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Improving Fatigue Life of LENS Deposited Ti-6Al-4V through Microstructure and Process Control

Prabhu, Avinash W
http://rave.ohiolink.edu/etdc/view?acc_num=osu1388768129

Abstract Details

2014, Master of Science, Ohio State University, Welding Engineering.
Laser Engineered Net Shaping (LENS) is a solid freeform fabrication process capable of producing net shape, custom parts through a layer-by-layer deposition of material using a laser energy source. LENS based Ti-6Al-4V parts are currently being explored for applications to aerospace and biomedical implant applications. To achieve satisfactory mechanical performance in the components, homogeneity of the microstructure and the physical structure is important. This study explores methods of improving the fatigue life of Ti-6Al-4V LENS deposits through the creation of defect free components with appropriate microstructure. The work focuses on the impact of beta grain refinement and the elimination of lack of fusion porosity defects on the fatigue life of the alloy. Further, a model is developed to predict the epitaxial grain growth in LENS builds of Ti-64. This model is used to augment the prediction capability of a simultaneous transformation kinetics (STK theory) based model developed previously. The beta grain refinement in the fatigue properties of Ti-64 is achieved through addition of boron in amounts < 3 wt%. The addition of boron is found to refine the columnar beta structure typically observed in LENS deposited Ti-64. However, at high concentrations of boron, it was difficult to discern the prior beta grain size to visualize the extent of grain refinement. The boron alloying further causes a significant change in the structure of alpha laths, leading the shorter and thicker individual laths. Grain boundary alpha is not observed in the microstructure on addition of boron above a certain threshold. The addition of boron is observed to improve the fatigue properties of deposited Ti-64 samples. The Ti-64 LENS builds are observed to contain lack-of-fusion porosity in the lower regions of the deposit close to the substrate. The effect of process parameters namely the power, travel speed, hatch width, pre-heating, powder flow rate, substrate surface quality, and the hatch path on the amount of porosity is analyzed through trial experiments followed by a more detailed design of experiments approach. At a fixed power setting of 400W, the hatch width and powder flow rate are observed to have a significant influence on the extent of porosity present in the deposit. The hatch width has been further optimized through a CFD based simulation based approach leading to an increased material efficiency and reduced time of building. These builds are observed to show a columnar beta grain structure growing epitaxially from the base of the deposit and consist of primarily basketweave alpha. The fatigue properties are analyzed at the combination of parameters determined through experiments and are found to show an improvement over previously reported values by 3 times. The epitaxial grain growth seen in the LENS deposits is modeled using the classical grain growth equation. The prediction is observed to be sensitive to the atomic mobility, starting grain size, activation energy for grain growth and alpha dissolution temperatures. This grain growth information is incorporated into simultaneous transformation kinetics (STK theory) based microstructure model to improve the predictions of microstructure fraction in Ti-6Al-4V LENS builds.
Wei Zhang (Advisor)
Dave Farson (Committee Member)
Sudarsanam Babu (Committee Member)
166 p.
Materials Science
Ti-6Al-4V; LENS; Additive Manufacturing; Laser Additive Manufacturing; Grain growth modeling; Boron alloyed Ti-6Al-4V;

Recommended Citations

Citations

  • Prabhu, A. W. (2014). Improving Fatigue Life of LENS Deposited Ti-6Al-4V through Microstructure and Process Control [Master's thesis, Ohio State University]. OhioLINK Electronic Theses and Dissertations Center. http://rave.ohiolink.edu/etdc/view?acc_num=osu1388768129

    APA Style (7th edition)

  • Prabhu, Avinash. Improving Fatigue Life of LENS Deposited Ti-6Al-4V through Microstructure and Process Control. 2014. Ohio State University, Master's thesis. OhioLINK Electronic Theses and Dissertations Center, http://rave.ohiolink.edu/etdc/view?acc_num=osu1388768129.

    MLA Style (8th edition)

  • Prabhu, Avinash. "Improving Fatigue Life of LENS Deposited Ti-6Al-4V through Microstructure and Process Control." Master's thesis, Ohio State University, 2014. http://rave.ohiolink.edu/etdc/view?acc_num=osu1388768129

    Chicago Manual of Style (17th edition)

osu1388768129
1,607
© 2014, some rights reserved.Creative Commons License Improving Fatigue Life of LENS Deposited Ti-6Al-4V through Microstructure and Process Control by Avinash W Prabhu is licensed under a Creative Commons Attribution-NonCommercial-ShareAlike 3.0 Unported License. Based on a work at etd.ohiolink.edu.
This open access ETD is published by The Ohio State University and OhioLINK.