Subscribe to our newsletter to receive the latest news and events from TWI:

Subscribe >
Skip to content

Dissimilar Joining of Steel to Aluminium for Automotive Use

A detailed review and practical trials were performed to investigate dissimilar joining of steel to aluminium for automotive applications.

The main objectives for this work were:

  • Assess all major mechanical technologies for dissimilar steel to aluminium sheet joining used in car body manufacturing
  • Perform practical trials to study mechanical fastening for dissimilar material joining
  • Provide users with a detailed guide allowing them to consider and select the most appropriate joining process fitting their materials, performance requirements, production speeds and economic restrictions
  • Assess the impact of hybrid (adhesive + fastener) solutions

Introduction

The latest statistics show that the transportation sector is the largest global contributor of greenhouse gas emissions[1],[2]. New regulations impose strict limits for greenhouse gas emission on new cars. As part of a long-term strategy for greenhouse gas emission reduction, new powertrain and alternative energy sources are being developed, however vehicle lightweighting is largely considered as the most appropriate short-term solution. Predictions show a reduction in carbon dioxide emissions of about 9g/km can be achieved by reducing the weight of a car by 100kg  [3]–[6]. The automotive industry is adapting “new” materials with improved strength-to-weight ratios, like ultra-high strength steels and aluminium castings and extrusions in a body in white (BIW).

For dissimilar joining of steel to aluminium in car body applications, a range of mechanical joining technologies are becoming established as reliable solutions. These include rivets (self-piercing and blind), flow drilling screws, friction welding elements, tacking elements and many more. However, each of these processes comes with a relatively high cost and restrictions upon the material combinations that can be joined, with material thickness, strength and formability being critical factors.

The dominant joining process used in steel car body construction over the last 100 years has been resistance spot welding, its popularity is due to its low cost, speed and flexibility.  However, despite its advantages, resistance spot welding has never been successfully applied in production for steel to aluminium joining.

Another aim of this work is to promote greater understanding of the mechanisms of steel to aluminium joining by resistance spot welding, with a view to developing an industrially applicable solution.

The inherent problem for welding dissimilar materials is that the materials are different in nature; crystal structure, melting point, specific heat, and thermal conductivity. In a dissimilar weld, these factors lead to local stresses and the formation of multi metal crystalline phases, referred to as intermetallic phases. In the case of Fe-Al intermetallic compounds, poor fracture toughness is a particular issue [7]–[9] leading to very weak joints. In this investigation the resistance spot welding of steel to aluminium was evaluated by exploring the effect of parameters responsible for the heat input. In turn, the effect of heating on intermetallic phase development and the resulting effect on mechanical properties was explained.

Selecting the most appropriate technology for a particular dissimilar materials joint is not straightforward, given that the following factors have to be taken into account:

  • Can the technology in question create the joint required?
  • Is the joint fit for purpose (in terms of its mechanical properties)?
  • Is the technology already in-house? Or does a new investment and validation process need to take place?
  • Does the new joint impact on the vehicle design (i.e. is the joint size and weight significant)?
  • How fast is the process?
  • How much does the manufacture of an individual joint cost?
  • What is the size and weight of equipment required, and does the production line layout need associated alteration / extension?
  • Does the process have a working NDT process?
  • Is on-line quality monitoring available?

Technologies

Clinching

Clinching  (Figure 1) is a high-speed, mechanical fastening technique for point joining of formable sheet metal components that deforms one material into the other to form an interlock. The sheets are initially clamped between the blankholder and the die assembly. The punch is then forced onto the sheets, and locally pushes them into the die. As the deformed sheets touch the bottom of the die, further downward movement of the punch forces the material to flow radially and form a button, forming the mechanical interlock which holds the sheets tightly together.

Clinch Riveting

The clinch riveting process (Figure 2) is an adaption of the conventional clinching process. The main innovation is that, instead of retracting the punch at the end of the process cycle, a consumable tip (or rivet) is used. This rivet remains in the joint giving extra reinforcement. Just like in the clinch joints, the punch pushes the rivet into the sheets. The material flow is like a clinch forming the interlock, but in this case the interlock is formed the rivet is part of the locking mechanism re-enforcing it.

Blind Rivets

Blind riveting (Figure 3) is a non-threaded mechanical fastening technique, particularly used when access to the joint is only available from one side or when the materials to be joined are not sufficiently formable for self-tapping technologies. The rivet is placed in a pre-formed hole and is generally of a tubular form with a head on one side, and a headed mandrel through it. As the mandrel is pulled back, it causes the tail of the rivet to flare against the reverse side of the sheets, providing the mechanical interlock between the original and the newly formed head. As the load on the mandrel increases, it breaks at a notch just behind the head, inside the rivet (closing off the hollow rivet). With some types, the mandrel is pulled through entirely (leaving an opening).

Self-Piercing Rivets (SPR)

Self-piercing riveting (SPR) (Figure 4) is a method of joining two or more pieces of sheet material, using a punching rivet. Unlike conventional (blind) riveting, SPR does not require a pre-drilled hole as the rivet makes its own hole as it is being inserted. Interlocking of the rivet into the material is considered to be the main parameter dictating joint strength, however the level of interlock required to achieve a good joint is highly dependent upon the ductility of the materials being joined.

Solid Punch Riveting

The solid punch riveting process (Figure 5) is an evolution of the established self-pierced riveting process. Instead of punching the upper sheet, and then relying on plastic deformation to form a locking joint into the lower sheet, a solid punch rivet punches out both sheets and fills the hole it has created. This offers a significant advantage if a joint is to be made in materials that have little or no ductility, such as high strength steels or higher strength aluminium grades.

Resistance Spot Welding

Resistance welding (Figure 6) can be defined as a process whereby a force is applied to sheet metal surfaces and in which the heat for welding is produced by the passage of electric current through the material at, and adjacent to, these surfaces. The process takes advantage of the natural electrical resistance of material to produce the heat. Sheet joining is thereby achieved by clamping of the sheet materials between copper alloy electrodes and convey the necessary electrical current through the work pieces. Heat is developed mainly at the interface between the sheets, eventually causing the material being welded to melt, forming a molten pool, the weld nugget. The molten pool is contained by the pressure applied by the electrodes and the surrounding solid metal.

When spot welding steel to aluminium, heat is generated in the higher resistance steel material and conduction into the aluminium melts the aluminium in contact with the steel, effectively forming a brazed joint. Hot dip galvanised steels (pure zinc coating) are the easiest to join to aluminium. Galvannealed steels (iron zinc coating) are more challenging and press hardening steels with an aluminium-silicon coating have not been successfully joined.

High Speed Tacking

High speed tacking (Figure 7) is a method where a hardened steel nail is driven through material plates by a high-speed punch. High speed joining is applied with extremely high forces and punches through multiple sheet combinations of both steel and aluminium. High speed joining has been demonstrated on materials of up to 1600MPa strength without the need for a pre-formed pilot hole, but can only be applied to very stiff assemblies, otherwise the forces exerted by the punch can distort the parts.

Flow Drill Screws (FDS)

The FDS process (Figure 8) uses a friction drilling mechanism to pierce a hole in formable materials (typically aluminium), before thread forming and locking the joint in place. This process is followed by:

  • Initially the sharp point of the screw is rotated at high speed on the upper sheet surface;
  • Friction heating softens the material and it begins to flow away from the screw tip;
  • With heating, the screw penetrates deeper into the material;
  • Once the screw has fully penetrated the materials, the rate of rotation is slowed down and the screw thread locks the screw in place;
  • As the material cools, thermal contraction tightens the grip of the material onto the threaded part of the screw.

Friction Element Welding

Friction Element Welding (Figure 9) is an innovative mechanical fastening method that uses a friction drilling element which, when rotated at speeds of approximately 5000/6000 rev/min, can pierce soft materials. The same element can also be used to create a friction stud or rotary friction weld, when it comes into contact with a hard material plate. The process was designed specifically to allow high strength joining of aluminium to steel, with several sheets of aluminium being pierced to join to one sheet of steel below.

Resistance Element Welding

The resistance element welding process (Figure 10) is a novel two step method to produce reliable high strength joints between steel and aluminium, taking advantage of the good punching performance of most common automotive grades of aluminium.

Adhesives

Adhesive bonding is defined as a process relying upon a third material, ‘an adherent’, that is able to form a bond between mating surfaces. The bonds formed are by chemical bonding (van der Waals forces or hydrogen bonding) or by mechanical bonding to the surface features of the substrates. Adhesive bonding is probably the most important of all dissimilar joining technologies and it is very unlikely that any structural joint between dissimilar materials will be used in a car body without an adhesive. The adhesive serves the roles of:

  • Increasing the joint area to give better load bearing and structural rigidity properties
  • Providing a watertight seal, vital for dissimilar joints, where the galvanic coupling between the materials can lead to aggressive corrosion
Figure 1 - Clinching
Figure 1 - Clinching
Figure 2 - Clinch Riveting
Figure 2 - Clinch Riveting
Figure 3 - Blind Riveting
Figure 3 - Blind Riveting
Figure 4 - Self-Piercing Riveting
Figure 4 - Self-Piercing Riveting
Figure 5 - Solid Punch Riveting
Figure 5 - Solid Punch Riveting
Figure 6 - Resistance Spot Welding
Figure 6 - Resistance Spot Welding
Figure 7 - High Speed Tacking
Figure 7 - High Speed Tacking
Figure 8 - Flow Drill Screws
Figure 8 - Flow Drill Screws
Figure 9 - Friction Element Welding
Figure 9 - Friction Element Welding
Figure 10 - Resistance Element Welding
Figure 10 - Resistance Element Welding

Conclusions

Assessment of mechanical fastening techniques for dissimilar metals joining in car bodies This project has completed an extensive comparative assessment of the leading mechanical fastening processes available to the automotive sector for dissimilar joining between steel and aluminium alloy sheets. This has included a comparison of process capability, joint performance, joint completion rate and process economics, finding that:

  • In terms of their relative capabilities, no single process can join the complete range of materials and joint configurations typical of modern body-in-white constructions.
  • There are large differences in the mechanical performance of the joints from process to process but, particularly when shear loaded, these differences are significantly reduced if the processes are used in conjunction with a structural adhesive.
  • There remains however large differences in the size and weight of the equipment used to execute these processes (and thus the manipulation, e.g. robotics, then needed), the size and weight of the resultant joint, and the economics of each process.
  • Consequently, for a given selection criteria, different processes would be chosen. For example:
    • Lowest cost – Clinching;
    • Fastest joint completion rate – Adhesive bonding or RIVTAC;
  • Highest shear strength – Adhesive bonding or EJOWELD;
  • Highest tensile strength – Blind rivets or EJOWELD;

Lightest equipment weight – Adhesive bonding or blind rivets   

More Information

See here for more information on the dissimilar joining of steel to aluminium within the automotive industry.

For more information on our findings and how we can help your business and processes please contact us.

 

References:

[1]     E. Georgina, “2016 UK Greenhouse Gas Emissions,” 2018.

[2]     E. Georgina, “2016 UK GREENHOUSE GAS EMISSIONS, FINAL FIGURES Statistical Release: National Statistics,” 2018.

[3]     C. U. Automotive, “Lightweight Vehicles And Powertrain Structures : Uk Opportunities.,” 2013.

[4]     Office for Low Emission Vehicles, “Driving the Future Today - A strategy for ultra low emission vehicles in the UK,” 2015.

[5]     Z. L. Kowalewski, “Dynamic Properties of Aluminium Alloys,” 2012.

[6]     Association Europian Aluminium, “CO 2 & Road Transport,” 2007.

[7]       J. V. Ryabov, “Fusion Welding of Aluminium to Steel,” 1969.

[8]     M. M. Atabaki, M. Nikodinovski, P. Chenier, J. Ma, M. Harooni, and R. Kovacevic, J. Manuf. Sci. Prod., vol. 14, no. 2, pp. 59–78, Jan. 2014.

[9]      L. Shao, Y. Shi, J. K. Huang, and S. J. Wu, Mater. Des., vol. 66, pp. 453–458, 2015.

For more information please email:


contactus@twi.co.uk