Cold metal transfer process is applied to join titanium and Q235 steel with copper filler metal. Scanning electron microscope (SEM), energy dispersive spectrometer (EDS) analysis, micro-hardness tests, and tensile strength test were performed to investigate the joining mechanism and strength of joints. The results show that the stacking order of two base metals affected the joining modes and strength. For top Q235 steel to bottom Ti-TA2 lapped joint, there was no distinct interface reaction layer between the steel base metal and the weld metal; dispersed TiFe 2 intermetalics (IMCs) IMCs between the steel base metal and the Ti base metal greatly improved the strength of joint; the tensile force of the joint could reach up to 93% that of steel-steel joint using the same welding parameters. Additionally, the joints were fractured in dimple mode at the steel base metal. For top Ti-TA2 to bottom Q235 steel lapped joint, the increasing volume fraction of Ti-Cu IMCs at the Ti-Cu weld metal interface contributed to the strength of joint degradation. The joints under tensile loading are initiated at the Ti-Cu weld metal interface between the weld metal and Ti base metal, then propagated to weld metal, finally fractured with brittle mode.
Titanium and titanium alloy, with their excellent corrosion resistance and high specific strength compared to steel, are widely applied in chemical and aerospace industries [1,2,3,4]. Zn coated steel is used due to their good corrosion resistance. The joints formed between titanium and steel have been widely used in chemical and nuclear industries, and both titanium and steel are greatly taken in manufacturing [5,6,7,8]. Therefore, joining titanium and steel is of great concern. However, it is difficult to successfully join titanium and steel directly based on the following two aspects: On the one hand, brittle intermetalics (IMCs) (TiFe and TiFe2) are formed due to low solubility of Fe in Ti (0.1 at. %, at room temperature) [1]. On the other hand, a significant mismatch of thermal expansion coefficients between these two materials results in large residual stress especially in the case of arc welding process with large heat input [9].
To obtain the Ti-steel joints, solid state joining methods such as diffusion bonding and friction welding were introduced. Friction welding process was used to inhibit the IMCs formed at the interface [5,10,11]. Diffusion bonding process was applied for the decrease in heterogeneities of chemical composition [12,13,14]. Although these solid-state joining methods were used to join titanium and steel, brittle Ti-Fe IMCs formed at interface are still unavoidable, which is compromised for the strength of Ti-steel joints. To further improve the mechanical properties of Ti-steel joints, various interlayers (Niobium, Tantalum, Silver, Nickel, and Vanadium) were applied for filler metal to reduce or even to prevent the formation of hard and brittle Ti-Fe IMCs [15,16,17,18,19,20]. Cherepanov et al. [15] investigated the strength of titanium-stainless steel joints formed by explosive welding with Nb and Ta foils as transition layers. It is shown that the highest ultimate tensile strength of 476 MPa and yield strength of 302 MPa were obtained with niobium foils. Lee [16] obtained the titanium-stainless steel joint with Ag-Cu alloy filler metal and an Ag interlayer. The results indicated that Ti base metal/TiAg/Ag-rich solid solution/steel base metal layered structure was formed in the welding joint. The joint was fractured along the layer with the ductile mode, and the strength was up to 440 MPa. Wang et al. [17] compared the microstructure and strength of electron beam welded titanium-steel joints with different filler metals (vanadium, nickel, copper, and silver), the results indicated that no Ti-Fe IMCs were formed at interface in the joints with nickel, silver, and copper filler metals. Additionally, the highest tensile strength of 310 MPa was obtained in the joint with silver filler metal. Reichardt et al. [18] used the laser deposition additive manufacturing method to fabricate Ti-6Al-4V-stainless steel gradient components with a vanadium interlayer.
Recently, ‘cold metal transfer’ (CMT) joining technique has been successfully applied to join dissimilar metals [21,22,23,24,25]. The key feature of the process is that the wire motion has been integrated into the joining process and into the overall control of the process. As a result, the lower heat input can be controlled, and thus the IMC formation and its thickness can be reduced as well, thereby enabling optimization of the joint strength. In the present study, due to low heat input of the key feature of CMT, CMT process was used to join pure titanium TA2 and hot dipped galvanized mild Q235 steel with ERCuNiAl copper-based wire as a filler metal to decrease the brittle Fe-Ti IMCs. Then the effects of the lapped sequence on joining mechanism and mechanical properties were investigated. To compare the weld appearance and mechanical properties, steel-steel lapped jointand Ti-Ti lapped joint with ERCuNiAl copper-based wire were also performed at the same welding parameters. In addition, reaction compounds, micro-hardness distribution, and fracture morphology of joints were investigated as well.