Precipitation Hardening Response Of Newly Developed Al-Mg-Cu-Zn Alloy And Its Formability

Non-heat-treatable 5xxx series of Al-Mg alloys are widely used in automotive industry due to their excellent formability and corrosion resistance.Because of the lower strength,Al-Mg alloys are used in the inner panel.Moreover,bake-softening is to occur during paint-baking after press-forming.The present 5023 Al alloy contains small addition of Cu to compensate the strength loss that happens in the paint-baking process.However,the yield strength after baking of the Al-Mg-Cu alloy is not high enough to satisfy safety standards.In order to further increase the strength of the Al-Mg-Cu alloy,we deliberately introduced Zn element in the 5023 alloy,and got the best out of the precipitation hardening potential of the alloying elements through optimization of aging treatment,and relative mechanisms were explained.Meanwhile,effects of Cu on the age hardening response and intergranular corrosion(IGC)of the newly developed alloy were analysed.In the end,influences of final cold reduction and intermediate thermo-mechanical treatment(ITMT)on the mechanical properties and formability were investigated.The conclusions are as follows:Results showed that Zn addition improved the age hardening response of the Al-5.2Mg-0.45Cu alloy during aging at 180? by stimulating the precipitation of GPB zones and S phase.The microstructure of the Al-5.2Mg-0.45Cu-xZn(x=0.6?1.2?2.0)alloys in peak aged condition consisted of needle-like S-A12MgCu phase which contained trace Zn atoms and coarse lath-like T-Mg32(AlZn)49 phase which contained trace Cu atoms.However,with increasing the content of Zn,natural aging of the alloys occured dramatically,which deteriorated the paint-baking(aging at 180? for 30min)hardening response of the alloys in T4 state.As for the Al-5.2Mg-0.45Cu-2.0Zn alloy,stable GP zones formed during pre-aging treatment at 80 ? for 12h,which not only improved the age hardening response of the alloy,but also enhanced the stability at room temperature,which could prevent bake-softening.The yield strength of the alloy after paint-baking was 271 MPa,and the increment of yield strength during paint-baking was 57MPa.The microstructure of the alloy in peak aged condition with pre-aging treatment only consisted of high density of fine and equiaxed T phase with the S phase disappeared.The mean composition of the T phase was Al43.9Mg43.7Zn3 7Cu8.7,while the Zn content in GP zone was significantly higher than that of Cu.The big difference of the Cu and Zn content between the GP zones and T phase was attributed to the much lower diffusion rate of Cu atom in the Al matrix compared with that of Zn atom.As for the Al-5.2Mg-0.45Cu-1.2Zn alloy,pre-aging treatment at 90 ? for 12h decreased the age hardening response of the alloy,and the microstructure of the alloy in peak aged condition consisted of S phase and T phase.Cu addition improved the age hardening response of Al-5.2Mg-0.45Cu-2.0Zn alloy significantly by stimulating the precipitation of GP zones during pre-aging treatment,which resulted in finer dispersion of the T phase in peak aged condition.Moreover,Cu addition narrowed the width of precipitation free zone(PFZ),which not only improved the yield strength of the alloy,but also enhanced the IGC resistance.In the aspect of electrochemistry,Cu addition could increase the corrosion potential and decrease the corrosion current density of the alloy,which also had beneficial effect on the IGC of the alloy in peak aged condition after pre-aging treatment.As for the mechanical properties and formability of the Al-5.2Mg-0.45Cu-2.0Zn alloy,on one hand,increasing final cold reduction resulted in finer recrystallized grain size to some extent,which improved the formabitliy but had no effect on the mechanical properties of the alloy.On the other hand,ITMT had obvious effect upon grain refinement,which also had no effect on the mechanical properties.ITMT improved the formability of the alloy by increasing the Erichsen(IE)value from 8.59mm of the alloy processed by traditional treatment to 9.20mm.As comparison,the IE values of the 5182 alloy in annealed state and 5023 alloy in T4 state were 10.34mm and 10.09mm respectively.