MOLDED METAL OBJECT AND METHOD TO MANUFACTURE THE SAME

Abstract: A molded metal object is manufactured in that one or more tubes and/or shells are prefabricated, which tubes and/or shells are intended to form channels and cavities, respectively, in the metal object. The tube or shell can be produced by thermal spraying of a thin layer (2) of a high temperature and corrosion resistant metal alloy onto a core body (1). The core body (1), e.g. consisting of sand, is removed, whereupon a thicker layer (3) is sprayed thereon. Thereafter, the tube (2, 3) or shell is mounted into a tool for making a sand mould. When making the latter, the tube (2, 3) or shell is also filled with sand, whereupon the desired metal object is molded and the sand is removed.

Description: ^ND MOLDED METAL OBJECT METHOD TO MANUFACTURE THE SAME The invention relates to a molded metal object and a method to manufacture the same. The background of the invention is a desire to achieve a molded metal object comprising one or more channels or cavities, the internal surfaces of which are formed by surface layers of a high temperature and corrosion resistant metal alloy. According to methods previously known, prefabricated, thermally sprayed tubes can be molded into an object with a remaining mandrel or core . Then it has only been possible to use tubes with a simple, especially cylindrical shape, since the mandrel or core must be removed after molding. The object of the invention is therefore to accomplish a molded metal object having at least one internal channel or cavity of a complicated configuration, and this object is achieved in that the wall surface of each channel or cavity consists of a prefabricated tube or shell, and in that the remaining portions of the metal object is made by molding in a sand mould with said shell or tube included in the mould and filled with sand. Hereby, it is possible, practically without any restrictions, to provide any desired configuration of the cavities or channels of the metal object. This is accomplished by removing the core body during or after the prefabrication of the tube or shell, and thereafter filling the tube or shell again with sand when making the sand mould. As appears from the sub-claims, the prefabricated tubes or shells may be made in accordance with the patent application relating to a "Shell and tubular object and method to manufacture the same" filed simultaneously herewith. The invention will be explained further below with reference to the appended drawings schematically illustrating an embodiment in which the tube is made by thermal spraying onto a sand core. Fig. 1 shows a sand core onto which a metal alloy is to be thermally sprayed; Fig. 2 shows the sand core having a sprayed, thin first layer; Fig. 3 shows the tube, to be molded into an object, after removal of the sand core and spraying a relatively thick second layer onto the first layer; Fig. 4 shows schematically a molded metal object with a tube molded therein so as to form a channel in the metal object. In a way known per se, a core body 1 is made of molding sand containing phenol resin as a binding agent. As shown in Fig. 1, this core body 1 has an external configuration corresponding to the desired, internal configuration of the tubular object, and additionally holding portions la, lb at each end for holding the core body. In the embodiment, the desired object consists of an exhaust channel in a cylinder head in a combustion engine. In accordance with Fig. 2, a relatively thin and essentially pore-free layer 2 of a metal alloy containing about 5% Al, 22% Cr and the rest mainly Fe (the contents are by weight) is sprayed onto the core body. When the layer thickness amounts to at least 0.5 mm, preferably about 1 mm, the spraying operation is interrupted, whereupon the sand core 1 is removed by heating the sprayed sand core in a furnace at about 6009C during 2-3 hours, and thereafter discharging the sand from the thin, tubular metal alloy layer 2. Thereafter, a second layer 3 is sprayed externally onto the first, self-supporting thin layer 2. In absence of a core body, the first layer 2 is permitted to expand and shrink freely due to variations of temperature, without any risk of tensions and accompanying cracking. The thickness of the second layer may be at least 1.5 mm. The spraying process can be performed without interruption for cooling. Different methods of spraying are possible. Preferably, the first, relatively thin layer 2 is sprayed by means of a wire-fed flame sprayer 80 as to form a dense layer. In case a surface layer with great purity is desired (small quantities of oxides and pores), the layer may be applied by plasma spraying. The second layer 3 may preferably be applied by means of a powder-fed flame sprayer in order to obtain a suitable quantity of pores and oxides. In this case a total quantity of 5 - 25X, preferably about 15%, pores and oxides is desired. However, the second layer 3 does not have to be of the same material composition as the first layer 2, but may consist of another metal alloy with high heat resistance or of a ceramic material. However, there are no requirements of corrosion resistance. Thereupon, the prefabricated tube 2,3 is mounted into a tool for the manufacture of a sand mould for the desired metal object. The tube 2,3 is again filled with sand, in order to enclose the corresponding part of the sand mould. Thereafter, the metal object is molded in a conventional way, wherein the tube 2,3 forms a channel, as schematically shown in Fig. 4. (In practice, the external outline of the metal object has a complicated geometry but is shown, for the sake of simplicity, as a box-like object). Thus, the channel formed by the tube 2,3 molded therein has an internal surface consisting of the first sprayed layer 2, which is esentially pore-free and resistant against high temperatures and corrosive gases. Therefore, this layer may serve as an exhaust channel in a combustion engine. On the other hand, the second layer 3 being located inside the surface layer 2 has more pores and oxides and is also thicker, and it will therefore serve as an effective heat barrier against the other material in the metal object. Naturally, by the method according to the invention, one may make metal objects of practically any configuration of internal cavities and channels by prefabricating shell or tube bodies of a corresponding geometry. Cavities may also form a part of the outside of the metal object, i.e. in order to form, together with another object, a space exposed to intense heat and corrosive gases, e.g. a combustion chamber, a valve chamber, an exhaust channel or the like. The tube or shell to be molded into the metal object can be produced by methods other than thermal spraying, such as e.g. a powder metallurgical process or plate pressing. It is also possible to press a thin plate into a tube or shell and therafter apply at least one layer of a heat and corrosion resistant metal alloy by thermal spraying. Compared with prior art technology, the following advantages are obtained: - complicated cavities or channels may be prefabricated quickly and simply and thereafter molded into a metal object; - the handling becomes easier and the waste will be less; - the molding precision is increased; - compared with applying ceramic material as a surface layer, the inventive method is cheaper and better due to good metal-to-metal adherence, and there are no problems with expansion due to varying temperatures.

Claims: CLAIMS 1. A molded metal object comprising at least one channel and/or cavity, especially intended to be incorporated in a combustion engine, c h a r a c t e r i z 8 d in that the wall surface of each channel or cavity consists of a prefabricated tube (2,3) or shell, said wall surface having a surface layer of a high temperature and corrosion resistant metal alloy, and in that the remaining portions of the metal object is made by molding in a sand mould with said prefabricated tube (2,3) or shell included in the mould and filled with sand. 2. A molded metal object according to claim 1, c h a r a c t e r i z e d in that each prefabricated tube (2,3) or shell comprises, on the one hand, a first, relatively thin and essentially pore-free layer (2) thermally sprayed onto a core body (i) and forming a wall surface in said channel or cavity, respectively, and , on the other hand, a second,relatively thick layer (3) being thermally sprayed onto the external surface of the first layer (2) upon removal of said core body (1). 3. A molded metal object according to claim 1 or 2, c h a r a c t e r i z e d in that said surface layer consists of 1-12% Al, 10-30% Cr, possibly small quantities of one or more elements in the group Si, Mn, Co, Y, Hf, possibly small quantities of oxides and nitrides, and the rest Fe. 4. A method to manufacture a molded metal object having at least one channel and/or cavity, especially intended to be incorporated in a combustion engine, c h a r a c t e r i z e d by prefabricating a tube or a shell, the dimensions of which correspond to said channel or cavity, mounting said prefabricated tube or shell into a tool for making a sand mould, wherein also said tube or shell is filled with sand, performing the molding and removing the sand. 5. A method as defined in claim 4, c h a r a c - t e r i z e d in that said prefabricated tube or shell is produced by thermal spraying a layer of a high temperature and corrosion resistant metal alloy onto a core body. 6. A method as defined in claim 5, c h a r a c t e r i z e d in that a second, relatively thick second layer is thermally sprayed onto said layer upon removing said core body. 7. A method as defined in claim 4, c h a r a c t e r i z e d in that said prefabricated tube or shell is produced by a powder metallurgical process. 8. A method as defined in claim 4, c h a r a c t e r i z e d in that said prefabricated tube or shell is produced by plate pressing.

Inventors: BJORKLUND, NILS-ERIC, CARLSSON, JAN-OLOV, LINDGREN, HAKAN, Björklund, Nils-Eric, Lindgren, Hâkan
Applicants: AB VOLVO, KANTHAL AB
Application year: 1986
Main classification: B22D19
IPC codes: F02F1, B22D19