Abstract:
The formation of spot welds in high strength steel workpieces is improved by using welding electrodes with annular, or donut shaped welding tips for the formation of the molten metal weld pool at the weld site. The annular poll solidifies to form an annular weld nugget. Preferably the donut shaped nugget has an internal diameter that is no more than three-quarters of the outside diameter. The method is applicable to carbon steel workpieces with allowing elements for strengthening, and to thick sheets of mild carbon steels.

Description:
TECHNICAL FIELD 
     This invention pertains to electrical resistance spot welds in high strength steel alloy workpieces. More specifically, this invention pertains to a method of making annular spot welds in sheet metal material of high strength steels. 
     BACKGROUND OF THE INVENTION 
     Low carbon steel sheet material has been used for many years to make automobile body panels, other body parts, and other vehicle components. Common low carbon steels, without strengthening alloy constituents, are readily shaped in sheet form and readily welded. In an effort to reduce vehicle weight, higher strength steels have been devised so that thinner and lighter steel sheet parts and other lighter steel components could be designed. In many applications it has been difficult to form suitable electrical resistance spot welds in such high strength steel components along with proper quality inspection in a high production rate manufacturing operation. 
     Repetitive spot welding operations are set-up to form one or more round weld nuggets between assembled high strength steel parts, typically using automated welding equipment. Opposing round electrodes engage opposite sides of the assembly under a controlled set-up force and a controlled welding current is momentarily passed between the electrodes and interposed workpiece to briefly fuse metal at the welding interface. When the current is stopped, the molten metal solidifies to form the familiar round weld nugget of the steel workpiece material. 
     The welding operation is set-up and often computer controlled to produce a continuous sequence of suitable welds in accordance with specified welding process parameters. It is an object of this invention to provide a method of producing repetitive spot welds in high strength steel alloy workpieces that further facilitates the formation of welded connections in such workpieces. 
     SUMMARY OF THE INVENTION 
     This invention is particularly applicable to resistance welding operations with respect to a class of high strength, relatively low alloy content, sheet steel materials that have been developed by the steel industry for automotive vehicle applications. These steels differ from other steel alloy compositions that are used in the automotive industry, such as stainless steels and tool steels, in that they have a relatively low content of alloying constituents including carbon. Representative high strength steels contain many, if not all, of the following elements in amounts that total to less than 2% to 4% by weight of the total composition: aluminum, carbon, chromium, manganese, molybdenum, niobium, nickel, phosphorus, sulfur, silicon, titanium and vanadium. Carbon is present, usually in the range of about 0.06–0.25% by weight and manganese is present, usually in the range of about 0.4–1.7% by weight. These high strength steels have a yield stress greater than 36,000 pounds per square inch (244 MPa). 
     It has been found that doughnut shaped resistance weld nuggets produced in high strength steels enhance properties of the welded joint and assembly. Such doughnut or torus shaped weld nuggets, when produced with a Di/Do ratio, suitably less than 0.75 provide desirable weld nugget strength characteristics in high strength steel workpieces. Di and Do refer to the internal diameter and external diameter of the weld nugget, respectively. High strength steel weld nuggets of doughnut shape are produced by using at least one annular (or doughnut tip shaped) welding electrode. 
     The annular shape of the high strength steel nugget appears to increase its strength such that if a welded high strength steel member assembly is pulled apart in a test intended to destroy a welded bond, the steel weld nugget is pulled from the surrounding workpiece steel material and the fracture is not interfacial, i.e. through the nugget itself. 
     The welding operation is set-up with two opposing doughnut shaped electrodes (or one doughnut electrode tip with a flat opposing electrode plate, if precise electrode alignment is difficult in setup of a welding assembly) and suitable clamping force and weld current determined. The outer diameter of the annular nugget will typically be larger than the round nugget that might otherwise be formed. Since the fuse area of the annular spot weld is substantially the same as the solid spot weld, no additional weld current is required. 
     Thus, this invention enables the formation of spot welds joining high strength steel sheets where the fused and re-solidified, alloy steel weld material nugget is strong enough to survive destructive testing of the welded interface. This method may also be used beneficially for spot welding of mild carbon steel sheets where at least one of the sheets is more than about three millimeters thick. Other objects and advantages of the invention will be understood from a description of preferred embodiments which follows. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is an exploded view of an assembly of two high strength steel alloy sheet workpieces and opposing copper electrodes with donut shaped welding tips. 
         FIG. 2  is a vertical cross-section of the assembly of  FIG. 1 , in the location and direction  2 — 2  indicated in that Figure, as the donut shaped weld nugget is produced. 
         FIG. 3  is a plan view cross-section of the assembly of  FIG. 2  in the  3 — 3  indicated location and direction. 
     
    
    
     DESCRIPTION OF PREFERRED EMBODIMENTS 
     High strength steels are commercially available and in sheet metal forms. In automotive applications the thickness of the sheet metal is often in the range of 0.6 mm to 4 mm. As stated above, these steels are iron based alloys with carbon, manganese and other alloying elements. The materials have various compositions as shown in the following Table. The steels are available in various grades identified as high strength low alloy steel (HSLA), transformation induced plasticity steel (TRIP), martensitic steel, dual phase steel and a Japanese grade material designated TS 590, a precipitation strengthened steel. There is variation in their overall composition and processing but each of such steels may be welded by the process of this invention. Carbon and manganese are common to each of the alloys and most also contain relatively small amounts, less than 0.1% each, of aluminum, chromium, copper, molybdenum, niobium nickel, phosphorus, sulfur, silicon, titanium and vanadium. The thermomechanical processing of the cast alloys to their sheet form, of course, contributes to their mechanical properties. 
     
       
         
               
               
             
               
               
               
               
               
               
               
               
               
               
               
               
               
               
             
               
               
               
               
               
               
               
               
               
               
               
               
               
               
             
           
               
                   
               
               
                 STEEL 
                 ALLOYING ELEMENT (wt %) 
               
             
          
           
               
                 GRADE 
                 Al 
                 C 
                 Cr 
                 Cu 
                 Mn 
                 Mo 
                 Nb 
                 Ni 
                 P 
                 S 
                 Si 
                 Ti 
                 V 
               
               
                   
               
             
          
           
               
                 HSLA 340 
                 0.01 
                 0.06 
                 0.02 
                 0.03 
                 0.49 
                 0.01 
                 0.03 
                 0.02 
                 0.01 
                 0.01 
                 0.1 
                 0.01 
                 &lt;0.01 
               
               
                 TRIP 800 
                 0.03 
                 0.2 
                 &lt;0.01 
                 0.01 
                 1.63 
                 &lt;0.01 
                 &lt;0.01 
                 0.03 
                 0.02 
                 &lt;0.01 
                 1.7 
                 0.02 
                 0.01 
               
               
                 Martensitic 
                 0.03 
                 0.25 
                 0.02 
                 0.02 
                 0.46 
                 &lt;0.01 
                 &lt;0.01 
                 0.03 
                 0.01 
                 0.01 
                 0.1 
                 0.06 
                 0.01 
               
               
                 Steel (M220- 
               
               
                 Grade 15) 
               
               
                 Dual Phase 
                 0.02 
                 0.11 
                 0.21 
                 0.01 
                 1.43 
                 0.05 
                 &lt;0.01 
                 0.03 
                 0.01 
                 &lt;0.01 
                 0.4 
                 0.02 
                 0.01 
               
               
                 (DP 600) 
               
               
                 TS 590 
                 0.02 
                 0.09 
                 0.18 
                 &lt;0.01 
                 1.64 
                 0.01 
                 &lt;0.01 
                 0.03 
                 0.03 
                 &lt;0.01 
                 0.3 
                 0.01 
                 0.06 
               
               
                   
               
             
          
         
       
     
       FIG. 1  is an exploded view, schematically illustrating a simplified assembly  10  of fragments of an upper sheet layer  12  of high strength steel and a lower sheet  14  of high strength steel. Poised above workpiece sheet  12  is upper, copper, electrical resistance welding electrode  16 . And positioned below lower sheet  14  workpiece is lower, copper, electrical resistance welding electrode  18 . In a welding operation setup for making a spot weld between sheet layers  12  and  14 , welding electrode  16  and welding electrode  18  are positioned on a welding axis in opposition to each other. Welding electrodes  16  and  18  are carried in a welding gun, or other suitable welding head, and are moved between an open position in which an assembly of sheet  12  and sheet  14  is placed between them, and a closed position where welding electrode  16  presses at a welding site on the upper surface of sheet  12  and welding electrode  18  presses at an opposing welding site on the lower surface of workpiece sheet  14 . 
     Welding guns are well known and not illustrated in the drawings of this specification. But the welding apparatus presses the welding electrodes  16  and  18  against sheets  12  and  14  with a predetermined welding force and conduct a predetermined momentary welding current through sheets  12 ,  14  to produce a spot weld. The welding apparatus then opens for re-positioning or removal of the sheets. 
     In the practice of this invention for welding high strength steel workpieces, electrode  18  has an annular welding tip  20 . Electrode  16  has a tip  22  of like annular shape. See  FIGS. 1 and 2 . Welding tips  20 ,  22  preferably have flat contact surfaces, as best seen in  FIG. 2 , for good face-to-face electrical contact with the engaged surfaces of sheet  12  and  14 . But each tip  20 ,  22  has a hollowed out portion  24  and  26 , respectfully, to produce the annular contact tip face. 
     The cross-sectional view of  FIG. 2  shows electrodes  16  and  18  in pressing contact with opposite surfaces of workpiece sheets  12  and  14 . Welding tip  22  of electrode  16  has a cylindrical relieved portion  26  so that the tip to surface contact with sheet  12  is annular in shape. Likewise, the tip  20  of electrode  18  has a cylindrical relieved portion  24  which permits annular tip face to surface contact with sheet  14 . As a suitable welding current is momentarily passed between welding electrode  18  and welding electrode  16  through the contacted regions of sheets  12  and  14 , an annular pool of molten metal is first formed. Then, as the electrodes are withdrawn, the pool rapidly dissipates heat into the surrounding metal and solidifies into nugget  28 . Weld nugget  28  is in the shape of a donut or torus, or like annular body, as seen in cross-section in  FIG. 2  and in plan view in the fragmentary  FIG. 3 . In accordance with this invention, the donut shaped weld nugget  28  has an external diameter, or outer diameter, indicated at D o  in  FIG. 3 . Weld nugget  28  also has an inner diameter, D i , as labeled in  FIG. 3 . The size and shape of electrode tips  20  and  22  is such that a weld nugget  28  is produced in which the inner diameter, D i , is no greater than 75% of the outer diameter D o . The goal of this welding practice is to produce an annular shaped weld nugget body that is likely to be larger in its outer diameter than a typical solid round weld nugget body that it replaces. Furthermore, the inner diameter of the donut shaped weld nugget body is less than three-quarters of the outer diameter so that there is suitable weld nugget volume and strength and face-to face contact with the sheet metal workpieces  12  and  14  that it joins. 
     Often workers in welding operations test the integrity or strength of the welded interface between two such sheet metal workpieces as sheets  12  and  14 . It is the intent and preferred embodiment of this invention that the annular weld nugget is sized so that when the respective sheet layers are gripped and forcibly pulled apart in a destructive test, the weld nugget itself remains un-fractured. The separation of the welded sheets in this destructive test occurs by one sheet, for example sheet  12 , being pulled away from the weld nugget body  28  that binds it to sheet  14 . In other words, the destructive test produces a sheet and weld nugget fragment like that depicted in  FIG. 3 . Weld nugget  28  remains in one of the steel sheets, here lower sheet  14 , and the weld nugget itself is not fractured. 
     Thus, it is found that the use of copper welding electrodes with flat, annular contacting tips can be suitably sized, in combination with suitable welding force, current and time parameters, to produce donut shaped weld nuggets. Such annular weld nuggets, with Di/Do of 0.75 or less, provide excellent interfacial welded bonds between high strength sheet metal workpieces. 
     The spot welding process of this invention was developed for welding high strength steels with alloying elements as described above. But it also has application to mild carbon steel (without additions of alloying elements for strengthening) especially when relatively thick sheets or sections (&gt;3 mm) of the steel is to be spot welded. Low carbon steels are iron based compositions containing carbon (up to, for example 0.15–0.25%) and usually manganese (for example, about 0.5%) with incidental impurities such as sulfur and phosphorus. 
     While the invention has been described in terms of a preferred embodiment it is appreciated that other embodiments could readily be adapted by ones skilled in the art. Accordingly, the scope of the invention is intended to be limited only by the following claims.