Abstract:
In welding joint surfaces of a high-hardness member and a low-hardness member to each other by use of a laser beam or an electron beam, the point of irradiation of the laser beam or electron beam is offset from the joint surfaces of the high-hardness member and the low-hardness member toward the low-hardness member by a predetermined distance, such that the melting provided by the beam is caused to spread from the low-hardness member to the high-hardness member. Thus, even if the two members different in hardness from each other are welded together by use. of the laser beam or electron beam, it is possible to avoid a poor weld, which would otherwise cause a cracking in the high-hardness member.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to an improvement in a process for beam-welding two members different in hardness from each other, that is, high and low in hardness, wherein joint surfaces of the high-hardness and low-hardness members are welded to each other by use of a laser beam or an electron beam. 
     2. Description of the Related Art 
     In the prior art, even when joint surfaces of high-hardness and low-hardness members are welded to each other by use of a laser beam or an electron beam, a point of irradiation of the laser beam or the electron beam is set on the joint surfaces of the two members, similarly as when two members having the same hardness are welded to each other. 
     However, in the welding of the joint surfaces of the high-hardness and low-hardness members to each other by use of the laser beam or electron beam, if the point of irradiation of the laser beam or electron beam is set on the joint surfaces of the two members, a poor weld often occurs for the following reasons: 
     (1) The heat input is too strong for the high-hardness member and for this reason, the high-hardness member is brought into a rehardened state, resulting in a cracking produced. 
     (2) Even if no cracking is produced immediately after the welding, a cracking may be produced in some cases due to a variation in temperature during service of the welded members. 
     (3) The melting of the high-hardness member during the welding is large and for this reason, a large amount of carbide is precipitated during solidification of the high-hardness member, which causes a cracking in the weld zone. 
     SUMMARY OF THE INVENTION 
     Accordingly, it is an object of the present invention to provide a process for beam-welding two members different in hardness from each other, wherein a poor weld as described above can be avoided to the utmost. 
     To achieve the above object, according to a first aspect and feature of the present invention, there is provided a process for joint surfaces of a high-hardness member and a low-hardness member are welded to each other by use of a laser beam or an electron beam, wherein a point of irradiation of the laser beam or the electron beam is set at a location offset from the joint surfaces of the high-hardness member and the low-hardness member toward the low-hardness member by a predetermined distance, so as to cause the melting provided by the beam to spread from the low-hardness member to the high-hardness member. The high-hardness member corresponds to a valve seat member  12  and a valve member  19  in an embodiment of the present invention, which will be described hereinafter, and the low-hardness member corresponds to a housing body  11  and a valve rod  18  in the embodiment. 
     With the first feature, the following merits can be achieved: 
     (1) when the welding using the laser beam or the electron beam is started, the melting first occurs in the low-hardness member and then spreads to a periphery of the low-hardness member to ultimately reach the high-hardness member. Therefore, the melting in the high-hardness member is advanced relatively slowly without reception of a direct heat input provided by the laser beam and hence, the high-hardness member is not brought into a rehardened state in which a cracking is liable to occur; 
     (2) the melting of the low-hardness member is larger than that of the high-hardness member and hence, the low-hardness member difficult to crack is molten into the high-hardness member, and a cracking-causing element in the high-hardness member is diluted with the low-hardness member; and 
     (3) the melting of the high-hardness member is smaller than that of the low-hardness member and hence, the variation in temperature up to the solidification of the high-hardness member is also relatively small and thus, the precipitation of a carbide from the high-hardness member can be inhibited. 
     Consequently, the joint zones of the high-hardness member and the low-hardness member can be welded, while avoiding a poor weld such as cracking of the high-hardness member to the utmost. Even during service of both the members, it is possible to prevent a cracking from occurring in the weld zones of the members. 
     According to a second aspect and feature of the present invention, in addition to the first feature, the high-hardness member is a spherical valve member of an electromagnetic fuel injection valve, and the low-hardness member is a valve rod welded to the valve member; and wherein the laser beam or the electron beam is emitted to the point of irradiation offset from the joint surfaces of the valve member and the valve rod toward the valve rod by the predetermined distance, so as to cause the melting provided by the beam to spread from the valve rod to the valve member. 
     With the second feature, the joint zones of the valve member and the valve rod of an electromagnetic fuel injection valve can be welded to each other, while avoiding a poor weld such as cracking to the utmost. In addition, even during service of the valve member and the valve rod, it is possible to prevent a cracking from occurring in the weld zones of the valve member and the valve rod. 
     According to a third aspect and feature of the present invention, in addition to the fist feature, the high-hardness member is a valve seat member of an electromagnetic fuel injection valve, and the low-hardness member is a valve housing body welded to a rear end of the valve seat member; and wherein the laser beam or the electron beam is emitted to the point of irradiation offset from the joint surfaces of the valve seat member and the valve housing body toward the valve housing body by the predetermined distance, so as to cause the melting provided by the beam to spread from the valve housing body to the valve seat member. 
     With the third feature, the joint zones of the valve seat member and the valve housing body of the electromagnetic fuel injection valve can be welded to each other, while avoiding a poor weld such as cracking to the utmost. In addition, even during service of the valve seat member and the valve housing body, it is possible to prevent a cracking from occurring in the weld zones of the valve seat member and the valve housing body. 
     According to a fourth aspect and feature of the present invention, in addition to any one of the first to third features, the distance of offsetting of the point of irradiation of the beam with respect to the joint surfaces is in a range of 0.5 to 1.5 mm. 
     With the fourth feature, the welding strengths of both the members can be ensured, while avoiding the cracking of the high-hardness member. 
     The above and other objects, features and advantages of the invention will become apparent from the following description of the preferred embodiments taken in conjunction with the accompanying drawings. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a vertical sectional view of an electromagnetic fuel injection valve for an internal combustion engine, which is made utilizing an embodiment of the present invention; 
     FIG. 2 is an enlarged view of an essential portion shown in FIG. 1; 
     FIG. 3 is a side view of an apparatus for beam-welding the essential portion shown in FIG. 2; and 
     FIG. 4 is a sectional view showing another embodiment of the present invention. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     The present invention will now be described by way of an embodiment with. reference to the accompanying drawings. 
     Referring first to FIGS. 1 and 2, a valve housing  10  of an electromagnetic fuel injection valve I for an internal combustion engine is comprised of a cylindrical valve housing body  11  (made of a magnetic material), and a bottomed cylindrical valve seat member  12  which is fitted and welded to an inner peripheral surface of a front end of the valve housing body  11 . The valve seat member  12  includes a valve bore  13  which opens into a front end face of the valve seat member, and a conical valve seat  12   a  connected to a rear edge of the valve bore  13 . An injector plate  15  made of a steel is welded over its entire periphery to the front end face of the valve seat member  12 , and have a plurality of (a pair in the illustrated embodiment) fuel injection bores  14  communicating with the valve bore  13 . 
     A movable core  16  is accommodated in the valve housing body  11 , and a spherical valve member  19  is welded to a valve rod  18  integrally and projectingly provided at a front end of the movable core  16 , such that the member  19  can be seated on the valve seat  12   a . Sides (in four directions) of the valve member  19  surrounding an axis of the valve rod  18  are each formed into a flat surface  19   a , so that a fuel flow path around the valve member is as large as possible. 
     A sliding-movement guide tube  20  (made of a non-magnetic material) is welded over its entire periphery to a rear end face of the valve housing body  11 , and the movable core  16  is slidably supported by an inner peripheral surface of the sliding-movement guide tube  20 . 
     A stationary core  17  is fitted and welded over its entire periphery to an inner peripheral surface of a rear end of the sliding-movement guide tube  20 , and the movable core  16  is opposed to a front end face of the stationary core  17  with a clearance corresponding to an opening stroke of the valve member  19 . 
     A small-diameter portion  21   a  of a stepped cylindrical coil housing  21  (made of a magnetic material) is fitted and welded to an outer peripheral surface of the rear end of the valve housing body  11 . A coil assembly  22  is accommodated in the coil housing  21  to surround a rear end of the valve housing  10 , the sliding-movement guide tube  20  and the movable core  16 . The coil assembly  22  comprises a bobbin  23  and a coil  24  wound around the bobbin  23 . The coil housing  21 , the coil assembly  22  and the stationary core  17  are sealed within a covering member  25  made of a synthetic resin. The covering member  25  is formed at its front end with a step  26  rising radially from the outer periphery of the valve housing body  11 , and a tapered stopper face  27  increased in diameter as being farther rearwards from an outer peripheral edge of the step  26 . A coupler  29  is integrally connected to an intermediate portion of the covering member  25 , and has a connecting terminal  28  connected to the coil  24 . 
     The stationary core  17  has a hole  31  communicating with the inside of the valve housing  10  through a through-bore  30  in the movable core  16 . Accommodated in the hole  31  are a coiled valve spring  32  for biasing the movable core  16  in the direction to seat on the valve seat  12 a, and a pipe-shaped retainer  33  which supports a rear end of the valve spring  32 . The retainer  33  is press-fitted to an inner peripheral surface of the hole  31 , and the set load of the valve spring  32  is regulated by regulating the depth of press-fitting of the retainer. Further, an inlet tube  34  is integrally connected to a rear end of the stationary core  17  to communicate with the hole  31  in the stationary core  17  through the pipe-shaped retainer  33 , and a fuel filter  35  is mounted in the inlet tuber  34 . 
     A sealing/positioning ring  39  made of a synthetic resin is fitted over an outer periphery of the valve housing body  11  exposed forwards from the step  26  of the covering member  25 , so that the sealing/positioning ring  39  abuts against the step  26 . A cap  42  made of a synthetic resin is resiliently mounted at a front end of the valve seat member  12 , and an O-ring  41  is mounted around the outer periphery of the valve seat member  12  between the cap  42  and the sealing/positioning ring  39 . 
     The cap  42  has an opening  44  in its front surface, so that it does not disturb the injection of fuel from the fuel injection bore  14 . 
     A supply port portion  52  of a fuel-dispensing pipe  51  is fitted over an outer periphery of the inlet tube  34  of the electromagnetic fuel injection valve I with a seal member  53  interposed therebetween. In this case, a resilient member  55  for urging the stopper face  27  into abutment against an intake manifold  5  is interposed between the fuel-dispensing pipe  51  and the intermediate step  54  of the covering member  25 . The fuel-dispensing pipe  51  has a mounting boss  56  at one side. The mounting boss  56  is secured to a support boss  58  on an outer surface of the intake manifold  5  by a bolt  59  with an insulator collar  57  interposed therebetween. In this manner, a state of close contact of the O-ring  41  with an inner peripheral surface of a mounting bore  7  is maintained. 
     In a state in which the coil  24  has been deenergized, the movable core  16  and the valve member  19  have been urged forwards by a biasing force of the valve spring  32 , whereby the valve member  19  has been seated onto the valve seat  12   a . Therefore, a high-pressure fuel supplied from the fuel-dispensing pipe  51  through the fuel filter  35  and the inlet tube  34  into the valve housing  1  is retained within the valve housing  1 . 
     When the coil is energized, a magnetic bundle produced by such energization runs in sequence through the stationary core  17 , the movable core  16 , the valve housing  10  and the coil housing  21 , and the movable core  16  is attracted to the stationary core  17  with the valve member  19  by a magnetic force, thereby opening the valve seat  12 a. Thus, the high-pressure in the valve housing  10  is injected through the fuel injection bore  14  toward an intake valve  6 . 
     The valve member  19  and the valve rod  19  having the movable core  17 , as well as the valve seat member  12  and the valve housing body  11  are welded to each other by a beam-welding process according to the present invention. 
     First, the beam-welding process according to the present invention for welding the valve member  19  and the valve rod  18  to each other will be described below with reference to FIGS. 2 and 3. 
     The valve member  19  is made of a high-hardness material which has been hardened. For example, the valve member  19  is made of a material cut from a martensite stainless steel (SUS440C) or an SK material and then subjected to a hardening. Therefore, the valve member  19  has a high wear resistance. On the other hand, the valve rod  18  is made of a low-hardness material. For example, the valve rod  18  may be made of a material similar to the material for the valve member  19 , but not modified, or a material cut from SVM, an austenite or ferrite stainless, which is not hardened. Each of joint surfaces F of the valve member  19  and the valve rod  18  is formed into a spherical surface. 
     In welding the valve member  19  and the valve rod  18  to each other, the movable core  16  which is integral with the valve rod  18  is retained by a first jig  63 , and the valve member  19  is brought into close contact with the valve rod  18  at the joint surfaces F, while being retained by a second jig  64 . 
     In such state, a laser torch  60  is disposed, so that a point P of irradiation of a laser beam B emitted from the laser torch  60  is a location offset from the joint surfaces F of the valve member  19  and the valve rod  18  toward the low-hardness valve rod  18  by a predetermined distance e. 
     When a laser beam B is then emitted from the laser torch  60  while synchronously rotating the first and second jigs  63  and  64 , the melting A first occurs in the valve rod  18  and then spreads to the periphery of the valve rod  18  to ultimately reach the valve member  19 , because the point P of irradiation of the leaser beam is the location offset from the joint surfaces F of the valve member  19  and the valve rod  18  toward the valve rod  18 . In this manner, the melting in the valve member  19  is advanced relatively slowly without reception of a direct heat input provided by the laser beam B and hence, the valve member  19  is not brought into a rehardened state. Moreover, the low-hardness material of the valve rod  18  is molten into the high-hardness material of the valve member  19 , and a crack-causing element in the high-hardness material of the valve member  19  is diluted with the low-hardness material. Further, the melting of the high-hardness material of the valve member  19  is smaller than that of the low-hardness material of the valve rod  18  and hence, the variation in temperature to the solidification of the high-hardness material is also relatively small and thus, the precipitation of carbide from the valve member  19  can be inhibited. 
     Consequently, while the first and second jigs  63  and  64  are being rotated in one rotation, the joint zones of the valve member  19  and the valve rod  18  can be welded, while avoiding a poor weld such as cracking to the utmost. Even during service of the valve member  19  and the valve rod  18 , it is possible to prevent a cracking from occurring in the weld zones of the valve member  19  and the valve rod  18 . 
     The process for beam-welding the valve seat member  12  and the valve housing body  11  to each other will be described below with reference to FIG.  2 . 
     The valve seat member  12  is made of a high-hardness material, as is the valve member  19 , and the valve housing body  11  is made of a low-hardness material, as is the valve rod  18 . 
     In welding the valve seat member  12  and the valve housing body  11  to each other, first, the rear end of the valve seat member  12  is fitted into the valve housing body  11  at a predetermined depth. In such state, the laser torch  60  is disposed such that the direction of the laser beam B emitted from the laser torch  60  is oblique with respect to the end face of the valve housing body  11 , and the point P of irradiation assumes a location offset from the joint surfaces F of the valve seat member  12  and the valve housing body  11  toward the low-hardness valve housing body  11  by a predetermined distance e. Even in this case, if the laser beam B is emitted from the laser torch  60 , while synchronously rotating the valve seat member  12  and the valve housing body  11  by jigs (not shown), joint zones of the valve seat member  12  and the valve housing body  11  can be welded to each other by an action similar to that in the welding of the valve member  19  and the valve rod  18 , while avoiding a poor weld such as cracking to the utmost. Even during service of the valve seat member  12  and the valve housing body  11 , it is possible to prevent a cracking from occurring in the weld zones of the valve seat member  12  and the valve housing body  11 . 
     FIG. 4 shows another embodiment of the present invention. To weld a low-hardness member  62  to a side of a high-hardness member  61 , the laser torch  60  is disposed such that the direction of a laser beam B emitted from the laser torch  60  is substantially parallel to the side of the high-hardness member  61 , and a point P of irradiation is at a location offset from joint surfaces F of both the members  61  and  62  toward the low-hardness member  62  by a predetermined distance e. If the laser beam B is then emitted from the laser torch  60 , joint zones of the high-hardness member  61  and the low-hardness member  62  can be welded to each other by an action similar to that in the previous embodiment, while avoiding a poor weld such as cracking to the utmost. Even during service of both the members  61  and  62 , it is possible to prevent a cracking from occurring in the weld zones of both the members  61  and  62 . 
     As a result of an experiment, it is desirable in each of the embodiments that the distance e of offsetting of the point P of irradiation of the laser beam B from the joint surfaces F of the high-hardness member  12 ,  19 ,  61  and the low-hardness member  11 ,  18 ,  62  toward the low-hardness member  11 ,  18 ,  62  is set in a range of 0.5 to 1.5 mm. The reason is as follows: If the distance of offsetting of the irradiation point P is shorter than 0.5 mm, the input of heat into the high-hardness member  12 ,  19 ,  61  by the laser beam is violent, resulting in a reduced cracking-preventing effect. If the distance of offsetting of the irradiation point P exceeds 1.5 mm, the melting of the high-hardness member  12 ,  19 ,  61  is too small, thereby making it difficult to ensure a welding strength. A distance e of offsetting most effective for ensuring a welding strength, while avoiding the cracking of the high-hardness member  12 ,  19 ,  61 , is approximately 1.0 mm. 
     Although the embodiments of the present invention have been described in detail, it will be understood that the present invention is not limited to the above-described embodiments, and various modifications in design may be made without departing from the spirit and scope of the invention defined in claims. For example, an electron beam may be used in the welding in place of the laser beam.