EP2949413B1

EP2949413B1 - A method of making a casting of a heat exchanger

Info

Publication number: EP2949413B1
Application number: EP15275144.2A
Authority: EP
Inventors: Shouxun Ji
Original assignee: Ideal Boilers Ltd
Current assignee: Ideal Boilers Ltd
Priority date: 2014-05-30
Filing date: 2015-05-29
Publication date: 2021-08-11
Anticipated expiration: 2035-05-29
Also published as: GB201409613D0; EP2949413A1

Description

The present invention relates to a casting method to manufacture heat exchangers. The heat exchangers may have cavities and/or channels inside, such as a complicated channel arrangement for cooling fluid.
As is known in the art, high pressure die casting (HPDC) is a technique which injects a molten material such as aluminium alloy into a metal mould at high speed under high pressure to cast a near-net-shape structure. In this process, when casting a product having intricate cavities, soluble cores are usually used. The soluble core is inserted into a mould prior to introducing a molten metal to flow into the mould cavity. The soluble core, which is typically made of a salt compound, reserves space in the mould as the liquid metal flows around it. The soluble core used in such a case must be able to withstand high pressure and high temperature because it may be subjected to a large force upon injection of a molten metal at high speed during mould filling and because a high static compressive pressure is applied until solidification is completed. After the molten metal has solidified, the soluble core can be dissolved in water or another liquid flowing in contact with the soluble core to leave a cavity of the desired shape and size. Therefore, the casting has a size and internal surface configuration generally identical to the outer surface configuration of the soluble core.
Soluble cores, such as those made of salt and other soluble materials, have been used in the casting industry for many years. Numerous methods are known for manufacturing soluble cores for these purposes. Molten salt can be injected into dies to form the salt cores. Other conventional casting processes, along with pressing technologies, are available to those skilled in the art for the manufacture of soluble cores which are used to provide cavities in finished cast metal castings.
U.S 3,963,818 describes a water soluble core for pressure die casting and a process for making the same. The process includes pre-drying a granular water soluble salt having a grain size of less than about 1000 microns so that the moisture content thereof becomes less than 1%. It also comprises the step of moulding under pressure the granular water soluble salt into a desired shape and volume at a pressure of between about 1.5 to 4 tons per square centimetre and, if necessary, sintering the moulded salt at a temperature of between about 100°-300°C. The core for pressure die casting acts as a cavity former within a casting and substantially consists of a water soluble salt having a compressive strength of between 15 about 800-1480kg per square centimetre, a bending strength of between about 200-370 kg per square centimetre, and a density of between 2.05-2.12.
U.S. 4,252,175 describes core units, for use in casting a cylinder block of an internal combustion engine, comprising a preformed cylinder liner which includes a cylindrical sidewall defining an interior bore and having a port in the sidewall. The first core unit is formed of a reducible material moulded upon the preformed liner, which first core unit includes a first main core portion which partially occupies the bore and a port core portion which extends through the port. A second core unit is formed of the reducible material separately from the first core unit and is assembled upon the cylinder liner, which second core unit includes a second main core portion in the bore and in mating alignment with the first main core portion, thereby forming a composite core assembly.
U.S. 4,586,553 describes a casting core used for the manufacture of difficultly accessible cavities in castings of aluminium or of one of its alloys. This is produced from a water soluble salt as a base substance and burnt sugar as a binding agent. During manufacture of the casting core the base substance is mixed with burnt sugar in aqueous or organic solution, pressed into moulds, and baked at elevated temperature.
U.S. 4,586,553 , describes a process for pressure casting a piston with a crown insert and a cavity. The crown is placed in the mould before casting and the soluble salt core forms a cavity in the piston. The salt core is held by a crown insert to position the salt core in the mould to prevent the salt core from moving during the pressure casting procedure.
U.S. 4,743,481 , describes a moulding process for articles having an irregular shaped internal passage. The process for making an article having an irregular internal passage utilizes a hollow polymer pre-form The pre-form is filled with a relatively incompressible filler material such as a powder or a fluid, which supports the pre-form when it is placed in a mould, such as an injection mould. The filler enables the pre-form to withstand high moulding pressures and prevents deflection and movement of the internal passage within the pre-form The shell, a layer of a polymer material is then moulded about the pre-form After the final article has been formed, consisting of the pre-form and the shell, the filler is removed for possible reuse.
U.S. 4,840,219 describes a mixture and method for preparing casting cores and cores prepared thereby. Casting cores are fabricated from a mixture comprising a molten salt having dispersed therein a particulate material which includes a first refractory material having a mesh size of 60-120 and a second refractory material having a mesh size of at least 200. The salts are preferably halides, carbonates, sulphates, sulphites, nitrates or nitrites of Group Ia and Group IIa metals and the refractory material may be selected so as to be non-reactive with the molten salt. Some preferred refractory materials include alumina and magnesium silicate.
U.S. Pat. No. 4,875,517, which issued to Donahue et al on Oct. 24, 1989 , discloses a method for producing salt cores for use in die casting. A pattern, identically proportional in configuration to the salt core to be produced, is initially formed from an evaporable foam material. The evaporable foam pattern is positioned in a mould and surrounded with an unbonded flowable material, such as sand. The pattern is contacted with a molten salt and the high temperature of the salt will vaporize the pattern, with the vapour being captured within the interstices of the sand while the molten salt will fill the void by vaporization of the foam to provide a salt core identical in configuration to the pattern. The salt core is subsequently used in a high pressure die casting operation to cast a metal part.
U.S. 4,922,863 describes a cast engine cylinder having an internal passageway and method of making same. A cast cylinder for an internal combustion engine is disclosed having an intake valve cavity located on one side of the piston bore, an intake bore for communication with a carburettor located on the other side of the piston bore, and an internal passageway cast there communicating the intake bore and the intake valve cavity. The internal passageway is curved and circumscribes a portion of the intake bore. A walled hollow tube having initially closed ends is embedded in the cast cylinder during casting as a permanently retained casting core. Subsequently, the ends of the embedded tube are machined open to communicate with the intake valve cavity and the intake bore, respectively, to define the internal passageway.
U.S. 4,904,423 describes a pressure moulding process using salt cores and a composition for making cores. The process involves pressure moulding an article around a hardened salt mould core made from a mixture of relatively low melting temperature salt and sand. The core is removed from the finished article by immersion of the article containing the core into a molten bath mixture of the core material to thereby melt the core out of the article. The process also recovers the core material and thus replenishes the bath for use in making additional cores. The bath is originally constituted by melting a suitable quantity of a dry premix of the salt and sand.
U.S. 5,165,464 discloses a method of high pressure casting hypereutectic aluminium-silicon alloys using a salt core. The method uses a salt core to form wear resistant articles, such as engine blocks. To produce an engine block, one or more solid salt cores is or are positioned within a metal mould with the space between the cores and the mould defining a die cavity. A molten hypereutectic aluminium-silicon alloy containing more than 12% silicon is fed into the die cavity. On solidification of the molten alloy, precipitated silicon crystals are formed, which are distributed throughout the wall thickness of the cast part and also on the surface bordering the salt cores which constitute the cylinder bores in the cast block. The salt cores are subsequently removed from the cast block by contact with a solvent such as water.
U.S. 5,303,761 describes a die casting process using casting salt cores. A process of providing a disposable core for use in a die casting processes is described. A molten salt material is cast into a core of a desired configuration under exacting conditions. The fluidity of the molten salt is controlled enabling casting the salt material into a core by a die casting method. The die casting method provides a core with a high surface finish and strength. The core is evenly cooled subsequent to it being cast and is maintained at an elevated temperature to maintain its surface finish and structural integrity. The cast core is inserted into the dies of a metal die casting machine to facilitate casting a metal product having internal forms not otherwise attainable. The core is removed from the metal product by simply dissolving and flushing the core out of the casting. The salt material may be reclaimed by a desalination process for further use.
U.S. 5,803,151 describes a soluble core method for manufacturing metal cast products. The improved soluble core, for die casting metals or metal matrix composites, is formed of a mixture of salt and up to about 20 weight % of ceramic material blended together to produce a homogeneous mixture and compacted under pressure to product a soluble core having little or no porosity. The ceramic material can be in the form of fibres, particulates, whiskers, and/or platelets, and has a melting temperature greater than that of the salt.
U.S. 5921312 discloses an improved soluble core for die casting metals or metal matrix composites formed from a mixture of salt and about more than 0 weight % and less than 20 weight % of ceramic material blended together to produce a homogeneous mixture and compacted under pressure to produce a soluble core having little or no porosity. The ceramic material can be in the form of fibres, particulates, whiskers, and/or platelets, and has a melting temperature greater than that of the salt. The core can include a thermally insulating outer ceramic coating to enable the core to withstand higher die casting temperatures than conventional salt cores. The improved soluble core is removable with hot water and/or steam and the core material can be reclaimed for reuse.
WO2002072328 discloses material used for the fabrication of composite parts of low density, water-soluble coring and tooling. A lightweight, strong composite coring material is disclosed which can be easily shaped and removed from cured composite parts. A lightweight, strong composite tooling material is also disclosed which is easily tailored to provide a specific coefficient of thermal expansion and thermal conductivity, thus providing a tooling material which can be matched to the composite structure and material being fabricated.
WO2009/092850 discloses a core for high- pressure die-casting, which core is intended to form a cavity in a high-pressure die-cast piece. The core includes a heart made from a disintegrable material, the external surface of which is formed from a reinforcement layer. The reinforcement layer is an essentially unified metal layer cast around the heart in a pattern. In addition, the metal of the metal layer is the same metal as that of the high-pressure die-cast piece, and is arranged to attach to the high-pressure die- cast piece.
WO2011/054920 discloses cores that are inserted into the die, when die-casting work pieces from metal, in order to maintain the cavities that are intended in the work pieces when the moulds are filled with the molten metal. The cores have to meet high requirements with regard to their dimensional stability and the ease with which they can be removed from the cavities. The salt-based cores can be produced by moulding and compacting a core material mixture, the core materials thereof being selected from at least one salt, at least one binder and optionally from auxiliaries such as additives, fillers, wetting agents and catalysts. The salt, the binder and the optionally used auxiliaries of the core material mixture are inorganic and the core materials are soluble with water as a solvent.
WO 2009/053248 A1 discloses a heat exchanger made using an insoluble core covered by sand.
Existing technology known to those skilled in the art for manufacturing and using salt cores has several inherent drawbacks. Salt cores are made by solidifying a salt melt or in a monolithic format. Cracks easily form on the surface of the core during manufacturing. These cracks can be filled with liquid metal during the injection of melt into the die cavity surrounding the core. This unwanted metal must later be removed. Secondly, positional issues arise when the soluble core is located in a steel die/mould used to make a high pressure die casting, normally of aluminium. It is often difficult to precisely locate the salt core within the steel die block prior to injecting liquid metal into the die cavity during the high pressure die casting process. Furthermore, the core is not suitable for use with thin and long shapes because of the strength of the core materials. Therefore, the cores are usually used for simple and shallow cavity and channels. Finally, the casting process is finished by injecting liquid metal into the die cavity to form the final shape. The casting is formed in one stage and normally has the same quality throughout the casting. Although the over moulding of one aluminium alloy on the core with the same type of materials or different types of alloys may be achievable in production, the integration of a portion with a different quality level and with water channels in-between is still difficult to achieve for high pressure die casting and thus satisfy the requirement of different mechanical performances in one piece of metallic casting.
The present invention has been made from a consideration of the above.
Aspects of the invention are recited in the appended independent claims.
Described herein is a casting method for casting a metal object by a high pressure die casting process on a core, the core having a component soluble to a solvent and a metal component non-soluble to the solvent, the soluble core component being cast on the surface of the metal core component to form a desired shape, wherein during casting the insoluble core component and the soluble core component are directly in contact with molten metal and remain in place within the solidified metallic casting, the method further comprising dissolving the soluble component with the solvent to form one or more cavity and/ or channels of the desired shape in the heat exchanger casting.
It is significantly beneficial to the metal casting process that the core of the invention, comprising integrated soluble and insoluble portions:-

i. reduces the likelihood of cracking on the surface of the core;
ii. allows the soluble core to be more accurately positioned within the steel casting die/mould used in high pressure die casting process;
iii. allows the core to be usable for long and thin channels with complex shapes; and
iv. allows the casting of metallic products with more than one type of aluminium alloy for different mechanical performances.

More particularly, in making a metallic casting of a heat exchanger on a metallic core that is integrated with at least one portion of soluble salt compound integrally cast on the insoluble metallic core component the soluble core component may form the shape of water channels and/or cavities in the heat exchanger casting.
In a particularly preferred embodiment of the present invention there is provided a method of making a heat exchanger by high pressure die casting on at least one metallic core that is integrated with at least one portion of one or more soluble salt compounds cast on the metal surface of the insoluble portion as part of the core to form the shape of water channels in the casting of heat exchanger. In this preferred embodiment the method involves making a metallic component with low porosity to provide the required performance of working under high temperature for better heat exchange capacity, casting one or more soluble salt compounds on the metallic component to form an integrated core that includes both soluble and insoluble portions, casting an external part via high pressure die casting on the integrated core to form an metallic casting to provide improved surface quality, in which the insoluble portion and the soluble portion of the integrated core are directly in contact with molten metal and remain in place within the solidified metallic casting. The method then involves dissolving the soluble portion of salt with water or another liquid to form the shaped channels in the heat exchanger casting.
In a particularly preferred embodiment of the present invention, the insoluble metallic component is made of metal, optionally from the same material as the external casting, optionally of aluminium alloy. The insoluble metallic component may also have at least one part extending out of the soluble portion to allow the component to be used for the purposes of positioning the integrated core within a die or mould block, prior to injection of molten metal, so as to provide more precise location of the soluble portion in the metallic casting. The die may be made of steel.
In a particularly preferred embodiment of the present invention, the integrated core is made by casting one or more molten salt compounds on the pre-made insoluble metallic component to form a part of surface with soluble salt and a part of surface with insoluble metallic component. As such, the metallic component acts as a support for the soluble portion of salts, which allows the soluble portion to be thin, long and in complicated shapes.
In a particularly preferred embodiment of the present invention, the insoluble metallic component may be made in different methods to that for the external casting. The insoluble metallic component is made in such a way that the component has lower defect levels to allow the component working at high temperature, in particular in contact with fire flame. The external casting made by high pressure die casting is characterised by high dimensional accuracy and high surface quality, allowing the final metallic casting to be optimized for the casting used in a heat exchanger.
Therefore, the present invention provides a method for casting a metallic product which comprises the process of making a metallic component with low porosity to provide the required performance of working under high temperature for better heat exchange capacity, casting soluble salt compounds on the metallic component to form an integrated core that includes both soluble and insoluble portions and casting an external part via high pressure die casting on the integrated core to form a metallic casting to provide improved surface quality. During the method the insoluble portion and the soluble portion of the integrated cores are directly in contact with molten metal and remain in place within the solidified metallic casting, dissolving the soluble portion of salt with water or another liquid to form the shaped channels in the heat exchanger casting.
The method further comprises using a part of the insoluble component out of the soluble portion of the integrated core and disposing the insoluble component, with the soluble portion formed around at least a portion of the insoluble component, in a metallic block die/mould having an internal surface which is shaped to form the metallic casting. The soluble portion is disposed at a location within the mould to create the cavity or channels in the metallic casting.
The method further comprises the steps of introducing a liquid metal to flow into the metallic block die/mould and around the insoluble component with the soluble portion formed around at least a portion of the insoluble component, allowing the liquid metal to solidify, removing the metallic casting from the metallic block die/mould with the soluble portion formed around at least a portion of the insoluble metallic component remaining in place within the solidified metallic casting, dissolving the soluble portion to form the cavity of the metallic casting made by insoluble portion in the integrated core and the external casting.
The casting method of the present invention is preferably not used in the manufacture of single piston disc brake caliper housings, metal pistons, metal cylinder blocks or other components for use in internal combustion engines. Further, and for the avoidance of doubt, the metal core component is not cast onto the non-soluble core component, but the soluble core component is cast onto the surface of the metal core component to form a desired shape. The soluble core component is also not formed into a desired shape and thereafter moulded onto the metal core component. In order that the present invention may be more readily understood specific embodiments thereof will now be described with reference to the accompanying drawings in which:-

FIG. 1 shows a heat exchanger made in accordance with the method of the present invention;
FIG. 2 is a sectional view along A-A of Fig. 1;
FIG. 3 is a sectional view along B-B of Fig. 1;
FIG. 4 is a sectional view along C-C of Fig. 1;
FIG. 5 is a view of the metallic component of a core that could be used to manufacture the heat exchanger of Fig. 1;
FIG. 6 is a view of a core made by casting a soluble portion on the insoluble metallic component of Fig 5, the core being useable to manufacture the heat exchanger of Fig 1; and
FIG. 7 shows the external casting for the heat exchanger of Fig. 1.

Referring to the drawings and specifically with reference to FIG. 1 a heat exchanger 10 comprises a combustion chamber 11 with a fluid intake port 12 and an outlet port 13. Complicated shapes and multiple layers of thin wall castings are visible. Fluid, such as cold water, enters the heat exchanger 10 through an intake passage 20 and flows out of the heat exchanger 10 through an outlet passage 21. The water is heated through the exchange of heat with the wall of the aluminium casting in contact with the water. The aluminium casting is heated by the flame of burned gas in the combustion chamber 11, in which the flame flows from the bottom of the casting to the top of the casting, during which the flame temperature is cooled down from a temperature of around 1500°C to a temperature of few hundred degrees. In the same time the water is heated from room temperature to a temperature around 60 to 100°C.
From FIGS. 2, 3 and 4, it can be seen that the cavity and channels are complex and in communication with the intake passage 20 and the outlet passage 21. The significantly thinned section of the wall also can be seen in FIGS. 2, 3 and 4.
In Fig. 2 the sectional view passes though the ribs 26 to separate the intake passage 20 and the outlet passage 21. Again it can be seen the heat exchanger is a complex shape comprising banks of cylindrical parts 35 in order to provide a maximum surface area for heat exchange.
FIG. 3 shows the intake passage 20 and the outlet passage 21. It can be seen that the intake passage 20 and outlet passage 21 are complex in shape with long and thin veins to form internal channels separated by a thin section 33.
FIG. 4 is a C-C section view of FIG. 1. It can be seen that the internal surface 25 of the intake passage 20 and the outlet passage 21 is significantly irregular and has a thinned width portion 34 approximately midway along its length.
FIG. 5 shows a metallic core 40 which may be used as an internal part of the heat exchanger casting 10 in FIG. 1. The metallic component 40 is also used as support for the soluble salt portion of the core forming water channels 41 in the final product. A number of channels on the surface of metallic core 40 are separated by thinned portion 48 and plates 33, 34 on the core which divide the core surface into channels that communicate with the intake passage 20 and the outlet passage 21 as described above in conjunction with FIG. 2, 3 and 4.
Conventionally, water channels in heat exchangers are made by using sand cores. In order to shake out the sand in the shaped casting, at least two windows needs to be opened in the outside of casting. After shaking out the sand, the windows need to be filled by another casting and joined together by welding. This not only increases the processing steps, but also increases the cost of component manufacturing. If the casting can be made for a monoblock structure, it will be beneficial for the improvement of functionality and the reduction of product cost.
The present invention provides a solution to this problem. If the intake passage 20 is formed through the use of a salt core the salt core would be installed in the die block to form a cavity. One end with a highly irregular shape having a thinned section would extend into the cavity and suffer the high impact and high pressure during die filling and solidification of aluminium melt. The thin sections are subjected to the potential of fracture. The solution of the present invention uses a part of casting as the core and the salt core is cast on the surface of the metallic core shown in FIG. 5 to achieve an integration of metallic portion and soluble portion on the core as shown in FIG. 6. It can be seen that the integrated salt core represented in the figures exhibits several possible advantages. Firstly, the central portion 41 that has a significantly thinned section which may be susceptible to breakage during handling of the salt core is directly cast on the metallic core and the support from the metallic core is sufficiently strong to avoid any breakage under impact of melt and pressure. Secondly, when the core is inserted into a die, into which molten metal will be injected, if the core end 42 of the integrated salt core is used to locate the core, it is difficult to be precisely located in relation to the cavity into which it is inserted because of its thin and short size. The use of the metallic core for the location of the integrated salt core is an important feature of the present invention.
The dimensional accuracy of the salt surface on the integrated core relies upon the precisely defined location of metallic core. Therefore the whole metallic core with a portion of salt on its surface can be accurately positioned in the die cavity. The core end 42 can be attached to the metallic core to rigidly form a part of the core in an attempt to accurately position the entire surface of the salt. In addition, the core end 42 can be attached to at least one insert 43 for the subsequent die casting, which is helpful to reduce the possible corrosion in the casting during application in boiler. For example, an iron insert 43 can be cast into an aluminium casting to extend the exchanger life during application. Preferably the iron insert 43 is not attached to the core end by moulding.
FIG. 6 shows the integrated core 30 made from a metallic component 40 and expendable portion 41 with the extended salt core 42 for the intake port 20 and the outlet port 13.
FIG. 7 shows the external casting 50 that is made at the final stage via high pressure die casting melt onto the integrated core 30 for the heat exchanger 10. The high quality of the casting surface can be achieved by high pressure die casting with a metallic die block.
The method starts with a functional block of metallic core 40. The first step of the process is to provide a mould to make the metallic core 40, which can be achieved by a casting process to achieve low level porosity and a refined microstructure, by which the casting can withstand the high temperature from the direct contact with fire flame. The surface of the metallic core needs to be cleaned with a proper location for the subsequent casting process. Then the metallic core 40 is inserted into a mould and a liquid soluble material, such as salt, is injected into the mould and around the insoluble metallic core 40 as described in Fig. 5. The soluble material is then allowed to solidify around the insoluble metallic core to form a soluble portion of the integrated core. This is shown in Fig. 6.
The core is then removed from the mould and inserted into a steel die block for a metal casting process. Molten metal of aluminium is then injected into the die cavity and around the integrated core, and the molten metal is allowed to solidify in the die cavity. After the molten metal is solidified, the metal casting is removed from the die. Then, the soluble portion of the integrated core is dissolved through the use of a liquid, such as water.
It is to be understood that the above described embodiment is by way of illustration only. Many modifications and variations are possible.
Although the present invention has been described in terms of the metallic core 40 used as insoluble support member, it should be understood that other configurations are also within the scope of the present invention. A preferred embodiment of the present invention incorporates an aluminium part with the same speciation for the metallic core and the external casting, but other insoluble materials can also be used. Furthermore, in a preferred embodiment of the present invention, the soluble portion of the core is made of salt. It should be understood that other soluble materials, as described in the prior arts can also be used. The soluble material can be disposed around the insoluble metallic core either by an injection moulding process in which molten salt is injected into a die, a standard casting process, or a pressing process. The particular technique used to dispose the salt around the metallic core is not limiting for the present invention.

Claims

A casting method for casting a heat exchanger casting by a die casting process, the method comprising casting molten metal onto a core, the core having a component soluble to a solvent and a metal component non-soluble to the solvent, the soluble core component being cast on the surface of the metal core component to form a desired shape, wherein during casting the insoluble core component and the soluble core component are directly in contact with the molten metal and remain in place within the resulting solidified metallic casting, the method further comprising dissolving and removing the soluble portion of salt with the solvent to form one or more cavities and/or channels of the desired shape in the heat exchanger casting.
The casting method of claim 1, wherein the metal core component comprises any of the following either alone or in combination:- steel, iron, copper, aluminium, or zinc alloy, or one or more alloys comprising any of the aforesaid.
The casting method of claim 1, wherein the metal core component comprises aluminium alloy.
The casting method of claim 1, wherein the metal core component has at least one part extending beyond the soluble component for location in the die block for positioning the core in the casting of the heat exchanger.
The casting method of claim 1, wherein the soluble core component comprises one or more salt compounds, preferably compact salt compounds.
The casting method of claim 1, wherein the soluble core component is formed on part of the surface of the metal core component which would otherwise be exposed to molten metal used to cast the metal casting.
The casting method of claim 1, wherein the soluble core component provides a shaped external surface of the soluble portion to be generally identical to a cavity in the metal casting.
The casting method of claim 1, wherein the soluble core component is removed by dissolving into liquid solution, preferably by dissolving in water.
The casting method of claim 1, wherein part of the core is brought into engagement with an internal surface of the mould to determine a position of the soluble portion within the mould.
The casting method of claim 1 comprising introducing the molten metal to flow into the mould and die block in direct contact relation with the surface of the insoluble portion of the metallic component in the core, where the surface of the insoluble portion is not covered by the soluble portion around at least a portion of the insoluble component, and allowing the molten metal to solidify around the core.
The casting method of claim 1 comprising introducing the molten metal via high pressure die casting.