Evolution of microstructure and mechanical properties during linear friction welding of AD730 TM Ni-based superalloy

Abstract

Linear friction welding (LFW) is an emerging manufacturing technology for joining of blades to disks or repairing turbine disks in gas turbines and jet engines. LFW has already been applied to Ti alloys for compressor (i.e. low temperature) operation applications but not to high temperature components. Thus, many challenges need to be addressed before full industrial application. Some of these challenges will be considered in this research. The focus of this study is to develop a better understanding of the fundamental mechanisms governing the evolution of the microstructure and high temperature mechanical properties of LFWed Ni-based superalloys in the case of a newly introduced alloy, AD730 TM. LFW experiments were carried out using different processing parameters. A methodology was developed for determining the optimum LFW parameters for the investigated alloy. The method can be extended and applied to other metallic alloys too. This consisted in estimation of the oscillation, frequency, friction and forge force using material plastic flow stresses, data reported in the literature and machine capability. Friction time was estimated based on an analytical thermal model. It was determined that the LFWed specimens with axial shortening of 3.5 mm were sound and free from oxide layers and microcracks. The Ni-based superalloy, AD730 TM, was solution treated at different temperatures. Semi-analytical model was developed to describe the dissolution processes of γ´ particles, and quantify their volume fraction. This model was used to predict temperature or γ´ volume fraction in different zones of LFWed sample. Reprecipitation mechanisms and kinetics of γ′ particles during cooling from supersolvus and subsolvus temperatures were studied. Then, kinetic model was proposed to quantify and predict the volume fraction of reprecipitated γ′ particles for high and low cooling rates representing the post weld cooling at different distances from the weld interface. Microstructure investigation using Scanning Electron Microscopy (SEM) revealed that γ′ volume fraction and size increased with the distance from the weld interface. These particles were totally dissolved at the weld interface, and only a monomodal re-precipitated γ′ distribution, less than 10 nm in size, was observed at the weld interface of the as-welded specimens. Electron Backscatter Diffraction (EBSD) maps showed that dynamic recrystallization occurred in the weld zone during LFW in as-welded samples. Thermo-mechanically Affected Zone (TMAZ) and weld zone sizes were estimated using EBSD maps, and were confirmed with microhardness measurements. Micohardness measurements on as-welded samples revealed a decrease in hardness in TMAZ due to dissolution of secondary γ′ particles, and an increase in hardness in the weld zone due to grain refinement. Examination of the weld interface and the flash did not reveal any occurrence of liquation (formation of resolidified zones). A new analytical model was developed that predicts the conditions for the occurrence or absence of liquation during LFW of Ni-based superalloys. Using this model, a general explanation is proposed that is able to explain the contradictory data reported in the literature. Post Weld Heat Treatment (PWHT), consisting of γ′ sub-solvus solution treatment followed by aging, was conducted on LFWed specimens. Microstructure evolution across the weld in the as-weld and PWHT conditions were studied using optical microscopy, SEM and EBSD. Mechanical properties of the LFWed specimens were evaluated using microhardness, tensile at room temperature and 650°C, as well as creep tests. Tensile tests on as-welded samples at 650°C revealed failure at Heat Affected Zone (HAZ) due to dissolution of γ′ precipitates. PWHTed joints showed higher tension and creep resistance than those of as-welded samples due to a combination of reprecipitation of γ′ precipitates at TMAZ and HAZ as well as grain growth at the weld zone. After PWHT, LFWed specimens failed in the base material under tensile tests. The PWHTed joints exhibited better ductility than those of base material at 850°C while they showed slightly lower creep life at 700°C in comparison to the base metal. Microstructure examination showed that cracks initiated at the interface of oxidized particles at 700°C. The decrease in creep resistance of AD730TM Ni-based superalloy at 850°C was related to a combination of the formation of Precipitate Free Zones (PFZ) in the vicinity of the grain boundaries (GBs) and microcracking assisted by oxidation. In addition, it was found that in the investigated temperature range, the PWHTed AD730TM had similar creep characteristics to UdimetTM720 Li and Inconel 738LC at low values of Larson Miller Parameter (LMP) and better creep properties than those of Inconel 617 alloy at higher LMP values.

Date	17 Jul 2018
Original language	American English
Awarding Institution	École de technologie supérieure
Supervisor	Mohammad Jahazi (Supervisor)