Study On The Fabrication Of Al-P Master Alloys By Powder Metallurgy

The disadvantages of the Al-P master alloy produced by casting are mainly as follows: AlP particles are coarse and present cluster-like morphology, their distribution is uneven in Al-P master alloy and phosphorus level is relatively low. A new technology preparing Al-P master alloy by powder metallurgy has been put forward. The microstructures of the Al-P master alloy fabricated by powder metallurgy were investigated, using optical microscopy (OM) and electron probe micro-analyzer (EPMA), etc. Moreover, the modification effect of the Al-P master alloy on hypereutectic Al-Si alloys was also investigated.The new technology of producing Al-P master alloy is as follows: the mixed powders of phosphorus-rich alloys (Cu-40Si-20P, Si-20Mn-20P and Cu-P) and aluminum are extruded into columned pallets, which are then put into a sintering furnace where the diffusion and reactions in pallets can occur at 500-660℃.The diffusion couples of phosphorus-rich alloys/Al was studied. At 500℃or so, phosphorus in phosphorus-rich alloys reacted with aluminum, which formed AlP. Specifically, AlP precipitated along the interface between phosphorus-rich and aluminum, dispersing distribution for Cu-8P master alloy, small block-like distribution for Cu-14P master alloy and fine flake-like distribution for Cu-40Si-20P master alloy.Based on the experiments mentioned above, the influences of such factors as the particle size of phosphorus-rich alloys, forming pressure, sintering temperature, sintering time and the degree of vacuum in furnace on the morphology and distribution of AlP particles were studied. It has been shown that if the forming pressure is low, the amount of ventage is large, so the samples are easily oxidized during the sintering process. On the other hand, if the forming pressure is high, the sintered samples is not prone to melt in the Al-Si alloy melts. In order to make AlP particles dispersed in the master alloy, the powder sizes of phosphorus-rich alloys should be controlled. The longer the sintering time is, the more sufficient the reaction is. When the reaction is sufficient, silicon phases can be observed. But at high temperature (below melting temperature of samples), the time of complete reaction is short. And the degree of vacuum is low, the severe oxidization prevents the samples from melting in the melt of Al-Si alloys.The optimum parameters of fabricating Al-P master alloys by powder metallurgy are as follows: paricles size of phosphorus-rich alloys are 50-75um, the forming pressure is 10MPa, the sintering temperature is 650℃and the sintering time is 40 minutes. The content of phosphorus in Al-P master alloys fabricated by powder metallurgy can reach up to 10%.Comparing with the Al-P master alloy produced by casting, the size of AlP particle is finer, the distribution is more homogeneous and the content of phosphorus is higher in the Al-P master alloy prepared by powder metallurgy. Adding the same amount of phosphorus into Al-Si alloy, the modification effects of these two kinds Al-P master alloys are equivalent. However, the modification time is reduced by 1/3 with adding the Al-P master alloy fabricated by power metallurgy.