The titanium alloy Ti-6Al-4V has been favoured by the aerospace industry for the past several decades due to its good combination of specific mechanical properties, alongside corrosion and fatigue resistance. Titanium alloys are naturally suited to the near net shape processing technique of Additive Manufacture (AM) due to both the inherent high cost of the raw materials, and the difficulties associated with machining the alloys. Unfortunately, the combination of Ti-6Al-4V with AM has been found to lead to undesirable microstructures with respect to large columnar prior β grains being found to grow potentially across the entire height of builds. This microstructure has been shown to lead to property anisotropy and poor fatigue resistance. However, it has recently been found that the integration of an additional process step that lightly deforms the deposited material between added layers leads to the refinement of this undesirable microstructure. This work characterises the effect that two different deformation processing techniques have on two different additive manufacturing processes; the effect of peening on a laser-powder AM technique, and the effect of rolling on an electric arc-wire AM technique. In both cases far more randomly textured prior β grains were found with an average grain size of > 100 µm rather than mm long columnar grains with a common growth direction formed in the non-deformed builds. The refined β microstructure was found to lead to a reduction in texture of the room temperature alpha phase. The low stains involved (>10%) indicated that the refined grain structures did not form by traditional recrystallisation mechanisms. In-situ EBSD measurements at temperatures spanning the alpha → β phase transformation have been used to observe the growth of new β orientations from crystallographic twins in the deformed microstructure that may explain the origin of the refined grains. New β orientations were observed to grow from twinned alpha colonies and from between alpha laths, where the new β is found to grow sharing a twinning relationship with the residual β. Simulation of both of the individual processing steps under laboratory conditions has been found to successfully replicate the refinement observed in process. Orientation analysis suggests that twinning of the residual β could lead to the texture observed in the refined grains. It is therefore suggested that the refined grains are formed from β twinned regions in the deformed material growing under the alpha → β phase transformation, as the material is heated by the next added layer during AM.
| Identifer | oai:union.ndltd.org:bl.uk/oai:ethos.bl.uk:713580 |
| Date | January 2017 |
| Creators | Donoghue, Jack |
| Contributors | Prangnell, Philip |
| Publisher | University of Manchester |
| Source Sets | Ethos UK |
| Detected Language | English |
| Type | Electronic Thesis or Dissertation |
| Source | https://www.research.manchester.ac.uk/portal/en/theses/hybrid-additive-manufacture-and-deformation-processing-for-large-scale-nearnet-shape-manufacture-of-titanium-aerospace-components(5bb9e7db-824e-46e2-a832-c2780e15d6b8).html |
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