Abstract:
Nickel based superalloys are an integral material for gas turbines, where their excellent
high temperature mechanical properties and corrosion resistance are utilised.
Due to the increasing costs of raw materials, manufacturers are interested in repairing
in-service and manufacturing defects in components. Unfortunately, superalloys
such as Rene 80 are highly susceptible to welding defects such as liquation cracking
and post-weld heat treatment cracking, which make repair welding highly difficult.
The aim of the research in this thesis was to develop an improved understanding
of welding defect production in nickel-based superalloys. In particular, the effect of
repair process and its parameters were examined, with the ultimate aim to produce
crack-free repair welds.
The main theme of the work involved a large parametric study of the process
parameter effects on welding defects in Rene 80 using a high power fibre laser. This
work determined an optimised range of parameters which reduced the incidence of
cracking. Furthermore, this work also identified a key relationship between the weld
bead geometry aspect ratio and the incidence of cracking. This relationship was
studied using neutron diffraction to determine the differences in strain and residual
stresses between two welds with identical heat input but different geometry. An
in-depth investigation of the cracks within the material, identified that as-welded
cracks formed via liquation of secondary phases such as carbides, γ/γ’ eutectics,
and secondary gamma prime. The post weld heat treatment cracks formed by the
strain-age mechanism in Rene 80.
From this work, a novel repair procedure avoiding the complications associated
with using lower strength filler metal was developed, based on the optimised welding
parameters. Finally, a number of advanced low heat input welding processes were
also investigated for repair of superalloys.
