| The 7N01 aluminum alloy(Al-Zn-Mg alloy)is a type of an age-strengthened aluminum alloy.It is widely used in modern aerospace and vehicle engineering due to its high fiber strength.However,this type of alloy is sensitive to stress corrosion to some extent.Furthermore,the frriction stir welding(FSW)technique is extensively utilized for joining 7N01 alloy.The performance of the friction stir weld joint is not only affected by the composition of the base metal alloy,but is also determined by the welding parameters.In this thesis,the friction stir welding process of 7N01 aluminum alloys with different alloying element contents is studied.The alloy with low alloying element contents is treated as A-type alloy while the alloy with high alloying element contents is considered as B-type alloy.The microstructure and corrosion resistance of the weld joint under different double-sided friction stir parameters is studied.The results show that the double-sided friction stir welding demonstrates good process adaptability to these two types of 7N01 aluminum alloys,and the small heat input shows minor effect on the re-dissolution of the precipitates.Analysis of the joint microstructure indicates that the weld nugget region becomes recrystallized and is composed of a large number of fine equiaxed crystals.However,as for the heat affected zone and the thermomechanical affected zone,grain coarsening and precipitates remelting occurs due to the thermal cycling,generating poor mechanical properties.Accordingly,it appears as a weak zone of the joint.The results of intergranular corrosion and electrochemical tests show that the corrosion resistance of the weld joints for these two different alloys is worse than that of the base metal.The weld nugget and base metal regions for B-type alloy with higher alloying element contentsdevelop a better corrosion resistance parameters is higher than that A alloy under the same welding process.The slow strain rate tensile corrosion results show that these two types of aluminum alloys have a high stress corrosion(SCC)sensitivity in 3.5%NaCl solution.The corrosion resistance of the base metal and joints for the A-type alloy with lower alloying elements is better than that of the B-type alloy.The fracture surface morphology of the A-type alloy base metal exhibits brittle fracture after the slow strain rate tensile corrosion.The fracture surface of the A-type alloy joint is characterized by ductile intergranular fracture or intersubgranular fracture.The formed secondary crack is obviously observed on the fractured surface of the B-type alloy base metal.These cracks propagates along the grain boundaris,exhibiting brittle fracture characteristics In addition,a number of black corrosion spots are observed on the macroscopic fracture surface for the B-type alloy in the 3.5%NaCl solution,and many impurity particles are present inside the large-sized dimples.A-type alloy owns a good stress corrosion resistance accompanying with a strength at the same time. |
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