Investigation of the effect of end mill-geometry on microstructure and indentation hardness of aluminum alloy AA6082
Micro-milling is one of the most promising technologies for micro-manufacturing of high tech components such as measurement technology modules for the highest frequencies in the terahertz range. A simple down-scaling from the conventional milling process is not possible among other things because of the size-effect. Therefore, the process of micro-milling must be well understood.
The objective of the present research is to investigate the impact of the mill geometry and tooth-feed on the micro-milling size effect. The surfaces were produced on AA6082 material using commercially available end-mills with two different cutting edge geometries (solid carbide (SC), cutting edge radius ca. 600 nm and monocrystalline diamond (MD), cutting edge raduis ca. 20 nm). The tooth feed was varied between 3 µm and 14 µm, while other parameters have been held constant. The tools' geometry (especially the cutting edge), as well as the produced surfaces have been thoroughly investigated (optical microscopy and SEM). Additionally, the indentation hardness of the surfaces has been determined as a function depth by using an indentation procedure with multiple partial unload cycles.
For surfaces produced with the SC mill, the strain hardened zone is over 200 nm and the maximum hardness is above 2000 MPa (more than 160%-increase of the bulk hardness). On the other hand, the surfaces produced with the MD mill show no significant strain hardening (layer thickness approx. 60 nm, max. hardness of 1500 MPa, ca. 125 % increase). Additionally, the cutting edge is replicated almost directly on the surface. Consequently, the cutting edge radius has a significant impact on the micro-milling size-effect. Apart from the increase of the cutting force, the mechanical properties of the surface are increased significantly. These findings are especially important for the high-precision manufacturing of measurement technology modules for the terahertz range.