| As one of the lightest structural materials in practical engineering applications,magnesium alloy can meet lots of our requirements for lightweight,so the improvement of the mechanical performance and processing level of magnesium alloy has become one of the major research topics.However,it is urgent for us to reduce the grain size of magnesium alloy,improve the morphology and distribution of the second phase and obtain uniform micro-structure.Although traditional plastic processing technologies such as forging,extrusion and rolling can refine the micro-structure to a certain extent,the plastic strain is small in the processing process and the grain refinement degree is low.Friction stir processing,as a large plastic deformation technology,can provide a new way to greatly refine grains,improve the distribution of precipitates and the plasticity of magnesium alloys.Also,compared with other large plastic deformation technologies,FSP can not only form a micron or sub-micron grade equiaxed grains,but also have a simple process and high efficiency.It does not require preheating of the work-piece and is not restricted by the external environment.FSP is an advanced processing technology with broad application prospects.In this subject,the Mg-4.3Al-0.7Zn alloy was selected as the research object.And the effects of different FSP parameters on defects,microstructure,texture and mechanical properties were studied.Besides,the relationship between process parameters and the average grain size of the stirring zone,between average grain size and micro-hardness,between average grain size and yield strength were established.This provides a reference for the process-structure-performance system of friction stir processing of magnesium alloys.Moreover,the previous studies on the microstructure evolution of FSP magnesium alloys mainly focused on the stirring zone,and lacked systematic and detailed analysis of the various processing areas.In this paper,the differences in microstructure,grain orientation and mechanical properties of FSP magnesium alloy according to different grain morphology zones were studied,and the actuation of different plastic deformation mechanisms in different regions and directions was analyzed,and the influence on tensile properties was further explained.The main conclusions are as follows:(1)The temperature field distribution in the friction stir process was studied.The peak temperature of the stirring zone is not the same as the distance to the processing start point is different.And it is concluded that the peak temperature of the stirring zone estimated by formula T/T0=0.6061×(ω2/v×104)0.0446 is closer to the actual value.(2)The variations of the average grain size in SZ was explored.It was found that FSP could effectively refine the grains.The grain size of BM is 47 μm.Due to sufficient dynamic recrystallization in the stirring zone during FSP,the SZ grain size is reduced to 3.35 μm at 800/60.Moreover,decreasing the rotation speed or increasing the travel speed will reduce the grain size.This paper proposes that the grain will become larger with the increase of ω2/(v×104).The estimation of the average grain size of the stirring zone can be closer to the actual value with the formula lnd=10.8-0.33Z.(3)The texture evolution of friction stir processing was studied.The textures formed in different positions are not the same.With the increase of the distance from the processed upper surface,the c-axis of the SZ grains changes at a certain angle with the PD direction,and the {0001} basal texture has a tendency to be gradually dispersed,and the texture strength gradually decreases.The {0001} basal texture of SZ is distributed along the cylindrical surface of the stirring needle,which is directly related to the flow state and stress state of metal.The texture of TMAZ presents an intermediate state from the texture of HAZ to the edge texture of SZ,and the texture of TMAZ is related to the formation of twins.(4)The general distribution of hardness in different FSP zones and the effects of various FSP parameters on hardness were studied.The average hardness of BM is about 60.8 Hv,and the hardness of SZ can reach up to 74 Hv.The general distribution of hardness in different FSP zones is that the hardness of SZ is greater than the hardness of BM,while the hardness of TMAZ and HAZ is slightly lower than the hardness of BM.Besides,the hardness of AS is lightly greater than the hardness of RS.The hardness will decrease with the increase of the grain size in SZ,in line with the Hall-Petch relationship,Hv=57.4+18.3d-1/2.(5)The difference of mechanical properties in different FSP zones and tensile direction were studied.The mechanical properties of the upper,middle and lower layers of the sample are obviously different,and the tensile properties of the specimens are also different when stretched in the PD and TD.Under the same processing parameters,the elongation in PD is greater than that in TD,while the yield strength and tensile strength in TD are higher than that in PD.This phenomenon is mainly related to the change of grain orientation.As the angle between the c-axis of the grain and the tensile stress direction changes,the ratio of the theoretical critical shear stress to the Schmidt factor becomes different.(6)The influence of different FSP parameters on the tensile properties and fracture behavior along the PD and TD were studied.The texture,grain size,and misorientation angle all make a big difference to the yield strength of magnesium alloys.Therefore,it is not comprehensive to use the Hall-Petch relationship with the grain size as the only variable to discuss the yield strength.After FSP,the magnesium alloy grains are significantly refined,but the strength of the processed magnesium alloy has not been improved,even lower than the strength of BM.However,the main factor affecting the yield strength is the grain size as the low angle grain boundaries,texture orientation and texture strength of magnesium alloy at different FSP parameters are relatively close.The yield strength improves with the decrease of grain size,which conforms to the Hall-Petch law.The formula of PD(σ0.2=31.2+61.2d-1/2)and the formula of TD(σ0.2=34.9+75.5d-1/2)are obtained.(7)The optimal parameters of FSP are explored.The optimal parameters for friction stir process of Mg-4.3Al-0.7Zn alloy should be the inclination angle of 2°,the downward pressure of 3.3 mm,the rotation speed of 1200 to 1600 rpm,and the travel speed of 60 mm/min. |
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