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Open AccessArticle

Fatigue Behavior of Non-Optimized Laser-Cut Medical Grade Ti-6Al-4V-ELI Sheets and the Effects of Mechanical Post-Processing

1
Institute of Materials Science, Technical University of Dresden, 01062 Dresden, Germany
2
Department of Materials Characterization and Testing, Fraunhofer-Institute for Material and Beam Technology IWS, 01277 Dresden, Germany
*
Author to whom correspondence should be addressed.
Metals 2019, 9(8), 843; https://doi.org/10.3390/met9080843
Received: 18 June 2019 / Revised: 22 July 2019 / Accepted: 26 July 2019 / Published: 30 July 2019
(This article belongs to the Special Issue Titanium Alloys for Biomedical Implants and Devices)
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Abstract

The study presented investigates the fatigue strength of the (α+β) Ti-6Al-4V-ELI titanium alloy processed by laser cutting with and without mechanical post-processing. The surface quality and possible notch effects as a consequence of non-optimized intermediate cutting parameters are characterized and evaluated. The microstructural changes in the heat-affected zone (HAZ) are documented in detail and compared to samples with a mechanically post-processed (barrel grinding, mechanical polishing) surface condition. The obtained results show a significant increase (≈50%) in fatigue strength due to mechanical post-processing correlating with decreased surface roughness and minimized notch effects when compared to the surface quality of the non-optimized laser cutting. The martensitic α’-phase is detected in the HAZ with the formation of distinctive zones compared to the initial equiaxial α+β microstructure. The HAZ could be removed up to 50% by means of barrel grinding and up to 100% through mechanical polishing. A fracture analysis revealed that the fatigue cracks always initiate on the laser-cut edges in the as-cut surface condition, which could be assigned to an irregular macro and micro-notch relief. However, the typical characteristics of the non-optimized laser cutting process (melting drops and significant higher surface roughness) lead to early fatigue failure. The fatigue cracks solely started from the micro-notches of the surface relief and not from the dross. As a consequence, the fatigue properties are dominated by these notches, which lead to significant scatter, as well as decreased fatigue strength compared to the surface conditions with mechanical finishing and better surface quality. With optimized laser-cutting conditions, HAZ will be minimized, and surface roughness strongly decreased, which will lead to significantly improved fatigue strength. View Full-Text
Keywords: Titanium alloys; Ti-6Al-4V-ELI; fatigue; laser cutting; post-processing; α’-martensite; HAZ; barrel grinding; notch; fracture Titanium alloys; Ti-6Al-4V-ELI; fatigue; laser cutting; post-processing; α’-martensite; HAZ; barrel grinding; notch; fracture
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This is an open access article distributed under the Creative Commons Attribution License which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited (CC BY 4.0).
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Reck, A.; Zeuner, A.T.; Zimmermann, M. Fatigue Behavior of Non-Optimized Laser-Cut Medical Grade Ti-6Al-4V-ELI Sheets and the Effects of Mechanical Post-Processing. Metals 2019, 9, 843.

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