Impact of joining dissimilar steel and aluminium for future automotive structures

Abstract

The transport sector is the largest contributor of greenhouse gas emissions globally [1],[2], and new limits for greenhouse gas emission from new cars have been imposed. It was shown that 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 therefore adapting “new” materials with improved strength to weight ratio, like ultra-high strength steels and aluminium castings and extrusions in the body in white (BIW) to achieve this goal. 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. The broad spectrum of technologies has different locking mechanisms, properties and weaknesses, however very little is known on the impact of the dissimilar properties on different joining mechanisms for comparison and selection purposes. This is due to the small handful of publications that discuss this impact. This work successfully identifies the limitations for the individual joining technologies for different material combinations, provide the necessary guidelines for technology selection and can be used as a detailed manual for guidance in technology selection. Results show, from a lightweight and economical viewpoint, that the clinching process offers the best solution as there is no added weight or cost of an additional element at the expense of low joint performance. However, if joint access is a requirement then blind rivet technology is the only possible solution, although this will be at the expense of added weight from the element and the need to have a pre-formed hole that must be correctly matched in both components in order to form the joint. If the requirement is performance alone, then piercing technologies provide a better solution. Among these piercing technologies self-piercing rivets offer the best strength performance, however, there is a trade-off with added weight, requirement of dual sided access for joining sheets and the specific requirement of the sheets’ properties. This reinforces the main principal conclusion drawn from this work, which is, that there is currently no solution that is able to provide all the required needs. Another available, but little unexplored technology is Resistance Spot Welding which offers the advantage of being a non-added weight process as it is an autogenous and it is potentially an extremely low-cost technology in addition to being a technology that is already established in nearly every high volume automotive production line for similar materials joining (particularly steel to steel joining). Since this is a fusion process, it is unavoidable to form different intermetallics of Fe-Al welding that will provide an increased challenge in obtaining structural high-quality welds. The inherent problem for welding Fe-Al dissimilar materials is that the materials are different in nature; crystal structure, melting point, specific heat, and thermal conductivity. In welding process, these factors lead to local stresses and the formation of different intermetallic phases where very hard low ductility crystal structures predominate. In a weld the presence of intermetallics leads to poor fracture toughness [7]–[9] giving way to very weak joints. Therefore the 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. According to literature; in order to form a good weld between aluminium and steel it is critical to understand and control the morphology, volume and location of the Fe-Al intermetallics present [8]. Intermetallic layer thicknesses greater than 10 μm seriously deteriorate the mechanical properties of the joints [10]. The agreed consensus in the literature is that the main phases present are Fe4Al13 and Fe2Al5 and the thickness of the intermetallic layer is governed by the temperature and time of the welding process. In this sense, this work focused on the resistance spot welding of steel to aluminium 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 investigated and explained. The focus was in tailoring the fundamentals of the process by indirect control of the local contact resistance of the interface by controlling the pressure to increase heat generation and localization in the centre of joints and improve weld size and quality. Results show that the parameters that govern joule heating are shown to strongly influence the weld size and performance. Increased welding time either in single pulse or dual pulse increased the heat input and therefore increased the weld size. For a given pulse duration, multiple pulses produced a larger weld size compared to its single pulse counterpart. Under optimal system parameters this work has shown that the weld size can be larger (8.8 mm) than the size of the electrode tip (8 mm). From this comprehensive piece of work, it was shown not only how to improve weld quality but also semi-empirical models were generated to predict weld size and performance. Overall, this thesis has built on and improved the joining process knowledge associated with Fe-Al joining, by providing a comprehensive manual for technology selection with both impact of joint performance as well as economical implications. Additionally, this work provided a technological milestone by enabling Fe-Al resistance spot welding by providing a basis for the continuation of the understanding how to control the process in order to target the necessary properties and the development of preliminary semi-empirical models to achieve the targeted properties.