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Review on Recent Trends in Composite Joints using Spot Welding Variants

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Submitted:

01 April 2024

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01 April 2024

Withdrawn:

04 April 2024

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Abstract
Traditional resistance spot welding (RSW) was unsuccessful in forming quality composite joints between steel or aluminium-polymer based composites. This led to the development of spot-welding variants such as friction stir spot welding (FFSW), ultrasonic spot welding (USW) and laser spot welding (LSW). The paper reviewed the differences in the bonding mechanism, spot weld characteristics and challenges involved in using these spot-welding variants. Variants of RSW used series electrodes arrangement, co-axial electrodes, metallic inserts, interlayers or external energy to produce composite joints. FFSW and USW used nanoparticles, interlayers or energy directors to create composite spot welds. Mechanical interlocking is the common composite joint mechanism in all variants. Each spot-welding variant had different sets of weld parameters and distinct spot weld morphologies. FFSW is the most expensive variant but commonly used for composite spot weld joints. USW has a shorter welding cycle compared to RSW and FFSW but can only be used for small components. LSW is faster than the other variants, but limited work was found on its use in composite spot weld joining. Use of interlayers in FFSW and USW to form composite joints are potential research areas recommended in this review.
Keywords: 
Subject: Engineering  -   Automotive Engineering

1. Introduction

In the recent decade, composite materials applications in transportation, medical equipment, sports equipment and electronics have increased tremendously. Industries that were, historically, depending mainly on metals, because of properties such as high strength, malleability and ductility, to manufacture the products have shifted to composite materials due lighter weight, enhanced strength and durability, high performance and reduced carbon emission[1,2]. In cars, a weight reduction of 10%, reduced fuel consumption by 3 – 7% with identical performance[3]. Global international aviation’s CO2 emission is being forecasted by International Civil Aviation Organisation (ICAO) to grow by a 300% - 700% by 2050 and aircraft with composite architecture is expected to contribute to 15% to 20% CO2 reduction by 2050[4].
In multi material design (MMD) composite materials are joined together or with metallic materials. This is a strategy that has been employed especially in automotive and aviation industries which involves integration of different materials in creating lightweight structure designs with improved crashworthiness and reduced CO2 emission[5,6]. Figure 1 shows the Mach-II lightweight vehicle Body in White (BiW) based on multi material design. Materials that are commonly used as lightweight materials are light alloys such as aluminium and magnesium, High Speed Steel (HSS) and composites[5,7,8,9]. The common joining technologies in multi material joining are fusion bonding or welding (fusion welding and solid-state welding), mechanical fasteners (bolting and riveting) and adhesive bonding [10] as illustrated in Figure 2.
Temporary joints which need disassembling and assembling of components use mechanical fasteners. As mechanical fasteners require holes to be drilled, these joints are prone to stress concentrations, which lead to reduced strength. The bolts and rivets further add the product weight affecting the lightweight design [12,13,14,15]. For permanent joints, either adhesive bonding or welding is used. Adhesive bonding involves extensive treatment of faying surfaces, depends on environmental factors, application temperature, curing time, type of adhesive and there is no universal adhesive to be used for all applications [16,17,18,19]. However, adhesive bonded joints showed higher joint stiffness, higher shear strength and better uniform load distribution compared to mechanically fastened joints or welded joints[20,21,22].
The other permanent joint option is welding which can be divided into fusion welding and solid-state welding. Fusion welding for example Resistance Spot Welding (RSW), Arc Welding (AW) and Resistance Seam Welding (RSSEW) involves faying surfaces of base metals to be fused by heating to melting points, to form coalescence during welding. Solid-state welding such as Friction Welding (FW) and Ultrasonic Welding (USW) involves joining the faying surfaces of base metal without heating to melting point. There are also other welding processes that do not fall into both the above categories; namely Laser Beam Welding (LBW) and Induction Welding (IW). Unlike joining with mechanical fasteners and adhesive bonding, welding has limited ability in joining dissimilar metals and metal-polymer combination. This is due to joining mechanism involved, i.e, heating or melting workpieces with differences in thermal conductivity and coefficient of the thermal expansion [23]. Furthermore, during solidification of the weld joints, intermetallic compounds (IMC) that are brittle, porosities and dendritic recrystallization will affect the integrity of the weld quality[24]. Even though mechanical fasteners and adhesive bonding are suitable for dissimilar and composite materials[18], welding process despite the above-mentioned limitations, are more suitable for process automation and faster process.
Spot welded joints are the common joints used in automotive industries. An automotive BiW generally will have around 2000 -5000 spot welds [25,26]. Traditionally, automotive BiWs when predominantly built using steel sheets, resistance spot welding (RSW) process was the preferred choices due to its inherent characteristics such as low cost as no filler is required for this process unlike the arc welding process, fast operation, and ease of being automated using robots[27,28,29]. However, when the concept of light weight vehicles was introduced and multi material design was incorporated, aluminium, magnesium and Carbon Fiber Reinforced Polymer (CFRP) were spot welded with steel. RSW uses the material electrical resistance to generate heat at the faying surfaces of metals to be joined. Non-ferrous metals such as aluminium, copper and magnesium have a very high thermal and electrical conductivities compared to steel, therefore concentrating the heat at the faying surfaces for metal melting to happen was difficult. Hence joining these materials with steel using RSW was challenging[30,31,32]. Welding aluminium with steel to form a dissimilar joint causes electrode deterioration and high energy consumption [33]. RSW was not able to join CFRP with metal as the former is an insulator[34].
Considering the limitations of the traditional RSW to join composite materials via spot welds, the process has been modified or new variants of the process were developed. Alternatively, the solid-state welding processes such as friction stir spot welding (FSSW) and ultrasonic spot welding (USW) and advance welding processes such as laser spot welding (LSW) were reported to have successfully spot-welded dissimilar materials to form composite/hybrid joints. This review paper will review recent studies in the different variants of spot-welding processes; RSW, FSSW, USW and LSW, used to form composite joints based on the following:-
a)
welding mechanism,
b)
weld characteristics,
c)
advantages and drawbacks of the processes
To the best knowledge of the author, such a comprehensive, scientific and organised study on different types of spot-welding processes for joining composite materials have not been published. With these review paper, detailed, organised and latest information on different spot-welding techniques will be made available to researchers and the industries. The review is also intending to provide researchers with new directions of research areas which will contribute to successful joining of composite materials with good quality spot welds.

2. Welding Mechanism

2.1. Resistance Spot Welding (RSW)

Resistance spot welding (RSW) was invented in 1886 by Elihu Thomson. The process involves overlapping metal sheets joined at the interface via spot welds. The sheets to be welded are clamped together by two water cooled copper electrodes by a clamping force as shown in Figure 3 during the squeeze cycle. Current then flows from the top electrode to the bottom electrode through the metal sheets. As the sheets interface has higher resistance to current flow, localised heating will be generated to melt the metals at the faying surfaces during weld cycle. The melted metal upon solidification during the hold cycle under electrode pressure and current turned off, will join the metal sheets with a spot weld. Figure 3 shows the spot-welding process and an example of a spot welded automotive body part.
The above arrangement is not capable to establish a proper MMD joint when one of the faying surfaces involves a composite such as CFRP. Therefore, the welding process is modified as given below:

2.1.1. Changes in Process Setup

Contrary to the traditional RSW setup, the two electrodes (+ve and -ve electrodes) are placed in series on the metal side as in Figure 4, to form a composite spot weld joint between austenitic stainless steel (SUS304) and carbon fibre reinforced thermoplastic (CFRTP)[35]. The joints were produced by current flowing only in the metal side. This current generates heat that melts the CFRTP at the faying surfaces by heat conduction thus produces joint between SUS304 with CFRP. The joint constituents depend on the materials joined. The bonding mechanism involves van der Waal forces and hydrogen bonds formed between metal oxide and polar functional groups of CFRP. The surface treatment of stainless steel enhanced joint strength. It was reported that SUS304 and polyphenylene sulphide based CFRP (PPS) did not produce a bond as CFRP(PPS) is non polar and only van der Waal forces were formed at the faying surface[35].
The co-axially arranged electrodes setup as in Figure 4 is also used to form metal-polymer spot welded composite joints where the electrodes placed on the metal side comprise of an outer cylindrical hollow electrode and the inner electrode with DIN EN ISO 5821 F1 geometry[34,36,37]. Current flows from the inner electrode to the outer electrode and the metal is heated. The heat is conducted to the polymer causing localised melting in the polymer. This generates bonding with the metal under the force exerted by the electrodes. Table 1 summarises work that used co-axial electrodes for spot welding composite joints.

2.1.2. Use of Interlayers and Metal Inserts

Another variant of RSW is Resistance Element Welding (REW), developed to produce multi material spot welds by including a third material in the form of metal insert (element) between the two materials to be joined as shown in Figure 5. A hole is created on the top material to position the insert. This technique invented by Volkswagen AG, used the conventional DC spot welding machines. The REW is a process that integrates the principles of thermal and mechanical bonding between insert and the bottom material. The ‘force and form locking’ establishes joint between the insert and the top material[38]. Interlayers such as zinc, nickel, Al-Mg alloy are used to improve weldability of composite materials with the use of RSW[39]. The interlayers in the form of thin film or powder are placed in the faying surface of the materials to be spot welded as shown in Figure 5. Limited work, however, was found in the use interlayers to spot weld metal-composite joints compared to the work on spot welding of metal-metal dissimilar joints with the use of interlayers. The interlayers in metal-metal joints overcome the development of the brittle intermetallic compounds (IMCs) and improve weld strength especially in aluminium-steel and aluminium-magnesium joints[39]. The joints are formed either by mechanical interlocking[40] or diffusion-reaction[41,42] mechanisms. The use of interlayers and metal inserts for composite spot weld joints are presented in Table 2.

2.1.3. Other RSW Variants

The other variants such as magnetic, shunt current, and inducted heat assisted resistance spot welding are presented in Figure 6 where additional energy is applied externally. Magnetically assisted resistance spot welding (MA-RSW) uses magnetic field from two permanent magnets that are attached to both top and bottom electrodes to create an electromagnetic stirring (EMS) force. The EMS force controls molten metal flow in the joint area hence enlarging nugget diameter compared to the nugget formed in RSW. MA-RSW used to spot weld silicon carbide with 2024 aluminium (SiCp/Al) expands the weld lobe while welding SiCp/Al composites[50]. This improved the joint strength compared to the use of RSW for same welding current.
The shunt current assisted resistance spot welding (SCA-RSW) and induction heat assisted resistance spot welding (IHA-RSW) were used to spot weld LITECOR® with DP600 steel[51,52]. The SCA-RSW involves the welding current flowing from the top electrode to bottom electrode, passing through an Al alloy shunt element hence bypassing the polymer material in LITECOR®. The heat from the shunting element will then be transferred to metallic sheet of the LITECOR® causing the polymer to be heated and weld joint is formed under electrode force. The IHA-RSW involves induction coil wound around the bottom electrode. The current flowing through the induction coil will generate magnetic field around the coil. Based on Lenz’s law, the magnetic field will produce eddy current in the bottom electrode. The eddy current will flow to the steel and heats the steel and via conduction melts the LITECOR® and form a spot weld joint under electrode pressure.
Variants of the RSW that uses Joule heating to form spot welds in composite joints, involves either changes in the electrode arrangement or electrode design [34,35,36,37], use of separate metallic elements at faying surfaces [43,44,45,46,47,48,53] or use of additional elements such as permanent magnets, induction coil and shunt tool for heat generation and transfer[50,51,52]

2.2. Friction Stir Spot Welding (FSSW)

Friction stir welding (FSW) was developed in 1991 by the Welding Institute to solve the problem associated with joining aluminium and its alloys [54]. Friction stir spot welding (FSSW) is a variant of FSW used in form spot welds. Unlike RSW where heating is produced due to material’s resistance to current flow, in FSSW heat in produced due to friction created between material and a rotating tool. Mazda Motor Corporation developed FSSW in 1993[55] to replace mass mechanical fastening processes such as riveting and to join dissimilar materials that were unable to be spot welded using RSW. A schematic diagram of FFSW is given in Figure 7 where there are three steps; i) plunge - the rotating tool is forced into the material till the tool shoulder touches the overlapped top material, ii) stirring - tool achieves a predetermined depth into the workpiece and maintain rotation in the workpiece. Frictional heat is generated and material closer to the tool is heated, softened and forms a solid-state spot weld. Finally, iii) the tool retracts from the materials. Because of tool design, the spot weld will have a keyhole in the middle which reduces the strength of the joints significantly [56]. Another variant of FFSW was developed by Helmholtz Zentrum Geesthacht, Germany in 2004 which is known as the refill FSSW to eliminate the keyhole in the joints. The tool design of the refill FSSW is complex as it is made from three components: the stationary clamp and rotating sleeve and probe. The process has four steps as illustrated in Figure 7, (i) tool with all three components touches the surface of the top material and the clamp presses the workpieces together, (ii) plunge – the sleeve rotates into the workpiece while the probe rotates aways from the workpiece, (iii) due to frictional heat, the material will melt and flow upwards into the sleeve. Sleeve then retracts, and the plasticized material is forced by the probe to refill the hole left by the sleeve and iv) tool is retracted.

2.2.1. Use of Nanoparticles and Interlayers in FSSW Composite Joints

The use of nanoparticles and interlayers at the joint area, similar to Figure 5, to create composite joints using FSSW is presented in Table 3.
Table 3 shows oxide ceramics-based nanoparticles improved composite spot weld joints’ strength by impeding grain growth. Polymer based interlayers formed chemical bonding and mechanical interlocking with metal to increase the strength of the joints. Limited work however, were reported in the used of interlayers for metal-polymer FSSW compared to use of interlayers in metal-metal FSSW. In metal FSSW, interlayers are metals, and the common interlayer is zinc[72,73,74,75,76,77].

2.3. Ultrasonic Spot Welding (USW)

Ultrasonic welding is a type of spot-welding process. It is a solid-state welding developed in 1940s to 1950s. The materials to be joined are positioned on an anvil and held by a normal clamping force exerted by a sonotrode. Shear vibration with high frequency and low amplitude is used to deform and shear surface asperities between two faying surfaces. This creates contact area between the materials and friction between faying surface to be joined, generates high temperature due to severe plastic deformation and a spot weld is created due to dynamic recrystallization[78]. The heat generated depends on the surface roughness and friction coefficient of both mating surfaces[79]. This type of spot-welding process is being widely used in lithium ion batteries in electronics and electric vehicles (EV) with the process being used to connect cell terminals and bus bars[80]. The ultrasonic spot welding can be divided into two types; ultrasonic plastic welding and ultrasonic metal welding as shown in Figure 8[10]. Ultrasonic metal welding is used for composite joints where metal is one of the materials and involves vibration is the transverse direction (parallel to weld area) and heat created due to friction of the surfaces without melting of materials. Ultrasonic plastic welding is used for polymer composite joints and the vibrations are in the longitudinal direction (perpendicular) to weld area and involves melting of polymer to form weld joints.

2.3.1. USW Process Variants

High pressure-amplitude ratio ultrasonic spot welding (H-USW) spot welded low glass temperature (Tg) thermoplastic carbon fibre reinforced epoxy through twisting the carbon fibres and polymer at the interface, and strengthening the bond between polymeric layers[81]. The ultrasonic spot weld setup called the differential ultrasonic spot (DUS) welding used a bigger diameter sonotrode compared to the anvil to create ultrasonic spot welds without the use of energy director (ED)s. The work concluded that the DUS setup created bigger spot welds with greater strength compared to the spot welds made from a pointed weld tip when the process was used to spot weld polyetherimide (PEI) [82,83]. Thermal profile analysis at the weld interface also showed that heating at interface is due to interfacial friction and the maximum temperature is related to the duration of ultrasonic vibration. The numerical analysis to analyse the temperature distribution during spot welding in a DUS welding concluded that the spot welding process involves two types of heating; a) initially as frictional heating to soften the weld interface and b) secondly viscoelastic heating for composite matrix decomposition[84]. The multi row ultrasonic spot-welding configuration to replace mechanical fasteners in joining composite joints was studied [85,86,87]. Multi row spot welded joints’ load carrying capability, was only about 10% lesser than the load carrying capability of mechanically fastened joints. The multi row spot welded joint was also found to have higher stiffness compared to the mechanical joints. Loading capability of spot-welded joints was improved by increasing the distance between rows in double rows but as the load is not uniformly distributed among rows, it was not beneficial to increase the rows more than 3.

2.3.2. Use of Interlayers and Energy Directors in USW Composite Joints

Energy directors (ED) was used to concentrate the heating at a spot at the sheets’ interface to form polymer based composite joints. ED generate localised heating through frictional and viscoelastic heating[88]. The common ED shapes are triangular ED, semi-cylindrical ED, rectangular ED and trapezoidal ED[89].The use of interlayers and energy directors in the spot welding of composite joints using USW have been presented in Table 4.
The energy directors and interlayers used in composite spot weld joints are polymer or polymer composite based. Mechanical interlocking is the main joining mechanism in metal-polymer composite joints[92]. Less work in the use of polymer interlayer for metal-polymer composites were found, even though the use of interlayer in the USW of dissimilar metals have been widely reported. Some of the interlayers reported in USW of dissimilar metals are silver (Ag)[98], copper (Cu)[99,100,101], aluminium (Al)[102,103,104,105,106,107], zinc (Zn)[108,109,110] and brass[111]; which is the only alloy based interlayer.

2.4. Laser Spot Welding (LSW)

Laser spot welding (LSW) is a process that uses laser beam to join two material surfaces at a single spot. The laser beam targets a small spot and transfers energy to melt and fuse material surfaces together. Unlike the RSW, FFSW and USW, LSW is a non-contact welding process with narrow heat affected zone[112]. Common lasers that are used as Nd:Yag pulsed laser, fibre laser or CO2 laser. Figure 9 shows the diagram of laser spot welding process.
LSW was used to form composite spot weld joints between Polyethylene terephthalate glycol (PETG) polymer and Macor glass ceramic. The heat from the laser beam, melts the glass ceramic and the ceramic solidifies while forming bubbles due to nucleation to form crystals. Microstructural changes happens only to the ceramic while the heat transmitted from ceramic to polymer melts the polymer to form the composite joint[112]. The use of LSW in joining copper with single walled carbon nanotubes (SWCNTs) involved the use of laser beam to melt the copper. Molten copper will mix with the solution based SWCNTs and solidifies to form joints with dispersed SWCNTs[113]. The SWCNT nanocomposites was embedded, prior to welding, into copper through the laser Surface Implanting (LSI) process. The pure copper spot weld strength was found to be lower than the spot weld strength of the copper-SWCNT.
Limited work on LSW of composite joints were observed. Hence the review studied LSW of similar/dissimilar metals and continuous laser welding (LW) of dissimilar materials to form composite joints as given in Table 5, to form the basic understanding of composite joint mechanism with LSW. There were two types of laser beam heating methods for composite joining process between metal and polymer named as the Laser Assisted Metal and Polymer (LAMP) process; transmission laser heating and conduction laser heating. Transmission laser heating involves the beam passing through the polymer and heating the interface while conduction heating involves the laser being targeted to the metal and heat is transmitted from the metal to polymer via conduction. Both heating processes are illustrated in Figure 10.
LSW of metal-polymer composite joints has no reported work. Table 5 has shown laser welding of composite joints are formed through chemical bonding and mechanical interlocking. Limited work was found in the use of interlayers in laser welding with polycarbonate being the sole polymer used as interlayer. LSW of dissimilar metal joints involved mechanical interlocking due to formed microstructures and intermetallic compounds after solidification.

3. Weld Characteristics

3.1. RSW Weld Characteristics

The welding parameters for RSW are current, time and electrode force. Higher current and weld time have been recommended to achieve an acceptable weld nugget diameter[49].There are positive correlation of molten zone depth with welding current and time and negative correlation with weld force[37]. The conventional Resistance Spot Welding (RSW) spot weld morphology for metal-metal joint is made of three distinct regions; the fusion zone (FZ), Heat affected zone (HAZ) and base metals (BM) as shown in Figure 11. The FZ is the weld nugget that is produced due to melting and re-solidification of the base metals. The HAZ is the area that did not melt but has undergone microstructural changes due to the heat at the adjacent FZ. For RSW of the metal-polymer composite joints, as heat flows from the metal to the polymer by thermal conduction in the series electrode arrangement and coaxially arranged electrodes, the molten region or the FZ was observed on the polymer side as in Figure 12. The spot weld morphology for series electrodes also showed extrusion of the top metal sheet into the lower polymer sheet as in Figure 13 due to the electrode indentation on the metal side. For coaxial electrodes, the FZ was in between the inner and outer electrodes, hence the spot welds as in Figure 13 did not have the conventional lens shape shown in Figure 11. Unlike the weld nugget in metal-metal joint that is created by melting and solidification, the metal-polymer weld nugget is formed due the mechanical interlocking or chemical bonding.
For RSW spot welds involving metal insert or Resistance Element Weld (REW), joints were formed between the metal insert and metal sheet as shown in Figure 14[45] [127] hence preventing the polymers from being damaged due to overheating

3.2. FSSW Weld Characteristics

FSSW’s welding parameters are tool rotational speed, plunge rate, plunge depth and dwell time. Rotational speed of tool is the most significant parameter in FSSW governing the strength of the composite weld joints[128,129,130,131,132,133]. Joining pressure is the second parameter to influence strength of joints[129]. Stirring/dwell time was found to be the least significant in the control of joint strength[130]. The disagreement on the FFSW parametric studies are due to a) use of different range of parameters, b) differences in tool dimensions, c) investigation on different polymer composites with different melting temperatures and glass temperatures and d) differences in the temperature produced in the process[134]. FSSW’s weld morphology is divided into three distinct regions; the stir zone (SZ), thermomechanical affected zone (TMAZ) and heat affected zone (HAZ) as seen in Figure 15. The stir zone is the spot weld nugget which is produced by stirring action of the rotating tool that bonds the two sheets together. The stirring generates the heat that changes the microstructure to fine equiaxed grains. The TMAZ is the zone that is thermally affected by the heat and the rotating tool and will have elongated and larger grains compared to the SZ.
In the joining of aluminium Al with CFRTP, tool plunge creates a keyhole on the aluminium side and joint was formed due to mechanical interlocking of melted CFRTP with Al Alloy as shown in Figure 16[136,137]. Materials are mixed together due to stirring at the stir zone[131]. Figure 17 shows the bond that was formed via mechanical interlocking between aluminium and CFRP with aluminium protruding into the carbon fibre polyphenylene sulphide (CF-PPS).
As for the FSSW made with interlayers, the interlayer materials form intimate bonding with the base materials with traces of the interlayer material found in the base materials. In some microstructures, hooks were observed at the metal-polymer interlayer interface and the hook was identified as the area where microcracks starts forming[68]. The bonding of the metal and polymer interlayer is via micro mechanical interlocking. In the case of FFSW with nanoparticles, the nanoparticles bond with the base materials at the interior and middle of the stir zone as shown in Figure 18.

3.3. USW Weld Characteristics

The process parameters for USW are welding force, vibration amplitude, frequency and vibration time. Low welding force and high vibration amplitude is recommended for aluminium and carbon fibre (CF)/PA6 composite joint[140,141]. Welding energy, welding force and vibration amplitude have significant effect on composite joint’s maximum shear load while the effect of hold time is insignificant[142]. Figure 19 shows the weld morphology between CF/epoxy and Al with a PA6 film in between both sheets at a lower energy level. Similar observation was also reported by in the joining of CFRP with steel[91].

3.4. LSW Weld Characteristics

LSW’s process parameters are laser power, pulse duration and laser beam diameter. Optimal laser peak power should be at 68-70% as lower power will create weaker joint and higher peak power will cause decomposition of PMMA and formation of bubbles at the interfaces[115]. Laser pulse duration has the most significant effect on joint strength followed by laser peak power and welding speed [120,126,143].In LSW, formation of bubbles in polymers or polymer based composites has been attributed to the spot weld formation between metal and polymer as shown in Figure 20. The bubbles that are formed at the interface bonds with the metal through the pyrolysis process[144].Mechanical interlocking was formed between the metal and polymer resin through the mixture of each other at the interface[145,146].

4. Advantages and Drawbacks of Processes

The variants of RSW have shown ability to spot weld composite joints. The inserts and interlayers used in RSW variants need to be resistive materials so that current can flow through these heating elements to generate heat. The metallic heating elements however might be relatively heavy, promote corrosion and create residual stress that create crack at the joint[147]. Magnetically assisted RSW (MA-RSW) produced spot welds with higher tensile strength compared to conventional RSW, hence has been proposed to be integrated with the welding gun[148]. Shunt current assisted RSW (SCA-RSW) and induction heat assisted RSW (IHA-RSW) techniques have the risk of overheating the polymer core in LITECOR®. SCA-RSW was also found not suitable for automation while the IHA-RSW, even though more effective in transferring heat to the weld zone compared to SCA-RSW, require proper placement of materials to be welded on the induction coil to prevent overheating of the polymer[51].
Comparative study between FSSW and RSW to join aluminium alloys has reported that with optimum welding parameters, FFSW joints had higher tensile shear strength compared to strength of joints made with RSW[149]. The FFSW has recently replaced RSW in the automotive industries to join Al alloy, steel alloy and polymer composites. Refill-FFSW even though gives improved weld strength compared to FFSW, the process is considered expansive and complex as the tool has three separate components that need to be controlled[150]. In FFSW, the reduced weld strength is mainly due to the weld thinning, keyhole defects and hook defect[151,152]. Other variations in the FFSW reported to join metal-polymer composite joints are threaded hole friction stir spot welding (THFSSW)[153,154] and static shoulder friction stir spot welding (SSFSSW) or pinless friction stir spot welding (PFSSW) [151,155]. Study between RSW, FSSW, PFSSW and THFSSW (at times referred as pre-hole FSSW (PHFSSW)) showed, FFSW gives better weld strength and has more flexibility on joining aluminium alloys and polymer composites yet is far expensive compared to RSW as shown in Figure 21 below.
USW can be used for mass production because of its high ability for automation. Recently, there has been a shift of interest on FSSW and high-power USW to replace RSW. However, only small components can be spot welded with USW due to the limits in power of the machines. Different clamping are required for different components and this increases production cost and inconvenience when used in manufacturing[146]. Compared to RSW and FSSW, USW has even shorter welding cycles, less energy consumption and higher efficiency[86,141]. LSW and RSW are very similar in process as both processes involve heating and melting the materials at the sheets interface. However, RSW uses an electrode to apply pressure at the sheets’ interface while LSW uses laser beam without any application of pressure at the interface. LSW was found to be 5 times faster in generating spots compared to RSW and can be easily automated. Comparison between micro-RSW’s and micro -LSW’s abilities to join thin foil of Inconel and thick steel showed that the achieved strength of a good weld using micro-LSW is higher than strength of a good weld obtained using micro-RSW[126]. Absence of subgrain region at the weld interface caused the HAZ in laser welding to be much smaller than HAZ in RSW. Furthermore, as the number of high angle grain boundaries (HAGB) were higher (90.89%) in LSW compared to RSW and with HAGB providing greater obstacle to dislocation gliding, laser welded joints had greater weld strengths compared to resistance spot welded joints under good weld conditions. As much as the LSW is favourable for welding in the hard-to-reach sections of an automotive or aircraft body parts, the welding is limited to the optical properties of the materials. For example, in glass fibre reinforced thermoplastic (GFRTP), depending on the glass fibre length and orientation, the fibres can scatter the laser beam and reduce the amount of radiation available for the melting of the matrix[157]. Table 6 shows comparison between the review spot welding processes.
This review has shown that within the period of 10 years (2013-2023), only a small number of publications have reported work on joining composite materials or to form composite joints using the spot-welding processes compared to the number of publications in the joining of similar and dissimilar metals within the same period. Figure 22 shows a comparison of number of publications reviewed in the review paper. More work seems to be concentrated on the FFSW and USW as solid-state welding has shown proven ability to join composite materials with vast difference in materials properties successfully compared to fusion welding. The use of interlayers including energy directors (ED) in RSW, FFSW and USW seems to be almost 50% of the total work reported.

5. Recommendation of Future Work

The review intends to identify new areas of studies in the use of spot welding to form composite joints. Even though joining composite materials and non-ferrous alloys such as aluminium and magnesium have been a challenging with RSW, the review found electrode modifications and use of interlayers have successfully spot-welded composite joints using RSW. The review also revealed that unlike the use of interlayer in RSW for dissimilar metals, limited work were reported in the use of interlayer for composite joints using RSW. One sided RSW is a newly developed process, developed for joining metal-polymer materials in the automotive industries. Coaxial one-sided resistance spot welding has been used for Al-CFRP and lower carbon steel-thermoplastic PA6 joints[34,36,37,158,159,160] and there are still prospects to study other combinations such as stainless steel-thermoplastic and magnesium-CFRP. Ren et.al[159] has stated that further studies are required on co-axial electrode materials combination as only the SUS404-CuCr combination for column and cylinder, have been used in all the reported work on coaxial one sided resistance spot welding. Due to limited reported work on the use of auxiliary joining elements such as insert in REW and interlayers for RSW of metal-composites, these are also areas that have scope for future investigations. Dharaiya et.al[161] has also raised concerns that the use of interlayers in RSW increases the weight of the BiW and affects production cost and proposed studies to identify critical locations on BiW to use interlayers rather than the entire BiW.
FSSW and USW are the solid-state spot-welding processes that have potentials in spot welding of composite joints and are extensively researched. Some of the potential research areas in FFSW and USW are:-
a)
The review on FSSW has found that work on the use of interlayers in metal-polymer or metal-composites spot weld joints are quite limited compared to the work on the use of metal interlayers in dissimilar metals joints. CFRP is the interlayer that has been used to most experimental studies. Limited work has been seen in the use of thermoplastics such as nylon and polyethylene. Furthermore, most interlayer related studies have only used aluminium, and no work has been done on joining magnesium with composite or high-strength-steel with composite.
b)
FFSW has been found to be a more suitable process to spot weld composite joints compared to RSW but as seen in Figure 21, FFSW is a more costly process than RSW. Even though other variants of FFSW have been introduced in the recent years, especially Refill-FFSW, limited work was found in the process improvement of this spot-welding process. An area in FFSW that will require further investigation especially in joining composite materials is the efficient tool design and optimum tool profile.
c)
Majority of the work reviewed in FFSW and USW on metal-polymer and metal-composite are purely experimental. There are still a lot of opportunity in finite element analyse (FEA) studies in 2D and 3D models especially in analysing the temperature gradient and stress distribution in the composite weld area during solid state spot welding especially with the use of interlayers and energy directors.
d)
Energy directors play a huge role in concentrating the heat in the weld area during USW. Even though there have been work reported on the use of different type of ED in composite spot weld joints, limited work or in fact no work have been found in the use of EDs for metal-thermoset joints.
e)
Another area of study which has research potential is the fatigue failure analysis of spot welds created by FSSW and USW. All the experimental work reviewed in the review for both FSSW and USW used static loading to analyse spot weld strength.
The reviewed work on LSW has found that only two work have been reported on the laser spot welding of metal-polymer and metal composites till date. Most of the laser welding work reported on composite joints as reviewed in Table 5 are involving seam welding. Limited work have been reported on the use on interlayers in LSW hence a potential research area in the future.

6. Conclusions

This paper reviewed the hybrid or composite spot weld joints made using the RSW, FFSW, USW and LSW to address the multi material design in automotive and aircraft structures. The formation of spot weld joints between metal-polymer or metal-polymer based composites using different spot-welding process variants included different combinations of welding parameters, variations in the machine setup and welding processes, use of metallic and non-metallic elements at the joining interface and different bonding mechanisms. The use of elements such as metal inserts, nanoparticles, interlayers and energy directors have shown that apart from able to join dissimilar materials together without causing any polymeric degradation and material damage, these elements have improved the spot weld strengths either by impeding grain growth, mechanical interlocking mechanism and chemical bonding. Mechanical interlocking is mainly due to the plastic deformation of the metal sheet which enters the polymer/composite region creating an anchoring effect (macro mechanical interlocking). Mechanical interlocking has also been formed due to the molten polymers penetrating into the micro cavities on the metal sheets (micro mechanical interlocking). Mechanical interlocking is the common hybrid bonding mechanism in RSW, FFSW, USW and LSW. FFSW and USW are the variants that have shown superior ability to produce composite joints. The former however is an expensive process due to the complexity in tool design and latter is only suitable for small components and will potentially increase manufacturing cost due to the need of different clamping. The review identified the use of elements such as interlayers and energy directors in hybrid/composite spot weld joints are however limited compared to the use of these elements in the dissimilar metal spot weld joints. Finally, future studies related to the spot welding of composite joints using the spot-welding variants were recommended.

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  128. Singh, J.; et al., PLA-PEKK-HAp-CS composite scaffold joining with friction stir spot welding. Journal of thermoplastic composite materials, 2021. 34(6): p. 745-764. [CrossRef]
  129. Goushegir, S.M., J.F. dos Santos, and S.T. Amancio-Filho, Influence of process parameters on mechanical performance and bonding area of AA2024/carbon-fiber-reinforced poly(phenylene sulfide) friction spot single lap joints. Materials & design, 2015. 83: p. 431-442. [CrossRef]
  130. Kumar, R.; et al., Thermo-mechanical investigations for the joining of thermoplastic composite structures via friction stir spot welding. Composite structures, 2020. 253: p. 112772. [CrossRef]
  131. Pandey, A.K., K.C. Nayak, and S.S. Mahapatra, Characterization of friction stir spot welding between copper and poly-methyl-methacrylate (PMMA) sheet. Materials today communications, 2019. 19: p. 131-139. [CrossRef]
  132. Bilici, M.K., M. Taşdemir, and M. Kurt, Relation between Friction Stir Spot Welding Parameters and Mechanical Properties of High Density Polyethylene/Glass Spheres Polymer Composites. Materials Science Forum, 2016. 860: p. 49-52. [CrossRef]
  133. Schäfer, H.; et al., Refill friction stir spot welding of thermoplastic composites: Case study on Carbon-fiber-reinforced polyphenylene sulfide. Thin-walled structures, 2023. 191: p. 111037. [CrossRef]
  134. Lambiase, F., A. Derazkola, and A. Simchi, Friction Stir Welding and Friction Spot Stir Welding Processes of Polymers-State of the Art. Materials (Basel), 2020. 13(10): p. 2291. [CrossRef]
  135. Shen, Z., Y. Ding, and A.P. Gerlich, Advances in friction stir spot welding. Critical reviews in solid state and materials sciences, 2020. 45(6): p. 457-534. [CrossRef]
  136. Ota, E.; et al., Friction stir spot welding of aluminum and carbon fiber reinforced thermoplastic using hybrid surface treatment improving interfacial properties. Materials & design, 2021. 212: p. 110221. [CrossRef]
  137. Ma, N.; et al., Thermo-mechanical modeling and analysis of friction spot joining of Al alloy and carbon fiber-reinforced polymer. Journal of materials research and technology, 2021. 12: p. 1777-1793. [CrossRef]
  138. Li, H.; et al., Influence of the rotation speed on the interface microstructure and joining quality of aluminum alloy 6061/CF-PPS joints produced by refill friction stir spot welding. Welding in the world, 2022. 66(5): p. 923-933. [CrossRef]
  139. Kurabe, Y., Y. Miyashita, and H. Hori, Joining process and strength in PVC friction stir spot welding with fabricating composite material at welding area. Welding international, 2017. 31(5): p. 354-362. [CrossRef]
  140. Liu, Z.; et al., Nonlinear friction behavior in ultrasonic welding of aluminum alloy to carbon fiber reinforced PA6 composite. Journal of materials processing technology, 2021. 296: p. 117230. [CrossRef]
  141. Rajalingam, P.; et al., Exploratory study on the effect of amplitude on ultrasonic spot welding of aerospace materials. Materials today: Proceedings, 2021. 45: p. 799-803. [CrossRef]
  142. Li, Y.; et al., An integrated process-performance model of ultrasonic composite welding based on finite element and artificial neural network. Journal of manufacturing processes, 2020. 56: p. 1374-1380. [CrossRef]
  143. Lambiase, F., S. Genna, and R. Kant, A procedure for calibration and validation of FE modelling of laser-assisted metal to polymer direct joining. Optics and laser technology, 2018. 98: p. 363-372. [CrossRef]
  144. Huang, Y.; et al., Interface formation and bonding mechanisms of laser welding of pmma plastic and 304 austenitic stainless steel. Metals (Basel ), 2021. 11(9): p. 1495. [CrossRef]
  145. Zou, P.; et al., Interfacial Microstructure and Formation of Direct Laser Welded CFRP/Ti-6Al-4V Joint. Metals (Basel ), 2021. 11(9): p. 1398. [CrossRef]
  146. Jiao, J.; et al., Laser welding process and strength enhancement of carbon fiber reinforced thermoplastic composites and metals dissimilar joint: A review. Chinese journal of aeronautics, 2023. [CrossRef]
  147. Russello, M.; et al., Resistance welding of carbon fibre reinforced PEKK by means of CNT webs. Journal of composite materials, 2023. 57(1): p. 79-94. [CrossRef]
  148. Li, Y.B.; et al., Review: Magnetically assisted resistance spot welding. Science and technology of welding and joining, 2016. 21(1): p. 59-74. [CrossRef]
  149. Karthikeyan, R. and V. Balasubramaian. Optimization of Electrical Resistance Spot Welding and Comparison with Friction Stir Spot Welding of AA2024-T3 Aluminum Alloy Joints.2017:p1762-1771. Elsevier Ltd. [CrossRef]
  150. Sioutis, I.; et al., Experimental evaluation of Refill friction Stir spot Welds (RFSSW) as crack arrest features in co-consolidated thermoplastic laminates. Composite structures, 2023. 309: p. 116754. [CrossRef]
  151. Bolouri, A., M. Fotouhi, and W. Moseley, A New Design for Friction Stir Spot Joining of Al Alloys and Carbon Fiber-Reinforced Composites. Journal of materials engineering and performance, 2020. 29(8): p. 4913-4921. [CrossRef]
  152. Ahmed, M.M.Z.; et al., Friction Stir Welding of Aluminum in the Aerospace Industry: The Current Progress and State-of-the-Art Review. Materials, 2023. 16(8): p. 2971. [CrossRef]
  153. Paidar, M.; et al., Pre-threaded hole friction stir spot welding of AA2219/PP-C30S sheets. Journal of materials processing technology, 2019. 273: p. 116272. [CrossRef]
  154. Karami Pabandi, H., M. Movahedi, and A.H. Kokabi, A new refill friction spot welding process for aluminum/polymer composite hybrid structures. Composite structures, 2017. 174: p. 59-69. [CrossRef]
  155. Yan, Y.; et al., Friction stir spot welding thin acrylonitrile butadiene styrene sheets using pinless tool. International journal of advanced manufacturing technology, 2018. 97(5-8): p. 2749-2755. [CrossRef]
  156. Lunetto, V., M. De Maddis, and P. Russo Spena, Pre-hole friction stir spot welding of dual-phase steels and comparison with resistance spot welding, conventional and pinless friction stir spot welding. International journal of advanced manufacturing technology, 2023. 129(5-6): p. 2333-2349. [CrossRef]
  157. Birtha, J.; et al., Optimizing the Process of Spot Welding of Polycarbonate-Matrix-Based Unidirectional (UD) Thermoplastic Composite Tapes. Polymers, 2023. 15(9): p. 2182. [CrossRef]
  158. Ren, S., Y. Ma, and N. Ma, Development of FEA-ANN-Integrated Approach for Process Optimization of Coaxial One-Side Resistance Spot Welding of Al5052 and CFRP. Journal of manufacturing science and engineering, 2022. 144(1). [CrossRef]
  159. Ren, S.; et al., 3-D modelling of the coaxial one-side resistance spot welding of AL5052/CFRP dissimilar material. Journal of manufacturing processes, 2021. 68: p. 940-950. [CrossRef]
  160. Ren, S.; et al., Digital Twin for the Transient Temperature Prediction During Coaxial One-Side Resistance Spot Welding of Al5052/CFRP. Journal of manufacturing science and engineering, 2022. 144(3). [CrossRef]
  161. Dharaiya, V., A. Panchal, and G.D. Acharya, Investigating feasibility of interlayers in Resistance Spot Welding of low-carbon steel sheets. SN applied sciences, 2021. 3(8): p. 749-15. [CrossRef]
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Figure 1. Vehicle Body in White (BiW) using Multi material design (MMD) [11].
Figure 1. Vehicle Body in White (BiW) using Multi material design (MMD) [11].
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Figure 2. Multi material joining methods.
Figure 2. Multi material joining methods.
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Figure 3. (a) Spot welding process and (b) spot weld on automotive part.
Figure 3. (a) Spot welding process and (b) spot weld on automotive part.
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Figure 4. (a) Series electrode arrangement [35] and (b) co-axially arranged electrodes [36].
Figure 4. (a) Series electrode arrangement [35] and (b) co-axially arranged electrodes [36].
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Figure 5. (a) Resistance Element Welding (REW)[43] and (b) Interlayer in the faying surface[44].
Figure 5. (a) Resistance Element Welding (REW)[43] and (b) Interlayer in the faying surface[44].
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Figure 6. (a) Magnetically assisted resistance spot welding (MA-RSW), (b) shunt current assisted resistance spot welding (SCA-RSW) and (c) inducted heat assisted resistance spot welding (IHA-RSW)[50,51].
Figure 6. (a) Magnetically assisted resistance spot welding (MA-RSW), (b) shunt current assisted resistance spot welding (SCA-RSW) and (c) inducted heat assisted resistance spot welding (IHA-RSW)[50,51].
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Figure 7. (a) Friction stir spot welding (FSSW) process [55] and (b) refill Friction stir spot welding (refill-FSSW) process[57].
Figure 7. (a) Friction stir spot welding (FSSW) process [55] and (b) refill Friction stir spot welding (refill-FSSW) process[57].
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Figure 8. (a) Ultrasonic plastic welding and (b) Ultrasonic metal welding [10].
Figure 8. (a) Ultrasonic plastic welding and (b) Ultrasonic metal welding [10].
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Figure 9. Diagram of laser spot welding process [112].
Figure 9. Diagram of laser spot welding process [112].
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Figure 10. LAMP joining mechanism[114].
Figure 10. LAMP joining mechanism[114].
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Figure 11. The microstructure of spot weld for metal-metal joint[26].
Figure 11. The microstructure of spot weld for metal-metal joint[26].
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Figure 12. Heat flow comparison between (a) metal-metal joint and (b) metal-polymer joint[37].
Figure 12. Heat flow comparison between (a) metal-metal joint and (b) metal-polymer joint[37].
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Figure 13. (a)Electrode indentation on metal side [35] and (b) Weld formed in between inner and outer electrodes [36].
Figure 13. (a)Electrode indentation on metal side [35] and (b) Weld formed in between inner and outer electrodes [36].
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Figure 14. Validation on spot weld simulation generated using metal inserts shows joint between insert and metal sheet [45].
Figure 14. Validation on spot weld simulation generated using metal inserts shows joint between insert and metal sheet [45].
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Figure 15. Macrograph showing various microstructural zones of conventional FSSW (a)Transverse cross-section, (b) Base metal, (c) Microstructural features in the SZ, TMAZ and HAZ[135].
Figure 15. Macrograph showing various microstructural zones of conventional FSSW (a)Transverse cross-section, (b) Base metal, (c) Microstructural features in the SZ, TMAZ and HAZ[135].
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Figure 16. FSSW aluminium- carbon fibre thermoplastic composite weld joint morphology[136].
Figure 16. FSSW aluminium- carbon fibre thermoplastic composite weld joint morphology[136].
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Figure 17. Refill - FSSW aluminium- carbon fibre thermoplastic composite weld joint morphology[138].
Figure 17. Refill - FSSW aluminium- carbon fibre thermoplastic composite weld joint morphology[138].
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Figure 18. (a) Cross section of FSSWed PVC joint with SiC particles at the middle section of SZ (red circle) and (b) Cross section of FSSWed PVC joint without SiC particles [139].
Figure 18. (a) Cross section of FSSWed PVC joint with SiC particles at the middle section of SZ (red circle) and (b) Cross section of FSSWed PVC joint without SiC particles [139].
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Figure 19. (a) PA6 interlayer was observed between CF/epoxy and aluminium (b) a mechanical interlocking between CF/epoxy and aluminium [90] (c) PA6 interlayer was observed between CF/epoxy and steel and d) ) a mechanical interlocking between CF/epoxy and steel [91].
Figure 19. (a) PA6 interlayer was observed between CF/epoxy and aluminium (b) a mechanical interlocking between CF/epoxy and aluminium [90] (c) PA6 interlayer was observed between CF/epoxy and steel and d) ) a mechanical interlocking between CF/epoxy and steel [91].
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Figure 20. (a) Laser welding of Titanium alloy -CFRP with weld area expanding from titanium to CFRP with molten zone at interface[145] and (b) laser welding aluminium alloy-PA with melt zone at PA side[116].
Figure 20. (a) Laser welding of Titanium alloy -CFRP with weld area expanding from titanium to CFRP with molten zone at interface[145] and (b) laser welding aluminium alloy-PA with melt zone at PA side[116].
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Figure 21. Comparison between FSSW, RSW and variants of FSSW[156].
Figure 21. Comparison between FSSW, RSW and variants of FSSW[156].
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Figure 22. Comparison on number of published work using different variants of spot welding processes.
Figure 22. Comparison on number of published work using different variants of spot welding processes.
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Table 1. Use of co-axially arranged spot welding electrodes.
Table 1. Use of co-axially arranged spot welding electrodes.
Reference Electrode materials Materials joined Results
Ren et.al[34] CuCr for inner electrode and SuS304 for outer electrode Surface treated Al5052 (silane coupling agent) -CFRP
a)
Joining mechanism is formation of covalent bond.
b)
CFRP overheating due to high current decreased the joint strength.
Szallies et.al[36] CuCr1Zr for inner and outer electrodes Low carbon steel(DX56)-thermoplastic(PA 6.6)
and aluminium(EN AW 6016)- thermoplastics(PA 6 GF47)
a)
The melting zone between inner and outer electrodes takes wave shape as the current and time increase.
b)
Surface treatment increased strength of joint via mechanical interlocking of the thermoplastic matrix to metal.
Ren et.al[37] CuCr for inner electrode and SuS304 for outer electrode Surface treated Al5052 (silane coupling agent) -CFRP
a)
Aluminium is forced into the molten CFRP by electrode force forming the molten zone.
b)
The molten zone which indicates the bonding area between Al-CFRP continued to grow on the CFRP side even during the cooling stage (current turned off) due to heat conduction from aluminium.
Table 2. Use of metal inserts and interlayers with RSW.
Table 2. Use of metal inserts and interlayers with RSW.
Reference Type of interlayer/metal inserts Materials joined Results
Shokati et.al[44] Titanium (Ti) powder interlayer Carbon-carbon C/C composite (2D and 3D) with Ti-6Al-4V alloy
a)
Molten Ti-6Al-4V penetrates into the grooves of composite (solid-liquid reaction) forming mechanical interlocking of both materials. The intrusion of molten Ti interlayer also contributed to mechanical interlocking.
b)
The grooved interface for composite reduced residual stress and increased the joint area, which in turn improved mechanical interlocking and the joint strength.
Troschitz et.al[45] Soft structural steel (S235JR) insert Glass fibre reinforced polypropylene ( GF/PP) and HC340LA steel
a)
The welding mechanism was due to melting and solidification at the insert-steel interface rather than the GFPP-steel interface.
b)
Joint strength depends on the insert geometry (head thickness).
Holtschke and Jutner [43] Boron alloyed heat treated steel 20MnB4 insert LITECOR® with ultra-high strength steel (UHSS) 22MnB
LITECOR® is a steel/polymer/steel sandwich material which is referred as SPS 		The welding mechanism was the melting and solidification at the insert-steel interface and no heat related damages to the LITECOR®.
Calado et.al[46] AISI304 stainless steel insert LITECOR® with LITECOR®
a)
The fusion at the interfaces between the skin sheets and the inserts with appropriate penetration depth and no damage to polymer core layers.
b)
Two different types of weld nugget formations observed depends on current and time; two weld nuggets, each between the insert and steel interfaces with heat affected zone in between due or a bigger nugget that extends from the top to bottom contact interfaces.
Schmal and Meschut[47] 20MnB4 coated with ZnFe coating insert LITECOR® with press hardened 22MnB5 with an AlSi150 surface
a)
Load bearing capacity of REW is higher than self-pierce riveted (SPR) and resistance spot shunt welded (RSSW) samples.
b)
The welding mechanism was the melting and solidification at the insert-steel interface.
c)
Electrode offset significantly affects nugget diameter and thermal damage of LITECOR®. Gap size and electrode polarity have no effect on nugget diameter, depth of penetration and thermal damage to LITECOR®.
Roth et.al[48,49] D04 steel and stainless steel 304 inserts Steel – fibre reinforced polymer
a)
The weldability lobe for the stainless steel insert was narrower than the weldability lobe for the DCO4 steel insert for the same weld time range.
b)
The welding mechanism was the melting and solidification at the insert-steel interface.
c)
Insert geometries affect temperature distribution at the centre of the weld, potentially damaging the composite.
Table 3. Use of nanoparticles and interlayers in FSSW.
Table 3. Use of nanoparticles and interlayers in FSSW.
Reference Nanoparticle/Interlayer Materials joined Results
Bagheri.et.al[58]



Abdollahzadeh.et.al[59]		 Silicon carbide (SiC) nanoparticle Al 2024 - pure copper C11000


Al 5083 - pure copper C11000
 
a)
The rotating tool extrudes the copper into aluminium creating a mechanical interlocking between materials.
b)
SiC nanoparticles prevent movement of dislocations at grain boundary by pinning effect leading to improved joint strength.
c)
Grain size of non-particle FFSW joint was coarse compared to the particle FFSW joint, leading to a lower tensile strength.
d)
SiC particles reduce the intermetallic compound (IMC) thickness which is formed by the diffusion of aluminium and copper and improves the weld strength.
Tebyani and Dehghani[60] Silicon carbide (SiC) nanoparticle IF steel-IF steel
a)
The grain size of particle FFSW joint was fine compared to the non-particle FFSW joint, leading to a higher tensile strength.
b)
SiC retards the grain growth via the Zener pinning effect.
Hong et.al[61]


Jeon et.al[62]		 Graphite nanoparticle Aluminium 552-H32 – Aluminium 6061-T4
Aluminium 5052 -H32 - Aluminium 5052 -H32
a)
Homogenous mixture of the graphite with aluminium forms a metal matrix composites (MMC) and improved joint mechanical properties.
b)
Carbon deposition enhanced mechanical properties (tensile load, ductility and toughness) of the FSSW joint compared to the joint between both aluminium alloys without carbon deposition.
Suresh et.al[63]



Enami et.al[64]



Hassnifard et.al[65]		 Aluminium oxide (Al2O3) nanoparticle Aluminium 7076-T6 - Aluminium 7076-T6

Aluminium AA2024-T3 - Aluminium AA2024-T3

Aluminium 7075-T6 - Aluminium 7075-T6
a)
A homogeneous mixture of Al2O3/Al at the joint zone, impedes growth of grain boundary due to heating creating joints with the high strength and ductile in nature.
b)
Alumina powder improved the weld joint strength compared to weld joint without alumina.
c)
Short dwell or longer dwell time reduces weld strength due to poor mixing of alumina with base metal or grain growth respectively.
d)
Higher content of alumina reduces tensile strength and ductility due to alumina agglomerations and less uniformity is dispersion in weld zone.
e)
Fatigue fracture mechanism is affected by alumina contents and applied load levels.
Sadeghi et.al[66] Titanium oxide nanoparticles (TiO2) IF steel-IF steel
a)
The highest nanoparticle content causes nanoparticle agglomeration leading to a weaker weld joint compared to the other lower nanoparticles contents.
b)
Lower nanoparticle contents, via Zener pinning effect inhibit grain boundary growth and increases the FSSW joint mechanical properties compared to a joint without nanoparticle content.
Xue et.al[67] Thermoplastic polyamide (PA6) interlayer Aluminium 5182 – Carbon fiber- reinforced bismaleimide (CF-BMI)
a)
Joining of aluminium alloy with CF-BMI was not possible due to inability of CF-BMI to react with metals.
b)
Chemical reaction between amide group and metal oxide forms C-O-Al chemical bond between PA6 interlayer and aluminium alloy.
c)
Melted PA6 bonds with CF-BMI through polymer diffusion and bonding between amide group with the bismaleimide matrix.
d)
PA6 interlayer improved weld strength due to improved interface fluidity and reduced interface defects.
Nasir et.al[68] Carbon fiber reinforced polymer (CFRP) interlayer Aluminium alloy 7075-T651 and titanium alloy Ti-6Al-4V.
a)
Joint is created through mechanical interlocking between aluminium and CFRP.
b)
The Ti-Al-C intermetallic compound (carbon is from CFRP interlayer) refines grain size hence improves the spot weld joint strength.
Khan et.al[69] Polyamide PA interlayer Aluminium alloy AA6061 – PP-Glass fibre reinforced polymer (GFRP-PP)
a)
Direct joining of AA6061 and GFRP-PP was not possible.
b)
Due to the carbonyl functional group, PA6 bonds with aluminium alloy. PA6 and PP via strong glass fibre network form a strong bond with aluminium.
Kalaf et.al[70] Carbon fiber reinforced polymer (CFRP) interlayer Aluminium alloy AA5052- aluminium alloy AA5052
a)
Bonding between aluminium and polymer is due to micro mechanical interlocking, when polymer melts and flows into crevices on aluminium, producing increased tensile strength of joint.
b)
The intermetallic compound (Al-Si-C) formed between aluminium and CFRP increases the microhardness of the composite joint.
Rana et.al[71] High density polyethylene (HDPE) interlayer Aluminium alloy AA5052-H32 – aluminium alloy AA5052-H32
a)
Hook formations at the unbonded regions of the joint are different for sheet interfaces without interlayer and with interlayer.
b)
Interlayer creates two interfaces (top sheet-interlayer and interlayer-bottom sheet) and one hook formed in each interfaces. For interface without interlayer, two hook is formed in only one interface.
Table 4. Use of energy directors and interlayers in USW for composite joints.
Table 4. Use of energy directors and interlayers in USW for composite joints.
References Energy director (ED)/interlayer (IL) Materials joined Results
Lionetto et.al [12,90] Polyamide 6 (PA 6) IL Aluminium AA5754 – Carbon fibre (CF)/epoxy
a)
The spot weld was formed by the melting of the PA6 due to frictional heat at faying surfaces.
b)
The bonding of aluminium and carbon fibre happens through mechanical interlocking when PA6 is pressed in aluminium under high sonotrode force.
Wang et.al[91] Nylon-6 (PA 6) IL Rolled cold steel (SPCC) – carbon fibre reinforced thermoplastic (CFRTP)
a)
PA6 interlayer fuses with CFRTP via interdiffusion of boundaries. This bonding occurs initially with a particular welding energy before the bonding on interlayer with SPCC.
b)
Interlayer and SPCC bond is created via micro mechanical interlocking due to the total effect of welding energy and pre heating temperature.
Conte et.al[92] Polyamide 6 (PA 6) IL Aluminium – Carbon fiber reinforced polymer (CFRP)
a)
Micro mechanical interlocking between CFRP and aluminium is the mechanism for joint formation when molten polymer flows and fills the aluminium topology.
b)
Surface treatment increased mechanical interlocking due to improved wettability of molten polymer when in contact with aluminium.
Zhao et.al[93] Polyphenylene sulphide (PPS) ED Carbon fibre reinforced polyphenylene sulphide (CF/PPS) - (CF/PPS) Bigger diameter sonotrode increased heating rate, reduced weld time due to the higher heating rate and produced a bigger weld area. This further leads to a higher ultimate failure load compared to the sonotrode with the lowest diameter
Alexenko et.al[94] PEEK ED with carbon fiber fabric (CFF) prepreg Polyether ether ketone PEEK-PEEK Bonding is formed due complete squeezing out of the ED and prepreg due to frictional heating and adhered with the PEEK material.
Tsiangou et.al[88] Polyetherimide (PEI) ED + an integrated PEI IL on the CF/epoxy Carbon fiber (CF)/PEI – CF/epoxy
a)
The ED deform and conform to surface irregularities. The resin in the ED flow above its Tg temperature under pressure of sonotrode, creating good contact on the overlapping surfaces and produce fully welded area.
b)
The use of the PEI ED in between CF/PEI and CF/epoxy+ PEI IL produced higher weld shear strength compared to CF/PEI and CF/epoxy+ PEI IL without ED.
c)
Large unwelded areas in the CF/epoxy adherend + PEI IL and degradation of the PEI resin were reasons for lower weld strength compared to the use of ED between the adherends.
Villegas et.al[95] Polyphenylene sulphide (PPS) ED Carbon fiber polyphenylene sulphide (CF/PPS) – CF/PPS
a)
ED melts under frictional heating due to sonotrode vibration and bond with the top and bottom adherends.
b)
Both flat ED and triangular ED had similar welding energy and maximum dissipated power at medium force and high amplitude combination. At lower force and amplitude, flat ED showed inefficient heat generation.
Palardy et.al[96] Polyetherimide (PEI) ED Carbon fibre/polyetherimide (CF/PEI) – CF/PEI
a)
Resin flow increased with the ED thickness leading to higher shear strength.
b)
In thinner ED, the heating and melting of ED and adherends happens simultaneously which will lead to overheating and degradation of welds. Thicker EDs however heats and melts before the adherends.
Tao et.al[97] Polyetheretherketone (PEEK) ED Carbon-fiber-reinforced polyetheretherketone (CF/PEEK) – CF/PEEK
a)
With the use of flat ED, heating rate at the interface is higher as heat generation is from friction and viscoelastic heating compared to without ED where heating is purely from friction effect.
b)
ED-less heating was not able to melt the PEEK resin creating incomplete fusion at interface and lower joint strength.
Kiss et.al[79] Polypropylene (PP) ED Polypropylene (PP)-PP
a)
With energy director, concentrated heat generated into a smaller heat affected zone, resulting lesser energy needed to create composite joint compared to joint made without ED.
b)
With the use of ED, shear strength was greater compared to joints made with ED.
Table 5. Laser Welding (LW) and Laser Spot Welding(LSW) on Metal-Polymer and Metal-Metal joint.
Table 5. Laser Welding (LW) and Laser Spot Welding(LSW) on Metal-Polymer and Metal-Metal joint.
Laser Welding of Metal-Polymer composite joints
Reference Laser Type / Heating method Materials Joined Results
Fernandes et.al[115] Nd:YAG pulse / conduction laser heating Polymethylmethacrylate (PMMA) - S235 galvanised steel
a)
Laser beam heats the steel, and the heat is conducted to PMMA to melt the polymer.
b)
Surface pre-treatment with sandpaper improves the mechanical interlocking at faying surfaces and therefore improves the weld strength.
Schricker et.al[116] Diode laser/ conduction laser heating Polyamide (PA 6, PA6.6) and polypropylene – high alloyed steel AISI304 and aluminium alloy AA6082
a)
Laser beam is focused to the metal surface and the heat is transfer through conduction to the polymer. The cooling and solidification at the faying surface will bond metal and polymer through metal interlocking and chemical bonding between oxide layer and polyamide.
b)
Metal sheet thickness decreases thermal efficiency due to heat loss leading to increase in energy per unit length to create molten zone.
Lambiase and Genna[117] Diode laser/ transmission laser heating Polycarbonate (PC) – stainless steel AISI304
a)
Laser beam passes through the transparent PC and heats the stainless steel. The PC melts through heat conduction and creates the bond between PC and steel via chemical bonding.
b)
Bubble dimensions at the faying area affect the weld strength where larger bubbles reduce the shear strength of the joint.
Lambiase and Genna[118]
 Diode laser/ conduction laser heating AA5053 aluminium alloy - Polyetheretherketone (PEEK)
a)
The laser beam heated the aluminium alloy and the PEEK was heated through heat conduction from aluminium.
b)
The joining mechanism is due to the penetration of aluminium into polymer.
c)
Increase in energy apart from increasing the joined area also causes PEEK degradation due to formation of bubbles. Bubbles reduces the strength of the joint.
Ma et.al[119] Diode laser/ conduction laser heating Carbon fiber reinforced polymer (CFRP) – aluminium with polycarbonate (PC) interlayer
a)
Laser heating was on the aluminium and heat is conducted to CFRP through PC.
b)
PC will melt and flow into CFRP and aluminium to form joint. Increase in PC interlayer improves the joint strength of CFRP and aluminium.
Huang et.al[120] Nd:YAG pulse/ conduction laser heating Polymethylmethacrylate (PMMA)- stainless steel AISI304
a)
The laser beam heats up the stainless steel and the heat is conducted to the PMMA to create a chemical bond between stainless steel and PMMA.
b)
Spot weld strength depends on the bubble sizes that are formed due to water vaporization and polymer thermal degradation.
Hussein et.al[121] Nd:YAG pulse / conduction laser heating & transmission laser heating Polymethylmethacrylate (PMMA) - stainless steel 304
a)
Transmission heating and conduction heating between stainless steel and PMMA created joints via polymer penetration into metal.
b)
Bubble formation increases the pressure and facilitate polymer penetration into metal.
c)
The morphologies of formed bubbles are different for transmission heating and conduction heating.
Meiabadi et.al[114] Nd:YAG pulse/ conduction laser heating AISI1008 low carbon steel – polycarbonate (PC)
a)
The steel was heated by laser beam and heat is transferred to PC via conduction to melt and bond PC and steel through chemical bonding of iron oxide and polymer molecules and hydrogen bonding as secondary bond.
b)
Bubbles formed in the polymer side is essential for penetration of polymer into metal, larger bubbles affect the joint strength.
Lin et.al[122] Fiber laser/ conduction laser heating Steel (DP590) – thermoset carbon fiber reinforced with polycarbonate (PC) interlayer
a)
Laser beam heats the steel and heat is conducted to the thermoset through the PC. The PC will melt and bond with thermoset via mechanical interlocking.
b)
Bubble expansion increases the pressure which cause molten PC to flow into DP590 forming a bond.
Laser Spot Welding of Metal-Metal dissimilar/similar joints
Reference Laser Type / Heating method Materials Joined Results
Pardal et.al[123] Fiber laser/ conduction laser heating Aluminium alloy 5083-mild steel Grade CR4
a)
The laser beam was focused to the top surface of steel and the textured surface was in contact with the surface of aluminum. The heat is conducted through the steel and transferred to aluminium to melt aluminium.
b)
Molten aluminium flows into the surface texture of steel creating interlocking between metals.
c)
The steel surface texture governs the amount of heat transferred to aluminium and the quality of bonding between the metals.
Chen et.al[124] Disc laser/ conduction laser heating Press hardened steel (22MnB5) - Press hardened steel (22MnB5) with Al-Si coating.
a)
Al-Si coating on press hardened steel formed δ ferrite strip at the weld notch and fusion zone hence contributing to low weld strength.
b)
Mechanical interlocking between steels is due to the formation of acicular ferrite at the fusion zone.
c)
Weld strength of joint made without Al-Si coating was found to be higher than weld strength of joints made with Al-Si coating.
Shengjie et.al[125] Fiber laser/ conduction laser heating Dual phase (DP)steel DP590 – aluminium alloy AA7075
a)
Twin spot focus splits the single beam into two separate spots to heat aluminium and steel separately.
b)
Molten aluminium flows into the gap between aluminium and steel, forming an intermetallic compound (IMC) that is responsible for joint formation.
Kumar et.al[126] Fiber laser/ conduction laser heating Inconel 718- steel 410
a)
The Inconel melts and penetrates into steel before solidification to form weld. The high angle grain boundaries at the weld interface prevent movement of dislocation hence produce stronger welds.
b)
Comparison in weld strengths between micro RSW and LSW showed that LSW produced joints with weld strength significantly higher than the strength of joints formed in micro-RSW.
Table 6. Comparison between RSW, FFSW, LSW and USW in terms of joining state, tool used for welding, source of heating and materials are commonly joined.
Table 6. Comparison between RSW, FFSW, LSW and USW in terms of joining state, tool used for welding, source of heating and materials are commonly joined.
Process Joint state Welding tool Source of heating Material
RSW Fusion Electrode Resistance Mainly ferrous and non ferrous metals
FSSW Solid state Pin/pinless rotating tool Friction Metal + Polymer composites
USW Solid state Sonotrode Friction Metal + Polymer composites
LSW Fusion - Laser beam Metal + Polymer composites
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