Surface Integrity of Al 7050-T7451 and Al 6061-T651 Induced by High Speed Milling 2005-01-3346
High speed milling (HSM) is widely used in aerospace and automotive industries in the fabrication of airframe or other various mechanical components. HSM induced surface integrity significantly impact the fatigue life of the machined components. However, surface integrity by HSM remains a “black hole” in the point of view of surface quality.
This paper presents a new approach to investigate surface integrity in HSM Al 7050-T7451 and Al 6061-T651 aluminum alloys. A design of experiment (DOE) method was adopted to conduct 54 cutting conditions with three levels of cutting speed, feed rate, and depth of cut. An acoustic emission (AE) sensor was mounted on the workpiece to monitor the HSM process and to correlate AE signals with surface integrity factors.
The results have shown that cutting speed has a dominant effect on surface roughness, the increase of cutting speed increases the arithmetic mean (Ra) for both aluminum alloys. Microhardness in machined near-surface (about 10 μm) is larger than the bulk materials due to the combined effects of strain hardening and size effect. The grain flows of both aluminum alloys cannot be observed in optical and SEM images. However, the grain structures can be seen in cross-sectional surface of Al 7050-T7451 samples. The strain hardened layer in both aluminum alloys has shown different etching resistance and demonstrated less voids and second phase particles. Residual stress in feed direction is tensile in near surface and quickly changes to compressive at about 20 μm below surface for Al 6061-T651 samples and about 35 μm for Al 7050-T7451 ones. The maximum compressive residual stress is in the subsurface. The AE signals of RMS, count, frequency, and amplitude are sensitive to cutting conditions. An apparent correlation between AE signals and surface integrity factors can be hardly observed.