Study On The Early-stage Precipitation Strengthening Mechanism And Plastic Instability Behavior Of The Al-Mg-Zn(-Cu) Alloy For Automotive Application

The traditional 5xxx series aluminum alloys are widely used in the inner panel of automotive applications due to their medium strength,excellent formability,and weldability.However,the plastic instabilities during the stamping process,namely the Lüders effect and Portevin-Le Chatelier(PLC)effect,and the subsequent bakesoftening behavior deteriorate the surface quality and the mechanical properties of this series of alloys,which restricts their use for automotive body exterior panels and structural components.Thus,improving the baking hardening response and surface quality of 5xxx series Al alloys is becoming an important research orientation.Recently,T-Mg32(AlZn(Cu))49 phase-strengthened Al-Mg-Zn(-Cu)alloys whose Mg/(Zn+Cu)ratios are above 1.0 have shown a promising agehardening response.However,the age-hardening mechanism underlying different heat treatments is not clear during the early-stage precipitation.Additionally,the lack of information on the early-stage precipitation has been an obstacle to the microstructural design and performance improvement of this series alloys.This dissertation focuses on the process optimization,micro structures and mechanical properties of age-hardened Al-Mg-Zn(-Cu)alloys employing hardness measurement,tensile testing,transmission electron microscopy,three dimension atom probe,and other tests.In this study,the early-stage precipitation and strengthening behavior of the Al-Mg-Zn(-Cu)alloy are systematically investigated and elucidated.The improvement mechanism of plastic instabilities during the stamping process is also explored.Additionally,the combined effect of predeformation and the pulsed electric current on improving the bake-hardening response is investigated.The influence of Cu addition and various early-stage aging process on the precipitation and strengthening behavior of the Al-Mg-Zn alloy was systematically studied.The higher number density of Cu-incorporated GP zones led to higher strength and bake-hardening response after pre-aging.Meanwhile,the GP zones were observed to act as sites for the formation of the T’-phase.Pre-aging prior to the natural aging facilitated stable GP zone formation,thereby exhibiting a rapid age-hardening response and enhancing the room temperature stability.Furthermore,the coarsening of the T’-precipitates was found after single-step aging at 180℃,which adversely affected the mechanical properties of the novel alloys.The effect of alloying elements,strain rate,and pulsed electric current on the plastic instabilities of the alloy was explored.The decisive measurement for suppressing the Lüders and PLC effect was based on the depletion of Mg atom concentrations in the matrix due to the formation of GP zones after pre-aging treatment,thereby weakening the interaction between solute atoms and dislocations during the stamping process.At the lower strain rate condition,the critical strain showed an inverse behavior in the T4P-temper Al-Mg-Zn-Cu alloy due to the impediment of GP zones and solute atoms to the dislocations,thereby improving the PLC effect of the alloy.Compared to conventional tensile testing,the pulsed electric current was introduced to improve the plastic instability behavior of the alloys.Based on the microstructural analysis and relevant mathematical model,this dissertation described the interaction of "dislocation-solute atoms" under the action of electric current.This dissertation shed light on predicting the precipitation strengthening of the newly developed Al-Mg-Zn(-Cu)alloys using a combination of microstructural observations and particle size distribution.The yield strengths of the alloy were quantified by adding the strengths of three individual distributions,wherein the precipitation strengthening was evaluated according to the different types of precipitates;the results calculated from the strengthening model are in reasonable agreement with the reported experimental data.The contributions of solid solution strengthening and precipitation strengthening were considerable to the yield strength,and the latter played a more significant role in promoting the bakehardening ability.It was found that the contribution of T’-phases to precipitation strengthening reached up to 176 MPa in Cu-modified alloy.Furthermore,the introduction of the pulsed electric current during the pre-deformation process significantly improves the bake-hardening response of the novel Al-Mg-Zn-Cu alloy.In our study,the bake-hardening response of the T4P-temper Al-Mg-Zn-Cu alloy reached up to about 180 MPa under this process.In summary,this dissertation reveals the early-stage precipitation behavior and the strengthening mechanism in the age-hardened Al-Mg-Zn(-Cu)alloys,providing new insights into the synergistic enhancement of the surface quality and bakehardening ability of the alloys,which makes this series of alloys very promising for applications in the transportation,marine naval and aerospace fields.