Effect Of Composite Addition On Microstructure And Mechanical Properties Of Mg-Al-Ca-Mn Alloy

Lightweight transportation has become one of the effective means to achieve energy conservation and emission reduction and to cope with energy crisis.As the lightest structural material for industrial applications,magnesium alloy can reduce weight by 20~30% on the basis of aluminum weight reduction,which could meet the urgent needs of lightweight of vehicle body.In order to further improve the lightweight effect of magnesium alloys in automotive applications,the development of high strength,especially high yield strength magnesium alloys has become one of the research hotspots.Rare earth alloying or increasing alloy contents is the main method to increase the strength of magnesium alloys currently.However,the disadvantages,such as increased cost and difficulty in deformation,greatly hinder the application of magnesium alloys.In this thesis,based on the addition of Cu,the effect of multi-addition on the microstructure and mechanical properties of Mg-Al-Ca-Mn alloy under rapid extrusion deformation process was investigated by further adjusting the content of alloying elements.In this thesis,microstructure and morphology of as-cast,as-homogenized and asextruded Mg-Al-Ca-Mn magnesium alloy with composite addition were characterized and analyzed by optical microscopy(OM),scanning electron microscope(SEM),energy dispersive spectrometer(EDS)and electron back-scattered diffraction(EBSD),to study the effects of composite addition on the microstructure,texture,aging behavior and mechanical properties of as-cast,as-homogenized and as-extruded Mg-Al-Ca-Mn magnesium alloy.The results show that:Cu addition can change the microstructure of as-cast Mg-Al-Ca-Mn alloy.With the increase of Cu content,the microstructure of the alloy changed from coarse dendrites to equiaxed crystals and the grain sizes were refined in a certain extent.Cu addition did not significantly refine the grain size of as-extruded Mg-Al-Ca-Mn alloys,but increased the dynamic recrystallization fraction of the alloys.The addition of Ca and the increase of Al content were beneficial to the recrystallization grain size refinement of as-extruded Mg-Al-Cu magnesium alloy with 0.3% Cu,and thus improved the microhardness of as-extruded and as-aged alloys.The average grain size of the as-extruded alloy was not significantly affected by addition of trace Zn and the increase of Al content,but for more uniform microstructure.During the extrusion process,the addition of Cu with trace amounts promoted the generation of twins,which hindered the movement of dislocations and reduced the fraction of low angle of grain boundaries,accompanied with the increase of high angle of grain boundaries.Besides,strong texture characteristics exhibited in the alloys.There was the maximum pole density values in the {0001} basal plane above all as-extruded alloys and the c-axis of most grains began to deflect in the extrusion direction.The addition of trace Cu element had limited effect on the average value of the schmid factor of the {0001} basal plane in the as-extruded alloy,however,with the increase of Cu content,the fraction of the slippage of the {0001} basal plane could be increased.Mg-0.9Al-0.4Mn-0.1Ca-0.8Zn-0.3Cu(AMXZC0803)as-extruded magnesium alloy had the best comprehensive mechanical properties at room temperature,with the yield strength,tensile strength and elongation being 198.6 MPa,254.9 MPa and 17.9%,respectively.The addition of trace Cu element had limited effect on the age hardening effect of as-cast and asextruded alloys,it can significantly improve the yield strength of extruded alloys,although the effects on the ultimate tensile strength and elongation can be ignored;trace Zn addition and the increase of Al content can increase the strength of the alloy due to solid solution strengthening.