**3.1 Resistance spot welding of similar and dissimilar advanced high strength steels (AHSS)**

Advanced High Strength Steels (AHSS) are generally have been used in the automotive body structures in automotive industries owing to the reduced vehicle weight, high strength safety requirements, good corrosion resistance, and improved crash resistance [7–10]. The numerous AHSSs under consideration are dual-phase (DP) steels, transformation-induced plasticity steel, complex phase steels, and martensitic steels. Pouranvari [11] investigated the failure mode transition from interfacial to pullout mode of DP600 steel and low carbon steel in both tensile-shear and crosstension loading conditions. They reported that increasing the carbon equivalent decreases the ductility ratio. Wang et al. [12] investigated the effect of martensite volume fraction and morphology on the dynamic mechanical properties of three DP steel sheets (DP600, DP800, and DP1000) and one MS steel (M1200). They found out that the increment of tensile strength decrease with the increase of martensite volume fraction. The ferritic plastic deformation dominated the fracture mode. **Figure 2** shows the obtained deformed microstructures and holes at various distances from the substrate surface at the strain rates of 10−3 s−1 and 103 s−1.

*Resistance Spot Welding: Principles and Its Applications DOI: http://dx.doi.org/10.5772/intechopen.103174*

**Figure 2.**

*The deformed microstructure and the holes at various distances from the surface of the M1200 at the strain rates of 10−3 s−1 and 103 s−1. (a) 0.1 mm (10−3 s−1). (b) 0.3 mm (10−3 s−1). (c) 0.5 mm (10−3 s−1). (d) 0.1 mm (103 s−1). (e) 0.3 mm (103 s−1). (f) 0.5 mm (103 s−1) [12].*

Hayat and Sevim [13] carried out the spot-welded joints of galvanized DP600 steel and found out that the fracture toughness of the welded joint varies with the welding current and the welding time. Also, the fracture toughness of spot weld is not only dependent on the nugget diameter but also on the sheet thickness, tensile rupture force, welding time, and current. Pal and Bhowmick [14] investigated the RSW characteristics of DP780 steel and found out that the maximum load-carrying capacity is affected by the mode of fracture, that is, interfacial fracture attributes lower load-carrying capability compared to plug and hole type fracture. Zhao et al. [15] carried out the RSW of similar DP600 joints and concluded that the electrode force has an obvious effect on the weld nugget size of the weld joint. And there is a critical electrode force with which the weld nugget size attains its maximum value. The mechanical properties enhanced with the increase in the welding current and time. The variation of the effect of electrode force on the nugget diameter and penetration rate and the relationship between the tensile shear load, and absorbed energy with the nugget diameter has been depicted in **Figure 3a** and **b**.

Banerjee et al. [16] evaluated the fatigue characteristics of resistance spot welded DP590 steel sheets and reported that the fatigue life of the joints depends on the nugget size, notch sensitivity, load regime, and associated shear and nominal stress conditions. In the high and intermediate load regimes, the fatigue performance can be correlated to the nugget diameter with the larger nuggets exhibiting better performance. Matlock et al. [17] studied the recent developments in AHSSs for automotive applications. Early dual-phase and TRIP steel research in the late 1970s and early 1980s evolved into the core ideas for these breakthroughs. Controlling austenite stability and volume fraction to make highly ductile TRIP steels was highlighted as a significant factor in the development of new third-generation AHSS. Khan et al. [18] carried out

**Figure 3.** *(a) The electrode force effect on nugget diameter and penetration rate and (b) the relationship between tensile shear load, energy with nugget diameter [15].*

the resistance spot weldability study of AHSSs like DP600, DP780, TRIP780, and 590R. They found out from the study that the typical inter-critical heat affected zone (ICHAZ) microstructure comprised of undissolved ferrite and dispersed martensitic islands. The TRIP steel exhibited some retained austenite within the ICHAZ. Also, AHSS produced superior tensile failure loads relative to HSLA. The interfacial fracture was observed during tensile testing of DP600, while button pullout failure modes occurred for HSLA 590R, DP780, and TRIP780. Shojaee et al. [19] investigated the mechanical properties and failure behavior of RSW joints in third-generation 980 and 1180 sheets of steel. They concluded that the tensile shear results (TSS) exhibited that welds can show IF mode and possess high load-bearing capabilities. This suggests that using failure mode as the primary criterion for gauging TSS tests is inaccurate and that load-bearing capacity is a better indicator of weldment performance under shear loads. **Figure 4** shows the Vickers microhardness maps across the weld cross section of (a) 3G-980 steel welded at 9.3 kA and 3G-1180 steel welded at 9.1 kA. And **Figure 4c** shows the hardness profiles extracted from the maps.

### **3.2 Techniques to improve the RSW joints**

Numerous techniques have been developed to enhance the joint strength of spotwelded joints of both similar and dissimilar metals. The techniques are listed as below:

### *3.2.1 Use of double pulsed current*

Soomro and Pedapati [20] studied the effect of second pulse current on the microstructure and mechanical behavior of RSW HSLA350 steel and concluded that the introduction of a second pulse current enhanced the energy absorption capability and tensile shear strength of the weld. Also, shear dimples with a low fraction of micro-cracks were observed compared with tearing ridges with a high fraction of micro-cracks ass seen in a single pulse weld. Soomro et al. [21] carried out both singlepulse and double pulse welding of DP590 steel and observed the maximum improvement of 62% in tensile peak load and 62.3% failure energy in double pulse welds compared with single pulse welds. Also, an increment of 3.7% at heat input (*Q*) = 0.25 and 13.8% at *Q* = 1 was observed in weld nugget size. Jahandideh et al. [22] investigated the effect of post-heating time and post-heating current on the weld quality

*Resistance Spot Welding: Principles and Its Applications DOI: http://dx.doi.org/10.5772/intechopen.103174*

#### **Figure 4.**

*(a) Vickers microhardness maps across the cross-section of (a) 3G-980 welded at 9.3 kA and (b) 3G-1180 welded at 9.1 kA. (c) Hardness profiles extracted from maps [19].*

of SAPH40 steel and concluded that post-heating time reduces interfacial fractures but with a lower rate. The post-heating stage does not have a significant effect on the shear-tensile strength of the welded joints and the failure mode for the tensile shear lap tests. **Figure 5a** shows the nugget hardness and failure modes at various weld parameters and **Figure 5b** and **c** depicts the typical fracture modes.

Eftekharimilani et al. [23] investigated the effects of single and double pulse RSW on the microstructures of an AHSS. The elemental distribution of phosphorous at the primary weld nugget edge of the double pulse welds is more uniform, according to the researchers. When the area is heated to a higher temperature, the distribution improves (i.e., a second current pulse of equal magnitude to the first). The mechanical properties also enhanced due to double pulsing and welds subjected to two equal current pulses show the highest maximum cross tensile strength and tensile shear strength and a favorable plug failure. Liu et al. [24] studied the effect of double-pulse RSW on the mechanical properties and fracture process of Q&P980 steel. They observed that martensite was the predominant microstructure in the weldment and this steel was susceptible to liquation crack formation. Also, the application of higher secondary current improved the tensile-shear strength and failure mode, while a medium value-enhanced cross-tensile strength and ductility ratio. **Figure 6** portrays the typical fracture modes in TSS tests and their corresponding fracture surfaces and cross-sections.

#### **Figure 5.**

*(a) Hardness of nugget and failure mode at various post-welding currents and times, (b) typical interfacial fracture, and (c) typical pullout fracture [22].*

*Resistance Spot Welding: Principles and Its Applications DOI: http://dx.doi.org/10.5772/intechopen.103174*

#### **Figure 6.**

*Typical fracture modes in TSS tests: (a)–(f) are fracture surfaces and cross-sections; (g)–(p) are magnified images located at locations (g)–(p), respectively [24].*

#### *3.2.2 Use of interlayers*

Ibrahim et al. [25] investigated the weldability study of A6061-T6 sheet to SS304 using Al-Mg alloy as an interlayer. They concluded that Al/steel dissimilar welds with an interlayer exhibited higher tensile shear force than those without interlayer. The tensile shear and fatigue strengths of RSW Al/steel dissimilar welds were higher than those of FSSW ones fabricated using a scroll grooved tool without a probe. Plug, shear, and upper Al sheet fracture were dominant at high, medium, and low load levels, respectively. Zhang et al. [26] carried out the thermo-compensated RSW of AA5052-H12 Al alloy and AZ31B Mg alloy using Zn as an interlayer. The addition of a Zn interlayer between the sheets does not affect the tensile properties of the Mg/Al dissimilar joints, and the tensile shear force

of the weld joint was improved to 219 N using a thermos-compensated method, whereas the peak load of the Mg/Al RSW joints and the Mg/Al with Zn interlayer RSW joints was only 33 and 727 N, respectively. Das et al. [27] carried out the RSW of AISI-1008 steel to Al-1100 alloy using graphene nanoplatelets (GNPs) coating as an interlayer. They reported an enhancement of ~124% in the weld strength in one of the welding parameters. There was also an increment in the hardness owing to the interplay of different strengthening mechanisms. Intermetallic compounds (IMCs) of Al-Fe like FeAl3, Fe2Al5, and Fe4Al13 were formed at the interfacial region of Al/Fe, which were brittle. **Figure 7a**–**d** presents the load vs. displacement plots of the bare and GNP coated specimens and also the percentage enhancement owing to the GNP addition.

Penner et al. [28] investigated the effect of gold-coated nickel interlayer on the mechanical and microstructural behavior of dissimilar Al-Mg resistance spot welds. They reported that no joints were produced using a bare Ni interlayer. The welds made with 24 kA current had an average peak load of 4.69 kN, which was as high as 88% of the optimized similar AZ31B welds. And the formation of Al-Mg IMCs was completely suppressed using a gold-coated nickel interlayer. Thus gold-coated nickel represented a promising approach in dissimilar RSW. Sun et al. [29] carried out the dissimilar RSW of AA5052 to AZ31 alloys with Sn-coated steel interlayer.

#### **Figure 7.**

*Load vs. extension plots of the (a) uncoated specimens, (b) graphene-coated samples processed at the best welding parameters, (c) failure energy of the samples, and (d) percentage increase in the peak load by graphene addition as compared to bare samples [27].*

*Resistance Spot Welding: Principles and Its Applications DOI: http://dx.doi.org/10.5772/intechopen.103174*

They reported that strong joints were achieved using the interlayer and it reached 88% of the maximum value of AZ31 similar RSW joints. The thickness of the Al-Mg IMCs reduced and also the voids reduced due to the long downslope time and maybe also the high boiling temperature of Sn. Das et al. [30, 31] studied the effect of multiwalled carbon nanotubes (MWCNTs) on the RSW of AISI-1008 steel joints. They concluded that an enhancement of ~45% in the joint strength was observed owing to

**Figure 8.**

*(a) schematic of the tensile shear specimen and fracture surfaces; fracture surface of the nugget on Ni (Al) and Mg side at welding current of 32 kA (b) and (c) and 42 kA (d) and (e) [32].*

the incorporation of MWCNTs interlayer. The failure energy is also enhanced with the increase of welding current and with the use of an interlayer. Sun et al. [32] carried out the RSW of dissimilar AZ31 Mg alloy to aluminum AA5754 with a commercially pure Ni as an interlayer. They summarized that increasing the welding current increased the nugget diameter and hence the joint strength increased to 36 kA. But defects such as cracks and porosity formed in the Mg/Ni interfacial region were excessively high at 42 kA. This led to the early fracture at the Mg/Ni interfacial region and a reduction in the joint strength. **Figure 8a**–**e** presents the schematic of the tensile shear specimen and fracture surfaces depicting clearly the interfacial mode of failure and metal expulsion.

Das et al. [33] investigated the effect of graphene nanoplatelets on the RSW of similar AISI-1008 steel joints and concluded that an enhancement of ~63% at a welding parameter was observed. Microhardness studies also reported an increase in the hardness with the incorporation of GNPs interlayer and also with the increase of welding current.
