TECHNOLOGY AREA(S): Air Platform, Ground/Sea Vehicles, Materials/Processes

ACQUISITION PROGRAM: Program Executive Office � Land Systems (ACAT I vehicle programs)

OBJECTIVE: This project will develop new material processing routes and technologies toward high-toughness, super-hard cubic boron nitride-based materials for use in friction stir welding (FSW) tool applications.

DESCRIPTION: FSW is a solid-state joining process that uses a third body tool to join two mating surfaces.� Heat is generated between the tool and material, which softens material to allow material mixing.� Currently, it is primarily used on aluminum structures that need superior weld strength without a post-weld heat treatment.� The Navy and Marine Corps have interest in developing tools for use in the joining and repair of high hardness steel and other hard materials that involve higher temperatures and fracture toughness than aluminum FSW; this will require the development of new, low-cost tool materials with longer lifetimes.

The proposed research will advance the current state-of-the-art of FSW tool materials.� The key aspects of the study will be: A) material selection utilizing theory/computational modeling to evaluate tool material compatibility; B) fabrication of material coupons; C) mechanical evaluation of test coupons; and D) fabrication and evaluation of friction stir welding tools.� The project will develop new bulk material or novel processing techniques for material fabrication.� This research has the potential to improve the efficiency of current FSW tool technology by developing lower cost processing techniques or identifying new compositions that lead to less expensive, longer lasting tools.

Current tool materials for FSW processing of steel are based primarily on cubic boron nitride (cBN).� These materials are difficult to process to full density due to the highly covalent nature of the bonding and the sensitivity of the reversible phase transformation at higher processing temperatures.� Metallic bond phases have been utilized to make dense multiphase �cermet �compositions that provide a tough, bond matrix phase (metal) with super-hard cBN particulate.� Previous studies have shown that smaller amounts of the bonding phase result in tool materials that have high hardness, but suffer from poor thermal stress response.� They demonstrate poor thermal cycling response, and typically fail in the initial plunge or during the extraction of the tool in the friction stir process.� Transient thermal loads drive the through-thickness stresses causing the tool to fail from poor strain response.� The addition of a more ductile bond phase alleviates this failure mode, but drives a more rapid wear response of the tool, again leading to shortened tool lifetimes.� Expensive refractory metals are currently used to provide tools with longer life, but that expense limits the commercial viability of FSW for more widespread industrial use.

PHASE I: Define and develop a concept/approach using computational tools for a new/optimized tool material composition, or novel processing technique to produce bulk materials of current compositions.� It is intended that the focus of the program be to target materials by understanding the thermochemistry of tool material in its use environment.� This includes investigating if the bond phase reacts with harder phases that may negatively impact tool performance at the use temperature of 1,000�C and exploring if the tool material reacts with the steel at those temperatures to negatively impact the as-fabricated weld properties.� Computationally guided materials selection to define the composition space will be of high importance.

The mechanical and thermal properties (modulus, fracture toughness, strength, coefficient of thermal expansion, and thermal conductivity) properties of the candidate tool materials (at room temperature (RT) and use temperature) can cover a wide range and there are tradeoffs for these tool properties.� However, as the Navy and Marine Corps desire improved tool life at lower cost, some properties of commercially available tool materials are listed as a reference.

To ensure progress in the Phase I plan, a key deliverable will be a sample material coupon (0.25� diagonal x 0.5� high) for independent Government testing and report of achieved material properties, with requisite documentation showing the rationale for selection (computational thermodynamics) and the description of the processing technology used to process the material.

Develop a Phase II plan.

PHASE II: Based on Phase I results, develop, demonstrate, and validate the proposed computational approach for new/optimized materials and/or processes.� This will include demonstrating optimized material composition(s) in large test builds in order to measure mechanical and thermal properties (at RT and elevated temperatures), and to characterize the microstructure and composition (grain size, porosity, phase identification/quantification).

Begin initial FSW studies in order to determine viability of the material for compatibility with the thermal and mechanical loads placed on the tool during use, as well as potential chemical interaction with the steel weldment.� It is recommended that the performer initiate work with experts in commercial FSW processing, as well as joining/repair OEMs to facilitate transition into Phase III.

Phase II will necessitate scale up of the process in order to produce larger size billets/blanks for thermal and mechanical property characterization as well as for extraction of sample tool shapes for initial FSW studies.� A Phase II option would involve supplying a government laboratory or FSW partner with the small tool shapes for plunge and extraction tests on steel (minimum 1� diagonal x 1� high).

PHASE III DUAL USE APPLICATIONS: Phase III will produce full-size FSW tools made from materials and processes developed under the program performing at equivalent speeds, feed rates, and test boundary conditions set forth during the program by the Navy (equivalent to or better than state-of-the-art tool materials performance) to industry partners or Navy Warfare Centers/DoD production/maintenance facilities.� Phase III will also plan to transition optimized materials compositions and/or processes to commercial suppliers through partnering agreement with OEMs, repair depots, etc.� FSW can be used in manufacturing and repairs of very hard materials in commercial industries as well.

REFERENCES:

1.� Rai, R., De, A., Bhadeshia, H. K. D. H., and DebRoy, T. �Review: friction stir welding tools.� Science and Technology of Welding and Joining. 16, 325-342. http://www.tandfonline.com/doi/abs/10.1179/1362171811Y.0000000023

2.� Dialami, N., Chiumenti, M., Cervera, M., and Agelet de Saracibar, C. �Challenges in Thermo-mechanical Analysis of Friction Stir Welding Processes.� Archives of Computational Methods in Engineering: State of the Art Reviews. 2017. 24, 189-225. https://link.springer.com/article/10.1007/s11831-015-9163-y

3.� Hanke, S., Lemos, G., Bergmann, L., Martinazzi, D., Dos Santos, J., and Strohaecker, T. �Degradation mechanisms of pcBN tool material during Friction Stir Welding of Ni-base alloy 625. Wear, 2017. 376-377, 403-408. http://www.sciencedirect.com/science/article/pii/S0043164817301898

4.� Sorensen, CD. �Progress in Friction Stir Welding High Temperature Materials.� Brigham Young University. http://fsrl.byu.edu/presentations/Progress%20in%20Friction%20Stir%20Welding.pdf

KEYWORDS: Friction Stir Welding; FSW