| Abstract Scope |
Friction Element Welding holds promising results among multi-material joining processes for high strength alloy joints. This novel process seeks to eliminate inherent limitations of traditional joining processes. Joining of materials with different thermal properties has its own challenges. Fusion welding processes like RSW fall short because of differences in melting temperatures of the base alloys. Differences in thermal expansion is also a limiting factor since it results in residual thermal stresses due to unequal cooling. Processes such as Self Piercing Riveting (SPR), Flow Drill Screwing (FDS), and clinching were developed to avoid using heat energy to melt the base materials. These processes form solid state joints by mechanical interlocking and hence overcome the limitations of fusion-based joining processes. This makes the use of lightweight and high strength materials possible in structural components of vehicles. Alloys such as dual and complex phase steels, high martensitic and boron steels, and magnesium alloys can be introduced along with lightweight, high strength aluminum alloys in order to significantly reduce weight. However, since processes like SPR and FDS are designed to form holes in high strength metal sheets, they are limited by thickness and strength of the base metals. In other research, it was found that for regular SPR processes, it is difficult to pierce rivet in 2 mm AZ31 magnesium sheet owing to its low ductility. They developed modifications to the process in order to increase the effectiveness of joint formation, resulting in increased joint strength. Similar limitations for SPR process were discovered, with limited capabilities to join only 2 mm thick sheets of 1000 MPa steel. Others described limitations of the FDS process with only 1 mm thick sheets of 600 MPa steel. This limitation occurs since the joining element must pierce through high strength sheets. Friction element welding overcomes this limitation and provides advantages of joining high strength base metals with relatively soft cover plates. The process is accomplished in four steps: 1) penetration, 2) cleaning, 3) welding, and 4) compression. Each step is controlled by a parameter such as spindle speed, axial force, time, and distance. The first step consists of the element piercing the top cover plate, with aluminum flowing upwards forming a pool of metal on which the element head is intended to seat. This pool of material is essential for quality metrics of the joint. The scope of this research focuses on an issue that arises during the penetration step, namely chip formation. Chipping is described as formation of whisker-like flares and thin and irregularly shaped pieces of aluminum near the shaft of the element. The chipping is prominent in thicker cover plates at relatively low rotational speeds. High speed imaging has revealed element walking of the element to be the probable cause of chipping. The presence of chipping in FEW joints is described to be the cause of accelerated corrosion of the joint. Therefore, the goal of this research is to eliminate chipping without the addition of any filler material while maintaining joint strength. Localized heating of the cover plate in the region where the element is designed to pierce has been explored as a viable and successful way of eliminating chipping. A design of experiments was conducted to assess the effectiveness of thermal augmentation during penetration step. The cover plate used was 7075-T6 aluminum with 3.175 mm thickness. The plate was heated to temperatures of 95°C - 425°C using a furnace. The heating process was followed immediately by the penetration step of the FEW process to assess chip formation. Results from different temperatures were compared visually with a baseline test that was conducted at room temperature. Although visual inspection showed some decrease in whisker formation at temperatures less than 200°C, chipping was still present. However, at higher temperatures, and especially for the maximum temperature of 425°C, both whiskers and chipping were
completely eliminated. The aluminum flowed in a perfectly donut shaped pool under the element head. The 425°C test was repeated for assessing repeatability of the process and yielded similar results every time. A scale for measuring the chipping is to be devised, which will provide quantification of effectiveness of the process along with visual inspection. The scale will account for factors such as size and density of whiskers, length and width of thin aluminum chips, and the overall size of the pool of metal under the element head. Friction Element Welding is an extremely rapid process, with cycle times of 1 – 1.6 seconds. The heating process is intended to be a supplementary process and hence is required to maintain fast heating in order to be effective in a production environment. Therefore, future stages of this research will be developed keeping in mind the development of a rapid heating system. Alternatives such as laser heating and induction heating are to be explored as viable heating option. Localized heating is a key aspect of developing the heating system, since widespread heating will result in unnecessary process energy consumption and cause changes in properties of the surrounding aluminum. |