The intermetallic γ-TiAl-based alloy, as a new type of lightweight high-temperature structural material, has the advantages of low density (3.9-4.2 g/cm³), high melting point, high strength, good oxidation resistance, and excellent creep at elevated temperature, and has gained wide attention in the application of high-temperature structural parts for aerospace vehicles and automobile engines. Since in practical applications, such as turbine blades, automotive turbochargers, and exhaust valves, the TiAl-based alloy needs to be connected to its own components or dissimilar metals and the research and development of the connection technology have become the key to the promotion and application of TiAl-based alloy. Diffusion bonding, argon-arc welding, infrared brazing, friction welding, laser beam welding and electron beam welding have been investigated with the aim of producing sound TiAl joints. However, the intrinsic brittleness of TiAl-based alloy often results in cracks, high residual stress, and poor mechanical properties of welds, limiting its more widespread applications in industry and commerce. Considering the poor plasticity of TiAl alloy, the control of welding cracks of TiAl-based alloy has become the keystone of the corresponding research.

The γ-TiAl based alloy with a chemical composition of Ti-42Al-2.5Cr-1Nb-0.7Si-0.5B is investigated here. The Ti42 is prepared using plasma arc melting in the form of button ingot. The button ingot has been melted for 4 times to improve its homogeneity. The 20 mm×20 mm×2.5 mm plates are taken from the ingot by electro-discharge machining (EDM) and subsequently cleaned and ground with the 800# SiC paper to remove oxides and machining remnants. The welding process is carried out with an IPG Ytterbium fiber laser, and a KUKA robot is used to control the laser beam movement in the x-y-z coordinate system. All plates have gone through three processes: pre-heating, welding, and post-weld heat treatment. The laser welding equipment and the welding process are shown in Fig. 1. The laser power used in the welding process is 5 kW, and the welding speed is 6 m/min. In order to prevent oxidation during welding, nitrogen and argon gases are used as shielding gases, and the effect of shielding gas on welding is investigated. The welds are visually inspected and investigated by X-ray radiographic inspection to detect cracks and porosity. The as-welded specimens are first sectioned, ground, and polished, and then mechanically polished with a vibrating polisher. The microstructure of the welded joint is observed using the FEI Quanta-200 field emission scanning electron microscope in the backscattering (BSE) mode, and the orientation relationship of its microstructure is analyzed using electron backscatter diffraction (EBSD).

The joints welded under nitrogen shielding gas have obvious penetrating cracks inside the joints and the weld surface is poorly formed [Fig. 2(a)]. The use of argon gas as the shielding gas makes a crack-free welded joint realized. The weld surface is well formed, and the weld is in an "X" shape as a whole [Fig. 2(b)]. When nitrogen gas is used as the shielding gas, the TiN phase exists in the top area of the weld. The phase compositions in the bottom area of the weld are α₂-Ti₃Al phase and γ-TiAl phase when argon gas is used as the shielding gas (Figs. 5 and 6). According to the Ti-Al binary phase diagram, there exists a transformation, β→α, in the alloy with Al concentration (atomic fraction) of 42% (Fig. 3). During the transformation from β to α, the α phase (hexagonal close-packed) and the β phase (body-centred cubic) maintain a specific crystal orientation relationship. This special orientation relationship is called the Burgers relationship, {0001}_α∥{110}_β and 〈112ˉ0〉_α∥〈111〉_β (Figs. 7 and 8). The results indicate that when there is no TiN phase in the weld zone, the welded joint is formed well without defects, and the weld zone is composed of Burgers α₂ phase. The complete solidification path of the weld zone is L→L+ β→β+ α→α+ γ→α₂+ γ. When there is TiN phase in the weld zone, the welded joint produces penetrating crack defects. The main reason is that TiN is a brittle intermetallic compound, which can easily cause cracks during laser welding. The weld zone is mainly composed of TiN dendritic phase and non-Burgers α₂ phase. Moreover, there exists a certain orientation relationship between TiN phase and α₂ phase: {111}TiN//{0001}α2(Fig. 10). The formation of TiN phase has a grain refinement effect on the weld structure. The complete solidification path of the weld zone is L→TiN+ L→TiN+ β→TiN+ α+ γ→TiN+ α₂+ γ.

A study on laser welding under different shielding gases is carried out with the Ti-42Al-2.5Cr-1Nb-0.7Si-0.5B alloy as the research object. A laser welding experiment is carried out with this alloy, and the effect of TiN on the phase transformation, texture, and grain refinement of laser welded joints is analyzed and studied. During laser welding of the TiAl-based alloy, the use of nitrogen and argon gases as shielding gases can have a great impact on the microstructure and properties. When using the argon gas, the welding process is stable, and a well-formed and crack-free welded joint is obtained. In contrast, when the nitrogen gas is used, nitrogen enters the melt pool to form the TiN dendrites, and penetration cracks can be easily produced in the joint because TiN is a brittle ceramic phase. The use of nitrogen gas as the shielding gas changes the traditional solidification path of the molten pool liquid metal. During the welding process, nitrogen enters the molten pool and the TiN dendrites are generated. In the subsequent cooling process, the β→α transformation is centered on the TiN dendrites, thus breaking the traditional Burgers orientation relationship and forming the orientation relationship of {111}TiN//{0001}α2. At the same time, the generation of TiN has a grain refining effect on the weld organization. The complete solidification path of liquid metal cooling during the welding process is L→TiN+ L→TiN+ β→TiN+ α+ γ→TiN+ α₂+ γ.