Mechanics of Materials
Lap Joining of Titanium to Galvanized Steel by Cold Metal Transfer Technology
2020 Volume 61 Issue 12 Pages 2312-2319
Details
2020 Volume 61 Issue 12 Pages 2312-2319
In this study, a cold metal transfer method with low heat input and no welding splash is used to weld the pure titanium TA2 and hot dipped galvanized mild Q235 steel with H08Mn2SiA wire. The effect of zinc coating on microstructure and properties of titanium/steel joints was investigated. The results show that an interface reaction layer with Ti–Fe intermetallics was produced between the titanium base metal and weld metal, even when a 2 mm offset distance of the welding torch towards the steel base metal was set. Zinc coating cannot improve mechanical properties of Ti/galvanized steel joint. It is attributed to the vaporization of the zinc coating, Zn element doesn’t react with Fe and Ti elements in the weld metal. The joints were fractured at the Ti–Fe interface reaction layer. It is not feasible to weld titanium and galvanized steel using the H08Mn2SiA wire, even though the cold metal transfer method and the galvanized steel with pure zinc coating were used.
Fig. 7 The formation process of Joint II.
In order to combine the performance of high corrosion resistance, high strength and light weight, titanium/steel welding parts are applied in the chemical, oil and power plant boilers industry.1–3) The weldability of dissimilar materials are depended on the physical and metallurgical properties of the base metals. There are major difference in the coefficient of linear expansion and thermal conductivity between titanium and steel, as shown in Table 1,4) which led to the formation of cracks eventually. Moreover, there is extremely low solubility between Fe and Ti, as shown in Fig. 1,5) and brittle Ti–Fe intermetallics (IMCs) cannot be avoided during joining titanium and steel directly.6,7) M. Ghosh et al. reported solid-state diffusion bonding of Ti–6Al–4V and type 304 Stainless Steel.6) The results found that Fe–Ti IMCs and Fe2Ti4O phases were identified in the reaction layers. C. Velmurugan et al. investigated the direct diffusion bonding of Ti–6Al–4V alloy and duplex stainless steel. In their study, the Fe–Ti interface reaction layers were formed in low temperature range.7) Additionally, the literatures also reported that brittle IMCs severely affected the mechanical properties of the welding joints.8,9) Therefore, the most important issue during titanium/steel welding processes is how to control the formation of brittle Ti–Fe intermetallic in the joint. Until now, most of the studies about joining titanium and steel have been concentrated on the decrease of brittle Ti–Fe IMCs. Two different routes were used to decrease the formation of Ti–Fe IMCs, which were full of much interest in many literatures.
Phase diagram of Ti–Fe.
One is the various welding method with the low heat input used in the welding process. When the heat input and interaction time are decreased, amounts of IMCs can be decreased in the welding process. Several studies use the relatively low heat input processes to join titanium and steel, such as diffusion bonding and friction welding. S. Kundu et al. investigated solid state diffusion joining between stainless steel and titanium and obtained sound joints when maximum heating temperature of 1000°C and holding time of 30 min were used.10) D. Podda joined commercially pure titanium and precipitation hardening stainless steel by diffusion joining method. The results found that the tensile strength and shear strength of the titanium/steel joints can achieve 108% and 87.6% that of pure titanium, respectively.11) P. Li et al. investigated the effect of friction time on mechanical and metallurgical properties of continuous drive friction welded Ti6Al4V/SUS321 joints. The results showed that the tensile strength increased with the increase of the friction time, and the maximum average strength of 560 MPa reached 90.3% that of the SUS321 base metal.12) X. Li et al. investigated the microstructure and mechanical properties of TC4/SUS321 joints welded by the rotary friction welding method. The results found that the morphology of IMCs transformed from dispersive pattern to laminar pattern with the increase the rotation speed from 400 to 1800 rpm. The reliable joints were obtained in the low heat input process.13)
The other is the filler metals which are used to decrease the formation of hardness brittle Ti–Fe IMCs. The sound titanium/steel joints were obtained by using the Ag, Ni, Cu, and V as the filler metal to join titanium and steel.2,14,15) J.G. Lee et al. bonded titanium and stainless steel by the Ag filler metal. The results found that the diffusion between Ti and Fe elements is completely prevented by the Ag interlayer, which inhibited the formation of any brittle Ti-based IMCs.2) Ting Wang et al. joined titanium and steel by the electron beam welding method with Ni, V, Cu and Ag filler metals. The results found that Ti–Fe IMCs are inhibited by Cu and Ag filler metals, the maximum tensile strength of the joint reached 310 MPa with Ag filler metals.14) X.H. Hao et al. investigated the feasibility of arc welding between titanium alloy and stainless steel with a copper-based filler metal. The results found that amounts of Ti–Cu IMCs can be formed, and the maximum shear strength of joint reached 107 MPa.15) In addition, R. Cao et al. investigated the feasibility of CMT welding Mg AZ31-galvanized mild steel. The results found that it is possible to join Mg to steel by CMT welding method. Because the galvanized mild steel was used, the zinc coating has a lower melting temperature than the steel and interacts with the Mg alloy molten to form a braze joint.16) A. Abdollahzadeh et al. investigated the microstructure and property of magnesium–aluminum alloy joint by adding zinc intermediate layer.17) A. Kar et al. performed the joining between aluminum and titanium by zinc intermediate layer.18) R. Cao et al. investigated the effects of Zn coating on welding-brazing process of Al-steel and Mg-steel dissimilar joints. The results showed that for the Al-steel joint, zinc coating has not participated the interface reaction between aluminum and steel, and for the Mg-steel joint, zinc coating reacted with Mg element to form the interface layer between magnesium and steel.19) Based on the above research achievements, zinc coating had significant influence on the interfacial microstructure and mechanical properties of dissimilar joints. Therefore, it is necessary to study the effect of zinc coating on microstructure and mechanical properties of titanium and steel joints.
The key feature of Cold metal transfer (CMT) technology is a wire motion which is integrated into the joining process and into the overall control of the process. The wire retraction motion assists the droplet detachment during the short circuit, thus the metal can transfer into the welding pool without the electromagnetic force. The heat input can be controlled, as a consequence, the IMCs formation and its thickness will be decreased, and the joint strength can be optimized as well.20) Gonçalo Pardal et al. joined stainless steel and titanium by CMT process using copper filler metal. The results found that the low heat input and the addition of copper improved the strength of the joints, which reached 200 MPa.21) Gang Mou et al. welded titanium alloy to stainless steel by CMT technology. The results found that the strength of the joints were determined by the Fe–Si–Ti intermetallics in the weld, which reached 294 MPa.22) So, it is feasible to join titanium and steel by CMT technology.
In this study, CMT welding of pure titanium TA2 and hot dipped galvanized mild Q235 steel was performed by using H08Mn2SiA wire, and the effect of zinc element on microstructure and properties of Ti/steel joints is discussed in detail.
Pure titanium TA2 sheet with dimensions of 125(L) × 50(W) × 1(T) mm3 and hot dipped galvanized mild Q235 steel sheet with the same dimensions were used in this study. The chemical composition of the base metals and the H08Mn2SiA welding wire with a diameter of 1.2 mm used in this study were shown in Table 2. The thickness of the galvanized layer is 10 µm.
The schematic welding equipment was shown in Fig. 2, titanium sheet and galvanized steel sheet were lapped with an overlap distance of 10 mm. The welding torch had a vertical distance of 10 mm from the base metal, and the angle between the welding torch and the lap seam was 45°. The offset distance of the welding torch towards the steel base metal is 0 and 2 mm. The welding direction was parallel to the length of the parent metal. High purity argon shielding gas with a flow rate of 15 L/min was used during the whole welding process.23,24)
Schematic diagram of welding equipment of CMT welding TA2/Q235 steel: (a) welding torch and work piece layout and (b) work piece configuration (dimensions in mm).
After the welding experiments, the cross section samples were cut from the lapped joint, and were prepared by conventional metallographic techniques. The specimens were ground using different types of abrasive paper, and polished with diamond suspension. Scanning electron microscopy (SEM, JSM6700F) equipped with an energy dispersive spectrometer (EDS) system were performed to analyze the microstructure of the weld joints.
The compositions of the chemical elements in the cross section samples were determined by EDS. The microhardness test and tensile shear test were carried out to evaluate the mechanical properties of joints. The microhardness was measured by HVT-1000A micro-hardness testing machine with the parameters (HV 0.1/10).25) The tensile properties were evaluated by a testing machine with a speed of 0.5 mm/min at room temperature. The sample positions of tensile specimens are shown in Fig. 3(a) and the dimensions and configuration of the tensile specimens are shown in Fig. 3(b). Three specimens were prepared for the testing experiment, the fracture morphology and the fracture side were observed by SEM.
Schematic and dimensions (in mm) for the tensile specimens: (a) sample position of the tensile specimens, and (b) dimensions and configuration of the tensile specimens.
To investigate the effect of zinc coating on the appearance and the microstructure of Ti/galvanized steel joints, extensive tests have been conducted. Process parameters of CMT welding Ti/galvanized steel were presented in the Table 3. For the purpose of comparison, sample #1 (Joint I) and sample #2 (Joint II) with different offset distance of the welding torch towards the steel base metal were used in this paper, and sample #1 (Joint I), sample #2 (Joint II), sample #3 (Joint III), sample #4 (Joint IV) and sample #5 (Joint V) with different heat input were used in this paper.
The weld appearance for Joint I, Joint II, Joint III, Joint IV and Joint V were shown in Fig. 4. Under no offset distance of the welding torch, penetration and a great number of cracks (yellow dotted lines in Fig. 4(a)) are existed on the weld appearance (Joint I), as shown in Fig. 4(a). The cracks are distributed in the whole welding seam, the brittle cracking sound can be heard after welding, and then the joints fracture from the root of welding seam near the titanium base metal. Thus, the effective connection of Ti/galvanized steel cannot be realized. Under the same heat input, a 2 mm offset distance of the welding torch toward the galvanized steel base metal was adopted to obtain the sound Ti/galvanized steel joint. The weld appearance for Joint II were shown in Fig. 4(b) and (c). There were little cracks (yellow dotted lines in Fig. 4(b)) in the front surface of the weld, as shown in Fig. 4(b). Figure 4(c) shows the back surface of the weld. A slight burning was occurred on the back appearance of the weld. Then the welding parameter are adjusted, Figs. 4(d)–(f) show the weld appearances of Joint III, Joint IV and Joint V with different heat input. There are a great number of cracks (yellow dotted lines in Figs. 4(d)–(f)) on the weld appearance. In other literature, the authors found that the tensile strength of Ti/stainless steel joints were decreased under the offset distance of the welding torch toward the TC4 side.22)
Weld appearances: (a) front surface of sample #1 (Joint I), (b) front surface of sample #2 (Joint II), (c) back surface of sample #2 (Joint II), (d) front surface of sample #3 (Joint III), (e) front surface of sample #4 (Joint IV) and (f) front surface of sample #5 (Joint V).
There are a great number of cracks on the weld appearance of Joint I, Joint III, Joint IV and Joint V, so this paper is focused on the microstructure, microhardness distribution and strength of Joint II. Figure 5 shows the microstructure of Joint II. The dotted blue box in Fig. 4(b) is presented the location of cross section (Joint II) in Fig. 5(a). Compared the melting point of TA2 and low carbon steel,4) under the action of arc, TA2 and low carbon steel base metals were molten, so interface I between Ti base metal and weld metal and interface II between weld metal and steel base metal were formed in the welding joint, as shown in Fig. 5(a). Although an offset distance of the welding torch toward the steel was used to obtain the sound lapped joint, there were bits of cracks in the joint, as shown in Fig. 5(b). The microstructure of three zones (interface I (zone A in Fig. 5(a))-the interface of Ti base metal to weld metal, interface II (zone B in Fig. 5(a))-the interface of weld metal to the steel, and the weld metal (zone C in Fig. 5(a))) were selected and analyzed by energy dispersive spectrometer (EDS). The microstructures of these zones were shown in Fig. 5(b), (c) and (d), respectively. The details of these zones were described in the followed paragraph.
The microstructure of Joint II: (a) the cross section (dotted blue box in Fig. 4(b)), (b) interface I (zone A in Fig. 5(a)), (c) interface II (zone B in Fig. 5(a)), (d) the weld metal (zone C in Fig. 5(a)), (e) the phase distribution map of Fig. 5(b), and (f) line analysis result along line II in Fig. 5(b).
The detailed view of the interface between Ti base metal and weld metal (interface I) was shown in Fig. 5(b). EDS was applied to analyze the composition and possible phases in various zones and the results are presented in Table 4. On the basis of the Ti–Fe binary phase diagram, along the transition zones (1→2→3→4→5→6), various phases can be formed from α-Ti solid solution and small TiFe IMCs (marked by point 1)→to TiFe IMCs and α-Ti solid solution (marked by point 2)→to TiFe IMCs and small α-Ti solid solution (marked by point 3)→to TiFe IMCs and TiFe2 IMCs (marked by point 4)→to TiFe2 IMCs and small α-Fe solid solution (marked by point 5)→to TiFe2 IMCs and α-Fe solid solution (marked by point 6), the phase distribution of interface I was schematized in Fig. 5(e). The EDS line analysis along line II in Fig. 5(b) was shown in Fig. 5(f). The results show that Ti and Fe elements were unevenly distributed at the interface between Ti base metal and weld metal (interface I), and the distinct Ti–Fe reaction layer was formed at the interface I. Enlarged feature of the interface II between weld metal and the steel base metal was shown in Fig. 5(c). There was no obvious reaction layer at interface II. The needle-shaped α-Fe (marked by point 7) were presented in the weld metal, as shown in Fig. 5(d).26)
To further reveal the microstructure of zone D in Fig. 5(a) and zones E, F, G, H in Fig. 5(b), the corresponded magnified figures were shown in Fig. 6. Figure 6(a) shows the interface between the Ti base metal and the steel base metal (zone D in Fig. 5(a)). The chemical composition of point 8 is 25.37 at% Fe, 42.91 at% Ti and 31.72 at% Zn. It is inferred that the Ti–Zn–Fe IMCs were formed at the interface between Ti base metal and the steel base metal. The detailed views of zone E, zone F, zone G and zone H (in Fig. 5(b)) were shown in Figs. 6(b), (c), (d) and (e), respectively. The blue zone I in Fig. 6(b) was the transition layer from the Ti–Fe interface reaction layer to the weld metal, which was composed of the α-Fe solid solution (marked by point 9). The TiFe2 and small α-Fe solid solution (marked by point 10) were formed in blue zone II in Fig. 6(b). The cystiform TiFe2 matrix (marked by point 11) were surrounded by the white TiFe2 and α-Fe solid solution (marked by point 12), as shown in Fig. 6(c). There were dual-phased IMCs of TiFe and TiFe2 (marked by point 13) in Fig. 6(d). The dendritic TiFe and α-Ti solid solution (marked by point 15) attached to the TiFe and small α-Ti solid solution matrix (marked by point 14), as shown in Fig. 6(e).
Based on the above analysis, the Ti/galvanized steel joint is composed of the interface between Ti base metal and weld metal (interface I), the weld metal and the interface between weld metal and the steel base metal (interface II). Figure 7 presents the formation process of Ti/galvanized steel joint (Joint II). Under the action of CMT arc, the liquid molten pool can be formed because of the melting of the wire and the part titanium base metal, and the evaporation of zinc coating is produced, as shown in Fig. 7(a). Due to the large temperature gradient at the interface between solid phase (titanium base metal) and liquid phase (weld metal) in the molten pool, interdiffusion between Ti and Fe atoms is taken place at the interface between Ti base metal and weld metal, as shown in Fig. 7(b). As the temperature of molten pool decreases, TiFe2 phase (L→TiFe2) is firstly precipitated from the liquid (weld metal) near the Ti base metal. Subsequently, TiFe phase is formed at 1317°C, at which the peritectic reaction is occurred (L+TiFe2→TiFe). At 1289°C, the eutectic reaction is occurred to form TiFe2 phase (L→TiFe2+α-Fe). Thus, Ti–Fe interface reaction layer and α-Fe solid solution are formed in the joint, as shown in Fig. 7(c). As can be seen from the phase distribution diagram (Fig. 5(e)) of the interface reaction layer, TiFe2 phase is mainly formed near the weld metal and TiFe phase is formed far from the weld metal. Because the growth rate of IMCs in the molten pool is determined by the maximum temperature and cooling rate of the molten pool during the welding process.27) Because the TiFe2 phase near to the weld metal is closer to the center of the molten pool, thus, its arc energy is higher during the welding process, which is beneficial to the growth of TiFe2 phase. In addition, the Gibbs free energy of TiFe2 phase is also higher than that of TiFe phase. Therefore, TiFe2 phase is priorly formed during the cooling process, and the growth rate of TiFe2 phase is also higher than that of TiFe phase. Figure 7(d) presents the SEM image of Ti/galvanized steel joint (Joint II).
The formation process of Joint II.
Figure 8 presents the microhardness variation of Joint II from Ti base metal, interface reaction layer to weld metal (along the line I in Fig. 5(a)). As shown in Fig. 8, the microhardness of Ti/galvanized steel joint (Joint II) was unevenly distributed. The average microhardness of the interface reaction layer is up to 800 HV, which reaches 381% that of Ti base metal (210 HV). It is mainly attributed to the formation of many hard and brittle Ti–Fe IMCs (TiFe and TiFe2) at the interface reaction layer. Moreover, the maximum microhardness is near to 1175 HV at the interface reaction layer. Compared with the SEM image and the phase distribution image (Figs. 5(b) and (e)), this microhardness value (1175 HV) should be the microhardness of TiFe2 phase. It was seen that the microhardness of TiFe2 phase (1175 HV) was higher than that of TiFe phase (546 HV). The experimental results were consistent with the reported literature.28) By contrast, the microhardness of the weld metal is more evenly distributed. The average microhardness of weld metal is 268 HV, which is higher than the average microhardness of Ti base metal (163 HV). It is attributed to the formation of α-Fe solid solution in the weld metal. According to the microhardness distribution characteristics (Fig. 8) of Ti/galvanized steel joint (Joint II), under the action of welding stress, stress concentration was produced at the interface between Ti base metal and weld metal (interface I), and the hard and brittle Ti–Fe IMCs (TiFe and TiFe2) made the interface layer produced cracks. Eventually, the interface between Ti base metal and weld metal (interface I) is becoming the weakest zone in the joint.
The micro-hardness along the line I in Fig. 5(a).
The above study revealed that cracks occurred in the weld metal. It was attributed to amounts of hard brittle IMCs formed between Ti base metal and the weld metal. Therefore, the strength of Ti/galvanized steel joint (Joint II) with H08Mn2SiA wire were especially low. The tensile load of Ti/galvanized steel joint could reach 1.44 KN, which was 51.6% of the tensile load of steel base metal under the same thickness of samples.29,30)
In order to further analyze the fracture reason, the fracture surfaces were observed by SEM. Figure 9(a) shows the fracture surface of Ti/galvanized steel joints, which was primarily dominated by the cleavage fracture. The EDS analysis results of phases were presented in Figs. 9(b) and (c), the fracture surface was composed of TiFe and TiFe2. It will be further identified from fracture side in Fig. 10. Figure 10(a) shows the fracture side of Ti/galvanized steel joint. The detailed view of the fracture side (zone J in Fig. 10(a)) was shown in Fig. 10(b). Combine the EDS analysis results of points 16 and 17 (Figs. 10(c) and (d)), the joint was fractured at the reaction layer with amounts of TiFe and TiFe2 IMCs, as shown in the red zone of Fig. 5(e). The TiFe and TiFe2 phase of reaction layer between the weld metal and Ti base metal is the weakest zone of the joint. The schematic of fracture mode for the Ti-steel joints was shown in Fig. 10(e). From the Fig. 10(e), the Ti/galvanized steel joint (Joint II) fractured along the direction of the red arrow.
The fracture analysis results of Joint II (a) SEM image of fracture; (b) EDS result of TiFe phase; (c) EDS result of TiFe2 phase.
The fracture analysis result of Joint II (a) SEM image of fracture side; (b) SEM image of zone J in Fig. (a); (c) EDS result of TiFe and TiFe2 phase (point 16); (d) EDS result of TiFe2 phase (point 17); (e) the fracture diagram of the Ti/galvanized steel joint.
TiFe and TiFe2 are regarded as brittle IMCs,31) therefore, stress concentration was likely resulted from the TiFe and TiFe2 phase at the interface between Ti base metal and weld metal (interface I), which lead to joint fractured in this zone. The formation of Ti–Fe IMCs at the interface between Ti base metal and weld metal (interface I) make the mechanical properties of the joint difficult to be controlled. Therefore, phase composition of the joint may be an important factor leading to the failure and fracture of Ti/galvanized steel joint (Joint II). On the other hand, due to the significant difference between the titanium and steel on the linear expansion coefficient and thermal conductivity, under the action of CMT arc, uneven heating of titanium and steel leads to different deformation of the two metals. The uniform contraction is occurred during the cooling process after welding, larger welding stress is produced at the joint, which leads to the joint fractured at a very lower applied load. Therefore, welding stress may be another important factor leading to the fracture of Ti/galvanized steel joint (Joint II).
In other references,19) the authors have found that the zinc coating can decrease the arc temperature, can decrease the number of Al–Fe IMCs during welding of Al and galvanized steel. But for Ti-steels, although CMT technology with lower heat input and the galvanized steel were used, the welding parameters are optimized, eventually Ti–Fe interface reaction layer and a few cracks are formed in the Ti/galvanized steel joint, which seriously deteriorates the mechanical properties of joint. It is attributed to the vaporization of the zinc coating, Zn element doesn’t react with Fe and Ti elements in the weld metal, zinc coating on the galvanized steel cannot prevent the formation of Ti–Fe IMCs. This result leads to Ti/galvanized steel joint failed and fractured at the interface reaction layer. Thus, the tensile properties cannot be improved even though the galvanized steel with pure zinc coating was used.
CMT welding of 1 mm thick TA2 and 1 mm thick hot dipped galvanized mild Q235 steel with a diameter of 1.2 mm H08Mn2SiA wire was carried out. Due to the formation of a large amount of hard brittle TiFe and TiFe2 IMCs and larger welding stress, cracks were formed in the Ti/galvanized steel joint.
This work was financially supported by National Nature Science Foundation of China (No. 51761027).
