Research on Roughness and Microhardness of C45 Material Using High-Speed Machining

Abstract

From the point of view of production, it is of fundamental importance to know the cutting parameters at which the new surface of the component was created because only in this way is it possible to understand the nature and properties of the created surface. Based on the information obtained, it is then possible to improve the processes used or to create machined surfaces with, if not zero, at least a minimum number of flaws. The main purpose of the article is to create a comprehensive overview of the behavior and properties of the selected material with a focus on the prediction of roughness depending on the cutting speed and depth of cut, Vickers microhardness evaluation, assessment of cutting tool wear, and assessment of the shape and structure of the resulting chip. The achieved results are recorded in graphical and verbal form, from which the necessary conclusions are drawn. From the performed analyses, a mathematical prediction of determining the quality of the machined surface was created, which reflects changes in roughness depending on the cutting speed in the three modifications (700 m/min, 1400 m/min, and 2000 m/min). Conclusions were also drawn regarding the characteristics of the resulting chip and the wear of the cutting edges depending on the change in cutting speed. The fluctuating course of surface roughness when changing the cutting speed can be considered a significant finding—at 1400 m/min, the surface roughness was expected to have a decreasing tendency; however, it increased; a decreasing tendency was not recorded until at speeds of 1800 m/min, but during this process, the material already crept.

1. Introduction

Intensification and optimization of the machining process require a set of knowledge about the behavior of the material in the machining process, about the phenomena of mutual interaction between the tool and the workpiece, and about changes in the properties of the material in the machining process. The development of acquired knowledge and the formulation of scientific postulates leads to a rational choice of machining conditions characterized by minimal consumption of human labor, electrical and other types of energy, and tool material. Knowledge of the influence of cutting parameters on the formation of deformations in the cutting zone is essential from the point of view of all aspects of the production process. The significance and importance of this issue are confirmed by the publications of the authors at the global level. The authors [1] dealt with the issue of the influence of cutting parameters on chip compression and plastic deformation in the turning process of carbon and tool steel. From the conducted research, cutting parameters were selected, through which it is possible to achieve an increase in process efficiency for the investigated materials and, at the same time, an improvement in surface integrity with the identification of the lowest possible plastic deformation in the cutting zone. The same issue was investigated by the authors [2] but within the high-speed cutting process. In their research, they implemented the theory of stress waves to describe the mechanism of chip formation, followed by a quantitative DEM (discrete element method) analysis through simulation in comparison with experimentally obtained data. The authors [3] devoted themselves to investigating the influence of cutting parameters on chip formation for fiber-reinforced plastics. The research was carried out through real application parameters implemented in the industry. The achieved results pointed to the tendency of increased chip size when setting lower cutting speeds. The issue is investigated in connection with mechanical loading and chip formation [4,5] and from the point of view of analyzing the microgeometry of the tool.

The authors in [6] examined coated tools under different cutting conditions in terms of wear and topography. The research of the surface topography depending on the cutting parameters was also described in [7], where the microstructure, hardness, and roughness in the process of conventional titanium milling were investigated. The effects of microlubrication and two alternatives of cryogenic cooling in the titanium hole-making process were also investigated [8], which is considered highly unsustainable, as it is disturbed by poor hole quality, high energy consumption, and is also accompanied by damage to tools, resulting in high costs for the production process. From the results, conclusions were identified with the determination of suitable parameters for the given machining processes, which ensured a higher overall efficiency of the production process. The same material was investigated, and the results of the performed experiment were described by the authors [9], but the authors focused on the study of surface topography in the process of high-speed machining. Based on the performed experiments, they came to the conclusion that the surface topography is significantly influenced by the depth of cut, while the feed rate and cutting speed do not have a significant effect on the topography. The issue of optimization of machining parameters of titanium alloys using the MOORA method (multi-objective optimization based on ratio analysis) is presented in the article by Abbas et al. [10], summarizing in the discussion part the knowledge about the possibilities of achieving a balance between achieving lower values of surface roughness at the highest possible permitted material removal rate. The correct setting of cutting parameters and conditions is not only a test for titanium alloys but also for aluminum [11,12] and nickel alloys [13,14], which are an integral part of the automotive and aerospace industries. Various methods can be implemented to set the optimal parameters, as described in article [15] for the Al7075 aluminum alloy machining process. The influence of cutting conditions on the topography of this alloy is also recorded by the authors [16], which describes the progressive machining of Al7075 using the MQL system. The mentioned research showed that the most significant influence on the topography of the Al7075 surface with the implementation of MQL (minimum quantity lubrication) has cutting feed and the least-cutting speed. The assessment of the machining process using MQL technologies was also described by the authors [17] with the derivation of a sustainable machining model. The machining parameters were also examined by the authors [18] in novel dry ice blasting cooling assisted milling in comparison with minimum quantity lubrication (MQL). The material AISI-52100 tool steel was also tested with the application of the new hybrid technology (MQL + CO₂) [19] in order to improve the machinability and sustainability of the production process. During the experiments, cutting forces, tool wear, residual stresses, surface roughness, surface hardness, and cutting temperature were investigated when changing cutting speed and feed per tooth using novel hybrid technology. The results of the experiment pointed to the improvement of the tool steel milling process (chip removal, cutting heat removal, and excellent lubrication), as the precise penetration of MQL lubrication and dry ice grains into the cutting site was achieved. In order to assess the influence of cutting conditions on topography, it is possible to implement a simulation method [20,21,22,23] as well as the Taguchi method [24,25,26], et cetera, in the development phase. Based on the above overview, the need to investigate cutting conditions in the process of conventional and progressive machining methods can be determined [27,28,29,30], while the focus of research is extended to a wide range of materials. The present research deals with the study of the C45 material, which is characterized by its broad-spectrum applicability in various industries and its economic efficiency. For this reason, it is necessary to gain knowledge about the behavior of C45 material not only during machining by conventional methods, as reported by the authors’ studies [31,32,33,34], but also the behavior of the material in progressive machining processes. However, the research of progressive machining of C45 material is focused on areas such as [35,36,37,38], while from a practical point of view and applicability in production processes in practice, the HSM is applied for other materials such [39,40,41] as it is necessary to focus on standard HSM (high-speed machining) processes [42], which are implemented in this research with subsequent analysis of the influence of cutting parameters on the formation of deformations in the cutting zone. The contribution of this article lies in

• Processing of predictions for the possibility of determining the quality of the surface of the machined components
• Identification of changes in surface roughness depending on the cutting speed
• Identification of the wear of the cutting edges due to the change in the cutting speed

2. Materials and Methods

In general, the transformations of the properties of solid objects are derived from the mechanism of conversion of the removed material into chips. The process of chip formation, newly emerging segments, begins with the alignment of wedge-shaped elements in front of the tool. The initial contact of the tool face with the segment being created is very short, and the contact length increases as the tool moves. At this stage, the movement between the tool face and the segment is almost damped until the surface of the segment touching the tool face becomes equal. In this phase, there is an increase in the intensity of heat transfer to the contact zone of the tool, and the chemical reaction between the tool and the chip is stimulated, which causes faster wear of the tool. The gradual formation of a new segment pushes the formed segment upwards. The contact between the created and the new segment is gradually displaced due to the alignment of the segment by the tool. In this process, it is possible to observe the movement of the segment along the face of the tool with periodic deceleration. One of the important parameters in the examination of the material removed in the chip machining process is the compression of the chip, which is expressed by the ratio of the chip thickness to the thickness of the cut layer of material. A zone of very intense plastic deformation is created in front of the tool face, which has a fibrous structure and slows down the movement of chip elements toward the tool face. In a certain range of cutting speeds, this layer is relatively stable, and direct contact occurs between this layer and the chip [43].

The mathematical model of chip compression (Figure 1) presents the relationship [43]:

$$k=\frac{\cos \left(\varphi -{\gamma }_n\right)}{\sin \varphi }[-]$$

(1)

• $\varphi$—angle of plastic deformation limit [°]

• γ_n—tool angle of the face in the normal plane [°]

In addition to the chip compression parameter, it is necessary to know the angle of plastic deformation limit, below which the chip separates from the machined material, which indicates the position of the boundary (imaginary) plane above which the material is plastically deformed. The mathematical expression for practical implementation is as follows:

$$tg\varphi =\frac{\cos {\gamma }_n}{k-\sin {\gamma }_n}[°]$$

(2)

Within the methodology, to determine the basic experimental conditions, it is necessary to consider other parameters which require mathematical formulations:

-

chip removal rate:

$$v_{ch}=v_c\frac{\sin \varphi }{\cos \left(\varphi -{\gamma }_n\right)} [\mathrm{m}/\min ]$$

(3)

-

chip deformation:

$${\gamma }_{sh}=\frac{\cos {\gamma }_n}{\cos \left(\varphi -{\gamma }_n\right)}=cotg\varphi +tg \left(\varphi -{\gamma }_n\right)[°]$$

(4)

-

shear speed:

$$v_{sh}=v_c\frac{\cos {\gamma }_n}{\cos \left(\varphi -{\gamma }_n\right)} [\mathrm{m}/\min ]$$

(5)

-

deformation rate in the shear plane:

$${\gamma }_{sh}=\frac{v_{sh}}{\Delta }=v_c\frac{\cos {\gamma }_n}{\cos \left(\varphi -{\gamma }_n\right)·\Delta }[°]$$

(6)

With the implementation of the above relations, the real basic conditions and machining characteristics (

Table 1. Input parameters of the experiment.

	Experiment Alternatives—Cutting Speed [m/min]
	700			1400			2000
Spindle Speed n [min−1]	13,934			27,866			39,808
Feed Speed vf [mm/min]	5573.6			11,146.4			15,923.2
Radial depth of cut ae [mm]	4			4			4
Actual chip thickness h [mm]	0.1
Medium chip thickness hm [mm]	0.047
Maximum angle of the cutter tooth mesh [°]	60
Specification of parameter “ap”
	Experiment alternatives—cutting speed [m/min]
	700			1400			2000
Axial depth of cut ap [mm]	0.5	1	1.5	0.5	1	1.5	0.5	1	1.5
Section S [mm2]	0.05	0.1	0.15	0.05	0.1	0.15	0.05	0.1	0.15
Chip width b [mm]	0.5	1	1.5	0.5	1	1.5	0.5	1	1.5
Maximal section Smax [mm2]	0.05	0.1	0.15	0.05	0.1	0.15	0.05	0.1	0.15
Average section Sm [mm2]	0.023	0.047	0.070	0.023	0.047	0.070	0.023	0.047	0.070
Specification of used material—C45
Tensile strength Rm [MPa]					570
Yield strength [MPa]					325
Density [kg.m−3]					7870
Thermal coefficient of expansion [K−1]					11.6 × 10−6
Thermal conductivity [W.m−1.K−1]					49

) for HSM (Figure 2) were determined in the preparatory phase.

The implementation of the experiment takes place in three alternatives—the first alternative 700 m/min, the second alternative 1400 m/min, and the third alternative 2000 m/min, with the modification of the basic cutting speeds, which are the primary attribute when using high-speed machining.

The CNC machine and DMG HSC 105 Linear machine with 90 m/min rapid traverse acceleration in 5-axis simultaneous machining were implemented in this phase. Technical data of used CNC machines are presented in

Table 2. Selected technical data of DMG HSC 104 Linear [44].

Work area x-y-z [mm]	1110 × 800 × 600
Swivel axes (A/B)	–10/+110
Rotary axis (C)	360
Maximum spindle speed [rpm]	40,000
Maximum feed rate [mm/min]	90,000
Maximum tool length [mm]	300
Maximum tool diameter [mm]	140
Power diagram

[44].

The cutting parameters were designed in conjunction with the choice of cutting tool (Figure 3). Since the experiment was oriented to connect the experimental area with practice, an available standard tool—a monolithic face-cylindrical cutter with a diameter of 16 mm—was designed, the substrate of which is nano grain carbide and the coating is multi-layered “X-5070 blue coated”.

The preparatory phase of the performed experiment was completed by modifying the semi-finished product from C45 material to the required shape and size. The C45 material that was used for the samples was not produced by drawing but by rolling. For drawn materials, it is known that the hardness ranges in the order of ±10. However, these materials are characterized by high internal stresses. Rolled material was used for the samples, and one part of the research pointed out the fact that the hardness in the material that was not previously heat-treated in the wide areas of the sample was different. If such material is processed in its original state, the different hardness can be one of the reasons for the rapid wear of the cutting tool, as it is very complicated to optimize the cutting parameters. The modification of the semi-finished product was performed on a band saw for the cutting operation and subsequently roughed on a CNC milling machine in the dimensions 100 × 100 × 20 [mm] (Figure 4).

The prepared test specimens were processed according to the specified cutting conditions listed in Table 1 using high-speed machining technology (Figure 5).

Analyzes of the influence of cutting parameters on the formation of deformations in the cutting zone were performed on the machined test specimens. A Mitutoyo SJ-400 digital surface roughness tester (Kawasaki, Japan) (Figure 6) was used for roughness analysis, a Neophot 21 light electron microscope (Oberkochen, Germany) (Figure 7) to evaluate the microstructure, and a CV-400 DAT hardness tester (Bjerringbro, Denmark) (Figure 8) to evaluate the Vickers microhardness in accordance with STN EN ISO 6507-1, STN EN ISO 6507-2, and STN EN ISO 6507-3.

For the evaluation of microhardness, the samples were prepared in a special way in accordance with existing standards, cold-covered in acrylic resin, and wet-sanded with SiC-based sanding papers with 500, 800, and 1200 grit. Prior to the observation, the samples were polished with a disc with artificial diamond fiber and a polishing suspension Al₂O₃ 5 μm followed by etching in 2% HNO₃ solution.

Vickers microhardness measurement was performed on each sample at three points—A, B, and C. The hardness value of the cutting tool was 2344.9 HV.

3. Results and Discussion

To assess the surface roughness, a graph of the dependence of the roughness Ra on the cutting speed and depth of cut was generated at a constant feed of f_z = 0.1 mm. Based on the graph (Figure 9), it can be stated that the highest roughness was recorded during machining with the value of the cutting speed v_c = 1400 m/min, at a depth of cut a_p = 0.5 mm and 1 mm. During the performed experiment, it was concluded that at a cutting speed of v_c = 1800 m/min and more, lower surface roughness was achieved, but the material crept.

In order to determine the surface quality, a graph shown in Figure 10 was also created. In the given graph, a mathematical prediction of determining the quality of the machined surface was also created with the following notation:

$$f\left(x\right)=p_1·x^2+p_2·x+p_3,$$

(7)

with 95% reliability and individual coefficients:

• p₁ = −0.004176
• p₂ = 0.1873
• p₃ = −0.4834

Microhardness was observed on the prepared samples at three basic points, always before the machining process and subsequently after the end of the machining process. A comprehensive overview of the achieved results is presented in

Table 3. Results of microhardness.

	vc = 700 (m/min)						vc = 1400 (m/min)						vc = 2000 (m/min)
	Before HSM			After HSM			Before HSM			After HSM			Before HSM			After HSM
	ap (mm)			ap (mm)			ap (mm)			ap (mm)			ap (mm)			ap (mm)
	0.5	1	1.5	0.5	1	1.5	0.5	1	1.5	0.5	1	1.5	0.5	1	1.5	0.5	1	1.5
A	198.2	261.8	218.4	210.1	206.0	212.7	245.8	179.0	210.7	187.1	215.6	175.2	181.9	764.4	224.0	221.3	624.7	303.3
B	201.3	177.2	232.8	205.2	262.1	207.5	225.4	168.3	208.5	193.3	162.7	219.1	172.5	279.3	199.3	215.0	270.3	222.5
C	213.0	183.7	192.5	203.5	217.3	200.1	218.4	187.9	228.7	208.8	185.4	210.3	210.1	210.6	210.2	217.8	204.7	240.2

.

For individual alternatives of cutting parameters, the basic machining parameters in the primary zone for high-speed machining processes were calculated, the values of which are given in

Table 4. Machining values for high-speed machining.

Parameter	Values—Experiment Alternatives
	vc = 700 [m/min]	vc = 1400 [m/min]	vc = 2000 [m/min]
Chip compression	1.16
plastic deformation limit angle	47°
chip removal rate	681.3 [m/min]	1362.6 [m/min]	1946.6 [m/min]
chip deformation	1.32°
shear speed	924 [m/min]	1848 [m/min]	2640 [m/min]
deformation rate in the shear plane	9240 [s−1]	18,480 [s−1]	26,400 [s−1]

.

Chips were assessed from a metallographic cut and under an electron microscope in each alternative of the research. During the first alternative, a smooth material removal process was observed in the cutting zone, with no significant changes observed in either the surface roughness or the microhardness (

Table 5. Machining values in the primary shear zone for the first alternative of high-speed machining (v_c 700 m/min, A, B, C, designation of measuring points for microhardness).

Depth of Cut [mm]	Before HSM	After HSM	Chip
0.5
1
1.5

). During the first analysis, only slight wear of the tool was recorded, assessing the condition before the start of the machining process and at the end of machining.

When analyzing the wear of the cutting tool in the first alternative (Figure 11), balanced wear of the cutting edges of the tool and minimal displacement of the cutting edge in the direction of the back was observed. During the experiment, tool wear analysis was examined after 5 min of continuous cutting. A shallow groove formed on the front of the tool.

During the analyses of the second alternative (

Table 6. Metallographic cutting results for the second alternative (v_c = 1400 m/min; A, B, C, designation of measuring points for microhardness).

Depth of Cut [mm]	Before HSM	After HSM	Chip
0.5
1
1.5

), deformation in the cutting zone was observed in the metallographic sections. During the removal of the 1 mm thick layers, the tool was significantly damaged (Figure 12), and the quality of the machined surface was low.

During the analysis of the third alternative, the mechanism of the initial deformation in the cutting zone before the cutting edge of the tool was also observed. Based on the obtained results (

Table 7. Metallographic cutting results for the third alternative (v_c = 2000 m/min; A, B, C, designation of measuring points for microhardness).

Depth of Cut [mm]	Before HSM	After HSM	Chip
0.5
1
1.5

), it is possible to observe the elongation of the grains into the shape of fibers. Chip deformation does not have the same intensity. There is a periodicity of deformation, which results in a “sawtooth” profile of the back of the chip. Heavily deformed grains break in front of the cutting edge. The figure shows that in the braked layer in front of the cutting wedge, there is a significant (up to fourfold) increase in the hardness of the material. This indicates a significant strengthening. This layer thus assumes the function of a cutting wedge when removing chips. At high temperatures, said layer in the deformation field appears to be plastic. Slides in the chip have a periodic character. Their frequency probably depends on the cutting speed and the geometry of the tool. There was a higher reinforcement in the shear zone than in the middle of the chip element. The plastic deformation limit is not really linear but bent towards the workpiece. From the metallographic cut, the flow of material over the braked zone can be observed. The plastic field has a small height, but the nature of the deformation is the same. On the front and partially back surfaces of the knife, an imprint of irregularities can be seen, into which the deformed workpiece material has penetrated. This testifies to the fact that the deformed material does not move immediately in contact with the tool, or more precisely, its speed is very slow. Gradually above the tool face, the speed of movement of the chip elements increases and reaches the speed of the chip. The microhardness measurement was performed only after the sample had cooled, i.e., after the process had ended. In the real chip formation process, the whole area is in a plastic state with different deformation intensities. It can be seen that the chip periodically shifts in its entire cross-section and changes to an elementary one, not a cohesive one. There is also a higher reinforcement of the material, especially in the area of the plastic field in front of the cutting edge of the tool.

In the tool wear analysis in the third alternative (Figure 13), the cutting edges of the tool were damaged to such an extent that permanent deformation occurred, and the tool was no longer able to continue the material machining process. The deformation occurred within a few seconds.

In the final part of the experiment, the shape of the chip formed during high-speed machining was observed under an electron microscope (Figure 14).

Based on the observations of the resulting chip, the following conclusions were drawn:

• The shape of the chip does not change significantly with the cutting speed, and the chips are short and coiled,
• Chips have a small cross-section, the cut is interrupted, and high temperatures (up to 1500 °C) occur during milling due to large plastic deformations and chip friction,
• With increasing cutting speed, a short cutting time of individual grains was recorded,
• The generated heat decarbonizes the surface of the workpiece, so cracks and changes in the structure occur, unfavorable tensile residual stresses in the surface layer of the machined surface are created,
• Blunting of the individual cutting edges (cutter teeth) causes a loss of cutting ability.

4. Conclusions

The experiment was carried out in three alternative cutting speeds—the first alternative 700 m/min, the second alternative 1400 m/min, and the third alternative 2000 m/min. In each alternative, the depths of cuts—0.5 mm, 1 mm, and 1.5 mm—were used for each cutting speed. The tested material was machined in practice with a standard tool, a monolithic face-cylindrical cutter with a diameter of 16 mm. The conducted experiments formed the basis for creating a mathematical prediction for determining the quality of the machined surface of the C45 material by HSM technology, which reflects changes in roughness depending on the cutting speed. The created mathematical prediction can be rewritten in the form: Ra = −0.004176v_c² + 0.1873 v_c − 0.4834, through which it is possible to realize the prediction of the resulting roughness for the given material depending on the cutting speed, for example, for a cutting speed of 1400 m/min it is based on the created relation predicted surface roughness 1.61 µm. A significant finding is the fluctuating course of surface roughness when changing the cutting speed:

• At 1400 m/min, the surface roughness was expected to have a decreasing tendency; however, it increased.
• A decreasing tendency was not recorded until at speeds of 1800 m/min, but during this process, the material already crept.

All presented features and structures provide a comprehensive overview of the research of problems and bring new knowledge to the academic community and for practice.