Influence of Friction Stir Welding (FSW) on Mechanical and Corrosion Properties of AW-7020M and Aw-7020 Alloys

Abstract

Friction welding associated with mixing the weld material (FSW-Friction Stir Welding ) is an alternative to MIG and TIG welding techniques for Al-alloys. This paper presents experimental results obtained from static tension tests on specimens made of AW-7020M and AW-7020 alloys and their joints welded by using FSW method carried out on flat specimens, according to Polish standards : PN-EN ISO 4136:2011 and PN-EN ISO 6892-1:2010. Results of corrosion resistance tests are also presented. The tests were performed by using the electrochemical impedance spectroscopy (EIS). EIS measurement was conducted with the use of three-electrode system in a substitute sea water environment (3,5% NaCl-water solution). The impedance tests were carried out under corrosion potential. Voltage signal amplitude was equal to 10mV, and its frequency range-100 kHz ÷ 0,1 Hz. Atlas 0531 EU&IA potentiostat was used for the tests. For the tested object an equivalent model was selected in the form of a substitute electric circuit. Results of the impedance spectroscopy tests are presented in the form of parameters which characterize corrosion process, as well as on Nyquist's graphs together with the best-fit theoretical curve. Analysis of the test results showed that the value of charge transfer resistance through double layer, Rct, for the FSW-welded specimen, was lower than that for the basic material, and that much greater difference was found in the case of AW-7020M alloy. The impedance spectroscopy tests showed that both the FSW-welded joints and basic material of AW-7020M and AW-7020 alloys were characterized by a good resistance against electrochemical corrosion in sea water environment, and that FSW-welded joints revealed a greater corrosion rate. The performed tests and subject-matter literature research indicate that application of FSW method to joining Al-alloys in shipbuilding is rational.