Ultrasonic in-process monitoring and feedback of resistance spot weld quality

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.

1 . An apparatus for monitoring welding in a resistance welder, the resistance welder including first and second electrodes, the apparatus comprising 
 an ultrasonic probe disposed within the first electrode that generates a burst of acoustic energy that is directed to a weld subject, receives acoustic energy reflected from the weld subject and generates an output signal in response to the reflected acoustic energy; and    a computer that receives the output signal, the computer determining a characteristic of a liquid zone that forms during welding based on a phase inverted acoustic wave reflected from an interface between the liquid zone and a solid portion of the weld subject proximal to the ultrasonic probe; the liquid zone solidifying into a weld nugget after welding current is turned off    
     
     
         2 . The apparatus of  claim 1  wherein the computer determines that the liquid zone formed based on the presence of the phase inverted acoustic wave in the reflected acoustic energy.  
     
     
         3 . The apparatus of  claim 1  wherein the weld subject includes at least two metal sheets, the two metal sheets disposed between the first and second electrodes, the computer determining a thickness of the liquid zone based upon a distance between a peak of the phase inverted acoustic wave and a peak of a second acoustic wave reflected from an interface between the liquid zone and a solid portion of the metal sheet distal to the ultrasonic probe.  
     
     
         4 . The apparatus of  claim 3  wherein the computer determines the distance between the peaks of the phase inverted acoustic wave and the second acoustic wave based on a time difference between the peaks of the phase inverted acoustic wave and the second acoustic wave as received by the ultrasonic probe.  
     
     
         5 . The apparatus of  claim 1  wherein the weld subject includes at least two metal sheets disposed between the first and second electrodes, the computer determining a position of the liquid zone relative to outer surfaces of the metal sheets based on a time difference between a peak of an acoustic wave reflected from an outer surface of the metal sheet proximal to the ultrasonic probe and a peak of the phase inverted acoustic wave and upon a time difference between a peak of an acoustic wave reflected from an interface of the liquid zone to a solid portion of the metal sheet distal to the ultrasonic probe and a peak of an acoustic wave reflected from an outer surface of the metal sheet distal to the ultrasonic probe.  
     
     
         6 . The apparatus of  claim 1  wherein the characteristic of the liquid zone includes at least one of size and position of the liquid zone.  
     
     
         7 . The apparatus of  claim 1  and further including the computer evaluating the characteristic of the liquid zone and determining whether to adjust a weld parameter to achieve a desired weld nugget based on its evaluation and upon determining to adjust the weld parameter, communicating an adjustment to the weld parameter to the weld controller.  
     
     
         8 . The apparatus of  claim 7  wherein the weld parameter includes at least one of weld time and weld current.  
     
     
         9 . The apparatus of  claim 1  wherein the weld subject includes at least two metal sheets, the two metal sheets disposed between the first and second electrodes, and the characteristic of the liquid zone includes at least one of width, height, position relative to surfaces of the metal sheets, temperature, melting time, and solidification time.  
     
     
         10 . The apparatus of  claim 2  further including a second ultrasonic probe disposed in the second electrode, the second ultrasonic probe producing a second output signal in response to acoustic energy transmitted through the weld subject indicative of a time of flight (TOF) of acoustic waves of the acoustic energy transmitted through the weld subject.  
     
     
         11 . The apparatus of  claim 10  further including the computer determining whether a TOF jump occurred and determining that a stick weld occurred based the absence of a TOF jump during welding.  
     
     
         12 . The apparatus of  claim 10  further including the computer determining whether a TOF jump occurred and upon determining that a TOF jump occurred, determining that the liquid zone began forming at the time that the TOF jump occurred.  
     
     
         13 . The apparatus of  claim 12  wherein the computer determines a time to instruct the weld controller to turn off welding current welding based upon a time that the liquid zone began forming.  
     
     
         14 . The apparatus of  claim 12  wherein the computer determines a size of the weld nugget based on a time that the liquid zone began forming and when welding current is turned off.  
     
     
         15 . The apparatus of  claim 12  wherein the computer determines a duration of liquid zone growth based on a time that the liquid zone began forming and when welding current is turned off, the computer determining whether a stick weld occurred based on the duration of liquid zone growth.  
     
     
         16 . The apparatus of  claim 1  wherein the computer displays on a display at least one of graphical and numerical representations of the characteristic of the liquid zone.  
     
     
         17 . In a resistance welder having a first electrode and a second electrode, a method of monitoring welding of a weld subject with the resistance welder, comprising: 
 a. generating bursts of acoustic energy with an ultrasonic probe disposed in the first electrode;    b. generating an output signal with the ultrasonic probe in response to acoustic energy reflected from the weld subject;    c. determining based on the output signal whether the reflected acoustic energy includes a phase inverted acoustic wave reflected from an interface between a liquid zone and a solid portion of the weld subject proximal to the ultrasonic probe; and    determining a characteristic of the liquid zone based on the phase inverted acoustic wave, the liquid zone solidifying into a weld nugget after welding current is turned off.    
     
     
         18 . The method of  claim 18  further including determining that the liquid zone formed based on the determination that the phase inverted acoustic wave is present in the reflected acoustic energy.  
     
     
         19 . The method of  claim 17  wherein the weld subject includes at least two metal sheets, the two metal sheets disposed between the first and second electrodes, the method further including determining a thickness of the liquid zone based upon a distance between a peak of the phase inverted acoustic wave and a peak of a second acoustic wave reflected from an interface between the liquid zone and a solid portion of the metal sheet distal to the ultrasonic probe.  
     
     
         20 . The method of  claim 19  further including determining the distance between the peaks of the phase inverted acoustic wave and the second acoustic wave based on a time difference between the peaks of the phase inverted acoustic wave and the second acoustic wave as received by the ultrasonic probe.  
     
     
         21 . The method of  claim 17  wherein the weld subject includes at least two metal sheets disposed between the first and second electrodes, the method further including determining a position of the liquid zone relative to outer surfaces of the metal sheets based on a time difference between a peak of an acoustic wave reflected from an outer surface of the metal sheet proximal to the probe and a peak of the phase inverted acoustic wave and upon a time difference between a peak of an acoustic wave reflected from an interface of the liquid zone to a solid portion of the metal sheet distal to the ultrasonic probe and a peak of an acoustic wave reflected from an outer surface of the metal sheet distal to the ultrasonic probe.  
     
     
         22 . The method of  claim 17  wherein the characteristic of the liquid zone includes at least one of size and position of the liquid zone.  
     
     
         23 . The apparatus of  claim 17  and further including determining based upon an evaluation of the characteristic of the liquid zone whether to adjust a weld parameter to achieve a desired weld nugget, and upon determining to adjust the weld parameter, adjusting the weld parameter.  
     
     
         24 . The method of  claim 23  wherein the weld parameter includes at least one of weld time and weld current.  
     
     
         25 . The method of  claim 17  wherein the weld subject includes at least two metal sheets, the two metal sheets disposed between the first and second electrodes, and the characteristic of the liquid zone includes at least one of width, height, position relative to surfaces of the metal sheets, temperature, melting time, and solidification time.  
     
     
         26 . The method of  claim 17  further including determining a time of flight (TOF) of acoustic energy transmitted through the weld subject based on a second output signal produced by a second ultrasonic probe disposed in the second electrode.  
     
     
         27 . The method of  claim 26  further including determining that a stick weld occurred based on the absence of a TOF jump during welding.  
     
     
         28 . The method of  claim 26  further including determining whether a TOF jump occurred and determining that the liquid zone began forming at the time that the TOF jump occurred.  
     
     
         29 . The method of  claim 28  including determining a time to turn off welding current based upon the time that the liquid zone began forming.  
     
     
         30 . The method of  claim 28  including determining a size of the weld nugget based on the time that the liquid zone began forming and when welding current is turned off.  
     
     
         31 . The method of  claim 28  including determining a duration of liquid zone growth based on the time that the liquid zone began forming and when welding current is turned off and determining whether a stick weld occurred based on the duration of liquid zone growth.  
     
     
         32 . The method of  claim 17  including displaying on a display at least one of graphical and numerical representations of the characteristic of the liquid zone.