η = R Rn Yansong ZHANG, Jie SHEN and Xinmin LAI

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1 , pp Influence of Electrode Force on Weld Expulsion in Resistance Spot Welding of Dual Phase Steel with Initial Gap Using Simulation and Experimental Method Yansong ZHANG, Jie SHEN and Xinmin LAI Shanghai Key Lab for Digital Auto-body Engineering, Shanghai Jiao Tong University, Shanghai, China. (Received on September 9, 2011; accepted on October 25, 2011) By increasing the electrode force, it would be an alternative way to avoid weld expulsion in resistance spot welding (RSW) of dual phase (DP) steel with initial gap. To investigate the influence of the electrode force on weld expulsion, a weld expulsion estimation method based on a finite element model were used to analyze the contact state, electrical field, temperature distribution and weld nugget formation process. The simulation results were validated by the experiments. It was shown that a great electrode force could be helpful to inhibit weld expulsion by improve the contact state between sheets. For 0.8 mm DP600 steel with 1.6 mm gap spacing, the minimum electrode force without expulsion is about 2.5 kn. For DP600 steels with different thickness from 0.8 to 1.8 mm, minimum electrode forces have been determined to inhibit the weld expulsion with different gap spacing to improve the weld quality of RSW. KEY WORDS: electrode force; Initial gap; weld expulsion; resistance spot welding; dual phase steel. 1. Introduction Dual phase (DP) steel has been widely used in the automotive industry to reduce weight and improve safety due to its good mechanical properties. Resistance spot welding (RSW) is still the first choice to join the DP steel in autobody assembly line because of the high efficiency and low cost. However, the weld expulsion in RSW of DP steel is a serious problem in the auto-body manufacturing. The reason is that DP steel has high resistivity, which would result in more joule heat in the nugget area during the welding process. 1) Pouranvari reported that weld expulsion would decrease the energy absorption capability of spot welds, which was attributed to the change of failure location due to excessive electrode indentation. 2) Regarding the importance of weld expulsion, many methods, such as neural network method and dynamic resistance method, have been developed for the detection and monitoring of expulsion in RSW. 3 6) Besides the resistivity of DP steel, the initial gap between sheets during the RSW process would also result in weld expulsion. The occurrence of the initial gap is due to the stamping springback, unexpected deformation of parts and assembling error in the auto-body assembly line. Therefore, it is necessary to study the effect of welding parameters on weld expulsion in resistance spot welding of DP steel with the initial gap. Some researchers have proved that the initial gap between sheets has great effect on the weld expulsion by changing the contact state between sheets and electrode. 7 9) To reduce the occurrence of weld expulsion, the adjustment of the welding current is proved to be a feasible way when spot welding of mild steel sheet with the initial gap. 10,11) However, it would not be feasible in RSW of DP steel due to the narrow weld lobe compared to that of mild steel. Therefore, an alternative method needs to be proposed for avoiding the weld expulsion in RSW process of DP steel with the initial gap. Electrode force, as an important welding parameter, could determine the contact state and dynamic resistance between sheets ) In current, there was little research on the effect of electrode force on weld expulsion in RSW of dual phase steel with initial gap. In our preliminary study, an estimation method is proposed to predict the weld expulsion when spot welding of dual phase steel with initial gap. 9) The method is focused on using the ratio of R n and R c, which is defined as η, to estimate the occurrence of weld expulsion (Eq. (1)). η = R Rn... (1) In Eq. (1), R n is the weld nugget radius and R c is the contact radius between sheets (Fig. 1). The weld nugget radius R n begins during the welding stage and would grow with the welding time. The contact radius R c forms by the electrode force during the squeezing stage, and it would exist during the whole welding and holding stage. η should be smaller than 1.0, otherwise the weld expulsion would happen. In this study, a weld expulsion estimation method based on a finite element model was inherited to research the influence of the electrode force on the weld nugget formation process and weld expulsion from our preliminary work. The contact state, electrical field and temperature distribution with different electrode force were studied to prove the c ISIJ

2 Table 1. Welding parameters for simulation. Electrode diameter (mm) Electrode force (kn) Welding current (ka) Time (ms) Squeezing Welding Hold ~ Fig. 1. Estimation of weld expulsion occurrence. Fig. 3. Contact state between sheets and electrodes during the squeezing stage. DP600. The initial gap spacing between sheets is 1.6 mm, which would make the weld expulsion happen. 9) The welding parameters for the specimen are listed in Table 1. The electrode force is selected from 2.2 kn to 4.4 kn to study the influence of the electrode force on the weld nugget formation and weld expulsion. Fig. 2. Finite element model and dimensions. effectiveness to inhibit the weld expulsion. Finally, the simulation results were validated by the experiments. The minimum electrode force for DP steels with different thickness and initial gap spacing was presented to avoid the occurrence of weld expulsion for auto-body manufacturing. 2. Finite Element Analysis of Influence of Electrode Force on Weld Expulsion with Initial Gap 2.1. Finite Element Model for RSW with Initial Gap In order to study the influence of electrode force on weld expulsion, a 3-D coupled axisymmetric finite element model for RSW with the initial gap between two sheets is built using the software ANSYS, shown in Fig. 2. The finite element model contains nodes and elements in order to guarantee the accuracy of simulation analysis. The main geometrical dimensions of the model are also shown in Fig. 2. Other detailed information about the model, such as the boundary conditions and computational procedure were introduced in our preliminary study. 9) The steel sheet for calculation is 0.8 mm galvanized 2.2. Simulation Results and Discussion Contact Radius (R c) between Sheets During the squeezing stage, the two sheets with initial gap would become contacted under the effect of electrode force. The contact state between two sheets is basically determined by the electrode force. Figure 3 show a typical contact state between the sheets and electrodes during the squeezing stage in RSW with initial gap when the electrode force is 2.2 kn. It could be seen that the contact position between two sheets, where the contact pressure distributes, is not located around the central region, but a short distance from the central line. It means the contact state between two sheets is an annular distribution. So, the distance from the left side of the contact distribution is called as internal radius, while the distance from the right side of the contact pressure distribution is called as external radius. The contact radius between two sheets (R c) is the external radius minus the internal radius. Figure 4 shows the distribution of the contact pressure along the contact radius with different electrode force for 0.8 mm DP600 with initial gap of 1.6 mm. It could be found that no contact would exist in the central part (contact radius R c<0.3 mm) of the contact interface between sheets even if the electrode force increased. The distribution of the contact pressure is an annulus on the contact interface with about 0.5 mm and 2.0 mm as internal and external radius. It could increase the contact radius between two sheets by decreasing the internal radius and increasing the external radius of the annular contact area when a large electrode force is applied ISIJ 494

3 Fig. 4. Contact pressure distributions with different electrode forces. Fig. 6. Distribution of electrical field with different electrode forces: 2.2 kn vs 4.4 kn. Fig. 5. Contact radiuses with different electrode forces during welding. Fig. 7. Temperature histories and distributions with different electrode forces. After the squeezing cycle, the steel would be softened due to the temperature rise by the joule heat during welding. So, the contact radius between sheets would change during the welding process. Figure 5 shows the change process of contact radius under the effect of different electrode forces in this stage. It could be seen that the contact radius would increase quickly during the early period of welding (earlier than 100 ms). After that, the contact radius would be stable until the end of welding. And the stable contact radius with 4.4 kn electrode force is bigger than that with 2.2 kn. It is because that the softened steel sheet would be easy to deform with a large electrode force due to the decrease of yield strength. This large deformation by great electrode force would make the steels contact well and increase the final contact radius between sheets Weld Nugget Radius (R n) during Welding During the welding stage, a welding current 9.0 ka would be applied and an electrical field in the sheets and electrodes would form to heat the steel sheets. Figure 6 shows the comparison of the typical distribution of electrical field in the welding stage with different electrode force. When the electrode force is 2.2 kn, the current flows disorderly and non-uniform due to the bad contact state on the interfaces between sheets and electrodes. Consequently, the rising of temperature would become abnormal and weld expulsion would trend to happen. 10) When a large electrode force of 4.4 kn is applied, the distribution of electrical field become basically orderly in the sheets and electrodes, especially around the contact area between two sheets. It is due to the improvement of contact state between steel sheets and would help the sheet temperature rise steadily to get a good weld nugget. The comparison of temperature histories with different electrode forces and temperature distributions at the welding time of 80 ms are shown in Fig. 7. When the electrode force is 2.2 kn, the temperature of the steel sheet at 80 ms has already reach the melting point (1 500 C) and a big weld nugget (about 4.5 mm diameter) has formed. This is because that the concentration distribution of weld current in the small contact area could generate great joule heat to make the sheet temperature rise up quickly. When the temperature reaches C, the weld nugget would begin to form and grow up around the contact interface between sheets. Meanwhile, when the electrode force increase to 4.4 kn, the weld nugget just begin to form under the same welding condition. It is due to the fact that the uniform distribution of electrical field by a large electrode force would decrease the current density and make the sheet temperature rise up slowly. The different distribution of electric and temperature field would change the weld nugget formation process and finally get a different weld nugget size. Figure 8 shows the weld nugget formation process with different electrode force ISIJ

4 Table 2. Chemical composition (Wt%) of steels. Steel C Mn P S Si Al Ni Mo Cr DP600 (HDG 60) Fig. 8. Weld nugget radiuses with different electrode forces during welding. Fig. 10. Experimental setup for RSW with initial gap. Fig. 9. η curves with different electrode forces during welding. when the welding current is 9.0 ka and the initial gap is 1.6 mm. It could be found that the formation time and the final size of the weld nugget is changed by increasing the electrode force. When the electrode force is 2.2 kn, the formation time is less than 20 ms. After increasing the electrode force to 4.4 kn, the formation time will delay to 60 ms. This result would lead to the decrease of the time for the weld nugget grow-up. So the final weld nugget size would diminish a little for a large electrode force η Curves with Different Electrode Forces Based on the estimation method for the expulsion occurrence in Fig. 1, the η curves with different electrode forces during the welding stage are calculated with Eq. (1) in Fig. 9. It could be seen that all the curves go up with the welding time due to the growth of the weld nugget. The trends of the η curves are basically similar to the weld nugget formation process. The starting time of the curves are delayed as a result of the late formation of the weld nugget with a large electrode force. When the electrode force is 2.2 kn, the η value would exceed 1.0 at 160 ms and the weld expulsion would happen. After increasing the electrode force to 3.3 or 4.4 kn, the whole curve would be located below the baseline (η=1.0) and no expulsion would occur. It is because that a large electrode force could not only increase the contact radius (R c) on the sheet interface, but also decrease the weld Fig. 11. Dimensions of specimen for RSW with initial gap. nugget radius (R n) during the welding process. So the η curve, as the ratio between R n and R c during the welding process, would go down when the electrode force increase. 3. Determination of Minimum Electrode Force without Weld Expulsion by Experiments 3.1. Experimental Preparation To investigate the influence of electrode force on weld expulsion and weld quality of RSW with initial gap for dual phase steel, galvanized DP600 steels with thickness of 0.8, 1.4, 1.8 mm were used in this study. The coating thickness is about 60 um. Chemical composition of this material is listed in Table 2. The different gap spacing is generated by putting different clearance gauge between two sheets (Fig. 10). The value of the gap spacing changes from 0 to 2.0 mm. The distance between the two clearance gauges are 30 mm, a common value between two weld spots in auto-body manufacturing. The dimensions of specimen are shown in Fig. 11. The welding equipment is composed by a servo gun and 2012 ISIJ 496

5 medium frequency direct current (MFDC) welder. The character of the servo gun could be helpful for the accuracy of the electrode force and improve the experimental results. The welding parameters (welding time and welding current) for DP600 steels with different thickness are listed in Table 3 according to the standards of American welding society (AWS). 15) The weld quality including weld nugget size and weld expulsion could be measured by metallographic test and peel test Experimental Results and Discussion Validation for η Value and Weld Expulsion To validate the effectiveness of the expulsion estimation by the η value, some experiment has been conducted using 0.8 mm DP600 steel with 1.6 mm gap spacing (Fig. 12). It is generally agreed that the occurrence of expulsion would lead to the rapid decrease of the weld nugget diameter under the same welding condition. The experimental result showed that, when the electrode force is 2.2 kn, the expulsion happened and the weld nugget diameter was only about 4.2 mm. While the electrode force increased to 4.4 kn, the weld nugget diameter would increase to 4.92 mm without expulsion. This is agreed with the η value, which is more than 1.0 when the electrode force is 2.2 kn, but smaller than 1.0 when the electrode force increases to 2.75 kn or more. These results proved that the η value could be effective to estimation the occurrence of weld expulsion. The minimum electrode force to inhibit the weld expulsion could be calculated by the η value when it is equal to 1.0. For 0.8 mm Table 3. Material DP600 Welding parameters for DP600 steels with different thickness. Thickness (mm) Electrode force (kn) Welding time (ms) Welding current (ka) DP600 steel with 1.6 mm gap spacing, the minimum electrode force is about 2.5 kn Minimum Electrode Force without Weld Expulsion with Initial Gap The initial gap spacing would make the steel sheets contact bad so that the weld expulsion would trend to happen. Different gap spacing value would lead to different contact state between sheets. So, a corresponding electrode force would be required to inhibit the weld expulsion for different initial gaps. Figure 13 shows the minimum electrode force without expulsion for 0.8 mm DP600 steel with different initial gaps. When the initial gap spacing is 0.4 mm, the contact state between sheets would change little. So, no expulsion would happen even if the electrode force keeps as 2.2 kn. However, more and more electrode force would be necessary to inhibit the weld expulsion when the initial gap spacing exceeds 0.8 mm and goes up to 2.0 mm. It is due to the fact that the great initial gap spacing would decrease the contact area a lot and make the η value bigger than 1.0. A great electrode force could make the sheets deform enough to improve the contact state. A steady and big contact area would be helpful to the weld nugget formation and no expulsion would happen. Additionally, some experimental results have been shown to confirm the simulation results in Fig. 13. It proves that the simulation results agree well with the experiment results. Since the increase of the electrode force is effective to inhibit the weld expulsion, it is necessary to confirm the relationship between the minimum electrode force without expulsion and the gap spacing for steel sheets with different thickness. The results in Table 4 present that the minimum electrode force to inhibit the expulsion in RSW with initial gap depends on the gap spacing and the sheet thickness. As shown, more electrode force without expulsion would be needed for dual phase steels when the sheet thickness and gap spacing increase. It is due to the fact that the thick sheet, the big gap spacing and the strong mechanical property of dual phase steel would produce great stiffness of the weld joint when the initial gap exist. The steel sheets would be hard to deform under the effect of the common electrode force to generate good enough contact state between steel sheets. Thus, a strong enough electrode force might be nec- Fig. 12. Expulsion occurrence based on the η values and experimental results. Fig. 13. Minimum electrode forces without expulsion for different initial gaps ISIJ

6 Table 4. Gap spacing (mm) essary to conquer these disadvantages and inhibit the weld expulsion for good weld quality. 4. Conclusions Minimum electrode force for DP600 without expulsion with initial gap. Minimum electrode force (kn) 0.8 mm DP mm DP mm DP (1) The weld expulsion estimation method based on the ratio of the weld nugget radius (R n) and the contact radius between steel sheets (R c) is effective to predict the occurrence of weld expulsion during RSW process. The analysis results are validated by the experiments. (2) It is effective to inhibit the weld expulsion by increasing the electrode force in RSW with initial gap. When the initial gap spacing increases, more electrode force would be needed to improve the contact state between sheets. For 0.8 mm DP600 steel with 1.6 mm gap spacing, the minimum electrode force without expulsion is about 2.5 kn. (3) For DP600 steels with different thickness from 0.8 to 1.8 mm, minimum electrode forces have been determined to inhibit the weld expulsion with different gap spacing to improve the weld quality of RSW. Acknowledgements This research was sponsored by Shanghai Rising-Star Program (11QA ) and Project supported by National Natural Science Foundation of China and Project supported by National Natural Science Foundation of China. REFERENCES 1) S. Dancette, V. Massardier-Jourdan, D. Fabrègue, J. Merlin, T. Dupuy and M. Bouzekri: ISIJ Int., 51 (2011), 99. 2) M. Pouranvari, A. Abedi, P. Marashi and M. Goodarzi: Sci. Technol. Weld. Join., 13 (2008), 39. 3) D. Ulrich and D. Jorg: ISIJ Int., 39 (1999), ) J. Wen, C. S. Wang, G. C. Xu and X. Q. Zhang: ISIJ Int., 49 (2009), ) J. Senkara, H. Zhang and S. J. Hu: Weld. J., 83 (2004), ) C. Ma, S. D. Bhole, D. L. Chen, A. Lee, E. Biro and G. Boudreau: Sci. Technol. Weld. Join., 11 (2006), ) H. Murakawa and J. X. Zhang: Trans. JWRI, 27 (1998), 75. 8) J. X. Zhang and H. Murakawa: Trans. JWRI, 27 (1998), 73. 9) J. Shen, Y. S. Zhang and X. M. Lai: Sci. Technol. Weld. Join., 15 (2010), ) H. Murakawa, J. X. Zhang and H. Nimani: Trans. JWRI, 28 (1999), ) P. Podrzaj, I. Polajnar, J. Diaci and Z. Kariz: Sci. Technol. Weld. Join., 11 (2006), ) M. Shome and S. Chatterjee: ISIJ Int., 49 (2009), ) K. Furukawa, M. Katoh, K. Nishio and T. Yamaguchi: Q. J. Jpn. Weld. Soc., 24 (2006), 3. 14) B. H. Chang and Y. Zhou: J. Mater. Process Technol., 139 (2003), ) American Welding Society: Recommended practices for test methods for evaluating the resistance spot welding behavior of automotive sheet steel materials, ANSI/AWS/SAE/D8.9 97, AWS, Miami, ML, USA, (1997) ISIJ 498