Kim March 2007 LAYOUT:Layout 1 2/8/07 2:30 PM Page 71 WELDING RESEARCH Estimation of Weld Quality in High-Frequency Electric Resistance Welding with Image Processing An image analysis algorithm can estimate the weld quality by using the weld image data during high-frequency electric resistance welding BY D. KIM, T. KIM, Y. W. PARK, K. SUNG, M. KANG, C. KIM, C. LEE, AND S. RHEE ABSTRACT. In high-frequency electric duction welding method, an induction coil surface is compressed by the squeeze resistance welding (HF-ERW), the weld is used in inducing high-frequency current roller, excreting oxided material from the quality is determined by the welding phe- to generate heat. In high-frequency resis- melting surface to generate a weld. The nomenon in the welding spot proximity. tance welding, on the contrary, a contac- molten part of the pipe generated by the Methods to eliminate or minimize defects, tor is applied to the workpiece to directly resistance heat contains oxided material therefore, include real-time monitoring of provide the current (Ref. 1). High- on account of exposure to the air. There- the weld quality and problem solving as frequency induction welding is primarily fore, if the oxided material is not com- problems arise. It is possible to estimate used in joining small-diameter steel pipes, pletely excreted during compression, and weld quality qualitatively by using the weld while HF-ERW is used in joining large- oxided material remains in the welding image data during HF-ERW. A method to diameter steel pipes. part as an impurity, the weld quality be- predict the weld quality using image pro- The characteristic of HF-ERW is the re- comes degraded. cessing in the heating process of the HF- sistance generated through the use of high- Haga (Ref. 2) used a high-speed cam- ERW point is proposed. An algorithm, frequency currents dissimilar from direct or era to observe the melting and joining which predicts the weld quality by using a low-frequency currents. In high- frequency mechanism around the V-convergence vision sensor to obtain the image of the electric welding, the resistance heat is con- point. It was found that even when me- welding spot proximity thus best express- centrated on the welding surface due to the chanic pressure was not applied to each ing the welding phenomenon, is devel- skin effect wherein the current is concen- section, the molten metal is excreted with oped. The heated area, which shows the trated on the conductor surface on account the oxided material due to the electro- image data with the highest correlation to of the high frequency, and the proximity ef- magnetic force. Also, the HF-ERW phe- the weld quality and the smallest amount fect wherein the current is concentrated on nomenon is classified into three types ac- of noise, is consequently calculated. the surface of two conductors as they ap- cording to the length and shape of the proach each other. The area affected by narrow gap. The first type of welding phe- Introduction heat can, therefore, be decreased and the nomenon is generated under low heat welding speed can be increased, producing input conditions when the approach speed In high-frequency electric resistance a welding method with superior productiv- exceeds the melting speed, a narrow gap is welding (HF-ERW), a high-frequency ity over the preceding methods. not formed, and the welding point is current in the range of 300~1000 kHz is Figure 1 shows an example of the direct formed near the V-convergence point. applied to the welding area, and squeeze contact method. The current is concen- The second type of welding phenomenon force is added to the workpiece heated trated on the surface of each side of the is generated under optimal heat input con- from the heat resistance. The HF-ERW pipe due to the skin and proximity effects ditions when the melting speed is equal to method is classified into the high- when a high-frequency current is applied the approach speed, the gap is generated, frequency induction method and the high- to the workpiece through a contactor. The and the welding point is formed at the end frequency resistance welding method, de- resistance heat generated by the current of the narrow gap thus producing the most pending on the method employed in the causes partial melting, and the melting favorable weld. The third type of welding application of high-frequency current on phenomenon is generated under high heat the workpiece. In the high-frequency in- input conditions when the melting speed exceeds the approach speed, the gap is too KEYWORDS large, and a momentary short circuit oc- D. KIM is with Joining & Coating Technology, curs before the welding point is formed. Shipbuilding & Plant R & D Institute, Samsung Haga (Ref. 3) has categorized weld de- Heavy Industries Co., Geoje-si, Korea. T. KIM, Y. High-Frequency Electric W. PARK, K. SUNG, and S. RHEE are with the Resistance Welding (HF-ERW) fects into cold weld and penetrator. The Department of Mechanical Engineering, Hanyang Weld Quality cold weld is a defect generated during the University, Seoul, Korea. M. KANG is with Ad- Image Processing first type of welding phenomenon when vanced Welding & Joining Research Team, the welding heat input is smaller than the KITECH Incheon, Korea. C. KIM is with Joining Heated Width Heated Area optimal heat input, while the penetrator is Research Group, POSCO, Pohang, Korea. C. a defect generated during the third type of LEE is with the Department of Mechanical Engi- welding phenomenon when the welding neering, Seoul National University of Technology, Seoul, Korea. heat input is larger than the optimal heat WELDING JOURNAL 71-s Kim March 2007 LAYOUT:Layout 1 2/8/07 2:31 PM Page 72 WELDING RESEARCH Fig. 1 — Schematic diagram of high-frequency electric resistance welding A system. input. External variables frequently affect perature sensor is used in the weld quality even when welding condi- this study to calculate and tions are preset and welding is performed monitor the two-dimen- to produce favorable weld quality without sional temperature distribu- defects. Real-time monitoring and control tion near the V-convergence of the welding process, thus, are necessary point. The data controlling in order to prevent weld defects and effi- the heat input in this ciently manage the production procedure. method, however, use the Based upon the results of Haga’s research temperature distribution (Ref. 3) identifying the least weld defects from only a particular line, in the second type of welding phenome- and thus the method does non and the most weld defects in the heat not present a solution to input conditions of the first and third types overcome the shortcomings of welding phenomena, Watanabe et al. found in the studies of Mi- (Ref. 4) have used the correlation between hara et al. (Ref. 5). the three types of welding phenomena To date, the research re- B found in HF-ERW and the frequency of sults regarding high- the welding currents to monitor the weld- frequency resistance weld- ing frequency and to control the heat input ing have shown that the in welding. electric resistance welding Mihara et al. (Ref. 5) have used a one- (ERW) weld quality is dimensional photodiode to calculate the closely correlated with the heated width of the welding part to control melting phenomenon the heat input, where the photodiode is set caused by the resistance to calculate the welding part 30 mm be- heat near the welding point. hind the V-convergence point. The heated The data calculated from width of the welding part is calculated, and the welding phenomenon of a feedback control system is established to ERW, however, are not maintain the value at a constant level. This highly reliable, and the method, however, retains a problem that ERW welding phenome- the changing V-convergence point accord- non, furthermore, fails to ing to the welding conditions is excluded actively respond to the from consideration, and heated width cal- changing V-convergence culated from a single point shows a high point. In order to overcome level of change according to the time thus such problems in this study, C undermining the correlation with the ac- active responses are made tual weld quality. In addition, the weld to the changing V-conver- Fig. 2 — Image of welding phenomena captured by a high-speed quality is determined by the welding gence point in HF-ERW, camera. A — Low heat input; B — optimal heat input; and C — mechanism near the welding point rather and a welding phenomenon high heat input. than the melting phenomenon behind the value highly correlated to welding point in HF-ERW. The data cal- the weld quality is obtained culated behind the welding point are, with a method to estimate the real time weld Raw Image Acquisition System therefore, not closely correlated to the quality proposed. Electric resistance weld- weld quality. Tatsuwaki et al. (Ref. 6) have ing image data taken with a high-speed A high-speed camera was used in order formulated a system using a two- camera are used in this study in order to to observe the melting phenomenon in dimensional pattern thermometer to con- develop an algorithm, which obtains and HF-ERW with the filming speed set at trol the heat input by calculating the two- calculates data showing the highest corre- 1000 fps to capture the welding phenome- dimensional temperature distribution of lation between the weld quality and weld- non around the V-convergence point. The the welding part. A two-dimensional tem- ing phenomenon.