Novel Ballistic Processing of Thick Films: Structural Evolution and Mechanical Behavior
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ULTRARAPID or extreme processes can give rise to unique microstructures and properties as opposed to conventional processing. This can be exemplified in processes such as rapid solidification and severe plastic deformation, where amorphous and nanostructured metals can be produced, respectively. Within the area of melt processing, planar-flow melt spinning (PFMS) and splat quenching[1,2] have been used to produce thick films and coatings for a variety of materials. PFMS involves the deposition of molten metal through a nozzle onto the surface of a rotating cold wheel, on which the metal solidifies into a thick film or ribbon.[3–5] The speed of the wheel has a significant impact on cooling rates and film thicknesses. Typical film thicknesses obtained in PFMS for metals can range from 8 lm to a few hundred micrometers, depending on the material being cast as well as on the processing conditions.[1,6] Moreover, cooling rates have been reported to range between 104 and 106 K/s,[7] while splat quenching can have cooling rates of 104 to 1010 K/s, again depending on conditions,[8] but typically produces ‘‘coat’’ thicknesses of 30 lm and above. In PFMS, the film thickness has been found to be inversely proportional to wheel speed.[7] The cooling rate can also be controlled by
D. HILLE, G. YOUSSEF, A. MOHAMMAD, and K. MORSI are with the Department of Mechanical Engineering, San Diego State University, 5500 Campanile Drive, San Diego, CA 92182. Contact e-mail: [email protected] Manuscript submitted October 30, 2019.
METALLURGICAL AND MATERIALS TRANSACTIONS A
external sources, such as laser energy.[9] Microstructures of Al films produced via PFMS have been primarily on the microscale, with grain sizes typically ranging from a few tenths of micrometers to a few tens of micrometers, with evidence of significant dislocation densities.[10] In this present article, we present a new process, ballistic processing (BP)/manufacturing,[11] where projectiles or carriers are accelerated at high speeds (which can exceed 400 m s 1) into a molten metal curtain, during which the projectile is rapidly coated with a film of molten material. The advantage of such an approach includes the ability to produce films with thicknesses much lower than those produced through PFMS or splat quenching, in addition to the ability to produce thousands to potentially millions of standalone microscale and nanoscale components almost instantaneously using masking techniques. Previous work by one of the authors[12] using low carrier speeds up to 36 m s 1 (on a low thermal conductivity wax carrier surface) has shown the feasibility of this technique in the production of thick films of Sn-0.7Cu down to 2-lm thickness.[13] Before ultrahigh speed is investigated, it is important to first understand the structural evolution at the lower speed regimes. In order to gain such in-depth understanding of this rapid process, a method for capturing ultrarapid morphological and microstructural transitions is needed. In the present study, we use carriers of different thermal conductiviti
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