Research On Heat Treatment Technology Of Metal Melt Based On Liquid Structure Changing

The melts of Al-12wt.%Sn, Al-12wt.%4wt.%Si alloy have been researched using the methods such as high temperature DSC, differential thermal analysis, thermal rate treatment processing and rapid solidification.The relationship of the structure and the solidification of the melts has also been analysised. the resistivity of Ga-Sb alloy in different ingredients and in different temperature was studied by the method of DC Four electrodes using the resistivity measuring instrument .The Al-12wt.%Sn alloy was tested by the DSC, there exist heat-sink valleys in temperature of 735-750℃and 845-858℃in the DSC curve, from which the overheating temperature of thermal rate treatment of alloy was determined that is 900℃and 780℃. The results of the thermal rate treatment showed that the structure of the alloy is refined obviously.The Sn phase also tend to be evenly tiny. At the same time, through studies influence of the time of preservation to the hot speed processing of alloy, the most melt that the holding time is short retained the structure of melt at 900℃, and the structure of sample melts is small in the process of quickly quenching to 700℃, due to the existence of relaxation phenomenon.The viscosity of Al-12wt.%Sn-4wt%Si melt was tested by viscosity measurement.The results proved that the viscosity of the melt appear discontinuous changes in 998K-968K and 1138K- 1103K with the drop of the temperature. The Al-12wt.%Sn-4wt.%Si alloy also was tested using DSC, the results proved that the curve still appeared absorptionn in 1013K and 1083K after the alloy was melted , these proved that there exists discontinuous structural transformations in the melt. The formation of Si-Si and Sn-Sn clusters with covalent binding,and Al, Si and Sn atoms combine into clusters and grow rapidly respectively caused discontinuous structural transformations in the melt in the two range of temperature. The temperature of mutations structural transition reflected by the viscosity and the temperature of discontinuous structural transformations attained by DSC are similar, but there are still some difference. This because that the heating system of the two kinds of measurement and the physical properties reflected by them are different during the test process. The solidification of Al-12wt.%Sn-4wt.%Si alloy at 963K, 1100K and 1200K was observed using the method of rapid Solidification after finding abnormal temperature,the results displayed that the higher of the overheating temperature,the tinier of the rapid solidification, especially the high content of white Sn phase, which present diffuse distribution of granular in 1200K after rapid solidification. This is due to the higher of the temperature, the smaller of the size of clusters of atoms in alloy melt, the clusters of atoms can decompose at higher temperatures,the melt tends to be uniform, rapid solidification can retain these characteristics of the melt to as-cast. Therefore, the higher of overheat, the more uniform and fine of the solidification in the range of test temperature .The resistivity- temperature curve of Ga-Sb melts in different ingredientsin was measured by d.c four-probe methord. The instrument was developed by ourselves. It was found that the resistivity of Ga-Sb melts increased with the increase of content of sb.The resitivity-temperature curves of Ga-Sb alloys keep linearity relation, besides that of Ga₅₀Sb₅₀ melt shows an anomalous change in 989-1008 K, The resitivity-temperature curves of the rest of the components are inlinear in the range above the melting temperature 50-100℃due to the produce of clusters of atoms. Ga₅₀Sb₅₀ is a kind of special compound, of which, crystal belongs to semiconductor compound, and acting force among atoms is of covalent bond with covalent tetrahedron coordination structure, but after melting, the electric resistance and temperature coefficient of this alloy displays typical metal characteristics, with acting force among atoms is of metallic bond. When the temperature drops to 1008K, the metallic bonds start to transform to covalent bonds at large quantity, at this moment, atom distribution is in confusional state, and the electronic movement is hindered. With increase of amount of covalent bonds, the electrons in melt that are originally in freedom state are gradually restricted, making motion ability of electrons decrease;therefore, the electric resistance will rise at 1008 K. The covalent bond is basically formed when the melt temperature reaches 998 K, by now, the atom distribution is in ordered state, and the resistance of electronic movement is reduced. Afterwards, the electric resistance will decrease with the reduction of the temperature till the melting point.