Research On Melt Temperature Treatment And Modification Of Hypereutectic Al-si Alloy

In developed countries, hypereutectic Al-Si alloy has a tendency to take place traditional materials because it is wear-resistant with volume stability. But in China, this material has not been popularized due to the difficulty to control the primary silicon microstructure. On the other hand, how to manipulate the solidification structure by controlling melt state, modification and other factors requires the improvement of fundamental solidification and solidification technology. This paper firstly explores the relationship between the melt state of Al-18%Si and its temperature by means of the resistivity method. Based on this obtained information, two groups of solidification experiments (overheating treatment of melt in one group and high-low temperature melt mixed treatment in another group) are carried out to explore the influences of the melt state on primary silicon and eutectic silicon respectively in the hypereutectic Al-Si alloy. This paper also creatively investigated into low-temperature melt which is in the semi-solid temperature zone and high-low temperature melt mixed treatment. What's more, the complex modification function of P-Re-B master alloy to Al-18%Si alloy is discussed here. On the whole, research contents and conclusions of this paper are as follows:1. The resistivity-temperature behavior of Al-18%Si melt (ρ-T curve) in the temperature-rising period reveals that between 800℃and 950℃above the liquidus, there is a peak with low and wide peak height in theρ-T curve. But between 950℃and 1050℃, there is a narrow but obvious temperature peak in theρ-T curve. Afterwards the resistivity of the melt enters into a new linear change phase. It is indicated that pre-transformation occurs in the first peak and irreversible liquid-liquid structural change occurs in the second peak with a result of a more even structural behavior.2. By dealing with Al-18%Si alloy with overheating treatment and solidification experiments, it is found the grain size of primary silicon, obtained from melt insulated at 950℃and rapidly cooled to 760℃, has no evident difference compared with the one solidified from insulated melt at 760℃. However, primary silicon which is obtained from melt insulated at 1150℃and rapidly cooled to 760℃, is obviously refined. The undercooling difference reflected by the cooling curve is basically in accordance with the law of solidification microstructure. 3. In high-low temperature melt mixed treatment; the high melt temperature is 1150℃while the low melt temperatures are respectively 620℃and 640℃among the semi-solid range as well as 700℃and 760℃above the liquidus line. And the proportion of melts at high temperature to the ones at low temperature is changed. The results show that primary silicon in the sample obtained by mixing melts at 1150℃and 640℃is the finest. In addition, the bigger the proportion of high-temperature melt is, the finer the crystal grain of primary silicon is. Seen from the mechanism, 640℃is the most favorable to fusing and proliferation in silicon dendritic crystal among the semi-solid range; and the rise of the proportion of high-temperature melt promotes fusing and proliferation of silicon crystal and increases the nucleus.4. After 640℃is decided as the low melt temperature, change the thermal history of high-temperature melts. The temperatures are 1150℃, 1150℃-950℃and 950℃respectively. It is found that high-temperature melts at 1150℃undergone irreversible transformation has better refining effect to primary silicon than the melts which have not undergone such transformation.5. Conduct compound modification to Al-18%Si by using P-Re-B master alloy. It has obvious positive influence on both primary silicon and eutectic silicon. Under the optimal P-Re-B compound modification technique (AlP0.3%+ AlRE2%+AlTiB1.2%), the average diameter of primary crystals is refined to 9.00μm from previous 33.7μm; their shapes change from coarser and brittle to more-edges-distributed fine particles with passivated edges; the eutectic silicon is also changed from acicular to fibrillar.To sum up, the results of this paper show it can avoid blindness and achieve better effects by carrying out heat treatment to melts based on the relationship between Al-18%Si melt structural state and temperature.