Estimation of thermal efficiency in capacitive discharge welding using inverse problems: experimental and numerical analysis of the effects of measurement with thermocouples

Abstract

Temperature measurement is critical in various engineering applications, particularly in processes involving high thermal gradients and rapid heating, such as welding. Accurate temperature data are essential for quality control, process optimization, and safety assurance in industrial operations. However, in fast and high-energy processes like Capacitor DischargeWelding (CDW), precise temperature measurement poses significant challenges, especially due to the difficulty of attaching thermocouples with minimal thermal contact resistance and high repeatability. This work presents an analysis of the process of attaching K-type thermocouples to metallic surfaces using a custom-designed CDW device. The development of this device aimed to overcome the scarcity of commercial equipment suitable for this purpose, improve the thermal contact between the thermocouple and the metallic surface, and reduce contact resistance, which directly affects the accuracy of temperature readings. Furthermore, the design of the equipment enabled better control and understanding of the electrical parameters, which were later integrated into the thermal analysis. Experimental and numerical methods were employed throughout the study. In the first stage, a simplified configuration using a single chromel wire was used to isolate and study heat transfer phenomena and validate the thermal model with fewer sources of uncertainty. This step was crucial for identifying and mitigating typical thermocouple measurement errors. In the second stage, the full welding of K-type thermocouple wires (chromel and alumel) was analyzed. A transient three-dimensional thermal model, including phase change, was developed and implemented using COMSOL Multiphysics®. To estimate the heat flux during welding, the nonlinear Function Specification Method was applied to the temperature data obtained from experiments. The study demonstrated that both the experimental design and the robustness of the thermal model significantly influence the accuracy of heat flux estimation. Using the estimated heat input and electrical energy delivered by the capacitor bank, the thermal efficiency of the process was calculated at 45%. Beyond enabling precise thermocouple attachment, the CDW process also served as a well-controlled case study for analyzing temperature measurement techniques and estimating heat flux in ultra-fast thermal events. The methods and findings from this research can be applied to improve thermal characterization in other manufacturing processes, such as micro-welding, additive manufacturing, and rapid thermal testing in the aerospace and automotive industries.