The Microstructure And Property Of Al-Cu-Si Ternary Eutectic Alloy During Directional Solidification

The research on solidification mechanism of multicomponent and multiphase alloysalways drew attention of solidification theory because of its difficulty, the solidification ofeutectic alloys was the most complicated and various among them, and existed generallyin the industrial production, especially the widely used Al-based alloys. In this paper,Al-Cu-Si ternary eutectic alloy was directionally solidified at a constant temperaturegradient5K/mm with the pulling-down speeds of1,3,5,12,20,30, and35mm/min. Theinfluence of pulling-down speeds on microstructure morphology, microstructure parameterand properties was analyzed by the means of optical microscope, scanning electronmicroscope, transmission electron microscope, microhardness tester and universalstretcher. The solidification mechanism of Al-Cu-Si alloy was discussed.It was found that the microstructural morphology of Al-Cu-Si eutectic alloy wasdeveloped with the increase of pulling-down speeds, when the pulling-down speedreached1mm/min, the microstructure was consisted of heterogeneous ternary eutectic,when the pulling-down speed exceeded1mm/min, the divorced eutectic consisted of Sibulks surrounded by binary eutectic (-Al+-Al2Cu) was formed during solidification,with the further increase of pulling-down speeds, the morphology of eutectic colonychanged as follows: cellular dendrite columnar dendrite some developed tips ofsecondary dendrite arms the dendrite framework with triple dendrite arms, when thepulling-down speed exceeded30mm/min, the equiaxed primary phase Al2Cu appearedand broke the directional growth of columnar dendrite. With the increase of pulling-downspeeds, the volume fraction of eutectic colony increased firstly, and then decreased atpulling-down speeds of1~30mm/min, the change of Si plates’ volume fraction wasopposite of eutectic colony. However, their volume fraction all decreased when thepulling-down speed exceeded30mm/min. The microhardness of eutectic colony increasedin the range of1~20mm/min, and then decreased when the pulling-down speeds exceeded20mm/min, the electrical resistivity of the alloy at room temperature decreased.Analyzing the morphology of Al-Cu-Si eutectic microstructure in approximateequilibrium solidification, the growth model of Al-Cu-Si ternary eutectic alloy was set up,the formation sequence of phases, Si Al2Cu-Al, was determined, and the growthdirection of twin crystal in Si plates was <110>.