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In this study, the size effect and mechanism on the macro-microstructure and mechanical properties of TA2-M pure titanium joints were systematically revealed by regulating the stirred friction lap welding shoulder diameter (10–15 mm). The results show that the shoulder diameter dominates the morphology evolution of the weld Stir Zone (SZ) through the frictional heat-force coupling effect. The Heat-Affected Zone (HAZ) exhibits multi-scale grain gradient coarsening characteristics due to heat conduction hysteresis. Microanalysis shows that the stirring head heat-force coupling induces non-uniform Dynamic Recrystallization (DRX). The local dislocation density difference leads to the preferential orientation and abnormal growth of grains. The Thermo-Mechanically Affected Zone (TMAZ) lacks the direct shear effect, and the degree of DRX is significantly lower than that of the SZ, which results in the formation of gradient crushing organization. As the diameter of the axial shoulder increases, the grain size of the SZ shows a non-monotonic evolution of first refinement and then coarsening, which follows the heat input-DRX threshold competition mechanism: under sub-threshold conditions (D < 12 mm), insufficient heat flow density suppresses the DRX process. Under suprathreshold conditions (D > 12 mm), heat accumulation triggers the grain coarsening dominated by the migration of the grain boundaries. The microhardness curve showed a “∧” distribution, and the tensile strength increased and then decreased with the increase of the shoulder diameter, reaching a peak value of 441.32 N/mm2 at D = 12 mm, which was consistent with the Hall–Petch theory of fine-grain strengthening. Fracture analysis showed that isometric and shear toughness nests coexisted, and the crack extension path was regulated by the microhardness gradient (SZ > TMAZ > BM zone), which confirmed the window effect of process parameter optimization and the plastic fracture characteristics. This study provides a theoretical basis for the design and performance control of the friction stir welding process of TA2-M pure titanium sheet.

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Regulation Mechanism of Shoulder Diameter on Dynamic Recrystallization Behavior and Mechanical Properties of TA2-M Pure Titanium Stir Friction Lap Welded Joints

  • 会议论文

  • 发表日期：

    2026年

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  • Zeyu Liu,
  • Bingxuan Chao,
  • Zhiyong Li,
  • Qipeng Li

摘要

In this study, the size effect and mechanism on the macro-microstructure and mechanical properties of TA2-M pure titanium joints were systematically revealed by regulating the stirred friction lap welding shoulder diameter (10–15 mm). The results show that the shoulder diameter dominates the morphology evolution of the weld Stir Zone (SZ) through the frictional heat-force coupling effect. The Heat-Affected Zone (HAZ) exhibits multi-scale grain gradient coarsening characteristics due to heat conduction hysteresis. Microanalysis shows that the stirring head heat-force coupling induces non-uniform Dynamic Recrystallization (DRX). The local dislocation density difference leads to the preferential orientation and abnormal growth of grains. The Thermo-Mechanically Affected Zone (TMAZ) lacks the direct shear effect, and the degree of DRX is significantly lower than that of the SZ, which results in the formation of gradient crushing organization. As the diameter of the axial shoulder increases, the grain size of the SZ shows a non-monotonic evolution of first refinement and then coarsening, which follows the heat input-DRX threshold competition mechanism: under sub-threshold conditions (D < 12 mm), insufficient heat flow density suppresses the DRX process. Under suprathreshold conditions (D > 12 mm), heat accumulation triggers the grain coarsening dominated by the migration of the grain boundaries. The microhardness curve showed a “∧” distribution, and the tensile strength increased and then decreased with the increase of the shoulder diameter, reaching a peak value of 441.32 N/mm2 at D = 12 mm, which was consistent with the Hall–Petch theory of fine-grain strengthening. Fracture analysis showed that isometric and shear toughness nests coexisted, and the crack extension path was regulated by the microhardness gradient (SZ > TMAZ > BM zone), which confirmed the window effect of process parameter optimization and the plastic fracture characteristics. This study provides a theoretical basis for the design and performance control of the friction stir welding process of TA2-M pure titanium sheet.