Novel hybrid joining technique for dissimilar metal combinations
In order to achieve further reduced CO2 emissions, automotive manufacturers are turning to multimaterial designs which reduce the vehicle weight. Traditional joining/welding methods struggle with these difficult material combinations, like aluminium to steel. Therefore, the ISOLDE Vinnova project, with additional support from XPRES, has examined several novel approaches to joining these materials, including a promising technique making use of a cold-sprayed interlayer
Project description
Multi-material designs, while enabling lighter weight vehicle designs, impose challenges for joining technology due to the metallurgical incompatibility of the materials. However, being able to use existing traditional joining technology, like resistance spot welding (RSW), is cost-effective for vehicle manufacturers. Therefore, novel approaches are needed in order to produce a sound joint in difficult material combinations like steel to aluminum.
In this project, alternative approches like project lock welding, and the use of a stainless steel interlayer applied with two different methods (directed energy deposition and cold spraying), were tested. Weld samples using industrially relevant RSW schedules were produced, and the microstructure and mechanical behavior of the welds were characterized.
Project results
Of the three novel approaches investigated, the use of a stainless (316L) interlayer deposited with cold spray on the aluminum substrate showed the best results. In particular, the best adhesion of the interlayer was obtained on the extruded aluminum as opposed to cast aluminum. SEM characterization of the interlayer showed the formation of a 2 µm – 6 µm thick reaction layer of Al-Fe and Al-Fe-Si intermetallic compounds at two interfaces: between the 316L CS interlayer and the Al-Si coating on the steel, and between the 316L CS interlayer and the extruded or cast aluminum substrate. These relatively uniform reactions layers that form during welding are what provide In cold-sprayed interlayer on extruded aluminum case, the RSW joint achieved the highest average maximum tensile force of 2.9 kN, thereby showing the most promise as a material combination that can benefit from the CS interlayer for the purpose of producing a fixation point before adhesive curing.