Investigation On The Corrosion Behaviour Of Friction Stir Welded Joint Of Al-Zn-Mg Alloys

Al-Zn-Mg aluminum alloy have been widely used in the aerospace industry due to their high specific strength,low-density.In the service process,especially in the coastal environment,Al-Zn-Mg aluminum alloy is prone to produce localized corrosion and cause great security threat.Therefore,it is very important to study the corrosion resistance of Al-Zn-Mg alloy to the development of modern industry.In this paper,the corrosion behaviour of different areas of Al-Zn-Mg aluminum alloy joint produced by friction stir welding process has been investigated.The susceptibility of intergranular corrosion,exfoliation corrosion and stress corrosion of friction stir welded joints are investigated by immersion test,polarization curve measurement and slow strain rate testing combined with transmission electron microscopy,scanning electron microscopy and metallographic test.And two kinds of artificial aging of joint were carried out to study the effect of artificial aging on corrosion corrosion performance.The corrosion resistance of the natural aging friction stir welded joint is investigated.The zone with highest corrosion resistance is the nugget zone(NZ),the next is the base material(BM),and corrosion resistance of the heat affected zone(HAZ)is lowest.The corrosion resistance of 160 arti℃ ficial aging joints is higher than 120 ℃ artificial aging joints,and the natural aging joints is lowest.The TEM test shows that the heat affected zone has dense second-phases,and continuous precipitation phases at the grain boundary are observed,and there is precipitation free zone around the grain boundary.The intergranular corrosion(IGC)behaviour and exfoliation corrosion(EXCO)are investigated.The results show that the HAZ is the most susceptible to IGC and EXCO,and the BM takes the second place,while the nugget zone(NZ)show high resistance to IGC and EXCO.The corrosion resistance of 160 arti℃ ficial aging joints is highest than others.The results of polarization curves show that the corrosion potential of NZ is highest and the corrosion resistance is the best,and the corrosion potential of the HAZ is the lowest and the corrosion resistance is the worst.The potential difference of natural aging joint different regions is the largest,and it’s corrosion resistance is the worst.The potential difference of 160 ℃ artificial aging joints different regions is the smallest,and it’s corrosion resistance is the best.Slow strain rate tensile test results show that the friction stir welded joints are fractured along HAZ or TMAZ.In the aging state,the stress corrosion sensitivity of 160℃ artificial aging joints is lowest,the stress corrosion sensitivity of the nature aging joints is highest.The results of fracture scanning show that the stress corrosion cracking is intergranular brittle fracture and the corrosion products appear at the edge of the fracture.It is a typical stress corrosion fracture characteristic,and some have secondary intergranular fracture.