Microstructure and Porosity of Laser Welds in Cast Ti-6Al-4V with Addition of Boron
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D with most other metallic materials, titanium alloys show superior specific strength and fatigue resistance.[1] This makes them advantageous in applications where light weight is an important parameter, such as in aerospace. Lighter components result in lower fuel consumption, which decreases both the environmental impact as well as costs. To be able to build optimal components, with regard to both cost effectiveness and lightweight design, welding is one of the key technologies for manufacture of large structural parts for aerospace applications. The microstructure of welded a-b titanium alloys is typically characterized by coarse prior-b grains occasionally with a continuous a layer in the prior-b grain boundaries, and an acicular microstructure consisting of
SAKARI TOLVANEN is with the Department of Industrial and Materials Science, Chalmers University of Technology, 41296 Gothenburg, Sweden. Contact e-mail: [email protected] ROBERT PEDERSON is with the Department of Industrial and Materials Science, Chalmers University of Technology and also with the Department of Engineering Science, University West, 46186 Trollhttan, Sweden. UTA KLEMENT is with the Department of Industrial and Materials Science, Chalmers University of Technology. Manuscript submitted October 25, 2017.
METALLURGICAL AND MATERIALS TRANSACTIONS A
fine a’ and/or a plates separated by thin layers of b phase.[2,3] The fine acicular microstructure contributes to the high strength of additively manufactured and welded titanium alloys but, on the other hand, these aspects reportedly contribute to the lowered ductility of Ti-6Al-4V welds.[4–6] Recent work[7–9] has indicated that addition of a small amount of boron to a titanium alloy can render a significantly finer microstructure in cast titanium alloys. Similar observations have been reported[10–12] in additive manufacturing where boron addition has been shown to refine the otherwise typically coarse columnar prior-b grain morphology. Reconstruction of the b grains, using the electron back scatter diffraction data of a phase, has proven to be a powerful method to characterize prior-b grain structure in titanium alloys.[13–15] In the present work, the influence of boron addition in titanium alloy laser beam welds is explored. The titanium alloy investigated here is the a-b alloy Ti-6Al-4V, and two modified versions of it with 0.06 wt pct B and 0.11 wt pct B addition, respectively. The same welding process parameters were used when welding the different alloy compositions. The following selected key features were investigated and quantitatively characterized with regard to the effect of boron addition: (i) prior-b grain size and a phase morphology in the weld zones, (ii) the porosity in the fusion zone, (iii) the weld geometry, and (iv) the effect of welding on the TiB particle size and distribution.
II.
measured in the FZ, the HAZ, and the BM, respectively. The shown values are an average of a minimum of 20 measurements. The porosity in the welds was examined with a Nikon XTEK XTH 225kV X-ray microt
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