Resistance Spot Welding: Principles and Its Applications

Abstract

Resistance spot welding is one of the primary welding techniques extensively utilized in the automotive and aviation industries. Some 2000–3000 spots are made in a single body of automobiles, which are numerically controlled nowadays. Resistance spot welding works on the principle of Joule’s law of heating, where the heat generated is directly proportional to the square of the welding current. This welding technique is generally used to join thin sheets of steel, titanium, aluminum, magnesium, etc. The welding of non-ferrous metals like aluminum and magnesium is quite tedious owing to their high thermal conductivities and the oxide formation on their surfaces. Thus extensive surface preparation is required before welding. Numerous limitations are also there in this welding technique which includes low strength of the joints and thickness limitation.

1. Introduction

Resistance spot welding (RSW) or simply spot welding is widely employed in automotive and aeronautical industries. This type of welding employs a tremendous amount of current and a very low voltage. Numerous types of metals like different grades of steels, aluminum (Al), magnesium (Mg), titanium (Ti), copper (Cu), and their alloys. Generally, thin sheets of similar or dissimilar metals are joined by RSW in lap joint configuration. RSW also suffers from a few limitations like liquation cracking, voids, misalignment, electrode wear, etc. Thicker sheets of metals are generally difficult to weld by RSW because the heat flows into the surrounding metal very easily.

2. Principles of resistance spot welding

2.1 Theory

Resistance spot welding is a fusion welding process that works on the principle of Joule’s law of heating, which states that: Q = I²Rt, where ‘Q’ is the amount of heat generated during RSW, ‘I’ denotes the welding current used, ‘R’ is the resistance setup at the interface of the metal sheets, and ‘t’ is the welding time employed. RSW technique uses two truncated cone/dome-shaped copper alloy electrodes to concentrate the welding current into a fixed small spot and to simultaneously clamp the sheets together without any misalignment. Thin sheets of metals used as workpieces are held together under pressure exerted by the electrodes. The thickness of the metal sheets generally varies between 0.5 and 3 mm. The enforcement of a large amount of current through the spot will melt the metal and form a weld. RSW allows a large amount of energy to be distributed to a specific location in a short period (approximately 10–100 milliseconds). This allows the welding to take place without overheating the rest of the metal sheet.

The resistance between the electrodes and between the electrodes and metal sheets, as well as the amplitude and duration of the welding current, control the amount of heat energy transferred to the produced spot. The amount of energy is chosen to match the sheets’ material properties like thermal conductivities, coefficient of thermal expansion, electrical conductivity, etc. Applying too little energy will not melt the localized region and sufficient strength will not be developed. Whereas, applying too much energy will melt too much metal, eject molten material and make a void rather than a spot [1]. Figure 1 depicts a schematic of a resistance spot welding process.

Figure 1.

Schematic of resistance spot welding.

2.2 Technique and equipment used

There are generally three stages in the resistance welding process which are stated as follows: (a) the electrodes are being brought to the surface of the metal sheets being welded and a slight amount of pressure is applied and (b) the welding current from the electrodes is then applied for a very short time after which the current is removed but the electrodes maintain the pressure to allow the weld metal to cool and solidify. The applied weld times normally range from 0.01 to 0.8 s depending on the thickness of the metal, the electrode force, and the electrode tip diameter [2, 3].

The resistance spot welding setup mainly consists of tool holders and copper alloy electrodes. The tool holders act as a mechanism to hold the electrodes firmly in place and also to support the cooling water hoses that are used for cooling the electrodes. Tool holding techniques generally include paddle-type light-duty, universal and regular offset. The electrodes are made up of low resistant highly conductive metals like copper and are manufactured in numerous designs such as truncated cone, dome, flat shapes depending on the application needed.

The metal sheets to be welded together are known as workpieces and should be a good conductor of electricity. The width of the metal sheets is limited by the throat length of the welding equipment and ranges typically from 5 to 50 inches (13–130 cm). The thickness can vary between 0.20 and 3 mm.

In the case of RSW, there are two critical components of the tooling system whose characteristics have a significant impact on the entire process: the gun and its kind, as well as the size and form of the electrode. The C-type gun is commonly employed in applications where the gun layout must be as stiff as possible due to large applied forces. This design provides great stiffness and tooling flexibility, as well as collinear electrode motion. The X-type arrangement, like the C type, provides minimal stiffness, even though the reachable workspace is significantly greater than the C-type. As a result, this architecture is highly frequent where tin and flat objects are processed. However, low flexibility is provided in terms of tooling, because the paths of the moving electrodes are not collinear, hence a dome-shaped electrode tip should be used.

Electrodes that are used in spot welding also vary in terms of their uses. For high heat applications, radial type electrodes are used, truncated tip electrodes are used for high pressure, eccentric electrodes for welding corners, offset eccentric tips for reaching into corners and narrow places, and lastly offset truncated into the workpiece itself.

2.3 Features of resistance spot welding

Resistance spot welding tends to work harden the material during the application of electrode force causing it to warp. This phenomenon leads to the reduction of the materials fatigue strength and may stretch the material as well as anneal it. The various defects of spot welding include internal cracking, liquation cracking at the interior of the weld nugget, and a bad appearance. The chemical properties affected include the metal’s internal resistance and its corrosive properties.

The welding times used are very short, which can cause electrode wear- they cannot move fast enough to keep the material clamped. During the first pulse, the electrode contact may not be able to make a good weld. The first pulse will soften the metal. During the interval between the two pulses, the electrodes will come closer and make better contact. Also, a higher welding current creates a huge magnetic field, and when the electric current and magnetic field intersect, a large magnetic force field is produced, which causes the melted metal to move very quickly, up to 0.5 m/s. As a result, the fast motion of the melted metal could substantially alter the heat energy distribution in spot welding. A high-speed camera can be used to observe the rapid motion of spot welding [4, 5, 6].

2.4 Power supply

The basic spot welding setup consists of a power supply, an energy storage unit (e.g., a capacitor bank), a switch, a welding transformer, and the welding electrodes. The capacitor bank acts as a supplier of high instantaneous power levels. The accumulated energy is dumped into the welding transformer when the switch is pressed. This transformer then reduces the voltage while increasing the current. The transformer’s main feature is that it reduces the amount of electricity that the switch can tolerate. The transformer’s secondary circuit includes the welding electrode. A control box is also present, which controls the switch and may also monitor the welding electrode voltage or current.

A large number of resistances are being set up in different regions, thus making the resistance offered quite intricate. Secondary winding, cables, and welding electrodes all have their resistances. The contact resistance between the welding electrodes and the workpiece is also a factor. There’s also the resistance of the workpieces and the resistance of the workpieces’ contact. Because contact resistances are typically high at first, the majority of the energy is wasted there. The heat created by the clamping force softens and smoothens the material at the electrode-material interface, resulting in better contact and lower contact resistance. As a result, more electrical energy will be transferred into the workpiece, and the junction resistance between the two workpieces will increase. The electrodes and the workpiece conduct the heat away as electrical energy is provided to the weld and the temperature rises. The most important need is to provide enough energy to melt a piece of the material within the spot without melting the entire spot. The perimeter of the spot will channel considerable heat away and keep the perimeter at a lower temperature due to thermal conductivity. Because less heat is transferred away from the inside of the spot, it is the first to melt. When a significant welding current is used, the entire spot melts, the material pours out, and a hole instead of a weld is formed.

The working voltage needed for welding is dependent on the resistance of the material to be welded, the sheet thickness, and the desired size of the nugget. When welding a 2 mm lapped joint, the voltage between the electrodes is only about 1.5 V at the start of the weld but can fall as low as 1 V at the end of the weld. This voltage reduction is due to the reduction in resistance owing to the workpiece melting. The open-circuit voltage from the transformer is higher than this which ranges from 5 to 22 V typically.

3. Literature review

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, 8, 9, 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 cross-tension 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.

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.

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].

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 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.

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].

3.2 Techniques to improve the RSW joints

Numerous techniques have been developed to enhance the joint strength of spot-welded 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 single-pulse 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 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.

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].

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 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.

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].

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. 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 multi-walled 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 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.

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].

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.

4. Applications of resistance spot welding

Resistance spot welding is typically used when joining particular types of sheet metal, welded wire mesh, or bare wire mesh. Thicker sheets are more difficult to spot weld owing to the dissipation of heat into the surrounding metal more easily. Spot welding is possible with aluminum alloys, but their higher thermal and electrical conductivities necessitate larger welding currents. This necessitates the use of larger, more powerful, and more costly welding transformers.

Spot welding is most commonly used in the automobile manufacturing business, where it is nearly routinely used to join the sheet metals of car frames. Spot welders can also be fully automated and many of the industrial robots found on assembly lines are spot welders.

Spot welding is also utilized in orthodontist clinics to resize metal “molar bands” used in orthodontics with small-scale spot welding equipment.

To create batteries, spot welding is also used to attach straps to nickel-cadmium, nickel-metal hydride, or lithium-ion battery cells. Spot welding thin nickel bands to the battery terminals connect the cells. This method prevents the battery from overheating, which could occur if traditional soldering was used.

Some design practices must be followed for spot welding like connecting surfaces should be free of contaminants such as scale, oil, and dirt to ensure quality welds.

5. Modifications of resistance spot welding

A modified version of resistance spot welding has been developed which is known as projection welding. The weld is localized in projection welding by using raised areas, or projections, on one or both of the metal sheets to be connected. Heat is concentrated at these projections, allowing for the welding of heavier parts or closer weld spacing. The projections can also be used to position the workpieces so that they are balanced. Studs, nuts, and other threaded machine parts are frequently welded to a metal plate using projection welding. Crossed wires and bars are typically joined with it. Multiple projection welds can be arranged by appropriate planning and jigging in this high-production technique [34].

6. Conclusions

This chapter dealt with the resistance spot welding technique and their various working principles and applications. Resistance spot welding is mainly used for the joining of thin metal sheets and their alloys. The difficulties associated with the joining of highly thermally conductive metals like aluminum and magnesium were also discussed. The power supply used was also discussed. Various techniques have also been discovered by researchers to enhance the joint strength of the weldments. The techniques included the use of double-pulse welding current and various metallic or non-metallic interlayers. Also, the broad field of applications of resistance spot welding was mentioned, and also a modified RSW technique which is the projection welding was also elaborated in this chapter.

Appendices and nomenclature

RSW

resistance spot welding

GNPs

graphene nanoplatelets

MWCNTs

multi-walled carbon nanotubes

IMCs

intermetallic compounds

AHSS

advanced high strength steels

DP

dual-phase

HSLA

high strength low alloy

TRIP

transformation induced plasticity

Q&P

quench and partitioning

ICHAZ

inter-critical heat affected zone