13th International Conference on Fracture June 16–21, 2013, Beijing, China -1- Solidification Cracking of IN 718 TIG Welds Myriam Brochu1,*, Alexis Chiocca1, Rafael Navalon-Cabanes1, Jean Fournier2 1 Department of Mechanical Engineering, École Polytechnique de Montréal, Montréal H3T 4A6, Canada 2 Special Process Development Group, Pratt & Whitney Canada, Longueil J4G 1A1, Canada * Corresponding author: myriam.brochu@polymtl.ca Abstract Solidification cracking was observed during a tungsten–inert gas (TIG) welding process used to join two thin sheets of nickel-based superalloy IN718. Microstructural analysis of cracked specimens showed a centerline grain boundary (CLGB) susceptible to limit the hot ductility of the welds. Different welding parameters were modified to avoid the formation of the CLGB; such as the current waveform, the heat input and the welding speed. Results show that it is possible to obtain a microstructure free of a straight CLGB when using a heat input below 135 J/mm. However, to achieve full penetration, the heat input cannot be decreased below 112 J/mm. The experimental results are presented in the form of a weldability map as proposed by Dye et al. and they can be used to predict the range of process parameters favorable to successfully TIG weld IN718 sheet metal. Keywords Fusion welding, solidification cracking, nickel superalloy, microstructure, process 1. Introduction 1.1. Context Nickel superalloy IN718 is used to manufacture gas turbine engine components such as disks, cases, shafts, blades, stators, seals, supports, tubes and fasteners [1]. This alloy is selected for its high temperature mechanical properties but also for its weldability, especially its resistance to strain age cracking [2, 3]. Nevertheless, a recurrent weld cracking problem was observed while TIG welding sheets of IN718. Furthermore, in all cracked welds, microstructural observation revealed the existence of a centerline grain boundary. Based on the hypothesis that the CLGB reduces the alloy resistance to solidification cracking [4], the objective of the research project is to identify process conditions that will produce a weld free of CLGB. Our research efforts were oriented toward the improvement of the welding parameters since the weld geometry and the materials chemistry are restricted. To reach our objective, several welding experiments were conducted as reported in section 1. The microstructure of the welds produced was examined and is reported in section 3. In section 4, the effects of the welding speed and of the welding power on the microstructure are discussed. Finally, the practical limitations of the proposed welding conditions are exposed and future work is planned. 1.2. Background: Solidification cracking Even though alloy 718 is nearly immune to strain age cracking, it is susceptible to other cracking mechanisms such as solidification cracking, heat affected zone liquation cracking, and ductility dip cracking [3]. Solidification cracking happens within the weld, when the deformations induced by the liquid-solid phase transformation and the thermal contraction are higher than the ductility of the mushy zone. This type of cracking occurs more specifically in the last stage of solidification, when the fraction of liquid is less than 10%. The remaining liquid forms a film between the grains and
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