Resistance spot welding control based on the temperature measurement

Transcription

1 based on the temperature measurement Primož Podržaj* and Samo Simončič In this paper a novel approach for temperature measurement during resistance spot welding is presented. The temperature is measured by two thermocouples mounted at the ends of both the electrode tips. The authors chose to mount them there because it was expected that it would be possible to measure the temperature at this point using a digital camera in the near future. The research was, therefore, motivated to obtain various pieces of information about the welding process from the measured temperature profiles. The measured temperature profiles showed a good correlation with the weld strength. Other phenomena, such as expulsion and electrode wear, can also be related to a temperature profile. Keywords: Resistance spot welding, Temperature measurement, Electrode wear Introduction Resistance welding (RW) can be defined as a group of welding processes that produce coalescence of the faying surfaces using the heat obtained from the resistance of the workpieces to the flow of electrical current in a circuit, in which the workpieces are a part of, and by the application of pressure. 1 There are several different versions of RW, but the most widely used is resistance spot welding (RSW). Resistance spot welding is one of the major welding technologies used in the appliance, electric and aviation industries. The automotive industry, however, is the major user. Resistance spot welding is widely used in an automotive body assembly. There are thousands of spot welds on an automobile body. 2,3 As their properties significantly affect the durability and crashworthiness of the vehicle, 4 improving their quality is an ongoing process in RSW research The most common approach for ensuring the weld quality is the selection of the appropriate welding parameters (welding current, welding time, welding force, etc.). There are however two problems associated with this kind of approach. 1 The first one is the fact that there is an ever increasing number of different materials being used in RSW. These materials can be combined (one sheet of one material and one sheet of another). If different materials and the varying geometry of the electrodes, different welding machine types, etc., are added together, an enormous number of welding schedules are needed to cover all the possible combinations. Although there is a lot of research going on in order to improve welding schedules, this type of approach to welding will always encounter the second problem. The welding schedule approach namely supposes that there are no disturbances during the Faculty of Mechanical Engineering, University of Ljubljana, Askerceva 6, 1000 Ljubljana, Slovenia *Corresponding author, primoz.podrzaj@fs.uni-lj.si welding process. Such a supposition is never exactly true. The level of some disturbances can be limited (by cleaning the surface, for example), but this is time consuming and expensive. As an alternative, feedback control was introduced to RW. Feedback control systems, as the name suggests, need a feedback signal in order to operate. An appropriate feedback signal should be relatively easy to measure on one hand and very well correlated with the weld quality on the other. Although weld quality is not a physical quantity and sometimes visual appearance of the spot welds is important as well, in general weld strength is the physical variable associated with the weld quality. The weld strength is primarily determined by the size of the welding nugget, which starts to form at the contact of the two workpieces and then grows outwards during the welding process. The majority of the feedback control systems are therefore based on the signals which are associated with the welding nugget growth and/or its size. The most commonly used signals are: N welding voltage, welding current and dynamic resistance welding force 15 electrode displacement ultrasound transmission N 21 acoustic and sonic emission There are also control systems which use information of several feedback signals. 25 Contrary to expectations temperature is one of the less researched feedback signals. The first attempts to use temperature dates back to The authors used the infrared emission in order to evaluate the surface temperature. The problem is however that special care must be taken in order to ensure constant surface emissivity and even when this is ensured, the dirt and fumes produced during welding can corrupt the signal. The focus has therefore shifted to systems that use thermoelectric voltage in order to measure the temperature. The easiest way of using the thermoelectric voltage ß 2013 Institute of Materials, Minerals and Mining Published by Maney on behalf of the Institute Received 18 March 2013; accepted 25 April 2013 DOI / Y Science and Technology of Welding and Joining 2013 VOL 18 NO 7 551

2 1 A schematic representation of temperature distribution during RSW is to use the electrode and the workpiece as a thermocouple. 27 The main disadvantage of this kind of approach is the fact that temperature can be measured only after the welding stops (during the cooling phase). Another problem appears because electrode and workpiece material are of course not selected from the thermoelectric point of view, so there are many nonlinearities present. 27 In this paper a thermocouple based measurement of RSW temperature is presented. Using thermocouples, temperature can be measured during the welding process as well. The materials used in thermocouples are optimised from the thermoelectric voltage point of view, so the temperature measurement is relatively easy. The main drawback is that the thermocouple has to be mounted somewhere, which is very inconvenient in a production environment. The authors decided to put it on the electrode tip. The main reason for this decision was the possibility of measuring the temperature at this spot using a digital camera. A digital camera was already used to measure electrode displacement using the same area of interest. 28 The thermal expansion of the electrode tips deforms the surface of the electrode tips and by observing the surface deformation it might be possible to measure the temperature. 29 Theoretical Background At the beginning of the welding process, the upper electrode moves toward the lower one so that the workpieces are pressed together with a certain welding force. After a predetermined time interval welding voltage is applied and as a consequence an electric current flows in a secondary circuit. The time and spatial distribution of the conversion of electrical energy into heat depends on the contact and bulk resistances in this circuit. At the beginning, the contact resistance between the workpieces is the major part of the total resistance. This implies that the most intense heat generation occurs at this spot and consequently a welding nugget formation starts here (see Fig. 1). The total amount of the generated heat can be calculated according to the following equation ðt w Q~ 0 i w ðþ t 2 Rt ðþdt (1) 2 Heat flows during RSW Despite the fact that due to losses only a smaller part of the total generated heat can be associated with welding nugget growth, equation (1) is usually the starting point for the RSW control. This is due to the fact that welding nugget size is very well correlated with the weld strength. The welding time t w and the welding current i w can be controlled precisely, whereas the resistance R(t) varies. As a consequence it is difficult to control the total amount of generated heat. As heat is correlated with temperature, by measuring the temperature signal the authors expected to obtain some information about the generated heat. The main problem with this approach is the fact that temperature at the contact of the two workpieces is very difficult if not impossible to measure. Besides that the temperature varies with position. As already mentioned in the Introduction, the authors decided to measure the temperature at the edge of the electrode tips because the authors expected that it would be possible to measure the temperature at this position using a digital camera as well. The temperature at this spot is the consequence of several heat flows (see Fig. 2). First of all heat is generated (P g ) because the electric current flows through the electrode tips. This heat flow stops when the welding current stops. There is also a heat flow from the much hotter welding nugget to the electrode tips (P in ). This flow has some time delay and continues to flow well after the welding current stops. Then there are heat losses from electrode tips to the surrounding air (P la ) and heat losses due to cooling water flow (P lw ). So for some volume of material around the thermocouple, the following equation could be written dt P g zp in {P la {P lw ~mc p (2) dt where m is the mass of the material being observed, c p its specific heat and T the temperature of this volume of material. Measurement Set-up The experiment was performed with a Gorenje Varstroj TIP VST 40 welding machine used in a combination with a Matuschek AutoSPATZM600L Medium Frequency Inverter Power Source. Science and Technology of Welding and Joining 2013 VOL 18 NO 7 552

3 3 Measurement set-up Electrode tips are a type B caps according to DIN (d mm, d mm, l mm, l mm, spherical contact surface with a radius of R575 mm). The workpieces were 2 mm thick steel sheets. The authors used three different types of materials: N 1?4462 duplex steel N 1?4016 ferrite steel N 1?4571 austenite steel. All the experiments were made with a welding time t w 5350 ms and a welding force F w N. The schematic representation of the measurement setup is shown in Fig. 3. The temperature was measured on the inner edges of both electrode tips using a K type thermocouple with a wire diameter of only 0?2 mm. The thermocouples were connected to a National Instruments NI 9213 thermocouple input module. The module has a built in coldjunction compensation. In a high-speed mode it is capable of measuring temperature with a sample rate of up to 1?2 khz. The National Instruments NI USB-6341 data acquisition card with a sampling rate of 500 khz was used to obtain the trigger signal. The thermocouple input module and the data acquisition card were both connected to a PC where the data acquisition program was executed within the LabView environment. The weld strength was measured using 120 mm6 40 mm specimens. They were tested on a ZWICK ROELL Z050 machine using tensile-shear testing. Results A typical temperature profile at the edge of the electrode tips is shown in Fig. 4. The welding current starts to flow 0?2 s after the trigger signal is obtained. Because the welding current flows for 350 ms, there is no heat generation (P g 50 W) after 0?55 s. The peak temperature is however measured at approximately 0?9 s. Therefore it can be concluded that the heat flow from the welding nugget to the electrode tip (P in ) has a major influence. The amount of the available heat from the welding nugget depends primarily on its size. As size increases with increasing welding current, it is expected that the measured temperature peaks will be higher, when welding is done using higher welding currents. This is confirmed by the temperature profiles shown in Fig. 5. There is a good correlation between peak temperature and welding current. An interesting phenomenon can be observed if the welding current range is extended to the expulsion region (see Fig. 6). A welding current of 4?5 ka is the last one without expulsion. It is nevertheless included in Fig. 6 for reference. There is a drop in peak temperature between 4?5 ka (no expulsion) welds and 5 ka welds (with expulsion). This can be attributed to the fact that 4 Temperature profiles for several welds made using duplex steel workpieces with a welding current of 5 ka expulsion is a process of removal of some molten material from the welding region. As a consequence there is less material which needs to be cooled. Therefore the heat flow P in is smaller. As the welding current increases further, the peak temperature increases as well. The authors assume that weld strength depends primarily on welding nugget size, a plot of weld strength versus peak temperature for different welding currents was made (see Fig. 7) A good correlation between weld strength and peak temperature can be observed. The only deviation from this trend occurs, when expulsion occurs (welding current of 5 ka). The results agree with the previous research. 30 If welding current is namely set just above the value that causes initial expulsion, the expulsion may occur toward the end of the weld time with little subsequent current flow. At higher welding current levels, expulsion tends to occur earlier in the welding period with current flowing during the remaining weld time. This effect is believed to cause the weld nugget to further increase in size. 30 As an example of a comparison of temperature profiles of two different materials, spot welds where one sheet is made of 1?4016 ferrite steel and the other of 1?4571 austenite steel were made. The results are shown in Fig. 8. The specific heat of the ferrite steel is 460 J/kg K 31 and in the case of austenite steel it is 500 J/kg K. 32 The thermal conductivity of the ferrite steel is 25 W/mK 31 and in the case of austenite steel it is 15 W/mK. 32 These values are important in explaining the peak temperature and the cooling rate of both materials. It can be observed that in general a higher peak temperature is reached at the electrode tip in contact with the ferrite steel. As the difference in specific heats of the two materials is not so big, the authors think that this is due to a much higher thermal conductivity of ferrite steel, because the heat flow from the specimen to the electrode tip (P in in Fig. 2) is related to it. It can also be seen, that ferrite steel cools down faster despite having a higher peak temperature. This can also be attributed to a much higher thermal conductivity of ferrite steel. Consequently during the cooling phase there is a more intense heat flow through a specimen itself and a less intensive heat flow to the electrode tips. Temperature profiles at different stages at electrode wear have been compared. A comparison between Science and Technology of Welding and Joining 2013 VOL 18 NO 7 553

4 5 Temperature profiles for different welding currents (duplex steel and welds without expulsion) 6 Temperature profiles for different welding currents (duplex steel and welds with expulsion) Science and Technology of Welding and Joining 2013 VOL 18 NO 7 554

5 7 Weld strength versus electrode peak temperature for 1?4462 duplex steel 8 Comparison of temperature profiles for ferrite and austenite steel Science and Technology of Welding and Joining 2013 VOL 18 NO 7 555

6 9 Temperature profiles versus electrode wear for austenite steel temperature profiles for a new electrode (after 30 spot welds) and a worn out electrode (after 1000 spot welds) is shown in Fig. 9. It is well known that welding current has to be increased due to electrode wear at certain stages of electrode life. Therefore it is no surprise, that lower peak temperatures were observed when worn out electrode tips were used. As the electrode wear is in general associated with the enlargement of the contact area between the electrode tips and the specimens, the authors expected a more intense heat flow from the specimen to the electrode tip (P in in Fig. 2) during the cooling phase, when a worn out pair of electrode tips is used. This assumption also turned out to be true as seen in Fig. 9. Conclusion Despite its very good correlation with the welding process, temperature is one of the less researched variables related to the RSW because it is considered too cumbersome to measure it in a production environment. Recent advances in the application of digital camera during RSW however present an opportunity to measure the temperature with a noncontact method which is well suited for a production environment. This method can be based on the measurement of the electrode tip deformation. This is the main reason why the authors have chosen to mount the thermocouple at the end of the electrode tip, although it might not be the best point from a thermal analysis point of view. The research presented in this paper has shown that electrode tip temperature is one of the RW process variables, which can give a lot of information about the process. It has been shown that its peak values are well correlated with weld strength. Besides that, phenomena such as expulsion and electrode wear can be observed on the measured temperature profiles. When spot welding of two different materials is performed, all of the most commonly measured variables (welding current and voltage, dynamic resistance, electrode displacement, welding force and ultrasound transmission) are unable to give data specific to a certain material. Temperature measurement at both the electrode tips was clearly able to give temperature profiles for a specific material in this combination. References 1. P. Podržaj, I. Polajnar, J. Diaci and Z. Kariž: Sci. Technol. Weld. Join., 2008, 13, Y. Luo, J. Liu, H. Xu, C. Xiong and L. Liu: Mater. Des., 2009, 30, M. Goodarzi, S. P. H. Marashi and M. Pouranvari: J. Mater. Process. Technol., 2009, 209, X. Sun, E. V. Stephens and M. A. Khaleel: Eng. Fail. Anal., 2008, 15, J. Jun and S. Rhee: Sci. Technol. Weld. Join., 2012, 17, G. S. Jung, K. Y. Lee, H. K. D. H. Bhadeshia and D. W. Suh: Sci. Technol. Weld. Join., 2012, 17, D. S. Safanama, S. P. H. Marashi and M. Pouranvari: Sci. Technol. Weld. Join., 2012, 17, E. Tolf, J. Hedegard and A. Melander: Sci. Technol. Weld. Join., 2013, 18, A. R. Jahandideh, M. Hamedi, S. A. Mansourzadeh and A. Rahi: Sci. Technol. Weld. Join., 2011, 16, P. Zhang, J. Xie, Y. X. Wang and J. Q. Chen: Sci. Technol. Weld. Join., 2011, 16, R. R. Patil, C. J. K. Anurag Tilak, V. Srivastava and A. De: Sci. Technol. Weld. Join., 2011, 16, Science and Technology of Welding and Joining 2013 VOL 18 NO 7 556

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