higher than 1200°C. The minimum threshold value remains constant in spite of the coated materials, as shown in Figure 11(b), while splash occurs when the estimated weld temperature exceeds 1400°C. The upper threshold value is lower than that shown in Figure 9(b) for organic zinc coated steels. This suggests that the surface condition can influence the upper threshold temperature of the critical splash condition. Alternatively, the normalized weld diameter can be estimated by the dynamic resistance curve calculated with the welding voltage and welding current because the contact diameter de is related to the dynamic resistance based on Equation 4. This alternative is especially effective on the low current and long weld time conditions like the class C conditions described on the RWMA (Resistance Welding Manufacturing Association) table. Table 3: Threshold Value for the Critical Splash Condition vs. the Stack Configuration *Threshold values were calculated based on the value for the 0.8+0.8 case The variation of the critical splash current values depend on the stack condition of workpieces can be explained by the temperature distribution along the plate thickness direction during welding. If the analytical solution can be applied for estimation of the temperature distribution, the peak temperature at the faying surface of each stack condition can be estimated with the experimental result on a two stack condition, and faying interface position of the stack. Estimated results of critical splash current for each stack are shown in the middle column of Table 3. The values were calculated based on the measured value under two sheet stack of 0.8 mm +0.8 mm. Temperatures in the left column in Table 3 show the measured results in each stack conditions. Each corresponding values closely coincide with each other. This suggests that if the information on the stack configuration of workpieces, the accuracy of threshold value for each critical splash condition expressed as the temperature can be improved. CONCLUSION A new monitoring and control system was developed based on the energy balance model in a weld part during resistance spot welding. Mathematical analysis deduced a new governing equation and procedure. The new concept, described in integral form, was proved with prior data and experimental results. This concept realizes a new adaptive control system that not only predicts splash occurrences in real time but also estimates weld diameter. Important conclusions of the present research are listed below: 1) A new governing equation of integral form was developed based on an energy balance model in a weld part during welding. 2) A new discrete equation can be applied to real time prediction of weld diameter and splash with at least two monitoring data, i.e., welding voltage, welding current, and one pre-measured value of total plate thickness. 3) If the measured weld size is normalized by the square value of the electrode force, there is good agreement between the normalized weld diameter and the peak value of mean weld temperature. 4) The maximum weld diameter is closely related to the applied electrode force for welding. 5) Only the upper threshold for defining critical splash current is influenced by the surface condition of workpieces. The lower threshold for minimum weld diameter is influenced by physical properties of the bulk materials. 6) Threshold values for splash occurrence in various stack configuration condition can be estimated with information on stack configuration and position of faying interface in the stack based on a measuring result of the mean weld temperature for two stack condition of equal sheets.. ACKNOWLEDGEMENT The authors are very grateful for the support provided by Kawasaki Robotics (USA) Inc., Osaka Denki Co., Ltd, Dengensha Mfg. Co., Ltd., and Robotron Inc. REFERENCES 1. Nishiguchi, K, Matsuyama, K., et al., 1987, Influence of current wave form on nugget formation phenomena when spot welding thin steel sheet, Welding in the World, 25 (11/12), pp 222-244. 2. 3. 4. Matsuyama, K. et al., 1990, Computer-Aided Monitoring System of Nugget Formation Process, The fifth Symp. of Japan Welding Society (JWS) in Makuhari, pp 577-582. Fujii, K., Matsuyama, K., et al., 1995, Resistance Spot Welding Monitors, Automotive Manufacturing International '95, pp 168-173. Matsuyama, K., Obert, R., Chun, J-H., 2001, Inverse method for measuring weld temperatures during resistance spot welding, SAE, SP-1621, pp. 131-137 5. Nishiguchi, K., Matsuyama, K., et al., 1978, A study on nugget formation process in resistance spot welding of multi stack plates, Com. on Resistance welding in Japan Welding Society, Doc. RW-148-78 (in Japanese). 6. Matsuyama, K., Chun, J-H., 2000, Splashing mechanism in resistance spot welding, Proc. of International Sheet Metal Welding Conference in Steringheight IX, Paper. 5-4 AN INTEGRATED FEA BASED PROCEDURE FOR WELD FIXTURE DESIGN Z. Yang, X. Chen, Y. Dong E. Martin*, D. Michael* ABSTRACT As mechanical forces and welding induced thermal stress are applied during welding, the strength of the weld fixture is one of the crucial factors determining a successful or unsatisfactory welding practice. In the present study, an integrated FEA based procedure for welding fixture design was established. In the integrated FEA procedure, the welding induced stress and distortion were simulated by sequentially coupling thermal and mechanical modeling using a comprehensive welding simulation package. The welding simulation results showed that thermal induced reaction forces were significant at the constraint locations, which must be considered for weld fixture design. The magnitudes of reaction forces varied depending on constraint locations during welding process. The obtained maximum reaction forces were then input into weld fixture structural analysis. The predicted stress and distortion from the fixture structure analysis provided a guideline for the weld fixture design. KEYWORDS Welding Simulation, FEA, Stress and Distortion, Fixture Design INTRODUCTION Residual stresses and distortions are two of the major concerns in welded structures. The welding induced residual stress can exceed the yield strength of the material and is detrimental to the integrity and fatigue life of the welded parts. Welding can also cause the welded products to distort significantly from their original shape, which may require costly post-weld treatment such as machining or straightening. In recent years, tremendous efforts have been made to reduce weld distortion. Various methods have been proposed to accomplish this, including precambering, pre-bending, preheating, and thermal tension. Thermal-mechanical FEA of the welding process is an emerging and rapidly maturing technique. Computer aided design of the welding process is becoming an efficient and effective approach to achieve high quality weld products in industry (Ref. 1-5). Welding simulation helps to optimize welding procedure (welding parameters, sequence, and weld joint geometry) and apply appropriate mechanical or thermal methods to reduce welding induced residual stress and distortion. In recent years, welding computer models have demonstrated the capability to reduce fabrication costs, improve weld quality and increase service durability by optimizing the weld process. Technical Service Division, Caterpillar Inc., 1311 East Cedar Hills Dr., Mossville, IL 61656-1875 |