Patent Publication Number: US-2009218067-A1

Title: Lost-wax method associated with piezocrystallization and a device for carrying out said method

Description:
TECHNICAL FIELD 
     The present invention relates to a lost-wax method associated with piezocrystallization and a device for carrying out said method. 
     BACKGROUND ART 
     A casting method by squeezing a metal into a mold associated with piezocrystallization is known from RU 2015829, wherein a molten metal is poured into a squeezing chamber for steel overheated substantially by 30-60° C. above a liquidus temperature and prior to squeezing is held until a metal skin freezes out on the squeezing chamber walls. 
     The disadvantage of this method consists in that holding metal in the squeezing chamber may bring the melt to the liquid-solid phase. Pressure is known to be most efficient when transferred in the liquid phase. Therefore, the pressure efficiency drops as the already cooled metal is poured from the squeezing chamber into the mold and further crystallization is under way. This may result in insufficiently poured-in thin areas of the cast, corrugated surface of the cast due to a loss of flowability or shrinkage cavities due to a shortage of the liquid phase for feeding, and in general in a significant loss of the cast product quality. 
     The prior art closest to the inventive method is a lost-wax method associated with piezocrystallization according to RU 2048954, wherein a pre-shaped investment mold is fixed on an upper table above a metal reservoir, and the melt is extruded at a melt crystallization temperature with a melt pressure being further maintained at 0.3-0.5 MPa until crystallization of the cast, wherein the mold filling rate and time is controlled by means of keeping the mass flow rate of the melt within 2 to 5 kg/s. 
     The disadvantage of the prior art method consists in a low strength of ceramic investment molds suitable for gravity die casting rather than pressure die casting if manufactured according to the common technique. The strength of the investment molds and accordingly the maximum possible operating pressure depends on the number of layers applied when manufacturing the multilayer molds, main materials and binders used, accurate adherence to the manufacturing conditions thereof, forming conditions thereof in a casting container, cast size and material, casting modes and other parameters. Therefore, the pressure value in the investment mold is difficult to calculate theoretically and an experimental check of the approved pressure is required based on the actual mold strength every time a new product is manufactured by this method. Besides, microfissures may form on the shells in the process of manufacturing the investment molds, for example, during baking of the same, which do not affect the cast product quality in case of gravity die casting but may cause the mold to crack when pressure is applied thereto. 
     A device is known from RU 2116865, comprising a metal reservoir mounted on a bottom table, a container with a investment mold fixed on an upper table, said metal reservoir being arranged as a base and a replaceable insert with a heat-insulating layer between their bottoms, and a container comprising a housing, a cover and a neck. Gas release openings are provided at the base of said replaceable insert. The disadvantage of the prior art device consists in that after metal is poured into the metal reservoir a crystallized layer of the melt (metal skin) begins to form throughout the height of its side walls preventing the punch to move into the metal reservoir and feeding the melt through the container neck to the mold which may in turn lead to incomplete filling of the mold. Given such design of the device, the effect of this skin, after the mold has been filled with the melt and it crystallizes, is particularly detrimental when the skin thickness within the metal reservoir on the punch way increases intensively and prevents metal from being extruded from the butt-end into the mold for feeding the cast as it shrinks at the end of crystallization. This lowers the cast product quality and reduces to a minimum the advantages of the pressure die casting since pressure is no longer applied to the melt but to the metal skin within the metal reservoir. 
     DISCLOSURE OF THE INVENTION 
     The unique technical problem to be solved by the claimed group of inventions is to improve the cast product quality by enabling more pressure to be applied to the melt within the reinforced investment mold obtained as a result of forming a crystallized layer of melt butting against its inner walls (metal sheath) along with reducing a negative effect of the skin crystallization process within the metal reservoir preventing the melt from being extruded into the investment mold. The unique technical effect of the method according to the claimed group of inventions is achieved by extruding the melt into the cavity of the shaped investment mold at a temperature above liquidus and a pressure enabling a maximum splash-free liquid metal flow rate as the investment mold is being filled, said pressure, after the investment mold has been filled with the melt, being maintained at a level obtained in the process of extrusion during crystallization of the melt layer butting against the investment mold walls, and whereafter said pressure being smoothly increased during crystallization of the entire melt to a pressure sufficient for further filling the mold in proportion to the amount of cast shrinkage. 
     The unique technical effect of the device according to the claimed group of inventions is achieved by providing the outer surface of the container neck with a removable thimble and mounting at the exposed edge of the metal reservoir insert a lined annular flange matching along its inner diameter with the outer diameter of said thimble, said lined flange having an inner diameter smaller than an inner diameter of said lined insert. The radial difference between the inner diameters of said lined insert and said thimble is greater that the total thickness of the single-side melt layer crystallized on the lined insert wall and the thimble wall in the process of pouring-in and holding the melt till the end of crystallization. 
     The above combination of features of the claimed method is novel and inventive in view of the following. The melt is extruded into the cavity of the shaped investment mold at a temperature above liquidus thereby enabling good flowability and contact of the melt with the mold walls for creating favorable heat transfer conditions as a metal sheath is formed on the inner surface of the investment mold. The pressure is determined by the absence of liquid metal splashing within the mold as it is filled at a certain rate. The maximum flow rate and thereby the pouring rate of liquid metal may be calculated theoretically [Borisov G. P., Pressure in Casting Control, Kyiv. Nauk. Dumka, 1988, page 121, Formula IV-18]. In practice (if, for example, a hydraulic drive is used) this calculated pressure may be determined by hardware and monitored using a pressure gauge on the hydraulic drive for moving the metal reservoir. In order to ensure a uniform thickness of the metal sheath eventually forming on the inner surface of the ceramic investment mold, a rapid mold filling is required. Upon filling the mold with the melt, a pressure is maintained within its cavity at a pressure level reached during extrusion. This condition is a minimum requirement for protection of the ceramic form against damage for the period of crystallization of the melt layer butting against the investment mold walls while the metal sheath is being formed on the inner surface of the ceramic investment mold. The thickness of said metal sheath is proportional to the cast crystallization time, and its value for the abutting layer may be taken as 5% to 10% of the crystallization time of the thinnest cast areas. Then, during the remaining crystallization time of the entire melt within the investment mold, pressure is smoothly increased to a level sufficient for further filling the mold in proportion to the amount of cast shrinkage. By smoothly increasing the pressure the risk of damaging the ceramic mold is reduced. Sufficient pressure is a pressure at which feeding the cast from the butt-end into the metal reservoir is ensured in proportion to the amount of shrinkage during crystallization. The amount of shrinkage for most of the conventional casting alloys is a known characteristic. Therefore, the sufficiency of pressure may be determined by the motion of the metal reservoir relative to the mold during crystallization, which is proportional to the amount of shrinkage, or by the absence of shrinkage defects when the cast product quality is tested. In general, the metallic sheath on the inner surface of the investment mold enables the operating pressure to be increased thereby ensuring a higher cast product quality. 
     The above combination of features of the claimed device is novel and inventive in view of the outer surface of the container neck being provided with a removable thimble enabling the same to be positioned within the metal reservoir without contacting the wall of the lined insert poured with the melt. As this takes place, the crystallized melt layer forming during pouring-in on the cool walls both of the lined insert and thimble will not prevent the same from moving relative to each other if a gap is provided between them. This gap is formed by the lined flange mounted at the upper edge of the removable lined insert and having an inner diameter smaller than an inner diameter of the lined insert. In turn, the thimble has an outer diameter matching by clearance fit with an inner diameter of the lined flange thereby providing a closed space as the melt is extruded into the investment mold. The gap width is selected once by experiment based on the longest process of pouring-in and holding the cast till the end of crystallization. It is taken radially greater than the total thickness of the single-side melt layer crystallized on the lined insert wall and the thimble wall in the process of pouring-in and holding the melt till the end of crystallization. The crystallized melt layer under the lower plane of the lined flange exhibits no significant resistance to the motion as it has no support provided under the same. The thimble is arranged to be removable since at the end of pouring-in process it is grasped along its lateral surface by the solidified butt-end and, as the bottom table is lowered with the metal reservoir, is removed from the container neck to stay within the metal reservoir. Therefore, the above combination of features of the claimed device enables unimpeded motion of the metal reservoir within the container neck under various rates and with possible stops for solving the technical problem of reinforcement of the investment mold by providing an additional metal sheath on the inner surface of the ceramic investment mold during crystallization of the melt layer butting against its inner walls. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  show a lost-wax casting device associated with piezocrystallization used in the proposed casting method. 
     
    
    
     The lost-wax casting device associated with piezocrystallization comprises a frame  1  with a stationary upper table  2  and a movable bottom table  3  driven by a hydraulic drive  4 . A container  5  with an investment mold  6  molded into a filler is rigidly fixed to the upper table. A removable thimble  8  is mounted in the neck  7  of the container  5 , said thimble being retained on the lower plane of the neck  7  by a seal  9 . The seal  9  also holds the filler within the container  5  and a pouring head with an opening  10  of the investment mold  6 . On the bottom table  3  a metal reservoir  11  is mounted comprising a base  12  with grooves  13  wherein a removable insert  14  with a gas-proof lining  15  is disposed with a minimum positive allowance. The seal  9  and the lining  15  may be made of a core sand mixture, for example, water glass mixture. Openings  16  for release of gas are provided in the upper part of the lateral element of the removable insert  14 . A flange  18  lined on a side of its cavity is mounted on the upper edge of the insert  14 . A running fit matched pair is formed by the thimble  8  and a central opening of the lined flange  18  along their outer and inner surfaces, respectively. Lead-in chamfers are provided on the thimble  8  and the flange  18  for mutual centering the lined flange  18  and the thimble  8 . The value “E” shown in the Drawing is greater that the total thickness of the single-side melt layer crystallized on the wall of the lined insert  14  and the wall of thimble  8  in the process of pouring-in and holding the melt till the end of crystallization. The bottom table  3  is mounted on the hydraulic drive  4  rod rigidly fixed to the frame  1 . 
     The device is also provided with standard automatic means for monitoring and controlling the hydraulic drive  4  operating parameters: travel, time, rod velocity, pressure (not shown). 
     BEST MODE FOR CARRYING OUT THE INVENTION 
     Next, a particular embodiment of the proposed method and device is described in detail with reference to the attached Drawing. The ceramic investment mold  6  is produced, baked according to a batch process and then placed into the container  5  with filler therein so that the filling opening  10  has its edge at the level of the end surface of the container  5  neck  7 . The thimble  8  is placed on the neck  7 , and the end surface of the neck  7  is molded with a water glass mixture leaving the filling opening to form the seal  9 . The container  5  is fixed on the upper table  2  of the frame  1  such that the neck  7  is disposed in axial alignment with the metal reservoir  11 . 
     Before the inner cavity of the removable insert  14  is mounted on the base  12  of the metal reservoir  11 , it is lined by a water glass mixture. The flange  18  lined by a water glass mixture is fixed on the top end of the insert  14 . The insert  14  mounted on the base  12  is centered relatively to the neck  7 . 
     The pouring rate of 0.15-0.2 m/s and the pressure within 0.2-0.3 MPa is initially set by the hydraulic drive equipment based on the calculated melt flow rate of 7-8 kg/s. 
     Casting is performed for rust-resisting austenitic steel. The melt is poured in the metal reservoir  11  at a temperature 25±1 C.° above the liquidus temperature to the level close to the flange  18  lining with the hydraulic drive  4  being immediately switched on and the liquid metal poured from the metal reservoir  11  into the mold  6 . After the mold  6  is filled the hydraulic drive  4  rod stops in up position. From now on, the melt is held under the predetermined pressure of 0.2-0.3 MPa for 6 to 8 seconds. Then, during the remaining full crystallization time of 1.3-1.5 min (as determined by computer modeling the pouring-in process) the pressure is uniformly increased to 5-6 MPa, a pressure generated by the hydraulic drive. During this time, the rod has moved by a small amount proportional to the amount of the metal shrinkage of 2% to 2.5%, however, it is this motion for feeding the cast at a relatively high pressure that eliminates shrinkage defects, ensure high density, mechanical characteristics, and ultimately high cast product quality. The device operates as follows. Prior to pouring-in, the container  5  with the investment mold  6  molded therein and the removable thimble  8  placed onto its neck  7  is rigidly fixed to the upper table  2  of the frame. As this takes place, the container  8  filler and the thimble  8  are retained by the seal  9 . The pre-lined removable insert  14  is mounted on the bottom table  3  into the grooves  13  of the base  12  of the metal reservoir. The lined flange  18  is mounted on the upper end of the insert  14 . By switching on the hydraulic drive  4  the metal reservoir  11  is caused to rise to the neck  7  with the thimble  8  thereon, which is centered relatively to the lined flange  18  by causing the insert  14  to move in a horizontal plane due to the minimum positive allowance in the grooves  13  of the base  12 . Then, the metal reservoir  11  is lowered into initial position. The device is ready to be operated. 
     Liquid metal is poured into the reservoir  11  approximately to the lower plane level of the flange  18  lining. The hydraulic drive  4  is switched on. The metal reservoir  11  with the insert  14  rises and enters with its flange  18  into the thimble  8  on the neck  7 , which through the filling opening  10  extrudes the liquid metal into the investment mold  6 . At the same time, gas (air) is forced out of the mold  6  through its sheath and out of the insert  14  through its gas-proof lining. After filling the investment mold  6  with the liquid metal, the hydraulic drive  4  rod stops in up position and is held for a few seconds. As this take place, the liquid metal layer in the metal reservoir  11  will crystallize along the walls of the insert  14  lining  15 , the flange  18  lining and on the neck  7  with the thimble  8 . After holding, the pressure in the hydraulic drive  4  increases to the operating pressure till the end of crystallization of the liquid metal within the whole cast space. Due to the difference “E” between the inner diameters of the lined insert  14    and the flange  18 , no grasping thereof occurs by the solidified metal layer so that the hydraulic drive  4  rod may move to provide feeding the cast with a liquid metal from the metal reservoir  11  till the end of crystallization. 
     After the crystallization process ends, the hydraulic drive  4  is switched to reverse operation. The metal reservoir  11  lowers to initial position entraining the thimble  8  due to the crystallized metal having been formed therein. Easy separation of the butt-end is facilitated from the bottom end of the neck  7  by the weakness of the seal  9  lining. The container  5  with the finished cast is removed from the upper table  2  of the frame  1  for extracting the cast. The insert  14  of the metal reservoir  11  is removed for replacing the lining. Another set comprising the container  5  with the thimble  8  mounted on the neck and the insert  14  of the metal reservoir  11  is fixed to the device, and the cycle is repeated. 
     INDUSTRIAL APPLICABILITY 
     The invention is applicable in foundry industry, namely, for lost-wax casting associated with piezocrystallization, substantially, metal ware casting.