Patent Publication Number: US-2009220820-A1

Title: Method of friction welding and friction welded part with at least two friction weld points

Description:
BACKGROUND OF THE INVENTION 
     1. Field of Invention 
     The present invention relates to a method for welding a first part with a second part by friction welding, wherein a hollow space in the component containing preferably small welding points is produced. The invention is also directed to a friction welded part produced according to the method of the present invention. 
     2. Description of Related Art 
     Forming of a connection between two parts by friction welding is known. Especially it is known, for example, as described in International Patent Application Publication WO 2006/034862 A1, in the case of a cooling duct piston, to connect the piston top with a piston shaft by friction welding. Also, from German Patent Application DE 10 2006 021 044 A1, it is known how to form the smallest weld beads as possible. 
     In certain applications, however, not only is produced a friction weld connection, but more than one connection, wherein it is very important that the internal hollow space is free of weld beads, if possible. 
     A typical embodiment is a piston with an oil cooling duct that should permit undisturbed oil circulation. However, all of the weld beads limiting the oil flow are disadvantageous for the cooling. 
     SUMMARY OF THE INVENTION 
     The present invention seeks to solve the problem of the friction welding of two parts with a structural part having a hollow space by producing an exactly defined small weld bead on the weld seam in the area of the hollow space surface. 
     According to the present invention, this problem is solved by a method of friction welding a first part to a second part by which as few welding points as possible are formed in a structural component having a hollow space by the internal connection surface of the first part to be welded being arranged with an internal connection surface of the second part to be welded by friction welding, so that they form a non-holohedral contact area; an external connection surface of the first part to be welded being reciprocally arranged with an external connection surface of the second part to be welded by friction welding so that they form a non-holohedral contact area, and by the first and second parts being friction welded together forming an internal and external friction welded connection so that the friction weld bead is essentially developed outside of the hollow space surface. 
     Because neither of the parts to be welded form a holohedral contact area before the welding, but bigger contact areas are formed only during the welding process, the formation of the weld bead on the parts to be connected can be controlled and defined in an exact way. 
     In order to achieve a non-holohedral contact area, at least one of the connection surfaces of the first or the second part to be welded is chamfered. Because the two connection surfaces to be welded do not exactly fit one on another before the welding process, they form a small contact area which, during the welding process, advantageously grows to a maximal contact area. Both the areas of the parts to be connected can be chamfered. However, it is preferred to weld by friction welding a planar area of one part with a chamfered area of another part. The chamfered area can have very different profiles, e.g., straight-lined, concave, convex, or acute. The shape of the chamfer depends on the alignment of the welding process and also depends on which kind of weld seam is to be achieved. If the weld seam is chamfered on one side, on the peripheral side where there is less material, a smaller or no weld bead is produced. On the peripheral side where the connection surfaces to be welded contact, there is more material. A respectively bigger weld bead is formed wherein its size can not be exactly predetermined. 
     If the connection surface to be welded is chamfered on two sides, that is, on two peripheries considered from the cross section, the contact area is in the middle and the size of the welding seams can be defined and controlled on both sides. 
     According to the present invention, the problem is solved by a friction welded part produced according to one of the methods described above and so that the internal and external welding areas are chamfered on the first part and/or on the second part before the friction welding. 
     In this way, the width of the weld bead can be produced in a defined size towards the direction of the chamfered side. The majority of the material accumulation is on the side on which a contact area is formed before and during the welding process. 
     In order to reduce the weld bead to a certain size in certain applications and to have to use only certain chipping or turning tools, the connected parts are assembled at least on one side by friction welding with no weld bead or with a very small weld bead. In this way, a weld bead can be completely eliminated within a predetermined width size. 
     The friction welded part is preferably a part in which a weld bead in a hollow space can pose a problem, for example, in a injection pump piston, in a piston for a combustion engine with oil duct or the like. 
     In order to achieve a uniform expansion when the friction welded par is heated and a long life cycle and good insertion characteristics, the material of the first and the second part are identical. 
     In order to ameliorate the quality of the friction welded parts while implementing other requirements, and at the same time to produce a very economical friction welded part, it can be an advantage if the material of the first and the second parts are of different alloys. For example, a finished or a high strength alloy in the upper part and for the second part an inferior/softer or less expensive material can be used. It shall be understood that the alloys used are steel or aluminum alloys or a combination between steel and aluminum alloys. 
     In case of a friction welded part, it is preferred that it is produced according to one of the methods described above, wherein the first part to be connected is a piston top and the second part to be connected is a piston ring. 
     It shall be understood that the above mentioned characteristics to be explained in the following, cannot be used only in the respectively indicated combination only, but also in the other combinations. 
     In the following, the invention is explained in detail on basis of a piston by referring to the accompanying figures, but the present invention is not intended to be limited thereto. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  shows sectional views of two piston parts to be connected with the method of the present invention and the two piston parts after they have been connected; 
         FIG. 2  is a sectional view of a peripheral portion of the piston parts of  FIG. 1  in the area of encircled detail A, prior to and after the formation of weld beads; 
         FIGS. 3-9  are views like that of  FIG. 2 , but showing various other embodiments of the invention. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     The upper portion of  FIG. 1  schematically shows a cross section of the top part  10  of a piston head and of a lower part  20  of the piston head, one over the other, before the welding process. The chamfer of the walls to be welded is clearly shown at the lower edges of the top part  10  of the piston head, so that the material accumulation during the friction welding process is formed outside of the oil duct  18 . The upper part  10  of the piston according to this embodiment shows the special design of an external chamfered welding surface  14  on the top part  10 , as shown in detail in  FIG. 2 . This chamfered welding surface  14  is put on a planar welding surface of the lower part  20  of the piston during formation the connection of the part  10 ,  20  by friction welding. As a result, there is a wedge-shaped gap before the friction welding. As a consequence, the contact areas of the top part  10  and lower part  20  to be welded do not completely fit one on the other. The holohedral (symmetrical) connection is only formed by the friction welding process with the result is that, there is less material  7 ,  19  on the sides of the edges facing into the duct  18  than on the opposite sides. The wedge-shaped gap form an angle of approximately 4-15°, preferably of approximately 5-10°, and most preferably of 7.5°. 
       FIG. 2  shows that a weld bead  16  is formed in a traditional way on the external part of the piston. In contrast, the friction weld bead  7  on the internal side has a substantially smaller size. In this way, according to this embodiment, a good cooling oil flow inside the oil duct is guaranteed. 
       FIG. 3  shows a section of an external wall of a piston in accordance with a second embodiment before and after the friction welding process. In this case, the external wall of the piston is shaped so that both of the facing edge surfaces  13  to be welded are arranged in an inwardly downward angle so that the material accumulation is formed on the external area of the piston. 
       FIG. 4  shows a section of a third embodiment of an external wall of a piston before and after the friction welding process. In this case, there are chamfers that create a recess in the area of the oil duct inside the piston walls, and in which the material of the weld bead can be accumulated after without limiting the internal width of the oil duct. 
       FIG. 5  shows a section of a fourth embodiment of the walls of the top and lower parts  10 ,  20  of the piston. Here, the facing edge surfaces have nearly matched inter-fitting profiles, the inner sides of which remain out of contact, as shown, both before and after the friction welding process, so that there is substantially less material accumulated in the area of the oil duct. 
       FIG. 6  shows a similar concept to that of the  FIG. 5  embodiment, the profiles being rounded instead of stepped. Here again, the inner sides of the inter-fitting profiles of the external wall of the piston parts  10 ,  20  remain out of contact, as shown, both before and after the friction welding process. 
       FIG. 7 ,  8  and  9  show sections of other embodiments of top and lower parts  10 ,  20  of a piston with inter-fitting profiles in which the inner sides of the inter-fitting profiles  13   a  of the external wall of the piston parts  10 ,  20  remain out of contact, as shown, both before and after the friction welding process and which are suitable for the friction welding with the material accumulations  22  of the welding process always being formed on the external wall of the piston. 
     As a consequence, the volume of the cooling duct  18  has no problems due to restricted volume and the friction weld bead  22  can be easily checked from outside by the piston. As a consequence, components which are faultily friction welded can be determined very quickly. 
     On the basis of the requirements, the material that the top part  10  is produced from can be a high strength or resistant material, for example, steel or aluminum. On the other hand, the lower part  20  can be produced from a more flexible or inferior steel or aluminum. For the rest, we refer to the figures depicting the invention as essential parts of thereof.