2. Material and Methods
The experimental investigations are focused on side milling of a particle-reinforced AMC. For the reason of the highly abrasive nature of the ceramic reinforcements the application of MCD as cutting material is beneficial to keep the tool wear in an acceptable range, supporting a controlled and steady wear progression, additionally enabling an appropriate modification of the tool geometry. In the experiments especially the effect of a modified clearance angle and rake angle of the minor cutting edge is examined.
The single-edged tools (Medidia GmbH, Idar-Oberstein, Germany) have a diameter of 3 mm and a clearance angle of the minor cutting edge of 1°, 2°, or 3° respectively. The influence of the rake angle of the minor cutting edge is investigated for the values −5°, 0°, and 5°. The cutting edge angle of the minor cutting edge is kept constant at 2°, resulting in a tool included angle of 88°. The cutting corner features a chamfer of 0.1 mm × 45° focusing on geometrical stability.
For a better understanding of the tool geometry
Figure 4a presents an overview of the cutting part of one of the MCD-tipped tools used, while a more detailed view on the rake face, the cutting corner, and the minor cutting edge is presented according to
Figure 4b. For the experimental investigations one tool of each type with the given specifications is used.
Scanning electron microscopy (SEM) is used subsequent to the cutting experiments to investigate the tool wear of the applied tools. Based on SEM images using a microscope of the type LEO 1455VP (Carl Zeiss Microscopy GmbH, Jena, Germany) the flank wear land width was assessed qualitatively.
For the manufacturing of the specimens, an AMC is used, which was developed within the framework of the Collaborative Research Center SFB 692 HALS. Hockauf et al. in detail investigated the fabrication process of the addressed composite material. Accordingly, the matrix consists of an aluminum wrought alloy comparable to the type EN AW-2017 (AlCu
3.9Mg
0.7Mn
0.6). As reinforcements SiC particles with a size of
< 2 μm and a proportion of 10 vol.% are used. Accordingly, 90% of the particles provide a diameter less than 2 μm. The composite is produced using a powder metallurgical route. Therefore, all components are mixed in powder state, high energy milled, and pressed hot isostatically. Subsequently, the compacted raw material is extruded to improve the mechanical properties. Eventually, a T4 heat treatment is applied [
26].
Table 1 presents the averaged specific values of relevant mechanical parameters determined by tensile tests in the direction of extrusion. Furthermore, hardness measured on an area perpendicular to the direction of extrusion is given.
Figure 5 presents cross sections showing the microstructure of the AMC in the longitudinal (extrusion) direction and in the cross direction. In general, a comparably homogeneous particle distribution as well as a statistical variation of the particle size are apparent. However, as a side effect of the extrusion process banding effects appear in the longitudinal direction. As a result, the anisotropy of the mechanical properties is increased. Concerning the experimental investigations, the influence of banding on the surface properties is limited, by aligning all surfaces to be machined perpendicular to the extrusion direction.
The side milling tests are carried out using a high-precision milling center of the type KERN Pyramid Nano (KERN Microtechnik GmbH, Eschenlohe, Germany). Flood cooling is applied throughout all experiments using emulsion as cooling lubricant based on Cimstar 501-Cimcool metalworking concentrate (Cimcool Industrial Products B. V., Vlaardingen, The Netherlands). The specimens, represented by cylindrical sections with a diameter of 12 mm and a length of 10 mm are clamped on a specifically designed device which incorporates a piezoelectric dynamometer of the type Kistler MiniDyn 9256A2 (Kistler Instrumente AG, Winterthur, Germany). Accordingly, in-process acquisition of data concerning the components of the resulting force in milling is enabled.
Figure 6 shows the experimental setup for the investigations.
Figure 7 illustrates the kinematics in side milling, applied for the cutting tests. In a first step, a CVD diamond-tipped auxiliary tool (Diamond Tooling Systems GmbH, Kaiserslautern, Germany) is used to create a square surface of (9 × 9) mm
2 at each specimen to benefit constant cutting conditions. Subsequently, each specimen is machined with the single-edged MCD-tipped tool corresponding to the experimental parameters. In this context, several tool paths according to the width of cut
result in the generation of the surface. Moreover, the milling operations are realized in down-milling, typically benefiting chip formation, thus resulting in enhanced surfaces.
The cutting parameters are kept constant with a cutting speed
of 250 m/min, a feed per tooth
of 0.015 mm, a depth of cut
of 0.25 mm and a width of cut
of 0.5 mm. Clauß, Nestler, and Schubert already conducted investigations on the influence of the cutting parameters when milling the same SiC particle-reinforced AMC. The investigations were realized using a CVD diamond-tipped double-edged tool [
23]. Intending to complement the existing data base and expanding the field of milling AMCs using MCD-tipped tools, the specific values of the cutting parameters are chosen comparable to medium levels of the previous investigations. For enhanced statistical reliability, each combination is realized in three repetitive experiments. The investigated combinations of the cutting parameters and the tool geometry aspects are summarized according to
Table 2.
The properties of the generated surface are evaluated applying different quantitative and qualitative methods. Regarding that, the roughness values in the feed direction as well as the valley void volume for each surface are taken into consideration. The 3D data of the generated surfaces are gathered using an optical laser scanning microscope of the type Keyence VK-9700 (Keyence Corporation, Osaka, Japan). The achieved profile or surface data are subsequently evaluated using the software MountainsMap® (Digital Surf, Besançon, France). In this context, the primary surface is levelled by subtraction-method, clipped to a field of 2 × 2 mm2 and filtered based on a denoising wavelength of 2.5 μm. Subsequently, roughness profiles with a measured length of 1.5 mm and an evaluated length of 1.25 mm are extracted and filtered using a cut-off wavelength of 0.25 mm. Each generated surface is evaluated referring to three roughness profiles and the areal evaluation of the porosity, represented by the value for .
The physical properties of the surfaces are evaluated using X-ray diffraction analysis. Regarding that, the -method is used, enabling the determination of the residual stress state. As instrument a Discover D8 (Bruker Corporation, Billerica, MA, USA) with an X-ray tube incorporating a cobalt anode is used.
The method is based on measuring the diffraction angle of the lattice structure of the matrix in the machined surface layer. In this context, specifically the crystallographic planes {311} are taken into consideration (
Figure 8). Comparing the values of the deformed and the undeformed grid parameters of the matrix material, residual stresses can be detected and evaluated. Regarding that, the elastic parameters for the X-ray diffraction are approximated with a Young’s modulus of
and a Poisson’s ratio of
. Moreover, the measurements are realized based on a circular measuring field with a diameter of 0.5 mm. However, the stress state of SiC particles is not respected. Regarding the presented investigations, primarily the first principal residual stress
is determined.
The identification of effects of the machining process on the microstructure of the surface layer is achieved by EBSD analysis. The analyses are performed using cross sections, specifically addressing the surface layer of the generated surface as well as the bulk material unaffected by the machining process. Regarding that, the achievable result concerning the EBSD pattern quality and thus the base of signals used for evaluation strongly depends on the preparation of the specimens.
In this context, Dietrich et al. described an appropriate preparation method for vibrational polishing with silicon oxide suspension for specimens being subjected to EBSD analysis. It is claimed that an improvement of the EBSD pattern signals can be achieved in contrast to oxide and electro polishing methods. According to experimental investigations comparing the different approaches it is concluded that vibrational polishing compared to the remaining methods enables very levelled and largely undeformed surfaces, thus providing high preparation quality [
27]. Consequently, all specimens for EBSD analysis are prepared according to the described method.
Hereafter, the grain size of the aluminum alloy matrix is investigated using the EBSD procedure. The EBSD system itself is integrated into a scanning electron microscope of the type NEON 40EsB (Carl Zeiss Microscopy GmbH, Jena, Germany) which is operated at 15 kV with a 60 μm aperture in high current mode. EBSD data sets are measured typically in regions of interest of (50 × 80) μm2 with a sampling size of 100 nm. The SiC particles appear brighter than the aluminum alloy matrix in secondary electron (SE) images. Accordingly, the SE signal is used to separate the particles from the matrix for filtering the EBSD data sets. Subsequently, the EBSD data are subjected to a slight clean up procedure comprising neighbor confidence index (CI) correlation and grain CI standardization with a minimum confidence index of 0.1. The parameters for grain size determination are set to 15° tolerance angle and a minimum of three hits per grain.
In addition to the quantitative evaluation of the grain size distribution based on the EBSD data sets, a qualitative assessment of the surface layer is realized by inverse pole figure (IPF) color maps. These allow the evaluation and description of the surface layer microstructure, thus providing additional information on grain distribution and characteristics, complementing the quantitative parameters.
3. Results and Discussion
The presented research focuses on the influence of the clearance angle and the rake angle of the minor cutting edge on the surface structure, the generation of surface imperfections, and the residual stress state of the generated surface. These properties primarily result from complex interactions between the composite material with its heterogeneous microstructure and the minor cutting edge within the cutting zone.
Subsequently to the experimental investigations the flank wear land width of the used tools was determined in a range of about 10 μm to 30 μm depending on the specific tool geometry. The strongest flank wear appeared at the tool with a clearance angle of 2° and a negative rake angle of −5°. Although providing comparably low values for referring to typically applied wear criteria (), a possible influence of the flank wear cannot be excluded referring to the resulting surface properties.
Figure 9 presents the influence of the clearance angle of the minor cutting edge on the roughness values for
of the generated surfaces. In this context, the error bars represent the standard deviation calculated in total based on nine separate roughness measurements gathered from three specimens machined with the same process parameters.
Accordingly, an increase of the clearance angle on average results in a slight yet insignificant decrease of the values for . The resulting mean values range around 0.2 μm, with all the generated surfaces featuring a mirror-like surface finish combined with characteristic milling patterns. Moreover, it is evident that especially the application of a clearance angle of 1° or 2° results in a stronger fluctuation of the specific values referring to the average roughness values. One reason for that is seen in the comparably low values for combined with the complex interactions between the reinforcing particles and the cutting edge. With a decrease of the surface roughness values there is a higher sensitivity towards particle fracture or pull-out. However, even taking the strong fluctuations into consideration any of the investigated tool geometries results in a reliable generation of surfaces with roughness depth values in the sub-micrometer range even below 0.4 μm. Moreover, despite the quite low average values the use of a clearance angle of 3° leads to a significantly lower fluctuation. Eventually, taking into consideration both the averaged surface roughness values as well as the fluctuations, the values for are comparable for the different clearance angles applied.
Regarding
Figure 10 the influence of the rake angle on the surface roughness values of the generated surfaces is presented. Similar to the investigation of different clearance angles the error bars represent the standard deviation taking into account nine separate roughness values.
The results indicate that the rake angle has only minor influence on the achievable surface roughness values. It appears that the highest average value, due to the comparably strong fluctuations results from milling with a clearance angle of 2° and a rake angle of 0°. Taking only the average values into consideration, no clear trend can be identified increasing the rake angle. The lowest values for on average result when using the tool with a clearance angle of 2° and a positive rake angle of 5°. Respecting the mean values as well as the fluctuations the achieved surface roughness values are in a comparable range. This is mainly attributed to the statistically randomized particle distribution within the composite. Accordingly, highly complex and partially random interactions with the cutting edge occur, even despite comparable cutting conditions. Interestingly, both a negative rake angle as well as a positive rake angle of the minor cutting edge lead to a reduced fluctuation of the roughness values. However, similar to the machining with different clearance angles, in any case the achieved surface roughness exhibits values below 0.4 μm and a mirror-like surface finish of the generated surface.
Figure 11 presents the influence of different clearance angles of the minor cutting edge on the resulting valley void volume
, representing the porosity of the generated surface. The error bars represent the standard deviation based on the separately evaluated surfaces of the three repetitive experiments for each combination of process parameters.
The highest average values result when applying the tool with a clearance angle of 1° and a rake angle of 0° of the minor cutting edge. With an increase of the clearance angle up to 2° the lowest value for is achieved in the investigated range. However, a further increase up to a clearance angle of 3° leads to higher valley void volumes. While no clear trend is identified concerning the averaged values, the fluctuations can be reduced applying clearance angles of 2° and 3° in the investigated range. This is mainly attributed to an interaction of the minor flank face and the generated surfaces in the tertiary cutting zone, intensified by decreased clearance angles.
Figure 12 indicates the influence of the rake angle of the minor cutting edge on the values for
. The error bars are determined similarly to the investigations for the different clearance angles.
Accordingly, it emerges that a rake angle of 0° combined with a clearance angle of 2° of the minor cutting edge results in the lowest average values for also enabling reduced fluctuations of the specific values compared to the other rake angles applied. In contrast to that, both the negative and the positive rake angle of −5° or 5° result in similar yet significantly higher average valley void volumes compared to a rake angle of 0°. Additionally, there is a considerably higher fluctuation of the specific values in both cases. It is assumed that the findings can be explained with altered stress regimes in the primary shear zone due to different wedge angles, resulting from the variation of the rake angle while keeping the clearance angle constant.
Figure 13 shows the influence of the clearance angle of the minor cutting edge on the resulting residual stress state. The initial stress state within the specimen material after manufacturing and prior to machining was investigated in advance of the cutting experiments. Accordingly, ten separate residual stress measurements evenly distributed over the complete batch of the investigated AMC material were realized. Influences of prior machining operations were removed preparing the measuring field using electro chemical machining (ECM). In this context, the averaged compressive residual stress state of the composite is represented by a dashed line. Moreover, the maximum and minimum values of the measuring sequence are marked with solid lines. On average, there are already initial residual stresses
of about −70 MPa within the bulk material fluctuating between −63 MPa and −79 MPa.
It can be seen that there is only a slight influence of the clearance angle of the minor cutting edge on the residual stress state of the generated surface. However, on average stronger compressive residual stresses are achieved with an increase of the clearance angle from 1° to 2°, although a further increase results in a reduction of the resulting absolute values of the compressive residual stresses. Taking the averaged values as well as the fluctuations into consideration, comparable values are achieved for the compressive residual stresses, independently of the applied clearance angle. Nonetheless, despite comparably strong fluctuations, in any case the absolute values of the compressive residual stresses are increased compared to the initial stress state.
Figure 14 depicts the influence of the rake angle of the minor cutting edge on the residual stress state of the generated surface.
The results indicate that milling with a tool featuring a positive rake angle of the minor cutting edge of 5° leads to stronger compressive residual stresses compared to a negative and a neutral rake angle of −5° or 0°, respectively. Moreover, it can be derived that the use of a negative as well as a positive rake angle results in a reduced fluctuation of the generated compressive residual stresses. However, even taking into consideration the comparably strong fluctuations when machining with a rake angle of 0°, the highest absolute values of the compressive residual stresses in the investigated range are evidently achieved with the tool comprising a positive rake angle. On average the highest achieved compressive residual stresses provide values of about −230 MPa, thus increasing the absolute value of the initial residual stress state up to about 290%. The results are primarily attributed to the different wedge angles resulting from the rake angle variation while keeping the clearance angle constant. As a result, contrary mechanical and thermal effects appear in the primary shear zone. On the one hand it is known that the residual stress state results from lattice deformations of the matrix material. These can be promoted by increased plastic deformation typically combined with an increase of the passive force component and a refinement of the surface layer microstructure. On the other hand, with a larger extent of plastic deformation in the cutting zone the internal friction within the microstructure increases as well, resulting in higher cutting temperatures. Consequently, initialized recrystallization effects, reduction of lattice deformation, and thus the degradation of residual stresses is assumed to be benefited.
Comparing the presented results, especially concerning the residual stress state with the findings of Clauß, Nestler, and Schubert [
23], interesting aspects can be derived with reference to the effects of different diamond cutting materials on the surface properties when machining AMCs. The milling of an identically composed SiC particle-reinforced AMC was investigated using double-edged CVD diamond-tipped tools and comparable cutting parameters. However, the achieved absolute values of the compressive residual stresses up to −141 MPa in most cases were lower compared to the compressive residual stress generated with MCD-tipped tools. This is primarily attributed to the differences of the cutting materials applied. In contrast to the MCD-tipped tools used in the presented investigations the CVD diamond grade applied by Clauß, Nestler, and Schubert [
23] features a polycrystalline structure. Accordingly, when using MCD as cutting material significantly lower cutting edge radii are achievable, resulting in a changed stress concentration within the cutting zone, evidently leading to increased compressive residual stresses of the generated surface. Furthermore, the heat conduction in diamond is based on oscillation, affected by the structural properties of the diamond material. Consequently, it is reasonable to assume that the typically polycrystalline CVD diamond cutting grades provide a lower thermal conductivity compared to MCD. Accordingly, when using MCD-tipped tools it can be stated that the influence of thermal effects as a result of the cutting process can be reduced as well.
Figure 15 constitutes the grain size distribution within the surface layer machined with a rake angle of −5°. The characterization is based on the EBSD data sets determining the assigned grain size distribution according to histogram analysis.
Taking the different plots into consideration, it can be concluded that the machining process typically leads to a larger proportion of finer grains compared to the microstructure of the bulk material. Accordingly, a strong effect is identified as a result of machining with the tool featuring a rake angle of −5° of the minor cutting edge. Compared to the unaffected bulk material, the microstructure of the surface layer provides a strongly increased fraction of fine grains with a peak in the range between 0.3 μm and 0.4 μm.
Figure 16 presents the grain size distribution within the surface layer generated with a rake angle of 5°.
In contrast, the microstructure of the surface layer significantly distinguishes from the textural pattern generated with a rake angle of −5°. The resulting peak values are found in two different ranges with maxima around 0.3 μm and 0.5 μm each exhibiting a lower grain fraction in the microstructure compared to the maxima when using a tool with a rake angle of −5°.
Although the assessment of the grain size distribution allows a quantification of the surface layer microstructure in terms of grain size fractions, it does not provide qualitative information on the composition of the microstructure near the generated surface. In this context, the IPF approach complementary allows the visual assessment of the microstructural composition in the surface layer combined with the grain sizes and distributions as well as the description of the crystallographic orientation of the single grains. Regarding that, the aluminum grains of the matrix alloy are presented in different colors according to the crystallographic orientation, the black areas represent SiC particles.
Figure 17 presents the surface as a result of machining with a rake angle of −5° and a clearance angle of 2°.
Figure 15 indicates that machining with a rake angle of −5° leads to the most significant change of the microstructure in terms of the dominant grain size fraction. However, based on the associated IPF image (
Figure 17) the distribution appears comparably homogeneous with a slight increase of the proportion of fine grains towards the surface. In contrast to that, the transformation of the overall microstructural composition according to
Figure 16 appears less significant. Furthermore, the plot is characterized by two specific values of grain diameter providing local maxima, in contrast to one dominating maximum with
Figure 15. Nonetheless, the related IPF image according to
Figure 18 constitutes a strongly distinguished composition of the surface layer, when applying a tool with a rake angle of 5°.
The dominating grain sizes and distributions of particles in a distance of about 5 μm to 10 μm from the generated surface appear comparable when applying tools with a rake angle of −5° or 5°, respectively. However, in contrast to the surface generated with a negative rake angle, the surface resulting from machining with a positive rake angle exhibits a distinctly formed band of fine-grained particles representing the generated surface. Moreover, stronger irregularities can be found in case of applying a positive rake with some grains appearing to be disconnected from the surface layer. This is primarily attributed to increased challenges, especially when detecting single grains with strongly reduced sizes. Eventually, a stronger deformation of the surface layer is observed compared to the surface layer produced with a rake angle of −5°. In this context, milling with the tool featuring a positive rake angle of 5° leads to the strongest compressive residual stresses in the investigated range according to
Figure 14. The EBSD analyses require the preparation of a cross section of the examined specimen that can lead to outbreaks in the transition area between the surface and the embedding material, particularly with high compressive residual stresses. Moreover, the equilibrium conditions prior to preparation are affected, influencing the degradation of the residual stresses and the deformation within the surface layer. However, the IPF image indicates a fine-grained affected surface layer with a thickness of at least 1 μm to 2 μm as a result of the cutting process.
4. Summary and Conclusions
Based on a fractional experimental design, investigations concerning milling of a particle- reinforced AMC are realized. The composite used consists of an aluminum wrought alloy similar to EN AW-2017 reinforced with 10 vol.% of SiC particles. For the cutting tests MCD-tipped single-edged tools are applied. The cutting experiments particularly address the influence of the clearance angle and the rake angle of the minor cutting edge on the properties of the generated surface.
Regarding that, the achieved roughness values for , the porosity represented by the valley void volume , and the residual stress state, especially the first principal residual stress are taken into consideration. The values for roughness and porosity are evaluated using optical surface data gathered with a confocal laser scanning microscope. The residual stresses are determined based on X-ray diffraction analysis according to -method. Additionally, EBSD analyses are realized to characterize the microstructure of the surface layer.
According to the achieved results an increase of the clearance angle of the minor cutting edge up to 3° leads to a slight reduction of the average surface roughness values combined with decreased fluctuations of the specific values. A decreased average valley void volume, thus representing reduced porosity is achieved by applying a rake angle of 0° as well as a clearance angle of 2°. Moreover, strong effects of a varied rake angle influencing the residual stress state are identified. In this context, a positive rake angle of the minor cutting edge of 5° leads to the strongest compressive residual stresses in the investigated range. Compared to the initial state of the bulk material the absolute values of the compressive residual stresses are increased up to about 290%. However, the surfaces generated by milling exhibit stronger compressive residual stresses for any used tool compared to the initial state. Concerning the microstructure of the generated surface, a positive rake angle involves a fine-grained surface layer with thickness values of at least 1 μm to 2 μm.
The presented investigations provide an enhanced insight into the relation between the machining process and the resulting surface properties when milling particle-reinforced AMCs with MCD-tipped tools. The adaptation of the tool geometry is identified as promising approach for the generation of surfaces with predefined properties. Moreover, the concept of more comprehensive correlations, respecting surface microstructure, physical properties, and the microstructural characteristics of the surface layer is introduced. However, ongoing research should further focus on an expansion of the in-depth understanding of the mechanisms leading to the determined surface properties.