Spot welding is one of the most widespread methods of connecting parts. For example, a Boeing 747 plane has over 300,000 spot welds. However, the use of spot welding in vital constructions without a reliable method and apparatus for testing the parameters of the nugget in a spot weld is impossible during welding as well as under working conditions in finished articles.
Known in the art is a method of non-destructive quality testing of spot welds (cf. a book by L. S. Feldman "Non-destructive Quality Testing of Glue-Welded Joints", "Tekhnika" Kiev, 1973), residing in that longitudinal ultrasonic oscillations are directed into the cast region at an angle to the surface of the test weld. Thereupon, an ultrasonic detector is displaced over the test weld. If the ultrasonic detector is situated over the zone of the test weld wherein the weld nugget is absent, in this case at the measuring device, there will appear a signal reflected from the contact surface of the welded parts. If the ultrasonic detector is situated over the zone of the test weld wherein the weld nugget is located, in this case the reflected signal will disappear. The diameter of the weld nugget is evaluated by measuring the distance between the points corresponding to disappearance and appearance of the reflected signal.
The apparatus realizing this method of non-destructive quality testing of spot welds comprises a master oscillator having a pulse generator connected to one of the outputs thereof and a sweep generator connected to another output. The output of the pulse generator is connected with the ultrasonic detector through a pulse amplifier. Connected to the outputs of the sweep generator and of the ultrasonic detector is a cathode-ray tube.
The master oscillator generates oscillations energizing the pulse generator and the sweep generator. From the pulse generator, high frequency electric pulses are supplied through the pulse amplifier to the ultrasonic detector and generate therein ultrasonic oscillations. These oscillations are applied to the test weld. In the absence of a weld nugget and with an air clearance between the welded parts, ultrasonic waves are reflected from the interface of the mediums and a signal is returned to the detector which is supplied to the cathode-ray tube controlled by the sweep generator. With a weld nugget present, ultrasonic oscillations pass through the fusion zone without any reflection.
Displacing the ultrasonic detector over the test weld, an instant of disappearance and appearance of the reflected signal on the face of the cathode-ray tube is recorded. The diameter of the test weld nugget is determined by measuring the distance between the points on the face of the cathode-ray tube, corresponding to instants of disappearance and appearance of the reflected signal.
The disadvantage of the aforementioned method and apparatus for non-destructive quality testing of spot welds is that, when there is an absence of a clearance between the welded parts with a poor fusion in the form of adhesion, a reflected signal is not produced. Moreover, for a number of materials using this method it is impossible to detect poor fusion in the form of adhesion which is one of the most dangerous welding defects.
For a number of materials having inhomogeneity of chemical composition in the weld nugget, poor fusion can be detected by means of a radio-graphic method.
Known in the art is a method of non-destructive quality testing of spot welds, wherein X-rays non-uniformly absorbed by various nugget regions because of inhomogeneity of chemical composition of the weld nugget are directed onto the test weld. X-rays pass through the test weld and fall onto the X-ray film whereon, after development, liquation rings are formed, the diameter of the weld nugget being evaluated according to the ring dimensions.
The apparatus realizing this method of non-destructive quality testing of spot welds comprises an X-ray source and a detector, such as an X-ray film, whereon the X-rays having passed through the tested weldment are recorded.
The disadvantage of the aforementioned method and apparatus for non-destructive quality testing of spot welds is that it is impossible to determine the nugget thickness because X-raying the tested material in a single plane. Besides, for materials which do not exhibit sharply defined inhomogeneity of chemical composition in the weld nugget section, it is also impossible to determine the diameter of the weld nugget, i.e. to detect such a dangerous and popular welding defect as poor fusion.
To ensure radio-graphic quality testing of spot welds formed of the materials which do not exhibit sharply defined inhomogeneity of chemical composition in the weld nugget section, so-called X-ray contrast materials are used which are introduced between the welded parts prior to welding and make it possible to obtain a contrast image of the weld nugget on the X-ray pattern. However, the use of X-ray contrast materials complicates the testing process which in turn results in a decrease of efficiency and in an increase of a testing cost price.
It is possible to increase the efficiency of the method and to decrease the equipment cost price by the use of an eddy current testing method which depends upon the fact that the nugget of the test weld and the welded material outside the nugget region have various values of electric conductivity.
Known in the art is a method of non-destructive quality testing of spot welds (cf. USSR Inventor's Certificate No. 336,587), wherein eddy currents are induced in the test weld nugget region which produce a secondary electromagnetic field in the weld zone. The electromagnetic field of eddy currents acts on the transducer varying its output signal according to the weld properties. The dimensions of the weld nugget are evaluated by measuring the output signal of the eddy current transducer.
The apparatus realizing this method of non-destructive quality testing of spot welds comprises a low frequency generator, a T-shaped LCR bridge connected to the output thereof, a main eddy current transducer included in the T-shaped bridge as an L-element, an electronic indicator of the bridge output signal, a phase shifter, frequency multipliers and a phase meter which are connected between the output of the generator and the main eddy current transducer.
Fixed in the center of the main eddy current transducer is an additional eddy current transducer which forms in conjunction with a capacitor a measuring circuit intended to test the depth of indentation. The measuring circuit of the additional eddy current transducer is connected to the high frequency generator and to the indentation depth indicator.
From the low frequency generator a sinusoidal voltage is applied to the input of the T-shaped symmetric bridge and to the phase shifter. The bridge is balanced when the transducer is placed on the reference comparison weldment. Thereupon, the transducer having the parameters which vary according to the weld quality, is placed on the test weld, and an unbalance signal dependent on the weld quality appears at the bridge output. The amplitude of the unbalance signal is measured by the electronic indicator. To measure the phase, the signal is taken directly from the main eddy current transducer and applied to the phase meter through the frequency multiplier. A reference voltage is supplied from the low frequency generator to the phase meter through the phase shifter and frequency multiplier. A voltage to the measuring circuit of the depth of indentation is supplied from a high frequency generator. The signal corresponding to the depth of indentation is supplied from the measuring circuit to the indentation depth indicator. The weld quality is evaluated according to the obtained values of the signal amplitude and phase.
The disadvantage of the aforementioned method and apparatus for non-destructive quality testing of spot welds is that, because of the use of two eddy current transducers disposed coaxially, overall dimensions of the detecting element are considerably increased and therefore it is impossible to test welds of small thickness.
Known in the art is a method of non-destructive quality testing of spot welds (cf. U.S. Pat. No. 3,526,829) characterized in that two pulsed electromagnetic fields are produced one of which is applied to the test weld and another is applied to the reference weld. Thereupon, the depth of penetration of two pulsed electromagnetic fields into the welds is measured by dynamic impedance measurement of the effect of the induced eddy currents on their applied electromagnetic fields.
The apparatus realizing this method of non-destructive quality testing of spot welds comprises eddy current transducers placed onto the reference and test welds, an impedance comparator unit having its inputs connected with the eddy current transducers, and a controlled switch having its output connected with a stored energy source. The output of the comparator unit is connected with a threshold circuit having its output connected with an information display unit wherein an information signal varying in accordance with any difference between the measured impedance values in the reference weld and the test weld is displayed in an acceptance-rejection form.
By means of this method and apparatus, it is possible to test spot welds of small thickness.
The disadvantage of the aformentioned method and apparatus for non-destructive quality testing of spot welds is that they provide quality testing of spot welds only in ferromagnetic materials.
Known in the art is a method of non-destructive quality testing of spot welds (cf. a paper "Apparatus for Quality Testing of Spot Welding of Aluminium Alloys" by V. S. Fastritsky, E. P. Rybalkina, and P. Sh. Fishkin in the book "Methods and Apparatus for Automatic Testing", issue 13, p.p. 18-24, Riga, 1975), characterized in that in the test weld zone a primary electromagnetic field is produced which induces in this zone eddy currents producing a secondary field, whereupon the phase value of the resultant electromagnetic field intensity is determined according to which the presence and quality of the nugget in a spot weld are evaluated.
The apparatus realizing this method of non-destructive quality testing of spot welds comprises a sinusoidal voltage generator, a reference channel connected to one of the outputs thereof and representing a phase shifter having its output connected to one of the inputs of a phase meter, and a measuring channel connected to another output of the generator and representing an unbalanced bridge circuit with a superimposed eddy current transducer included into one of its branches and with its output connected to another input of the phase meter.
The voltage from the sinusoidal voltage generator is supplied to the phase shifter which serves to set the phase of the reference voltage, and to the bridge circuit with one of its branches including the eddy current transducer placed upon the test weld. The transducer impedance resistance, and hence the output signal of the bridge circuit wherein it is included, vary according to the weld quality. The bridge circuit is adjusted so that the phase of the output voltage is not dependent upon the size of the clearance between the transducer and the weldment, but is determined only by variations in the electric conductivity of the tested zone, which in turn is dependent upon the weld quality. From the output of the phase shifter the signal is applied to one of the inputs of the phase meter, and from the output of the bridge circuit it is fed to the second input of the phase meter. Placing the transducer upon the reference weld, the phase of the reference voltage is varied by the phase shifter so that the phase shift between the reference and measured voltages in case of a quality weld should be equal to zero. Thereupon, the transducer is placed onto the test weld, and the weld quality is determined according to indications of the phase meter.
The disadvantage of the aforementioned method of and apparatus for non-destructive quality testing of spot welds is that the output signal of the transducer contains only the resultant information on the dimensions of the weld nugget and on presence of defects therein with the result that in some cases wrong results of the weld quality evaluation may be obtained. For example, with the reduced dimensions of the weld nugget and with cracks present therein, the signal obtained at the output of the apparatus can be the same as for a quality weld nugget. A similar result can be obtained for a complete infusion with a clerance between welded parts. This disadvantage also restricts possible control of welding conditions because of lack of data on the weld nugget diameter and thickness.
Besides, the apparatus should be previously adjusted by placing the superimposed eddy current transducer upon the reference weld. However, for a number of materials, such as aluminium-magnesium alloys, a reference weld can not be defined by non-destructive methods.
Thus, eddy current methods of and apparatus for non-destructive quality testing of spot welds make it possible to obtain a summary characteristic of a spot weld dependent upon variations in the whole volume of the weld nugget without differentiating the effect of variations in the diameter and thickness of the spot weld.
Such characteristic of a spot weld does not allow for reliable testing of weldments with different thicknesses and of weldments formed of composite alloys.