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Substrate Oxide Effects in the Adhesion of Supersonically Accelerated Aluminum Particles

Swetaparna Mohanty1, Carmine Taglienti1, Wanting Xie1,2, Victor K. Champagne3, and Jae-Hwang Lee1

1 Department of Mechanical and Industrial Engineering, University of Massachusetts, Amherst,Massachusetts 01002, USA.

2 Department of Physics, University of Massachusetts, Amherst, Massachusetts 01002, USA.

3 United States Army Research Laboratory, Aberdeen Proving Ground, Maryland 21005, USA.

Supersonic particle deposition, or cold spray, is a deposition process of solid feedstock powders, employed in additive manufacturing processes to fabricate parts and structures with melting temperatures below the powder’s. In this process, a supersonic stream of microparticles are accelerated and subjected to microscopic collisions with a target substrate. At these high speeds, the kinetic energy of the particles leads to extreme plastic deformation at the interfaces of both the microparticle and the target substrate, which eventually enables solid-state consolidation/adhesion. Study of impact dynamics, bonding mechanism and critical velocities help us in understanding the cold spray process in a better way. It has been generally understood that the oxide layer plays a significant role in the bonding process of a metallic particle to a metallic surface. This oxide surface acts as a barrier preventing the formation of a metallic bond between the particle and the substrate. Thus, bonding is considered to be governed by the ability of the collision to remove this oxide barrier and expose the pure metals to each other for metallurgical bonding.

In this study, as a model system, on an, aluminum 6061 substrate with three different thicknesses (5, 15, 25 nm) of aluminum oxide layers was prepared using atomic layer deposition. The dynamic behavior of the single microparticles was quantified by using a micro-ballistic method: advanced laser induced projectile impact test (α-LIPIT). In the α-LIPIT experiments, aluminum 6061 microparticles (~ 20 µm diameter) were accelerated to controlled high speeds (50 – 1,100 m/s) and impacted with the substrate. Rebound speeds after impact/collision and coefficients of restitution were precisely quantified using an ultrafast microscopic imaging technique. Moreover, critical velocities of different oxide thicknesses were measured. With the increase in the surface oxide thickness, the rebound velocities were found to increase. The cross-sectional images from focused ion beam milling showed that the thicker oxide acted as a barrier especially near the collision center. Furthermore, the regions of bonding were found towards the edges of the collision region, supporting the idea that the oxide is more easily moved out of the way to expose the pure metals for bonding from the regions closest to the edge. Effects of surface oxidization in the adhesion mechanism can help to better understand and optimize the cold spray process.

* This material is based upon work supported by the United States Army Research Laboratory under Grant No. W911NF-15-2-0024

Extreme plastic deformation of nanostructured copolymer micro-particles in additive manufacturing

Ara Kim1, Wanting Xie

1, Kathy Zhu1, and Jae-Hwang Lee1

1Department of Mechanical and Industrial Engineering, University of Massachusetts, Amherst, Massachusetts 01002, USA.

Among additive manufacturing techniques, cold spray utilizes high-velocity impacts of solid microparticles or feedstock powders. Cold spray with polymeric microparticles have been largely unexplored due to the inherent complexities arising from its strong strain-rate-dependency. We envisioned that multi-phase polymers or phase-separated block copolymers (BCPs) consisting of two or more mechanically distinctive nano-scale phases can be tailored to have high-strain-rate mechanical properties, which are favorable to additive manufacturing via cold spray.

In order to gain deep insight into the material’s behavior at the high-strain-rate regime, controlled impact tests at velocities that are relevant for cold spray applications, we performed single-particle impact experiments of polystyrene-block-polydimethylsiloxane BCP microparticles because polystyrene and polydimethylsiloxane phases are glassy and rubbery, respectively. Using the laser-induced projectile impact test (LIPIT) method, a single BCP microparticle was accelerated to high velocities (100 - 600m/s) and the microparticle’s collision with a rigid substrate was observed with an ultrafast imaging system using femtosecond illumination pulses. Coefficients of restitution and collision-induced extreme plastic deformation features of the BCP particle were investigated for different impact conditions. Using electron microscopy and focused ion beam milling, we also demonstrated impact-induced morphological changes of the BCPs.

* This material is based upon work supported by the National Science Foundation under Grant No. CMMI-1760924

In-Situ STEM Observations of Phase Transformations and Grain Refinement in Single Cold Spray Splats 

Benjamin A. Bedard1, Seok-Woo Lee1, Avinash M. Dongare1, Harold Brody1, Victor K. Champagne2, and Mark Aindow1 

1. Department of Materials Science and Engineering, Institute of Materials Science, University of Connecticut, 97 North Eagleville Road, Storrs, CT 06269-3136

1. U.S. Army Research Laboratory, Weapons and Materials Research Directorate, Aberdeen Proving Ground, Aberdeen, MD. 

Cold Spray deposits and coatings have been studied extensively through the use of ex-situ heat-treatment and subsequent microscopy. Considerably less effort has been devoted to the study of these materials using in situ microscopy techniques. It is known that cold spray produces coatings and parts with a high degree of plastic deformation; coupled with the metastable microstructural state of the as-atomized powders, this leads to coatings that may behave rather differently than bulk cast or wrought analogues of the same material. In this work, we present proof-of-principle in situ scanning transmission electron microscopy (STEM) heating studies on the nature and kinematics of phase transformations and grain refinements in single cold spray splats. It is shown that static recrystallization happens extremely rapidly within a narrow band of highly-sheared material along the periphery of the splat. Secondary phase dissolution and coarsening are recorded and quantified through area fraction and equivalent radius measurements with respect to time during isothermal in-situ heat treatments. From these data, estimations of diffusion coefficients can be extracted in the interfacial region of single cold-spray splats. These experiments demonstrate the applicability and utility of in situ STEM heating experiments for determining the characteristics of various thermally activated phenomena that occur within cold-sprayed material. 

 Biography of the presenter 

Benjamin Bedard is a fourth-year PhD student in the University of Connecticut’s Materials Science and Engineering Program. Ben began investigating advanced powder-processed aluminum alloys in the Aindow Microscopy Lab as an undergraduate in the Spring of 2013. He received his BS in Materials Science and Engineering at the University of Connecticut in the Spring of 2015. 

Modeling the Single Particle Impact Microstructural Evolution during Cold Spray of Metallic Powders at the Mesoscales 

Sumit A. Suresh1, Seok-Woo Lee1, Mark Aindow1, Harold Brody1, Victor K. Champagne2 and Avinash M. Dongare1, 

 1. Department of Materials Science and Engineering, Institute of Materials Science, University of Connecticut, 97 North Eagleville Road, Storrs, CT 06269-3136

1. U.S. Army Research Laboratory, Weapons and Materials Research Directorate, Aberdeen Proving Ground, Aberdeen, MD. 

The science behind particle-substrate adhesion in cold spray has been thoroughly investigated by experiments and computational modeling alike. Studies have shown interfacial material ejection or “jetting” to be a characteristic feature of well-adhered cold-sprayed splats. Even though adiabatic shear instability and shock wave interactions have gained popularity as the theoretical mechanisms behind jetting, studies involving atomistic mechanisms and microstructure-based modeling is lacking. In this work, we employ the Quasi-Coarse-Grained Dynamics (QCGD) method, a framework that retains the crystalline features of the microstructure and uses scaled classical interatomic potentials to accelerate molecular dynamics to the mesoscales. The QCGD simulations demonstrate that the pressure wave propagation and interaction with the edge of the particle/substrate interface, along with the localization of temperature and high strains at the edge, initiates the formation of a jet. Long term evolution of microstructure also shows several new satellite grains at the particle/substrate interface as a consequence of dynamic recrystallization, a feature backed by several experimental reports. A comparison of pure aluminum particle impacts to multicomponent systems involving copper and tantalum impacted under similar conditions will be presented.  

 Biography of the presenter 

Sumit Suresh is a graduate student (Ph.D. program) in the department of Materials Science and Engineering at the University of Connecticut. He obtained his Bachelor of Technology degree in Metallurgical and Materials Engineering from Indian Institute of Technology Roorkee in 2012. In 2015, he received his Master of Technology degree in Process Engineering from Indian Institute of Technology Bombay. 

Mechanical Properties of Supersonic-Impacted Al-6061 Microparticles 

Tyler J. Flanagan1, Benjamin A. Bedard1, Avinash M. Dongare1, Harold D. Brody1, Aaron Nardi2, Victor K. Champagne2, Mark Aindow1, Seok-Woo Lee1 

1. Department of Materials Science and Engineering & Institute of Materials Science, University of Connecticut, 97 North Eagleville Road, Unit 3136, Storrs, CT 06269-31316, USA

1. U.S. Army Research Laboratory, Weapons and Materials Research Directorate, Aberdeen Proving Ground, Aberdeen, MD, 21005, USA 

The cold spray process utilizes a high-pressure gas jet to accelerate metallic particles to supersonic velocities, and these particles bond to the substrate upon impact. The mechanical characterization of impacted particles is of great interest as their mechanical properties are strongly related to those of bulk cold sprayed coatings. Here, in-situ micro-compression/tension results are presented from impacts in single-pass cold spray trials of as-atomized and heat-treated Al-6061 powders. We observed the following results. (1) The flow stress increases substantially after supersonic impact due to strain hardening that results from the increase in dislocation density. (2) The flow stresses of the splats are not dependent on the impact velocity under the conditions considered here, presumably due to the saturation of dislocation density. (3) The flow stresses of the splats are not dependent on the initial powder microstructure because the extremely high dislocation density dominates the effects of other microstructural features. (4) The ductility of the splats is dependent on the initial powder microstructure. Homogenized powder particles give higher ductility splats due to dissolution and coarsening of cell boundary phases, which allows for slip transfer across the boundaries. We note that our approach can be used more generally to study other supersonic impacted metallic powder particles. Furthermore, our results will not only give an important insight in fundamental understanding in mechanical behavior of post-supersonic-impacted metallic materials, but also will be greatly useful to understand bulk mechanical behavior of cold-spray deposits by providing the microscopic viewpoint of cold spray processes. 

 Biography of the presenter 

Tyler Flanagan is a graduate student (Ph.D. program) in the department of Materials Science and Engineering at the University of Connecticut. He obtained his Bachelor of Arts degree in Physics from Clark University in 2014. 

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