Critical assessment 27: dissimilar resistance spot welding of aluminium/steel: challenges and opportunities

ABSTRACT

Dissimilar joining of aluminium and steel, especially using resistance spot welding as a critical process in vehicle manufacturing, is a key challenge for multi-materials lightweight design strategy. Controlling the formation and growth of Al₅Fe₂ intermetallic is the outstanding issue for producing high strength crash-resistance Al/steel dissimilar resistance spot welds. This critical assessment highlights the current understating regarding factors affecting the joint properties and approaches to control the interfacial reaction. Finally, the unresolved scientific challenges are discussed with the goal of shedding light on the path forward to produce reliable metallurgical bonding between aluminium and steels for automotive application.

KEYWORDS:

• Dissimilar metal welding
• Al/Fe welding
• spot welding
• intermetallic compound

Introduction

Lightweight design concept in the automotive industry is one of the key strategies to address the growing global demands for energy-saving and CO₂ emissions reduction [1,2]. Manufacturing a weight-reduced automotives with improved fuel consumption as well as enhanced crash safety is the key driver behind materials evolution in automotive industry [3,4]. Lightening can be achieved by the use of advanced high strength steels and low-density materials with high strength to weight ratio such as aluminium, magnesium and carbon fibre reinforced polymer in vehicle body [4]. In addition to current steel-intensive lightweight design approach, the multi-materials lightweight (MML) design strategy in which both advanced high strength steels and low-density alloys are used is becoming a novel approach towards environmentally sustainable transport [5,6]. The idea behind MML design is to select the best mass efficient materials for each part of the body-in-white which fulfils the given service requirement [7] (i.e. right material for right place [8]).

Since joining is a critical enabling manufacturing technology, the dissimilar joining of steel and low-density alloys (i.e. Al and Mg alloys) is an important area of research [7,9–12]. As Al alloys are currently the most promising light materials, the dissimilar joining of Al/steel is the key materials combination in the context of MML design. Dissimilar metallurgical joining of Al and steel can be achieved via formation of three distinct types of interfaces including solid/solid interface (e.g. diffusion bonding [13], magnetic pulse welding [14], ultrasonic welding [15], friction stir welding [16–19], where welding peak temperature is below the melting temperature of Al), solid/liquid interface (arc brazing [20], laser brazing [21] and resistance spot welding [22], where Al alloy experiences melting and wets the solid steel) and liquid/liquid interface (where the heat input is so high enabling melting of both Al and steel side). The key metallurgical challenges of dissimilar welding of Al/steel includes differences in thermal properties (e.g. melting point, coefficient of thermal expansion, thermal conductivity, electrical resistively), presence of Al oxide and intermetallic compound (IMC) formation at joint interface [23–31]. Due to limited mutual solubility of Fe and Al (Figure 1), the formation of intermetallic compounds the joint interface is inevitable. The formation of IMC is essential for establish a strong atomic-scale metallurgical bonding at Al/Fe interface. However, the brittleness of IMC coupled with the presence of internal stress created during thermal joining process due to large differences between their thermal properties make Al/steel dissimilar joint very susceptible to cracking and brittle failure during service [25,26]. It is interesting to note that the Al-rich IMCs, Al₅Fe₂ and Al₁₃Fe₄, which have low symmetry crystal structure [16], are more brittle than Fe-rich ones (e.g. Fe₃Al). Since, the diffusivity of Al in Fe is much higher than vice versa [25]; the formation of Al-rich IMCs at Al/steel interface is promoted compared to the Fe-rich. The experimental researches have shown that η phase (Al₅Fe₂), which is an Al-rich orthorhombic IMC phase with very high hardness (ca. 1000–1150 HV [25,26,32,33]) and low fracture toughness (ca. 2–5 MPa m^0.5 [34,35]), is the prominent component in such Al/steel dissimilar joints as a result of its rapid growth kinetics facilitated by the open and anisotropic crystallographic arrangement [36–38]. Therefore, controlling the thickness of IMC layer is the key to success in producing reliable joints particularly in crash-resistance components. The total thickness of reaction layer which is governed by the parabolic diffusion-controlled growth of the η phase [36] is a function of bonding process thermal cycle (i.e. peak temperature and dwell time) as well as composition of Al/steel base metals and coating composition of steel.

Figure 1. Al-rich part of the Fe–Al phase diagram. Modified from Li et al. [31]. The approximate peak temperature at solid/liquid interface (T_S/L) during RSW is highlighted.

Resistance spot welding (RSW), as the dominant welding process in automotive industry, plays critical role in vehicle manufacturing [39]. Therefore, a fundamental knowledge of the metallurgical transformations during dissimilar Al/steel RSW and their consequent on the joint failure behaviour is required to achieve sound, strong and reliable joints. Therefore, this assessment is focused on the metallurgical challenges during Al/steel joining via resistance spot welding.

Metallurgical challenges during Al/steel resistance spot welding

Figure 2 shows schematic representation of macrograph of dissimilar Al/steel joint welded using direct RSW. During Al/steel RSW dissimilar joining the following metallurgical phenomena occurs:

1. Melting phenomena in the base sheets and formation of a liquid/solid interface: The relative bulk resistance and thermal conductivities of Al and steel play important roles in bond formation mechanism. The heat generation in the welds is dominated by Joule heating in the steel sheet. Due to presence of thermal gradient between steel and Al, coupled with the high thermal conductivity of Al sheet, the generated joule heat in steel conducted into the Al sheet. This factor along with low melting point of Al alloys leads to liquid nugget formation in Al sheets. Depending on the amount of heat generation/loss in steel sheet, an isolated nugget in steel sheet can be formed [42,43]. The Al/steel interface is liquid/solid interface suggesting that the metallurgical bonding of Al/steel using RSW is achieved by a brazing mechanism [44–46].

2. Wetting of solid steel by liquid aluminium: Wetting is the key prerequisite for solid–liquid reaction and joint formation. The presence of Al oxide can significantly affect the wettability. During RSW the combined effect of heat and force can help to break-up the oxide layer. Wetting is established where the oxide layer is broken [46,47].

3. Dissolution of steel in Al melt and IMC formation at the joint interface: To achieve equilibrium at solid/liquid interface, the composition of solid/liquid is modified via dissolution of the base metal. This occurs by diffusion of iron into Al melt. These processes supersaturate the liquid in Fe which eventually leads to formation Al-rich IMC resulting an atom-atom metallurgical bonding [26].

Figure 2. Schematic of macrostructure dissimilar resistance spot welds between aluminium and steel along with details of interfacial reaction zone. The bonding zone length, IMC thickness and indentation in Al sheet are determining factors for weld mechanical performance. When reaction layer (lower right part [40]) is thicker than a critical value, the joint fails via crack propagation through brittle IMC. Controlling the thickness of Al₅Fe₂ phase (left part [41]), the dominant phase in the IMC layer, is of crucial importance for producing strong and reliable dissimilar Al/steel welds.

Mechanical properties of Al/steel RSW depend on how aforementioned stages can be controlled. The factors controlling the mechanical properties can be summarised as follows [40,47–57]:

• Bonding zone length: This is defined as the width of liquid aluminium nugget, which is a function of heat development at sheet/sheet interface in Al side. This determines the load bearing area.

• Thickness of IMC layer: This is a function of kinetics of liquid/solid interface reaction which is complicated of welding thermal cycle, composition of reacting base materials and coatings. The presence of thick IMC layer is the primary source for producing weak joints especially when the joint interface experiences mode I loading condition (e.g. under cross-tension loading).

• Electrode indention in Al sheet: The depth of electrode indentation in Al side is a crucial parameter to control during welding [58] that affects the stress state in the joint zone and thus influences the mechanical strength [59]. This factor is a function of heat input, electrode force and electrode geometry.

• Defects at joint interface: Defects includes formation of porosity/void in Al nugget adjacent to the joint interface, caused mainly by Al solidification shrinkage and melt expulsion and steel vaporisation, and presence of lack of wetting regions, where the IMC layer is discontinuous, cased primarily due to the un-even Al oxide removal [46,47]. It is of note that the formation of Kirkendall-porosity is minimised during RSW due to rapid cooling as welding as well as the presence of a compressive loading of joint [28].

The failure mode of spot welds, as an important qualitative measure for joint energy absorption during failure, depends on several factors including weld physical attributes, weld metallurgical characteristics as well as the loading condition [60–64]. Three fracture paths can be identified for Al/steel RSW:

1. Interfacial failure via crack propagation through IMC layer, promoted when IMC thickness is larger than the critical value.¹ In this situation, the energy-consuming crack-interception at the irregular η/steel interface becomes less frequent and the joint strength is dictated by the low fracture toughness of IMC layer [28]. In this condition, the thickness of IMC layer governs the joint strength.

2. Interfacial failure via rupture mainly between the intermetallic phases and the Al base material. This failure is promoted when the IMC layer is thin or un-bonded regions and pore/void are presented at the joint interface.

3. Pullout failure via nugget withdrawal from Al sheet. Large bonding zone length coupled with thin and continuous IMC is required to obtain pullout mode, the most favourite mode. It is of note that obtaining pullout failure mode due to severe Al sheet thinning is not preferred. When the IMC thickness is less than the critical value, it can be expected that bonding zone length is the decisive factor in determining the joint properties.

Approaches to improve the joint properties

Controlling welding heat input: the imperative prerequisite

Welding heat input which is determined primarily by welding current, welding time and electrode force is crucial for joint properties by affecting several factors including (i) the L/S interface temperature (T_S/L) and thus bonding zone length, (ii) thickness of IMC layer which is affected by both T_S/L and dwell time above effective reaction temperature, (iii) indentation depth in Al sheet, (iv) void/porosity formation at the joint interface due to liquid Al expulsion and Zn-coat vaporisation and (v) HAZ softening phenomena in Al sheet due to grain coarsening, loss of strain/precipitation hardening effects. To optimise the welding parameters all of these factors should be considered. Low welding times and high welding currents help to minimise the IMC growth.

Cover plate: a means for controlling L/S interface temperature

As mentioned above the bonding zone length is one of the determining factors for mechanical strength Al/steel RSW. To enlarge the bonding zone, the L/S interface temperature (T_S/L) should be increased. Due to low heat generation in and high thermal conductivity of Al, during conventional RSW, the increased T_L/S can be achieved merely by utilisation of enormously high welding current which accelerates electrode degradation process and increases the indentation. An alternative approach is to use cover plate technique [41,65–67]. Cover plate is a high electrical resistance-low thermal conductivity material (such as steel) which is placed on the Al sheet. When electrical current is passed through cover plate/Al/steel, the generated Joule heat in the cover plate is effectively conducted towards the Al/steel interface resulting in increased T_S/L (200–300°C higher than when welding without using cover plate, according to finite element simulation) and hence enlarged bonding zone [41,65–67]. The main benefit of this technique is to improve joint mechanical properties, caused by enlarged bonding zone, under relatively low welding currents. This is accompanied by low electrode indentation and electrode wear. It is of note that this technique does not have a significant impact on the growth of IMC. The effectiveness of this approach depends on thermal/electrical properties and thickness of the cover plate.

Zinc coating: improving wetting by fluxing action

One of the key obstacles to achieve strong bond in Al/steel RSW is the presence of un-wetted/un-bonded regions (i.e. absence of reaction layer) due to presence of Al oxide. Removing the oxide layer needs a high heat input, resulting in the formation of a thick Fe–Al IMC layer at the joint interface, making it impossible to obtain satisfactory joint strength [46,47]. The presence of zinc, as a protective coating on the steel surface, can be regarded as beneficial due to two distinct features:

1. Fluxing behaviour: The Zn-coating is quickly dissolved by the liquid Al at low temperature, facilitated by low temperature Al–Zn eutectic reaction, leaving a ‘clean’ steel surface without any contamination such as oxides, ensuring good wetting and thus more uniform growth of the reaction layers [37,46,47,57,68–70]. The key to successful fluxing during RSW is squeezing out the oxide-containing melt to the nugget periphery by electrode pressure [46,47]. This is where the electrode shape and geometry can play role [47]. This beneficial Zn-induced effect can be diminished at high welding heat input due to Zn vaporisation [57,68].

2. Reducing IMC growth: Generally, it is shown that the presence of Zn in the liquid phase induces an accelerating effect on the IMC growth [37]. However, in the case of RSW, the presence of Zn-coating decreases the contact resistance and lowers TS/L temperature [57]. This coupled with the fact that melting of Zn-coating lower the effective heat input available for interface reaction resulted in formation of thinner but more regular IMC at the joint interface [37,57].

It should be mentioned that the composition and microstructure of the coating can affect its fluxing behaviour and IMC growth. It seems that the fluxing behaviour of the coat is controlled by its melting temperature, while, the thickness of IMC layer is affected by the electrical resistivity of the coat as well as the reaction path. For example, the cleaning action of Zn was more pronounced for Zn–5Al–2Mg galvanised coating due to its lower melting point [71]. Dissimilar RSW of Al/galvannealed (Zn–10Fe) steel represented less effective Zn-induced fluxing, due to higher melting point of the coating, with thicker reaction layer, due to its higher electrical resistivity and different reaction path [47]. With optimising the welding parameters and electrode geometry, a joint with high cross-tension strength can be achieved in RSW of Al/Zn-coated steel. It should be noted that the problems associated with Zn-coating, porosity formation induced by Zn vaporisation [57] and surface breaking cracking induced by liquid metal embrittlement, should be considered during welding optimisation process.

Silicon: the miracle element for Al/steel welding

It is well-known that Si additions can be used to reduce the Al₂Fe₅ layer growth rate when the Al–Si alloy is liquid [28,36]. The anatomy of reaction zone in Al–Si(L)/steel(S) interface is Al alloy/Al₈Fe₂Si(τ₅)/Al₁₃Fe₄(θ)/Al5Fe2(η)/steel [36]. Although, the mechanism of Si effect on inhibiting the reaction layer growth is not clear, it has been proposed that it can be attributed to Si atoms occupying structural vacancies on the c-axis of the η phase resulting in reduced atomic mobility and hence hindered IMC growth rate [18,36]. Moreover, the formation of thin layer of τ₅ at the liquid/solid interface, as an inhabiting layer, acts as a barrier to further diffusion, thus suppressing rapid growth of Fe–Al MCs.

The use of Si-containing Al-based filler metal is a viable approach to reduce IMC thickness during arc/laser welding of Al/steel [20–24]. Same approach can be utilised in dissimilar resistance spot welding of aluminised (Al–Si) coated steel and Al alloy [72,73], in which the dissolution of coating enters Si in the Al melt. It is shown that formation and growth IMC during Al/steel RSW can be further inhibited by using an Al–Si coated nitrogen-containing steel. It is claimed that an IMC free weld nugget periphery with desired pullout failure mode during cross-tension test can be obtained due to combined effect of Si and formation of a nitrogen-rich inhibition layer [74] on the steel surface which prevents inter-diffusion between Fe and Al. It has been proved that using Al–Si coating on steel is a promising solution for welding problem associated with dissimilar welding of Al/Si.

The idea of using Al–Si (e.g. Al–12Si eutectic alloy) thin foil between Al and steel sheet interlayer as a brazing material [75,76], based on the Si-induced IMC growth deceleration, failed to significantly improve strength under the same welding conditions as for conventional process. Some improvement in tensile-shear properties can be achieved at extremely high welding current. Nevertheless, this idea needs to be evaluated further.

Bi-metal transition materials: a promising solution

The use of aluminium clad steel as a transition material between Al and steel can be considered as a reaction barrier which can inhibit the metallurgical reaction at Al/steel interface [9,55,77–80]. Two separate weld nuggets are formed at steel/steel and Al/Al interfaces. The shape of weld nugget in Al sheet suggests that the Joule heat for nugget formation on the aluminium side is conducted from the steel side. A thin IMC layer is also formed at Al/steel clad interface due to welding heat input. To reduce the susceptibility to interfacial failure from clad interface, the width of transition joint should be large enough to reduce the stress on the clad interface during loading [9,80]. The key for producing reliable joints is controlling the IMC growth at Al/steel clad interface, the size of weld nugget in Al side and the electrode indentation depth at Al side. These factors are in turn governed by the welding heat input, thickness of insert metal and Al/steel cladding ratio [55,77–80]. This technique can produce strong welds with static/fatigue strength equal to that of in similar Al/Al RSW even in cross-tension loading conditions. Nevertheless, the introduction of additional insert material brings in weight increase, which deviates from the requirement by automobile lightening.

Research roadmap: where do we go from here?

Producing crash-resistance and in-service reliable dissimilar Al/steel spot welds plays key role in manufacturing multi-materials lightweight vehicle. The widespread application of Al sheets in steel-dominant multi-materials lightweight automotive depends on how aforementioned challenges can be addressed during RSW. The bottleneck is to obtain a defect free thin IMC at the joint interface without considerable electrode indentation/HAZ softening in Al sheet.‏The research activity on this subject over the past years has led to a greatly improved understanding of Al/steel welding problems; however several key problems and issues remain to be addressed. The following points should be considered in the future research and development:

1. Brittleness of IMC layer vs. loading conditions: One of the shortcomings of the published research on RSW of Al/steel is the lack of sufficient information on the cross-tension behaviour of these joints, as the most relevant quasi-static loading condition for this brittle joint. It should be emphasis that the weld notch in the tensile-shear and cross-tension tests experiences mode II and I loading, respectively. This means that the cross-tension is more sensitive to the presence of brittle microstructure at the joint interface compared to the tensile-shear test [39,81]. It seems that tensile-shear strength depends strongly on the bonding zone length rather than IMC thickness. However, the cross-tension strength is strongly affected by the low fracture toughness IMC and thus the thickness of reaction layer play key role in determining the failure behaviour. An enabling reliable technique for Al/steel spot welding should be able to produce joints with improved cross-tension properties. Otherwise, the effectiveness of the approach is questionable. Moreover, there are limited studies on the evaluating the mechanical properties of Al/steel RSW under fatigue and impact loading conditions. Therefore, the major challenges remain in demonstrating the integrity and mechanical performance of dissimilar joints to the degree of confidence required for the operation in crash situation.

2. Lack of systematic research on the role of base metal composition: The composition of the base materials plays critical role in the growth of reaction layers [37,82,83]. For example, it has been shown that total thickness of IMC layer in RSW of Al to austenitic stainless steels is thinner than when RSW of Al to low carbon steel [50]. Also, it has been reported that the IMC thickness in dissimilar RSW of steel/Al–Mg alloy is a function of Mg content (increasing the Mg content in Al sheet accelerates the growth of IMCs) [48]. However, most of the research has been conducted on multi-component steel and Al alloy system that can confound the determination of any systematic solute effect. Therefore, it is clear that there is need for systematic investigations on the effect of base metals compositions, particularly C, Mn and Si content in the steel substrate and Cu, Zn, Mg and Si in the Al alloys. Systematic experimentation coupled with computational thermodynamic calculations are needed to develop a fundamental knowledge regarding the role of base alloys composition of IMC growth. This provides a basis to move towards design alloys specifically tailored for dissimilar metal joining.

3. Design sophisticated protective coating for steel to control the formation and growth of IMC: Considering the effective role of Zn and Al–Si coating in reducing the growth rate of IMC during Al/steel RSW, it is worthy to investigate (a) the detailed mechanism behind this achievement, (b) the influence of coat composition (Zn-based, Al-based, Sn-based) as well as coat thickness on interface reaction metallurgy. Moreover, thermodynamic moulding can be used to design new protective coating with specific composition which can lead to formation of IMCs with less brittleness or reduced growth rate via either changing the interface reaction path or through acting as diffusion barrier. In addition to the role of coating, employing an interlayer is common approach in dissimilar materials joining. Some interlayer have been attempted for improving RSW of Al/steel including pure Al (to reduce the Mg content in the melt Al alloy) [48], Al–Si (to use the beneficial effect of Si) [72,73], Al–Mg (to act as a brazing material to fill the gap between sheets) [56] without sufficient metallurgical/mechanical characterisations. To design/choose proper interlayer for RSW, the electrical properties of the interlayer should be considered, in addition to its compatibility with the base metals as well as its effect on the interface reaction.

4. Need for developing integrated computational weld modelling-from process to performance: The development of integrated process-microstructure-properties models is a key in the today's welding engineering. While, some reliable models are available for RSW process simulation, there is limited works on modelling of IMC formation during dissimilar Al/steel RSW [42,43,52]. Development of integrated modelling for Al/steel RSW requires fundamental understanding of thermodynamic and kinetics of IMC formation as well as fracture mechanics of IMC-containing joints.

5. Corrosion: Galvanic corrosion is a critical challenge in dissimilar welding of Al/steel [84] which is less investigated in previous researches. The galvanic corrosion in Al/steel RSW can be minimised when using bi-metal transition material. Moreover, hybrid weld-bonding technique in which the adhesive can act as a seal can improve the corrosion resistance of the dissimilar joint [85]. However, the effect of adhesive on heat generation and IMC formation requires further investigations.

6. Research on alternative welding processes: Various solid-state welding techniques are available for dissimilar Al/steel welding. Among them, friction stir spot welding (FSSW) has potential to become a competitor to resistance spot welding process. However, there is no comparative study regarding metallurgical/mechanical characteristics dissimilar joints made by both RSW and FSSW. Despite, the lower interface temperature during Al/Fe which helps in minimising the reaction layer thickness, the longer processing time in FSSW encourages IMC growth. Despite the fact that FSSW has found some commercial applications, this process is time consuming and costly compared to the RSW. Moreover, there is no comprehensive mechanical characterisation (especially in mode I loading) regarding the application of FSSW for dissimilar joining of Al/steels to a necessary level to ensure fitness for purpose. Time and cost effective joining of Al/steel using FSSW will require considerable further development.

Acknowledgements

The author would like to thank Professor Harry Bhadeshia for his kind invitation to write to this critical assessment and his helpful comments.

Disclosure statement

No potential conflict of interest was reported by the author.

ORCID

Majid Pouranvari http://orcid.org/0000-0002-9603-447X

Notes

1 No specific critical IMC thickness is reported for Al/steel welding. The reported value ranges between 1 and 10 µm. Indeed, the critical IMC thickness is a function of base material composition, the brittleness of the IMC and its continuity as well as loading condition.