WO2005118201A1

WO2005118201A1 - Apparatus and method for nondestructive testing of the joint quality of stud welded joints

Info

Publication number: WO2005118201A1
Application number: PCT/EP2005/052524
Authority: WO
Inventors: Klaus Gisbert Schmitt; Harald Knetsch
Original assignee: Newfrey LLC
Current assignee: Newfrey LLC
Priority date: 2004-06-04
Filing date: 2005-06-02
Publication date: 2005-12-15
Anticipated expiration: 2006-12-04
Also published as: DE102004028607A1; DE102004028607B4

Abstract

Method and apparatus for nondestructive testing of the joint quality of fastened workpiece joints whereby the apparatus comprises a device (12) for heating a first workpiece (14) that is joined to a second workpiece (16), and a device (18) for optical measurement of a heat distribution, whereby the device for heating is an inductor (28) which is capable of producing a magnetic field (38) that acts on the first workpiece.

Description

APPARATUS AND METHOD FOR NONDESTRUCTIVE TESTING OF THE JOINT QUALITY OF STUD WELDED JOINTS

The present invention relates to a method for nondestructive testing of the joint 5 quality of fastened workpiece joints having the following steps: joining a first workpiece to a second workpiece; heating the first workpiece that is joined to the second workpiece, in particular to a temperature of 50 °C to 500 °C; and optical detecting of a heat distribution in a region where the first workpiece is joined to the second workpiece. 10 The present invention further relates to an apparatus for nondestructive testing of the joint quality of fastened workpiece joints having a device for heating a first workpiece that is joined to a second workpiece, and a device for optical detecting of a heat distribution. Such a method and such an apparatus are known from the document DE 199 15 20 219 A1. Various test/measurement methods for quality control of fastened joints, especially spot welded joints, exist in the prior art. The heat development in the zone in which the spot welded joint is to be produced is decisive in the quality of a spot welded joint, wherein a sufficiently large 20.. molten pool between the parts or workpieces to be joined-.must be achieved in particular. For example, if an electric current, which is supposed to cause the development of heat, flows not only through the zone where the spot welded joint is to be created, but also through other connections (which usually are called "shunts"), this can seriously impair the quality of the developing spot weld connection and possibly 25 prevent the spot welding from occurring at all. There is a need for continuous evaluation of the quality of spot welded joints, especially during the course of automated fabrication processes, in order to be able to detect inadequate spot welded joints immediately and eliminate them. Sampling inspections of spot welded joints, which are often destructive, are not considered 30 adequate under some circumstances to ensure the desired quality of spot welded joints. One prior art method is characterized by mechanical strength testing of stud welded joints. This testing is carried out in accordance with statistical principles. A first such test method is the known chisel method, in which the weld joint is mechanically loaded to the tearing point. Conclusions regarding other, untested weld joints are drawn from this. In addition, there are nondestructive testing methods, such as ultrasound and X-ray structure analyses. However, these represent labor-intensive analysis methods. Known from DE 196 50 883 A1 is a method for contactless and nondestructive testing of welded seams on moving strips. In this process, two plastic strips are heated and then joined by means of pressure rollers. Immediately after the joining, one of the surfaces of the layered composite structure thus obtained is thermographically measured "online" in the form of lines. The temperature distribution can be derived from a thermographic image consisting of lines scanned in this way. A uniform heat distribution is obtained when the strips are homogeneously joined material-to-material. A disadvantage of this method, however, is that inspection of the joint quality must take place directly following the actual joining process so that the heat used during joining can be exploited for thermographic purposes. To this end, the thermal scanner must be located relatively close to the location where joining takes place. But this closeness can falsify the thermal scanner measurements, since they can be influenced by the radiated heat of the actual joining process. In DE 199 20 219, which was already mentioned above, a production line is proposed in which two workpieces are areally joined to one another by point weld contacts. In order to inspect the quality of a weld point, a heat source is installed further downstream, which delivers a specific quantity of heat per unit of time to the weld point under test by means of a heat flow. Located still further downstream is a heat sensor, which measures the temperature or its distribution. Here, too, the heat diffusion of an intact weld joint to its environment is utilized to determine quality.

Lasers or microflames are proposed as heat sources. A disadvantage of this testing method is that large amounts of energy are needed in order to heat the (possibly already cooled) workpieces sufficiently to carry out the thermal quality testing. It is therefore an object of the present invention to create an apparatus and a method for nondestructive testing of the joint quality of fastened workpiece joints wherein energy is saved and the inspection process can be automated. It is especially desirable to be independent of the fastening location. This object is attained with a method of the type initially mentioned, wherein the heating is accomplished by generating a magnetic field that penetrates the second workpiece. This inductive heating has the advantage that the energy used is converted nearly completely into the heat energy that is needed for testing the quality of the joint. The energy taken from the magnetic field through induction is converted into heat energy which is generated within the first workpiece by eddy currents. According to a preferred embodiment, the magnetic field is generated by the application of a voltage, in particular an alternating-current voltage, preferably at or above 1 kHz. The application of an AC voltage raises the level of heat generated by the induction in the workpieces under test. It is advantageous if the second workpiece is also penetrated by the magnetic field during heating. In this way, heat is generated not only in the first workpiece, but also in the second workpiece. Thus, a heat flow not only takes place from the first workpiece toward the second workpiece, but also in the opposite direction. In this way, the heat distribution necessary for measurement can be achieved more quickly. The heat distributes itself more evenly. It is further preferred for the heating to occur after the joining with respect to time. This has the great advantage that quality testing of the weld joints can take place significantly later than the actual joining of the workpieces. This is especially important when the testing method is to be used in a production line, such as in the automotive industry for example. It is thus possible to decouple the individual work stations within a line. In this way, flexibility in laying out a production line can be increased. Especially good results are achieved when the joining is accomplished through welding, particularly arc welding. In this type of joint, a material-to-material connection of the two workpieces takes place. The material-to-material connection intensifies the flow of heat between the workpieces to be joined. The heating time of the weld joint under test can be shortened in this manner, with the result that the basic cycle can be accelerated. In accordance with another preferred embodiment, in order to generate the magnetic field, an inductor is brought into the vicinity of the first workpiece such that the magnetic field produces sufficiently large eddy currents in the first workpiece to be able to carry out the testing method according to the present invention. The inductor can thus be movably mounted so that it can be moved even closer to the workpieces to be heated. It can be removed again after the inductive heating process, however, in order to make room for exchanging the workpiece to be tested, especially when the workpieces to be joined are bulky in design. This measure also helps to save energy during heating, while still retaining the greatest possible degree of flexibility with regard to exchange of the workpieces. The object is further attained by an apparatus of the type initially mentioned, in which the device for heating is an inductor which is capable of producing a magnetic field that acts on the first workpiece. The provision of an inductor permits inductive heating of the joint to be tested. Inductive heating makes it possible to save energy for heating. According to another preferred embodiment, the inductor is arranged on a side of the second workpiece, which is joined to the first workpiece, where the first workpiece is located, and the device for optical detecting is arranged on a side opposite this side. This type of arrangement has the advantage that the device for optical detecting is arranged such that the heat generated by induction flows toward this device. Jn this way, the connections between the workpieces can be observed or evaluated especially well. It is further preferred for a control and analysis device to be provided, which is coupled to the inductor and the device for optical detecting. The control and analysis device permits direct analysis of the data on a heat distribution obtained by the device for optical detecting. The control component can have a regulating effect on the apparatus, for example in order to compensate for external influences such as temperature fluctuations or heating of the inductor. By preference, the device for optical detecting is an infrared camera, in particular which works with wavelengths in a range from 8 μm to 14 μm or 1 μm to 2 μm. Infrared thermography at low temperatures (e.g. T < 100°C) can require a camera that is operated at a wavelength of 8 μm to 14 μm (depending on the material of the workpieces). Both steel and aluminum emit infrared light only to a small degree. This emitted light has a wavelength of approximately 8 μm to 14 μm. Infrared thermography at relatively high temperatures (e.g. T < 300 °C) permits the use of cameras that are preferably operated at a wavelength of 1 μm to 2 μm. In this range steel, for example, has optimum emission of infrared radiation, and negative surface effects disappear nearly entirely. Moreover, extremely small temperature differences can be visually represented with an infrared camera. This is of particular advantage for the application at hand. According to a preferred embodiment, the inductor is connected to a voltage source, which in particular provides an AC voltage of at least 1 kHz, wherein the AC voltage can be, for example, a sinusoidal voltage, a triangular wave voltage, or a square wave voltage. As already mentioned above, inductive heating is especially fostered by provision of such a voltage. It is further preferred if the inductor is a circular coil, which can be arranged essentially coaxial to a longitudinal axis of the first workpiece. A circular coil generally has a central opening, into which the first workpiece can be introduced, especially when said workpiece is of corresponding design. A circular coil is also advantageous when the first workpiece is a stud. The central opening of the circular coil can then surround the bolt nearly completely. The magnetic field lines extending through the middle of the coil can thus pass through the bolt or the joint to be inspected quite well. It is also advantageous when the inductor is composed of a ferromagnetic material, in particular which is designed in the shape of a yoke. When a yoke-shaped inductor is used, it is generally magnetized through a coil. The provision of a yoke makes it possible to apply the magnetic energy at a different location from the place where the coil that generates the magnetic field is located. The shape of the yoke, in particular, can be of any desired design. This increases the flexibility insofar as that even workpieces of complex design can be surrounded by a yoke of correspondingly complex design as needed in order to make for optimal inductive heating of the workpiece. According to another embodiment, the inductor is coupled with a temperature sensor. An additional temperature sensor may be provided for the first workpiece in order to capture the heat thereof. Both temperature sensors can be connected to the control and analysis device. These measures make it possible to better account for temperature fluctuations during analysis of the heat distribution. Temperature fluctuations occur especially when the basic cycles of the test are shortened. In such a case, the inductor will heat itself and can thus represent a source of error in the analysis. To make it possible to account for residual heat from the joining process, the temperature of the first workpiece can also be measured. Using this measurement, the control and analysis device can, firstly, appropriately compensate the measurement result, and secondly intervene to regulate the testing process, for example with respect to the strength of the heat-inducing magnetic field. In this way, constant conditions can be created for each individual measurement process. It is a matter of course that the features mentioned above and those explained below can be used in other combinations in addition to those described, or in isolation, without departing from the scope of the invention. Example embodiments of the invention are shown in the drawing and are explained in detail in the description below. Shown are: Fig. 1 an apparatus for nondestructive testing of the joint quality of workpiece joints according to the present invention; Figs. 2a and 2b heat distributions as can be seen from the perspective along line ll-ll in Fig. 1 ; Fig. 3 another embodiment of an inductor for an apparatus according to the present invention; and Fig. 4 a flowchart to illustrate the method according to the present invention. In the description that follows of the preferred embodiments of the invention, identical reference characters always designate identical elements. The apparatus according to the present invention is generally labeled 10. Fig. 1 shows an apparatus 10 for nondestructive testing of the joint quality of, e.g., a stud welded joint. The apparatus 10 comprises a device 12 for heating, e.g. a stud 14, which is joined (in a material-to-material way) to a workpiece 16. In Fig. 1 , the stud 14 is joined from above to the workpiece 16 arranged beneath it. The joining station is not shown in Fig. 1. The representation in Fig. 1 shows a sectional view through the heating device 12 and the workpieces 14 and 16. An infrared camera 18 is arranged opposite the workpiece 16. The arrows 20 and 22 schematically indicate the field of view of the camera 18. The camera 18 "views" the workpiece 16 from below in Fig. 1. The stud 14 was joined to the workpiece 16 from the opposite side. The camera 18 is additionally connected to an analysis unit 24. Images of heat distributions can be taken with the camera 18. Images of such heat distributions are shown schematically in Figs. 2a and 2b, which are explained in more detail below. Images of heat distributions can be analyzed with the analysis unit 24. For this reason, the analysis unit 24 is equipped, e.g., with a storage unit for storing image data and a processing unit (not shown), such as with a microprocessor, for example. The analysis unit 24 is connected in turn to a control unit 26 by a line 25. The control unit 26 regulates the energy supply of the device 12. The heating device 12 preferably comprises a ring-shaped coil 28, which is connected to the control unit 26 by a line 29. The coil 28 is mounted on a preferably movably supported holder 30. The holder 30 can also be composed of a coil form of the coil 28 itself and is preferably made of a nonmagnetic material. The inductor winding or coil winding is shown in Fig. 1 by rectangles which contain an oval with a cross or a dot. The winding is designed such that a current flowing in the winding flows into the plane of the drawing in the half shown on the left in Fig. 1 and flows out of the plane of the drawing in the half of the coil shown on the right in Fig. 1. In accordance with the right-hand rule, a current flow through the coil thus induces a magnetic field that is oriented downward, as shown in Fig. 1. Magnetic field lines are schematically indicated in Fig. 1 by the arrows 38. The magnetic field lines 38 run through a central opening 32 in the coil 28. For reasons that will be explained later, the apparatus 10 can also include a first temperature probe 34, which is coupled to the analysis unit 24. In the embodiment shown in Fig. 1 , the first temperature probe 34 is connected to the analysis unit 24 through a line 37, the control unit 26, and the line 25. Alternatively, the first temperature probe 34 can also be directly connected to the analysis unit 24. The apparatus 10 can also include a second temperature probe 36, which is likewise connected to the analysis unit 24 through a line 37, comparable to the first temperature probe 34. With the aid of the two temperature probes 34 and 36, temperature fluctuations within the system can be identified and compensated by the analysis unit 24. The control unit 26 comprises a current or voltage source (not shown) with which the coil 28 is supplied with energy. Application of a suitable voltage causes a current to flow in the coil 28. The flow of current in the coil induces the magnetic field 38. Provided that the stud 14 was arranged within the coil opening 32 beforehand, the magnetic field 38 induces eddy currents within the (electrically conductive) stud 14, since the stud is penetrated by the magnetic field 38. The eddy currents within the stud, in turn, generate the desired heat (on account of friction). The energy that is required to generate heat is thus drawn from the magnetic field 38 of the coil 28. The heat thus generated diffuses toward the workpiece 16 in order to establish thermal equilibrium with the surroundings. Such a heat flow is schematically indicated by an arrow 40. The heat can of course only flow between the stud 14 and the workpiece 16 where the two have established a preferably material-to-material connection with one another. In the ideal case, the stud 14 is joined to the workpiece 16 with full material- to-material contact. In this case, the heat distribution is homogeneous. Figs. 2a and 2b illustrate such a homogeneous heat distribution 48 in a black and white representation. The heat distribution is preferably displayed in color to make it possible to better differentiate between different temperature levels. The temperature levels can be determined from the temperature scale 50 shown on the right in Figs 2a and 2b. Figs. 2a and 2b each show an image from the infrared camera 18 along the line ll-ll from Fig. 1. The representations in Figs. 2a and 2b thus show images as they are obtained by means of the infrared camera 18. In the heat distribution 48 in Fig. 2b, an approximately circular, bright center 52 can be seen. This center 52 corresponds approximately to the contact area of the stud 14 with the workpiece 16. Since the connection shown in Fig. 2b has been made with full material-to-mate ial contact, a homogeneous heat distribution establishes itself. The temperature level decreases radially toward the outside. The reason for this is that preferably only the stud 14 is inductively heated. An entirely different situation presents itself, however, when the connection between the stud 14 and the workpiece 16 has not been made with full material-to- material contact. This is the case, for example, when air inclusions have occurred during the process of melting the workpieces (i.e., during joining). At the places where air inclusions have occurred, heat is transferred more poorly. Such a situation is shown in Fig. 2a. It can clearly be seen in Fig. 2a that the bright center 52 in the heat distribution 48 has discrete darker areas 54. These darker areas indicate a lower temperature. A reduced flow of heat thus also occurs in these locations. Fig. 2a thus represents a qualitatively bad connection. Fig. 3 shows an alternative embodiment of a heating device 12 according to the present invention. In addition to the coil 28, a field former or yoke 60 of a preferably ferromagnetic material is provided in this embodiment. The inductor shown in Fig. 3 generates a magnetic field 38 that is preferably oriented perpendicular to the longitudinal axis A of the stud 14. The yoke 60 likewise has an opening 32 into which the stud 14 can be introduced. In contrast to the inductor of Fig. 1 , the inductor in Fig. 3 only excites eddy currents in the stud 14. The desired heat flow 40 occurs nonetheless. The provision of the yoke 60 has multiple advantages. Firstly, only the stud 14 is inductively heated. The magnetic energy of the magnetic field 38 is essentially concentrated between the legs of the yoke 60. In this way, the energy yield or conversion can be made still more effective, since essentially only the stud 14 is inductively heated. The magnetic field 38 outside the opening 32 is negligibly small with respect to the capability of inductively heating. Needless to say, the stud 14 represents merely an example embodiment of a first workpiece that is joined or welded to the second workpiece 16. The stud 14 described up to now is rotationally symmetric in design and has an elongated stud shaft. However, the shape of the first workpiece can also be of any other desired design. This may potentially make it necessary to change the shape of the yoke 60. This is in particular the case when the shape of the workpieces to be joined together is complex. The shape of the yoke 60 shown in Fig. 3 is only by way of example, however. Any other desired shape is possible. Fig. 4 shows a flowchart of the method according to the present invention. First the workpieces to be joined are joined together, especially by welding (step S1 ). Next, an inductor is brought into the vicinity of the joint to be tested in order to inductively heat at least one of the workpieces (step S2). After the inductive heating, a heat distribution is optically measured by means of the infrared camera 18 shown in Fig. 1. The data thus obtained can then be analyzed by the control and analysis unit 24, 26 shown in Fig. 1 (step S3). It is a matter of course that the inductor can be built in many ways. A nonuniform distribution of the magnetic field lines can be advantageous, since this results in an introduction of heat that is largely concentrated on one of the workpieces and thus generates the desired heat flow. Th e energy for inductive heating is preferably provided with alternating currents having frequencies greater than 1 kHz. The alternating current can have different waveforms. A sinusoidal alternating current is preferably used. In this way, a controlled, very finely metered flow of heat can be reproducibly introduced into the joint and can generate temperature distributions of preferably 50 *€ to 500 °C on the back of the workpiece. Other temperatures are possible, however. The temperature distribution on- the back of the workpiece, i.e., precisely opposite the joint location, is measured and visualized by the infrared camera. The outer contour of the joint zone is clearly differentiated from the neighboring regions by color (shown only in black and white in Figs. 2a and 2b). The areas of the defect locations within the joint zone differ clearly in color from the areas which represent the material-to-material connection areas of the joint zone. The analysis is generally performed through an integral calculation of the different proportional areas in the joint zone. The ratio of areas of the defect locations to areas of the material-to-material connection can be represented as a percentage. Moreover, the position of defect locations can also be analyzed if it is critically important for the dynamic quality of a connection whether a defect is located at the edge or in the middle of the joint zone. These analyses can be performed for each individual joint in an automatic system, for example in a production line. It is also possible to test only statistically selected joints. Depending on the test result, the joint can be reworked at another location on the production line. In conclusion, it is noted that the present invention makes possible nondestructive, nondamaging testing of fastened joints, in particular stud welded joints. The analysis of joint quality can be performed with computer support. The control and analysis unit 24, 26 can be provided as a single device, or, as shown in Fig. 1 , in modular form.

Claims

Method for nondestructive testing of the joint quality of fastened workpiece joints comprising the following steps: joining (S1 ) a first workpiece (14) to a second workpiece (16); heating (S2) the first workpiece (14) that is joined to the second workpiece (16), in particular to a temperature of 50 ° C - 500 ° C; and optical measurement (S3) of a heat distribution (48) in a region where the first workpiece (14) is joined to the second workpiece (16); characterized in that the heating (S2) is accomplished by generating a magnetic field (38) that penetrates the first workpiece (14).

2. Method according to claim 1 , characterized in that the magnetic field (38) is generated by the application of a voltage, in particular an alternating-current voltage of preferably at least 1 kHz.

3. Method according to claim 1 or 2, characterized in that the second workpiece (16) is also penetrated by the magnetic field (38) during heating.

4. Method according to one of claims 1 - 3, characterized in that the heating occurs after the joining with respect to time.

5. Method according to one of claims 1 - 4, characterized in that the joining is accomplished through welding, particularly arc welding.

6. Method according to one of claims 1 - 5, characterized in that, in order to generate the magnetic field (38), an inductor (28) is brought into the vicinity of the first workpiece (14) such that the magnetic field (38) produces eddy currents in the first workpiece (14).

7. Apparatus (10) for nondestructive testing of the joint quality of fastened workpiece joints having a device (12) for heating a first workpiece (14) that is joined to a second workpiece (16), and a device (18) for optical measurement of a heat distribution (48), characterized in that the device (12) for heating is an inductor (28) which is capable of producing a magnetic field (38) that acts on the first workpiece.

8. Apparatus according to claim 7, characterized in that the inductor (28) is arranged on a side of the second workpiece (16), which is joined to the first workpiece (14), where the first workpiece (14) is located, and the device (18) for optical measurement is arranged on a side opposite this side.

9. Apparatus according to claim 7 or 8, characterized in that a control and analysis device (24, 26) is provided, which is coupled to the inductor (28) and the device (18) for optical measurement.

10. Apparatus according to one of claims 7 - 9, characterized in that the device (18) for optical measurement is an infrared camera (18), which in particular works with wavelengths in a range from 8 μm - 14 μm or 1 μm - 2 μm.

11. Apparatus according to one of claims 7 - 10, characterized in that the inductor (28) is connected to a voltage source (26), which in particular provides an AC voltage of at least 1 kHz, wherein the AC voltage can in particular be a sinusoidal voltage, a triangular wave voltage, or a square wave voltage.

12. Apparatus according to one of claims 7 - 11, characterized in that the inductor (28) is a circular coil (28), which can be arranged essentially coaxial to a longitudinal axis (A) of the first workpiece (14).

13. Apparatus according to one of claims 7 - 11 , characterized in that the inductor (28) is composed of a ferromagnetic material, in particular which is designed in the shape of a yoke.

14. Apparatus according to one of claims 7 - 13, characterized in that the inductor (28) is coupled with a temperature sensor (36).

15. Apparatus according to claim 14, characterized in that an additional temperature sensor (36) is provided to measure a temperature of the first workpiece (14).

16. Apparatus according to claim 9 and 15, characterized in that the temperature sensors (34, 36) are connected to the control and analysis device (24, 26).

17. Apparatus according to one of claims 7 - 16, characterized in that the first workpiece (14) is a stud (14).