Research On Thermal Treatment Technique Of Hypereutectic Al-Si Alloy Melt

In order to connect the research of melt structure with the study of melt thermaltreatment and phosphorus modification of hypereutectic Al-Si alloy, the structure of Al-18Simelt was researched by DSC and the measurement of resistivity. The results show that thereexists an exothermic peak in the temperature range from780to875℃in the DSC curve ofalloy, which suggests that the melt structure changes in this temperature range. In the meltingstate, the resistivity of Al-18Si melt increases linear with the rising of temperature in general.However, the resistivity of melt presents discontinuous changes in the temperature range of777-790℃,819-835℃and945-971℃, which reveals that the melt has different structurein different temperature range. According to the information of melt structure revealled byDSC curve and resistivity testing, the melt was overheated before and after the changes ofmelt structure respectively. The results show that the size, morphology and distribution ofprimary silicon in solidified structure are changed in different superheating temperature andholding time, and the morphology of eutectic silicon can be affectted by superheatingtemperature, which suggests that there is a certain relationship between melt structure andsolidified structure.The effects of different superheating temperature, different holding time and differentsolidified rate on microstructure and mechanical properties of Al-18Si alloy were discussedby treating the melt with thermal-rate treatment technique. The results show that the sizes ofprimary silicon and eutectic silicon tend to decrease first and then increase with the rising ofsuperheating temperature and the prolonging of holding time，and the mechanical propertiesof the alloy are the opposite. When the superheating temperature is930℃and the holdingtime is20min，the morphology of primary silicon evolves from plate-like appearance to fineblocky shape, the average size of primary silicon is decreased from approximately70μm to16μm, the morphology of eutectic silicon evolves from needle-like appearance to finegranular shape, the tensile strength and the brinell hardness are increased by15.38%and8.08%，respectively，compared with the alloy obtained by conventional casting technique. Thissuggests that thermal-rate treatment of the melt can modify the structure and improve themechanical properties of Al-18Si alloy. The mechanical properties of alloy can be enhanced by decreasing the size, improving the morphology and uniformizing the distribution ofprimary silicon and eutectic silicon. The size and the number of primary silicon are inverselyproportional to the solidified rate in the same melt state. This suggests that a lower solidifiedrate is in favour of the precipitation of primary silicon, while the precipitation of primarysilicon is restrained by a faster solidified rate.The effects of phosphorus modification on microstructure and mechanical properties ofAl-18Si alloy were discussed by adding Al-3.5P master alloy into melt. The results indicatethat the alloy can be refined well by phosphorus modification at770℃compared with theone modified at930℃, which suggests that the effect of phosphorus modification is greatlyaffected by modified temperature. Moreover, the effects of the complex treatment ofthermal-rate treatment and phosphorus modification on microstructure and mechanicalproperties of Al-18Si alloy were discussed, when Al-3.5P master alloy was added into themelt at930℃before the melt was treated by thermal-rate treatment technique, the grains ofprimary silicon gathered and the mechanical properties of alloy depressed. When Al-3.5Pmaster alloy was added into the melt at770℃after the melt was treated by thermal-ratetreatment technique at930℃, the microstructure of primary silicon modified and themechanical properties of alloy improved. To compared this technique with conventionalcasting technique, the temperature of numerous nucleation dropped8.7℃and thesupercooling degree increased10.9℃in the solidification of primary silicon, the tensilestrength and brinell hardness of alloy increased by23.02%and11.79%, respectively.The effects of the combination of alloying and thermal-rate treatment on microstructureand mechanical properties of Al-18Si alloy were discussed by way of adding Cu and Mg intomelt and treating the melt by thermal-rate treatment. To compared the Al-18Si-1.5Cu-0.6Mgalloy with Al-18Si alloy treated by thermal-rate treatment technique, TN、TMand TGin thesolidification of primary silicon increased to some extent and the eutectic platform declinedslightly, the tensile strength and brinell hardness of alloy increased by43.99%and40.20%,respectively. In addition, the effects of alloying and the complex treatment of thermal-ratetreatment and phosphorus modification on microstructure and mechanical properties ofAl-18Si alloy were studied. The results show that primary silicon can be refined to fineblocky shape, and the size of primary silicon is refined to approximately16μm when Al-3.5P master alloy was added into Al-18Si-1.5Cu-0.6Mg melt at770℃after the melt was treatedby thermal-rate treatment technique at930℃. Compared with Al-18Si alloy treated byconventional casting technique, the supercooling degree in the solidification of primarysilicon increased8.5℃, the supercooling degree of eutectic reaction raised5.6℃, thetensile strength and brinell hardness of alloy increased by75.94%and66.59%, respectively.