformation led to denser embryonic AGG grains and decreased <i>V</i>/<i>N</i>, consequently decreasing the AGG grain sizes. The modified model accurately predicts the occurrence of AGG and provides effective solutions to inhibit AGG during PWHT.</p>

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Modified Humphreys’ Model-Based Investigation of the Mechanism of Abnormal Grain Growth in Friction-Stir Welded Joints in A356 Aluminum Alloy

  • 期刊论文

  • 发表日期：

    2025年2月9日

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  • Li-Jia Tan,
  • Zhao Yang,
  • Jian-Qi Lu,
  • Kang-Hsien Fan,
  • Yong-Jie Su

摘要

Friction-stir welding of A356 aluminum alloy is an important process in the new-energy vehicle industry. However, the abnormal grain growth (AGG) that occurs in the stir zone (SZ) during post-weld heat treatment (PWHT) hinders further improvement in the mechanical performance of the joints. The aim of this study is to clarify the fundamentals and find appropriate methods to inhibit AGG during PWHT. Accordingly, Humphreys’ model was modified by incorporating the strain-induced grain boundary migration mechanism that was characterized by electron backscatter diffraction technology. The percentage of the embryonic AGG grains (N) and their mean relative growth rate (V) were proposed and used to predict the AGG grain sizes. The results demonstrated that the AGG grain sizes increased as the magnitude of V/N increased. As the pinning force (PZ) increased, the AGG grain sizes increased in the SZ, resulting in fewer coarse AGG grains in the SZ. When PZ exceeded the grain boundary migration driving force of all the grains, grain growth was eliminated. An increase in the difference in the stored strain energy among grains by pre-deformation led to denser embryonic AGG grains and decreased V/N, consequently decreasing the AGG grain sizes. The modified model accurately predicts the occurrence of AGG and provides effective solutions to inhibit AGG during PWHT.