The Study On The Modification Of Hypereutectic Aluminum-Silicon Alloy Using Thermal Analysis Technology

The hypereutectic Al-Si alloy is widely used in manufacture of automotive engine piston and other parts, but the large primary silicon and the long acicular eutectic silicon exist in the unmodified alloy, which affect the mechanical properties of the alloy greatly. Modifications are widely used in industry in order to improve the mechanical properties. Among the methods of detecting the effect of the modification, thermal analysis has great theoretical and practical significance. In this paper, with the improvement of the hardware and software of the original eutectic Al-Si alloy thermal analysis system, a new system which can measure the modification effect of the hypereutectic Al-Si alloy within 5 minutes was developed.By the use of the I-7018 data acquisition module and the I-7520 signal converter module, a new system was developed. The collection of the original temperature data could be completed accurately through serial communication, three commonly used methods of serial communication under VC environment had been compared, the multi-threaded category–CserialPort was selected to control the serial communication. The serial communication could be completed not only through the RS232 serial port but also through the USB interface with the conversion line, so that the speed, flexibility and convenience of the data transmission were greatly improved. The filtering of the original temperature data was implemented by calculating average and medial value, the derivative curve was calculated using three points central difference method, then the adjacent average filtering method was chosen to filter the derivative curve. According to the cooling characteristics of the hypereutectic Al-Si alloy, corresponding algorithm was adopted to identify the thermal analysis characteristic values of the alloy.Using the thermal analysis system, the effect of primary silicon modification of the Al-20Si-1.5Cu-1Ni-0.5Mn alloy newly developed by our group was studied in order to find the relationship between the effect of the modification and the thermal analysis characteristic values, it is found that,ΔTG ( the difference of the primary silicon growth temperature) has a good relationship to the average grain size of the primary silicon, ifΔTG was between 35℃and 45℃, the primary silicon was well modified, ifΔTG was lower than 30℃, the alloy was unmodified.The effect of eutectic silicon modification of the Al-20Si-1.5Cu-1Ni-0.5Mn -0.4Mg alloy had also been studied, it is found that, ifΔTEG(the difference of the eutectic silicon growth temperature) was between 6.5℃and 7.3℃, the eutectic silicon was well modified, ifΔTEG was lower than 4.3℃, the alloy was unmodified,ΔTEG had a good relationship to the effect of the modification of the eutectic silicon.