Effects of Interlayer on the Microstructure and Mechanical Properties of Resistance Spot Welded Titanium/Steel Joints: A Short Review

: In this paper, the influence of interlayer on titanium/steel dissimilar metal resistance spot welding is reviewed from the aspects of macroscopic characteristics, microstructure and interface bonding properties of the joint. Previous studies have demonstrated that TiC, FeTi and Fe 2 Ti intermetallic compounds with high brittleness are formed in the joint during titanium/steel welding, which reduces the strength of the welded joint. Researchers proposed different interlayer materials, including Cu, Ni, Nb, Ta, 60%Ni-Cu alloy and BAg45CuZn. Firstly, adding an interlayer can weaken the diffusion of Fe and Ti. Secondly, the interlayer elements can combine with Fe or Ti to form solid solutions or intermetallic compounds with lower brittleness than Fe–Ti compounds. Finally, Cu, Ni, Ag, etc. with excellent ductility can effectively decrease the generation of internal stress, which reduces the formation of defects to improve the strength of the joint.


Introduction
Titanium is an industrialized metal with the advantages of high specific strength and strong corrosion resistance, but the high cost limits its application in industry to some extent.Connecting Ti and steel together to form a composite metallic structure can effectively solve this problem.The application of titanium/steel welded structures in metal components combine the advantages of strong corrosion resistance of titanium and low cost and high strength of steel, so titanium/steel welded structures are widely used in petrochemical, marine and transportation fields [1,2].Welding is an important process for manufacturing titanium/steel bi-metal structures [3].
Resistance spot welding (RSW) technology is a method that uses the resistance heat generated by the current through the base material to heat the base material until the base material melts, forming solder joints after solidification to achieve metallurgical bonding [4][5][6].A schematic diagram of the device is shown in Figure 1, and the main process parameters are current, energizing time and electrode pressure [7][8][9].The welding process is divided into three successive stages.First, the electrode applies pressure to the workpiece surface to ensure that the plates are in close contact with each other.Second, the current passes through the workpiece at the power-on stage, and the generated heat makes the workpiece locally melt to form a melt pool.Finally, the molten pool solidifies to form a welded joint in the cooling stage.The melted metal begins to shrink considerably at this stage, so it is necessary to continue applying pressure by the electrode to form a denser welded joint.The method has the advantages of short heating time, small material deformation, low production cost and easy automation, which does not require other fill materials to form joints [10][11][12][13].At the same time, RSW can not only weld steel materials but can also weld aluminum, copper and other non-ferrous metals and alloys, which is the most common Metals 2024, 14, 429 2 of 14 pressure welding method with a wide range of applications [14][15][16][17].The disadvantage is that it is currently limited to welding of thin plates [18,19].
During titanium/steel welding, TiC, FeTi and Fe 2 Ti intermetallic compounds with high brittleness and hardness are formed at the welded joint, reducing the strength of the welded joint and making it prone to fracture [20][21][22][23].The addition of an interlayer can effectively solve this problem.Firstly, the interlayer can effectively block the diffusion of Ti, Fe and C [24,25].Secondly, the interlayer metal does not combine with titanium or steel to form brittle intermetallic compounds, or the resulting compounds have a brittleness better than Fe-Ti intermetallic compounds [26,27].At present, the interlayer mainly used by researchers are Cu [28][29][30], Ni [31,32], Ta [33], V [34], Nb [35,36], Ag [25] and Al [37][38][39][40].For example, Ni and Fe do not form intermetallic compounds.Although Ni and Ti form intermetallic compounds, their brittleness is lower than that of Fe-Ti intermetallic compounds, so the addition of Ni as an interlayer can improve the joint performance [41].
Metals 2024, 14, 429 2 of 14 but can also weld aluminum, copper and other non-ferrous metals and alloys, which is the most common pressure welding method with a wide range of applications [14][15][16][17].
The disadvantage is that it is currently limited to welding of thin plates [18,19].
During titanium/steel welding, TiC, FeTi and Fe2Ti intermetallic compounds with high brittleness and hardness are formed at the welded joint, reducing the strength of the welded joint and making it prone to fracture [20][21][22][23].The addition of an interlayer can effectively solve this problem.Firstly, the interlayer can effectively block the diffusion of Ti, Fe and C [24,25].Secondly, the interlayer metal does not combine with titanium or steel to form brittle intermetallic compounds, or the resulting compounds have a brittleness better than Fe-Ti intermetallic compounds [26,27].At present, the interlayer mainly used by researchers are Cu [28][29][30], Ni [31,32], Ta [33], V [34], Nb [35,36], Ag [25] and Al [37][38][39][40].For example, Ni and Fe do not form intermetallic compounds.Although Ni and Ti form intermetallic compounds, their brittleness is lower than that of Fe-Ti intermetallic compounds, so the addition of Ni as an interlayer can improve the joint performance [41].The research progress on the use of interlayers in titanium/steel resistance welding is reviewed in this paper.Firstly, the appearance characteristics of the titanium/steel joints obtained by RSW are briefly introduced.Secondly, the effect of the addition of an interlayer on the microstructure of the welded joint is discussed, including the type of compound and the thickness of the compound layer.Regarding defects in the welded joint, the influences of adding an intermediate layer on the defect characteristics at welded joints are summarized.Regarding the mechanical properties of welded joints, the effects of the type and thickness of the interlayer on the shear strength and hardness of the welded joint are discussed.Finally, problems in the research of titanium/steel RSW are presented and future research directions are proposed.

Macroscopic Characteristics of Welded Joints
The titanium/steel weld sample obtained by RSW has certain indentations on both sides of the base material, and the shape of the welded joint transverse truncation surface is approximately elliptical [43,44].The cross-section morphology of the joint has the following characteristics: Firstly, the base material interface inside the molten core has disappeared, and the molten core that is formed is macroscopically homogeneous.The flowing molten metal is mixed under the action of electromagnetic force and then solidifies to form an approximately homogeneous welded joint [45,46].Secondly, the welded joint shifts significantly to the titanium side.The resistivity of pure titanium is 0.42 µΩ•m, which is much higher than that of low carbon steel (0.13 µΩ•m).The melting point of titanium is similar to The research progress on the use of interlayers in titanium/steel resistance welding is reviewed in this paper.Firstly, the appearance characteristics of the titanium/steel joints obtained by RSW are briefly introduced.Secondly, the effect of the addition of an interlayer on the microstructure of the welded joint is discussed, including the type of compound and the thickness of the compound layer.Regarding defects in the welded joint, the influences of adding an intermediate layer on the defect characteristics at welded joints are summarized.Regarding the mechanical properties of welded joints, the effects of the type and thickness of the interlayer on the shear strength and hardness of the welded joint are discussed.Finally, problems in the research of titanium/steel RSW are presented and future research directions are proposed.

Macroscopic Characteristics of Welded Joints
The titanium/steel weld sample obtained by RSW has certain indentations on both sides of the base material, and the shape of the welded joint transverse truncation surface is approximately elliptical [43,44].The cross-section morphology of the joint has the following characteristics: Firstly, the base material interface inside the molten core has disappeared, and the molten core that is formed is macroscopically homogeneous.The flowing molten metal is mixed under the action of electromagnetic force and then solidifies to form an approximately homogeneous welded joint [45,46].Secondly, the welded joint shifts significantly to the titanium side.The resistivity of pure titanium is 0.42 µΩ•m, which is much higher than that of low carbon steel (0.13 µΩ•m).The melting point of titanium is similar to that of steel, so more resistance heat is generated on the titanium side during welding, making the melting amount of titanium during welding greater than that of steel [47,48].Thirdly, a large number of holes and cracks are formed inside the welded joint.During the solidification process, the metal in the molten pool is subjected to local stress and strain due to volume shrinkage, resulting in the metal in the molten pool being unable to fill the entire molten core after it solidifies; consequently, holes are formed in the welded joint [49,50].Because the linear expansion coefficient of titanium is different from that of steel, the contraction rate of the two is different when the temperature is reduced.As a result, a large amount of internal stress is generated in the welded joint, which aggravates the expansion of cracks and holes.[51].Finally, the holes and cracks are significantly reduced after adding a suitable interlayer, which significantly improves the performance of the welded joint [52].

Interfacial Microstructure without Interlayers
During spot welding of titanium/steel, Fe-Ti intermetallic compounds with high brittleness and hardness are formed in welded joints [53].The welded joint mainly consists of TiFe 2 +α-Fe eutectic structure near the steel side, TiFe columnar crystal and TiFe+α-Ti eutectic structure near the titanium side.As is shown in Figure 2a, the band TiFe+α-Ti with thickness of 12 µm and the columnar TiFe with thickness of 50 µm are observed near the titanium side.The microstructure near the steel side is shown in Figure 2b; the M layer near the Q235 steel is the supersaturated solid solution product of Ti element in Fe element with a thickness of about 6 µm, and the N layer is TiFe 2 +α-Fe with a thickness of 30-50 µm.The composition of the columnar crystal in the fusion zone is TiFe.
Metals 2024, 14, 429 3 of 14 that of steel, so more resistance heat is generated on the titanium side during welding, making the melting amount of titanium during welding greater than that of steel [47,48].Thirdly, a large number of holes and cracks are formed inside the welded joint.During the solidification process, the metal in the molten pool is subjected to local stress and strain due to volume shrinkage, resulting in the metal in the molten pool being unable to fill the entire molten core after it solidifies; consequently, holes are formed in the welded joint [49,50].
Because the linear expansion coefficient of titanium is different from that of steel, the contraction rate of the two is different when the temperature is reduced.As a result, a large amount of internal stress is generated in the welded joint, which aggravates the expansion of cracks and holes.[51].Finally, the holes and cracks are significantly reduced after adding a suitable interlayer, which significantly improves the performance of the welded joint [52].

Interfacial Microstructure without Interlayers
During spot welding of titanium/steel, Fe-Ti intermetallic compounds with high brittleness and hardness are formed in welded joints [53].The welded joint mainly consists of TiFe2+α-Fe eutectic structure near the steel side, TiFe columnar crystal and TiFe+α-Ti eutectic structure near the titanium side.As is shown in Figure 2a, the band TiFe+α-Ti with thickness of 12 µm and the columnar TiFe with thickness of 50 µm are observed near the titanium side.The microstructure near the steel side is shown in Figure 2b; the M layer near the Q235 steel is the supersaturated solid solution product of Ti element in Fe element with a thickness of about 6 µm, and the N layer is TiFe2+α-Fe with a thickness of 30-50 µm.The composition of the columnar crystal in the fusion zone is TiFe.Liu et al. [54] observed an M layer with a thickness of 30-50 µm near the steel side, which is composed of TiFe2+α-Fe, and the columnar crystal structure of TiFe was formed near the core of the M layer.A layer of TiFe+α-Ti with a thickness of about 12 µm was observed near the titanium side, denoted as the N layer.The formation of a columnar crystal structure with a thickness of about 50 µm is found in the molten core near the N layer, and the columnar growth direction changes to the heat dissipation direction as shown in Figure 3. Liu et al. [54] observed an M layer with a thickness of 30-50 µm near the steel side, which is composed of TiFe 2 +α-Fe, and the columnar crystal structure of TiFe was formed near the core of the M layer.A layer of TiFe+α-Ti with a thickness of about 12 µm was observed near the titanium side, denoted as the N layer.The formation of a columnar crystal structure with a thickness of about 50 µm is found in the molten core near the N layer, and the columnar growth direction changes to the heat dissipation direction as shown in Figure 3.

Ni Interlayer
When a Ni interlayer is added between 304 stainless steel and TC4 for spot welding, the Fe-Ti compound in the joint is replaced by a Ti-Ni compound with lower brittleness, which improves the performance of the joint [55].The melted Ni is squeezed to the edge of the molten core, forming a thinner layer of α-Ti+Ti2Ni and a thicker layer of TiNi compounds [56].In the welded joint close to the Ti plate, a layered structure of α-Ti+TiFe with a width of about 20 µm is formed, and the TiFe2+α-Fe layer is observed near SUS304 whose width is about 3 µm.The center of the welded joint is mainly composed of columnar TiFe and TiFe2 as shown in Figure 4.

Cu Interlayer
Fe-Ti intermetallic compounds are the main structures formed in the welded joint near both sides of the base material after adding a Cu interlayer [57].The side near the titanium is composed of 15 µm thick TiFe and α-Ti, the composition near the steel side is TiFe2+α-Fe, with a thickness is 10 µm, and the center of the welded joint is mainly TiFe and TiFe2.The melted Cu foil is extruded to the edge of the molten core to form a CuTi2+CuTi region [58].As shown in Figure 5, the movement of copper to both sides is inhibited after the Cu interlayer is fixed to the steel surface using an ultrasonic vibration device.Fe-Ti compounds were significantly reduced, which only formed a compound zone with thickness of 2 µm near the interface of Q235 side, and the core zone was mainly TiCu and TiCu2 [59].

Effects of Adding an Interlayer 3.2.1. Ni Interlayer
When a Ni interlayer is added between 304 stainless steel and TC4 for spot welding, the Fe-Ti compound in the joint is replaced by a Ti-Ni compound with lower brittleness, which improves the performance of the joint [55].The melted Ni is squeezed to the edge of the molten core, forming a thinner layer of α-Ti+Ti 2 Ni and a thicker layer of TiNi compounds [56].In the welded joint close to the Ti plate, a layered structure of α-Ti+TiFe with a width of about 20 µm is formed, and the TiFe 2 +α-Fe layer is observed near SUS304 whose width is about 3 µm.The center of the welded joint is mainly composed of columnar TiFe and TiFe 2 as shown in Figure 4.

Ni Interlayer
When a Ni interlayer is added between 304 stainless steel and TC4 for spot welding, the Fe-Ti compound in the joint is replaced by a Ti-Ni compound with lower brittleness, which improves the performance of the joint [55].The melted Ni is squeezed to the edge of the molten core, forming a thinner layer of α-Ti+Ti2Ni and a thicker layer of TiNi compounds [56].In the welded joint close to the Ti plate, a layered structure of α-Ti+TiFe with a width of about 20 µm is formed, and the TiFe2+α-Fe layer is observed near SUS304 whose width is about 3 µm.The center of the welded joint is mainly composed of columnar TiFe and TiFe2 as shown in Figure 4.

Cu Interlayer
Fe-Ti intermetallic compounds are the main structures formed in the welded joint near both sides of the base material after adding a Cu interlayer [57].The side near the titanium is composed of 15 µm thick TiFe and α-Ti, the composition near the steel side is TiFe2+α-Fe, with a thickness is 10 µm, and the center of the welded joint is mainly TiFe and TiFe2.The melted Cu foil is extruded to the edge of the molten core to form a CuTi2+CuTi region [58].As shown in Figure 5, the movement of copper to both sides is inhibited after the Cu interlayer is fixed to the steel surface using an ultrasonic vibration device.Fe-Ti compounds were significantly reduced, which only formed a compound zone with thickness of 2 µm near the interface of Q235 side, and the core zone was mainly TiCu and TiCu2 [59].

Cu Interlayer
Fe-Ti intermetallic compounds are the main structures formed in the welded joint near both sides of the base material after adding a Cu interlayer [57].The side near the titanium is composed of 15 µm thick TiFe and α-Ti, the composition near the steel side is TiFe 2 +α-Fe, with a thickness is 10 µm, and the center of the welded joint is mainly TiFe and TiFe 2 .The melted Cu foil is extruded to the edge of the molten core to form a CuTi 2 +CuTi region [58].As shown in Figure 5, the movement of copper to both sides is inhibited after the Cu interlayer is fixed to the steel surface using an ultrasonic vibration device.Fe-Ti compounds were significantly reduced, which only formed a compound zone with thickness of 2 µm near the interface of Q235 side, and the core zone was mainly TiCu and TiCu 2 [59].

Nb Interlayer
When Nb is used as the interlayer for RSW, the Nb layer in the welded joint does not completely disappear [60].As shown in Figure 6, a dendrite layer with a thickness of about 20 µm was observed near the steel side and was denoted as the R layer, which was composed of solid solutions of Nb and Ti in Fe.The researchers found a serrated reactant layer with a thickness of about 15 µm near the Nb side, denoted as the S-layer, and composed mainly of FeNb.The center of the welded joint consists of α-Fe and a relatively small amount of Fe2Nb.There is a layer of about 50 µm thickness on the titanium side, referred to as the T layer, which is composed of Fe and Nb in the solid solution of Ti [54].Zhang et al. [61] found 8 µm thick Fe2Nb+α-Fe and 14 µm thick Fe2Nb on the Nb/Q235 side, and detected a 25 µm thick compound layer on the Nb/Ti side, which is the supersaturated solid solution of Nb in Ti.

Nb Interlayer
When Nb is used as the interlayer for RSW, the Nb layer in the welded joint does not completely disappear [60].As shown in Figure 6, a dendrite layer with a thickness of about 20 µm was observed near the steel side and was denoted as the R layer, which was composed of solid solutions of Nb and Ti in Fe.The researchers found a serrated reactant layer with a thickness of about 15 µm near the Nb side, denoted as the S-layer, and composed mainly of FeNb.The center of the welded joint consists of α-Fe and a relatively small amount of Fe 2 Nb.There is a layer of about 50 µm thickness on the titanium side, referred to as the T layer, which is composed of Fe and Nb in the solid solution of Ti [54].Zhang et al.

Nb Interlayer
When Nb is used as the interlayer for RSW, the Nb layer in the welded joint does not completely disappear [60].As shown in Figure 6, a dendrite layer with a thickness of about 20 µm was observed near the steel side and was denoted as the R layer, which was composed of solid solutions of Nb and Ti in Fe.The researchers found a serrated reactant layer with a thickness of about 15 µm near the Nb side, denoted as the S-layer, and composed mainly of FeNb.The center of the welded joint consists of α-Fe and a relatively small amount of Fe2Nb.There is a layer of about 50 µm thickness on the titanium side, referred to as the T layer, which is composed of Fe and Nb in the solid solution of Ti [54].Zhang et al. [61] found 8 µm thick Fe2Nb+α-Fe and 14 µm thick Fe2Nb on the Nb/Q235 side, and detected a 25 µm thick compound layer on the Nb/Ti side, which is the supersaturated solid solution of Nb in Ti.

Ta Interlayer
In the spot welding of pure titanium and stainless steel SUS304 using Ta foil as the interlayer, Qiu et al. [62] found a light gray band I zone near Ta with a thickness greater than 10 µm, which is the eutectic structure of FeTa+Ta; Region II is about 1 µm of A Fe 2 Ta.A supersaturated solid solution III region of Ta with a thickness of about 8 µm in α-Ti is observed in Ti near the Ta/Ti interface as shown in Figure 7.

Ta Interlayer
In the spot welding of pure titanium and stainless steel SUS304 using Ta foil as the interlayer, Qiu et al. [62] found a light gray band Ⅰ zone near Ta with a thickness greater than 10 µm, which is the eutectic structure of FeTa+Ta; Region Ⅱ is about 1 µm of A Fe2Ta.A supersaturated solid solution Ⅲ region of Ta with a thickness of about 8 µm in α-Ti is observed in Ti near the Ta/Ti interface as shown in Figure 7.
Zhang et al. used BAg45CuZn as the interlayer for RSW of TC4/304SS, and added three different adsorption layer structures of V/Cr [64], V + Nb/Cr [65,66] and V + Mo/Cr + Mo [67] between the substrate and the interlayer, respectively.The experimental results show that Fe-Cr, Fe-Nb, Ti-V, Mo-V, Mo-Cr and other phases with low brittleness can replace part of the original Fe-Ti phase with the intervention of Cr, V, Nb and Mo elements to effectively improve the strength of the joint.In the spot welding of pure titanium and stainless steel SUS304 using Ta foil as the interlayer, Qiu et al. [62] found a light gray band Ⅰ zone near Ta with a thickness greater than 10 µm, which is the eutectic structure of FeTa+Ta; Region Ⅱ is about 1 µm of A Fe2Ta.A supersaturated solid solution Ⅲ region of Ta with a thickness of about 8 µm in α-Ti is observed in Ti near the Ta/Ti interface as shown in Figure 7.
Zhang et al. used BAg45CuZn as the interlayer for RSW of TC4/304SS, and added three different adsorption layer structures of V/Cr [64], V + Nb/Cr [65,66] and V + Mo/Cr + Mo [67] between the substrate and the interlayer, respectively.The experimental results show that Fe-Cr, Fe-Nb, Ti-V, Mo-V, Mo-Cr and other phases with low brittleness can replace part of the original Fe-Ti phase with the intervention of Cr, V, Nb and Mo elements to effectively improve the strength of the joint.Zhang et al. used BAg45CuZn as the interlayer for RSW of TC4/304SS, and added three different adsorption layer structures of V/Cr [64], V + Nb/Cr [65,66] and V + Mo/Cr + Mo [67] between the substrate and the interlayer, respectively.The experimental results show that Fe-Cr, Fe-Nb, Ti-V, Mo-V, Mo-Cr and other phases with low brittleness can replace part of the original Fe-Ti phase with the intervention of Cr, V, Nb and Mo elements to effectively improve the strength of the joint.

Defects in Ti-Steel Joints without Interlayer
When no interlayer is added, the microstructure of the spot welded joint of TC4/Q235 is shown in Figure 9, and it can be seen that the welded joint is full of macroscopic cracks and pores [63].During the solidification process, the metal in the molten pool is subjected to local stress and strain due to volume shrinkage, resulting in the metal in the molten pool being unable to fill the entire molten core after it solidifies; consequently, holes are formed in the molten core.Because the linear expansion coefficient of titanium is different from that of steel, the contraction rate of the two is different when the temperature is reduced.Therefore, a large amount of internal stress is generated in the welded joint, which aggravates the expansion of cracks and holes.

Defects in Ti-Steel Joints without Interlayer
When no interlayer is added, the microstructure of the spot welded joint of TC4/Q235 is shown in Figure 9, and it can be seen that the welded joint is full of macroscopic cracks and pores [63].During the solidification process, the metal in the molten pool is subjected to local stress and strain due to volume shrinkage, resulting in the metal in the molten pool being unable to fill the entire molten core after it solidifies; consequently, holes are formed in the molten core.Because the linear expansion coefficient of titanium is different from that of steel, the contraction rate of the two is different when the temperature is reduced.Therefore, a large amount of internal stress is generated in the welded joint, which aggravates the expansion of cracks and holes.

Influence of Adding Cu Interlayer on Defects in Ti-Steel Joints
Figure 10 shows the cracks in the welded joint that are approximately perpendicular to the bonding interface during spot welding with Cu as the interlayer [58].This configuration occurs because the electrode pressure applied in the vertical direction of the base material counteracts the tensile stress of the molten metal in that direction, so the force acting on the molten metal is mainly parallel to the tensile stress of the interface direction.The larger the molten pool is, the greater is the tensile stress on molten metal during cooling and solidification, and the longer the duration of solid-liquid phase during solidification, resulting in more cracks in welded joint.

Influence of Adding Ni Interlayer on Defects in Ti-Steel Joints
The morphology of the welded joint after adding the Ni intermediate layer is shown in Figure 11, and SUS304 adjacent to the welded joint protrudes into the Ti plate to form a concave surface [56].Because the thermal conductivity of SUS304 is greater than Ti, a large temperature gradient is formed along the plate direction during the welding process.

Influence of Adding Cu Interlayer on Defects in Ti-Steel Joints
Figure 10 shows the cracks in the welded joint that are approximately perpendicular to the bonding interface during spot welding with Cu as the interlayer [58].This configuration occurs because the electrode pressure applied in the vertical direction of the base material counteracts the tensile stress of the molten metal in that direction, so the force acting on the molten metal is mainly parallel to the tensile stress of the interface direction.The larger the molten pool is, the greater is the tensile stress on molten metal during cooling and solidification, and the longer the duration of solid-liquid phase during solidification, resulting in more cracks in welded joint.

Defects in Ti-Steel Joints without Interlayer
When no interlayer is added, the microstructure of the spot welded joint of TC4/Q235 is shown in Figure 9, and it can be seen that the welded joint is full of macroscopic cracks and pores [63].During the solidification process, the metal in the molten pool is subjected to local stress and strain due to volume shrinkage, resulting in the metal in the molten pool being unable to fill the entire molten core after it solidifies; consequently, holes are formed in the molten core.Because the linear expansion coefficient of titanium is different from that of steel, the contraction rate of the two is different when the temperature is reduced.Therefore, a large amount of internal stress is generated in the welded joint, which aggravates the expansion of cracks and holes.

Influence of Adding Cu Interlayer on Defects in Ti-Steel Joints
Figure 10 shows the cracks in the welded joint that are approximately perpendicular to the bonding interface during spot welding with Cu as the interlayer [58].This configuration occurs because the electrode pressure applied in the vertical direction of the base material counteracts the tensile stress of the molten metal in that direction, so the force acting on the molten metal is mainly parallel to the tensile stress of the interface direction.The larger the molten pool is, the greater is the tensile stress on molten metal during cooling and solidification, and the longer the duration of solid-liquid phase during solidification, resulting in more cracks in welded joint.

Influence of Adding Ni Interlayer on Defects in Ti-Steel Joints
The morphology of the welded joint after adding the Ni intermediate layer is shown in Figure 11, and SUS304 adjacent to the welded joint protrudes into the Ti plate to form a concave surface [56].Because the thermal conductivity of SUS304 is greater than Ti, a large temperature gradient is formed along the plate direction during the welding process.

Influence of Adding Ni Interlayer on Defects in Ti-Steel Joints
The morphology of the welded joint after adding the Ni intermediate layer is shown in Figure 11, and SUS304 adjacent to the welded joint protrudes into the Ti plate to form a concave surface [56].Because the thermal conductivity of SUS304 is greater than Ti, a large temperature gradient is formed along the plate direction during the welding process.The temperature field is characterized by the contraction of the higher isotherm towards the core, while the lower isotherm spreads out.As a result, SUS304 close to the welded joint has a large thermoplastic deformation under the action of electrode pressure to form a concave surface.In the direction perpendicular to the interface, the tension stress parallel to the bonding interface is the cause of the crack because the electrode pressure of the welded joint in the solidification stage cancels out the tension stress in this direction.

Influence of Adding Nb Interlayer on Defects in Ti-Steel Joints
Liu et al. [54] found some hole defects in Ti/Nb/Q235 joints, as shown in Figure 12.The solidification of molten pool metal occurs in the maintenance stage, which stops the electric heating and continues the pressure.At this time, the solid base metal around the molten metal has a binding effect on it, and the welded joint is cracked due to the stress.During the grinding process of the sample, the crack spreads to the region of the brittle intermetallic compound, which causes the metal to fall off from the crack to form long holes.
Metals 2024, 14, 429 8 of 14 The temperature field is characterized by the contraction of the higher isotherm towards the core, while the lower isotherm spreads out.As a result, SUS304 close to the welded joint has a large thermoplastic deformation under the action of electrode pressure to form a concave surface.In the direction perpendicular to the interface, the tension stress parallel to the bonding interface is the cause of the crack because the electrode pressure of the welded joint in the solidification stage cancels out the tension stress in this direction.

Influence of Adding Nb Interlayer on Defects in Ti-Steel Joints
Liu et al. [54] found some hole defects in Ti/Nb/Q235 joints, as shown in Figure 12.The solidification of molten pool metal occurs in the maintenance stage, which stops the electric heating and continues the pressure.At this time, the solid base metal around the molten metal has a binding effect on it, and the welded joint is cracked due to the stress.During the grinding process of the sample, the crack spreads to the region of the brittle intermetallic compound, which causes the metal to fall off from the crack to form long holes.

Influence of Adding Ta Interlayer on Defects in Ti-Steel Joints
Figure 13 shows part of the unfused interlayer metal in the welded joint and the crack perpendicular to the base material in the central region when Ta foil is used as the interlayer [62].Because the coefficient of linear expansion of Ta is smaller than that of Ti and 304 stainless steel, and the center area of the welded joint is last solidified, there is no excess liquid metal to fill the central area after the edge metal solidifies, so it cracks under the action of tension.

Influence of Adding Ta Interlayer on Defects in Ti-Steel Joints
Figure 13 shows part of the unfused interlayer metal in the welded joint and the crack perpendicular to the base material in the central region when Ta foil is used as the interlayer [62].Because the coefficient of linear expansion of Ta is smaller than that of Ti and 304 stainless steel, and the center area of the welded joint is last solidified, there is no excess liquid metal to fill the central area after the edge metal solidifies, so it cracks under the action of tension.

Influence of Process Parameters on Shear Strength of Joints
The shear load increases first and then decreases with increases in welding current, welding time and electrode pressure.With an increase in welding current or welding time, the heat generated by the joint increases, the molten metal increases, the diameter of the molten core increases, and the bearing area of the joint increases, so the shear strength of the joint increases.When the welding current is too large or the welding time is too long, splashing will occur in the joint, making the electrode indentation deep and reducing the joint performance.As the cooling time of the joint becomes longer, the columnar crystals continue to grow, resulting in the shear load of the joint decreasing [53].
With an increase in electrode pressure, the contact between the base material is more sufficient, which improves joint-forming.When the electrode pressure is too large, the molten metal moves towards the interface under the action of pressure to form a large number of brittle compounds along the outer edge of the molten core, which reduces the strength of the welded joint [58].

Influence of Interlayer Type on Shear Strength of Joints
Ta, Nb or V reacts with Ti to form solid solutions, and the brittleness of the intermetallic compounds formed by combining with Fe is lower than that of Fe-Ti intermetallic compounds.After using Nb as the interlayer in titanium/steel spot welding, the maximum shear load of the joint reaches 4.3 kN, which is 19.4% higher than that of 3.6 kN without adding interlayer [54].Qiu et al. [62] added Ta foil in the middle of Ti and SUS304 stainless steel, and the welded joint shear resistance reached a maximum of 7.4 kN.When Nb is used as the interlayer material for micro-RSW, the shear load of the joint is increased from 0.9 kN to 1.24 kN, an increased of 38% [68].When V + Mo/BAg45CuZn/Cr + Mo is used as the interlayer to weld TC4/304SS, the joint strength reaches a maximum of 235 MPa [64].
Cu, Ni or Cr reacts with Ti to form intermetallic compounds with low brittleness and high ductility, but forms only solid solutions with Fe.Under the optimal welding parameters, the maximum shear resistance of Ti/Ni/SUS304 welded joint can reach 5.62 kN [56].When Cu is used as the interlayer material, the maximum shear load of the joint reaches 7.5 kN, which is 62.6% higher than that of 4.61 kN without the interlayer [59].The various interlayers used by different researchers are tabulated in Table 1.

Influence of Process Parameters on Shear Strength of Joints
The shear load increases first and then decreases with increases in welding current, welding time and electrode pressure.With an increase in welding current or welding time, the heat generated by the joint increases, the molten metal increases, the diameter of the molten core increases, and the bearing area of the joint increases, so the shear strength of the joint increases.When the welding current is too large or the welding time is too long, splashing will occur in the joint, making the electrode indentation deep and reducing the joint performance.As the cooling time of the joint becomes longer, the columnar crystals continue to grow, resulting in the shear load of the joint decreasing [53].
With an increase in electrode pressure, the contact between the base material is more sufficient, which improves joint-forming.When the electrode pressure is too large, the molten metal moves towards the interface under the action of pressure to form a large number of brittle compounds along the outer edge of the molten core, which reduces the strength of the welded joint [58].

Influence of Interlayer Type on Shear Strength of Joints
Ta, Nb or V reacts with Ti to form solid solutions, and the brittleness of the intermetallic compounds formed by combining with Fe is lower than that of Fe-Ti intermetallic compounds.After using Nb as the interlayer in titanium/steel spot welding, the maximum shear load of the joint reaches 4.3 kN, which is 19.4% higher than that of 3.6 kN without adding interlayer [54].Qiu et al. [62] added Ta foil in the middle of Ti and SUS304 stainless steel, and the welded joint shear resistance reached a maximum of 7.4 kN.When Nb is used as the interlayer material for micro-RSW, the shear load of the joint is increased from 0.9 kN to 1.24 kN, an increased of 38% [68].When V + Mo/BAg45CuZn/Cr + Mo is used as the interlayer to weld TC4/304SS, the joint strength reaches a maximum of 235 MPa [64].
Cu, Ni or Cr reacts with Ti to form intermetallic compounds with low brittleness and high ductility, but forms only solid solutions with Fe.Under the optimal welding parameters, the maximum shear resistance of Ti/Ni/SUS304 welded joint can reach 5.62 kN [56].When Cu is used as the interlayer material, the maximum shear load of the joint reaches 7.5 kN, which is 62.6% higher than that of 4.61 kN without the interlayer [59].The various interlayers used by different researchers are tabulated in Table 1.

Influence of Interlayer Thickness on Shear Strength of Joints
The shear strength of the material increases first and then decreases with the increase in interlayer thickness.As the number of Nb interlayers increases, the shear strength of welded joints also increases first and then decreases [61].When the thickness of the Nb layer is 300 µm, the joint shear load reaches a maximum of 6.29 kN.Zhang et al. [70] found that the shear strength of the joint increased first and then decreased with the increase in the thickness of the Cu foil, reaching a maximum of 324 MPa when the thickness was 0.4 mm.When a nickel interlayer was added by spraying and the thickness reached 27 µm, the shear strength reached 254.71 MPa [55].

Influence of Interlayer Type on Hardness of Joints
The joint hardness varies with the type of interlayer.The introduction of a Ni interlayer allows the more malleable Ni3Ti to replace Fe2Ti, resulting in a decrease in hardness to 500-600 Hv [68]; Yu et al. [59] found that the hardness of the welded joint with an added Cu interlayer decreased from 15.3 GPa to 7.9-9.1 GPa, due to Ti-Cu with lower hardness replacing Ti-Fe with higher hardness.At the same time, the internal stress of the welded joint is reduced, which is beneficial to the mechanical properties of the welded joint.

Summary and Outlook
This article reviews the influence of adding the interlayer on the macro defects, microstructure and mechanical properties of the joints in the titanium/steel resistance welding process.The following main conclusions can be drawn: (1) The fracture mode of titanium/steel RSW joints is a brittle fracture, and the fractured surface is mainly composed of FeTi, Fe 2 Ti and TiC.(2) During the welding process, the molten pool undergoes volume contraction during solidification, resulting in local stress and strain due to the constraints of the surrounding solid metal.The solidified metal in the molten pool cannot completely fill the entire molten nugget, so defects such as holes and cracks are formed in the welded joint.(3) When the temperature decreases, the significant disparity in the coefficient of linear expansion between titanium and steel causes a significantly different rate of contraction, which leads to the generation of local stress and exacerbates the extension of defects.The excellent ductility of Cu, Ni and Ag can effectively alleviate the generation of internal stress and reduce the formation of defects.(4) Elements such as Cu, Ni, Cr, Nb, Ta and V can form solid solutions or intermetallic compounds with Fe or Ti, and the intermetallic compounds formed have lower brittleness than Fe-Ti compounds, thus increasing the joint strength.(5) By comparing the mechanical properties of joints in direct RSW of titanium/steel with those in titanium/steel RSW with the addition of interlayers, it is evident that the addition of Cu, Ni, Nb, Ta, 60% Ni-Cu alloy or BAg45CuZn layers results in a significant increase in shear strength of the joints.When Cu is used as the interlayer, the maximum shear load of the joint reaches 7.5 kN, which is 62.6% higher than that of 4.61 kN without the interlayer.The addition of a Ta interlayer makes the shear load reach 7.4 kN.By adding a Nb interlayer, the maximum shear load can reach 6.62 kN, and the shear strength of a Ni interlayer can reach 254.71MPa by the spraying method.When V + Mo/BAg45CuZn/Cr + Mo is used as the interlayer to weld TC4/304SS, the maximum shear strength of the welded joint can reach 235 MPa.
At present, there are still some problems in the research of titanium/steel RSW.On the one hand, the interlayer of a single element is difficult to meet the needs of titanium/steel dissimilar metal welding.On the other hand, during welding, the melted interlayers are squeezed to the edge under the action of electric force in the spot-welding process, which leads to the failure of the interlayer.
In view of this, the future research directions of titanium/steel resistance welding are suggested to include the following aspects.Firstly, the influence of various interlayer elements on the interface bonding quality of titanium/steel dissimilar welds should be thoroughly researched, including the effects of various interlayer elements on intermetallic compound growth, defect formation and nugget size.Secondly, conducting research on the interface bonding mechanism and interface bonding performance of dissimilar titanium/steel welding using low-cost high-entropy alloys as interlayers is worthwhile.High-entropy alloys contain multiple alloying elements, allowing for the full utilization of the advantages and combined effects of various alloying elements, thereby achieving excellent performance in titanium steel welding joints.Finally, it is worthwhile to study the use of multi-element and multi-layer interlayers which could be more suitable for controlling the interfacial reactions on both the Ti-interlayer interface and the Fe-interlayer interface in titanium/steel dissimilar metal welds, while effectively controlling the spattering of molten intermediate layers, to achieve excellent Ti-Fe joint properties.

Figure 2 .
Figure 2. SEM images of Ti/Q235 joint (a) near the Ti side (b) near the Q235 side-Reprinted from Ref. [53].2018 Journal of Welding Technology.

Figure 2 .
Figure 2. SEM images of Ti/Q235 joint (a) near the Ti side (b) near the Q235 side-Reprinted from Ref. [53].2018 Journal of Welding Technology.

Figure 3 .
Figure 3. SEM images of the Ti/Q235 joint (a) near the Q235 side and (b) near the Ti side-Reprinted from Ref. [54].2021 Springer.

Figure 4 .
Figure 4. SEM images of Ti/Ni/SUS304 joint (a) at the molten core left end, (b) near the Ti side and (c) near the SUS304 side-Reprinted from Ref. [56].2018 Transactions of Materials and Heat Treatment.

Figure 3 .
Figure 3. SEM images of the Ti/Q235 joint (a) near the Q235 side and (b) near the Ti side-Reprinted from Ref. [54].2021 Springer.

Figure 3 .
Figure 3. SEM images of the Ti/Q235 joint (a) near the Q235 side and (b) near the Ti side-Reprinted from Ref. [54].2021 Springer.

Figure 4 .
Figure 4. SEM images of Ti/Ni/SUS304 joint (a) at the molten core left end, (b) near the Ti side and (c) near the SUS304 side-Reprinted from Ref. [56].2018 Transactions of Materials and Heat Treatment.

Figure 4 .
Figure 4. SEM images of Ti/Ni/SUS304 joint (a) at the molten core left end, (b) near the Ti side and (c) near the SUS304 side-Reprinted from Ref. [56].2018 Transactions of Materials and Heat Treatment.

Figure 11 .
Figure 11.Morphology of Ti/Ni/SUS304 spot welding joint -Reprinted from tions of Materials and Heat Treatment.

Author Contributions:
Conceptualization, methodology, supervision, project administration, writing-review and editing, C.Z.; validation, formal analysis, investigation, data curation, writingoriginal draft preparation, visualization, Y.L.All authors have read and agreed to the published version of the manuscript.Funding: The authors are grateful for the financial support from Innovation Group Project of Southern Marine Science and Engineering Guangdong Laboratory (Zhuhai) (No. 311021013).This work was also supported by the fund of State Key Laboratory of Clean and Efficient Turbomachinery Power Equipment, State Key Laboratory of Long-Life High Temperature Materials (No. DTCC28EE190933), National Natural Science Foundation of China, grant number 51605287, and the Natural Science Foundation of Shanghai, grant number 16ZR1417100.

Table 1 .
Summary of the types of interlayers used by researchers in the titanium/steel spot welding process.