Abstract
The aluminium alloy studied in this paper is the heat treatable AA7075 which has zinc as the main alloying element. 7xxx aluminium alloys have the best strength performance among all commercial series and AA7075 has tensile strength above 500 MPa. The outstanding strength properties open the possibility to use this alloy in automotive industry as a possible alternative material for car body elements instead of steel. However, their limited formability properties mean obstacle when a complicated shape car body panel is formed, since the elongation is limited only between 5 and 11%. In order to successfully form these sheets into the demanded geometry, hot forming should be used, and thus the body panels should be formed when the AA7075 is in a solution heat treated condition, such as using the HFQ® process. Then, after assembling the body elements, when resistance spot welding is among the most common joining methods, the AA7075 body parts should get the artificial ageing. It means that welding is followed by artificial ageing as a kind of post weld heat treating which can partially compensate the softening of this alloy during welding. Softening is considered among the most challenging weldability issues similarly to the high melting point oxide layer at the surface which also reduces the load bearing capacity of RSW joints. During the performed experimental program, three routes were investigated. The RSW experiments are performed on 1-mm-thick AA7075 sheets. The spot-welded joints were examined by macroscopic testing, tensile shear and hardness tests.
Highlights
Due to their relatively low density and good resistance to corrosion, aluminium alloys are among the best engineering materials used in automotive industry [1,2,3]
The elemental composition of aluminium surface was tested by glow-discharge optical emission spectroscopy (GDOES)
After pre-treatment with the alkaline immersion followed by HF acidic pickling, the surface is mostly free of oxidation that was formed during the previous solution heat treatment (SHT) process
Summary
Due to their relatively low density and good resistance to corrosion, aluminium alloys are among the best engineering materials used in automotive industry [1,2,3]. The assembled car body is generally moved to the paint shop, where the formed and joined aluminium sheets get a heat input according to the heat cycle during the paint bake This heat input can be partially used for ageing in order to reach the demanded mechanical properties of the AA7075 alloy [9]. It means that the aluminium sheets are joined in a solution annealed (softened) and formed condition. Since the artificial ageing is partially connected to the painting of the car-body; the joint connections (RSW, FSW, clinching, adhesive bonding etc.) should be prepared before It means that joints are heat treated as well, which might influence the mechanical properties of the weld metal and the heat-affected zone [10].
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