Consideration of decision guidelines for welding condition by analytic solution of heat conduction equation

Abstract

Application of advanced high strength steel sheet (AHSS) to car bodies is expanding to achieve lighter and stronger bodies. However, it sometimes becomes a problem that spot welded joints of AHSS exhibit low CTS (= Cross tension strength). Therefore some studies have been made on enhancement of CTS for AHSS. Welding with post heat of a short time is one of effective methods for improving CTS. In this study, we experimentally clarified appropriate welding conditions using hot-stamped steel sheets of 1470MPa grade first. And then we tried temperature estimation and phenomenon analysis by analytic solution of heat conduction equation, in order to derive decision guidelines for welding condition. Consequently, the following results were obtained: 1) Improvement of CTS by over 30% in comparison with that without post heat was obtained when appropriate 1^st cooling time; t_c1 and post heat time; t_h2 were chosen depending on the sheet thickness, and also current ratio (= current in post heat; I_h2 / current in 1^st welding; I_h1) was around 0.9. 2) By means of the analytic solution, equations of temperature estimation for four steps (1^st welding, 1^st cooling, post heat and final cooling) were gained. 3) The phenomenon analysis with the temperature estimation led to each recommended condition for parameters such as t_c1, t_h2 and I_h2/I_h1 on each sheet thickness. 4) These conditions analytically recommended showed a good accordance with them experimentally clarified.