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3.4. Aspects of surface integrity
Apart from the surface roughness, shown in Figure 2, other aspects of surface integrity were analyzed: subsurface deformation and distribution of residual stresses into the machined surface. The test samples for both subsurface deformation and residual stress testing were generated with new tools. Subsurface deformation was studied on a cross-section of the samples after their polishing and subsequent etching, see Figure 8. When characterizing deformation it is possible to distinguish a zone of severe deformation, adjacent to the machined surface, having significant bending and elongation of grain boundaries and slip lines in the direction of cutting. This major deformation zone normally extends to 10-15 μm. Both, intensity of deformation in this zone and thickness of the zone increase with the cutting speed.
AFM topography of (a) crater and (b) wear land on worn out UCBN tool (vc=350 m/min, f=0.2 mm/rev)
It can be assumed that a combination of chemical and abrasive wear plays dominant role. High temperature developing on the crater leads to reaction of both cBN and binder with the workpiece material as well as their softening and a subsequent removal of reaction layer by abrasive particles of Inconel 718, thus smooth and uniform surface. According to Proskuryakov [4] temperature on the tool clearance is lower by 200-250 ºC than on the rake, which implies that intensity of chemical reactions is limited, if existent. Hot hardness of cBN in this temperature range is significantly higher than that of TiC-based binder. According to Ståhl [18] hardness of
TiC at temperatures 850-950 ºC is only 520 HV. Based on the above results it is assumed that highly abrasive TiC and NbC carbides in the matrix of Inconel 718 tend to abrade the binder at higher rate than cBN, exposing the later (Figure 7.b).
. SEM of subsurface deformation after machining with CCBN tool (vc=250 m/min, f=0.1 mm/rev, new tool)
Application of coated PCBN tools resulted in more severe grain elongation, but thickness of this zone was affected to a lesser degree. These effects can be explained by the increase of the process temperature with increase in cutting speed and insulation effect of the coating. Rise of the temperature results in a loss of material strength and increase of its plasticity, allowing for more intensive deformation. Effect of higher edge radius of the coated tools additionally contributes to the ploughing action and mechanical constituent of subsurface damage.
When analyzing residual stresses after machining it was found that both axial and tangential stresses have exhibited a “hook” type profile (see Figure 9), which is typical for high speed machining with PCBN tools. Significant difference between reported data when machining with cemented carbide tools [15] and the current study was found for residual stresses on the surface. Surface residual stresses were found to be compressive for both PCBN tools, on the contrary to undesired tensile stresses for cemented carbide tools.
V. Bushlya et al. / Procedia CIRP 3 (2012) 370 – 375 375
Residual stresses profile for (a) uncoated and (b) coated PCBN tools (vc=300 m/min, f=0.1 mm/rev, new tool)
The main factor responsible for the generation of surface compressive stresses in the workpiece is the tool geometry, large nose radius in particular. Round inserts with diameter of 12.7 mm were applied throughout the tests, which leads to a significant thinning of the chip area. This, in turn, results in considerable reduction of local cutting temperature in the surface formation region of the tool-workpiece interface [12]. This in turn reduces the thermally-related tensile component of residual stresses as a whole. On the other hand large nose radius leads to multiple deformation of the machined surface [19] thus increasing compressive mechanical-related stresses. Similar effect of nose radius on surface residual stresses was observed by Arunachalam et al. [8].