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Title: Metallurgical performance of hyper-joints in composite to metal joining
Author: Oluleke, Rotimi
ISNI:       0000 0004 6495 0996
Awarding Body: University of Manchester
Current Institution: University of Manchester
Date of Award: 2015
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The use of composites materials in aerospace applications can provide significant weight reduction. However, in airframe designs composites are frequently required to be joined to metal components, which is a challenging issue owing to the very different thermo-physical properties of the two classes of materials. In many cases adhesive bonding has insufficient durability and the requirement for large lapped areas and mechanical fasteners adds considerable weight, reducing the mass savings associated with the introduction of composite structures. A promising approach for improving joint performance is to surface engineer locking features on to the metal part which then integrate with the composite laminate to increase shear load transfer, both via better adhesion and also by mechanical "fit" throughout the thickness of the composite. Such hybrid joints are known as "hyper-joints". For this work, three main techniques are currently being investigated for generating the required surface features. These are; (i) sculpting surfaces using power beam local surface melting techniques (Surfi-Sculpt), (ii) building surface protrusions by additive layer manufacturing (AM) and (iii) arc percussive welding process. The present work aims to provide understanding of the critical metallurgical interactions during the growth/production of the surface engineered features and how some build parameters might affect the eventual joint integrity, durability and performance. To enable the use of the arc-percussive welding process as a viable manufacturing route for these hyper-joints, optimisation of its process parameters were studied in the course of this work. Further work on the effect of process parameters such as voltage, travel velocity, gap setting and time delay on the quality of the weld were also investigated in this study. Of the above mentioned process parameters, the voltage across the capacitor and travel velocity of the actuator have been found to have far greater effect on the weld quality. More importantly, the travel velocity of the actuator was also found to help determine the shape, size and distribution of the melt pool. Based on the systematic study and analysis of all of the process parameters involved, an optimised process window has now been proposed. Microstructural characterisations of hyper-joint samples made via the three manufacturing routes were performed in the course of the work. The observed microstructures were related to the process history and the process parameters. Most importantly it was for found that in common with most welding and AM processes, columnar prior β grains developed in all the samples studied by epitaxial-regrowth either along the build or weld directions. It was also found that the fusion zone of the similar titanium alloy (Ti-6Al-4V to Ti-6Al-4V) arc-percussive welded samples were characterised by extremely fine acicular alpha' martensitic platelets formed as a result of the high cooling rate associated with the process. On the other hand, the fusion zone of the dissimilar titanium alloy (Ti-6Al-4V to β21S) arc-percussive welded samples was characterised by an extremely fine cellular structure prompted by constitutional supercooling. Finally the microstructures observed in the Surfi-Sculpt samples were found to be dependant on the swipe pattern and duration. In order to assess the performance of hyper-joints made via two of the candidate manufacturing routes, tensile testing of standard and modified tensile samples was performed. The result indicated that on average the strength of these hyper-joint pins were within nominal values expected of the Ti-6Al-4V and β21S alloys. It was also found that with optimum weld conditions, failure occurred only along the gauge length for the arc-percussive welded samples whilst the strength of the AM samples degrade as the gauge diameter decreases as the effect of surface defects became prominent. Results are also reported on a novel method for testing the shear strength and properties of individual hyper-pins manufactured via this process. Interpretation of the shear test was assisted by finite element modelling. The shear test results indicated that the arc-percussive welded samples outperformed the SLM-AM samples thereby giving credence to the process as a viable means for making the pins.
Supervisor: Prangnell, Philip Sponsor: Not available
Qualification Name: Thesis (Ph.D.) Qualification Level: Doctoral
EThOS ID:  DOI: Not available
Keywords: Hyper-Joint ; Ti-6Al-4V ; ß21S alloy ; FEA Modelling ; Shear Testing ; Metallurgical Performance and Composite to Metal Joining