| High Si Al-Si alloys are those of which the Si content is more than 20 wt.%.Due to its light weight,friction resistance,heat resistance,corrosion resistance,low thermal expansion coefficient and excellent castibility,high Si Al-Si alloy has become an important casting aluminum alloy which has aroused considerable attention.It is mainly used in manufactural fields,such as automobile,watercraft,high speed train,aircraft and so on.However,coarse primary Si lump forms with the increasing of Si content,which damages the mechanical property of High Si Al-Si alloy severely and limits its industrial application,leading to a lack of high Si Al-Si alloy compositions both at home and abroad as well as many difficulties in the formation of high Si Al-Si alloy.On the basis of experimental research,optical microscopy,X ray diffraction,scanning electron microscopy,thermal analysis,room and high temperature mechanical property testing were applied.In this thesis,high Si Al-Si alloy and its corresponding rheological die casting were investigated,the effects of Cu content on the microstructure,mechanical property and hot tearing susceptibility of die casting Al-22Si-0.4Mg alloy were demonstrated,Al-22Si-2.5Cu-0.4Mg alloy adapted to high pressure die casting and its corresponding rheological slurry preparation method and rheological die casting process were developed,the influences of rheological die casting processing parameters on the microstructure of Semi-solid slurry,microstructure and mechanical property of rheological die casting sample were illustrated.The results are shown as follows:Significant effects of Cu content on the microstructure,mechanical property,and hot tearing susceptibility of die casting Al-22Si-0.4Mg alloy were found.With the increasing of Cu content,the amount of Al2 Cu increased with the morphology transformation at the same time.When Cu content was low(1.5wt.%),Al2 Cu existed as particle.When Cu content was moderate(2.5wt.%),Al2 Cu existed as thin and short rod.When Cu content was high(≥3.5wt.%),Al2 Cu existed as thick and long rod or big lump.When Cu content was more than 3.5wt.%,Al2 Cu formed continuous intergranular net.The addition of Cu had no effect on the grain size,morphology and distribution of primary Si,eutectic Si and α-Al.As Cu content increased(1.5-4.5wt.%),the room temperature mechanical property rose gradually,from 197 MPa to 230 MPa.The room temperature elongation rate showed a tendency of decreasing with Cu content increased.Constrained hot tearing rod method was adopted to study the hot tearing susceptibility of Al-22Si-x Cu-0.4Mg alloy,which showed that with the increasing of Cu content,the hot cracking susceptibility value of Al-22Si-0.4Mg alloy increased,from 16 to 48,240,272.The liquid high pressure die casting formability of Al-22Si-0.4Mg alloy was greatly affected by Cu content.When Cu content was less than 2.5wt.%,the hot tearing susceptibility was low and had no influence on the die casting formation property.The die casting sample did not fracture during solidification.When Cu content was more than 2.5wt.%,the hot tearing susceptibility increased severely,leading to the fracture of sample during solidification.Considering both comprehensive mechanical property and die casting formability,the optimal Cu content in Al-22Si-0.4Mg alloy was 2.5wt.%.The rheological slurry preparation optimization was carried out by using a self-made low superheat pouring shearing field method with corresponding apparatus.The rotation speed of the barrel and the melt pouring temperature were investigated and optimized.The Semi-solid microstructure of Al-22Si-2.5Cu-0.4Mg alloy was greatly influenced by the rheological slurry preparation processing parameters.It showed that the optimal melt pouring temperature was 800oC and the rotation speed of the barrel was 90r/min.Under the optimalcondition for preparation,the average particle size of primary Si was about 30 μm and the primary Si dispersed evenly.Meanwhile,the influence of melt pouring temperature and rotation speed of the barrel on the microstructure of rheological slurry was studied.The effects of rheological die casting processing parameters on the microstructure,comprehensive mechanical property and die casting formability of Al-22Si-2.5Cu-0.4Mg alloy were investigated by comparing with liquid high pressure die casting.Based on the room temperature mechanical property and die casting formability,optimal rheological die casting parameters and liquid high pressure die casting parameters were given.When in the rheological die casting situation,the optimal rotation speed of the barrel was 90r/min and the optimal melt pouring temperature was 760oC.When in the liquid high pressure die casting situation,the optimal melt pouring temperature was 680oC.Compared with liquid high pressure die casting,rheological die casting obviously refined the primary Si and dispersed primary Si evenly,decreasing the average particle size of primary Si from 45μm to 27μm.Besides,it passivated the eutectic Si to some extent.It improved the room-temperature mechanical property of alloy from 204 MPa to 225 MPa,enhancing the 300oC mechanical property from 102 MPa to 116 MPa.Meanwhile,it decreased the hot tearing susceptibilityand improved the die casting formability of alloy.As for Al-22Si-2.5Cu-0.4Mg alloy,rheological die casting can apparently refine its microstructure and improve its mechanical property,which makes it possible to beapplied in industrial production. |
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