Effect of V Interlayer on Microstructure and Mechanical Properties of TC4/QP980 Coaxial Dual⁃Beam Laser Welded Joint

Objective The joining components of titanium alloy and steel have been widely used in aerospace, automobile manufacturing and other fields because of their high strength and light weight. However, the formation of Ti-Fe intermetallic compounds seriously affects the mechanical properties of joints welded with titanium alloy and steel dissimilar metals, which limits their further application. The selection of appropriate welding methods and the use of the interlayer or filler materials are of great significance for obtaining efficient and reliable titanium alloy/steel joints. Dual-beam laser welding can accurately control the composition of intermetallic compounds in dissimilar metal welding, and is more stable than single laser welding. The use of single interlayer or filler material can inhibit the formation of Ti-Fe intermetallic compounds, but other intermetallic compounds might be formed. Therefore, in this study, the dual-beam laser welding method is used and V is added as an interlayer on the basis of using CuSi3Mn1 filler wire to optimize the interface structure of the joint and improve the mechanical properties of the joint. Methods TC4 titanium alloy and QP980 high-strength steel are used as the base materials, and CuSi3Mn1 alloy is used as filler wire. A novel welding method with coaxial pulsed-continuous dual-beam laser heat source assisted by V interlayer is adopted. The microstructure and mechanical properties of the joints with different V interlayer thicknesses are compared. The macroscopic morphology of the joint is observed by optical microscope, and the mechanical properties are tested by universal tensile testing machine. The element distribution and phase composition of the interface microstructure of TC4/QP980 joints with and without V interlayer are tested and analyzed by electron probe X-ray micro-analyzer. Results and Discussions The V interlayer-assisted dual-beam laser welding process can obtain TC4/QP980 welded joints with smooth weld surface and high strength. When the thickness of the intermediate layer is small, the high energy heat input of the dual-beam laser destroys the V interlayer, resulting in a large influx of CuSi3Mn1 filler wire and forming a thick intermetallic compound reaction layer between the V interlayer and TC4. In contrast, when the thickness is large, there are pores and incomplete welding (Fig. 2). The joints with a 0.10 mm-thick V interlayer have the better mechanical properties, and the maximum tensile shear load reaches 457 N/mm (Fig. 3). The steel-side interfaces are all composed of Fe solid solution (ss) + Cu solid solution (Figs. 4 and 5). However, the microstructures of the TC4-side interfaces are different. TC4-side interface without V interlayer is mainly composed of TiCu and Ti5Si3intermetallic compounds with large hardness and brittleness, and the thickness of the intermetallic compounds layer is about 200 mu m (Fig. 6). The microstructure of the TC4-side interface using V interlayer is optimized to two layers, and the total thickness is reduced to about 110 mu m (Fig. 7). The upper layer is composed of alpha-Cu/(Fe, V, Cu, Si) ss/V, and the thicknesses of the ss layers are less than 2 mu m. The microstructure of the lower layer is V/(Ti, V) ss+Ti2Cu/Ti2Cu+TiCu/(Ti, Cu) ss+Ti2Cu/alpha-Ti. The formation of Ti2Cu, which has lower brittleness than TiCu, and the solution bonding both improve the bonding strength of the TC4-side interface. Conclusions In this study, a novel welding method with coaxial pulsed-continuous dual-beam laser heat source assisted by V interlayer is proposed. The main problem of TC4 titanium alloy and QP980 high-strength steel welding is that it is easy to form Ti-Fe intermetallic compounds between them, which leads to stress concentration and finally the joints fracture. The thickness of the V interlayer is adjusted to match the dual-beam laser welding process. The maximum tensile properties can be obtained when the thickness of the V layer is 0.10 mm. The application of a V interlayer can improve the interface microstructure and reduce the thickness of the bonding layer to about 100 mu m. Overall, the proposed coaxial pulsed-continuous dual-beam laser welding method assisted by V interlayer can effectively suppress the formation and thickness of intermetallic compounds in TC4 and QP980 high-strength steel welded joints, optimize the interface microstructure, and thus improve the mechanical properties of welded joints. This study provides a new theoretical support for solving the problem of dissimilar metal welding of titanium alloy and steel, and a reference for the application of dual-beam laser welding technology in high-performance dissimilar metal structures.