Measuring and testing – Vibration – By mechanical waves
Patent
1995-01-31
1999-07-06
Oda, Christine K.
Measuring and testing
Vibration
By mechanical waves
73602, G01N 2904
Patent
active
059200146
DESCRIPTION:
BRIEF SUMMARY
DESCRIPTIONS
1. Technical Field
The present invention relates to a process for assessing welding operations and, in particular, resistance welding using an ultrasonic source, which impinges ultrasonic waves onto the weld region and using an ultrasound receiver.
The invented process is not only applicable in resistance welding, but also in a variety of very different welding operations, such as laser welding, gas welding, etc.
2. State of the Art
"Materials Evaluation" No. 47 of August 1989, pages 935-943 describes a process in which the weld region, respectively the weld spot, are sonically inspected during the entire resistance welding process.
The resistance welding process is made up of, i.a., three phases. The first phase is the so-called prepress period. During this period, there is no current flowing. The electrodes close, raising in this way the electrode power of 1-3 KN. Then follows the current flow phase during which the sheets are heated. The post-press period or cooling phase follows the current flow phase. In this phase, the weld nugget cools. It is not until then that the weld electrodes open. All three phases are usually of the same length.
The ultrasonic inspection occurs in the aforementioned method using a longitudinal ultrasonic transmitter in the frequency range of 2 MHz which is disposed within the weld electrodes on the electrode bottom. The ultrasonic inspection starts during the prepress period, in which there is no current flowing, and lasts until the end of the post-press phase. In particular, the ultrasonic transmissivity of the weld spot is assessed during the post-press phase, with the duration of the cooling being determined until attainment of the minimum ultrasonic transmissivity and being correlated with the volume of the molten weld nuggets.
Minimum ultrasonic transmissivity occurs, according to the opinion expounded in this article, at the point of conversion of the iron from the austenitic state to the ferritic state (Curie point) as a consequence of raised ultrasonic absorption at this point of conversion.
A drawback of this process is, however, that the welding process is already terminated and the molten weld nugget cooled when the weld nugget assessment is carried out. The welding process can no longer be influenced. A welding that was detected to be faulty can, however, at most be rewelded.
An apparatus for monitoring a resistance spot welding operation, in which the weld spot is ultrasonically inspected from the inner electrode bottom when the current is flowing with the ultrasonic signal reflected at the opposite electrode being received and evaluated, is known from DE-AS 26 55 415.
This printed publication says nothing about the type of wave that was used; however, the course of the ultrasonic transmissivity depends essentially on the type of wave that was used so that the assumption that the sonic transmissivity of the weld spot during the welding operation first attains a relative maximum, then drops to a minimum and in the further course rises once more to a second maximum cannot be followed.
The first maximum of the ultrasonic transmissivity is explained by the rise in temperature in the sheets to be welded improving the ultrasonic contact between the electrodes and the sheet as well as between the sheets. The subsequent minimum is supposed to result from the fact that the weld material melts. The molten iron raises ultrasonic absorption and reduces sonic transmissivity. The rise in ultrasonic transmissivity during the further course of the welding operation is not explained although this rise, which was not understood prior to the present invention being an element thereof, is essential for the assessment of the welding operation. The assessment process determines the extent of the renewed rise of the ultrasonic transmissivity. The size of the weld nugget is determined from this measured value. The physical explanation for this principle is contradictory in itself. The process is based on the measurement of the difference between two ultrasonic transmissivi
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McMaster, R. C. Nondestructive Testing Handbook. N.Y., The Ronald Press Cany, 1959. pp. 43-15 to 43-17.
Carlin, B. Ultrasonics. N.Y., McGraw-Hill Book Company, Inc., 1949. pp. 4-5, 40-41.
Fraunhofer -Gesellschaft Zur Forderung der Angewandten Forschung
Oda Christine K.
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