​​Laser-TIG Hybrid Welding of 7075 Aluminum Alloy Using Matching Filler Wire​

From the paper "7075 Aluminum alloy welded by laser-TIG hybrid with homogeneous filler wire: Microstructure evaluation and molten pool behavior" published in the international journal "Optics and Laser Technology" by researchers from Dalian University of Technology.

01 Paper Introduction
7075 aluminum alloy, due to its high strength, low density, and excellent corrosion resistance, is widely used in aerospace and rail transit fields, such as aircraft skins and high-speed train brackets. However, this alloy has poor weldability and high sensitivity to hot cracking, which severely limits its reliability in structural joints. Traditional riveting processes are inefficient and add significant weight, making it necessary to adopt welding technologies to achieve lightweight and integrated manufacturing. Existing research mainly employs friction stir welding (FSW), laser welding (LW), or TIG welding, but these methods suffer from defects such as porosity, cracking, and element burn-off. Laser-TIG hybrid welding, through the synergistic action of laser and arc heat sources, offers high energy utilization, stable forming, and fewer defects, becoming an effective solution to the welding challenges of 7075. This paper adopts a laser-TIG hybrid welding process with homogeneous Al-Zn-Mg-Cu filler wire to systematically study molten pool behavior, microstructural evolution, and mechanisms for improving mechanical properties.

02 Full-Text Overview
This study addresses the problems of cracking and strength loss in 7075 aluminum alloy welding by proposing an integrated strategy combining laser-TIG hybrid welding with homogeneous Al-Zn-Mg-Cu filler wire: the synergistic heat source of laser and arc reduces molten pool cooling rates and enhances melt convection, while the filler wire replenishes Zn and Mg elements that are prone to burn-off, achieving crack-free formation in single-pass thin-plate welding. EBSD and TEM analyses show that the weld center forms a twinned dendritic structure with finely dispersed η′ strengthening precipitates, achieving a tensile strength of 475 MPa, which is 80.8% of the base metal—significantly superior to existing processes—offering an efficient and reliable new route for high-strength aluminum alloy welding.

03 Figures and Analysis
Figure 1 shows the macroscopic forming quality of the 7075 aluminum alloy weld after laser-TIG hybrid welding. The weld surface is smooth, with no obvious macroscopic cracks or spatter, and a typical fish-scale pattern appears on the upper surface, indicating the obvious effect of pulsed laser action; cross-section observations show a regular weld profile with good fusion and high dimensional consistency, confirming the process stability and excellent forming effect.

Figure 1. Macrostructure of laser-TIG hybrid welds of 7075 aluminum alloy using Al-Zn-Mg-Cu filler wire: (a) Top surface of the hybrid weld; (b) Root surface of the hybrid weld; (c) Cross-section of the hybrid weld.

Figure 2 presents the grain morphology, structural distribution, and grain size of the 7075 aluminum alloy laser-TIG welded joint in the XOY plane. Results show: the base metal is equiaxed; the fusion zone exhibits columnar grains; the weld center consists of slender feather-like twinned dendrites, showing clear directional solidification characteristics. Pole figure analysis shows the weld zone has strong 〈100〉 and 〈111〉 textures, with {111} as the twin plane, indicating twin relationships between grains. The average grain sizes in the fusion zone and weld zone are 33 μm and 26.7 μm, respectively, reflecting the grain-refining effect of thermal cycling.

Figure 2. XOY plane EBSD scanning image of a laser-TIG hybrid weld joint, i.e., a horizontal cross-section of the laser-TIG hybrid weld joint: (a) Inverse pole figure (IPF); (b) Pole figure of the heat-affected zone; (c) Pole figure of the weld zone; (d) Pole figures of grains A and B; (e) Grain size in the heat-affected zone; (f) Grain size in the weld zone.

Figure 3 shows the molten pool flow evolution induced by pulsed laser during laser-TIG hybrid welding. The laser significantly enhances Marangoni convection in the pool, raising central temperatures and intensifying flow, forming two vortices recirculating from the center to the edges, with a maximum flow speed of 0.41 m/s—much higher than the 0.028 m/s under arc-only welding. This strong convection promotes twinned dendrite formation, improving structural uniformity and weld quality.

Figure 3. Longitudinal cross-sections of the molten pool before and after pulsed laser application: (a) t = 299.2 ms; (b) t = 300.8 ms; (c) t = 302.8 ms; (d) t = 304.8 ms; (e) t = 306.8 ms; (f) t = 308.8 ms. 

Figure 4 shows that fracture mainly occurs near the fusion line, with cleavage facets and a small number of dimples, exhibiting mixed intergranular and transgranular brittle fracture, with overall low ductility.

Figure 4. Tensile fracture: (a) Sample fracture location; (b) Fracture macromorphology; (c) (b) Yellow-boxed area; (d) (c) Yellow-boxed area.

04 Conclusion
Using laser-TIG hybrid welding with homogeneous Al-Zn-Mg-Cu filler wire, the 7075 aluminum alloy weld consists of columnar grains in the fusion zone and twinned dendrites in the weld zone; the solidification parameter G×R decreases and G/R increases along the thickness direction, with central undercooling increasing. Coupled with molten pool convection peaking at 0.41 m/s, these factors jointly promote twinned dendrite growth. Eutectics form as a network in the fusion zone and as long chains in the weld core. Pulsed laser stirring ensures uniform element distribution, with η′ strengthening precipitates semi-coherent with the aluminum matrix dispersed within grains. The joint’s ultimate tensile strength reaches 475 ± 11.1 MPa, 80.8% of the base metal, with fracture along the fusion line in a brittle manner, demonstrating that homogeneous filler wire effectively suppresses cracking and significantly improves overall performance.

**--Cite the article published by 高能束加工技术 on August 11, 2025, in the WeChat public account "High-Energy Beam Processing Technology and Applications."