(1.7)--Chapter 8 Special casting金属材料工艺基础.doc

Chapter 8 Special Casting Processessand casting is not suitable nor economical in many applications where the special casting processes would be more appropriate. In the following pages the details of some of the commonly used special casting methods would be described.8.1 Precision Investment Casting8.1.1 ProcessThis is the process where the mould is prepared around an expendable pattern. As shown in Fig.8.1, the first step in this process is the preparation of the pattern for every casting made. To do this, molten wax which is used as the pattern material is injected under pressure of about 2.5 MPa into a metallic die which has the cavity of the casting to be made. The wax when allowed to solidify would produce the pattern. To this wax pattern, gates, runners and any other details required are appended by applying heat.Fig. 8.1 Schematic of Precision Investment CastingTo make the mould, the prepared pattern is dipped into a slurry made by suspending fine ceramic materials in a liquid such as ethyl silicate or sodium silicate. The excess liquid is allowed to drain off from the pattern. Dry refractory grains such as fused silica or zircon are 'stuccoed' on this liquid ceramic coating. Thus a small shell is formed around the wax pattern. The shell is cured and then the process of dipping and stuccoing is continued with ceramic slurries of gradually increasing grain sizes. Finally when a shell thickness of 6 to 15 mm is reached, the mould is ready for further processing. The shell thickness required depends on the casting shape and mass, type of ceramic and the binder used.The next step in the process is to remove the pattern from the mould, which is done by heating the mould to melt the pattern. The melted wax is completely drained through the sprue by inverting the mould. Any wax remnants in the mould are dissolved with the help of the hot vapor of a solvent, such as trichloroethylene.The moulds are then pre-heated to a temperature of 100 to 1000 , depending on the size, complexity and the metal of the casting. This is done to reduce any last traces of wax left off and permit proper filling of all mould sections which are too thin to be filled in a cold mould.The molten metal is poured into the mould under gravity, under slight pressure, by evacuating the mould first. The method chosen depends on the type of casting.Other pattern materials used are plastics and mercury in place of wax. In the process called Mercast, the mercury is kept under -57 where the mercury is frozen. The complete mould preparation is to be undertaken at a temperature below -38 . The main advantage of mercury as a pattern material is that it does not expand when changed from solid to liquid state as wax. But the main disadvantage is keeping the pattern at such low temperature, which is responsible for its diminishing use.8.1.2Advantages（1）Complex shapes which are difficult to produce by any other method are possible since the pattern is withdrawn by melting it.（2）Very fine details and thin sections can be produced by this process, because the mould is heated before pouring. （3）Very close tolerances and better surface finish can be produced. This is made possible because of the fine grain of sand used next to the mould cavity.（4）Castings produced by this process are ready for use with little or no machining required, This is particularly useful for those hard-to-machine materials such as nimonic alloys.（5）With proper care it is possible to control grain size, grain orientation and directional solidification in this process, so that controlled mechanical properties can be obtained.（6） Since there is no parting line, dimensions across it would not vary.8.1.3 Limitations （1）The process is normally limited by the size and mass of the casting. The upper limit on the mass of a casting may be of the order of 5 kg.（2）This is a more expensive process because of larger manual labour involved in the preparation of the pattern and the mould.8.1.4 Applications This process was used in the older days for the preparation of artifacts, jewelry and surgical instruments. Presently the products made by this process are vanes and blades for gas turbines, shuttle eyes for weaving, pawls and claws for movie cameras, wave guides for radars, bolts and triggers for fire arms, stainless steel valve bodies and impellers for turbo chargers.8.2 Permanent Mould Casting8.2.1 ProcessIn all the processes that have been covered so far, a mould need to be prepared for each of the casting produced. For large scale production, making a mould for every casting to be produced may be difficult and expensive. Therefore, a permanent mould, called 'die' may be made from which a large number of castings, anywhere between 100 and 250,000 can be produced, depending on the alloy used and the complexity of the casting. This process is called permanent mould casting or gravity die casting, since the metal enters the mould under gravity.Fig. 8.2 Schematic of Precision permanent CastingThe mould material is selected on the consideration of the pouring temperature, size of the casting and frequency of the casting cycle. They determine the total heat to be borne by the die. Fine grained grey cast iron is the most generally used die material. Alloy cast iron, C20 steel and alloy steels (H11 and H14) are also used for very large volumes and large parts. Graphite moulds may be used for small volume production from aluminum and magnesium. The die life is less for higher melting temperature alloys such as copper or grey cast iron.For making any hollow portions, cores are also used in permanent mould casting. The cores can be made out of metal, or sand. When sand cores are used, the process is called semi-permanent molding. The metallic core cannot be complex with under-cuts and the like. Also, the metallic core is to be withdrawn immediately after solidification; otherwise, its extraction becomes difficult because of shrinkage. For complicated shapes, collapsible metal cores (multiple-piece cores) are sometimes used in permanent moulds. Their use is not extensive, because of the fact that it is difficult to securely position the core as a single piece as also due to the dimensional variations that are likely to occur. Hence, with collapsible cores, the designer has to provide coarse tolerance on these dimensions.The mould cavity should normally, be simple without any undesirable drafts or undercuts which interfere with the ejection of the solidified castings. In designing the permanent moulds, care should be taken to see that progressive solidification towards the riser is achieved. If the casting has heavy sections which are likely to interfere with the progressive solidification, mould section around that area may be made heavier around that area to extract more heat. Chills supported by heavy air blast may also be used to remove the excess heat. Alternatively, cooling channels may be provided at the necessary points to get proper temperature distribution. The likely problems with the cooling water circulation are the formation of scales inside the cooling channels and their subsequent blocking after some use.The gating and risering systems used are very similar to that of the sand casting. In fact to get the proper gating arrangements, it may be desirable first to experiment with various gating systems in sand casting and then finally arrive at the correct gating system for the metallic mould.The moulds are coated with a refractory material to a thickness of around 0.8 mm. The coatings are used to increase the mould lifel by preventing the soldering of metal to the mould,l by minimising the thermal shock to the mould material, andl by controlling the rate and direction of the casting solidification.The coatings normally are mixtures of sodium silicate, kaolin clay, soap stone and talc. The coatings are both insulating type and lubricating type. The main requirement of a coating is that it should be inert to the casting alloy. The coating may be applied by spraying or brushing. It must be thick enough to fill up any surface imperfections. The coatings can be applied thicker at surfaces which need to be cooled slowly, for example, sprues, runners, risers and thin sections. The maximum thickness of a coating required is about 0.8 mm.Under the regular casting cycle, the temperature at which the mould is used depends on the pouring temperature, casting cycle frequency, casting weight, casting shape, casting wall thickness, wall thickness of the mould and the thickness of the mould coating. If the casting is done with the cold die the first few castings are likely to have misruns till the die reaches its operating temperature. To avoid this, the mould should be preheated to its operating temperature, preferably in an oven.The materials which are normally cast in permanent moulds are aluminum alloys, magnesium alloys copper alloys, zinc alloys and grey cast iron. The sizes of castings are limited to 15 kg in most of the materials. But, in case of aluminum, large castings with a mass of up to 350 kg have been produced. Permanent mould casting is particularly suited to high volume production of small simple castings with uniform wall thickness and no intricate details.8.2.2 Advantages（1）Because of the metallic moulds used, this process produces a fine grained casting with superior, mechanical properties.（2）They produce very good surface finish of the order of 4 microns and better appearance.（3）Close dimensional tolerances can be obtained.It is economical for large-scale production as the labor involved in the mould preparation is reduced. （4）Small cored holes may be produced compared to sand casting.（5）Inserts can be readily cast in place.8.2.3 Limitations（1）The maximum size of the casting that can be produced is limited because of the equipment.（2）Complicated shapes cannot be produced.（3）The cost of the die is very high and can only be justified for large scale production.（4）Not all materials are suited for permanent mould casting essentially because of the mould material.8.2.4 Applications Some of the components that are produced in permanent moulds are, automobile pistons, stators, gear blanks, connecting rods, aircraft fittings, cylinder blocks, etc.8.3 Die CastingDie casting involves the preparation of components by injecting molten metal at high pressures into a metallic die. Die casting is closely related to permanent mould casting, in that both the processes use reusable metallic dies. In die casting, as the metal is forced in under pressure compared to permanent molding, it is also called 'pressure die casting'. Because of the high pressure involved in die casting, any narrow sections, complex shapes and fine surface details can easily be produced.In die casting, the die consists of two parts. One called the stationary die or cover die which is fixed to the die casting machine. The second part called the ejector die is moved out for the extraction of the casting. The casting cycle starts when the two parts of the die are apart. The lubricant is sprayed on the die cavity manually or by the auto lubrication system. The two die halves are closed and clamped. The required amount of metal is injected into the die. After the casting is solidified under pressure the die is opened and the casting is ejected.The die casting machines are of two types: hot chamber die casting and cold chamber die casting. The main difference between these two types is that in hot chamber, the holding furnace for the liquid metal is integral with the die casting machine, whereas in the cold chamber machine, the metal is melted in a separate furnace and then poured into the die casting machine with a ladle for each casting cycle which is also called 'shot'.8.3.1 Hot chamber process Fig. 8.3 Schematic of a hot chamber die casting machineA typical hot chamber die casting machine is shown in Fig. 8.3. In this, a gooseneck is used for pumping the liquid metal into the die cavity. The gooseneck is submerged in the holding furnace containing the molten metal. The gooseneck is made of grey, alloy or ductile iron, or of cast steel. A plunger made of alloy cast iron and which is hydraulically operated, moves up in the gooseneck to uncover the entry port for the entry of liquid metal into the gooseneck. The plunger can then develop the necessary pressure for forcing the metal into the die cavity. A nozzle at the end of the gooseneck is kept in close contact with the sprue located in the cover die.The operating sequence of the hot chamber process is presented in Fig. 8.4. The cycle starts with the closing of the die, when the plunger is in the highest position in the gooseneck, thus facilitating the filling of the gooseneck by the liquid metal. The plunger then starts moving down to force the metal in the gooseneck to be injected into the die cavity. The metal is then held at the same pressure till it is solidified. The die is opened, and any cores, if present are also retracted. The plunger then moves back returning the unused liquid metal to the gooseneck. The casting which is in the ejector die is now ejected and at the same time the plunger uncovers the filling hole, letting the liquid metal from the furnace to enter the gooseneck.Fig. 8.4 operating sequence of the hot chamber process 8.3.2 Cold chamber processThe hot chamber process is used for most of the low melting temperature alloys such as zinc, lead and tin. For materials such as aluminum and brass, their high melting temperatures make it difficult to cast them by hot chamber process, because gooseneck of the hot chamber machine is continuously in contact with the molten metal. Also liquid aluminum would attack the gooseneck material and thus hot chamber process is not used with aluminum alloys. In the cold chamber process, the molten metal is poured with a ladle into the shot chamber for every shot. This process reduces the contact time between the liquid metal and the shot chamber.The operation sequence shown in Fig. 8.5 is similar to hot chamber process. The operation starts with the spraying of die lubricants throughout the die cavity and closing of the die when molten metal is ladled into the shot chamber of the machine either manually by a hand ladle or by means of an auto ladle. An auto ladle is a form of a robotic device which automatically scoops molten aluminum from the holding furnace and pours into the die at the exact instant required in the casting cycle. The metal volume and pouring temperature can be precisely controlled with an auto ladle and hence the desired casting quality can be had. Then the plunger forces the metal into the die cavity and maintains the pressure till it solidifies. In the next step, the die opens. The casting is ejected. At the same time the plunger returns