Study On The Microstructure Evolution And Surface Cracking Prediction During Hot Extrusion Of Mg-Al-Ca-Sr Magnesium Alloy

The addition of alkaline earth elements Ca and Sr can not only improve theperformance but also reduce production costs of Mg-Al-Ca-Sr magnesium alloy,which meets the requirements of lightweight and energy saving for vehicle. However,Mg-Al-Ca-Sr magnesium alloy presents poor formability at room temperature. Studyon the hot forming and microstrure controlling of the alloy will help to provideguidance for the processing degsin.The hot deformation behavior, microstructure evolution and extrusion processingoptimization of Mg-Al-Ca-Sr alloy have been studied by experimental and numericalmethods. Hot compression tests were conducted to develop the processing map whichcan be adopted to analyze the relationships between deformation process parametersand microstructure evolution. The constitutive model of Mg-Al-Ca-Sr alloy based ondynamic recrystallization was developed to predict the microstructrual evolutions ofthe alloy during hot compress and extrusion processing. The surface crackingphenomena in the extrusion processes of Mg-Al-Ca-Sr alloy were also predicted. Thereliability of simulation results were valided by comparing to the experimental results.Following results were obtained:The analysis of processing maps indicated that different efficiency domains andflow instability regions presented various microstructural evolution. The efficiency ofpower dissipation changed in a small range at high deformation temperature and lowstrain rate. It clearly decreased high deformation temperature and high strain rate. Theefficiency of power dissipation increased gradually with increasing of deformationtemperature and decreasing of strain rate. The flow instabilities of Mg-Al-Ca-Sr alloywere induced by the localized flow and fracture in recrystallization domains. Theextruded Mg-Al-Ca-Sr alloy presented finer grain size, which showed bettermechanical properties. The extrusion parameter of billet temperature lower than350℃and ram speed less than2.7mm/s should be adopted to obtain fine grain structureand higher strength for the component.The strain hardening and softening parameters of the modified L-J dislocationdensity model were solve by adopted K-M dislocation density model to process theflow stresses data which were obtained from hot compression tests. The constitutivemodel of Mg-Al-Ca-Sr alloy based on dynamic recrystallization was developed. The microstructure evolution during hot extrusion processes were simulated by cellularautomation method based on the finite element software platform-DEFORM3D. Thesimulated results were verified by experiments, and the predicted and experimentalresults shared well agreement, which indicated that the predicted results were accurateand reliable.The Cockcroft and Latham criterion, Normalized Cockroft and Latham criterion,Brozzo criterion, Freudenthal criterion and Z parameter method (considering initialtemperature and strain rate) were adopted to predicted the surface crackingphenomena of extrusion profile in the extrusion simulation processing ofMg-Al-Ca-Sr magnesium alloy. The results indicated that the Z parameter methodcould effectively predicte the surface quality of the profiles in the extrusion process,as a result of the consideration of the deformation temperature and strain rate effectson the surface cracking phenomena. There were some limitations for the predictionresults with the other four criteria.