Linear friction welding of AD730™ Ni-based superalloy to additively manufactured Inconel 718

Abstract

Linear friction welding (LFW) is an emerging solid-state welding technology for joining of blades to disks (blisks) or repairing turbine disks in gas turbines and air engines. In recent years, LFW has also been applied to the assembly of components made of Nickel (Ni)-based superalloys, as well as in many other engineering applications, ranging from automotive to agriculture, and allowing for new designs and applications. The LFW process is well suited for joining dissimilar alloys as they do not suffer from weld shrinkage and cracking which are very common in fusion-based techniques. On the other hand, Selective laser melting (SLM) as a laser powder bed fusion (LPBF) method is being developed as an economically viable technology for fabricating Ni-based superalloys with the complex geometries in aerospace industries and also as an advanced technique for component repair. Gas turbine blades are known for their complex geometries and high time to market delay. The additive layer manufacturing can reduce drastically the time to market. The focus of this PhD work was to study the suitability of the LFW process to join two superalloys fabricated with two different methods: SLMed Inconel 718 (IN718) superalloy and the newly introduced forged AD730TM Ni-based superalloy. Each alloy had individual initial and final microstructures which had to be understood. Thus, many challenges and questions needed to be addressed. Some of these challenges will be considered in this PhD research. The focus of this study is to have a better understanding of the evolution of the microstructure and mechanical properties of LFWed AD730TM to SLM IN718. LFW experiments were carried out using a model from literature based on 1D heat transfer simulation, which was developed and adapted in this work for the dissimilar welding. A methodology was adapted to determine the LFW parameters for achieving an acceptable joint with free defects for the investigated alloys. The method can be extended and applied to other metallic as well. The method consisted in estimation of friction and forge forces by using the plastic flow stresses and data reported in the literature as well as the capability of LFW machine. Friction time was estimated based on an analytical thermal model for both alloys and their mechanical properties at high temperatures. It was determined that the LFWed specimens with axial shortening more than 2 mm and optimum to 3.5 mm were free from oxides and microcracks. Hot ductility of the forged polycrystalline Ni-based superalloy, AD730TM was investigated in the temperature interval 1050–1240 °C. The nil strength (NST) and nil ductility temperatures (NDT) were determined by hot tensile testing using the GleebleTM 3800 known a convenient method for weld thermal simulations. The tests plan and designing the mechanical tests had been done by other CM2P members. The influence of heating rate, representing the LFW thermal cycle, on hot ductility behavior of the alloy was also studied. The microstructure and the fracture mode of samples were examined. The influence of heating rate on the extent of grain boundary liquation and void formation was determined. It was shown that the significant ductility loss near the NDT point could be related to the reduction of surface tension at interface of the grain boundary and the matrix. In addition, the contribution of hard precipitates (e.g., grain boundary MC carbides), voids, and cavities on damage mechanisms responsible for ductility loss was discussed. The influence of high heating rates on the evolution of the secondary phases in the microstructure of SLM IN718 was investigated. The microstructural characterizations using Scanning Electron Microscopy (SEM) revealed different regions in the heat-affected zone (HAZ) of the welded additively manufactured specimens. A combination of thermal analysis by DTA and dilatometry was used to show the precipitation and dissolution of the secondary phases and microstructural features. The dissolution of γ" and δ phases were delayed under high heating rates and shifted to higher temperatures. The Laves phase at the interdendritic regions was decomposed in specific zones near the surface of the sample. A possible mechanism based on the influence of heating rate on Nb diffusion in the interdendritic regions and core of the dendrites was proposed to interpret the observed changes in the microstructure. Microscopic analysis and XCT-Scan image showed successful joints free of micro-porosity, micro-cracking, and oxide layers. The microstructures variations were also evaluated, particularly in terms of grain size and misorientation changes. Dynamic recrystallization (DRX) occurred on both sides of the dissimilar weld line, and it was found that Discontinuous DRX (DDRX) and Continuous DRX (CDRX) recrystallization took place in the weld zone (WZ) and in the thermomechanical-affected zone (TMAZ), respectively. A clear change in the size and local grain misorientations levels were related to a greater degree of strain-induced in a homogenized sample and the increasing and the effect of the solid solution strengthening mechanisms caused by a partial dissolution of the second phase strengthening particles in the matrix. The SEM observations and EBSD maps showed that dynamic recrystallization occurred in the weld zone during hybrid LFW in samples in as-welded condition on both sides. Close to the weld line, the dissolution of γ'/γ" and Laves phases and grain refinement occurred which reveals the combined effects of both compressions loading and high temperature on recrystallization on drastic diffusion of chemical elements in the WZ. It is shown that the size, volume fraction, and shape of secondary phases increased and changed from the WZ to the base metal (BM). The measured microhardness, in the WZ, indicated that the strength of AD730TM alloy depends significantly on the grain size and also possibly on ultrafine precipitates. The strength in SLM IN718 was dominated by the shape (or size) and the presence of secondary phases. A post-weld heat treatment (PWHT) cycle was performed on dissimilar LFWed samples. The microstructure and hardness of the joint after the PWHT were studied and compared to those of LFWed samples. The precipitation hardening mechanism of γ' + γ" is the main mechanism used to increase the mechanical properties of SLM IN718 alloy. These particles coarsened during heat treatment at 980 °C and double aging. The thermomechanical history of LFWed joints can affect the microstructure of IN718 alloy such as morphology of δ phase after solution treatment (ST) from the plate-like in the weld zone (WZ) to the needle-like in the base material (BM). It was found that in AD730TM, nanometric size γ' particles re-precipitated close to the weld line during rapid cooling after welding was completed. The developed PWHT could homogenize the hardness on both alloys and all welding zones. The maximum hardness difference between each zone was 15-20 Vickers for the WZ and the BM. The maximum hardness difference between two alloys was 19 Vickers which means the PWHT, in two steps, could provide a better homogeneity for a dissimilar joint than one-step heat treatment which has been used in literature.

Date	16 Jul 2021
Original language	American English
Awarding Institution	École de technologie supérieure
Supervisor	Mohammad Jahazi (Supervisor) & Farhad Rézaï-Aria (Co-supervisor)