Weld Cracking of Precipitation Hardening Ni-based Superalloys - Investigation of repair welding characteristics and susceptibility towards strain age cracking
Doktorsavhandling, 2020

High temperature resistance and strength requirements make nickel-based superalloys the material of choice for the hot section of aero engines. Fabrication in terms of combining wrought and cast parts in the manufacturing of hot structural components enables component optimisation via the use of wrought high-strength parts, where geometrical constraints allow, and cast parts to produce complex geometries. Such an approach requires that the materials involved are weldable. Due to the complex microstructure of precipitation hardening nickel-based superalloys, welding comes with the risk of weld cracking, more specifically solidification cracking, heat affected zone (HAZ) liquation cracking and strain age cracking (SAC). While the first two types require a liquid phase to be present, SAC occurs during heating to post-weld heat treatment, in which age-hardening reactions coincide with the relaxation of weld residual stresses. Increasing engine operating temperatures as well as the intermittent cycling of land-based gas and steam turbines motivates research on the weldability of highly temperature-stable alloys.

Hence, the main objective of this work has been the investigation and analysis of microstructural changes and their effect on weldability in terms of susceptibility towards weld cracking of the nickel-based superalloys Haynes® 282® and ATI 718Plus®. This has been addressed by the means of repair-welding studies and a simulative test approach using a Gleeble system. Microstructural changes were found to significantly affect HAZ cracking in cast ATI 718Plus®, where high amounts of Laves phase showed an increased resistance towards cracking. Haynes® 282® shows good weld-cracking resistance, as no HAZ cracks were present after multi-pass weld operations and subsequent post weld heat treatments. A simulative Gleeble test was developed to provide more data on ductility in the SAC temperature range and its dependence on ongoing microstructural changes during thermal exposure. Comparison with Waspaloy showed that the high resistance of Haynes® 282® towards SAC is correlated with the moderate age-hardening kinetics of the alloy and the rapid formation of a grain boundary strengthening carbide network. Furthermore, grain size was found to be a major factor affecting ductility and hence SAC susceptibility.

ATI 718Plus®

Nickel-based superalloys

hot cracking

weldability

post-weld heat treatment

Gleeble

welding

weld cracking

strain age cracking

Waspaloy

Haynes 282

Opponent: Associate Professor Boian Alexandrov, The Ohio State University, USA

Författare

Fabian Hanning

Chalmers, Industri- och materialvetenskap, Material och tillverkning

A Review of Strain Age Cracking in Nickel Based Superalloys

7th Swedish Production Symposium 25.-27.10.2016 Lund,; (2016)

Paper i proceeding

Weldability of wrought Haynes® 282® repair welded using manual gas tungsten arc welding

Welding in the World, Le Soudage Dans Le Monde,; Vol. 62(2018)p. 39-45

Artikel i vetenskaplig tidskrift

Advanced microstructural characterisation of cast ATI 718Plus®—effect of homogenisation heat treatments on secondary phases and repair welding behaviour

Welding in the World, Le Soudage Dans Le Monde,; Vol. 64(2020)p. 523-533

Artikel i vetenskaplig tidskrift

F. Hanning, A.K. Khan, O. Ojo, J. Andersson. Effect of short-term isothermal exposure on the ductility signature of Waspaloy in the temperature range of 750-950°C - a comparison with Haynes® 282®

F. Hanning, G. Singh, J. Andersson. The effect of grain size on the susceptibility towards strain age cracking of wrought Haynes® 282®

Aircraft engines are composed of a multitude of materials, with the design being led by the aim to reduce the engine weight while at the same time increasing its performance. Light materials such as titanium and carbon fibre are used in colder parts of the engine, while nickel-based superalloys are used in the hot sections due to their good temperature resistance and high strength. Traditionally, structural components such as the engine housing have been produced as a single, large cast part. This approach limits the possibility of weight savings because of lower strength of cast parts. Wrought material on the other hand has higher strength which would enable a reduced component weight. Complex shapes can however only be realised by extensive machining, if at all possible.
The so-called assembly approach uses small cast and wrought parts that are joined together by welding. Component weight can be reduced by using wrought material where high strength is required, while cast parts are used in places where strength requirements are lower but complex shapes are needed instead. Such a manufacturing concept requires a good weldability of the used materials. Welding of nickel-based superalloys is however more complicated as for example the joining of construction steel. Their complex microstructure requires the careful selection and control of welding parameters, and cracking often occurs during welding. Weld cracking can furthermore occur during the post weld heat treatment, an operation carried out to obtain uniform mechanical properties in the whole component after welding. This provides the background for research on the weldability of nickel-based superalloys. The aim of this work was to study what type of weld cracks occur during welding and how the microstructure of the material affects their formation.
For this, welding tests were carried out using manual tungsten inert gas welding. Investigating actual welds enables looking at conditions that are close to real application. The analysis showed that the microstructure before welding has a large effect on crack formation during welding. The presence of the Laves phase, which is typically considered detrimental for the properties of the material, reduced the formation of cracks during welding. To further study how the microstructure of the material can cause weld cracking, another part of the work was focussed on developing a test method that can simulate the thermal cycle during and after welding. The results contribute to better understanding the interrelationship between microstructural changes during heat treatments and the likelihood of crack formation. Knowledge about how microstructure changes affect the susceptibility towards weld cracking can be used to adapt production processes and could ultimately help to avoid cracking problems during welding.

Ämneskategorier

Bearbetnings-, yt- och fogningsteknik

Annan materialteknik

Metallurgi och metalliska material

Styrkeområden

Materialvetenskap

ISBN

978-91-7905-258-4

Doktorsavhandlingar vid Chalmers tekniska högskola. Ny serie: 4725

Utgivare

Chalmers tekniska högskola

Online

Opponent: Associate Professor Boian Alexandrov, The Ohio State University, USA

Mer information

Senast uppdaterat

2020-03-25