| The melt-treatment technology and grain refinement are crucial steps for the production of Mg-RE-Zr castings with high quality.Although the Zr has the potential refinement and has been widely used for this kind of Mg alloy,the effects of melt process factors on grain refinement and mechanism for grain refinement fading are unclear.Simultaneously,there are limit comprehensive studies so far on the fatigue properties,mechanical properties at elevated temperature as well as creep resistance.Therefore,the melt processes and the effects of grain size on mechanical properties of a Mg-Nd-Zn-Zr alloy were systematically investigated in this thesis.The factors influencing grain refinement were analyzed.The effects of grain size on fatigue properties at room temperature were also investigated and the behaviors of crack initiation and propagation were analyzed.The effects of grain size of Mg-Nd-Zn-Zr alloy on mechanical properties and creep at 250 ? were measured and the critical grain size for requirement of designing was determined.The changing rules for contents of soluble Zr and insoluble Zr d uring melting were studied.The mechanisms of grain refinement fading for Zr-containing Mg alloys were explored.A new protective gas used for melting of Mg alloys was developed for replacing SF6 containing gas,and its protective mechanism was analyzed.The casting processes for a typical casting were optimized by numerical simulation.Finally,the melt process and the protective behavior of the developed gas were tested by pouring the typical casting.This is both theoretical and actual important in improving the technique level of Mg alloy in defense field.The investigations on the melting processes of the Mg-Nd-Zn-Zr alloys indicated that without stirring,the Zr could not dissolve into the melt even after a long time isothermal holding and enough Zr addition,resulting in a coarse a nd columnar structure with an average grain size of 2400 ?m.After 10 mins stirring,the soluble Zr was up to 0.68 wt% and grain size decreased to 62?m,and the grain structure was evolved into equiaxed one.However,the melting temperature in the range of 730-830 ? had little influence on the dissolution of Zr.With increasing the content of Zr,the morphology of grain changed from columnar to equiaxed,and the critical value of Zr content for this change was determined to be 0.14 wt%.The relationship between soluble Zr and grain growth restriction factor(GRF)was established.When soluble Zr was less than 0.54 wt%,no Zr-rich core was observed.The single layer Zr-rich core formed with further increasing of soluble Zr content and the double-layer Zr-rich core or multi-layer Zr-rich core appeared when the soluble Zr content was more than 0.68 wt%.The concentration of soluble Zr had little change during isothermal holding.However,the total Zr concentration lowered with prolonged time.At the same,the g rain size increased,leading to a fading of grain refinement.The settling down of Zr particles was attributed to the fading of grain size refinement by calculating the relationship between the settle down of Zr particles and grain size.The insoluble Zr h ad a significant effect on the grain refinement,determining by the amount of Zr addition.It could be up to 60 wt% on the contribution of grain refinement when 3.2 wt% Zr was added into the melt.Therefore,its effect on grain refinement was more than soluble Zr.The fatigue tests were carried out in order to investigate the effect of grain size on the fatigue lives.There was a wide span in the fatigue lives under the same applied stress,and this span became more big with the decreasing applied stress.The grain refinement reduced the span of fatigue lives,resulting in an enhanced fatigue strength.The fatigue strength increased from 60 MPa to 70 MPa when grain size decreased from 596?m to 94?m.The Weibull distribution was used to analyze the fatigue live distribution.For the coarse-grained Mg-Nd-Zn-Zr alloy,the shape parameters,m,under loading of 90 MPa and 80 MPa,were 0.93 and 0.91,respectively,showing an exponential distribution for the density function,where,for the fine-grained alloy,the shape parameter was 1.05 at 90 MPa,indicating a near exponential distribution.The effect of grain size on the fatigue crack propagation showed that the threshold value,Kth,for crack propagation increased from 1.74 MPam1/2 to 3.23 MPam1/2 when the grain size increased from 94?m to 596?m.At the same stress intensity factor,the fine-grained alloy showed a faster crack propagation rate than that of coarse-grained alloy,especially at the early stage of crack propagation.The crack propagation rate was increased with the enhanced fatigue frequency for both the coarse-grained and fine-grained alloys,but the sensitivity of frequency to crack propagation rate was lowered for the coarse-grained alloy.The crack propagation route for the coarse-grained alloy moved in zigzags,especially at the early stage,meanwhile,the fine-grained alloy showed a relatively smooth route.The fatigue cracks mainly propagated transgranularly for both coarse-grained alloy and fine-grained alloy with a small amount of intergranular ones.Brach appeared during crack propagating.The slip bands were obviously observed in front of the crack tip for the coarse-grained alloy.A systematic study has been made on the effect of grain size on the tensile and creep properties of Mg-Nd-Zn-Zr based on the analysis of its microstructure.The tensile strength,yield strength and the elongation to failure were all increased with the decreasing grain size at grain size ranging from 102-920?m.Both tensile strength and yield strength followed the Hall-Petch relationships while the ductility increased linearly with decreasing grain size at 250 ?.The steady-state creep rate at 250 ? increased significantly with decreasing grain size.The creep processes were controlled by dislocation glide and climb.The effect of grain size on creep is related to the applied stress where the grain size effect exponent showed a linear dependency on applied stress.To satisfy the design criteria for both the tensile and creep properties for long-term service at 250 ?,the maximum grain size of Mg-Nd-Zn-Zr alloy should be limited to about 174?m.In order to reduce the inclusion forming during melting and pouring by flux protection,a new mixed gas,consisting of SO2 and CO2,was developed for protection of the melt of Mg alloys.The structure of the protective film was analyzed and the protective mechanism was also determined.The mixed gas exhibited a good protection at the temperature range of 710~800 ? for Mg-Nd-Zn-Zr alloy.Finally,the filling process was simulated by a commercial software to optimize the casting processes.A typical casting was cast under the protection of the mixed gas and the refinement process developed in order to validate the refinement process and protection of the mixed gas.The surface and internal qualities as well as the mechanical properties of the casting satisfied the design criteria. |
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