Abstract:
A piston particularly adapted for heavy-duty diesel engine applications is fabricated from separate parts having circumferentially extending joining surfaces that are heated prior to bonding to an elevated temperature sufficient to enable bonding of the joining surfaces, and thereafter the joining surfaces brought into contact with one another and twisted to attain a permanent metallurgical weld at the interface of the joining surfaces.

Description:
BACKGROUND OF THE INVENTION  
         [0001]    This application is a Continuation of U.S. patent application Ser. No. 10/701,274, filed Nov. 4, 2003.  
           [0002]    Various methods are known for bonding separately formed portions of a piston in order to yield a piston structure. One such process is friction welding in which one portion of the piston is rotated at high speed while pressed against the other portion, with the resulting frictional energy generating sufficient heat to bond the portions together. Other techniques include resistance welding, induction welding, and the like in which, after the portions are brought into contact with one another, an energy flux is introduced across their joining surfaces which causes them to be heated sufficiently to join the surfaces to one another.  
           [0003]    U.S. Pat. No. 5,150,517 is an example of friction welding, whereas U.S. Pat. No. 6,291,806 is an example of typical induction heating wherein the coils are presented to the sides of the contacting joining surfaces to induce energy and thus heat at the interface. Such side presentation of the induction coils has a tendency to heat the regions of the joining surfaces near the edges of the material adjacent the induction coils at a faster rate than those regions further from the coils, thus producing a variation in the heat flow and heat affected zone in the area of the material adjacent the interface. In a demanding, highly loaded application such as pistons for diesel engines, it would be desirable to provide a weld joint that is uniform in its heat affected zone across the interface so as to minimize any variation in strength and integrity of the material.  
           [0004]    U.S. Pat. No. 6,155,157 discloses a piston having first and second portions which are joined across two radially spaced sets of joining surfaces by means of friction welding. It will be appreciated that such an architecture would present a challenge to joining the portions by induction welding, since access to the regions where the joining surfaces are located is limited and, in the case of the internal cooling gallery, inaccessible to the positioning of any induction coil next to the mated joining surfaces. Based on the known existing technology in the field of pistons, a suitable technique for induction welding such complex architectures of pistons as those shown in the aforementioned &#39;642 patent is not known to be in existence, and certainly is not known to be used due to the practical difficulties in adapting such induction heating technology to complex piston designs with multiple radially spaced joining surfaces.  
           [0005]    Outside of the field of heavy-duty pistons, induction heating is used to join simple structures, such as butt-welding metal tubes that carry petroleum products. Such tubing is a simple, single walled cylindrical structure having flat, planer end faces. To join one end face to another, an induction coil is introduced between the end faces, and the end faces are heated to an elevated temperature, after which the coil is withdrawn and the end faces brought into engagement with one another to achieve a weld joint. Preferably, once the surfaces are brought into contact, they are twisted a small amount (a few degrees) to attain more intimate union of the weld surfaces. Surprisingly, the inventors have discovered that the induction welding technique heretofore limited to joining simple single walled cylindrical petroleum piping can be improved to be successfully employed to join complex piston structures in a manner to attain a strong, high integrity joint with a uniform but minimal heat affected zone across the interface of the joining surfaces.  
         SUMMARY OF THE INVENTION  
         [0006]    A method of making a piston according to a first aspect of the invention includes fabricating first and second portions of the piston each having at least two joining surfaces. The portions are supported with the joining surfaces in spaced relation to one another. While spaced, the joining surfaces are heated to an elevated temperature and thereafter the heat discontinued and the joining surfaces brought into contact with one another to form a metallurgical bond across the joining surfaces.  
           [0007]    According to another aspect in the invention, a method is provided for making a piston in which a joining surface of a first piston portion is supported in spaced relation to a joining surface of a second piston portion and, while spaced, the surfaces are heated and then brought together to form a metallurgical bond.  
           [0008]    According to still a further aspect in the invention, a piston is provided having first and second portions with mating joining surfaces joined by an induction weld joint and having a heat affected zone which is uniform across the joint.  
           [0009]    The invention has the advantage of providing a simple, low-cost method for welding multi-piece pistons.  
           [0010]    The invention has the further advantage of providing a low-cost, high integrity weld joint that has a small and uniform heat affected zone adjacent the weld joint.  
           [0011]    The invention has the further advantage of providing an induction heating method which permits precise control of the heating of the joining surfaces of the two piston parts, such that the joining surface of each piston part is not overheated or underheated during the heating of the joining surfaces to an elevated bonding temperature.  
           [0012]    The invention has the further advantage of heating the joining surfaces of the piston portions, while spaced apart from one another, for a more precise, uniform and controlled heating of the surfaces as compared to heating the surfaces after they are joined to one another. With friction welding, for example, a piston having upper and lower crown parts with adjoining surfaces provided at the end faces of radially spaced inner and outer wall sections of the portions necessarily result in the outer wall being heated relatively more than the inner wall since the outer wall diameter is greater and thus rotates at a greater angular speed than that of the inner wall and consequently generates frictional heat at a greater rate than that of the heat generated at the inner wall. Unlike friction welding, induction heating makes it possible according to the invention to precisely control the relative heating of the inner and outer walls of such pistons, thereby providing more uniform weld joints as between the inner and outer walls.  
           [0013]    Controlling the heating of inner and outer walls of the piston which are joined by the method of the invention avoids excessively heating the outer wall where the ring grooves are formed to better control the heat flow in the ring belt region as compared to friction welding.  
           [0014]    Another advantage of induction heating according to the invention is that it requires relatively low compression force to join the parts following induction heating as compared to friction welding in which the heat needed for welding is generated by relative rotation of the parts while under relatively high compression loads (about 1,000 psi vs. 20,000 psi for friction welding). Consequently, the fixturing and equipment needed to hold and support the parts for induction welding according to the invention need not be as substantial as that required for friction welding. Moreover, the architecture of the piston is liberated somewhat since the structure does not have to withstand the heavy compression loading which is imparted during friction welding and which often exceeds the loading experienced during use of the piston. Consequently, thinner sections and lighter weight pistons are possible with induction welding at a cost savings to the manufacturer and recognized fuel and emission efficiencies by the user of such pistons.  
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0015]    These and other features and advantages of the present invention will become more readily appreciated when considered in connection with the following detailed description and appended drawings, wherein:  
         [0016]    [0016]FIG. 1 is a perspective view of upper and lower piston parts prior to welding;  
         [0017]    [0017]FIG. 2 is a view like FIG. 1 showing the parts fixtured and their joining surfaces heated;  
         [0018]    [0018]FIG. 3 is a plan view of the heating coil used in FIG. 2;  
         [0019]    [0019]FIG. 4 is a cross-sectional view through the parts of FIG. 2;  
         [0020]    [0020]FIG. 5 is a view like FIG. 2 but showing the parts moved into contact with one another and twisted following heating;  
         [0021]    [0021]FIG. 6 is a perspective view of the final machined piston;  
         [0022]    [0022]FIG. 7 is a cross-sectional view taken along lines  7 - 7  of FIG. 6; and  
         [0023]    [0023]FIG. 8 is an enlarged fragmentary sectional view showing a heating coil positioned nearer to the joining surface of one of the piston parts than to the other.  
     
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT  
       [0024]    A piston constructed according to a presently preferred embodiment of the invention is shown generally at  10  in the drawings and is fabricated of at least two parts which are formed separately from one another in a manner to provide at least one and preferably at least two sets of circumferentially extending mateable joining surfaces which are initially spaced apart from one another and heated to a temperature sufficient for welding the parts, after which the heating of the surfaces is terminated and the surfaces joined to one another to effect a permanent weld between the parts.  
         [0025]    In the illustrated embodiment, the piston  10  includes a first part  12  and a second part  14 . Both parts  12 ,  14  are fabricated of metal, and preferably steel alloys, although the invention is not limited to these materials. The first and second parts may be cast, forged, fabricated of powder metal or any other process for making metal parts. The alloys used for the first and second parts  12 ,  14  may be the same or different, and thus the temperature at which the first and second parts need to be heated in order to effect welding of the materials may be the same or different, depending upon the requirements of a particular application.  
         [0026]    In the illustrated embodiment, the first part  12  comprises and upper crown part of the piston  10 , and the second part  14  is illustrated as a lower crown part of the piston  10  that complements the upper part  12  such that when joined, the parts  12 ,  14  make up the piston  10 .  
         [0027]    The first part  12  has an upper wall  16  formed with a combustion bowl  18  and, optionally one or more valve pockets  20 . The combustion bowl  18  may be symmetric about a longitudinal axis A of the piston  10 , or may be non-symmetrical as illustrated, if called for by a particular application. The valve pockets  22  are non-symmetrical with respect to the lower part  14 . In other words, the valve pockets  20  and combustion bowl  18  are formed to have a particular position or orientation relative to the lower part  14 , such that the angular location of the valve pockets  20  and combustion bowl positions  18  relative to the lower part  14  is critical to the operation of the piston  10  if such non-symmetrical features are provided to the piston  10 .  
         [0028]    The upper part  12  is formed with an inner annular wall  22  extending downwardly below the combustion bowl  18 , and an outer annular wall or ring belt  24  that is spaced radially outwardly of the inner wall  22  and depends from the upper wall  16 . The inner and outer walls  22 ,  24  are formed at or near their ends with respective joining surfaces  26 ,  28 . The joining surfaces  26 ,  28  are circumferentially extending and preferably continuous and formed symmetrically with respect to the longitudinal axis A, such that the joining surfaces  26 ,  28  are concentric about the axis A.  
         [0029]    Prior to welding of the first part  12  to the second part  14 , the first part is preferably machined, and still further preferably final machined to provide a final finished surface to the combustion bowl  18 , any valve pockets  20 , the joining surfaces  26 ,  28 , and annular cooling gallery recess  30  disposed between the inner and outer walls  22 ,  24  and extending upwardly from the joining surfaces  26 ,  28  toward the upper wall  16  to the outside of the combustion bowl  18 , and an inner dome  32  extending radially inwardly of the inner wall  22 . As will be described below, the piston  10  is formed with a series of ring grooves in the outer ring belt  24 , but such ring grooves are preferably machined into the piston  10  following joining as will be explained.  
         [0030]    The second lower crown part  14  of the piston  10  is formed with a pair of pin bosses  34  extending downwardly from a neck  36  and formed with a set of pin bores  38  coaxially aligned along pin bore axis B. The neck  36  is formed with an inner annular wall  40  and an outer annular wall  42 . The inner and outer walls  40 ,  42  are formed with respective joining surfaces  44 ,  46  which are circumferentially extending and preferably continuous and which align and mate with the joining surfaces  26 ,  28 , respectively, of the inner and outer walls  22 ,  24  of the upper crown part  12 . As best illustrated in FIG. 2, the joining surfaces  26 ,  28  of the upper crown part  12  and the joining surfaces  44 ,  46  of the lower crown part  14  are preferably contained in respective common planes to allow for easy introduction and removal of a heating coil between the parts as will be described below. However, while the planer arrangement of the joining surface is preferred, the invention is not limited to such an arrangement, and the joining surfaces can be arranged in different planes and have a variety of shapes, so long as the surfaces mate with one another (e.g., the mating surfaces being conical, stepped, or the like).  
         [0031]    Prior to welding the lower crown part  14  to the upper crown part  12 , the lower crown part  14  is preferably machined, and still more preferably final machined such that a final finish is formed on the pin bores  38 , the neck  36 , including a cooling gallery recess  48  disposed between the inner and outer walls  40 ,  42  and extending downwardly from the joining surfaces  44 ,  46  to a bottom wall  50  that extends between and joins the lower ends of the inner and outer walls  40 ,  42  and is preferably formed as one piece therewith. The lower crown part  14  further includes a piston skirt  52  that is fabricated as a single, immovable structure with that of the lower crown part  14  and is fixed immovably to the pin bosses  34 . Inner and outer surfaces  54 ,  56  of the piston skirt  52  are final machined prior to welding, as are inner and outer faces  58 ,  60  of the pin bosses  34 . The pin bores  38  may further be final machined to include a ring groove  62  used for retaining a wrist pin within the pin bores  38  during operation of the piston  10 .  
         [0032]    The outer walls  24 ,  42  of the upper and lower crown parts  12 ,  14  may be formed adjacent their free ends with a radially reduced or neck region  64 ,  66  that is thinner and cross section in the region of the wall  24 ,  42  immediately away from the necked regions  64 ,  66 . The joining surfaces  28 ,  46  are formed at the free ends of the necked regions  64 ,  66  according to the preferred embodiment, such that when the crown parts  12 ,  14  are joint as illustrated in FIG. 4, an oil drainage groove  68  is formed in the piston immediately above the pin bosses  34 , and a weld joint  70  is formed across the oil drainage groove  68  at the location of the joining surfaces  26 ,  44  and  28 ,  46 , respectively.  
         [0033]    Turning now to further details of the welding operation, FIG. 2 shows the separately formed, pre-machined upper and lower crown parts  12 ,  14  fixtured with their respective joining surfaces  26 ,  28  and  44 ,  46  in axially aligned but spaced relation to one another. A heating coil, and preferably an induction heating coil  72 , is extended into the space between the upper and lower crown parts  12 ,  14  and the coil  72  energized to induce heating of the joining surfaces to elevate them to a temperature sufficient to enable the joining surfaces to be bonded metallurgically to one another by means of an induction weld joint. Once heated to a sufficient elevated temperature, the heating coil  72  is quickly removed as illustrated in FIG. 4 and the upper and lower crown parts  12 ,  14  are relatively moved axially toward one another bringing their respective joining surfaces  26 ,  44  and  28 ,  46  into united engagement with one another while at a temperature sufficient for bonding. According to the invention, the joining surfaces of both the inner and outer walls are simultaneously heated to the appropriate bonding temperature or temperatures in a single operation by means of the heating coil  72 . Preferably, the heating coil  72  comprises an induction heating coil which, when energized, induces a flow of electrons in the inner and outer walls to cause localized heating of the joining surfaces to an elevated bonding temperature, while the majority of the inner and outer wall material remains largely unaffected by the induction heating (i.e., is not raised to such an elevated temperature or for that matter to a temperature that would cause a change in microstructure of the material). Consequently, the induction heating produces a very controlled heat affected zone (HAZ)  74  which is substantially uniform across the width of the inner and outer walls.  
         [0034]    Once the upper and lower crown parts  12 ,  14  have been heated and brought into contact with one another, the parts  12 ,  14  are preferably twisted by a relatively small amount to mix or smear the joining surfaces to achieve a very high integrity metallurgical union or bonding of the upper and lower crown part materials across the weld joint interface  70 . The upper and lower crown parts  12 ,  14  are twisted in the range of a few degrees to less than one revolution, and preferably on the order of about 2-4 degrees. In the case where the upper or lower crown parts include asymmetrical features, such as the valve pockets  20  or offset combustion bowl  18 , it is important that they be properly oriented with respect to the pin bore axis B in the final piston. Accordingly, the position and fixturing of the crown parts  12 ,  14  is carefully controlled such that prior to joining the features are misaligned with the axis B by an amount that, following twisting, brings the features into proper orientation with respect to the pin bore axis B.  
         [0035]    As shown in FIG. 6, following welding, a final machining operation is performed on the piston  10  to provide a series of ring grooves  76  in the ring belt  24 . The ring grooves  76  are preferably above the oil drainage groove  68  and thus the weld joint  70  is positioned in the outer wall  24 ,  42  below the lowest of the ring grooves  76 .  
         [0036]    As a result of welding the upper and lower crown parts  12 ,  14  a closed oil gallery  78  is formed between the crown parts  12 ,  14 , bounded by the inner and outer walls  22 ,  40 ;  24 ,  42 , the upper wall  16 , and the bottom wall  50 , and the weld joint  70  is exposed to the oil gallery  78 . The crown parts  12 ,  14  may be formed or machined with appropriate oil feed and drainage passages into the oil gallery  78  which may advantageously be formed prior to welding as with the other final machined surfaces described previously.  
         [0037]    It will be appreciated that since the joining surfaces  26 ,  28  and  44 ,  46  are heated by the heating coil  72  prior to joining the surfaces, rather than heating after the surfaces are joined, a direct and uniform heating of the joining surfaces is attainable and highly controllable. FIG. 8 illustrates a situation in which, because of different materials, geometries, or the like, the joining surfaces of the upper and lower crown parts would not heat uniformly if the coil were positioned an equal distance from each of the sets of joining surfaces. In the illustrated example of FIG. 8, the joining surfaces  26 ,  28  of the upper crown part  12  require a greater amount or more intense heating than that of the lower crown part, and thus the induction coil  72  is biased or shifted toward the joining surfaces  26 ,  28  so as to be relatively closer to the upper crown part than to that of the lower crown part. In this way, it is assured that the mating joining surfaces are properly heated to their required respective bonding temperatures, even when the bonding temperatures of the two parts may be different or one part may require more energy than the other part to attain a given bonding temperature. By shifting the coil  72  toward the part that requires more heating and away from the part that requires less heating, the appropriate equilibrium position can be achieved to minimize overheating and prevent underheating of the parts prior to bonding. This ability to control the relative heating of the upper and lower crown parts enables the upper and lower crown parts  12 ,  14  to be fabricated of different materials having different bonding temperatures, or architectures of the same or different material calling for different heating requirements in order to arrive at the appropriate bonding temperature at the appropriate time for joining with the complementing part.  
         [0038]    The parts  12 ,  14  are preferably fabricated of steel, and more preferably of SAE  4140  grade. The parts  12 ,  14  are tempered prior to welding to provide a tempered martensite structure having a hardness in the range of 28-34 R c . The hardness of the weld joint at the center is in the range of 35 to 50, and preferably toward the low end of the range. With controlled pre-heating, by the induction coil, of the joining surfaces the hardness of the weld joint can be controlled to within 38-42 R c . The pre-heating effectively “soaks” the joining surfaces and penetrates the heat below the surface. This has the benefit of reducing the “quenching” action of the weld zone material following joining, with the goal of avoiding the formation of untempered martensite at the center, but rather bainite. The 4140 material has the benefit of a suppressed TTT curve that allows for controlled cooling within a reasonable time (i.e., seconds).  
         [0039]    Obviously, many modifications and variations of the present invention are possible in light of the above teachings. It is, therefore, to be understood that within the scope of the appended claims, the invention may be practiced otherwise than as specifically described. The invention is defined by the claims.