Effect of Heat Treatment on the Al-Cu Feedstock Powders for Cold Spray Deposition
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COLD spray is an emerging solid-state material deposition technique in which small feedstock powder particles (5 to 40 lm) are accelerated to high velocities in the 300 to 1200 m/s range before impacting onto a substrate using a supersonic gas jet under temperatures below the melting point of the feedstock material.[1] In the last decade, growing interest in depositing aluminum alloys for additive repair and manufacturing using cold spray techniques has resulted in a substantial literature in this field.[2–7] Moreover, several studies on the microstructures of the cold-sprayed aluminum alloys, such as AA2618,[8] AA6061,[9–11] and AA7075,[12–15] have been reported in the literature. The majority of the feedstock powders for cold spray are produced using gas atomization,[16] in which the cooling rates can reach 104 to 105 K/s.[17] Gas atomization, a rapid solidification process, will produce microstructures in a highly nonequilibrium state.[18–20] Al alloy feedstock materials produced by the gas atomization process usually present a cellular or dendritic solidification microstructure
TIAN LIU, WILLIAM A. STORY, and LUKE N. BREWER are with the Metallurgical and Materials Engineering Department, University of Alabama, Tuscaloosa, AL. Contact e-mail: [email protected] Manuscript submitted December 16, 2018. Article published online May 1, 2019 METALLURGICAL AND MATERIALS TRANSACTIONS A
with alloying elements segregated at the cell boundaries as intermetallics.[10,20–23] The solute atoms contained in these intermetallics are unavailable for the formation of strengthening phases which are the primary strengthening mechanism for heat-treatable aluminum alloys.[24] This cellular solidification microstructure with intermetallics distributed along the cell boundaries could also be detrimental to the ductility of the resultant cold-sprayed materials. Solution treatment could eliminate the solute segregation in the feedstock powders for cold spray deposition. However, it is not an easy task to solutionize a large amount of feedstock powders for cold spray. To do so, the powder material must be heated above the solvus temperature of the material to accomplish complete dissolution of the intermetallics. This solution treatment must be followed by quenching at a sufficiently high cooling rate to prevent the re-precipitation of intermetallics in the powders. She et al.[25] described a high-temperature fluidized bed furnace for the heat-treatment of powders in a recent patent, in which the powders were heat treated by a fluidizing gas to avoid sintering and oxidation of powders. However, the importance of cooling rate was not discussed in the method of She et al.[25] Rokni et al.[26] have recently applied heat treatment to AA5056 powders using a fluidized bed heat-treatment process with flowing nitrogen, and problems like sintering of powders were negated in their method. Furthermore, they found that heat treatment of the powders enhanced the ultimate
VOLUME 50A, JULY 2019—3373
tensile strength and ductility in the resultant deposit.
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