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Combined EBSD-Investigations and In-situ Tensile Tests of a Direct Metal Deposited Ti6Al4V-Alloy
Norwegian University of Science and Technology, Faculty of Natural Sciences and Technology, Department of Materials Science and Engineering.
2013 (English)MasteroppgaveStudent thesis
Abstract [en]

Two blocks made of Ti6Al4V material produced by Norsk Titanium Components ned Direct Metal Deposition (DMD) production technology were delivered for this investigation. The main difference between the blocks was the different waiting time implemented in the production parameters. The different waiting time implied that the material were allowed to cool to a deisred Interpass Temperature (IT). The blocks are referred to as T200 and T600 after their lowest IT of <200 degrees celsius and 500-700 degrees celsius, respectively. Sintef Manufacturing Raufoss reported elongations of 5% and 8% in the deposition-direction of the two blocks, which was lower than the 10% elongation obtained for the building-direction. The objective was to find any explanations for the impaired ductility in the deposition-direction, and why T200 have a lower elongation than T600. To reveal differences in micro- macrostructure and deformation-mechanisms between the blocks was combined EBSD+In-situ. tensile tests the main tool. Complementary studies with optical microscopy of etched in-situ specimens were also performed. The work started with an extensive macroetching of different planes relative to the deposition-direction. This revealed a 3D-image of the epitaxially growing solidification structure of columnar prior beta grains. Measurements of grain size and morphology did not indicate significant differences between the blocks. Later was EBSD selected to prior beta grain boundaries for specimens at the center of the blocks. Measurements of the thickness of primary alfa phase also indicated small differences between the blocks. From this it was concluded that the different ITs lead to small differences in solidification structure and the amount of primary alfa phase in the center of the blocks. The EBSD + in-situ tensile tests were carried out for specimens fabricated from the blocks in the deposition direction, such that investigation could be performed in the XZ-plane. The force was applied parallel to the deposition direction X. The in-situ specimens in block T200 had a slightly higher position in the block giving a finer microstructure because of faster cooling rates. The T200 specimens also contained to deposition layers in contrast to only one in T600 specimens. This made a direct comparison difficult. However, from the four in-situ specimens investigated was the following concluded: specimens from T200 deformed more homogeneously on a macroscale. Correspondingly was more activity observed at prior beta grain boundaries for T600. The more active grain boundaries in T600 may result from the different cooling rates from the peak temperature. Slower cooling rates will give smaller prior beta grains decorated with more homogeneous alfa phase along the boundaries of T600 specimens. The frequently reported detrimental mismatch between soft and hard HCP-grains in titanium alloys were also observed. However, the orientation and morphology of these grains seems to play an important role. Cracks growing along prior beta grains were always located at the side having a non-Burgers orientation relationship. The lamellar basketweave microstructure also seem quite effective to stop cracks from growing further, because of the plates having distinct orientations. This work has also uncovered the transition in microstructure between the substrate plate and first deposition layer. The microstructure develops from a very fine bimodal plate structure, through a equiaxed region with small amounts of primary alfa phase, to early stages of the columnar beta grain configuration. The first columnar beta grains are very small in size compared to higher up in the block, and they contains very small amounts of primary alfa phase. However, the basketweave microstructure develops relatively close to the substrate plate.

Place, publisher, year, edition, pages
Institutt for materialteknologi , 2013. , 154 p.
URN: urn:nbn:no:ntnu:diva-22438Local ID: ntnudaim:8996OAI: diva2:649682
Available from: 2013-09-19 Created: 2013-09-19 Last updated: 2013-09-19Bibliographically approved

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