Phase-field study on microstructure formation in Mar-M247 during electron beam welding and correlation to hot cracking susceptibility

Abstract

Ni-base superalloys are frequently used for cast components in the aero-engine and power generation industries. For joining and repair of these components, beam welding is often the method of choice in industrial praxis. However, precipitation-strengthened nickel alloys generally present poor weldability as a consequence of their high weld cracking susceptibility, with high segregating alloys like Mar-M247 even being considered unweldable. Therefore, strong efforts are taken on optimizing techniques and parameters to reduce crack formation during welding of these alloys. Optimization of welding parameters can be assisted by virtual modelling methods through different scales. To be able to focus onto the factors which eventually are responsible for crack formation during welding, comprehensive modelling of the whole process chain is required, starting from a realistic model of the base material and a simulation of the heat source on the macro-scale, and including melting and microstructure formation during welding on the micro-scale. Then, based on the thermal history and the exact microstructure, cracking susceptibilities during solidification can be deduced by hot cracking models adapted to the specific conditions. In this paper, results of microstructure simulations are presented for the technical superalloy MAR-M247 using the phase-field software MICRESS with coupling to Calphad databases. Based on prior phase-field simulations of equiaxed and columnar microstructures of the base material as well as results of a macroscopic simulation of the heat source, melting and subsequent solidification of MAR-M247 has been simulated for two different welding parameter sets. As-weld microstructures are compared to experimental welds, and the virtual hot cracking susceptibility, obtained from the simulation results using a modified Rappaz–Drezet–Gremaud (RDG) hot cracking criterion, is discussed against experimental crack observations.

Phase-field simulation of the formation of new grains by fragmentation during melting of an ABD900 superalloy

“…The fragmentation phenomenon had been observed by the authors in 2D-simulations of alloy melting under various conditions [22,23], especially when melting homogeneous or homogenized materials, but also for remelting not homogenized material like in this study. However, our systematic approach to reproduce this behaviour with 3D-simulations in this work has failed so far.…”

“…The second simulation step was performed using the results of the first simulation as initial microstructure. Reading of the structure was achieved without information loss by a technique using the restart output (.rest) of the former simulation [21,22]. The initial microstructure was shifted in negative z-direction according to the thickness of 40 µm of the new layer.…”

3D Minimum Channel Width Distribution in a Ni-Base Superalloy

“…Phase-field simulations of microstructure evolution in general are frequently employed for the description and optimization of the physical and mechanical material behavior at mesoscale [32]. With respect to the present context, we mention the successful simulation application to the solidification of Ni-base superalloys [33][34][35] as well as to microstructure evolution during heat treatment [36][37][38]. Phase-field simulations operate with explicit microstructures, based on a diffuse interface description.…”

Impact of electromagnetic stirring on the gas metal arc welding of an MAR-M247 superalloy

“…Laser welding [23], electron-beam welding [24], friction stir welding [25,26], and other advanced welding methods generate less welding stress, thereby helping to reduce welding hot cracks; however, these welding methods are not yet practically applicable in repair welding. Recently, electromagnetic welding (EMS) has come to be recognized as an effective method for facilitating bead formation, as well as enhancing the microstructure and mechanical properties of the weld [27][28][29][30][31][32][33].…”