Abstract:
A method of welding a stud ( 11 ) is provided. In another aspect of the present invention, a welding system is provided for a weld stud ( 11 ). A further aspect of the present invention employs a weld stud ( 11 ) with a substantially conical end section ( 29 ). Still another aspect of the present invention includes a welding method, wherein an aluminum or aluminum alloy stud ( 11 ) is brought into contact with an aluminum or aluminum alloy base material ( 14 ), voltage is applied between the stud ( 11 ) and the base material ( 14 ), the stud ( 11 ) is lifted slightly off the base material ( 14 ), an arc is generated, the tip of the stud ( 11 ) and the section of the base material ( 14 ) to be melted are melted, pressure is applied to the tip of the stud ( 11 ) and the section of the base material ( 14 ) that has been melted and the stud ( 11 ) and base material ( 14 ) are welded together, the current is divided into at least three stages and incrementally increased from the beginning to the end while the main arc is generated, and/or the molten tip of the aluminum or aluminum alloy stud ( 11 ) is applied under pressure to the molten base material ( 14 ) in under five milliseconds after the arc current has been cut off.

Description:
BACKGROUND AND SUMMARY OF THE INVENTION 
   The present invention relates to a method and device for welding an aluminum or aluminum alloy stud and, more specifically, to a welding method and device in which a stud is brought into contact with a base material, voltage is applied between the stud and the base material, the stud is lifted slightly off the base material, an arc is generated between the stud and the base material, the tip of the stud and the section of the base material to be melted are melted, pressure is applied to the tip of the stud and the section of the base material that has been melted, and the stud and base material are welded together after the current has been cut off. 
   In a well known method, a stud is brought into contact with a base material, voltage is applied between the stud and the base material, the stud is lifted slightly off the base material, an arc is generated between the stud and the base material, the tip of the stud and the section of the base material to be melted are melted, pressure is applied to the tip of the stud and the section of the base material that has been melted, and the stud and base material are welded together after the current has been cut off. In Japanese Utility Model Application Disclosure No. 5-49172 and Japanese Utility Model Application Disclosure No. 6-48967, a pilot arc with a small amount of current is generated, the main arc with a large amount of current is generated, the tip of the stud and the section of the base material to be melted are melted, pressure is applied to the tip of the stud and the section of the base material that has been melted and the stud and base material are welded together (the so-called drawn arc method). In automobile manufacturing, aluminum and aluminum alloy vehicle frames and bodies are also increasingly being used because of their lighter weight. In Japanese Utility Model Application No. 63-173583, a T stud consisting of a large-diameter head and a rod-shaped shaft is welded to a vehicle frame, and a clip for securing a member such as wiring is fastened to the T stud. 
   Technologies have already been developed to weld T studs to vehicle bodies and frames when the bodies and the T studs are made from iron-based metals. In these welding technologies, the T stud is welded to the body or frame while maintaining a constant level of strength. For example, when the iron-based T stud  1  in  FIG. 1  has a 5-mm diameter head  2  and a 3-mm long, 3-mm diameter rod-shaped shaft  3 , the height of the stud  1  below the neck after being welded to a base material  5  such as an iron-based body is about 2.6 mm, the reinforcing height (h) of the reinforcement  6  is less than 1 mm, and the diameter (d) of the reinforcement  6  is less than 5 mm. In this way, enough space remains on the shaft  3  of the molten T stud  1  to accommodate the clip disclosed in Japanese Utility Model Application Disclosure No. 63-173583, and attach the clip properly and securely. 
   However, this technology was developed to weld an iron-based T stud to an iron-based base material. When the base material of the body or frame consists of an aluminum-based metal such as aluminum or an aluminum alloy, it is difficult to weld an iron-based T stud to it. If a stud consisting of an aluminum-based metal such as aluminum or an aluminum alloy is welded in the same manner as an iron-based stud, the proper height below the neck, reinforcing height, and welding spot diameter cannot be reliably obtained. In addition, the strength after welding varies and a stable welding strength cannot be obtained. Therefore, the purpose of the present invention is to provide a stud welding method and device able to reliably obtain the desired welding profile and welding strength even when the stud is made from aluminum or an aluminum alloy. 
   In accordance with the present invention, a method of welding a stud is provided. In another aspect of the present invention, a welding system is provided for a weld stud. A further aspect of the present invention employs a weld stud with a substantially conical end section. Still another aspect of the present invention includes a welding method, wherein an aluminum or aluminum alloy stud is brought into contact with an aluminum or aluminum alloy base material, voltage is applied between the stud and the base material, the stud is lifted slightly off the base material, a pilot arc with a small amount of current is generated, the main arc with a large amount of current is generated, the tip of the stud and the section of the base material to be melted are melted, pressure is applied to the tip of the stud and the section of the base material that has been melted and the stud and base material are welded together, and the current is divided into stages and incrementally increased as the main arc is generated from beginning to end, and/or the molten tip of the stud is applied under pressure to the molten base material in under five milliseconds after the main arc current has been cut off. 
   The present invention also includes a welding device, wherein an aluminum or aluminum alloy stud is brought into contact with an aluminum or aluminum alloy base material, voltage is applied between the stud and the base material, the stud is lifted slightly off the base material, a pilot arc with a small amount of current is generated, the main arc with a large amount of current is generated, the tip of the stud and the section of the base material to be melted are melted, pressure is applied to the tip of the stud and the section of the base material that has been melted and the stud and base material are welded together, the current is divided into stages from beginning to end and incrementally increased as the main arc is generated, and/or the molten tip of the stud is applied under pressure to the molten base material in under five milliseconds after the main arc current has been cut off. As a result, the desired height below the neck in the stud after welding (L in  FIG. 2 ) is reliably obtained, the welding strength is high, and the reinforcement height (h in  FIG. 2 ) and the diameter of the melted section of the base material (d in  FIG. 2 ) are kept within the proper range. 
   In another aspect of the welding device and method of the present invention, there are three steps and the main arc in the first step is a small-current arc effective at removing oil from the surface and surroundings of the section of the base material to be melted, the main arc in the second step is an intermediate-current arc for melting the tip of the stud and the section of the base material to be melted and for keeping the area of the section of the base material to be melted within a predetermined range, and the main arc in the third step is a large-current arc for melting the tip of the stud and the section of the base material to be melted into each other deeply. A further aspect of the present invention uses a T stud having a large-diameter head and a rod-shaped shaft, and the profile of the end of the shaft is conical with a flat tip. As a result, the arc is concentrated in the center, the reinforcement does not tilt to one side, and the height of the reinforcement is kept from getting shorter. A pilot arc is not absolutely necessary. 
   Still another aspect of the present invention includes a welding method, wherein an aluminum or aluminum alloy stud is brought into contact with an aluminum or aluminum alloy base material, voltage is applied between the stud and the base material, the stud is lifted slightly off the base material, an arc is generated, the tip of the stud and the section of the base material to be melted are melted, pressure is applied to the tip of the stud and the section of the base material that has been melted and the stud and base material are welded together, the current is divided into at least three stages and incrementally increased from the beginning to the end while the main arc is generated, and/or the molten tip of the aluminum or aluminum alloy stud is applied under pressure to the molten base material in under five milliseconds after the arc current has been cut off. As a result, an aluminum-based stud can be welded properly. 
   Similarly, in still a further aspect of the present invention, a welding device or system is used wherein an aluminum or aluminum alloy stud is brought into contact with an aluminum or aluminum alloy base material, voltage is applied between the stud and the base material, the stud is lifted slightly off the base material, an arc is generated, the tip of the stud and the section of the base material to be melted are melted, pressure is applied to the tip of the stud and the section of the base material that has been melted and the stud and base material are welded together, the current is divided into stages and incrementally increased from beginning to end as the arc is generated, and/or the molten tip of the aluminum or aluminum alloy stud is applied under pressure to the molten base material in under five milliseconds after the main arc current has been cut off. As a result, an aluminum-based stud can be welded properly. 
   Various embodiments of the present invention are advantageous over prior devices. For example, because the main arc current of the present invention is divided into stages and increases incrementally in this welding method, the welding area of the section of the base material to be melted is kept within the desired range when the tip of the stud is being welded to the section of the base material to be melted, the tip of the stud and the section of the base material to be melted are melted into each other deeply, and the heat introduced to the stud and base material remains constant. Because the tip of the stud is applied under pressure to the base material in less time and the short current is controlled during this time, the splattering of molten metal is reduced by the pinch effect (a phenomenon in which the large current flowing through the molten fluid constricts the fluid, the constriction reduces the flow and lessens the constriction, and the lessening of the constriction once again constricts the molten fluid). As a result, the desired height below the neck in the stud after welding (L in  FIG. 2 ) is reliably obtained, the reinforcement height (h in  FIG. 2 ) and the diameter of the melted section of the base material (d in  FIG. 2 ) are kept within the proper range, and high welding strength is maintained. Additional advantages and features of the present invention will become apparent from the following description and appended claims, taken in conjunction with the accompanying drawings. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
       FIG. 1  is a front view of an iron-based T stud of the prior art; 
       FIG. 2  is a front view of the iron-based stud in  FIG. 1  after welding; 
       FIG. 3  is a block diagram of the circuit in the stud welding device of the present invention; 
       FIG. 4  is a front view of the aluminum-based T stud in the present invention; 
       FIG. 5  shows graphs of the timing when an arc is generated by the stud welding device of the present invention wherein (A) is a graph showing the change in the arc current over time, (B) is a graph of the stud lift distance over time, and (C) is a graph of the arc voltage between the stud and the base metal over time; and 
       FIG. 6  is a chart showing the preferred weld parameters for different studs and materials. 
   

   DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
   The following is an explanation of working examples of the present invention with reference to the drawings.  FIG. 3  is a block diagram of the circuit in a welding device  10  for welding an aluminum or aluminum alloy stud to an aluminum or aluminum alloy base material. The stud welding device  10  contains a collet  13  for holding a stud  11  at the tip, a welding gun  17  with a lift coil  15  as the lifting means for lifting the stud  11  held by the collet  13  off the base material  14 , and a power source  18  connected to the welding gun to supply a specific amount of power between the stud  11  and the base material  14 . It is not necessary, but the stud can also be welded in an inactive gas atmosphere such as an argon gas atmosphere. A stud-surrounding member such as a ferrule (not shown) can be installed on the collet  13  holding the stud. 
   A control device  19  is connected to the power source  18  and the welding gun  17 . The stud welding device  10  is a so-called drawn-arc stud welding device in which a stud  11  is brought into contact with a base material  14 , voltage is applied between the stud and the base material, the stud is lifted slightly off the base material, a pilot arc with a small amount of current is generated, the main arc with a large amount of current is generated, the tip of the stud  11  and the section of the base material  14  to be melted are melted, pressure is applied to the stud and the section of the base material that has been melted and the stud and base material are welded together. Therefore, the control device  19  has to control the supply of power from the power source  18  to the welding gun  17  so a pilot arc and subsequent main arc are formed between the stud  11  and the base material  14 . It also has to operate the lift coil  15  in the welding gun  17  so the stud  11  is lifted off the base material  14  to a specific height and the pilot arc and subsequent main arc are generated. After a certain amount of time, the lift coil  15  has to be turned off so the stud  11  is brought into contact with the base material  14  forcibly. In the present invention, a drawn-arc welding device in which a pilot arc precedes a main arc does not have to be used. Any welding device that generates an arc between the stud and base material for arc welding can be used. In the following explanation of the working example, however, a drawn-arc stud welding device is used. 
   The control device  19  has an arc voltage detector  21  for detecting the voltage between the stud  11  and the base material  14  and outputting a signal depending on whether the stud is in contact with the base material or lifted off the base material. It also has a current detector  22  for detecting the welding arc current supplied from the power source  18  to the welding gun  17 . The detection signals outputted from detectors  21  and  22  are sent to a sequence controller  23  for controlling the sequence of operations required to perform stud welding. The output from the current detector  22  is in-putted to the sequence controller  23 , and the sequence controller  23  controls the power source  18  so the current is reduced for the pilot arc and increased for the main arc. 
   In the present invention, the sequence controller  23  divides the supply of current into three stages from beginning to end while the main arc is being generated and increases the current incrementally. In dividing the main arc current into three stages, the sequence controller  23  performs control operations so the main arc in the first step is a small-current arc effective at removing oil from the surface and surroundings of the section of the base material to be melted, the main arc in the second step is an intermediate-current arc for melting the tip of the stud and the section of the base material to be melted and for keeping the area of the section of the base material to be melted within a predetermined range, and the main arc in the third step is a large-current arc for melting the tip of the stud and the section of the base material to be melted into each other deeply. 
   The control output from the sequence controller  23  does not have to be inputted to the power source  18 . It can also be inputted to a lift coil controller  25  for turning the lift coil  15  on and off. The lift coil controller  25  turns on the lift coil  15  to lift the collet  13  in the welding gun  17  against resistance from an internal spring and raise the stud  11  a specific height with respect to the base material  14  and hold the stud there while the pilot arc is generated and the stronger main arc is generated. After the main arc has properly welded the tip of the stud  11  and the section of the base material  14  to be welded, the sequence controller  23  turns off the current to the lift coil  15  for bringing the stud  11  into contact with the base material  14  forcibly. When the power is turned off, the collet  13  is lowered by the spring action of the internal spring, and the stud  11  is brought into contact with the base material  14  forcibly. 
     FIG. 4  is a detailed depiction of the stud  11  used in the present invention. The stud  11  is made from aluminum or an aluminum alloy. It is a T stud with a large-diameter head  26  and a rod-shaped shaft. In order to be welded in the same manner as the iron-based stud  1  in  FIG. 2 , for example, the head  26  has a diameter of 5 mm and the shaft  27  has a diameter of 3 mm. The length of the shaft  27  before welding is 3.3 to 3.7 mm, or 0.3 to 0.7 mm longer than the iron-based stud  3  in  FIG. 2 . This allows for welding with the base material deep enough to obtain the appropriate welding strength. The tip  29  of the shaft  27  on the stud  11  is conical with a flat end. The tip surface  30  is flat with a 1.5 mm to 2 mm diameter, and the tapering angle α of the conical section is between 5 and 10°. The formation of a cone with a flat end concentrates the arc in the center, keeps the reinforcement from tilting to one side, and keeps the height of the reinforcement from getting shorter. By forming the tip  29  of the aluminum-based stud  11  of the present invention in this manner, the height of the stud  1  below the neck after being welded is about 2.6 mm, the reinforcing height (h) of the reinforcement is less than 1 mm, and the diameter (d) of the reinforcement is less than 5 mm. As a result, the welding strength is as high as the iron-based stud shown in  FIG. 2 . 
   The following is an explanation of the operation of the stud welding device  10  in the present invention with reference to  FIG. 5(A)  through (C). When a welding start signal is sent to the sequence controller  23  in the control device  19  from the switch (not shown), the constant-voltage pilot arc current is supplied from the power source  18  to the aluminum-based stud  11  and the aluminum-based base material  14  in the initial stage denoted by Phase I in  FIG. 5(A) . A signal is also sent to the lift coil controller  25 , the lift coil  15  is activated, and the stud  11  is gradually lifted off the base material  14  against the resistance acting on the collet  13  as shown in  FIG. 5(B) . Once lifted, the pilot arc is generated. The stud  11  is kept at a specific height for a specific period of time. When the stud  11  is lifted from the base material  14 , as shown in  FIG. 5(C) , a constant-level arc voltage is generated between the stud  11  and the base material  14 . This is detected by the voltage detector  21  and sent to the sequence controller  23 . The sequence controller  23  then makes sure the stud  11  is lifted off the base material  14 . 
   After the pilot arc has been generated, the sequence controller  23  increases the current and supplies the main arc current from the power source  18  in the first stage to the stud in the second phase denoted by Phase  11  in  FIG. 5(A) . The main arc current in the first stage is set at a small-current arc effective enough at removing oil from the surface and surroundings of the section of the base material  14  to be melted. When performing the welding in an inactive gas atmosphere, the water component is scattered and does not contaminate the section of the stud to be melted. The small-current arc in the first stage is effective enough to perform pre-welding processing. 
   Next, the sequence controller  23  increases the current from the power source  18  and supplies the main arc current in the second stage to the stud  11  in the third phase denoted by Phase III in  FIG. 5(A) . The main arc current in the second stage welds the tip  29  of the stud  11  and the section of the base material  14  to be welded. This intermediate-current arc keeps the area of the section of the base material  14  to be melted within a predetermined range and positions the section to be melted with high precision. 
   In the fourth phase denoted by Phase IV after the third phase denoted by Phase III in  FIG. 5(A) , the sequence controller  23  increases the current even more and supplies the main arc current in the third phase from the power source  18  to the stud  11 . The main arc current in the third stage is large enough to perform deep welding on the tip of the stud  11  and the section of the base material  14  to be welded. This is sufficient to weld the sections to be welded. The sequence controller  23  has a reference table stored in RAM or ROM memory containing data related to the welding of various types of studs and base materials. In Phases I, II, III and IV, the sequence controller  23  uses the signals from the voltage detector  21  indicating the stud  11  has been lifted as the initiation signals, and sets the proper timing and current levels accordingly. The power source  18  is a chopper high-frequency power source. The size of the current outputted is controlled by signals from an external source using pulse wave modulation (PWM). Therefore, the sequence controller  23  can set the appropriate pilot arc current and main arc current for the various stages and the appropriate length of time for the various stages based on the type of stud and base material being used. 
   When the third main arc in Phase IV is terminated, the main arc current from the power source  18  is stopped. In the present invention, the sequence controller  23  operates the lift coil controller  25  so the molten tip of the stud is forcibly brought into contact with the molten section of the base material to be welded in under 5 milli-seconds. Because the sequence controller  23  can check the reference table to determine when to end Phase IV (the third main arc stage), the current to the lift coil Is stopped at the appropriate time before the end of the process, and a signal is sent to the lift coil controller  25  to forcibly bring the tip of the stud  11  into contact with the molten section of the base material  14  to be welded in under 5 milliseconds in Phase IV or after the third main arc stage has ended. In the present invention, the amount of time in which the tip of the stud is brought into contact with the base material is shortened, and the short current is limited to a brief period of time. Because the short current is brief, the splattering of molten metal is reduced by the pinch effect (a phenomenon in which the large current flowing through the molten fluid constricts the fluid, the constriction reduces the flow and lessens the constriction, and the lessening of the constriction once again constricts the molten fluid). In testing, the period of forcible contact was conducted within 0 and 4 milliseconds of ending the current. This significantly reduced the amount of splattering of molten metal. 
   The time T in  FIG. 5(B)  is the range of time in which the tip of the stud is brought forcibly into contact with the base material after the main arc current has been terminated. After Phase IV in  FIG. 5(A) , the current does not go down to zero immediately after the current is cut. Because some power remains in the circuit for supplying power to the stud  11  and the base material  14 , the current cutoff time is denoted by dotted line  33  in the Figure. A stud contact signal from the sequence controller  23  is sent to the lift coil controller  25  during time  34  in  FIG. 5(B)  before the time  30  the current is cut. When the stud makes contact, the arc voltage in  FIG. 5(C)  goes to zero. This is detected by the voltage detector  21 . The sequence controller  23  receives the signals from the voltage detector  21  and begins the contact timing. 
   When an aluminum or aluminum alloy stud  11  is forcibly brought into contact with an aluminum or aluminum alloy base material  11 , as shown in  FIG. 2 , the welding obtained is similar to that of a iron-based stud welded to an iron-based base material. In testing, the height of the aluminum-based stud  11  below the neck was about 2.6 mm, the reinforcing height (h in  FIG. 2 ) of the reinforcement  6  was less than 1 mm, and the diameter (d in  FIG. 2 ) of the reinforcement was less than 5 mm. A high welding strength was also maintained. 
   Because, in the present invention, the main arc current is divided into stages and increases incrementally in this welding method, the welding area of the section of the base material to be melted is kept within the desired range when the tip of the stud is being welded to the section of the base material to be melted, the tip of the stud and the section of the base material to be melted are melted into each other deeply, and the heat introduced to the stud and base material remains constant. Because the tip of the stud is applied under pressure to the base material in less time and the short current is controlled during this time, the splattering of molten metal is reduced by the pinch effect (a phenomenon in which the large current flowing through the molten fluid constricts the fluid, the constriction reduces the flow and lessens the constriction, and the lessening of the constriction once again constricts the molten fluid). This stabilizes the stud after welding at the desired height below the neck, keeps the height of the stud reinforcement and the diameter of the section to be welded within the appropriate ranges, and maintains a high welding strength. In addition, the aluminum-based stud is a T stud consisting of a large-diameter head and a rod-shaped shaft, and the profile of the end of the shaft is conical with a flat tip. As a result, the arc is concentrated in the center, the reinforcement does not tilt to one side, and the height of the reinforcement is kept from getting shorter. 
   The preferred weld parameters for the present invention are shown in  FIG. 6 . The weld parameters are first shown for different materials used with the previously disclosed T-stud. For example, when A7N01 is used for the base material  14  and A5056 is used for the stud  11 , the weld current voltage is maintained at 18 volts, the lift height is maintained (for the lift motor or coil position relative to the workpiece although the welded tip may actually change as melting occurs) at 2.3 millimeters, the step  1  welding current average is maintained about 100 amps for about 20 milliseconds, the subsequent step  2  welding current average is maintained about 170 amps for about 10 milliseconds, and the subsequent step  3  welding current average is maintained about 290 amps for about 13 milliseconds. The final group of weld parameters are shown for a T5 Christmas (“Xmas”) Tree style weld stud. The shape of this type of stud is disclosed in U.S. Pat. No. 5,461,209 entitled “Stud Bolt” which issued to Yamada et al. on Oct. 24, 1995, and is incorporated by reference herein. 
   Various aspects of the present invention have been disclosed but other embodiments can be used. For example, the preferred method and device can be used for weld studs which have differing shapes, such as those without a T-shaped or enlarged head, although some of the advantages may not be achieved. Furthermore, the stage timing, volts, amps and distances can be varied depending upon the specific stud and base material dimensions and materials utilized. While various materials and dimensions have been disclosed, it will be appreciated that other materials and dimensions may be readily employed. It is intended by the following claims to cover these and any other departures from the disclosed embodiments which fall within the true spirit of this invention.