Abstract:
Methods are disclosed for producing melt supply pipes. In one illustrative implementation, there is provided a method for producing a melt supply pipe, composed of an inner ceramic pipe and an outer steel pipe fitted to the inner pipe, the melt supply pipe directed to connecting a melting furnace and a plunger sleeve of a die casting machine. Moreover, the method may include forming a Ni alloy layer over the inner circumferential surface of the outer steel pipe, burying the outer pipe in and bonding the surface of the Ni alloy layer to TiC particles, and fitting the inner ceramic pipe into the outer pipe with the TiC particles bonded to the inner circumferential surface.

Description:
This application is a divisional of U.S. application Ser. No. 11/565,771 filed Dec. 1, 2006, now U.S. Pat. No. 8,333,920 issued on Dec. 18, 2012. U.S. application Ser. No. 11/565,771 claims priority to Japanese Patent Application No. 2005-348830 filed Dec. 2, 2005. The entirety of all of the above-listed Applications are incorporated herein by reference. 
    
    
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to a melt supply pipe for supplying a molten aluminum alloy from a melting furnace to a plunger sleeve of a die casting machine in aluminum die casting. 
     2. Background Art 
     In conventional die casting machines, a ladle method has been commonly employed for supplying a molten aluminum alloy to a plunger sleeve. According to the ladle method, a molten aluminum alloy is drawn from a melting furnace by means of a ladle and supplied to a plunger sleeve. 
     As a technique to take the place of the ladle method, a melt supply pipe method has recently been attracting attention which involves directly connecting a melting furnace and a plunger sleeve with a melt supply pipe, and supplying a molten aluminum alloy through the melt supply pipe to the plunger sleeve. Mixing of an Al oxide film or solid broken pieces into a molten aluminum alloy can be significantly reduced with the melt supply pipe method as compared to the conventional ladle method. The melt supply pipe method thus has the advantage that it can provide a higher-quality die-cast product. 
     A conventional melt supply pipe, which has so far been used to connect a melting furnace and a plunger sleeve, has such a structure that a heater is wrapped around a ceramic pipe. A ceramic material is used for a melt supply pipe because the material has high melting loss resistance to a molten aluminum alloy. 
     While a ceramic pipe is thus strong to a molten aluminum alloy, it is weak to impact and can be broken by its vibration during operation or by erroneous handling upon its maintenance. Further, only an insufficient load can be applied on the connecting portions of such a breakable ceramic pipe, which could cause leakage of a molten aluminum alloy from the connecting portions. 
     The applicant has proposed a molten aluminum alloy-contact member having enhanced melting loss resistance to a molten aluminum alloy, comprising a steel base, a Ni alloy layer formed on the steel base, and TiC bonded in a particulate state to the surface of the Ni alloy layer (Japanese Patent Laid-Open Publication No. 2005-264306). 
     Further, a melt supply pipe is known which has such a structure that a ceramic or graphitic pipe is encased in a steel pipe for the purpose of covering the breakableness of the inner pipe. However, because of a large difference in thermal expansion coefficient between the steel pipe and the ceramic or graphitic pipe, a large gap can be formed between the inner and outer pipes due to their different thermal expansions. A molten aluminum alloy will easily intrude into the gap, which may result in melting loss of the steel pipe and formation of holes therein in a short period of time. 
     SUMMARY OF THE INVENTION 
     It is therefore an object of the present invention to solve the above problems in the prior art and provide a melt supply pipe for aluminum die casting which is strong to mechanical impact and is excellent in the melting loss resistance to a molten aluminum alloy and which has a significantly extended life, and a method for producing the melt supply pipe. 
     In order to achieve the object, the present invention provides a melt supply pipe for connecting a melting furnace and a plunger sleeve of a die casting machine, comprising an inner ceramic pipe and an outer steel pipe fitted to the inner pipe, wherein a Ni alloy layer is formed over the inner circumferential surface of the outer steel pipe, and TiC particles are bonded to the surface of the Ni alloy layer. 
     In a preferred embodiment of the present invention, the TiC particles have an average particle diameter of 10 to 500 μm, and are bonded to the Ni alloy layer in such a state that the particles are not fully covered with the Ni alloy layer but partly protrude from the surface of the Ni alloy layer. 
     The Ni alloy preferably has the composition of 2.6 to 3.2% of B, 18 to 28% of Mo, 3.6 to 5.2% of Si and 0.05 to 0.22% of C, with the remainder being Ni and unavoidable impurities. 
     In a preferred embodiment of the present invention, gaps in the TiC particles are filled in with powder comprising at least one of boron nitride (BN), alumina (Al 2 O 3 ), zirconia (ZrO 2 ) and silicon nitride (Si 3 N 4 ). 
     In a preferred embodiment of the present invention, a pair of fibrous sheet members, composed of an inorganic material having the property of expanding by heating, is sandwiched between the inner ceramic pipe and the outer steel pipe at both ends of the pipes. Preferably, the gap formed between the inner ceramic pipe and the outer steel pipe and defined by the sheet members, is filled with a spherical or particulate ceramic filler. 
     The present invention also provides a method for producing a melt supply pipe, composed of an inner ceramic pipe and an outer steel pipe fitted to the inner pipe, for connecting a melting furnace and a plunger sleeve of a die casting machine, comprising the steps of: forming a Ni alloy layer over the inner circumferential surface of the outer steel pipe; burying the outer pipe with the Ni alloy layer in TiC powder, and heating the pipe and the powder under vacuum in a vacuum heating oven to a temperature at which a liquid phase is generated from the Ni alloy, thereby bonding the TiC particles to the surface of the Ni alloy layer; and fitting the inner ceramic pipe into the outer pipe with the TiC particles bonded to the inner circumferential surface, thereby assembling the melt supply pipe. 
     According to the present invention, the outer steel pipe can protect the inner ceramic pipe from mechanical impact and, in addition, enables application of a sufficient clamp load on the terminal connecting portions of the melt supply pipe, thereby preventing leakage of a molten aluminum alloy. Furthermore, owing to TiC particles densely scattered over the inner circumferential surface of the outer pipe, the present melt supply pipe has significantly enhanced melting loss resistance to a molten aluminum alloy. Thus, the melting supply pipe of the present invention, having both high impact resistance and high meting loss resistance, can enjoy a significantly extended life. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a cross-sectional diagram showing a melt supply pipe for aluminum die casting according to a first embodiment of the present invention; 
         FIG. 2  is an enlarged illustration of the portion A of  FIG. 1 ; 
         FIG. 3  is a diagram corresponding to  FIG. 2 , illustrating the case of filling in the gaps in TiC particles with fine ceramic particles; 
         FIG. 4  is a vertical sectional diagram showing a melt supply pipe for aluminum die casing according to a second embodiment of the present invention; 
         FIG. 5  is a cross-sectional diagram taken along the line V-V of  FIG. 4 ; 
         FIG. 6  is a diagram illustrating a method for producing a melt supply pipe for aluminum die casting according to the present invention; and 
         FIG. 7  is a diagram illustrating the step of fitting an inner pipe into an outer pipe in the method for producing a melt supply pipe for aluminum die casting according to the present invention. 
     
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     Preferred embodiments of the present invention will now be described in detail with reference to the drawings. 
       FIG. 1  is a cross-sectional diagram showing the structure of a melt supply pipe according to a first embodiment of the present invention. In  FIG. 1 , reference numeral  10  denotes an inner ceramic pipe and reference numeral  12  denotes an outer steel pipe. The inner pipe  10 /outer pipe  12  integral structure of the melt supply pipe is obtained by fitting the outer pipe  12  to the inner pipe  10 . 
     As shown in  FIG. 2  which is an enlarged view of the portion A of  FIG. 1 , the entire inner circumferential surface of the outer steel pipe  12  is coated with a Ni alloy layer  13 , and the surface of the Ni alloy layer  13  is covered with a myriad of titanium carbide (TiC) particles. The TiC particles  14  are bonded in a particulate state to the Ni alloy layer  13  such that they partly protrude from the surface of the Ni alloy layer  13 . Preferably, the gaps in the TiC particles are filled in with fine ceramic particles  15  comprising at least one of boron nitride (BN), alumina (Al 2 O 3 ), zirconia (ZrO 2 ) and silicon nitride (Si 3 N 4 ), as shown in  FIG. 3 . The fine ceramic particles  15  can improve the melting loss resistance of the base Ni alloy layer  13  to which the TiC particles  14  are bonded. 
     According to the melt supply pipe of this embodiment, which employs the combination of the inner ceramic pipe  10  and the outer steel pipe  12 , the outer steel pipe  12  can protect the inner ceramic pipe  10  from external mechanical impact and, in addition, enables application of a sufficient clamp load on the terminal connecting portions of the melt supply pipe, thereby preventing leakage of a molten aluminum alloy. 
     Furthermore, the TiC particles  14  are bonded to the Ni alloy layer  13  formed over the inner circumferential surface of the outer steel pipe  12 . The TiC particles  14  have the property of repelling a molten aluminum alloy. By utilizing this property, direct contact of a molten aluminum alloy with the steel material, constituting the main body of the outer pipe  12 , can be prevented and the melting loss resistance of the outer pipe can thus be enhanced. Further, the TiC particles  14  are made to partly extrude from the surface of the Ni alloy layer  13 . This can increase the contact angle with a molten aluminum alloy, thereby enhancing the property of repelling the molten aluminum alloy. 
     In the structure that the TiC is bonded in a particulate state to the Ni alloy layer  13  and densely scattered over the layer, a large thermal stress will not act on the TiC particles  14  even when the outer pipe  12  thermally expands or contracts. Thus, the TiC particles  14  hardly peel off and, therefore, the melting loss resistance can be maintained for a long period of time. Though  FIG. 2  schematically shows the TiC particles  14  lining up side by side, there is actually a case in which the TiC particles  14  are piled up in multiple layers. 
     The base Ni alloy layer  13 , to which the TiC particles  14  are bonded, itself has poor melting loss resistance to a molten Al alloy. The melting loss resistance can be improved by attaching the fine ceramic particles  15  to the Ni alloy layer  13 , as shown in  FIG. 3 . Since the attached fine ceramic particles  15  are present such that they fill in the gaps in the TiC particles  14 , the fine ceramic particles  15  hardly fall off upon contact with a molten aluminum alloy. It is possible that the fine ceramic particles  15  may adhere also to the surfaces of the protruding portions of the TiC particles  14 . 
     The inner pipe  12 , on the other hand, can be made to resist melting loss for a long period of time by selecting a ceramic material having excellent melting loss resistance to a molten aluminum alloy. A preferable ceramic material may comprise at least one of Al 2 O 3 , SiC, Si 3 N 4 , MgO, Al 2 TiO 5 , ZrO 2 , and sialon. 
     A melt supply pipe for aluminum die casting according to a second embodiment of the present invention will now be described with reference to  FIGS. 4 and 5 . 
     In the melt supply pipe of the second embodiment, a pair of fire-resistant sheets  16  is sandwiched between the inner ceramic pipe  10  and the outer steel pipe  12  at both ends of the pipes, and the gap formed between the inner and outer pipes and defined by the fire-resistant sheets  16  is filled with ceramic balls  17 . 
     The fire-resistant sheet  16  is a sheet member composed of inorganic fibers having the property of expanding by heating. Preferably, each fire-resistant sheet  16  extends over the entire circumference, and the outer end of the sheet is aligned with the end surfaces of the inner pipe  10  and the outer pipe  12 . The balls  17  are a spherical filler formed of a ceramic material comprising at least one of Al 2 O 3 , SiC, Si 3 N 4 , MgO, Al 2 TiO 5 , ZrO 2 , and sialon. It is also possible to use a particulate filler instead of the balls  17 . 
     According to the second embodiment, there is no gap between the inner ceramic pipe  10  and the outer steel pipe  12  at both ends of the pipes because of the presence of the fire-resistant sheets  16 . Even when a gap is formed between the inner pipe  10  and the outer pipe  12  upon heating by a molten aluminum alloy, due to a difference in thermal expansion coefficient between the pipes, the fire-resistant sheets  16  can prevent the molten aluminum alloy from intruding into the gap. 
     Since the internal gap defined by the fire-resistant sheets  16  and the inner and outer pipes  10 ,  12  is filled with the balls  17 , the weight of a molten aluminum alloy flowing in the inner ceramic pipe is supported by the balls  17 , so that application of the weight of the molten aluminum alloy on the inner pipe  10  can be prevented. 
     A description will now be made of a method for producing the melt supply pipe for aluminum die casting, according to the present invention. 
     The inner ceramic pipe  10  and the outer steel pipe  12  are prepared in advance, and the melt supply pipe is produced by the following procedure: 
     First, the Ni alloy layer  13  is formed by thermal spraying on the inner circumferential surface of the outer pipe  12 . Thereafter, a vessel containing TiC powder  20  is prepared, and the outer pipe  12  is entirely buried in the TiC powder  20 , as shown in  FIG. 6 . 
     The vessel, containing the TiC powder  20  and the outer pipe  12  buried in it, is placed in a vacuum heating oven, and heated under vacuum to a temperature at which a liquid phase is generated from the Ni alloy, thereby bonding TiC particles  14  to the surface of the Ni alloy layer  13 . 
     By the heating in this step, the TiC particles  14  are bonded to the Ni alloy layer in such a state that they protrude from the surface of the Ni alloy layer  13 , as shown in  FIG. 2 . In this connection, it is undesirable if the TiC particles  14  become entirely covered with the melting Ni alloy in the heating process. In order not to entirely cover the TiC particles  14  with the Ni alloy but to strongly bond the TiC particles  14  to the Ni alloy layer  13  with the particles partly exposed on the surface of the Ni alloy layer  13 , the average particle diameter of the TiC particles  14  is preferably made within the range of 10 to 500 μm. 
     When the average particle diameter of the TiC particles  14  is smaller than 10 μm, it is difficult to control the temperature during the vacuum heating so that the TiC particles  14  may not be entirely covered with the liquid phase of the Ni alloy. The intended melting loss resistance will not be obtained if the TiC particles  14  are entirely covered with the liquid phase of the Ni alloy. 
     When the average particle diameter of the TiC particles  14  is larger than 500 μm, on the other hand, the liquid phase of the Ni alloy will cover only lower portions of the particles with small contact area, resulting in weak bonding strength between the Ni alloy layer  13  and the TiC particles  14 . Accordingly, the TiC particles  14  will easily fall off. 
     After the bonding of TiC particles  14  to the Ni alloy layer  13 , the outer pipe  12  is subjected to a process comprising applying a slurry of a mixture of a binder and a fine ceramic powder comprising at least one of boron nitride (BN), alumina (Al 2 O 3 ), zirconia (ZrO 2 ) and silicon nitride (Si 3 N 4 ) to the inner circumferential surface of the outer pipe  12 , and burning the ceramic powder into the inner circumferential surface. 
     As shown in  FIG. 3 , the TiC particles  14  can be bonded to the Ni alloy layer  13  with high strength through generation of the liquid phase from the Ni alloy. Further, because of good wetting between the liquid phase and the TiC particles  14 , a large number of TiC particles  14  can be densely bonded to the Ni alloy layer  13 . 
     Next, as shown in  FIG. 7 , the inner pipe  10  is inserted into the outer pipe  12 . Prior to the insertion, the fire-resistant sheets  16  are placed on the inner circumference surface of the outer pipe  12  at its both ends such that each sheet  16  extends over the entire circumference. After inserting one end of the inner pipe  10  and before inserting the other end into the outer pipe  12 , the ceramic balls  17  are filled into the gap between the inner pipe  10  and the outer pipe  12 . Thereafter, the inner pipe  10  is completely inserted into the outer pipe  12  till the other end of the inner pipe  10  reaches the fire-resistant sheet  16 . 
     The thus-produced melt supply pipe was fixed in an actual die casting machine to carry out a durability test by repeating a casting cycle of supplying a molten aluminum alloy from a melting furnace through the melt supply pipe to a plunger sleeve of the die casting machine. The test conditions were as follows: the type of molten aluminum alloy, JIS AC4CH, the melt temperature, 72° C.; and the temperature of a melt supply pipe heater, 720° C. Comparative durability tests were also carried out under the same conditions but using, instead of the present melt supply pipe, a comparative ceramic melt supply pipe  1  (composition: 70% SiC/30% Si 3 N 4 ) (comp. test 1) or a comparative melt supply pipe  2  composed of an outer steel (JIS S45C) pipe and an inner graphitic pipe thermally inserted into the outer pipe (comp. test 2). 
     As a result, a connecting portion of the comparative melt supply pipe  1  broke and the melt began to leak out after about 40,000 shots in comp. test 1. In comp. test 2, a connecting portion of the comparative melt supply pipe  2  broke by melting loss and the melt began to leak out after about 8000 shots. The early melting loss in comp. test 2 is considered to be caused by early formation of a gap between the graphitic pipe and the steel pipe due to a large difference in thermal expansion coefficient therebetween. Thus, intrusion of the melt into the gap may have caused melting loss of the steel pipe. In contrast, no defect, such as melting loss, was found in the melt supply pipe of the present invention even after 120,000 shots, and the operation could be continued.