Method of bonding a valve seat

Abstract: A method for producing a valve seat (19) within a cylinder head unit (11) provided with valve openings (13a,14a) associated with respective valve guide holes (11a) for receiving valve guides (15,16) for associated intake or exhaust valves (17,18) opening and closing said valve openings (13a,14a), respectively, comprises the steps of: (a) placing a valve seat base material onto a surface of said openings of said cylinder head unit, (b) pushing an electrode against the end face of said valve seat base material opposite to said cylinder head unit (11) with a pushing direction matched with an axis (C) of said intake or exhaust valve (17,18), whereby said electrode being adapted to apply electricity to said cylinder head unit (11) through said valve seat base material, and (c) applying a finishing treatment to said bonded pieces to receive the desired valve seat (19).

Description: This invention relates to a method for producing a valve seat within a cylinder head unit provided with valve openings associated with respective valve guide holes for receiving valve guides for associated intake or exhaust valves opening and closing said valve openings, respectively, and a device conducting said method. A method for producing a valve seat as defined in the preamble of claim 1 is known from patent abstract of Japan, volume 95, no. 004, and JP-A-07 103070, filed in the name of the applicant of this patent application. This prior art document also discloses a device in accordance with the preamble of independent claim 15. EP-A-0 195 177 discloses a device for induction heating of a valve seat comprising an inductive rod to be inserted into a bore of a cylinder head unit. The inductive head is coupled with a heat source. When inserting the rod into the valve shaft, the desired alignment of the inductive head and the bores is achieved. A conventional cylinder head body for engines is mainly made of Al alloy, and a valve seat is provided where the valve face of an intake or exhaust valve engages the cylinder head body. Since the valve seat engages the intake or exhaust valve repeatedly and is subject to high temperature, the valve seat is made of sintered Fe alloy of excellent wear resistance and high temperature strength, press-fit in the recess formed in the intake or exhaust port opening of the cylinder head on the combustion chamber side as shown in FIG. 17, and finished by grinding so as to be held integrally in the cylinder head body. FIG. 17 is an enlarged sectional view of a portion of a conventional cylinder head with a valve seat press-fit therein, numeral 1 designating a cylinder head body, numeral 2 a press-fit type valve seat, and numeral 3 a recess for the press-fit valve seat. However, the cylinder head with a valve seat 2 of different material press-fit in the cylinder head body 1, must have an appropriate dimensional allowance for the valve seat 2 not to come off even when the cylinder head temperature went high during running. As a result, the wall thickness between adjacent ports must be kept large to a certain degree and dimension reduction between ports cannot be effected. That is, the press-fit type valve seat 2 has a great difficulty in designing port-related dimensions of the cylinder head due to its large sectional area. Further, the heat conductivity of Fe-based sintered alloy, the material of valve seat 2, is lower than that of Al alloy, the material of the cylinder head body 1, and the valve seat 2 has an appropriate thickness for preventing deformation during press-fitting as well as has a minute clearance between the valve seat 2 and the cylinder head body 1, therefore heat resistance will be high when heat is transmitted from the intake or exhaust valve face and exhaust gas to the cylinder head 1. As a result, cooling capacity of the cylinder head will be insufficient, which might cause abnormal combustion and excessive temperature rise of the valve. Furthermore, since the thermal expansion coefficient of the material of the valve seat 2 is larger than that of Al alloy, the material of the cylinder head body 1, the minute clearance between the cylinder head body 1 and the valve seat 2 is widened at a temperature higher than a certain value and accordingly heat transmission to the cylinder head body 1 will be further hindered. As a result, the valve seat 2 may be heated excessively and wear resistance will be apt to be lowered. In order to eliminate the disadvantage in press-fitting the valve seat 2 as described above, a method has been suggested in which valve seat material of high heat resistance, high wear resistance, and high anticorrosion is melted by heat using laser as a heat source, clad over the portion of the cylinder head body 1 to be used as a valve seat, and the clad layer is finished by machining to form a valve seat (for example, see Japanese Unexamined Patent Publication Sho 62-150014). FIG. 18 shows a valve seat formed using this laser clad method. FIG. 18 shows an enlarged sectional view of a valve seat portion of a cylinder head formed using the laser clad method, numeral 4 designating the valve seat portion, numeral 5 the bond interface between the cylinder head body and the valve seat portion 4, and numerals 6, 7 the melt reaction layer formed in the vicinity of said bond interface 5. Further, in order to raise the bonding efficiency between the clad layer and the cylinder head body in a cylinder head with a valve seat formed by the laser clad method, one method has been suggested in which laser cladding is performed after a plastically deformed layer has been made in advance by, for example, a pressure roller on the surface of the cylinder head body to be bonded (for example, see Japanese Unexamined Patent Publication Hei 2-196117). However, even if the valve seat is formed using the laser clad method, a problem is left in bond strength associated with melting and cladding of the valve seat material. This is because a portion of the cylinder head body 1, close to the bond interface, also melts when the valve seat material is melted by heat. That is, said portion of the cylinder head body 1 solidifies after it has melted so that gas is generated and left as blow holes in the melt reaction layer 7, or the melted Al alloy contracts when solidifying, whereby cavities may be produced in said melt reaction layer 9. In addition, this laser clad method has the disadvantage of being liable to be affected by cavities and impurities in the Al alloy. Moreover, the bond strength of the bonded portion formed by melting and solidifying of material is liable to decrease when the engine is kept running for a long time in a condition in which the temperature of the valve seat portion goes high. Accordingly, it is an objective of the present invention to provide an improved method for producing a valve seat as indicated above which facilitates the manufacture of valve seats having a small section area as well as a small heat resistance and simultaneously ensuring a reliable bonding strength. According to the invention, this objective is solved by a method as defined in claim 1. One possibility to match the pushing direction with the axis of said intake or exhaust valve is given by advancing a guide rod coaxially aligned with said electrode such that said guide rod enters said valve guide hole and simultaneously guides said electrode. According to another embodiment of the invention, the pressing force and/or said electricity are applied according to a predetermined pattern. In order to check whether the valve seat base material is fitted correctly, it is advantageous to rotate said electrode after the steps (a) and/or (b) . In order to control the direction of the eutectic alloy removed from said bonded portions, it is advantageous to control the direction and the magnitude of a magnetic flux in the magnetic field caused by energization. According to a still further embodiment of the invention, after step (b) a sampling test is carried out by applying a tensile force to the bonded valve seat base material. This will save production time and costs and especially enhances the reliability of said cylinder head because scrap cylinder heads may be eliminated before the final finishing treatment and especially before using same for an internal combustion engine. It is a further objective of the present invention to provide a device conducting the method as indicated above ensuring manufacture of reliable valve seats having an enhanced bonding strength. According to the invention, this further objective is solved by a device as defined in claim 15. According to an embodiment of the invention, this means for ensuring the coaxial alignment between the electrode and said valve guide hole is a guide rod slidingly received in a through hole of said electrode and being capable to be inserted into said valve guide hole of said cylinder head unit. According to another embodiment of the invention, said first means is a lower platen fixed to a lower portion of said base frame and said second means is an upper platen disposed upwardly of the lower platen for vertical movement so as to be able to come into contact with the lower platen, whereby the upper platen is fixed to the lower end of a rod being an end portion of the cylinder device mounted on the upper portion of the base frame vertically. When a displacement meter for measuring displacement of the first and second means or upper and lower platen, respectively, with respect to each other is provided, it is possible to control the sinking rate of said valve seat base material into said cylinder head to always ensure constant quality of valve seats. In order to control the direction of a eutectic alloy removed from the bonded portions, it is advantageous when said device comprises a shield made of a ferromagnetic material for controlling the direction and magnitude of magnetic flux in the magnetic field caused by energization. Other preferred embodiments of the present invention are laid down in further dependent claims. According to the invention, energization causes resistance heat at the contact portion between the valve seat base material and the cylinder head body, temperature rises at the interface of the contact portion where said valve seat base material and the cylinder head body are pressed against each other, so that atoms on both sides of the interface will diffuse to each other. As a result, a eutectic alloy layer made of material metal of the film of the valve seat base material and material metal of the cylinder head body is produced in the vicinity of said interface. This eutectic alloy layer is changed by said resistance heat to a liquid phase because of its low transformation temperature, and removed to the outside from the bonded portion together with the material metal of the cylinder head body developing plastic flow by heating, as the valve seat base material is pushed against the cylinder head body. In this way, contact between the Fe-based sintered alloy of the valve seat base material and material metal of the cylinder head body causes their atoms to diffuse to each other, and in this state, the valve seat base material is embedded in the port opening. According to the invention, pushing force is exerted in the axial direction of the port opening at this time, so that not only said atom diffusion phenomena are brought about uniformly over the entire circumference of the valve seat base material, but also pores associated with the atom diffusion, or defective structure associated with deformation can be avoided. In addition, pushing the electrode according to a certain pattern of pushing force and energizing according to a certain pattern of electric current make it possible to control the rate of plastic flow in the cylinder head body. According to a further embodiment of the invention, a valve guide hole is formed on the valve axis so that the pushing direction of the electrode exactly coincides with said valve axis. In the following, the present invention is explained in greater detail with respect to several embodiments thereof in conjunction with the accompanying drawings, wherein: FIG. 1 is a sectional view of a valve seat portion of a cylinder head, with a valve seat bonded using a bonding method according to the invention; FIG. 2 is a sectional view showing a valve seat base material being placed on a port opening, depicting only a part of a cylinder head body and a valve seat base material on an enlarged scale; FIG. 3 is a front view of a pressure devise for use in performing the bonding method according to the invention; FIG. 4 is a side view of a pressure devise for use in performing the bonding method according to the invention; FIG. 5 is a sectional view of an electrode being engaged with the valve seat base material: FIG. 6 is a diagram showing the patterns of pushing force and electric current, and the amount of sinking; FIG. 7 is a sectional view of an alloy layer being produced, said alloy layer consisting of material metal of the film of the valve seat base material and material metal of the cylinder head body; FIG. 8 is a sectional view of material metal of the cylinder head body developing plastic flow; FIG. 9 is a sectional view of the valve seat base material being embedded into the cylinder head body; FIG. 10 is a sectional view of the valve seat portion being finished by machining; FIG. 11 is a plan view showing an embodiment using the shield device; FIG. 12 is a graph of relationship between electric resistivity of an aluminum alloy for the cylinder head body and separation load of the valve seat member; FIG. 13 is a graph of relationship between thermal conductivity of an aluminum alloy for the cylinder head body and separation load of the valve seat member; FIG. 14 is a graph of relationship between solidus temperature of an aluminum alloy for the cylinder head body and separation load of the valve seat member; FIG. 15 is a graph of relationship between 0.2% yield strength of an aluminum alloy for the cylinder head body and separation load of the valve seat member; FIG. 16 shows sectional views of other embodiments with bonded portions of different shapes, FIG. 16(a) and (b) are examples in which only valve seat base materials are formed with convex portions, and FIG. 16(c) an example in which only the cylinder head body is formed with a convex portion; FIG. 17 is an enlarged sectional view of a portion of a conventional cylinder head with a valve seat press-fit therein; and FIG. 18 is an enlarged sectional view of a valve seat portion of a cylinder head, with the valve seat formed using laser clad method. An embodiment of the invention will be described below with reference to FIG. 1 through FIG. 10. FIG. 1 is a sectional view of a valve seat portion of a cylinder head, with a valve seat bonded using a bonding method according to the invention, FIG. 2 is a sectional view showing a valve seat base material being placed on a port opening, depicting only a part of a cylinder head body and a valve seat base material on an enlarged scale. FIG. 3 is a front view of a pressure devise for use in performing the bonding method according to the invention, FIG. 4 is a side view corresponding to FIG. 3, FIG. 5 is a sectional view of an electrode being engaged with the valve seat base material, FIG. 6 is a diagram showing the patterns of pushing force and electric current, and the amount of sinking, FIG. 7 is a sectional view of an alloy layer being produced, said alloy layer consisting of material metal of the film of the valve seat base material and material metal of the cylinder head body, FIG. 8 is a sectional view of material metal of the cylinder head body developing plastic flow, FIG. 9 is a sectional view of the valve seat base material being embedded into the cylinder head body, and FIG. 10 is a sectional view of the valve seat portion being finished by machining. In these figures, numeral 11 designates a cylinder head body of a four-stroke engine, which is molded with Al alloy, formed with a combustion chamber recess 12 of a dome-like shape facing downward, and with an intake port 13 and an exhaust port 14 each having an opening at the recess 12. In the upper wall portion of said intake and exhaust ports 13, 14 are fitted an intake valve 17 and an exhaust valve 18 through valve guides 15, 16, and at the openings of both ports 13, 14 are bonded valve seats 19 , respectively. Said valve guide 15, 16 are press-fit in valve guide holes lla' formed in the cylinder head body 11. The valve guide hole lla' are formed with their axes C coinciding with the axes of the openings 13a, 14a of the intake and exhaust ports 13, 14. The valve seat 19 shown in FIG. 1 is a seat in which a valve seat base material of a ring shape is bonded to the cylinder head 11 using a bonding method of this invention and finished by machining. Said valve seat base material is designated symbol 20 in FIG. 2. The valve seat base material 20 consists of a ring body 21 made of Fe-based sintered alloy covered with a Cu film 22. However, it is not compulsary that said ring body 21 is provided with such a film or coating. It is sufficient that the materials of said cylinder head body 11 and said ring body 21 may form an eutectic alloy, as described later. As for said ring body 21 of this embodiment, the Fe-based sintered alloy is infiltrated with Cu for the purpose of avoiding development of internal resistance heat during energization as described later. Said Cu film 22 is formed by electroplating the ring body 21 so as to be 0.1-30µm in thickness. For the Al alloy constituting the cylinder head body 11, material of AC4C specified in JIS is used in this embodiment. The valve seat base material 20 is formed, as shown in FIG. 2, into a shape such that a part of its outer circumferential surface faces the opening 13a or 14a when it is placed on the opening 13a or 14a of the intake or exhaust port 13 or 14. By the way, in FIG. 2, the cylinder head body 11 is shown with the bottom (the face at which the combustion chamber recess 12 is open) upward. More specifically, the outer circumferential surface 20a of the valve seat base material 20 is inclined in such a manner that its outside diameter becomes smaller toward the cylinder head body 11, the bottom surface 20b is inclined in the direction nearer to the cylinder head body 11 toward the axial center of the valve seat base material 20, and the outer circumferential surface 20a and the bottom surface 20b are joined together to form a convex surface. In FIG. 2, the convex surface is designated by symbol 20c. On the other hand, at the portion of said opening 13a or 14a facing said convex surface 20c is formed a convex portion 23 in which the inside diameter of the intake or exhaust port 13 or 14 become smaller partially. That is, when the valve seat base material 20 is placed on the opening 13a or 14a as shown in FIG. 2, the convex surface 20c comes into contact with the convex portion 23 of the cylinder head body 11. The inner circumferential surface of the valve seat base material 20 consists of a slant surface 20d inclining in such a manner that the inside diameter of the valve seat base material 20 becomes smaller toward the cylinder head body 11, and a axial surface 20e extending axially from the inner edge of the slant surface 20d. A pressure device 24 shown in FIG. 3 and FIG. 4 is used to bond the valve seat base materials 20s to the cylinder head body 11 on said openings 13a and 14a. The pressure device 24 has a lower platen 26 fixed to the lower portion of a base frame 25, and an upper platen 27 is disposed upwardly of the lower platen 26 for vertical movement so as to be able to come into contact with the lower platen 26. The upper platen 27 is fixed to the lower end of a rod 28a which is the end portion of a cylinder device 28 mounted to the upper portion of the base frame vertically. Said upper and lower platens 26, 27 are supplied with electricity from a power supply (not shown) through conductors 26a, 27a. The conductor 27a connected to the upper platen 27 is adapted to be bent or moved vertically in response to the vertical movement of the upper platen 27. In this embodiment, the upper platen acts as an anode and the lower platen as a cathode. On the upper portion of the base frame supporting said cylinder device 28 is mounted a laser displacement meter 30 for measuring displacement of the upper platen 27, said displacement meter measuring, using a laser beam, the distance from a reflection member 29 fixed to the front portion of the upper platen 27. To bond the valve seat base material 20, first is fixed on said lower platen 26 an underside electrode 31, on which is mounted fixedly the cylinder head body 11. At this time, the cylinder head body 11 is positioned with the combustion chamber recess 12 side upward and with the axis of the port opening, on which the valve seat base material 20 is bonded, coinciding with the axis of a rod 28a of said cylinder device 28. Then, as shown in FIG. 5, a guide rod 32 is inserted, from the combustion chamber recess 12 side, into the valve guide hole 11a of the port on which the valve seat base material 20 is bonded. The guide rod 32 is made of a metallic rod 32a covered with insulating material such as alumina, and has a length such that it protrudes from the end face of the cylinder head body 11 on the combustion chamber side when inserted into the valve guide hole 11a and held in place by a stopper 32c. Said insulating member 32ba is formed, in this embodiment, using a method in which ceramic material such as alumina is flame sprayed and then finished by polishing. However, although not shown in the drawings, it is still possible to fixedly connect said guide rod 32 to an upper electrode 33. In turn, on the port opening is placed the valve seat base material 20, on which is laid the upper electrode 33. The upper electrode 33 is formed with a through hole 33a for fitting said guide rod 32 at the axial center of its cylindrical metallic body, and at the lower end portion, with a tapered surface 33b adapted to be in close contact with said slant surface 20d (FIG. 2) of the valve seat base material 20 and a positioning circumferential surface 33c adapted to be in close contact with the axial extending surface 20e over its entire circumference. On the lower end portion of this upper electrode 33 is fixed a magnetic body 33d for magnetically attracting the valve seat base material 20. That is, when said guide rod 32 is inserted into said through hole 33a, the upper electrode 33 is positioned coaxially with the axis of the port opening of the cylinder head body 11, and when said tapered surface 33b and the circumferential surface 33c are brought into close contact with the valve seat base material 20, the valve seat base material 20 is also positioned coaxially with the port opening. In this way, after laying the upper electrode 33 on the valve seat base material 20, the upper electrode 33 is turned so as to check whether the valve seat base material 20 is fitted reliably. Then, the cylinder device 28 is driven to move the upper platen 27 downward and bring it into close contact with said upper electrode 33. At this time, the bottom surface of the upper platen 27 and the top surface of the upper electrode 33 are adapted to be parallel to each other. Then, said cylinder device 28 is driven to move the upper platen 27 downward and press said valve seat base material 20 against the cylinder head body 11 with a certain pushing force. Since the movement of the upper electrode 33 is restricted by the guide rod 32, the direction of the pushing force exerted on the valve seat base material 20 coincides with the axis of the port opening 13a or 14a. Therefore, the valve seat base material 20 is pressed coaxially with the port opening 13a or 14a. The pushing force is changed according to the pattern of pushing force shown in solid line in FIG. 6. That is, a first pushing force P1 of a certain lower value is applied at early stage of the bonding process and then a second pushing force P2 of a certain higher value is applied till the downward movement is completed. When the upper platen 27 becomes stable after application of the first pushing force P1, the distance between said laser displacement meter 30 and the reflection member 29 is measured by the displacement meter and recorded as a sinking movement starting point of the upper platen 27. When time T1 has elapsed after application of the first pushing force P1, a voltage is applied between said upper and lower platens 27, 26, so that an electric current flows through the upper electrode 33, valve seat base material 20, cylinder head body 11, and lower electrode 31. The current flows from the upper electrode 33 toward the cylinder head body 11, and the current value is changed according to the pattern of the current value shown in dash line in FIG. 6. That is, the current valve is increased first, then decreased near to 0, and then increased further again before reduced to 0 during the time said pushing force is still applied at the final stage of the bonding process. At this time, the convex surface 20c of the valve seat base material 20 is in contact with the convex portion 23 of the cylinder head 11 and the contact area between these two components is very small, so that when electric current is applied as described above, electric resistance becomes large enough to develop resistance heat at the contact portion. The heat will be transmitted over the entire interface between the valve seat base material 20 and the cylinder head body 11. When the temperature of the interface between the valve seat base material 20 and the cylinder head body 11 rises, atoms in the material metals (Cu in the Cu film 22 and Al alloy in the cylinder head body 11) pressed against each other in solid phase, start moving actively and diffusing mutually between two materials. To what extent the film of aluminum oxide on the surface of the cylinder head body 11 will hamper this atom diffusion phenomena, is not clear. As a result of mutual atom diffusion described above, the crystalline structure near the interface turns to eutectic alloy between Cu in the Cu film 22 and Al alloy in the cylinder head body 11, that is, into the state capable of changing from solid phase to liquid phase at lower temperature than pure Cu does. The state near the interface at the time is shown schematically in FIG. 7, and the portion where said eutectic alloy layer is produced as a result of the mutual atom diffusion, is designated by symbol A. When the temperature near said interface rises higher and a part of said eutectic alloy layer turns into liquid phase, the atom diffusion phenomena become more active and the interface between solid phase and liquid phase will expand together with the growth of the eutectic alloy layer. While liquidization of the eutectic alloy layer proceeds, the Al alloy of the cylinder head body 11 adjacent to the eutectic alloy layer develops plastic flow (plastic deformation) because of the valve seat base material 20 being pressed and its own temperature rise due to said resistance heat. Since the plastic flow develops approximately symmetrically in the vertical direction in FIG. 7 with the internal contact portion as a center, said liquidized eutectic alloy is removed from the contact portion to the outside in association with said plastic flow. FIG. 8 shows the removed portion of the eutectic alloy in symbol B. At this time, a part of the Cu film 22 of the ring body 21 is turned into Al eutectic alloy and removed from the contact portion, therefore a part of the ring body 21 comes into contact with the Al alloy, which brings about the atom diffusion phenomena between these materials. The portion developing atom diffusion is shown in symbol C in FIG. 8. A part of the eutectic alloy layer being removed from the contact portion and the Al alloy developing the plastic flow, cause the valve seat base material 20 to sink into the cylinder head body 11. When the time comes to the point T3 in FIG. 6 after the valve seat base material 20 has begun sinking, the pushing force is increased to the value of said second pushing force P2. The increased pushing force causes the increase in the rate of plastic flow in the Al alloy, thereby increasing the volume of the eutectic ally removed. As a result, new eutectic alloy consisting of Cu-Al alloy is produced in a contact portion where no reaction has occured so far, the phenomena described above is repeated and these eutectic alloy layers are liquidized to be removed. In the course of this process, the area where mutual atom diffusion occurs, will expand in the interface between Fe-base sintered alloy, the material of the ring body 21, and Al alloy. When the time comes to the point T4 in FIG. 6 after the second pushing force P2 has been applied, the current value is lowered temporarily near to 0 and raised again to the original value. Lowered current value will prevent heat development temporarily to hold plastic flow in check, so that increase in the sinking rate of the valve seat base material 20 is lowered temporarily. The aim of temporarily lowering the current value is to prevent the Al alloy from melting by heat. The current value is raised to the original one as described above, and then lowered gradually to 0 during the time from point T5 to T6. Not only during the time the current is flowing, but after the current is shut off, said reaction proceeds till the temperature falls to the point where the reaction is impossible to occur; the phenomena of generation of the eutectic alloy layer → liqudization → removal associated with plastic flow, and the phenomena of mutual atom diffusion between Fe-based sintered alloy and Al alloy, occur simultaneously, while the valve seat base material 20 continues to sink; almost all the circumferential surface is embedded into the cylinder head 11 as shown in FIG. 9. When the amount of sinking has almost stopped to increase (point T7), pushing operation by the cylinder device 28 is stopped, the final position of the upper platen 27 is determined, using the laser displacement meter 30, from the distance between the displacement meter and the reflection member 29, then the upper platen 27 is moved upward, and the cylinder head body 11 is dismounted from the pressure device 24. The mean current value and the total energization time are calculated by the time all the procedure is completed. Next. height difference between the sinking movement starting point and the final point is calculated to determine the total amount of sinking of the valve seat base material 20. If this value does not fall in the range of a predetermined allowable value D (see FIG. 6), the bonding process is regarded as defective. In this embodiment, the allowable value D is set to be about 0.5mm-2mm. Usually, the allowable value of about 1mm-1.5mm is preferable, depending on the material of the cylinder head body 11. The cylinder head which has passed the sinking amount determination described above, also receives a judgment whether said mean current value and total energization time are within the range of allowable values, and only if this judgment is favorable and also the cylinder head passes a sampling test described below, the lot of the cylinder head is machined to be finished. The sampling test is performed, for example, on every production lot of the valve seat base materials 20; tensile force is applied to the valve seat base material 20 after bonding in the state shown in FIG. 9, in the direction of the valve seat 20 separating from the cylinder head 11. More specifically. a pulling Jig is hooked to the valve seat base material 20 at the inner circumferential edge of the bottom surface 20b (FIG. 2), a tensile testing machine (not shown) exerts a tensile force on the Jig in said direction, and if the load is higher than a predetermined separation limit at the time the valve seat base material 20 is separated from the cylinder head body 11, the test sample is judged to be acceptable. Not only the tensile separation test described above but a heat endurance test or a heat shock test may be conducted. The heat endurance test is performed after the cylinder head 11 in the state of FIG. 9 is kept in a furnace of 300 °C and atmospheric condition for 24-200 hours and cooled at room temperature. In the heat shock test, the cylinder head body 11 in the state of FIG. 9 is heated to 300°C in the furnace of 300°C and atmospheric condition, taken out from the oven, and then immersed immediately in ice water of 0 °C. This procedure is repeated 10 times, and separation, cracks, etc. are checked before the separation test is performed. In the finishing process, an unnecessary portion is removed from the cylinder head body 11 bonded with the valve seat base material 20, for example, by grinding as shown in FIG. 10. The finishing process removes the unnecessary portion of the ring body 21 together with the Cu film 22, and the valve seat 19 bonded to the cylinder head body 11 through the atom diffusion area shown in symbol C in FIG. 10, is obtained. The metallographical bonding between Al alloy of the cylinder head body 11 and Fe-based sintered alloy of the valve seat 19 in a cylinder head as described above, is essentially different from the mechanical bonding in the metallographecally separated state without atom diffusion. Further, it is different from the metallographical bonding such as resistance heat bonding in which materials to be bonded are locally melted to form liquid alloy using electrical resistance heat generated in the interface, and cooled to be solidified by deenergization. That is, the metallurgical bonding of a cylinder in this embodiment is performed without leaving melt reaction layer between different materials and provides a continuous structure through mutual atom diffusion in the interface between two materials. In the bonding process between the valve seat base material 20 and the cylinder head body 11 described above, if the eutectic alloy is removed from the bonded portion in a particular direction under the influence of magnetic field caused by energization, it is advisable to dispose a shield 34 in the vicinity of the upper electrode 33. FIG. 11 is a plan view showing an embodiment using the shield device, with an upper electrode 33 mounted on a cylinder head body 11 together with a valve seat base material 20. The same or equivalent components in this figure as described in FIG. 1 through FIG. 10 are designated by the same symbol and detailed explanations are omitted. The shield 34 is made of Fe-based ferromagnetic material,formed into a semi-cylindrical shape, and disposed with the top of the circumferential surface facing the base frame 25 of the pressure device 24. The direction of the base frame 25 is shown in an arrow head in the figure. The shield 34 makes it possible to control the direction and the magnitude of magnetic flux in the magnetic field caused by energization, and accordingly the direction of the eutectic alloy removed from the bonded portion. In creating this invention, the inventor used various aluminum alloys for the material of the cylinder head body (11) to which the valve seat member (20) is joined, conducted the separation test for each of the alloys, and found aluminum alloys which provide a separation force to stand the use for the valve seat for engines. The test results are shown in FIgs. 12 to 15. In FIGs. 12 to 15, the symbols ○ and  represent the material AC4C used in this embodiment. The symbols ⋄and ◆ represent the material AC4B, and the symbols □ and ▪ represent the material AC2B. Both materials AC4B and AC2B are specified in JIS and typically used in cylinder head bodies for engines. By the way, the material AC4B is an Al-Si-Cu-based aluminum alloy, and the material AC2B is an Al-Cu-Si-based aluminum alloy. The symbols ○, ⋄, and □ represent the results when the separation tests were conducted in the state the valve seat member (20) was joined to the cylinder head body (11) as it was. The symbols , ◆, and ▪ represent the results of the separation tests after the cylinder head body (11) joined with the valve seat member (20) was let stand in the atmospheric 300°C heating oven for 24 hours and air-cooled. The test samples were determined acceptable when the separation force in the separation test was equal to or exceeded 20 KN. As shown in FIG. 12, the test results for the material AC4C having electric resistivity in the range of 7 - 7.5 x 10 -2 µ Ω ·m which is smaller than that of other aluminum alloys proved acceptable in the two separation tests described above. This seems to have resulted from the fact that the resistance heat is conducted more easily over the entire boundary surface in comparison with other materials. As shown in FIG. 13, the material AC4C having heat conductance in the range of 150 to 160 W/m·°C which is greater than that of other aluminum alloys proved acceptable in the two separation tests described above. This seems to have resulted from the fact that the resistance heat is conducted more easily over the entire joining boundary surface in comparison with other materials. As shown in FIG. 14, the material AC4C having solidus temperature in the range of 550 - 560°C which is higher than that of other aluminum alloys proved acceptable in the two separation tests described above. The solidus temperature is the temperature at which the aluminum alloy transforms from solid phase to liquid phase. The reason for the material AC4C having a relatively high solidus temperature having proved acceptable seems that a molten layer is less likely to be formed than in other materials. As shown in FIG. 15, the material AC4C having 0.2% yield strength at 400 °C in the range of 30 - 35 MPa which is smaller than that of the material AC4B proved acceptable in the two separation tests described above. The 0.2% yield strength at 400 °C is the stress at residual plastic deformation of 0.2% when an aluminum alloy is subjected to a tensile strength test at 400 °C. The reason for the material AC4C having a relatively small yield strength having proved acceptable seems to be that plastic flow is more likely to occur in AC4C than in AC4B. While the foregoing embodiment has shown an example in which the valve seat base material 20 and the port openings 13a, 14a of of the cylinder head body 11 are formed with convex portions (the convex surface 20c, the convex portion 23) and these convex portions are bonded to each other, both of said components are not necessarily formed with the convex portions, but only one of them may. These embodiments are shown in FIG. 16(a)-(c). FIG. 16 shows sectional views of other embodiments with bonded portions of different shapes, FIG. 16(a) and (b) are examples in which only valve seat base materials are formed with convex portions, and FIG. 16(c) an example in which only the cylinder head body is formed with a convex portion. The same or equivalent components in this figure as described in FIG. 1 through FIG. 10 are designated by the same symbol and detailed explanations are omitted. In the example shown in FIG. 1 (a), the valve seat base material 20 is formed into a shape equivalent to the foregoing embodiment, and the portion of the port opening 13a or 14a to be engaged with the valve seat base material 20 is in the shape of a flat slanting surface. In the example shown in FIG. 16(b), the valve seat base material 20 is formed in such a manner that the outer circumference is adapted to be fitted in the port opening 13a or 14a. The shape of the inner circumferential surface of the valve seat base material 20 is different from the foregoing embodiment, and formed into an axially extending surface 20e over its entire depth. In the example shown in FIG. 16(c), the portion of the valve seat base material 20 to be engaged with the convex portion 23 of the cylinder head body 11 is in the shape of a flat slanting surface. Forming either one of the valve seat base material 20 and the cylinder head body 11 with a convex portion as shown in FIG. 16(a)-(c), will provide results equivalent to the foregoing embodiment. If both of said components are formed with convex portions as shown in this embodiment, material metal of the cylinder head 11 tends to develop plastic flow easily and to relatively a large extent in volume, so that the area of the bonded portion (contact interface) of the valve seat base material 20 can be expanded, which provides high bond strength. In the foregoing embodiment is shown an example in which AC4C is used as a material for cylinder head body 11, Cu-infiltrated Fe-based sintered alloy for the ring body 21 of the valve seat base material 20, and Cu coated by electroplating for the film of the ring body 21, but the materials and the methods of forming films are not restricted to those described in this embodiment. That is, for the cylinder head body 11, materials generally used in the cylinder head for engines, such as AC4B, AC2B, etc. may be adopted. For the ring body 21, any kind of Fe-based sintered alloy may be used, and sintered metals with similar electrical conductivity to that of Cu, may be employed. For the film of the ring body 21, any kind of metal that will produce eutectic alloy with material metal of the cylinder head body 11, may be used. In selecting these materials and film forming methods, the most economical options are adopted in this embodiment in view of the cylinder head being manufactured as an industrial product. Moreover. in the embodiment described with reference to FIG. 1 through FIG. 10, an example has been shown of detecting the amount of sinking of the valve seat base material 20 at the final stage of the bonding process, the magnitude of sinking may be measured continuously from the beginning of the pushing process and determined whether it is within the tolerable range from time to time. In so doing, any defective can be detected in the middle of the bonding process, so that time loss due to defective products spending the same working time as good ones, may be avoided. As described above, one method of bonding a valve seat comprises placing a valve seat base material on a port opening of a cylinder head body, the valve seat base material being a ring body made of sintered Fe alloy covered with a film of metallic material, and subsequently pushing an electrode against this valve seat base material with the pushing direction matched with the axis of the valve, the electrode being adapted to apply electricity to the cylinder head through the valve seat base material. Therefore, energization causes resistance heat at the contact portion between the valve seat base material and the cylinder head body, temperature rises at the interface of the contact portion where said valve seat base material and the cylinder head body are pressed against each other, so that atoms on both sides of the interface will diffuse to each other. As a result, a eutectic alloy layer made of material metal of the film of the valve seat base material and material metal of the cylinder head body is produced in the vicinity of said interface. This eutectic alloy layer is changed by said resistance heat to a liquid phase because of its low transformation temperature, and removed to the outside from the bonded portion together with the material metal of the cylinder head body developing plastic flow by heating, as the valve seat base material is pushed against the cylinder head body. In this way, contact between the Fe-based sintered alloy of the valve seat base material and material metal of the cylinder head body causes their atoms to diffuse to each other, and in this state, the valve seat base material is embedded in the port opening. Pushing force is exerted in the axial direction of the port opening at this time, so that not only said atom diffusion phenomena are brought about uniformly over the entire circumference of the valve seat base material, but also pores associated with the atom diffusion, or defective structure associated with deformation can be avoided. Therefore, the valve seat can be bonded to the cylinder head body through mutual atom diffusion instead of press-fitting, and said mutual atom diffusion will develop almost uniformly over the entire circumference of the valve seat due to the direction of the pushing force during bonding, so that high bond strength can be obtained. Further, dimensional allowance for press-fitting is not necessary compared to the case in which the valve seat is press-fit in the cylinder head body, so that the sectional area of the valve seat portion can be reduced, which provides larger freedom in designing the structure around the port area in the cylinder head. In addition, resistance against heat transmission through the valve seat is small, so that abnormal combustion in the engine, or wear of damage of the valve seat due to its thermal degradation, can be prevented. Furthermore, no melt layer is produced between the cylinder head body and the valve seat, so that the number of defects near the bond interface between said two components is reduced to a large extent compared to the conventional bonding process using the laser clad method, and the valve seat can be bonded to the cylinder head body with high bond strength. In addition, even if the engine is kept running for a long time under the operating condition in which the temperature of the valve seat portion goes high, bond strength does not fall substantially. According to another bonding, a valve seat matching the pushing direction of the electrode with the axis of the valve is performed by inserting the electrode into a guide rod fitted to be held in a valve guide hole in the cylinder head. Therefore, a valve guide hole is formed on the valve axis, so that the pushing direction of the electrode exactly coincides with said valve axis. Therefore, the direction of the pushing force coincides with the axis of the port opening with simple structure. A still further method of bonding a valve seat comprises placing a valve seat base material on a port opening of a cylinder head body, the valve seat base material being a ring body made of sintered Fe alloy covered with a film of metallic material, subsequently pushing an electrode against this valve seat base material according to a predetermined pattern of pushing force, and applying electricity to the cylinder head body through the valve seat base material according to a predetermined pattern of current value a predetermined time after the pushing operation. Therefore, energization causes resistance heat at the contact portion between the valve seat base material and the cylinder head body, temperature rises at the interface of the contact portion where said valve seat base material and the cylinder head body are pressed against each other, so that atoms on both sides of the interface will diffuse to each other. As a result, a eutectic alloy layer made of material metal of the film of the valve seat base material and material metal of the cylinder head body is produced in the vicinity of said interface. This eutectic alloy layer is changed by said resistance heat to a liquid phase because of its low transformation temperature, and removed to the outside from the bonded portion together with the material metal of the cylinder head body developing plastic flow by heating, as the valve seat base material is pushed against the cylinder head body. In this way, contact between the Fe-based sintered alloy of the valve seat base material and material metal of the cylinder head body causes their atoms to diffuse to each other, and in this state, the valve seat base material is embedded in the port opening. Thus, the valve seat can be bonded to the cylinder head body through mutual atom diffusion instead of press-fitting. In this bonding process, the electrode is pushed to the valve seat base material according to a certain pattern of pushing force and an electric current is applied according to a certain pattern, so that the rate of plastic flow in the cylinder head body will be controlled, Fe-based sintered alloy will be brought into contact with the cylinder head body securely, and the area in which said mutual atom diffusion occurs will be expanded, whereby high bond strength can be obtained. Therefore, dimensional allowance for press-fitting is not necessary compared to the case in which the valve seat is press-fit in the cylinder head body, so that the sectional area of the valve seat portion can be reduced, which provides larger freedom in designing the structure around the port area in the cylinder head. In addition, resistance against heat transmission through the valve seat is small, so that abnormal combustion in the engine, or wear of damage of the valve seat due to its thermal degradation, can be prevented. Furthermore, no melt layer is produced between the cylinder head body and the valve seat, so that the number of defects near the bond interface between said two components is reduced to a large extent compared to the conventional bonding process using the laser clad method, and the valve seat can be bonded to the cylinder head body with high bond strength. In addition, even if the engine is kept running for a long time under the operating condition in which the temperature of the valve seat portion goes high, bond strength does not fall substantially.

Claims: Method for producing a valve seat (19) within a cylinder head unit (11) provided with valve openings (13a, 14a) associated with respective valve guide holes (11a) for receiving valve guides (15) for associated intake or exhaust valves (17,18) opening and closing said valve openings (13a, 14a), respectively, comprising the steps of: a) placing a valve seat base material (20) onto a surface of said openings (13a, 14a) of said cylinder head unit (11), b) pushing an electrode (33) against the end face of said valve seat base material (20) opposite to said cylinder head unit (11) with a pushing direction matched with an axis (C) of said intake or exhaust valve (17, 18), c) passing electric current through said electrode (33), said valve seat base material (20) and said cylinder head unit (11) to cause resistance heat at an interface between said valve seat base material (20) and said cylinder head body, d) applying a finishing treatment to said valve seat base material and said cylinder head body (11) to receive the desired valve seat (19). characterized in that in step c) a film of eutectic alloy is formed at said interface by atom diffusion of material metal of the said valve seat base material and material metal of said cylinder head unit; the electronic current passed through said electrode is controlled to melt the eutectic alloy while preventing a temperature which would cause the melting of the material metals and the electrode is pushed to push the valve seat base material onto the cylinder head, thereby causing a plastic flow of the eutectic alloy which is thereby removed to the outside. Method according to claim 1, characterized by advancing a guide rod (32) coaxially aligned with said electrode such that said guide rod enters said valve guide hole (11a) and simultaneously guides said electrode for matching the pushing direction with the axis (C) of said valve (17, 18). Method according to claim 1 and 2, characterized in that either the electrode (33) or the cylinder head unit (11) or both are moved towards each other. Method according to at least one of the preceding claims 1 to 3, characterized in that in step (a) said valve seat base material (20) and said opening (13a, 14a) contact each other along a circumferential line and that this line of contact is provided by a convex portion (20c) of said valve seat base material (20) and/or a convex portion (23) of said opening (13a, 14a). Method according to at least one of the preceding claims 1 to 4, characterized in that during step (a) said electrode magnetically attracts said valve seat base material (20) for placing said valve seat base material (20) on the surface of said valve opening (13a, 14a). Method according to at least one of the preceding claims 1 to 5, characterized in that after steps (a) and/or (b) the electrode is rotated for checking whether the valve seat base material (20) is fitted correctly. Method according to at least one of the preceding claims 4 to 6, char acterized in that that in the course of pushing said electrode (32) against the end face of said valve seat base material (30) the pressing force between said valve seat base material (20) and said cylinder head unit (11) is increased from a first pushing force (P1) to a second pushing force (P2). Method according to claim 7, characterized in that the pattern of the applied electricity starts when a time has lapsed after application of the first pushing force (P1) whereby the value of the electricity first increases, then decreases near to zero and after this increases again before reduced to zero during the time said second pushing force (P2) is still applied. Method according to claim 7 or 8, characterized in that the second pushing force (P2) is applied when it is recognized that the valve seat base material (20) has begun to sink. Method according to at least one of the preceding claims 1 to 9, characterized in that the direction and the magnitude of a magnetic flux in the magnetic field caused by energization is controlled to control the direction of the eutectic alloy removed from the bonded portions. Method according to at least one of the preceding claims 1 to 110, characterized in that the magnitude of sinking of the valve seat base material (20) into the opening (13a, 14a) is measured continuously during the whole bonding process. Method according to at least one of the preceding claims 1 to 11, characterized in that said valve seat base material (20) is made of an Fe-based sinter alloy being provided with a coating (22) of a metal alloy being capable of forming an eutectic alloy with that cylinder head unit (11). Method according to at least one of the preceding claims 1 to 12, characterized in that the material of said cylinder head unit (11) is selected out of the group consisting of AC4C, AC4B and AC2B as set forth in the Japanese Industrial Standard (JIS). Method according to at least one of the preceding claims 1 to 13, characterized in that after step (b) a sampling test is carried out by applying a tensile force to the bonded valve seat base material (20). Device conducting the method as defined in claim 1, comprising a base frame (25), a first means (26) for accommodating said cylinder head unit (11) and a second means (27) provided with an electrode (33) being capable of engaging said valve seat base material (20) placed on a surface of said valve opening (13a, 14a), whereby said first means (26) and said second means (27) are capable of moving relatively to each other, characterized by a guide rod (32) for ensuring the coaxial alignment between the electrode (33) and said valve guide hole (11a), said guide rod (32) being slidingly received in a through hole (33a) or being fixed to said electrode (33) and being capable to be inserted into said valve guide hole (11a) of said cylinder head unit (11), wherein said guide rod (32) is made of a metallic rod (32a) covered with insulating material (32b) and having a stopper (32c) being capable of abutting against said cylinder head unit (11). Device according to claim 15, characterized by a magnetic body (33d) for magnetically attracting the valve seat base material (20). Device according to at least one of claims 15 to 16, characterized in that said first means is a lower platen (26) fixed to a lower portion of said base frame (25) and that said second means is an upper platen (27) disposed upwardly of the lower platen (26) for vertical movement so as to be able to come into contact with the lower platen (26) whereby the upper platen (27) is fixed to the lower end of a rod (28a) being an end portion of the cylinder device (28) mounted on the upper portion of the base frame (25) vertically. Device according to at least one of claims 15 to 17, characterized by a displacement meter for measuring the displacement of the first and second means or upper and lower platen (26, 27), respectively, with respect to each other. Device according to claim 18, characterized in that said displacement meter comprises a laser beam unit and a reflection member (29) fixed to a front portion of the upper platen (27). Device according to at least one of claims 15 to 19, characterized in that said electrode (33) is rotatable for checking whether the valve seat base material (20) is reliably fitted. Device according to at least one of claims 15 to 20, characterized by a shield (34) made of a ferromagnetic material for controlling the direction and magnitude of magnetic flux in the magnetic field caused by energization in order to control the direction of an eutectic alloy removed from the bonded portions. Device according to claim 21, characterized in that said shield (34) has a semi-cylindrical shape being disposable with a top of a circumferential surface facing the base frame (25).

Inventors: ADACHI SHUHEI, INAMI JUNICHI, ADACHI, SHUHEI, INAMI, JUNICHI
Applicants: YAMAHA MOTOR CO LTD, YAMAHA HATSUDOKI KABUSHIKI KAISHA
Application year: 1996
Main classification: F01L3
IPC codes: F02F1, F01L3