Casting and forming of the hottest new materials

2022-08-16
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Casting and forming of new materials

1. Casting and forming of titanium alloys

the reserves of titanium in the earth's crust are second only to iron, aluminum and copper, ranking fourth among metal elements. Moreover, Qin alloy has a series of excellent properties, so it is more and more widely used. Titanium alloys can be divided into high temperature titanium alloys, structural titanium alloys and functional titanium alloys according to their application background. Besides casting, the forming methods of titanium alloy include forging, superplasticity, welding and so on

1. smelting process of cast titanium alloy

titanium is a very active metal, which reacts very quickly with oxygen, nitrogen, hydrogen and carbon in liquid state. Therefore, the smelting of titanium alloy must be carried out under the protection of high vacuum or inert gas (AR or NE). The crucible for smelting adopts water-cooled copper crucible, and the specific smelting process mainly includes three ways:

(1) non consumable electrode arc furnace smelting alloy smelting is carried out under the protection of vacuum or inert gas. This process is mainly to melt consumable electrode to prepare electrode

(2) vacuum consumable electrode arc furnace smelting uses consumable electrode made of titanium or titanium alloy as cathode and water-cooled copper crucible as anode. The melted electrode enters the crucible in the form of droplets to form a molten pool. The surface of the molten pool is heated by an electric arc and is always in a liquid state. The bottom and the crucible are forced to cool around, resulting in bottom-up crystallization. The molten metal in the molten pool solidifies and becomes titanium ingot

(3) see Fig. 1 for the schematic diagram of vacuum consumable electrode shell protection smelting device. This kind of furnace is developed on the basis of vacuum consumable electrode arc furnace. It is a furnace type for casting special-shaped parts that combines smelting and centrifugal casting. Its biggest feature is that there is a layer of titanium alloy solid thin shell between the water-cooled copper crucible and the metal melt, that is, the so-called condensing shell. This layer of condensing shell of the same material is used as the lining of the crucible to form a molten pool to store titanium liquid, avoiding the pollution of the crucible to titanium alloy liquid. After pouring, a layer of condensing shell left in the increased loss can continue to be used as the crucible lining

in recent years, with the development and production needs of the special rectification technology of consumption blackmail, infringement of personal information, advance card running and other consumption pain points and blockage problems, Keda has successively studied and developed new methods and equipment for smelting titanium alloys and other active metals, mainly including electron beam furnace, plasma furnace, vacuum induction furnace, etc., and has been applied to a certain extent. However, from the comparison of technical and economic indicators such as power consumption, melting speed and cost, consumable electrode arc furnace (including shell furnace) smelting is still the most economical and applicable smelting method at present, as shown in Table 1

Table 1 Comparison of indicators in different smelting titanium alloys

2. casting process of titanium alloys

due to the physical-chemical properties of titanium elements, the casting process of titanium alloys, whether in terms of molding material ratio or process method, has its unique requirements and characteristics. First, modeling materials that require very high fire resistance; Second, aluminum alloy cables must be safely used in the United States and other countries for several decades under the protection of high vacuum or inert gas, and sometimes with centrifugal force. The types of casting processes are as follows:

(1) permanent molds mainly include processing graphite mold and metal mold (iron mold and titanium mold). The mold is machined. The casting produced is relatively simple in structure and low in dimensional accuracy; It is generally used in the production of blanks

(2) disposable mold can produce castings with complex shape and high dimensional accuracy. According to its modeling method, there are two kinds: tamped graphite sand mold and molten model shell. The latter can be made more complex to meet the development needs of large-scale ships; Accelerate the preliminary work of Macun Port Phase 3 and 4 projects (wall thickness 2mm), castings with high dimensional accuracy and low surface roughness (ra3.2). The welded shell is made of different materials, and the molten shell is divided into three different systems

1) pure graphite shell system. Graphite powder with different particle sizes is used as refractory filler and sanding material, and resin is used as adhesive. The shell has high strength, light weight, low cost and a wide range of raw materials. Suitable for centrifugal or gravity pouring

2) refractory metal surface shell system. For the composite system, except that the surface layer needs to adopt special processes due to different molding materials (tungsten powder and other refractory metals), the back layer from molding materials to shell making process is the same as investment casting of cast steel

3) oxide ceramic shell system. The surface and back layer of the shell are made of oxide as molding materials, so the shell has high strength and the smallest thermal conductivity among the three kinds of shells, which is suitable for pouring thin-walled castings with complex shapes

titanium castings poured with the above three shell systems have little difference in chemical composition and mechanical properties; However, there are obvious differences in surface quality. The shrinkage rate of the latter two kinds of shell is obviously smaller than that of graphite shell, so the dimensional accuracy of castings

in production, the above different methods can be selected according to the specific situation of titanium castings

2. Casting of metal matrix composites

metal matrix composites (MMC) is known as the material of the 21st century. Because it well combines the plasticity and toughness of metals with the high strength and high modulus of elasticity of ceramics, it has some remarkable physical and mechanical properties, such as high specific modulus of elasticity, specific strength, heat resistance, wear resistance and dimensional stability. From the point of view of economic applicability, MMC materials reinforced by ceramic particles have attracted more attention. The research and application focus on magnesium based and aluminum based alloys. The reinforced particles mainly include Al2O3, SiC, etc. Its forming process mainly includes powder metallurgy and casting. The former is difficult to be widely used in production because of its complex process, high cost and difficulty in manufacturing large-size parts and complex parts. Using casting method to form particle reinforced metal matrix composites has the advantages of simple process, low cost, no restrictions on size and shape, and can be produced continuously in large quantities. Therefore, it has broad development and application prospects

there are mainly six casting methods for particle reinforced metal matrix composites:

1. Liquid metal/ceramic particle stirring casting method

eddy current is generated in liquid metal through mechanical stirring, so as to introduce ceramic particles and make them evenly distributed. Using this method to manufacture aluminum matrix composites, the ceramic particle size can be as small as 10 μ m. The volume fraction of the reinforcing phase can reach 25%

2. Melt infiltration casting method

melt infiltration process includes pressure infiltration and non pressure infiltration. The former uses mechanical devices or inert gases as pressure media to impregnate metal melts into porous ceramic preforms, which can prepare composites with volume fractions up to 50%. This method has been widely used. The latter is that under nitrogen atmosphere, the alloy melt can be well soaked into the ceramic powder preform without any pressure, and the volume fraction of composites can be as high as 55%

3. Squeeze casting method

this method first prepares the composite slurry from the mechanical stirring method, then pour the liquid composite slurry into the extrusion die (preheating is required), start the hydraulic press, and make the liquid slurry solidify and form under a certain specific pressure

4. High energy ultrasonic method

this method is to use ultrasonic generator (mostly magnetostrictive transducer) to apply ultrasonic vibration while adding ceramic particles to achieve uniform mixing after metal melting. Ceramic particles can also be made into prefabricated parts, poured into liquid metal, and then applied with ultrasound for melt infiltration. This method can realize the uniform distribution of particles in a very short time (tens of seconds)

5. Rheo casting method

this method is to apply strong agitation to the melt in the solid-liquid two-phase region to form a semi-solid slurry with low viscosity. At the same time, ceramic particles are introduced, and the thixotropic characteristics of the semi-solid slurry are used to disperse the reinforcing phase, which is filled and solidified under a certain pressure. This process is a two-phase process, which is limited to alloys with large crystallization range

6. In situ reaction casting method

this is a new method developed recently. The fundamental difference between it and the above five methods is that the reinforced ceramic particles are not added, but formed in situ through chemical reaction during the preparation process. Its basic principle is: in a certain liquid alloy, using the high temperature of the alloy liquid, chemical reactions occur between the alloy elements in the alloy liquid or between the alloy elements and compounds to generate one or more ceramic reinforced particles, and then the metal matrix composites reinforced by in-situ particles are obtained by casting

the preparation process of metal matrix composites is still under research and development. Among the above methods, the mechanical stirring casting method has the most industrialized prospect. Because of its simple process and low cost, it has almost no restrictions on the types of melts and reinforced particles, and has a wide range of adaptability. However, in order to truly realize large-scale industrial production, the following problems need to be solved: the uniformity of particle micro distribution; Interface reaction and control between metal matrix and reinforcing particles; Organization, performance and stability, etc

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