Research On Behaviors And Monitoring For Heat Generation And Transfer In Micro Resistance Spot Welding Of Enameled Wire

Micro resistance spot welding(MRSW)of enameled wire is widely applied in the electronics manufacturing industry,since it is an efficient process for insulation painting removal and connection of fine enameled wire by Joule heat.Factors influencing MRSW for enameled wire joining are complex.With the miniaturization and high integration of electronic devices,the joining reliability has become more serious,which affects the performance and service life of devices.Nowadays,there is a lack of systematic and in-depth understanding for MRSW of enameled wire,manufacturing process and welding quality heavily rely on manual inspection,which has become a main obstacle to the improvement of the quality of related devices in the electronics manufacturing industry.In this paper,a systematic study has been carried out for behaviors of heat generation of electrode and thermal contact at interfaces as well as monitoring of electrode wear state and abnormal welding quality,with an aim at clarifying the rules of MRSW process of enameled wire and enhancing the quality monitoring of MRSW.First,an electrical-thermal-mechanical coupled finite element model for MRSW of enameled wire was developed.In the model,changes in contact at welding interfaces and the temperature-dependent properties of materials were taken into account and the measured current was applied as the excitation of Joule heat.The simulation results of heat generation and transfer were verified by experimental measurement.It provides a research tool for the behavior analysis of heat generation and transfer for MRSW of enameled wire.The behavior of heat generation of electrode in MRSW of enameled wire was systematically studied.Effects of tip wear of electrode for MRSW on the resistance,heat generation and temperature increase of electrode tip and welding quality were analyzed with highlight.It shows that with the intensification of electrode wear,the resistance,heat generation and temperature rise of electrode tip can increase from 0.74 mΩ,3.18 J,1095 ℃ to 0.93 mΩ,3.45 J,1194 ℃.The regulation of heat generation of electrode was discussed.The good suitability of constant voltage control mode for micro joining of enameled wire was clarified.The information characterization for heat generation of electrode during production was analyzed by dynamic resistance and Joule heat generated at electrode.The behavior of contact heat transfer with thermal decomposition of insulation painting in MRSW of enameled wire was investigated.The evolution of thermal contact coefficient at the electrode/workpiece and the workpiece/workpiece interface was quantitatively studied through the combination of experimental analysis and simulation trials.It shows a significant change in contact heat transfer at interfaces before and after the removal of insulation painting.The increase in thermal contact coefficient from the beginning of welding to obvious overheating can reach more than 14 times.The temperature evolution at the tip of MRSW electrode and welding area of fine workpieces,elastoplastic deformation of fine enameled wire,current density distribution of electrode tip and increase in workpiece temperature with electrode tip wear were simulated.The results show that the maximum temperature of enameled wire when obtaining qualified welds is about 750 °C,which is lower than the melting point of pure copper.The temperature at pad center can rise from 523 °C to 791 °C(overheating)with electrode wear.In view of the relation between heat generation behaviors and electrode wear,between heat transfer behaviors and welding quality,monitoring for electrode wear based on state division and grey prediction and for welding quality based on anomaly detection was explored.The maximum of dynamic resistance and its rate with heating number were extracted as electrode wear features.The process of change in electrode wear was quantitatively evaluated.The class-imbalance distribution that unqualified welds were obviously fewer than qualified welds in production was explored.Similarity features were extracted from dynamic resistance and Joule heat generated at electrode.The performance of three anomaly detection models(Isolation Forest,One-Class Support Vector Machine and Local Outlier Factor)to recognize unqualified welds was compared and analyzed.The results show that the similarity features can be used to effectively distinguish the feature distribution of incomplete fusion welds and qualified welds.The quality classification model based on Isolated Forest is able to detect incomplete fusion welds and qualified welds with balance.Their identification accuracy is 86 %and 90 % respectively.It provides an intelligent classification technique for quality monitoring in MRSW of enameled wire.