The Structure-activity Relationship Of Semi-solid A356 Al Alloy Composite Process And The Mechanism Research Of Its Synergistic Refinement

During the solidification of semi-solid A356 Al alloy,adding the RE and being based on the eutectic reaction of L→α-Al+RExAly,the design of the composite process and the research of its structure-activity relationship were completed,and the effect of the process on the synergetic refinement of solidified microstructure of Al alloy was also discussed.A new idea of isothermal holding process design was mastered with the help of the attributes of the Al-RE eutectic reaction,and based that,through the optimization of the experimental process and the step-by-step verification,the process properties and corresponding experimental parameters in the final composite process were determined.Therefore,the entire thinking logic of this study is to select different attributes refining process according to the order of grain nucleation and growth by combining process design with experimental verification,so as to find the right solution for the problem and get the effective refining effect.A new dual gradient（n-order）isothermal holding process was designed,which was only used in the research field of multiple rare earth refinement of solidified microstructure of semi-solid Al alloy.The solidification process of the alloy corresponds to a temperature range,so a new type of thermal insulation technology which could make the eutectic reactions of the multiple rare earth be related to this temperature range was designed to optimize the characteristics of the nucleation substrate and enlarge the refining capacity of rare earth in the solidification temperature range of the alloy.According to the process planning procedure,the multiple rare earth of Gd,Y,Nd,and Ce were selected,and the dual gradient（fourth-order）isothermal holding process of 645℃/30 s-635℃/40 s-625℃/50 s-615℃/60 s was constructed.The process is replaceable in terms of rare earth species and chemical reaction types,flexible in terms of rare earth numbers and gradient temperature orders,and controllable in the occurrence state of eutectic reaction.The effects of x%Nd and electromagnetic stirring on the refinement of the primary phase and the tensile properties of the semi-solid A356 Al alloy were studied.The relationship between x%Nd and NdxAly including their corresponding laws of physical and chemical evolution may be summarized as follows:when the x%Nd（x=0.1⁰.8）is changed,possibly according to the phase transitions of NdAl₂-NdAl₃-Nd₃Al₁₁-NdAl,its morphology follows the evolution of"（block-small bean）-（particle-short rod）-（needle-strip）-（plate）",and the characteristics of the nucleation efficiency about those NdxAly regarding as the heterogeneous substrates are presented"NdAl₂（semi-coherent,the effective nucleation）-NdAl₃（11.53%,Secondary nucleation）-Nd₃Al₁₁（6.13%,close to the most effective nucleation）".It speculate that the different NdxAly have different influence on the refinement of primary phase:the NdAl₂ is more inclined to inhibit the growth of primary phase,the NdAl₃ has an effect on promoting nucleation of primary phase and inhibiting its grain growth（but the inhibitory effect is greater）,and the Nd₃Al₁₁ mainly is focused on promoting the nucleation of primary phases.The optimized process composed of 0.6%Nd+（30 Hz-15 s-4 A）can obtain more remarkable refinement and strengthening effect.The corresponding average grain equal-area circle diameter is 87.15μm,the shape factor is 0.55,the number of effective primary phase is 70,the average tensile strength of the alloy is 218.80 MPa,and the elongation is 8.3%.The effect of multiple rare earth（Gd,Y,Nd,Ce）on the gradient refinement of the solidified structure of the semi-solid Al alloy was verified using the dual gradient（fourth-order）isothermal holding process.The primary and secondary effects and the change ranges of multiple rare earth on the different refining characteristic parameters are different:D:Y>Ce>Gd>Nd;F:Y>Ce>Gd>Nd;n:Y>Gd>Ce>Nd,but in general,the effects on D and N are greater than F.The optimal proportion of multiple rare earth is:0.5%Gd+0.4%Y+0.2%Nd+0.2%Ce,and the corresponding product proportion order is:α-Al_（Nd）>α-Al_（Ce）>α-Al_（Y）>α-Al_（Gd）>>GdAl₃>YAl₃>Al₁₁Ce₃>Nd₃Al₁₁,in which the proportion ofα-Al is more than 99%,while the RExAly is less than 1%.The dual gradient（fourth-order）isothermal holding process can promote the formation of eutectic products with the high density（α-Al）or the diversification（RExAly）to improve the refining ability of RE.The primary phase is obtained with the equal-area circle diameter of 85.118μm,shape factor of 0.786,the number of 75.A new composite process was designed and its structure-activity relationship was also explored.Different refinement processes have different influence on the nucleation basement.According to the order of grain nucleation and growth,the refinement processes with specific attributes are designed separately,and the dual gradient（fourth-order）isothermal holding process is used to link them,so as to build a new composite process of"chemical refinement+gradient isothermal holding+physical refinement".The structure-activity relationship of the composite process was clarified,and they were the innovativeness of the structure and the superiority of the effect.The feasibility of the composite process design was verified by experiments,and its the synergistic refinement effect on the solidification structure of Al alloy was studied.Using the triple refinements,the composite process can increase the number of primary nucleus in the same batch to promote the relatively uniform and stable nucleation,and achieve the expected results of synergetic refinement.Under the condition of composite process[（0.5%Gd+0.4%Y+0.2%Nd+0.2%Ce）+（645℃/30 s-635℃/40 s-625℃/50 s-615℃/60 s）+（30 Hz-15 s-4 A）],the average diameter of primary phase is 80.408μm,the shape factor is 0.746,the number of effective grains is86,and the synergistic refining effect is remarkable.