The Behavious And Interaction Of Added Elements During The Melt Treatment Of Casting Al-Si Alloys

With the development of manufacturing such as automobile, aviation and navigation industry, the needs of casting Al-Si alloys are increasing greatly because they have the characters of low density, low expansion coefficient, excellent wearing and corrosion resistance, and satisfied casting properties. But in the normal microstructures of casting A1-Si alloys, usually consist of coarse primary silicon and needle-like or lamellar eutectic silicon, which greatly aggravated the mechanical properties of these alloys. So the control of the morphology and size of Si phase have became the most important and necessary processing for these alloys before using them. Generally, P had been used extensively to change the morphology and size of primary silicon, and elements such as Sr, RE(Rare Earth elements), Sb are used to transform the morphology and size of eutectic silicon. But it was also found all of them have their own shortcomings, for example, p-modified hypereutectic A1-Si castings can't meet the requirements of high strength and wear-resistance, Sr-modification increased the formation of gas porosity in the casting products. Modification effect of RE and Sb are sensitive to the solidification cooling rate, so the compound modification of these element had been the aim of many researchers. Evidently, systemic study of the interaction of these additional elements is the premise to obtain effective ways to improve these problems.Elements including P, Sr, RE, Bi and Sb were selected to study their interaction on the morphology and size of Si phase, mechanical properties and melt hydrogen melt in the present dissertation. New modification methods for casting aluminum-silicon alloy with excellent compound effect were obtained, moreover, the interaction mechanics was analyzed preliminarily. The main research results are the following:1,In the hypereutectic A390 alloy, both Sr and RE element promote the modification effect of P on Si phases.●Among all the compound modification, Sr-RE-P modification obtain the best comprehensive effect, the size of primary silicon modified by this method is the smallest, and RE-P get the secondly effect, then is the Sr-P compound modification, all of the compound modification have better effect than P-modified. ●Addition of RE elements optimized the modification effect of E Firstly, compared with P-modified A390 alloy, the primary silicon phases in the compound modified A390 alloy are refiner, and the morphology of most eutectic silicon phases changed from long needle-like to short rod-like. Moreover, the agglomerate rate of primary silicon phases is reduced. A390 alloy modified with 0.6%RE (La/Ce=2) and 1.0%P obtained the best modification effect, the average size of primary silicon decreased to 36μm, and the maximum size decreased to 52μm.●When the alloy was added with Sr plus P, Contrary results were observed with different adding sequence of modifiers. If adding P firstly, then adding Sr (P-Sr compound modification), the primary silicon phases appear rough star-like, there has no modification effect on primary silicon, but the morphology of the eutectic silicon was rod-like. Contrarily, if adding Sr firstly (Sr-P compound modification), there was excellent modification effect in the alloy, both the shapes of primary and eutectic silicon phases are benefic to the alloy.●Formation of complex morphology of primary silicon is due to different agglomerate manner of silicon clusters. When the melt added with P, there would have abundant tiny insoluble AlP compounds, and act as the nuclei of silicon clusters, so the primary silicon phases present regular shape. But with prolonging of holding time, the collision probability of silicon clusters would increase, they are easily to get together, as a result, the primary silicon present complex shape. After added RE elements, they are readily to absorb at the surface of silicon clusters, and impede the agglomerate of silicon clusters in a certain extent, and optimize the modification effect of P. when the P-modified melt was then added with Sr, Sr with react with AlP, and the primary silicon present rough shape, but if the Sr-modified melt was added with P, because a great deal of Sr was burned during the course of enhancement of melt temperature, and the residual Sr would absorb at the surface of silicon clusters, and hinder the agglomerate of silicon clusters. So have an better modification then that of P-modified melt.2,In the hypoeutectic alloy, RE,Sr and Sb elements present excellent promoting effect:●In the microstructure of A356 alloy modified with 0.01%Sr, eutectic silicon showed typical no-modified shape(rough plate-like), and in the A356 alloy added with 0.2%RE, eutectic silicon showed under-modified shape, some is plate-like, most of them are short rod-like. But in the microstructure of the A356 alloy modified with 0.01%Sr and 0.2%RE compound modifier, eutectic silicon showed typical excellent modification shape. Moreover, with 0.1%Sb and 0.2%RE compound modifiers, the microstructure presented the same rule. Its modification is superior to those modified with 0.15%Sb and 0.2%RE.●Among all the melt treatment methods, hydrogen content of melt modified with RE-Sb compound modifier reached the lowest, the RE-modified is secondary, and then is the Sb -modified one. The hydrogen content of the melt modified with Sr increased greatly, and the hydrogen content of the melt increased linearly with holding time. However, the melt modified with RE-Sr compound modifier has lower hydrogen content than that of Sr-modified or unmodified.●Compared with unmodified A356 alloy, surface oxide film of RE-modified melt is compacter; there is almost no crack in the film. However, the addition of Sr destroys the surface's integrity and compactness, a large amount of crack traces are easily to be seen in the surface of the melt modified with Sr, and the width of the crack reached to above 100μm. The surface oxide film of RE-Sr compound modified melt has some cracks, but both the crack quantity and size are decreased greatly compared with Sr-modified one. There has no crack in the Sb-modified and RE-Sb compound modified surface oxide films. And the microstructure of the RE-Sb compound modified melt is compacter than that of RE-modified and Sb-modified melts.3,If the Bi element addition is proper, it will benefit the properties of hypereutectic A1-Si alloy greatly:●Bi and Si represent strong interaction. The morphology and size of the silicon phase have impact directly on the solidification behavior of liquid rich of Bi element. The more evenly of the primary silicon distributed and the smaller of its size, the more evenly distribution of Bi will gained. In the microstructure of hypereutectic alloy which solidified at the permanent mould which installed a water-cooling device at the bottom, most Bi phases distribute evenly in the boundary of primary silicon phase, and some of them are around the eutectic structures. Results showed that when the addition amount of Bi element was 2-3%, the primary silicon size reached the lowest value and the average size was only 35μm.●During the course of friction, Bi element is easily to remelt because its melting point is low to 271℃, and form an effective lubricating layer, consequently, improved the frictional behavior of hypereutectic Al-Si alloy. After alloy added with 3%Bi, the running-in time reduced 37.5%; the friction factor of dry friction reduced 12%; the wearing capacity reduced 25%. And the wearing surface temperature reduced 32%.4,applications of aluminum compound modification technology●In the microstructure of the A390 alloy modified with Sr-RE-P compound modifier, the average size of primary silicon phases is 24μm, the maximum size is 47μm, and the modification effect can last for 4hours at least, the mechanical properties of the part named 'two-way swash plat', meet the designed requirements, and the manufacturing cost reduced more than 50%, the passable percent of the casting reached 100%, and passed the installation test.●After modified with RE-Sr compound modifier, density ofA356 RPT sample enhanced greatly, improved the pinhole-problem of the Sromodified A356 alloy greatly. At the same time, the eutectic silicon phases changed to fine spot-like. Both the mechanical properties and bench test meet the designed requirements of the aluminum auto-wheel. The modification with RE-Sr compound modifier is a feasible technology for reducing the aluminum auto-wheel rejection rate.●Our development self-lubricating and wear-resisting hypereutectic A1-Si alloy has stratified mechanical properties, reached the American standard of the A390 alloy. Conditional compressor installed the part of two-way swash plat made of this alloy had running for 920hours continuously, meet the designed requirements of the conditional compressor. In summary, this new alloy combined the outstanding friction performance of FA2 alloy and the good mechanical performance of A390 alloy.It becomes an idea material for producting the part of the two-way swash plat.