Additive Manufacturing Feedstock Designed for Uniform Printing of Metallic Builds

Description:

TECHNOLOGY AREA(S): Materials 

OBJECTIVE: Develop a feedstock metallic alloy that, despite the complex thermal processing cycle of the additive manufacturing process, enables 3D printing of builds with consistent mechanical properties and reliable performance. 

DESCRIPTION: Additive manufacturing (AM) is of considerable interest to the Army as a prototyping method and a means to enable fabrication at the point of need (e.g. the Rapid Equipping Force's Mobile Expeditionary Lab [1]). While AM processing methodologies are well established for polymers, there are considerable challenges in creating reliable metallic components via 3D printing. Similar to what is observed in welding, a heat affected zone (HAZ) develops in the underlying layers of a build. This creates many potential problems: residual stresses, interior porosity due to lack of fusion and undesirable precipitate or grain structures [2-3]. In addition, the orientation of the part during the printing process creates significant anisotropy, a subtlety which many operators may not take into account [4]. While some issues can be mitigated by post-processing, this is not always desirable or available in remote locations. The issue of reliability is a key obstacle to the fabrication of metallic components via AM [2]. Determining the correct processing parameters for a metallic build is difficult and time consuming. Often, especially in a theater of war/remote location, it is not practical to develop a printing procedure when there is a very urgent need for a specific component or tool. The bulk of research into AM metallurgy has been devoted to adjusting engineering alloys such as Ti 6-4 to be more amenable to the AM process [2,3]. This is because AM offers several potential advantages to fields such as aerospace or biomedical that demand complex parts of very specific alloys. Not all potential applications of AM require such high performance alloys. A soldier in a remote location requires a simpler but more reliable process to meet and adapt to evolving mission requirements in the field. Performance of the additive manufacturing process needs to be straightforward, with minimal a-priori technical knowledge of materials and manufacturing. It must be possible to quantify, with a high level of confidence, the reliability of the resulting product in order to appropriately assess the risk associated with its utilization as a field expedient solution. Currently, soldiers are using AM to print polymer tools and components to meet temporary or very specific needs [1,5]. An alloy designed to enable the simple production of metallic objects would be an asset by avoiding the prolonged process development normally required. The key features of such an alloy should be resistance to the formation of coarse processing defects, avoidance of undesirable precipitate or grain structures, and material properties as isotropic as possible with respect to build direction. Development of such an alloy is the objective of this STTR. This alloy should provide a means of simple and rapid production of basic structural tools or hard to source parts (e.g. containers, or frames) that address temporary or mission specific needs of the Army. 

PHASE I: The proposal team will perform an exploratory study to investigate various alloy systems as potential candidates for a consistent additive manufacturing (AM) feedstock. The team should identify potential AM feedstock compositions, with suitability judged on the potential avoidance of interior processing defects, mechanisms to minimize anisotropy/residual stresses, and ease of developing the processing procedures. If necessary, the use of metallic elements with a low melting temperature such as lead and cadmium can be explored, although it is expected proper care will be taken to mitigate potential health hazards. CALPHAD (or equivalent computational thermodynamics tools) phase diagrams of candidate alloy systems should be produced and used to guide the selection process. These calculations should be adjusted to account for the highly variable conditions inherent to AM processing. Potential alloys should be evaluated under conditions that mimic the AM process to directly observe the solidification behavior of the alloy, compositional segregation and precipitate structures that occur. These experimental samples can be fabricated by either AM or powder metallurgy. At the end of Phase I the goal is to demonstrate a clear correlation, in terms of phases identified and micro-structural features observed, between experimental results and computational predictions that justifies the future development of the alloy. 

PHASE II: The team will focus on the experimental and computational efforts to understand the micro-structure and evolution of the selected composition(s) in response to the additive manufacturing (AM) process. For clarification of any of the terms used to describe the Phase II tasks, (e.g., X-direction), please reference the ISO/ASTM 52921 and ASTM F2924 standards. Test components should be fabricated and characterized for both micro-structural and mechanical properties, with adjustments to the composition and processing procedure being made as necessary. The objective is the processing and testing of a minimum of twelve tensile specimens from a single powder or feedstock lot that meet the following criteria. The tensile specimens should be consistent with a standard size ASTM E8/E8M configuration, with six being printed in the X-direction orientation and six oriented in the Z-direction. Visual, ultrasonic, and radiographic examination should reveal no exterior or interior flaws that will compromise structural strength in any of the twelve specimens. The ultimate tensile strengths (UTS) of the samples should have a coefficient of variation of 3.5% or less between parts printed in the same orientation and be within 90% of the UTS of commercial alloys of comparable composition. A hypothesis test comparing the tensile results of the different build orientations should indicate that the mean strengths are not significantly different with a confidence of 95%. After completion of this criteria, a second set of twelve tensile standards should be fabricated on a separate AM machine. The statistical differences in results between these specimens and those obtained from the previous machine should be quantified. 

PHASE III: The proposal team will develop the manufacturing process for commercial production of the alloy. Further adjustments to eliminate machine to machine variability should be made. Any treatments or practices necessary during or after the printing process will be determined and documented for the end user. Certification procedures for the powder or feedstock will be established. If necessary, substitutions for elements with potential health hazards, such as lead or cadmium, used in earlier experiments will be made. Software tools necessary to enable an operator to produce a component utilizing the AM process will be developed. At the end of Phase III, the new additive manufacturing alloy should be available for commercial and military purchase and use. The alloy can consist of powder, wire, or whichever feedstock type is best suited for the specific type of AM process it was developed for. The key aspect of the alloy is that it should enable relatively simple printing of metallic components in a manner analogous to what can be done currently with polymers. Ideally, an operator without a degree in materials and/or metallurgical engineering should be able to design and print a build, and utilize it for its intended purpose, with minimal consideration of complex factors such as internal stresses and warping, anisotropic material properties, or severe processing defects. Examples of components that might be built from this alloy include simple tools (silverware, screwdrivers, etc.), furniture, or complex prototypes or designs that would be difficult to machine. 

REFERENCES: 

1: Cox, Matthew, "Mobile Labs Build On-the-Spot Combat Solutions", Military.com, Aug. 2012, http://www.military.com/daily-news/2012/08/17/mobile-labs-build-on-the-spot-combat-solutions.html

2:  Frazier, William, "Metal Additive Manufacturing: A Review", Journal of Materials Engineering and Performance, Vol 23(6), June 2014, pp. 1917-1928

3:  Zhang, Meixia

4:  Liu, Changmeng, Shi, Xuezhi

5:  Chen, Xianping

6:  Chen, Cheng

7:  Zuo, Jianhua

8:  Lu, Jiping

9:  and Ma, Shuyuan, "Residual Stress, Defects and Grain Morphology of Ti-6Al-4V Alloy Produced by Ultrasonic Impact Treatment Assisted Selective Laser Melting", Applied Sciences, Vol 6(11), Nov. 2016, article 304

10:  Wells, Lee J.

11:  Camelio, Jaime A.

12:  Williams, Christopher B.

13:  White, Jules, "Physical Security Challenges in Manufacturing Systems", Manufacturing Letters, Vol 2(2), April 2014, pp. 74-77

14:  Cox, Matthew, "Army Sees Rapid Prototyping as Key to Rapid Innovation", Defensetech.org, April 2015, https://www.defensetech.org/2015/04/01/army-sees-rapid-prototyping-as-key-to-rapid-innovation/

KEYWORDS: Additive Manufacturing, Metals, 3D Printing, Uniformity, Feedstock, Alloy Development, Manufacturing Process, Manufacturing Knowledge 

CONTACT(S): 

Michael Bakas 

(919) 549-4242 

michael.p.bakas.civ@mail.mil 

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