Can we control the (local) microstructure in additively manufactured Ti64 by varying the EBM beam scan strategy?

Significance 

Computer Aided Design (CAD) modeling has remarkably simplified how artists, engineers and scientist present their work/models by enabling 3D printing. The method, technically termed Additive Manufacturing (AM), is of significant interest in large-scale manufacturing. In fact, over the past two decades, AM has moved to the forefront of industrial and academic research due to advancements in AM instrumentation and technology, also opening up AM for a wider selection of materials. Despite this progress, several challenges remain in metal-based AM. Electron beam melting powder bed fusion (EBM PBF) is an AM process that employs a high-energy electron beam as a fine-controlled moving heat source to melt pre-alloyed metal powder deposited on a substrate in a ‘layer-by-layer’ process. During EBM, the spatiotemporal variations in energy density due to the rastering beam affects the thermal gradient and solidification cooling rate of the melt pool, which subsequently results in the typical columnar microstructure commonly found in AM builds. However, the entire range of variations in microstructure, which can be obtained by varying thermal transients by controlling process parameters, is not yet fully understood. Therefore, correlations between AM process parameters and resulting local microstructure need to be identified and strategies need to be developed that generate localized and controlled thermal transients by manipulating EBM process parameters, which can aid in the design of builds with homogenous microstructures.

Of particular interest is the Ti64-based parts with complex geometries. This makes AM of Ti64-based components with site-specific microstructural control a topic of substantial scientific interest. Review of previously published research reveals that early efforts attempted to address the outlined challenges; however, these studies were very limited. In fact, most of these studies focused on Ni-based superalloys. On this account, The Ohio State University researchers: Meiyue Shao (PhD candidate), Dr. Sriram Vijayan (postdoctoral fellow) and led by Professor Joerg Jinschek, in collaboration with Prof. Sudarsanam (Suresh) Babu (UT/ORNL Governor’s Chair of Advanced Manufacturing Professor) and Dr. Peeyush Nandwana at the Oak Ridge National Laboratory, used three different EBM beam scan strategies, i.e. the standard raster scan, ordered spot scan, and random spot scan patterns, to fabricate three identical Ti-6Al-4V blocks. Their work is currently published in the research journal, Materials and Design.

In their approach, a systematic analysis of microstructure and texture was carried out on Ti64 AM builds of similar geometry but fabricated using three different beam scan strategies. These beam scan strategies were employed to vary thermal transients influencing solidification parameters and thereby changing the Ti64 microstructure. Scanning electron microscopy and large-area electron backscatter diffraction experiments were performed to gather statistically significant, site-specific microstructural and crystallographic texture data across multiple build planes in and from all three samples. The data was then analyzed using a consistent semi-automated image analysis approach.

The authors reported that both spot scan strategies resulted in coarser α laths and smaller prior β grains with width and height < 1/3 of the value of the typical large columnar grains, observed in raster scan samples. The combined fraction of type 2 and type 4 α/α LBs measured in the three samples was found to be between 0.50 and 0.85, which is greater than the expected combined fraction of ~0.36 for a random distribution of α variants. This suggested the presence of a mild to weak variant selection in EBM Ti64.

In summary, the study demonstrated the use of three different EBM PBF beam scanning strategies to systematically investigate their effect on variations in the microstructure and crystallographic texture. The results showed that microstructure of EBM builds can be modified in a site-specific manner using different EBM beam scan strategies. In a statement to Advances in Engineering, Professor Joerg Jinschek, the lead author, explained that their findings proved that statistically significant variations in microstructural features observed in the three EBM Ti64 samples indicate that the build experiences faster cooling rate and fewer thermal gyrations above the β-transus when using the standard raster scan strategy, compared to when using a spot melt strategy, such as the ordered spot scan or the random spot scan strategy.

Acknowledgement:

This project is supported by the Australia–US Multidisciplinary University Research Initiative (MURI) program, under ONR award number N00014-18-1-2794 and project “Rationalization of Liquid/Solid and Solid/Solid Interphase Instabilities During Thermal-Mechanical Transients of Metal Additive Manufacturing.

microstructure in additively manufactured Ti64-Advances in Engineering

Reference

Meiyue Shao, Sriram Vijayan, Peeyush Nandwana, Joerg R. Jinschek. The effect of beam scan strategies on microstructural variations in Ti-6Al-4V fabricated by electron beam powder bed fusion. Materials and Design: volume 196 (2020) 109165.

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