This thesis presents a microstructural evaluation of the gamma titanium aluminide alloy Ti-48Al-2Mn-2Nb (at.%) joined by diffusion bonding, electron beam welding and linear friction welding. The aim of the project was to evaluate a number of different joining techniques in order to gain an understanding of the type of microstructures generated and, ultimately, the advantages and disadvantages of each technique. The diffusion bonding behaviour of the intermetallic compound Ti-48Al-2Mn-2Nb (at.%) was studied as a function of various parameters including; starting microstructure, temperature, surface finish and time. During diffusion bonding, small y-based grains nucleated at the interface and grew into the matrix forming a 'double necklace' grain structure. Backscattered electron imaging (BSEI) and transmission electron microscopy (TEM) identified the presence of oc 2 -based particles, both at along the original boundary and between the newly formed interfacial y grains and the matrix. The formation of a 2 particles was believed to be related to the redistribution of a 2 -stabilising elements, notably oxygen, during bonding. Post bond heat treatment in the a phase field («1400°C) was found to be an effective way of replacing the original double necklace grain structure with a fully lamellar structure that exhibited good grain growth across the original boundary. The electron beam weldability of the alloy was studied with particular emphasis on reducing cold cracking associated with the weldment cooling through the ductile to brittle transition temperature (DBTT) at approximately 700°C. It was found that the amount of cold cracking could be reduced and eliminated by reducing the cooling rate of the weldment through the DBTT via in-situ electron beam heating. Tensile testing of the welds showed that although the welds appeared to be crack-free they were considerably weaker than the matrix, this suggests the welds contained a high degree of residual stress. The application of different preheats to the samples produced a range of cooling rates within the weldment which proved ideal for studying the various transformation products of the high temperature a phase. The retained ot 2 , massively transformed y and wheatsheaf / feathery structures were all studied in detail by TEM, with particular emphasis on the oc-»massive y transformation. The analysis culminated with the proposal of a qualitative CCT curve for the various transformation products from the a phase. Analysis of the a -> massive y transformation suggests the massive phase developed in the parent a from a coherent, grain boundary nucleus which subsequently grew into an adjacent grain by movement of an incoherent interface. A feathery structure was also observed in the welds. This structure consisted of lamellar colonies that appeared to gradually Tan out' at angles between 1-30°. No conclusive results were obtained to explain how the feathery structure developed, although a number of mechanisms were proposed including; the growth of lamellae on distorted a planes, non-coherent growth and the growth of colonies via an intermediate massive transformation. Examination of linear friction welds identified the presence of three different types of microstructure. The high deformation associated with the forging cycle produced extensive dynamic recrystallisation at the weld centre. The microstructure within this region was predominantly fine grained equiaxed y (50-100 nm). Away from the weld centre the matrix appeared severely deformed, causing extensive twinning and bending of the original lamellar structure. In some cases, regions were observed containing a high density of oxide - nitride based particles indicating the welding parameters were insufficient to cause complete extrusion of flash material from the weld.
| Identifer | oai:union.ndltd.org:bl.uk/oai:ethos.bl.uk:633099 |
| Date | January 1996 |
| Creators | Godfrey, Stuart Paul |
| Publisher | University of Birmingham |
| Source Sets | Ethos UK |
| Detected Language | English |
| Type | Electronic Thesis or Dissertation |
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