Inertia welding nickel-based superalloy: Part I. Metallurgical characterization

Abstract

This article describes a quantitative study of the microstructure of nickel-based superalloy RR1000 tube structures joined by inertia welding. One as-welded and three post weld heat-treated (PWHT) conditions have been investigated. The samples were characterized mechanically by measuring the hardness profiles and microstructurally in terms of ␥ grain size, ␥ Ј precipitate size and volume fraction, stored energy, and microtexture. Electron backscatter diffraction (EBSD) was used to characterize highangle grain boundaries (HAGB) and the variation of microtexture across the weld line. The coherent ␥ Ј precipitates were investigated over a range of scales on etched samples in a field emission gun scanning electron microscope (FEGSEM), using carbon replicas in a transmission electron microscope (TEM) and from thin slices by means of high-energy synchrotron X-rays. Dramatic changes in the microstructure were observed within 2 mm of the weld line. In this region, the hardness profile is influenced by changes in grain size, ␥ Ј volume fraction, ␥ Ј particle size, and the work stored in the material. Further away, the observed hardness variation is still significant although only minor microstructural changes could be observed. In this region, the correlation of microstructure and hardness is less straightforward. Here, a combination of small microstructural changes appears to give rise to a significant change in strength. No significant texture or grain distortion was observed in the extensively plastically deformed region due to recrystallization.

Characterisation of y' Across Inertia Friction Welded Alloy 720Li

“…It should also be noted that the normalized intensity of the (110) superlattice reflection gave essentially the same profile with a minimum observed at z = 1.2mm. Previous high energy synchrotron measurements carried out on inertia friction welded RR1000 (similar γ′ volume fraction) in the as-welded condition showed a similar profile [4] with an trough in intensity close to the weld line and an intensity increase towards the weld line. as-welded Figure 3 shows that when approaching the weld line, the hardness starts to decrease from about 2.5mm away.…”

“…This results in a heavily modified HAZ microstructure. These microstructural changes include changes in the γ′ distributions, γ grain size, misfit between γ and γ′ and work hardening [4,5]. Hardness and microstructural mapping of aswelded conditions of IFWs are essential for understanding the welding process.…”

“…It is a critical step in the process because it is important to relieve the residual stresses without compromising the microstructure/ mechanical properties. In terms of microstructural changes, the PWHT mainly effects the distribution of fine γ′, which, in turn, has a significant effect on the hardness of the HAZ [4].…”

On the diffusion mechanisms of fine-scaleγ′ in an advanced Ni-based superalloy

“…The blank was then subjected to a cooling of 10 K/min to produce a multi-modal PSD. To reveal the microstructure, the sample was polished and etched using a two-part etchant to remove the γ phase [17]. Scanning electron microscope (SEM) imaging was performed using a Philips XL30 FEGSEM at an accelerating voltage of 10 kV and at a working distance of 7.5 mm.…”

Developing Damage Tolerance and Creep Resistance in a High Strength Nickel Alloy for Disc Applications

“…For welded components, the heat treatments after quenching and welding have been designed to optimise the relief of residual stresses whilst ensuring that the size of tertiary gamma prime 25 , which is produced from the specified ageing heat treatment, is maintained. 720Li -700ºC, 500 MPa Alloy #1 -725ºC, 500 MPa Figure 7.…”