| Hypoeutectic and hypereutectic Al-Si alloys have obtained wide application because of their fine casting characteristics, strength,corrosion-resisting properties and low thermal expansion coefficient.Phosphorus modification for Al-Si alloy can improve its high temperature strength, wear-resisting ability, machining properties and decrease its thermal expansion coefficient.Currently, Al-P master alloys are mainly adopted for phosphorus modification, which contain AlP compound that has formed. It can be known from former researches that AlP is a stable compound, and that if it has existed in the master alloy it will quickly disperse and directly become the nucleus of primary silicon after the master alloy is added into Al-Si melt. However, experiments show that it is not entirely the case. Therefore, more work has been done in this area.Many analytic devices such as high-scope video microscope(HSVM), electron probe micro-analyzer(EPMA),field emission scanning electron microscopy (FESEM),and X-ray diffraction (XRD) were used to study the behavior of AlP in Al-Si melt.It was found that the melt temperature, holding time and cooling speed all have great influences on the size and morphology of AlP. When other parameters are fixed, AlP is shown as block when the temperature is low, and grows to thick flake after the temperature goes up.The extension of holding time and the decrease of cooling speed can both increase the size of AlP. These all demonstrate that AlP experiences the processes of dissolution, precipitatation and growth.Moreover, when those plate-like AlP particles are added into the melt, they can turn into small AlP blocks in a very short period of time with a huge decrease of dimensions, which can show that AlP also disassociates in the melt.Therefore, AlP experiences the processes of disassociation, dissolution, precipitatation and growth.In general,the most common way to judge the modification effect of phosphorus-containing master alloys for Al-Si alloys is to observe the microstructure after modification, which cannot guarantee the objectivity of the decision.Therefore, in this work, phosphorous absorption rate, an evaluation criterion that can be ascertained quantitatively, is adopted to provide a more fair evaluation for the modification effects of master alloys. This can satisfy the consumers of the master alloys with the specific requests about phosphorus amount or phosphorous absorption rate during the application of master alloys.Phosphorus amount in the specimen is detected by photoelectric direct-reading spectrometer and then phosphorous absorption rate can be calculated.It is found that phosphorous absorption rate can be affected by plenty of factors.Phosphorous absorption rate will show certain regularity with the change of parameters such as temperature, holding time and adding amount. When the temperature rises, phosphorous absorption rate increases, which in the mean time indirectly proves the dissolution process of AlP in Al-Si melt;With the prolongation of holding time, phosphorous absorption rate will gradually increase with a tendency to stabilize, and then will slowly decrease because of factors like burning; When raising the adding amount, the phosphorous absorption amount is sure to increase, but the absorption rate will decrease.Phosphorous absorption rate will also change after Al-P master alloys are processed.When Al-Si-P master alloys are re-melted under different temperature, the one under a higher temperature that contains AlP flakes has a higher phosphorous absorption rate than that of the one under a lower temperature that contains AlP blocks; Phosphorous absorption rate exhibits a large increase after Al-P master alloy is extruded under heat.Phosphorous absorption rate also has an obvious increase when adding other master alloys such as Al-Ti-B, Al-Ti-C-B and Al-Ti-B-C during the modification using Al-P, due to the introduce of external particles.Phosphorous absorption rate is studied when the components of Al-P master alloys change. It can be found that with the increase of the silicon amount in Al-Si-P master alloys, phosphorous absorption rate will first increase and then decrease, and the dividing point is the eutectic composition; Al-Si-P master alloys containing TiB2/TiC particles have higher phosphorous absorption rate than those without such particles;Al-Zr-P master alloy also has a great modification effect with a relatively high and stable phosphorous absorption rate.
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