Welding is the most widely used process for joining metallic structures. But it is an aggressive procedure that introduces long-range distortion, residual stress localized material inhomogeneity, geometric discontinuities and defects. It is unsurprising that weldments are “the weakest link” determining the life and safety of most engineering structures.
Our research spans the use of conventional and advanced welding processes including narrow gap arc-welding, electron beam welding, laser welding and laser-hybrid welding for joining both similar and dissimilar metals. Using state-of-the-art measurement and modelling techniques we study the microstructure, mechanical properties, defectiveness and residual stress in benchmark test specimens and complex components. We are particularly interested in the role of residual stress and microstructure on the life and integrity of welded joints under complex service loading conditions. We closely collaborate with industry, academia and several international partners in this area.
An example of our pioneering work is application of digital image correlation (DIC) to measure the spatial variation in tensile properties across welded joints (our procedure produces several hundred local stress-strain curves from a single test). We are now developing this powerful technique to measure cross-weld creep deformation properties at high temperature.
Diffusion bonding is a method for joining similar and dissimilar metals for which conventional welding processes have proved unsuccessful. It is also used for joining metal to ceramics in fabricated bi-material components. The main applications of this method are in the electronics, defence, transport, aerospace and power generation industries.
Our diffusion bonding facilities are used for joining various combinations of metals, alloys and ceramics. For example, current projects are studying nickel-based superalloy joints for jet engines, bi-material components for nuclear power plants and high-precision joining of micro-electronic devices.
Structural materials used in the core of nuclear power plants have to withstand the combined effects of high temperature, harsh irradiation and corrosive/erosive operating conditions. Oxide-dispersion-strengthened (ODS) materials offer potential advantages in terms of their high-temperature stability and resistance to radiation damage.
We are working on the development of ODS systems, fabricated by a commercial mechanical alloying, powder metallurgy process. Our study is on high chromium (9 to 12%) steels having ferritic-martensitic microstructures, using low-activation or reduced activation alloys; that is removing elements such as Mo, Nb and Ni and specific impurities from the steels.
Laser shock peening is a novel technique for surface processing. A high-energy laser induces shock waves that cause plastic dilation resulting in compressive residual stresses in the vicinity of surface. The technique offers deeper compressive residual stress fields with better surface finish, leading to improved fatigue performance, when compared with conventional shot peening.
We are researching the optimisation of laser peening parameters on order to generate uniform deep compressive residual stress fields in thin aluminium plates for potential application in the aerospace and automotive industries. The production of a compressive, biaxial stress field is difficult in thin sections because of the interaction of the incident stress waves with the back surface of the section being peened.