An investigation into the near-threshold crack growth behaviour and metallurgy of inertia friction welded nickel-based superalloy RR1000

Viki, Mabutho (2018). An investigation into the near-threshold crack growth behaviour and metallurgy of inertia friction welded nickel-based superalloy RR1000. University of Birmingham. Eng.D.

Viki2018EngD.pdf
Text - Accepted Version
Restricted to Repository staff only until 31 July 2033.
Available under License All rights reserved.
Download (23MB) | Request a copy

Abstract

Comprehensive microstructural characterisation and mechanical assessments have been conducted in inertia friction welded nickel-based superalloy RR1000. Using a novel experimental approach, threshold stress intensity factor (Kth, Kth) and near threshold crack growth behaviour (da/dt versus K, da/dN versus K) were measured at a temperature of 650C in both air and vacuum. The focus of this project is to understand the influence of welding introduced weld-line features from an earlier welding process (single stage pressure welding, SSPW), and the influence of a new welding process introduced to clean up the weld (2-stage pressure welding, 2SPW) on threshold values and crack growth behaviours. Previous work has shown the considerable success of 2SPW in clearing hafnia inclusions from the weld interface. However, it introduced variability in microstructure and residual stresses. As a result, the threshold values obtained from this work betray a dependency on grain size, grain shape, residual stresses and sample orientation in relation to inner and outer diameter. Of these factors, it was evident that the influence imposed on the threshold values by sample orientation was a function of the residual stresses, which were in turn heavily influenced by welding parameters. It was also demonstrable that the size and shape of the grains as well as the width of the primary gamma prime free zone (PGPFZ) was related directly to the welding conditions. In particular, it was observed that very high second phase pressure typically results in squashed elongated low aspect ratio fine grains and a very narrow PGPFZ. Microstructural studies and micro-hardness profiles confirmed the occurrence of gamma prime evolution during rapid recrystallization at the weld interface. To assess the fatigue threshold and fatigue crack growth resistance of both SSPWs and 2SPWs, tests were conducted in vacuum following an air pre-crack at 650oC. On interrogating the fracture surfaces, it was observed that, for SSPWs, there were varying degrees of crystallographic faceting ahead of the air intergranular pre-crack. This crack growth mechanism was not present on 2SPWs tested under identical conditions. Reasons for the occurrence of these facets in SSPWs is unclear at present, however, it was noted that they did not extend beyond 16MPam1/2.

Type of Work:

Thesis (Doctorates > Eng.D.)

Award Type:

Doctorates > Eng.D.

Supervisor(s):