| The mechanical properties of metal materials are determined by its composition and structure,and the initial solidification structure of metals after smelting has significant effect on the follow-up structure in heat treatment process,rolling process and its final structure.However,coarse solidification structure often forms because of the unreasonable heat transfer condition after pouring the molten metal.In addition,the shrinking character of metal solidification frequently leads to defects such as porosity and shrinkage.Therefore,fine and compact structure of metals is one of the targets that are always pursed in the casting process.In this work,super gravity was applied in metal solidification process with focused on the refining effect of super gravity on the solidification structure.Simultaneously,the elements macrosegregation behavior was also paid attention in metal solidification process.In this research,the refining effect and refining mechanism of super gravity on Al-Cu alloy and Cu-Sn alloy,which have lower melting points,have been systematically studied firstly.Then the macrosegregation behavior and the macrosegregation mechanism of elements in super gravity field were totally investigated and discussed.Later,the combined refining effect of super gravity and grain refiner Ti and Ce on pure A1 has been exployed.Finally,Preliminary investigation was conducted concerning the refining effect and the macrosegregation behavior of steels in super gravity field,which have higher melting point,with the hope of providing theoretical guidance for broadening the application of super gravity technology on improving metals'solidification structure.The results about the solidification structure refinement of Al-Cu and Cu-Sn alloys by super gravity showed that super gravity could effectively refine the solidification structure of the two alloys.The solidification structure of Al-4.5wt%Cu and Cu-11wt%Sn alloys became refined with the increasing gravity coefficient,with coarse columnar grains changing to fine equiaxed grains and also the dendritic grains changing to globular grains as a result of the primary dendrite and the secondary dendrite becoming short.The refining effect of super gravity on pure metal(pure Al and Cu)was obviously not as good as alloys(Al-Cu alloy and Cu-Sn alloy).In addition,the refining effect became better with increasing the solute content.After Al-4.5wt%Cu alloy was solidified in super gravity field of G=800,the tensile strength markedly increased to 220MPa compared with that of 136MPa in normal gravity field.Simultaneously,when Cu-llwt%Sn alloy was solidified in super gravity field of G=600,the tensile strength increased to 521MPa from that of 265MPa in normal gravity field.The study concerning with the refining mechanism of super gravity on the solidification structure of Al-Cu alloy and Cu-Sn alloy indicated that because there is density difference between the precipitated solid and the remining liquid in the solidification process of Al-Cu alloy and Cu-Sn alloy,"Crystal Rain"caused by super gravity happened in the solidifying melt,which is the main refining mechanism of solidification structure by super gravity.And it was only at the initial solidification stage that the "Crystal rain" could happen,meaning that the solidification structure could be refined only when super gravity was applied at the initial solidification stage and it made no sense with appling super gravity at the later stage.With the increasing solute content,the nucleation rate increased and the density contrast increased between the initial precipitated solid and the liquid metal.As a result,the "Crystal Rain" caused by super gravity became heavier,leading to more refined structure finally.The macrosegregation behavior of solute elements in Al-Cu alloy and Cu-Sn alloy in super gravity field has been systematically studied,and the results showed that macrosegregation of solute Cu along the direction of super gravity and solute Sn along the opposite direction of super gravity was generated after Al-Cu alloy and Cu-Sn alloy were solidified in super gravity field,respectively.The macrosegregation became severer with the increasing gravity coefficient and solute content.The macrosegreation of solute Cu in Al-Cu alloy in super gravity field could be attributed to gravity segregation and channel segregation.At the early solidification stage,the more solute-riched liqud settled to the bottom for its higher density,causing the macrosegregation.This gravity segregation casued by the density difference among liquid contributed the main macro segregation at early solidification stage.At the later solidification stage,the solute-riched liquid remining at the interdendrite flowed to the bottom of the casting driven by super gravity,which contributed to the main macrosegregation at the later solidification stage.The combined refining effect of pure A1 by super gravity and grain refiner Ti and Ce has been exployed,and the results showed that the solidification structure of pure A1 can be effectively refined by combination of super gravity and grain refiner Ti and Ce.With gravity coefficient G=500 and 0.15wt%Ti or Ce,the solidificaton structure of pure Al changed from coarse columnar or equiaxed grains to fine equiaxed grains.The refining effect became better with increasing gravity coefficient and content of grain refiner.The macrosegregation phenomenon of grain refiner appeared along the direction of super gravity after combined refinement.The macrosegregation became severer with the increasing gravity coefficient and refiner content.The results related to solidification structure refinement and macrosegregation behavior of steel materials in super gravity field illustrated that super gravity could effectively refine the solidification structure of Fe-1.5C-1.5Si-1B high speed steel,Fe-lwt%C high-carbn steel and Fe-0.09wt%C low-carbon steel.In super gravity field of G=600,the solidification structure of Fe-1.5C-1.5Si-1B high speed steel changed from coarse columnar grains in normal gravity field to fine equiaxed grains,with the primary dendrite and secondary dendrite shortening and changing to globular grains.In super gravity field of G=70,the solidification structure of Fe-1wt%C high-carbon steel changed from extremely developed dendrite in normal gravity field to much refined dendrite.In super gravity field of G=70,the austenite grains were obviously refined for Fe-0.09wt%C low-carbon steel.After solidified in super gravity field of G=600 for Fe-1.5C-1.5Si-1B high speed steel,elements C and B were slightly segregated to the up position of the casting and element Si was segregated to the bottom of the casting.When Fe-lwt%C high-carbon steel was solidified in super gravity field of G=70,element C was slightly segregated to the up position of the casting.There is no obvious macrosegregation of element C for Fe-0.09wt%C low-carbon steel in super gravity field of G=70. |
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