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.

What is claimed is: 
     
         1 . An apparatus for monitoring welding in a resistance welder, the resistance welder including first and second electrodes operating on a weld subject, wherein the weld subject includes at least two metal sheets, the two metal sheets disposed between the 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 the weld subject, receives reflected acoustic energy reflected from the weld subject in response to the burst and generates an output signal in response to the reflected acoustic energy, the reflected acoustic energy including 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   a computer that receives the output signal, the computer determining a characteristic of the liquid zone that forms during welding based on determining that the output signal includes the phase inverted acoustic wave, the computer determining the characteristic based upon a change in slope of a curve of a time of flight value of the phase inverted acoustic wave, 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, wherein the ultrasonic probe receives a third acoustic wave reflected from an interface between the two metal sheets, and wherein the computer monitors a peak of the third acoustic wave over time to evaluate the liquid zone.   
     
     
         2 . The apparatus of  claim 1  wherein the computer determines that the liquid zone was formed based on the presence of the phase inverted acoustic wave in the reflected acoustic energy. 
     
     
         3 . The apparatus of  claim 2  wherein the computer monitors a change in the thickness of the liquid zone over time. 
     
     
         4 . The apparatus of  claim 2  wherein the computer monitors a change in the thickness of the liquid zone over time based upon a distance between the peak of the phase inverted acoustic wave and the peak of the second acoustic wave. 
     
     
         5 . The apparatus of  claim 2  wherein the reflected acoustic energy includes acoustic energy reflected off an outer surface of the weld subject, wherein the outer surface of the weld subject is distal to the ultrasonic probe, the computer configured to determine the characteristic based upon the acoustic energy reflected off the outer surface of the weld subject. 
     
     
         6 . The apparatus of  claim 5  wherein the computer is configured to determine an initiation of the interface between the liquid zone and the solid portion of the metal, to monitor a change in thickness of the liquid zone over time and to monitor solidification of the liquid zone. 
     
     
         7 . 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 of acoustic waves of the acoustic energy transmitted through the weld subject. 
     
     
         8 . The apparatus of  claim 7  further including the computer determining that the liquid zone began forming based upon a change in slope of the time of flight of acoustic waves of the acoustic energy transmitted through the weld subject. 
     
     
         9 . The apparatus of  claim 8  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. 
     
     
         10 . The apparatus of  claim 8  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. 
     
     
         11 . The apparatus of  claim 8  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. 
     
     
         12 . The apparatus of  claim 1  wherein the computer monitors the peak of the third acoustic wave to determine symmetry of the weld relative to the interface between the two metal sheets. 
     
     
         13 . The apparatus of  claim 1  wherein the computer monitors an amplitude of the peak of the third acoustic wave to determine a lateral size of the liquid zone. 
     
     
         14 . The apparatus of  claim 1  wherein the computer monitors an amplitude of the peak of the third acoustic wave and an amplitude of the peak of the phase inverted acoustic wave to monitor a width of the liquid zone. 
     
     
         15 . The apparatus of  claim 1  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. 
     
     
         16 . 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. 
     
     
         17 . The apparatus of  claim 1  wherein the characteristic of the liquid zone includes size and position of the liquid zone. 
     
     
         18 . 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. 
     
     
         19 . The apparatus of  claim 18  wherein the weld parameter includes weld time and weld current. 
     
     
         20 . 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. 
     
     
         21 . The apparatus of  claim 7  wherein the computer is configured to analyze a curve over time of the time of flight during the welding process to determine the characteristic. 
     
     
         22 . The apparatus of  claim 21  wherein the curve over time of the time of flight is analyzed during different times in the welding process. 
     
     
         23 . The apparatus of  claim 21  wherein the time of flight is a time required for acoustic energy to pass through the weld subject in one direction. 
     
     
         24 . The apparatus of  claim 21  wherein the time of flight is a time required for acoustic energy to pass through the weld subject and reflect back through the object. 
     
     
         25 . The apparatus of  claim 7  wherein the computer is configured to determine whether a change in slope of a time of flight curve over time occurred and to determine that a stick weld occurred based the absence of the change in slope. 
     
     
         26 . The apparatus of  claim 7  wherein the computer is configured to determine whether a change in slope of a time of flight curve over time occurred, and to determine that a change in slope occurred and to determine that the liquid zone began forming at the time that change in slope occurred. 
     
     
         27 . 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 reflected acoustic energy reflected from the weld subject in response to the burst and generates an output signal in response to the reflected acoustic energy, the reflected acoustic energy including 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   a computer that receives the output signal, the computer determining a characteristic of the liquid zone that forms during welding based on determining that the output signal includes the phase inverted acoustic wave, the computer determining the characteristic based upon a change in time of flight value of the phase inverted acoustic wave, 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, the computer determining whether a time of flight jump occurred and determining that a stick weld occurred based the absence of a time of flight jump during welding.