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<p>Titanium alloys, characterized by their excellent specific strength, corrosion resistance, and other remarkable properties, serve as critical materials in deep-sea engineering and the fabrication of key equipment. As deep-sea exploration advances, understanding the formation and fracture mechanisms of titanium alloy welded joints has grown increasingly significant. This study focused on TC4ELI titanium alloy and adopted the narrow-gap laser-arc hybrid welding process to systematically explore the influence laws of different heat inputs on the weld formation, microstructure and mechanical properties. Through the combination of numerical simulation and experimental verification, the laser-arc hybrid heat source model has been confirmed to effectively reproduce the welding process. The predicted molten pool morphology is highly consistent with the experimental observations, and the temperature field is symmetrically distributed along the center of the weld seam. With the increase of heat input, the peak temperature in each area of the joint rises and the cooling rate decreases, resulting in a significant temperature gradient difference between the weld metal and the base metal. The welded joint was well formed without any defects. The center of the weld seam was mainly composed of coarse <i>β</i> columnar crystals and intragranular <i>α</i>′ martensite. The heat-affected zone contained equiaxed <i>α</i> phase, <i>β</i> phase and <i>α</i>′ martensite. The grains in the coarse-grained zone were coarser and the acicular <i>α</i>′ martensite was denser. The microhardness shows an “M” shape distribution: the hardness of the base material is the lowest, the hardness of the weld layer is higher than that of the base material, and the HAZ hardness fluctuates significantly. Among them, the hardness of the bottom weld layer is the highest under a heat input of 4.28&#xa0;kJ/cm. In terms of tensile properties, the average strength of the joint exceeds 1100&#xa0;MPa, reaching 1166&#xa0;MPa when the heat input is 4.28&#xa0;kJ/cm. All fractures occur in the fusion zone, presenting sawtooth-shaped crack propagation and ductile fracture characteristics, which are attributed to the formation of needle-like <i>α</i>′ martensite near the fusion line. In the room-temperature impact performance, the average impact energy of the base material is 37&#xa0;J. When the heat input is 5.96&#xa0;kJ/cm, the impact energy of the joint is 42&#xa0;J and that of HAZ is 30&#xa0;J, both of which are higher than those of other heat inputs. With the increase of heat input, the impact energy at the center of the weld seam first rises and then decreases, the HAZ change is not obvious, and the fracture surface shows a mixed toughness and brittleness characteristic. In addition, there were no cracks on the bent surface under different heat inputs, indicating that this process can obtain a joint structure with excellent plastic toughness. The research results provide a key basis for the process optimization and performance regulation of laser-arc hybrid welding of TC4ELI titanium alloy.</p>

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Study on Microstructural Evolution and Fracture Mechanism of Titanium Alloy Narrow Gap Laser-Arc Hybrid Welded Joints Under Different Heat Inputs

  • 期刊论文

  • 发表日期：

    2025年12月1日

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  • Hongpeng Liu,
  • Xuetian Ding,
  • Deyang Zhang,
  • Jun Wang,
  • Yonglin Ye,
  • Yong Zhao,
  • Feiyun Wang

摘要

Titanium alloys, characterized by their excellent specific strength, corrosion resistance, and other remarkable properties, serve as critical materials in deep-sea engineering and the fabrication of key equipment. As deep-sea exploration advances, understanding the formation and fracture mechanisms of titanium alloy welded joints has grown increasingly significant. This study focused on TC4ELI titanium alloy and adopted the narrow-gap laser-arc hybrid welding process to systematically explore the influence laws of different heat inputs on the weld formation, microstructure and mechanical properties. Through the combination of numerical simulation and experimental verification, the laser-arc hybrid heat source model has been confirmed to effectively reproduce the welding process. The predicted molten pool morphology is highly consistent with the experimental observations, and the temperature field is symmetrically distributed along the center of the weld seam. With the increase of heat input, the peak temperature in each area of the joint rises and the cooling rate decreases, resulting in a significant temperature gradient difference between the weld metal and the base metal. The welded joint was well formed without any defects. The center of the weld seam was mainly composed of coarse β columnar crystals and intragranular α′ martensite. The heat-affected zone contained equiaxed α phase, β phase and α′ martensite. The grains in the coarse-grained zone were coarser and the acicular α′ martensite was denser. The microhardness shows an “M” shape distribution: the hardness of the base material is the lowest, the hardness of the weld layer is higher than that of the base material, and the HAZ hardness fluctuates significantly. Among them, the hardness of the bottom weld layer is the highest under a heat input of 4.28 kJ/cm. In terms of tensile properties, the average strength of the joint exceeds 1100 MPa, reaching 1166 MPa when the heat input is 4.28 kJ/cm. All fractures occur in the fusion zone, presenting sawtooth-shaped crack propagation and ductile fracture characteristics, which are attributed to the formation of needle-like α′ martensite near the fusion line. In the room-temperature impact performance, the average impact energy of the base material is 37 J. When the heat input is 5.96 kJ/cm, the impact energy of the joint is 42 J and that of HAZ is 30 J, both of which are higher than those of other heat inputs. With the increase of heat input, the impact energy at the center of the weld seam first rises and then decreases, the HAZ change is not obvious, and the fracture surface shows a mixed toughness and brittleness characteristic. In addition, there were no cracks on the bent surface under different heat inputs, indicating that this process can obtain a joint structure with excellent plastic toughness. The research results provide a key basis for the process optimization and performance regulation of laser-arc hybrid welding of TC4ELI titanium alloy.