Effect Of Heat Treatment And Friction Stir Processing On Microstructure And Stress Corrosion Resistance Of Copper Alloy

In order to meet the needs of building China into a maritime power,people pay more and more attention to the service safety of materials in the marine environment.Copper alloy has excellent corrosion resistance and mechanical properties,and is widely used in the manufacture of Ship systems,wear plates,gears and bearings and other large engineering parts.The corrosion resistance of copper alloy is closely related to its microstructure.The microstructure of copper alloy can be changed by heat treatment and friction stir treatment,which affects the stress corrosion cracking behavior of copper alloy.Therefore,two kinds of marine equipment materials CuZn₃₉Pb₃ alloy and CuAl₉Ni₄Fe₄Mn₂ alloy were selected to study the stress corrosion behavior.The effect of heat treatment on the microstructure and stress corrosion resistance of CuZn₃₉Pb₃ alloy and the effect of heat treatment and friction stir treatment on the microstructure and stress corrosion resistance of CuAl₉Ni₄Fe₄Mn₂ alloy were studied.The main conclusions are as follows（1）The SCC susceptibility of CuZn₃₉Pb₃ alloy depends on the size and proportion ofαphase,the continuity ofβphase distribution and the distribution of free Pb.The continuous distribution ofβphase will cause poor corrosion resistance,free Pb phase is easy to gather at the interface ofαphase andβphase,the coarseness ofαphase and continuous distribution ofβphase will lead to the decrease of phase interface area,lead to the aggregation of Pb phase at the interface,reduce the interfacial bonding stress,and further reduce the stress corrosion resistance of materials.The slow strain rate tensile（SSRT）test shows that the stress corrosion resistance of the annealed alloy is the worst;after aging at 450℃,the fineαphase in the alloy splits relative toβphase,the content ofβphase is the least,the free Pb is homogenized,and the comprehensive mechanical properties and stress corrosion resistance are the best.（2）The strength of stress corrosion cracking tendency of CuAl₉Ni₄Fe₄Mn₂ alloy in corrosion solution is related to the content of easily corroded structure in the alloy.Continuous layered ki II/αeutectoid structure andβphase are the sensitive areas of stress corrosion.In the normalized state,due to the existence of a large number of untransformedβphase,the precipitatedαstructure is coarse,and the ki II/αeutectoid structure at theαphase interface shows the lowest mechanical properties and stress corrosion resistance.However,the kiii/αeutectoid structure andβphase are less in the alloy aged at 450℃and 550℃,and the stress corrosion sensitivity is low.（3）After friction stir treatment,the microstructure of CuAl₉Ni₄Fe₄Mn₂ alloy changes and the microstructure in the stir zone is refined.Compared with the as cast alloy,the ultimate tensile strength of CuAl₉Ni₄Fe₄Mn₂ alloy after friction stir treatment is improved,the elongation after fracture is reduced,and the stress corrosion sensitivity is reduced.After heat treatment,the microstructure of CuAl₉Ni₄Fe₄Mn₂ alloy was optimized,the stress corrosion sensitivity of the alloy was further reduced,the elongation did not change,and the strength of the alloy decreased.Before and after heat treatment,the fracture position of the specimens changed after SSRT test.Before heat treatment,the specimens were broken in the heat affected zone,and after heat treatment,the specimens were broken at the junction of the heat engine effect and the stirring zone,which was attributed to the dissolution of the corrosiveβphase in the heat affected zone.