The Influence of Heat Treatment on Nanoscale Microstructure and Crystal Orientation of 7055 Aluminum Alloy Before and After High-Speed Milling

摘要

This paper is intended to examine changes in the microstructure and crystal orientation of 7055 aluminum alloy before and after cutting. Single-factor cutting speed test was designed and implemented to investigate the influence of three heat treatment processes, T6, T87 and T815, on the microstructure and crystal orientation of 7055 aluminum alloy before and after cutting. Results showed that, before cutting, T6-state microstructure had uniform grain size with pinning in theta' phase; T815-state grains were obviously elongated as a result of predeformation; T87-state grains also displayed some elongation, but their overall elongation was not as long as that of T815-state grains; there was a dislocation in the TEM microstructure after both T87 and T815. After cutting, T6-state initial grains were elongated; their horizontal and longitudinal sizes were 46 and 92 mu m, and the low-angle boundary (LAB) and high-angle boundary (HAB) densities of T6, T87 and T815-state grains were , , , , and mu m(-1). The crystal structure and orientation relationship of T6-state alloy after aged for 4, 8 and 12 h was theta'', theta', and many mixed regions of theta'' and theta', were observed along {001}alpha. After aged for 12 h, the T8-state microstructure along [001]alpha and [011]alpha was roughly the same as that after aged for 4 h, except that the share of theta' particles along [011]alpha had increased from 55 to 90% while theta'' particles along [001]alpha had reduced a little. After aged for 12 h, the precipitated particles of the cutting layer of T815-state alloy along [001]alpha were all theta' phase while those along [011]alpha were composed of theta' and Omega phases. From the boundary microstructure, before cutting, the grain boundary of T6-state alloy was a continuous one with no obvious non-precipitate zone; the grain boundary of T87-state alloy displayed some discontinuity as a result of predeformation, and quite a lot of the precipitated particles were concentrated on the boundary; the grain boundary of T815-state alloy was a discontinuous one, but the non-precipitate zone on the boundary was not as wide as that of T87-state alloy. After cutting, T6-state alloy had the widest non-precipitate zone of all at about 42 nm. The non-precipitate zone of T6-state alloy was 25 nm wide, and the particles were mainly grown theta' particles, and theta particles incoherent to the aluminum matrix. The non-precipitate zone of T815-state alloy was the narrowest at approximately 15 nm.