Study On Compositional Design,Microstructure And Properties Of Novel High Strength-toughness Al-Mg-Zn-Cu Aluminium Alloy With Good Weldability And Corrosion Resistance

Traditional 5000 series Al-Mg alloy is widely used in vehicle and ship industry due to their excellent corrosion resistance,weldability and formability and relatively high specific strength.However,Al-Mg alloys belongs to non aging strengthening alloy,and the alloy strength is relatively low,which can not meet the needs of aviation industry.7000 series Al-Zn-Mg(-Cu)and 2000 series Al-Cu series alloys are widely used in aviation field because of their high strength-toughness.However,the higher hot cracking susceptibility of these alloys during the traditional fusion welding limits the further development of alloys.Thus,the weldability of alloys need to be solved urgently.In addition,the corrosion resistance of these alloys at peak aging state is relatively poor,and their corrosion resistance also needs to be further improved.The strength of the novel Al-Mg-Zn alloy can increased to about 500 MPa by composition design,and the intergranular corrosion(IGC)resistance can be greatly improved under peak aging(T6)condition through process optimization.Besides,the addition of trace Cu(mass fraction of Cu<0.5wt%)in Al-5.1Mg-2Zn alloys can play the role of strengthening the baking paint.Through the composite addition of Zn and Cu and the optimization of aging process,the effect of Cu on the precipitation behavior of the alloy was deeply analyzed,and the strengthening mechanism and corrosion mechanism of the alloy were revealed.Therefore,the development of high strength-toughness Al-Mg-Zn-Cu alloy with good weldability and corrosion resistance is the trend of the development of multi-purpose aluminum alloy in the future,which also puts forward higher requirements for the comprehensive properties of the aluminium alloys.In this work,the novel Al-Mg-Zn-Cu alloys with(Zn+Cu)/Mg ratio below 1.5 were prepared by adding Cu element into the age-hardening Al-Mg-Zn alloy.The alloys with a(Zn+Cu)/Mg ratio below 1.5 are different from traditional 2000,5000 and 7000 series aluminium alloys.The microstructure and properties of the novel Al-Mg-Zn-Cu alloys alloy were analyzed by hardness measurement,Differential Scanning Calorimetry(DSC)analysis,metallographic observation,Scanning Electron Microscope(SEM)and Transmission Electron Microscope(TEM).The mechanical properties,welding hot cracking susceptibility(HCS),corrosion resistance and fracture toughness of the alloy were studied in detail.The strengthening and toughening mechanism of the novel alloys,the factors which affected the welding hot cracking susceptibility and the corrosion mechanism are clarified in our work.The results show that the novel Al-Mg-Zn-Cu alloys are mainly strengthened by the T-Mg32(Al,Zn,Cu)49 phase.The tensile strength and yield strength of the T6 treated alloys can reach to 570 MPa and 500 MPa respectively,while maintains a relative higher elongation;which is equivalent to that of 7075-T6 alloy,and the specific strength of Al-Mg-Zn-Cu alloys is higher.Furthermore,by introducing deformation strengthening,combine with the T-phase precipitation strengthening effect,the ultra-high strength Al-Mg-Zn-Cu alloy was prepared.The tensile strength and yield strength of the alloy were 746MPa and 696MPa respectively,and the elongation was about 8%.Meanwhile,the HCS,corrosion resistance and fracture toughness of T6 alloy were studied.The results show that the HCS of T6 Al-Mg-Zn-Cu alloy is better than that of 7075 alloy,and the HCS is related to the effective solidification range,the liquid fraction at the end of solidification,the width of the mushy zone and the internal stress.Based on SKK criterion,a modified criterion for hot cracking during welding is established.When the hot cracking sensitivity is greater than 1,hot cracking occurs;otherwise hot cracking will not occur.The IGC resistance of the alloy is mainly determined by the continuity of the grain boundary precipitates(GBPs).The results show that the fracture toughness is related to the grain boundary precipitates,PFZ width and recrystallization fraction.Through high temperature regression and thermomechanical treatment(HRTMT),the aluminum alloy with excellent comprehensive properties was obtained,which has high strength-toughness,corrosion resistance and weldability.These results will provide guidance for the composition design,sheet preparation,microstructure design and performance regulation,the improvement of welding performance,corrosion resistance and fracture toughness of the age-hardening Al-Mg-Zn-Cu alloys.