Session: 02-01-01: 7th Annual Conference-Wide Symposium on Additive Manufacturing: Metals I

Paper Number: 96493

96493 - Physics-Based Microstructure Modeling for Grain Tailoring and Refinement in Wire Arc Additively Manufactured Ti-6Al-4V Alloy 

This paper presents the advances in wire arc metal additive (WAAM) processes and efforts in grain tailoring during the process for desired size and shape. WAAM is a promising process for fabrication various metal components like aluminium, nickel alloys, steel and chiefly titanium. WAAM can cause fabrication and post-processing time reduction in comparison with traditional processes. WAAM is a novel method that allows for greater production rates (several kg/hr) and allows for the bulk production of large components. However, the high cooling rates and thermal gradient of the fusion-based metal additive manufacturing process often leads to an almost exclusively columnar grains microstructure (especially in titanium-based alloys), which can result in anisotropic mechanical properties and are, in consequence, undesirable.It is well known that the microstructure of the product depends on its thermal history during the fabrication process. The WAAM process involves a repeated thermal cycle of heating and cooling which produces meta-stable microstructures and inhomogeneous composition in the fabricated part. WAAM fabricated Ti-6Al-4V samples show anisotropic properties with lower strength and higher elongation values in the build direction (Z) compared to deposition direction (X), which is mainly attributed to the grain size of α lamellae and the orientation of the elongated prior β grains. In titanium alloys, the local solidification of small melt pools during layer deposition can result in epitaxial growth and the formation of columnar grains as heat is primarily extracted through the previously deposited solidified layer, often across a steep thermal gradient. The emergence of grain boundary α that forms during the subsequent solid-state transformation along aligned prior-β columnar grains can result in highly anisotropic properties and poor ductility in components produced by AM. The issue of large directional grains can be addressed either with the addition of potent nuclei, a solute that promotes constitutional supercooling or a combination of the two. Nuclei are naturally present in liquid metals and are the starting point of every grain. Introducing additional potent nucleant particles by inoculation would facilitate grain refinement by increasing the total number of grains and therefore reducing the average grain size.Columnar grain growth occurs parallel to the direction of heat transfer from a pre-existing solid region and must be restricted to obtain homogeneity and to facilitate grain refinement. By using potent nuclei and a solute in unison, grain refinement can be optimized to produce many nucleation events. Nucleant particles are effective in improving the β-grain density through constitutional supercooling for cast titanium alloys. We conducted coupled thermal and microstructure simulations to study the importance of solutes like Cr. and/or nuclei phases like TiB on final grains size and topology of the printed Ti-6Al-V titanium alloy with WAAM method. The simulation software has a module for 3D thermal simulation of WAAM method and the elements birth and death numerical method used for updating the simulation domain to consider the receiving molten metal from the metal wire during each time step of the simulation. The software uses Cellular Automata to simulate the solidified grain topology and grain growth according to the related local heating and cooling curve. Simulation results show that the final microstructure of Ti-6Al-4V alloy at points without adding nucleants (solute and nuclei phase) will be columnar. By adding some solutes (Cr), the final microstructure is finer but still remains columnar. The new grains will start to nucleate at the grain boundaries of older grains in heat-affected zone (HAZ) and will continue to grow to fill the whole sample. In this case, the whole sample will have an equiaxed microstructure. If the temperature is high enough to keep the sample in the HAZ, the new grains will start to grow. The grain growth in our simulation is based on the local surface energy and grain growth kinetics. Although the final microstructure is equiaxed for all scenarios, adding solute and nuclei phases will change the final average size of the grains and hence achieve grain refinement.

Presenting Author: Tugrul Ozel Rutgers University

Presenting Author Biography: Tuğrul Özel is a Professor of Industrial and Systems Engineering. Prof. Özel 's research focuses on manufacturing, 3D printing, laser material processing, automation, physics-based machine learning and AI. Prof. Özel published four edited books; Intelligent Machining (2009), Micromanufacturing: Design and Manufacturing of Micro-Products (2011), Biomedical Devices: Design, Prototyping and Manufacturing (2016), Modern Manufacturing Processes (2020), over 200 peer-reviewed articles that received high number of citations. He is the Editor-in-Chief of International Journal of Mechatronics and Manufacturing Systems, Associate Editor of Journal of Manufacturing Science and Engineering, Editorial Board Member of several other journals. He served on the committee of over 60 conferences, delivered 80 invited, keynote, and planetary talks at international conferences. His research funded by the US-NSF, NASA, DoC-NIST, TTRF Foundation, and industry. Prof. Özel supervised over 60 graduate and visiting students in these areas.

Authors:

Tugrul Ozel Rutgers University
Hamed Shokri Rutgers University- New Brunswick
Hamed Hosseinzadeh University of South Carolina