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Steel to Aluminium Joining for the Automotive Industry

Thu, 26 September, 2019

TWI has been working on a project to investigate dissimilar joining of steel to aluminium for automotive lightweighting applications.

The relevance comes from new environmental laws set by the European Union which are forcing car manufacturers to follow strict limits for greenhouse gas emissions on new cars.

Predictions made show a reduction of carbon dioxide emissions of about 9g/Km can be achieved by reducing the weight of a car by 100kg. Therefore, weight saving presents itself as a faster approach to reduce greenhouse effects.

Making a car lighter is not straight-forward because the car needs to be lighter without losing structural integrity, while also taking into account the final price of the car.

Therefore there is the need to use ‘new’ materials with improved strength-to-weight ratio. However, when adopting a new material, a significant increase in vehicle cost can be incurred due to the extra complexity of the production processes required as a result of the required joining processes. In many cases, this extra cost means that adoption of the new material is simply economically unviable. 

The inherent problem associated with joining dissimilar materials is that the materials are different in nature, so they will inevitably have different thermal, mechanical, crystalline and atomic properties. In the particular case of steel to aluminium joining, the intended property to take advantage is the difference in weight, since aluminium has 1/3 the density of iron. However the differences in melting point, specific heat, and thermal conductivity can lead to local stresses and subsequently to different types of defects. These challenges only take account of the solid state properties, whereas in a weld there is the unavoidable formation of intermetallic phases (intermetallics) that are extremely brittle, leading to very poor joints. The difference in thermal conductivity, specific heat and coefficient of thermal expansion can create a joint with residual stresses, weld shrinkage, weld distortion and deformation which undermine the joint even further.

TWI’S work focusses on an evaluation of current joining technologies for dissimilar material joining. With this intent relevant technologies including Blind riveting; Clinching; Clinch-riveting; Flow drill blind rivet; Flow drill screws; Friction element welding; Refill friction Spot welding; Resistance spot welding; Self-piercing riveting; Solid punch riveting; Tack high speed joining will be investigated, with and without a structural adhesive.

The main objectives for this work included:

  • Assessment of all major mechanical technologies for dissimilar steel to aluminium sheet joining
  • Production of the most complete guide to the use of mechanical fastening for dissimilar material joining available in the public domain
  • Providing 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
  • Assessing the impact of hybrid methods (adhesive + joint element)

For more information please email: