It is possible to join aluminium easily to most metals using mechanical fastening or adhesive bonding. However, to weld aluminium to steel requires other techniques such as arc and friction welding which is further explained below.
Aluminium (and its alloys) are much lighter than steels, having a density of around 2.70 g/cm3 compared to a range of 7.75 to 8.05 g/cm3 for steels. This means that a comparable volume of steel is approximately three times heavier than aluminium.
Many industries utilise steel for a range of structural applications. However, owing to the density of steel, there is a significant weight penalty associated with its use. New environmental laws are forcing transport industries to follow strict limits on greenhouse gas emissions. One way to aid emission reduction is to reduce the weight of a vehicular structure. Replacing various steel structures with aluminium alloys is now of much industrial importance. In many applications, it is not necessarily possible to replace the entire steel structure with aluminium alloys, so there is a need to join the two materials.
Aluminium alloys can be joined to steels relatively easily using techniques such as adhesive bonding, mechanical fasteners or brazing, but when superior structural integrity is required, welding is preferred. However, welding of aluminium alloys to steel is difficult.
Aluminium alloys and steel are very different with respect to metallurgy and physical properties, such as thermal conductivity and melting temperature. Generally, steel’s melting temperature is around 1370°C, more than twice that of aluminium that melts at around 660°C. Apart from their widely differing melting points, each of these metals is virtually insoluble in the other. In the molten state, they react to form brittle intermetallic phases. It is clear that the above issues can present challenges in fusion welding, such as arc welding of steel and aluminium. The resulting welded joints would have unsatisfactory properties and, owing to their brittle nature, are often undesirable for many industrial applications.
The application of fusion welding processes to join steel to aluminium is well-known to be difficult due to the dissimilar melting points, thermal conductivities, expansion coefficients and the tendency to form brittle intermetallic compounds. As Fe solubility in Al is very low (around 0.04wt%), at temperatures >350°C, when Fe diffusion into Al becomes significant, precipitation of Fe-Al intermetallic compounds commences. Significant intermetallic precipitation can occur well below the melting point of aluminium (660°C for pure Al). The exact degree of intermetallic precipitation is driven by diffusion and is dependent upon the time and temperature history of the interacting Fe and Al interface.
The use of lasers to create a brazed type joint between steel and aluminium is a logical step, as the high intensity of heat in a small area generated by a laser means that a stable brazing environment can be locally generated and quickly moved to create a joint with minimum time for diffusion to drive excessive intermetallic compound formation. The Fe-Al phase diagram shows the range of hard intermetallic phases that can be formed, namely; Fe3Al (892HV), FeAl (470HV), FeAl2 (1060HV), Fe2Al5 (1013HV) and FeAl3 (892HV). These phases are characterised by extremely high hardness, close to zero ductility and very poor fracture toughness. Consequently, if a thermally produced joint between steel and aluminium must contain some or all of these phases, the thickness of the intermetallic compound layer should be as thin as possible if a good mechanical performance is to be achieved in the joint.Certain approaches must be taken to arc weld steel and aluminium, with the aim of avoiding intermetallic compound formation. The first is to use an aluminium coating on the steel. This can be achieved by dipping the steel into molten aluminium (hot dip aluminising). Once coated, the aluminium can be arc welded to the aluminium coating. Care must be taken to prevent the arc from heating the coated aluminium to an excessive temperature, otherwise there is potential for intermetallic compound formation.
Bimetallic transition inserts are another means to reduce intermetallic formation when fusion welding. The inserts are comprised of one part aluminium and another part steel, bonded together using rolling, explosion welding, friction welding, flash welding or hot pressure welding. The bimetallic transition joint is then individually welded to the bulk aluminium and steel. Typically, the bulk aluminium is welded to the aluminium part of the transition insert first, as this creates a larger heat sink for when the bulk steel is arc welded to the steel half of the transition insert.
The primary aim when joining these materials is to keep the welding temperature as cold as possible and minimising the exposure time of the weld to high temperatures. This is why processes such as friction welding (primarily rotary friction welding) are used to produce bimetallic transition inserts between aluminium alloys and steel bulk components.
Rotary friction welding is a solid-state joining process which works by rotating one workpiece relative to another while under a compressive axial force. The friction between the surfaces produces heat, causing the interface material to plasticise. The compressive force displaces the plasticised material from the interface, promoting metallurgical joining mechanisms. By not entering the liquid state, friction welds remain much cooler during processing. Moreover, friction welding is rapid, preventing long exposure times of the weld to high temperatures. Consequently, friction welding is used commercially to join a range of dissimilar materials, as intermetallic compound formation is significantly reduced.
Despite the benefits of friction welding for reducing intermetallic formation between aluminium alloys and steels, care must still be taken with parameter selection. Often, when welding steel and stainless steel to an aluminium alloy an interlayer of pure aluminium is used, which drastically reduces intermetallic formation. Intermetallic compounds between friction welded steels and aluminium alloys are iron-aluminium based, consequently, it would be expected that the brittle compounds would also be formed between the steel and pure aluminium, but this is not the case. Pure aluminium is much softer than aluminium alloy. This means that the temperature required to get the softer pure aluminium to flow and form a weld is much lower than that of an aluminium alloy. The lower temperatures help to reduce brittle compound formation.
Owing to the difficulty in producing strong welds between these materials, many commercial applications for joining aluminium alloys to steel involve mechanical fasteners. When using mechanical fasteners, and depending on the application, care must be taken to prevent galvanic corrosion. Galvanic corrosion preferentially occurs on the aluminium alloy. To prevent this, insulation of the aluminium alloy from the steel is required, which usually occurs by using an insulating coating or paint.
How can TWI Help?
TWI has a long history of working with its Members to overcome the challenges involved with joining dissimilar material combinations, including the use of resistance spot welding, friction welding, laser welding, electron beam welding, and brazing, among many other processes.
If you need help with any of these processes get in touch by sending us an email: