Types of Refractories for Zirconium Alloy Precision Casting
Types of Refractories for Zirconium Alloy Precision Casting
precision casting refers to melting metal and casting it to be made of refractory materials. Inside the shell, After the metal solidifies, the mold is removed and the sand is removed to obtain the required alloy products. Precision casting technology can be used to cast castings with complex shapes, which are close to the final shape of the parts after forming. However, due to casting type shell in direct contact with the alloy melt, therefore, like zirconium and Zirconium metals with high melting points and strong chemical activity such as alloys, type shell material the choice is the key to ensure the quality of castings. Zirconium and Zirconium the melting point of the alloy is(1852±10)℃, and the chemical activity is very high at high temperature, and it can react with most refractory materials in the molten state, resulting in the formation of a pollution layer on the alloy surface. For this reason, the selection of refractory materials that do not react with the zirconium melt as the shell for precision casting is to obtain high quality. Zirconium the key to casting. Can be used zirconium and Zirconium refractory materials for alloy precision casting mainly include graphite, refractory metals, refractory compounds, oxides, etc.
1 Graphite
graphite reacts weakly with active metals such as titanium and zirconium, as early1950s western countries have used graphite zirconium Production from Shell castings. However, graphite shell also there are some shortcomings, for example, oxidation starts at about 430°C, and the gas adsorption capacity is strong, and it needs to be degassed at high temperature in a vacuum furnace before casting; On the other hand, graphite has high thermal conductivity, it is easy to produce chilling, and it is easy to cause microcracks on the surface of the casting. Liu Hong feather study in precision casting, zirconium melt and graphite shell the results show that due to the high porosity, fast cooling speed and poor yield of graphite, obvious cold separation and cracks appear on the surface of zirconium alloy sample, zirconium and Graphite Shell the thickness of the contamination layer is about 20 μm. Xie Huasheng and others have also obtained similar research results. In addition, the C element pair zirconium the corrosion resistance of the alloy is extremely to the disadvantage, this also limits the application of graphite materials in zirconium alloy precision casting.
2 Refractory metal
mo, Ta, W, Nb and other refractory metals with higher melting points on titanium, zirconium Active metal has good stability and can be used to make precision casting. Shell surface layer. In-shell preparation in the process, a tungsten or tungsten oxide surface layer is generally formed on the ceramic surface, thereby reducing the molten metal with the shell reaction. At present, the use of this process to prepare type shell mainly by applied to titanium and precision casting of titanium alloy. Basche using Tungsten Compounds to Ceramics type shell progress infiltration, and then calcination in a reducing atmosphere to reduce the compound of tungsten, thereby coating the surface of the refractory material with tungsten, and then avoid titanium at high temperatures the reaction with refractory materials. But, this tungsten surface type there are also some shortcomings in the shell. For example, the shell must be made by solvent dewaxing, which will cause harm to the environment and human health; In addition, it will be deposited after dewaxing. On the surface of the shell the mold material is easy to react with metal and generate pores on the surface of the casting.
3 Refractory compound
refractory compounds include carbides, borides, sulfides, nitrides, etc. Since these refractory compounds are easily oxidized in air, they are used type shell material the roasting shall be carried out under vacuum. BN is a special refractory material with excellent performance, but Gao Yonghui et al. TiNi when studying the interfacial reaction of the alloy, it was found that element B and element n were found in TiNi the diffusion in the melt is more serious, which will lead to alloy embrittlement after mixing. Indacochea etc, Mcdeavitt such as through molten zirconium in ZrC, HfC, ZrN the wettability experiments on a variety of refractory compounds were studied, and the interfacial reactions between them were studied. The results showed that: ZrN A chemical reaction with Zr produces a new phase, and other refractory compounds and melting zirconium between although no obvious transition zone appeared, but elemental diffusion occurred. Therefore, can refractory metals and refractory compounds be used zirconium further research is needed in precision casting of alloys.
4 oxide
oxide is currently the most widely used in precision casting technology at home and abroad. The shell material. Oxide type shell material it has great advantages. On the one hand, it has low thermal conductivity and good thermal insulation, which can reduce the cooling speed of the alloy melt and ensure better melt. The filling type; On the other hand, the oxide will not be oxidized during the sintering process, does not need vacuum or special atmosphere protection, and is not easy to adsorb gas. It has certain chemical stability to molten metal at high temperature. There are many types of oxide ceramics commonly used in industry, but most of them are not suitable for casting materials for active metals such as titanium and zirconium. At present, more research on cast materials for active metals include Al2O3, CaO, ZrO2, Y2O3, etc.
Al2O3 can be roughly divided into industrial alumina, high purity alumina, fused alumina (brown corundum, white corundum) and sintered alumina. Among them, white corundum has excellent performance, high Al2O3 content, less impurities, good chemical stability and high temperature resistance, and is currently the most mature ceramic studied type shell material one. Since Al2O3 is associated TiAl the coefficient of thermal expansion is very similar, so it can effectively reduce the cause. TiAl the probability of fracture due to low plasticity at room temperature is ideal. TiAl refractory materials for alloy precision casting. However, Zhou Wenbin studied zirconium melt and Al2O3 type shell during the interface reaction, it was found that the diffusion depth of O element into the alloy was about 60 μm,Al element was about 100 μm, and a large number of cold cracks appeared on the surface of zirconium alloy castings, and molten zirconium for refractory materials the erosion and penetration are serious, and the casting effect is not ideal. Therefore, Al2O3 is not suitable for casting active metals such as titanium and zirconium. Shell surface coating, but can try as a back coating.
CaO it is an alkaline refractory material with a melting point of up to 2570 ℃, good high temperature performance, strong corrosion resistance to alkaline slag, and can play a purification role in metal refining, with low price and wide sources. Zhang Xiyan et al think CaO it is a very thermodynamically stable refractory material, no the presence of a pair of titanium melts the pollution problem. But, CaO it is extremely hydrophilic. If it is not well preserved, it is easy to hydrate, cause the shell to crack, Material failure. Therefore, preparation shell the CaO raw materials must be densitized and stored in vacuum. Even so, it is difficult to completely prevent hydration. In order to make use CaO good performance, and prevent its hydration, can be CaO as a doping material or stabilizer material.
ZrO2 is one of the most widely used refractory materials today, but it is used zirconium the production of castings is still relatively small, mainly used to prepare titanium alloy precision casting shells. The U.S. Bureau of Mines was the first to apply ZrO2 to the preparation of high-temperature alloy melt precision. Cast shell, type shell surface layer ZrO2 is used for the refractory materials of the adjacent surface layer, and small and medium-sized thin-walled titanium castings with high precision and qualified quality are poured, but there are concentrated pores on the surface of the castings. Liu hongyu ZrO2 is shell material cast zirconium after the alloy, it was found that the O element diffusion from the shell to the surface of the alloy substrate, the thickness of the diffusion zone is about 35 μm. In addition, ZrO2 has 3 crystal forms and is a polycrystalline phase-converted oxide. At low temperature, it is monoclinic. Crystal the structure (m-ZrO2) is gradually transformed into tetragonal phase (t-ZrO2) when it is higher than 1000 ℃, and all of them are transformed into tetragonal phase when it is 2370 ℃, above 2370 ℃ to melting point temperature is cubic crystal phase (c-ZrO2). Due to the phase change transformation, ZrO2 is accompanied by volume shrinkage during heating and heating, and volume expansion during cooling, so in the preparation of ZrO2 type shell crystal stabilization treatment must be carried out ensure that the shell is stable cracking. Commonly used stabilizers are MgO, CaO, Y2O3, CeO2, etc. Zhou Wenbin Research zirconium melt and ZrO2 ( CaO stable), ZrO2(Y2O3 stable) type shell during the interface reaction, it was found that the Zr atoms produced by the decomposition of ZrO2 have good compatibility with the matrix Zr, and the mutual penetration of the two will lead to uneven surface of the sample, but the refractory material is dissolved into zirconium alloy. Less solution, ZrO2 ( CaO stable) type shell the thickness of the contaminated layer is about 45 μm;ZrO2(Y2O3 stable) type shell the thickness of the contaminated layer is about 35 μm.
Y2O3 is currently the most stable refractory used in precision casting of active metals. Type shell material one. Suzuki et al. in the traditional mullite shell surface after spraying a layer of Y2O3, it is used to cast Ti-6Al-4V alloy. The surface hardening degree and oxygen content of the obtained titanium alloy casting are ideal, but Y2O3 coating and shell surface the binding force is not good, and it is easy to peel off during the pouring process. Xi'an Pump Valve General Factory has developed a zirconium and Zirconium the production method of alloy large castings is machined. Graphite shell surface coated Y2O3 ceramic coating can effectively reduce graphite shell chilling acts to improve liquid metal the filling capacity to reduce defects such as cold separation marks on the surface, so as to produce zirconium castings with smooth surface and small pollution layer thickness. Shenyang Foundry Research Institute brush manually or the spray gun spraying method will Y2O3 or ZrO2 is coated on the cavity surface of cast iron, cast steel or cast copper metal mold to improve zirconium quality of alloy castings. However, the higher price of Y2O3 and its poor thermal shock resistance limit its application. Mcdeavitt such as research molten Zirconium and interfacial reaction of Y2O3 ceramic tabletting, it is found that Y2O3 and melting zirconium only the diffusion of elements has occurred between them, and there is no obvious transition area. Zhou Wenbin Research zirconium melt and Y2O3 type shell during the interface reaction, it was found that Y2O3 decomposed into Y atom and O atom respectively and diffused into Zr matrix, but Y and Zr did not form an intermediate phase, Zr and Y2O3 type shell the thickness of the contaminated layer is about 25 μm. And Liu Hongyu to Y2O3 type shell the results show that: Zr and Y2O3 type shell the pollution layer thickness is50~60 μm. The results of Xie Huasheng and others show that: Zr and Y2O3 type shell the thickness of the pollution layer is about 30 μm, the content of alloy element Zr in the matrix is stable, and the content in the reaction layer decreases. It is speculated that Zr may have diffused to shell among them. Preliminary analysis, although the literature has studied zirconium melt and Y2O3 in precision casting process type shell interface reaction, but used zirconium alloy type, type shell assembly the method and the preparation method of refractory materials are different, which causes zirconium melt and Y2O3 type shell the thickness of the contamination layer varies.
BeO it has the characteristics of high thermal conductivity, high melting point, high strength, good insulation, high chemical and thermal stability, and is a good melting Be, Zr, Ti crucible materials such as high purity metals. Mcdeavitt research such molten Zirconium and BeO the interface reaction of ceramic tabletting found that, BeO with melting zirconium only the diffusion of elements occurred between them, and there was no obvious transition area. But BeO in the process of manufacturing and use, it is easy to cause harm to personnel safety and the environment, and has been listed as a carcinogen, and some countries and organizations have BeO limitations have also been raised in the use of materials.
