Friction stir welding of AA 2198 T3 rolled sheets in butt configuration[Saldatura in configurazione testa a testa di lamiere in lega 2198 T3 mediante procedimento friction stir welding]

Friction-stir welding (FSW) is a solid-state joining process where, in contrast to the fusion welding, the material to be welded does not reach the melting point. Due to its interesting features, a lot of research activities have been carried out on FSW of different materials. In particular, FSW of AA 2198 aluminium alloy has gained wide interest for the fabrication of lightweight structures requiring a high strength-to-weight ratio and a good corrosion resistance. In this study, rolled plates of AA 2198 T3 aluminium alloy are friction-stir welded in butt configuration varying two fundamental process parameters: rotational and welding speeds. An empirical model based on regression analysis is developed to predict the mechanical strength, in particular yield and tensile strength, of the joints as function of the process parameters under investigation. For the development of the empirical model, a 32 full factorial designs with the yield and tensile strength as observed responses is used. Statistical tools such as analysis of variance, F tests, Mallows' CP, coefficient of determination etc. are used to build and to validate the developed models. By using the desirability function approach, the optimum process parameters to simultaneously obtain maximum possible yield and tensile strength are found within the investigated range. The developed models can be effectively used to predict the stationary forces and the mechanical proprieties of the joints at 95% confidence level. Finally, to validate the possible use of FSWof 2198 in the in the aerospace industry, joints welded using the optimum process parameters before identified are tested under stress corrosion cracking and fatigue. Results show that, unlike the base material, the optimized joints are not attacked by pitting if subjected to a tensile stress in a corrosive environment; furthermore, their fatigue life is comparable to that of the base material and longer than that showed by joints welded using traditional techniques, as MIG or TIG.