Laser beam welding of dissimilar aluminium alloys of 2000 and 7000 series: effect of post- welding thermal treatments on T joint strength C. Badini 1 , M. Pavese* 1 , P. Fino 1 and S. Biamino 1 Two experimental aluminium alloys (belonging to 2000 and 7000 series respectively) were welded using the laser beam welding (LBW) technique and an Al–Si alloy as filler. Different combinations of pre- and post-welding thermal treatments were proposed. The change of strength during aging treatments was investigated by microhardness measurements carried out on the weld and on the regions of the alloys not affected by the LBW process. The microstructure of the welded specimens was studied by scanning electron microscopy and energy dispersive X-ray spectroscopy. The strength of the joints after different thermal treatments was checked by T pull tests. Different thermal treatments resulted in different final strengths of the joint. Keywords: Laser beam welding, Dissimilar aluminium alloys, Thermal treatment, T pull test Introduction Transportation industries more and more require materials for lightweight structures, suitable for saving weight and thus reducing fuel consumption and atmo- spheric pollution. For this reason, aluminium alloys are increasingly employed in automotive industry and aeronautics. Aluminium alloys joining techniques should be improved by developing low cost and high efficiency production processes. Aircraft industries demand for substituting conventional joining technolo- gies like mechanical fastening, riveting and arc welding with innovative methods giving some advantages in large scale production. For instance, mechanically fastened stiffeners of aircraft fuselage shells should be substituted by stringers attached to the skin by quicker and more efficient joining methods. At present, to this purpose, two alternative joining techniques are under investigation: laser beam welding (LBW) and friction stir welding (FSW). The FSW method, based on the production by friction of a thermomechanically plasti- cised zone in the materials to be welded, shows some advantages over traditional welding techniques invol- ving material melting. In particular, it allows greatly reducing the formation of porosity and residual stresses and the risk of hot cracking. This method has been successfully adopted for welding particular aluminium alloys, otherwise not easily weldable using conventional fusion techniques (like those belonging to the 2000 or the 7000 series), or even for joining dissimilar aluminium alloys. 1–5 On the other hand, this method needs expensive equipments, and it is easily applicable only for joining plates or components with quite simple shape. LBW is recognised as a more mature joining technique. 6 In principle, LBW has to face the drawbacks common to every fusion welding method: porosity due to the decrease of hydrogen solubility occurring during the solidification of the molten zone, hot cracking resulting from solidifica- tion shrinking and thermal stresses and mechanical strength of both fusion and heat affected zones lower than that typical of the alloy submitted to the welding process. However, these drawbacks can be overcome. Porosity within the weld can be avoided, or at least reduced, by a correct preparation of the materials to be welded and the use of a protective atmosphere. Tendency for solidification cracking can be reduced both by modifying the composition of alloys normally considered as ‘unweldable’ 7 and by altering the weld composition through the addition of a filler wire of proper composi- tion. 8,9 In any case, a tight control of the LBW process parameters (like welding speed and filler feedrate) is required, in particular for laser welding of 2000 and 7000 series alloys. Among the fillers that have been tested, Al– 12Si alloy proved to give good results. This filler limits the content of Mg in the molten pool, hindering the precipitation of the Mg 2 Si phase, which is thought to enhance the solidification cracking susceptibility. 8 Finally, the mechanical strength and ductility of the weld can be improved by tailored pre- and post-welding thermal treatments. 9 The Nd:YAG laser, owing to its characteristic low wavelength (1?06 mm), provides good coupling with aluminium alloys, which adsorb the laser efficiently and thus can be welded with speeds of several m min 21 . LBW of several aluminium alloys have been widely investigated in the last years 6,10–12 , and also dissimilar alloys of 6000 series have been joined by this techni- que. 8,9 However, 6000 series aluminium alloys are widely 1 Dipartimento di Scienza dei Materiali e Ingegneria Chimica, Politecnico di Torino, corso Duca degli Abruzzi 24, 10129 Turin, Italy *Corresponding author, email matteo.pavese@polito.it ß 2009 Institute of Materials, Minerals and Mining Published by Maney on behalf of the Institute Received 16 July 2008; accepted 30 September 2008 DOI 10.1179/136217108X372559 Science and Technology of Welding and Joining 2009 VOL 14 NO 6 484