Size effects in ultraprecision machining of aluminum alloys: Conventional AA6061-T6 and RSA 6061-T6

Minimum chip thickness determination by means of cutting force signal in micro endmilling

Journal of Manufacturing Processes

Volume 68, August 2021, Pages 136-157

R.G. Jasinevicius, J.A. Otoboni, I. Basso, M.H. Militão Dib 

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6061 aluminum has been widely used in the manufacture of bicycle frames, providing these frames with a lifetime warranty. 6061 aluminum is also used in the automotive, aerospace industries. In this paper, 6061 aluminum is analyzed in the ultraprecision machining process, focusing on the finished surface, comparing the conventional AA6061-T6 with RSA6061-T6.

Abstract

The metallurgical modification in non-ferrous metal alloys is carried out to adapt not only physical or mechanical properties to their applications, but also to improve their machinability. In the case of aluminum alloys for optical applications, the reduction in grain size is an alteration aimed at adapting the dimensions of the microstructure to the dimensional scale used in the machining conditions. The dimensions of the cutting section are often much smaller than the average grain size, which causes anisotropic effects on both the material removal mechanism and surface integrity. The objective of this study is to compare the effect of the material's microstructure, in this case, grain size, on the removal mechanism, energy dissipation and surface generation involved in the ultraprecision diamond turning of aluminum alloy 6061-T6 using micrometric and submicrometric cutting conditions. Cutting forces results showed that below feedrate of 10 μm/rev there is a reversal effect on the cutting forces, thrust force becomes greater than the main cutting force. The specific cutting energy reaches values one order of magnitude higher than conventional values when machining with nanometer range undeformed chip thickness. The impurities, hard inclusions, void formation and the effect of anisotropy will be assessed and discussed by explicating the influence on surface finish at nanoscale level. Based on observations of optical profilometry and scanning electron micrographs, the machined surfaces and chips removed were assessed and the effects of the aforementioned factors on the cutting mechanism were identified. Under nominal undeformed chip thickness smaller than 286 nm the machined surface does not show parallel groove marks left by the diamond tool. As the undeformed chip thickness decreases, the friction coefficient becomes high, leading the energy dissipation due to rubbing action at the tool-chip interface to move downward to cutting edge portion. Below this value the surface generation mechanism is attributed to a burnishing of the round part of the tool cutting edge and the portion adjacent to tool flank face. It was observed that the microstructure and metallurgical process involved to obtain each type of aluminum alloy play an important role on the performance of the cutting process and on the surface formation. Under chip cross sectional area greater than 50 μm2 hard particles dragged by the cutting tool are observed on machined surface. This causes the surface roughness to increase.

Keywords: Diamond tool, Aluminum alloy 6061-T6, Energy dissipation, Roughness, Burnishing, Size effect

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Highlights

• Grain size and hard inclusions affects surface roughness of conventional AA6061-T6

• A frictional size effect is evident for undeformed chip thickness values below 1 μm.

• The size effect is evident for Specific Cutting energy for undeformed chip thickness values below 1.15 μm.

• The minimum surface roughness values obtained for both microstructures was 2.8 nm Sa.

• Burnishing is the main mechanism of surface formation under nanometer undeformed chip thickness.