Friction stir spot welding (FSSW) is a spot-like solid-state joining process suitable for welding structural components in overlap configuration that has already found its place in commercial applications in the automotive industry. In the past few years, the use of conventional spot welding in automotive applications has become more complex due to the extensive use of lightweight alloys that has been considered as promising materials to replace steel in order to reduce vehicle weight improving fuel economy as well as vehicle performance. FSSW has a potential to replace these conventional methods as well as to reduce manufacturing steps and lower overall costs. In FSSW the rotating tool is not traversed and instead creates the joint by plunging into and retracting out a rotating tool of the overlapping sheets - a keyhole depression remains at the weld centreline. Frictional heat is generated from the contact between the tool and the material to be welded resulting in softening of this material. The softened material is stirred to form a metallurgical connection which is aided by the forge action applied by the shoulder contacting the upper sheet surface. FSSW has been performed in two different automotive 1.5 mm-thick aluminium alloys - AA5754-H22 and AA6082-T6 in similar and dissimilar combination using different joining parameters (e.g. plunge rate, rotational speed and dwell time). Plunge depth has been held constant at approximately 2.3 mm (0.2 mm of shoulder plunge depth and 2.1 mm of pin plunge depth) corresponding to approximately 75% of the combined thickness (3 mm) of the overlapping sheets. FSSW connections have been accomplished using same tool geometry - cylindrical 3 flute pin (5 mm diameter and 2.1 mm length) and flat shoulder (15 mm diameter). Mechanical behaviour has been investigated in terms of cross-tension and tensile shear testing. The fracture surface of the FSSW connections has been investigated using a stereoscope. Three different failure modes have been observed in tensile shear specimens: shear fracture through the bonded region; non-circumferential nugget pull-out as well as circumferential nugget pull-out. Similar failure modes have been found for the cross-tension specimens. Relationships between mechanical performance and joining parameters have been established for the stir spot joints investigated. The preliminary results have shown that best mechanical behaviour has been obtained by using lower plunge rates and rotational speeds. An important feature in terms of productivity has been observed: at low plunge rates an increase in dwell time has resulted in a decrease in the tensile shear and cross-tension maximum forces; the opposite behaviour has been found at high plunge rates, i.e., increasing dwell time has increased both maximum forces.
Effect of joining parameters on static strength and failure mechanisms of friction stir spot welded automotive aluminium alloys
2010
15 Seiten, 10 Bilder, 7 Tabellen, 47 Quellen
Conference paper
English
Friction stir welding of automotive aluminium alloys
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