The work aims to address the issues of poor sidewall fusion, high porosity sensitivity, and significant differences in thermal cycles during multi-pass welding in narrow-gap TIG welding of 20 mm thick TC4 titanium alloy medium-thick sheets caused by the narrow groove, by systematically investigating the effects of welding current, welding speed, wire feed speed, and tungsten electrode oscillation speed on the weld formation and mechanical properties of the root and filler layers through a layer-specific optimization strategy, and clarifying the regulation mechanisms and priorities of process parameters for each layer. Multi-pass welding experiments were conducted using a narrow-gap oscillating arc TIG welding system. Orthogonal experiments with four factors and three levels were designed for the root layer and filler layer, respectively. The cross-sectional area, penetration depth, and tensile strength were taken as evaluation indicators. Range analysis and variance analysis were employed to determine the primary and secondary effects and significance of each factor. Combined with metallographic observation, the influence mechanism of heat input on the microstructure evolution in the fusion zone and heat-affected zone was revealed. For the root layer, wire feed speed was the dominant factor affecting the cross-sectional area (R=89.82); welding speed had the most significant effect on tensile strength (R=25.9, P=0.031), and tungsten electrode oscillation speed also showed a significant effect (P=0.028); welding current was the main factor influencing penetration depth (R=1.4). The optimized parameters were welding current of 180 A, welding speed of 120 mm/min, wire feed speed of 0.8 m/min, and oscillation speed of 60 mm/s, corresponding to a heat input of 1 080 J/mm, achieving a tensile strength of 910 MPa. For the filler layer, tungsten electrode oscillation speed had an extremely significant effect on tensile strength (R=38.97, P<0.001), and wire feed speed had a significant effect (P=0.015). The optimized parameters were welding current of 200 A, welding speed of 120 mm/min, wire feed speed of 1.5 m/min, and oscillation speed of 55 mm/s, corresponding to a heat input of 1 300 J/mm, achieving a tensile strength of 908 MPa. Metallographic analysis showed that as the heat input increased from 775 J/mm to 1 178 J/mm, the microstructure of the root layer weld evolved from fine acicular martensite to a mixed α+β structure and then to coarsened Widmanstätten structure, while grains in the heat-affected zone gradually coarsened. The filler layer exhibited a similar evolution trend, but grain coarsening was more pronounced due to different heat dissipation conditions. In conclusion, the layer-specific optimization strategy effectively resolves the problems of poor sidewall fusion and porosity defects, clarifying that the root layer requires a balance between penetration and strength, while for the filler layer, the focus should be put on molten pool stirring and filler metal matching. Heat input is a key factor in microstructure evolution; tensile strength first increases and then decreases with increasing heat input, reaching peak values at approximately 890 J/mm for the root layer and 1 040 J/mm for the filler layer. Within an appropriate heat input range, welded joints with fine microstructure and excellent mechanical properties can be obtained, providing a systematic experimental basis and theoretical support for the process design of multi-pass welding of thick sheets.
Key words
titanium alloy /
narrow gap welding /
orthogonal experiment /
process parameters /
heat input
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Funding
Shandong Provincial Natural Science Foundation (ZR2022QE184); Hunan Provincial Natural Science Foundation (2024JJ6174)
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