Excessive weld pitch results in overheating with localized melting of the surface material. It can be calculated from Fig.11 that welds of good quality are obtained in the optimized range from 2.0 to 9.0 rev/mm, (Ref. 12). The operating parameters of friction stir welding for 6061-T6 Al plate is shown in Fig. 12. The quality of the joints is judged from the weld appearance and presence of any internal defects. Low rotational speeds do not produce stable welds because of the insufficient heat generation. On the other hand, high rotational speed produces excessive heat and causes unsatisfactory appearance and internal defects. Typical weld appearance under different welding conditions is shown in Fig.13. Good weld joints are obtained for rotational speeds ranging from 344 to 637 rpm and welding speeds from 95 mm/min to 330 mm/min. For the welding parameters 151 rpm and 229 mm/min heat input is too low to produce sufficient heat to induce the plastic flow of material. This can cause additional force to be applied to the traversing pin, leading to pin failure, Fig13 (b). Figures 13 (c) and 13 (d) show the weld appearance with severe surface defects caused by excessive heat input under the conditions of 914 rpm and 140 mm/min, and 914 rpm and 330 mm/min. The cross-sectional morphologies of the friction stir welds for different welding parameters, shown in Fig.14. An onion-ring-type inner structure consisting of concentric ovals proves that the nugget is well-developed, Fig.14 (a). A small void is found on the cross section of the welds created under the welding conditions of 914 rpm and 95 mm/min Fig.14 (c, d). Tensile test results show that the good weld quality is obtained for the rotational speed from 344-637 rpm and welding speeds 95-330 mm/min, (Ref. 13). The shapes and sizes of all of the different zones (HAZ, TMAZ plus weld nugget) are very hard to predict because they depend on the tool design and the welding parameters, and particularly on the welding speed (Ref. 12). a) b) c) d) a) 637 rpm and 190 mm/min; b) 151 rpm and 220 mm/min; c) 914 rpm and 140 mm/min d) 914 rpm and 330 mm/min Figure 13. Typical weld appearances of the friction stir welds under different welding conditions It is found that the welding speed has a larger effect on the weld zone size than the rotational speed. The width of the zones (HAZ, TMAZ, and nugget) is the largest, about 60mm under the conditions of 914rpm and 95mm/min, while the smallest is about 45mm under the conditions of 344 rpm and 330mm/min. a) 416 rpm and 133 mm/min; b) 637 rpm and 95 mm/min; c) 914 rpm and 95 mm/min Figure 14. Cross-sectional morphologies of the friction stir welds for different welding parameters CONCLUSION Using an infrared camera, the temperature distribution in front of the shoulder can be obtained for different welding parameters. Rotational and linear speeds can be adjusted during the welding in order to keep the temperature in front of the shoulder constant. The optimal range of parameters that produce satisfactory weld quality includes rotational speeds from 344 rpm to 673 rpm and welding speeds from 95 mm/min to 330 mm/min. The influence of the rotational speed and the linear speed on the temperature changes is presented. The temperature change is very small for variations in rotational speed from 95 to 330 mm/min. The welding speed affects the surface temperature in front of the shoulder, and the temperature 0.5mm from the surface under the shoulder, more than does the rotational speed. The influence of the linear speed on the cooling rate and zones' (HAZ, TMAZ plus weld nugget) sizes is also observed. It has to be pointed out that this work is still in its embryonic stage. Some other sensing techniques will also be investigated. These are very attractive topics for future research in Friction Stir Welding and also very helpful for industrial applications. ACKNOWLEDGEMENTS The authors would like to express sincere thanks to Dr. M. Song for providing help in performing the calculations. This work was financially supported by the U.S Department of Education, Grant No. P200A80806-98. and the American Welding Society through the Graduate Fellowship Grant. REFERENCES 1. 2. 3. E. D. Nicholas and W. M. Thomas, A review of friction processes for aerospace applications, Int. J. of Materials and Product Technology, Vol. 13, Nos 1⁄2, 1998. G. Bruggemann and Th. Benziger, The combination of thermography and image treatment for monitoring of quality assurance during laser welding, Independent nondestructive evaluations 33, no.7; 453 Lukens, W. E. and Morris, R. A., Infrared temperature sensing of cooling rates for arc welding control, Welding Journal, Vol. 61, No. 1, pp.27-33 (1982) 4. 5. 6. 7. 8. 9. 10. 11. 12. 13. Z.Loftus, R. Venable and Glynn P. Adams, Development and Implementation of a LoadControlled Friction Stir Welder, The First International Symposium on Friction stir welding, CA, USA, 14-16, June 1999. R. Jeffrey Ding, Force Characterization on the welding pin of a friction stir welding Mahoney, M. W. and C. G. Rhodes, J. G. Flintoff, R. A. Spurling, and W. H. Bingel, W. Tang, X. Guo, J. c. McClure and L. E. Murr, Heat Input and Temperature Distribution J.R. Davis, ASM Specialty Handbook-Aluminum and Aluminum Alloys, ASM J.H. Ouyand and R. Kovacevic., Material Flow and Microstructure in the Friction Stir O. Frigaard, O Grong and O.T. Midling, Modeling of the Heat Flow Phenomena in A. P. Reynolds, W. D. Lockwood and T.U. Seidel, Processing-Property Correlation in J. H. Ouyang, R. Kovacevic. M. Vlant and D. Jandric, Experimental Studies on Friction P. Threadgill, Friction Stir Welds in Aluminum Alloys - Preliminary microstructural A REAL-TIME MONITORING AND CONTROL SYSTEM FOR RESISTANCE SPOT WELDING K. Matsuyama, R. Obert#, J-H. Chun ABSTRACT A new monitoring and control algorithm has been developed based on an integral form of an energy balance model to realize a low-cost real time monitoring and control system for resistance spot welding. The system captures welding voltage, welding current, and total plate thickness to calculate the mean temperature of a weld during welding. It predicts both weld diameter for the non-destructive evaluation of weld quality and splash occurrence for improvement of the working environment. After training with two stack welds of equal plates, the system can handle two stack welds of unequal thickness, multi stack welds, and other thickness welds without modification of the program parameters. KEYWORDS Resistance spot welding, Prediction of weld diameter, Prediction of splash occurrence, Integral form, Energy balance model, Quality monitoring, Improvement of working environment INTRODUCTION Industry belief has it that the occurrence of splash, or expulsion, yields good information on weld melting. Its evaluation has been used as quality assurance in resistance spot welding. Splash, however, causes some deterioration of the working environment and the quality of welds. Making a weld for comparison purposes is also wasteful of expensive energy. Furthermore, maintenance costs are higher than necessary because the metal powders caused by the splash degrade the moving parts of production robotics. A new procedure addresses the problems inherent in splash during resistance spot welding. It solves deterioration issues based on the idea that splash or expulsion occurs when a weld overheated, even shortly, during welding. part is The authors analyze the facts of splash with numerical simulations and experiments (Ref. 1). The article demonstrates that splash occurs when the corona bond zone at the faying interface suddenly melts. The article also suggests that continuous monitoring or prediction of weld part temperature, i.e., of the dynamic behavior of the temperature rising pattern, is important in predicting splash caused by overheating. Temperature patterns and history can be continuously predicted if a monitoring procedure is associated with a prediction system based on a numerical simulation program as an identification routine of the weld part temperature (Ref. 2). The system described in the referenced paper Mechanical Engineering, Massachusetts Institute of Technology, 77 Massachusetts Ave, Cambridge, MA 02139 #MS Student, Massachusetts Institute of Technology, 77 Massachusetts Ave, Cambridge, MA 02139 |