The need for lightweighting in automobiles has forced automakers to shift from heavy steel bodies to lighter aluminum structures, necessitating the advancement of multi-material joining techniques. Friction Element Welding (FEW) is one such friction-based, multi-material joining technique used to join overlapping parts. Even though there are multiple techniques to join aluminum to steel, FEW is the only joining approach that is suitable for joining aluminum to ultra-high-strength steels. It is a thermo-mechanical joining technique in which two dissimilar metals are joined using a rotating element and the compressive force that is applied on the joining surfaces.
FEW includes four processing steps through which the materials are joined: penetration, cleaning, welding, and compression. In the penetration step, the element penetrates the top Al sheet through axial force and rotation; in the cleaning step, the element removes any coating from the surface of the steel sheet; in the welding step, a friction weld is formed between the steel and the element; in the final compression step, a compressive force is exerted to extrude the Al sheet under the element head and close any gap in between the sheets. It is desired to have the aluminum material constrained under the element head; however, when using suboptimum parameters, there is chip formation that protrudes outside the head when joining specific aluminum alloys. This chip formation is disadvantageous since it promotes crevice corrosion. Despite having a very rare occurrence only in certain aluminum alloys, this defect potentially requires additional post-processing such as the application of sealant, decreasing its effectiveness within a production environment. This research identifies and discusses the current understanding of why chipping occurs in those aluminum alloys and reviews the optimum parameters that could be employed to reduce the formation of chips.
The materials chosen for this experimental study were AA 7021 – T79 of 2.5 mm thickness and a proprietary cast aluminum alloy of 3 mm thickness with e-coat. The elements selected for this experiment were 6.5-P-10, polygon-shaped with a shaft length of 6.5 mm. The backing sheet that was used for all the experiments is JAC980 (a DP980 steel). Since the chipping effect is only influenced by the aluminum top sheet, the steel backing sheet material is irrelevant to this study.
Chipping was found only to occur during the penetration step of the welding process. Therefore, only the parameters used during this step were varied. The end loads that were chosen for this experiment were 5 kN, 8 kN and 10 kN. The RPMs that were chosen for this experiment were 5000, 7000, and 8000 RPM. Since only the penetration step was under investigation, the parameters were limited to just these factors.
For every endload and RPM, three tests each for AA 7021 – T79 and e-coated Al alloy were conducted to check the results for repeatability. All of the experiments were captured and recorded using an Olympus i-speed 2 high-speed camera. An incision was made on the downholder in order to enable viewing of the friction element being welded onto the upper substrate. A few of the tests were extended for all four FEW steps to confirm that the trend is the same.
Determination of the cause of chipping was primarily performed considering the high-speed video footage. The use of a high-speed camera with a modified downholder provided unprecedented visualization of the penetration step during welding. It was found that for increasing RPM of the spindle speed, the chipping was greatly reduced. Also, as the endload on the friction element increased, less chip formation was experienced. Since there is no quantitative metric to quantify the chip formation, the degree of chipping must be determined qualitatively by the operator. Though difficult to describe the chipping behavior, it was simple to relatively rank the results.
It could also be seen that the degree of chip formation was higher in the proprietary cast aluminum with the e-coat than the Al 7xxx alloys that were tested at all the different parameters in which they were compared. Therefore, the chipping found in the cast aluminum alloy could be attributed to the e-coat on the surface. To verify this, the same proprietary cast aluminum alloy without the e-coat was tested for all parameters. The uncoated cast aluminum alloy showed less chip formation compared to the e-coated aluminum alloy. This leads to an important conclusion that the chipping behavior is indeed affected by the material property such as the coatings on alloys and not only by the operating parameters. In addition to the effects of coatings, it is hypothesized that the material strength can be directly related to the chipping behavior. Chipping was found to occur in thicker, high strength aluminums. During the penetration step, there is a significant temperature increase. It is possible that thicker materials are not able to reach the same temperature as thin materials due to the larger thermal mass. Future work will focus on validating this hypothesis through thermal measurements.
The tests were also extended to Al 7075 - T6 and Al 6061 as the top sheet for the same operating parameters and conditions. Both the samples showed reduced chip formation at higher spindle speeds. This chipping behavior has been found unique only to certain alloys of aluminum. This process is otherwise extremely robust and yields exceptional results for other materials. Therefore, we can conclude that the degree of formation of chips reduces as the spindle speed increases and shows maximum reduction at higher endloads on the friction element.