Abstract:
A method and apparatus for ultrasonic in-process monitoring and feedback of resistance spot weld quality uses at least one transducer located in the electrode assembly transmitting through a weld tip into an underway weld. Analysis of the spectrum of ultrasonic waves provides the operator with an indication of the size, thickness, location, dynamics of formation and quality of the spot weld. The method presents a fundamentally new physical approach to the characterization of the spot weld quality. Together with transmission mode it includes new modes of operation of ultrasonic probes such as a reflection mode and simultaneous use of transmission and reflection modes, and a new physical interpretation of the signal analysis results.

Description:
BACKGROUND 
       [0001]    1. Field of the Invention 
         [0002]    The present invention relates to an ultrasonic transducer associated with an electrode for real-time monitoring and feedback in a welding process. 
         [0003]    2. Description of Related Art 
         [0004]    The nondestructive testing of spot welds in real time using ultrasound has many advantages over other nondestructive approaches. Prior art arrangements include the insertion of the ultrasonic probe into the weld electrodes, with the acoustic energy sent through the weld subject. Then the analysis of the transmitted and/or reflected signal is performed in order to make some conclusions about the quality of the weld. 
         [0005]    U.S. Pat. No. 3,726,130 discloses a probe glued to the back surface of the welding electrode. The probe generates a shear wave and receives a reflection from the primary solid-liquid interface of the weld. This method allows the determination of the penetration depth of the liquid zone into the subject. However, this method only gives information about one side of the weld, telling nothing about the opposite side. Also, as weld electrodes must be frequently changed or refreshed, this arrangement of the probe on the surface of the removable electrode can make it impractical or susceptible to damage in an industrial setting. 
         [0006]    U.S. Pat. No. 4,099,045 discloses an acoustic wave undergoing multiple reflections within a weld subject. Evaluation of the degree of attenuation of the wave provides some information about the spot weld. This is an empirical approach which requires a collection of data for each particular case. The method enables prediction of the quality of the weld by comparison with previous results. 
         [0007]    U.S. Pat. No. 6,297,467 discloses an electrode assembly incorporating ultrasonic probes and its basic principles of operation, and is hereby incorporated herein in its entirety. 
         [0008]    It would be advantageous to provide a method of using acoustic waves to directly measure the dynamics of formation of weld and critical parameters which define the weld quality without comparing weld characteristics with previously tabulated results. 
       SUMMARY 
       [0009]    In an embodiment of the present invention, a spot welder has either two electrode assemblies containing ultrasonic probes (for transmission mode or for combination of transmission and reflection modes) or just one electrode assembly (for purely reflection mode). During welding the ultrasonic probe from first electrode assembly generates a burst of acoustic energy. In transmission mode, one portion of this acoustic energy passes through a weld zone and then is picked up by the second probe located in the second electrode assembly. In reflection mode, another portion is reflected by the weld subject and is received by the first ultrasonic probe. The third option includes simultaneous operation of transmission and reflection modes. Both ultrasonic probes in the electrode assemblies then emit an output electrical signal to the computer. The computer processes the received signals and outputs the information about the weld geometry and the “time history” of the weld. This information is used by the computer software to make a decision whether it is necessary to change welding parameters in real time to provide the quality output. This comprises the feedback stage of ultrasonic in-process welding quality control. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0010]    The present invention will become more fully understood from the detailed description and the accompanying drawings, wherein: 
           [0011]      FIG. 1  is a schematic representation of an apparatus for ultrasonic in-process monitoring and feedback of resistance spot weld quality according to the invention. 
           [0012]      FIG. 2  is an enlarged view of an electrode assembly of  FIG. 1 . 
           [0013]      FIG. 3  is an enlarged representation of reflected waves within the weld zone of  FIGS. 1-2 . 
           [0014]      FIG. 4  is an oscillogram of reflected waves within the system of  FIGS. 1-3 . 
           [0015]      FIG. 5  is a side view of a three-layer weld subject. 
           [0016]      FIG. 6  is a dynamic graphing of the oscillograms representing the welding process. 
           [0017]      FIG. 7  is a pair of graphs showing a time of flight for good quality weld, comparing actual results with a model. A good weld is indicated by a change of slope highlighted by the circle on the right graph. 
       
    
    
     DETAILED DESCRIPTION 
       [0018]    Referring to  FIG. 1 , an apparatus  100  for ultrasonic in-process monitoring and feedback according to the invention includes a computer  110 , analog-digital converter (ADC)  120 , pulser-receiver  130 , monitor  140 , weld controller  150  and acoustic transmitter-receiver probes  166 ,  176  mounted within weld electrodes  160 ,  170 . 
         [0019]    Each of the electrodes  160 ,  170  includes a probe  166 ,  176  capable of emitting and receiving acoustic waves  10 ,  20  and  30 . The computer  110  sends commands to the weld controller  150  and the pulser-receiver  130 . The weld controller  150  clamps the electrodes  160 ,  170  and starts welding. Simultaneously, the pulser-receiver  130  sends electrical pulses to a probe  166  located in one of the electrodes. The electrical energy is converted into mechanical energy in the form of acoustic waves  10 . 
         [0020]    The waves  10  propagate through the first and second metal layers  60 ,  80  and the weld zone  70  as long as the metal is transparent to sound waves. The weld zone  70  includes a liquid metal zone and adjacent areas. 
         [0021]    The probe  166  emits incident wave  10 . Part of the wave  10  is reflected by the weld zone  70  and shown as reflected wave  20 . Reflected wave  20  is received by the same probe  166 , and received by the pulser-receiver  130 . A portion of wave  10  shown as wave  30  passes through weld zone  70  and is picked up by probe  176 . The received acoustic wave  30  is also sent to the pulser-receiver  130 . Pulser-receiver  130  forwards the signals to the ADC  120 . The digitized signal is sent to the computer  110  and processed with special signal processing software. The results of signal processing and signal analysis can be output to the monitor  140  or stored as a computer file. 
         [0022]    Referring to the  FIG. 2 , the incident and reflected waves  10 ,  20  (shown in  FIG. 1 ) pass through the several media. The incident wave  10 , generated by the probe  166 , propagates through a cooling water column  40  which works as a couplant for the acoustic waves. The wave  10  propagates through the body of the electrode  160  and enters the first metal sheet  60 . Part of the wave  10 , as wave  30 , passes through the weld zone  70 , body of the second electrode  170  and water column  42 . The weld zone  70  is defined as a bulk volume of the welded metal sheets located between the tips of the two electrodes and the volume of the dynamic molten region including all interfaces between all substances in this zone. Then the wave  30  induces electrical signals in the piezoelectric crystal of the probe  176  that are returned to the pulser-receiver  130 . 
         [0023]    Wave  20 , a portion of the wave  10 , reflects from the weld zone  70 . Referring to  FIG. 3 , the incident wave  10  reflects from the interface of the electrode  160  and sheet-metal  60  as ray  13 . The reflection of wave  10  from the interface of the first sheet-metal  60  and liquid zone  75  is shown as ray  14 . The reflection from the interface of first sheet-metal  60  and second sheet-metal  80  is shown as ray  15 . The reflection from the interface of liquid zone  75  and second sheet-metal is shown as ray  16 . The reflection from the interface of second sheet-metal  80  and electrode  170  is shown as ray  17 . All these reflected rays  13 ,  14 ,  15 ,  16 ,  17  come back to the probe  166  as reflected wave  20 . Wave  20  generates the electrical signal in the piezoelectric crystal of the probe  166 . 
         [0024]    All of the received signals come to the pulser-receiver  130 , and then are forwarded to the ADC  120  and then to the computer  110 . 
         [0025]      FIG. 4  depicts an oscillogram of the reflected wave  20 , showing each of the reflected components illustrated as rays  13 ,  14 ,  15 ,  16 ,  17 . Using the oscillogram, it is possible to determine the geometry and position of the liquid portion of the weld zone  70  at given moment during welding. Signals  13  and  17  are used as the reference points to locate the position of the upper interface of the first plate and the lower interface of the second plate. 
         [0026]    Signal  14  inverts its phase when the wave strikes solid-liquid interface. The impedance of the liquid is lower than that of the solid metal. Such kind of impedance mismatch gives rise to the phase inversion of the reflected signal  14 . 
         [0027]    Signal  16  is reflected from liquid-solid interface. Its phase is always inverted with respect to signal  14  because phase inversion happens only when impedance of second medium is less than that of the first. 
         [0028]    Signal  15  comes from the interface of the two sheet-metals. The greater the lateral size of the molten region the weaker this signal is due to the reduced reflecting area within the range of the main lobe of the wave generated by the probe  166 . 
         [0029]    The phase inversion of the reflected signal is the crucial feature which allows say that one really deals with the solid-liquid interface and not with some artifact. Calculation of the distance between the reflected signal peaks  14  and  16 , on the time scale t, provides information about the thickness of the liquid zone  75 . The positioning, on the time scale t, of the peak  14  with respect to peak  13 , and the peak  16  with respect to peak  17 , gives an indication of the position of the liquid zone relative to the outer surfaces of the sheet-metal  60 ,  80 . Position of the peaks  14 ,  16  with respect to peak  15  further provides an indication of the position of the liquid zone relative to the interface between sheet-metal  60 ,  80 . This information is of particular importance in the welding of high strength steel, when position of the nugget is not symmetrical with respect to the central contact interface. 
         [0030]    The comparison of the amplitude of peak  15  and peak  14  provides information about the lateral size of the liquid zone  75 . The smaller the amplitude of peak  15  the bigger area is covered by the weld liquid zone  75  in lateral directions. Disappearance of the peak  15  indicates that the liquid zone  75  is equal to or bigger than the main lobe width of the incident wave  10 . When the width of the main lobe is known, it is possible to calculate a minimum lateral size of the liquid zone  75  by evaluation of the amplitude of peaks  14 ,  15 . 
         [0031]    A similar analysis can be applied to the composition of three and more sheets put together. An example of such an arrangement is shown in  FIG. 5 . 
         [0032]    Specially designed signal processing software allows the removal of unwanted noise, helping to distinguish the reflections from all interfaces. With the waves  10  sent through the weld  70  during the whole process of welding, the resultant data stream illustrates the dynamic picture of liquid zone growth and its position with respect to the outer interfaces of the sheet-metal  60 ,  80 . 
         [0033]    Referring to  FIG. 6 , the arrival time of signals reflected from different interfaces at different moments of welding is presented. Before welding started, only signals  13 ,  15 ,  17  are visible. When welding starts, the velocity of wave propagation in the metal decreases. This leads to the increase of propagation time of the wave in the metal. When melting begins, two additional interfaces appear, a solid-liquid interface  14  and liquid-solid interface  16 . The moment of appearance  18  of these signals is the moment of the beginning of melting of metal. The elapsed time from the moment of appearance  18  until the weld current is turned off at the moment  19  is the time of growth of the liquid zone. 
         [0034]    After the current is turned off, the liquid zone shrinks until the sheet-metal has re-solidified. The weld nugget is formed in place of the liquid zone. The weld nugget is defined as the volume of metal which used to be liquid zone during welding. At the moment of re-solidification  20 , the interfaces  14 ,  16  disappear. The elapsed time  19 - 20  is the solidification time, a further indicator of the weld nugget strength. 
         [0035]    A further method of in-process monitoring is based on using the transmission mode and is characterized in  FIG. 7 .  FIG. 7  the time of flight (TOF) of the series of signals  30  through the weld subject of  FIG. 1 . Referring to the  FIG. 7 , this delay is not linear throughout the welding process. As the material warms up, the sound velocity decreases through the material so that the delay of the transmitted signal increases. The melting of the liquid zone, the change in phase from solid to liquid, results in a discontinuous change of the physical properties of the material. These properties include resistivity and, notably, sound velocity, as is reflected in the discontinuity of the TOF graphs of  FIG. 7 . 
         [0036]    This abrupt change of the properties can be monitored by measuring the delay of the wave passing through the weld before and after melting moment. The moment of the beginning of melting is seen on the time of flight curve shown on  FIG. 7  as a change of the slope of the curve to the higher values. The abrupt increase of the signal delay at a certain moment of welding corresponds to the beginning of melting of the welded plates. The computer  110  determines the exact moment of the start of melting. The time from this moment up to the end of welding is the time of liquid pool growth. The time of liquid pool growth characterizes the weld nugget size. The computer  110  uses the time of the beginning of melting to instruct the weld controller  150  to discontinue the current at an appropriate time for adequate weld growth. 
         [0037]    This time of flight (TOF) jump does not occur in a stick weld. In stick weld the mating surfaces are weakly bonded in the weld zone—when the sheets are peeled apart there is no nugget present. Thus this technique is capable of distinguishing stick weld from one in which the weld nugget has formed. It can also qualitatively characterize the weld by measuring the time between the start of melting and the moment the current is off. The through transmission mode distinguishes between a stick weld and a proper weld based on the exact duration of liquid zone growth. 
         [0038]    While the invention has been described in the specification and illustrated in the drawings with reference to a preferred embodiment, it will be understood by those skilled in the art that various changes may be made and equivalents may be substituted for elements thereof without departing from the scope of the invention as defined in the claims. In addition, many modifications may be made to adapt a particular situation or material to the teachings of the invention without departing from the essential scope thereof. Therefore, it is intended that the invention not be limited to the particular embodiment illustrated by the drawings and described in the specification as the best mode presently contemplated for carrying out this invention, but that the invention will include any embodiments falling within the scope of the appended claims.