Study on the Microstructure and Mechanical Properties of Rheo-squeeze Casting Mg-Nd-Zn Alloys

Author:Chen Yu Shi

Supervisor:wu guo hua


Degree Year:2018





Mg-RE alloys have been widely used in the automobile and aerospace industries because of their good room and high temperature mechanical properties,creep resistance and corrosion resistance.Among them,Mg-Nd-Zn alloys present good comprehensive mechanical properties and castability with low content of RE elements and cost.Therefore,these alloys are very attractive for the applications in the automobile,aerospace and military or weapon industries.Generally,Mg-Nd-Zn alloys were produced by permanent mold casting(PMC)and low-pressure sand mold casting(LSMC).However,it is difficult to avoid the porosities and produce components with thin-wall and complex shape for the PMC technology.And the grain size of the LSMC alloys was larger than that cast by a metal mold with a low production efficiency.In order to overcome the above shortcomings and improve the mechanical properties of the alloy,it is important to develop new techniques for forming the Mg-RE alloys.It is also beneficial to the further applications of the Mg-RE alloys.Rheo-squeeze casting is a new promising near-net forming technology.In this technology,semisolid non-dendritic slurry is prepared,and then the slurry is formed by squeeze-casting to obtain components.It can not only refine the grain size,reduce and even eliminate the shrinkage and porosities,improve the mechanical properties of the alloy,but also can form the components with large and complex shape.The present studies focus on the rheo-squeeze casting of Al alloys,the related investigate on the Mg alloy is scarce.The mechanical properties of the Mg-Nd-Zn alloys are expected to be enhanced by using rheo-squeeze casting,and the study on the formation mechanism of semisolid non-dendritic primary particles and the strengthening mechanism provides theoretical and experimental basis for expanding the further application of Mg-RE alloys and rheo-squeeze casting technology.In this study,the electromagnetic stirring(EMS) was used to prepare the Mg semisolid slurries,then the slurries were formed by squeeze casting.The effects of stirring parameters(stirring time,applied voltage and rotational frequency) on the formation of Mg-Nd-Zn alloy semisolid slurry,the influences of rheo-squeeze casting processing parameters(mold temperature,pressure) on the microstructure and mechanical properties of Mg-Nd-Zn alloy,the effects of minor Y addition on the microstructure and mechanical properties of NZ30K alloy were systematically investigated.The results are listed below.The microstructure evolution and formation mechanism of non-dendritic primary particles of Mg-Nd-Zn alloys during EMS treatment(rotating magnetic field)were revealed.The influence of the Zr particle on semisolid non-dendritic slurry and the effect of EMS parameters on the alloy melt were verified.Mg-3Nd-0.2Zn(NZ30)and Mg-3Nd-0.2Zn-0.4Zr(NZ30K)alloy melt were stirring,the effect of different parameters on the semisolid microstructure was studied.The results showed that after being treated by EMS,alloy melt with uniform solute solubility field and temperature field was obtained.The morphology of primary a-Mg phases in the NZ30 alloy slurries evolved from dendrite to three types:rosette,dendrite,and spheroid.The main phenomena occurred in the NZ30 alloy slurries are nucleation and dendrite fragment in early stages,and ripening in latter stages.The dendrite fragmentation criterion could be used to study the formation mechanism of non-dendritic particles during the rheocasting process.As for NZ30K alloy,the Zr particle distribution in the slurry was improved,and the content of active Zr particles which promoted the nucleation was increased after EMS treatment.An ideal semisolid microstructure with small and spheroidal particles was obtained in the NZ30K alloy slurries.Nucleation and subsequent ripening were the main phenomena during solidification under stirring.It was found that with addition of Zr paricle in the Mg-Nd-Zn alloy,the main formation mechanism of the primary particle during EMS treatment was transformed from dendrite fragmentation to directly nucleation.The data for solid fraction with stirring time could be fitted to linear equations,which can be used as a process control in the rheocasting process.The data for the particle size of NZ30K alloy at different stirrings times could be fitted using a ripening model.The increase of applied voltage and rotational frequency made the primary a-Mg phases initially refined and then coarsened.A new RSC tehchnology suitable for preparation of Mg-Nd-Zn alloy was developed,and the optimal processing parameters were obtained.The optimal processing parameters were stirring time 120-180 s,applied voltage 300-350 V,rotational frequency~20 Hz for NZ30 alloy,and stirring time 30-180 s,applied voltage~350 V,rotational frequency~20 Hz for NZ30K alloy.The average particle size of NZ30 alloy could be refined from~879 to~457 μm.The relationship between processing parameters(including applied voltage and rotational frequency)and the undercooling,melt temperature could be obtained based on the thermodynamics theoryThe effect of mold temperature on microstructure and mechanical properties of rheo-squeeze casting(RSC)NZ30K alloy was revealed.The results indicated that the rise of mold temperature contributed to the increase of particle size,and the decrease of dislocation density,eutectic phase size and secondarily solidified structre.In addition,the alloy density was increased and more compact microstructure was obtained.Therefore,the mechanical properties of the alloy were increased with the mold temperatrue rising from 200℃ to 300℃.However,the mechanical properties were decreased with the mold temperatrue further rising.Grain boundary strengthening and second phase strenghening were the main strengthening contributions in the as-cast alloy.After T6-treated,the chang of mechanical properties of the alloy showed the same tendency with the as-cast alloy,and the main strengthening contributions were grain boundary strengthening and precipitation strengthening.The influence of applied pressure on microstructure and mechanical properties of rheo-squeeze casting NZ30K alloy was revealed.The relationship among applied pressure,undercooling and nucleation rate N=nkT/h exp was obtained based on the Clausius-Clapeyron equation.It was found that when the undercooling was less than 13.8℃,the nucleation rate of the NZ30K alloy were increased with increasing pressure.In addition,the solid solubility of Nd and Zn in the a-Mg matrix,and dislocation density were increased with increasing applied pressure.The mechanical properties of as-cast alloy were initially increased and then decreased with increasing pressure.The increase in the mechanical properties was mainly attributed to the refinement of the particles and more compact microstructure obtained.The further decrease of tensile properties was related to the local stress concentration.While the properties of T6-treated alloy were increased continuously and the yield strength(YS),ultimate tensile strength(UTS)and elongation could reach 165 MPa,309 MPa and 5.7%,respectivelyThe different strengthening mechanisms of the Mg-Nd-Zn alloy prepared by different methods including PMC,CSC(conventional squeeze casting)and RSC were investigated.Compared with the PMC alloy and CSC alloy,the grain size and the amount of eutectic structure in the RSC alloy were smaller.Moreover,the eutectic phases were uniformly distributed among the particles in the RSC alloy.Therefore,the RSC alloy presented the best comprenhensive mechanical properties.After aging treatment,β’ phase was the main precipitate in the PMC alloy and RSC alloy,while two types of precipitates including β" phase and β’ phase were observed in the RSC alloy.Compared with the T6-treated PMC sample,the T6-treated RSC sample presented higher mechanical properties due to the effect of grain refinement.Compared with the T6-treated CSC sample,the T6-treated RSC sample showed higher mechanical properties due to the larger precipitation strengthening contribution.The effect of minor Y addition on microstructure and mechanical properties of PMC and RSC Mg-Nd-Zn alloy was revealed.With the increase of Y content,both the average grain size of the PMC and RSC alloys were refined.In addition,some Mg24Y5 phase formed in the as cast Y-containing alloys and this phase was uniformly distributed in the grain boundaries of 0.39 wt% Y-containing alloy.The precipitation strengthening contribution was increased with increasing Y content.After aging treatment,the amount of Zn2Zr3 phase was incteased,and the maximum was reachend in the 0.39 wt% Y-containing alloy due to the segregation of Zn atom.0.39 wt.%Y-containing alloy exhibited the best combination of strength and elongation in all conditions.In particularly,the yield strength of the PMC alloy was significantly enhanced in the temperature range of 200-300℃(~150 MPa).The improvement in mechanical properties was attributed to grain refinement,solution strengthening and enhanced precipitation hardening of the Zn2Zr3 phase and β’ phase.Furthermore,an Arrhenius model was established to quantify the effect of deformation temperature and strain rate on the flow stress of the PMC 0.39 wt% Y-containing alloy:σ=(169.69(?) exp(223045/RT))0.098.The stress exponent value was higher than the Y-free alloy.Compared with the PMC alloy,the grain size of 0.39 wt%Y-containing alloy was signiciantly refined after RSC process,and the mechanical properties were enhanced.After T6 treated,the YS,UTS and elongation of the RSC 0.39 wt% Y-containing alloy were up to 168 MPa,329MPa and 8.7%.