Mechanical behaviour of self-piercing riveted aluminium joints
The introduction of alternative materials and in particular aluminium alloys, for vehicle body applications has impelled the development of new joining techniques. Traditional joining methods such as spot-welding and arc-welding are being challenged. Self-piercing riveting has attracted considerable interest by the automotive industry and has been used as an alternative to spot-welding in vehicle body assembly. However, self-piercing riveting is a relatively new joining method and as such it is not well understood. The aim of this project was therefore to develop an understanding of the mechanical behaviour of self-piercing riveted joints. The effects of paint-baking, shelf-life, pre-straining and surface condition of the sheet material on the joint quality and behaviour were therefore examined. Aluminium alloy sheet materials, 5754 and AA6111, were used in this investigation. The project began with a metallographic inspection of cross-sections of samples that were joined under different conditions in order to examine the effect of process variables on the joint quality. This part of the investigation led to the identification of suitable setting parameters that produced joints which, by metallographic inspection, were of good quality. It was also observed that some process variables, such as sheet thickness combination, rivet and die design and setting force, affected the joint quality and therefore needed to be taken into consideration in the choice of the processing parameters. Subsequent work focused on mechanical testing. Lap shear, T-peel, pull-out and fatigue tests were carried out in order to examine the mechanical behaviour and to analyse the failure mechanisms of the joints. The work showed that the strength, the thickness and the surface condition of the riveted sheets affected the strength and the failure mechanisms of the joints. The joint strength was also observed to be dependent on the rivet and anvil design as well as the setting force. In addition, the joint strength and behaviour differed as the specimen geometry thus emphasising the need for a test standard for self-piercing riveted joints. Paint baking led to a marginal and insignificant reduction in the static strength, whilst resulting in a reduction in the fatigue strength of the joints as a consequence of recovery of the 5754 alloy and the removal of the wax-based surface lubricant. The effect of 3%, 5% and 10% pre-straining of the 5754 sheet on the quality and performance of the self-piercing riveted joints was also examined. It was established that it was possible to produce joints of good quality, higher strength and superior fatigue performance by using the same setting parameters as for joints without additional pre-straining. An investigation of the effect of the shelf-life of AA6111 indicated that this only had a minor and insignificant effect on the joint quality and behaviour. It was therefore deduced that the quality and performance of joints would not be compromised even after an AA6111 self-life of 21 months. The effect of the interfacial characteristics on the joint quality and behaviour was examined by placing a PTFE layer at the interface between the riveted sheets. It was observed that the PTFE insert significantly reduced the joint strength and changed the failure mechanism. Three distinct failure modes, referred to as rivet pull-out, rivet fracture and sheet material failure, were observed during this investigation. All shear tested samples failed by rivet pull-out. The same failure mechanism was the only one observed for the pull-out tests. The failure mechanism for the peel test depended on the thickness of the rivet sheet. For joints with a (1 mm+2mm)/(0.9mm+2mm) combination, fracture of the thinner sheet material dominated the failure mechanism, whilst for joints with a (2mm+2mm) combination, rivet pull-out was the only failure system. Rivet fracture and sheet material failure were also observed during fatigue testing. Examination of samples following fatigue testing led to the observation of fretting which had not been reported by previous investigators working with self-piercing rivets. Fretting had an important effect on the fatigue strength and fatigue failure mechanisms. Inspection of fatigue fractured samples which were tested at maximum applied loads ranging from 50% to 85% of the ultimate shear load of the joints exhibited fretting scars at three different interfaces. Flange-face fretting was observed to take place at one side of the interface between the two riveted sheets and led to the formation of mainly A1203 debris. Pin-bore fretting was observed to occur between the rivet shank and the aluminium alloy sheet and led to debris containing oxides of aluminium and iron together with the oxides of zinc and tin from the wear of the corrosion protective coating of the rivet. Both types of fretting were affected by the applied load and the surface condition of the riveted sheets. Further examination indicated that fretting contributed to the initiation and propagation of fatigue cracks. The failure modes during fatigue testing were affected by the fretting behaviour and were dependent on the applied load and the interfacial conditions. A PTFE layer introduced a very low coefficient of friction leading to a significantly reduction in the amount of fretting. However, this was accompanied with a change in the load transfer mechanism resulting in rivet fracture and a shorter fatigue life. The paint-baking process led to the removal of the wax-based surface lubricant and fretting cracks therefore initiated at an earlier stage of the fatigue test. In addition, fretting also led to a significant work-hardening of the riveted sheets. It was observed that there was an increase in microhardness at the regions immediately below the fretting area from the riveted sheets. The depth of the work-hardened area below the fretting interface after different periods of fretting represented the depth of damage as a result of fretting fatigue. It was therefore further indicated that fretting played an important role in the fatigue behaviour and would probably affect the crash behaviour of the joints. The effect of secondary bending, an inherent feature of lap joints, was examined and analysed using strain gauge measurements. It was established that secondary bending contributed to the failure mechanism and led to a significant reduction in the fatigue strength of such joints. Using the experimental data an analysis has been carried out to predict the fatigue strength in the absence of secondary bending.