Thermomechanical Welding (TMW)

All metallic materials consist of grains which are just a few hundredths of a millimeter. Alloys with finer grains usually have higher strength and toughness in comparison to materials with coarser grains. Therefore, the grain refinement leads to weight decrease of components which is associated with economic and ecological benefits. Among the grain refinement processes, thermo-mechanically controlled processes (TMCP) are the most efficient technique for the majority of alloys. During a TMCP, metals are subjected to plastic deformation which leads to deformed grains with high stored energy supporting the nucleation of new grains in further treatments at high temperatures (Recrystallization). The grains are usually prone to coarsening at high temperatures what is called grain growth. To inhibit the coarsening of the recrystallized grains, TMCP is associated with an accelerated cooling process subsequently after recrystallization. Alloys subjected to a welding process can experience grain growth due to the exposure to high temperatures. Thus, the deterioration of the strength and toughness of the welded material cannot be avoided. Various techniques like welding arc pulsation, electromagnetic and ultrasonic weld-pool stirring have been proposed in the literature to refine grains during welding. However, these techniques are restricted to specific welding conditions and alloy compositions, and are exclusively applicable to the molten weld metal (WM). However, besides the WM, grain growth occurs also in the adjacent area of the WM which is called heat affected zone (HAZ). This project proposes Thermo-Mechanical Welding (TMW), comparable to TMCP, and targets on refining the grain size in both the WM and HAZ regions. To estimate the appropriate time for deformation and appropriate cooling rate, a fundamental investigation of thermodynamics, kinetics, and mechanisms of the grain growth and recrystallization is required. Accordingly, comprehensive modeling will be performed comprising FEM, analytical simulation and cellular automata (CA). In parallel, physical simulation using the thermomechanical GLEEBLE system and validation experiments with the versatile TMW machine, which was constructed by the applicant, are envisaged. Within the proposed project two types of steels representing high strength low alloyed steel (HSLA) and austenitic stainless steel will be investigated. Also, along with the ex-situ metallographic techniques, a new in-situ technique, so-called LUMet, will be used to measure the grain size and kinetics of recrystallization within cooperation with the University of British Columbia. The innovation of the project lies in a new combined deterministic and probabilistic CA method to simulate grain growth and recrystallization and also the introduction of a comprehensive grain refinement method for the welding process. Improvement of the welding joint microstructures via TMW would yield to an improvement of the properties (strength, toughness) and Non-destructive test capability of the weldment.

This project aimed to minimize grain growth during fusion welding by proper mechanical treatment during cooling. Therefore, the concept of thermomechanical rolling is transferred to welding. The mechanisms of microstructure refinement were investigated detailled and the parameters of the TMW process were optimized. The tests were carried out on an austenitic stainless steel (AISI 304L) and a high-strength low-alloy steel (S700MC) with different degrees of deformation. Welding speed and current were systematically varied to quantify the influence of heat input. The impact force and frequency were examined to properly adjust hammer shape and size in order to optimize the influence on the microstructure development. Among other things, load cells, high-speed cameras and high-resolution methods were used for this purpose. The ferromagnetic phases in the weld seam were quantified and the change in mechanical properties after the TMW process was determined using hardness measurements. In addition to weld metal and heat-affected zone a TMW weld seam has also a deformed zone. In order to investigate the microstructural development in the weld metal using a model, compression tests were carried out on weld metal at different temperatures and annealing tests were performed on the cold-formed as-received material. Main project results are: a) Development of an adaptive heat source model for analytical description of the transient temperature distribution b) Influence of thermal and mechanical induced oscillations on the microstructure c) Effect of dynamic and static recrystallisation on grain refinement d) Modelling of grain coarsening of fully recrystallised austenite e) Transformation of weld metal considering transformation of deformation induced martensite and dissolution and globularisation of -Ferrite f) FE model for the simulation of TMW incl. experimental validation The results led to two PhD theses (1 completed), five master's theses (3 completed), five peer-reviewed journal publications and four conference presentations. Three international guest students also contributed to the project. Overall, this project has led to valuable findings in thermomechanical welding, as mechanisms are better understood and partly can be described quantitatively. There are still open questions such as what is the influence of the optimized hammer geometry and parameters on the microstructure development. The role of -ferrite in the discontinuous dynamic recrystallization (DDRX) of austenite and its effects on grain coarsening has also not yet been fully clarified. Modeling the microstructure development using cellular automata (CA) can make a valuable contribution to clarify those questions.