
==== Front
Heliyon
Heliyon
Heliyon
2405-8440
Elsevier

S2405-8440(24)13368-3
10.1016/j.heliyon.2024.e37337
e37337
Research Article
Effect of ball nose flank wear on surface integrity in high-speed hard milling of AISI 4340 steel using MQL
Hassanpour Hamed hamed.hassanpour@iau.ac.ir
a⁎
Rasti Amir b
Khosrowshahi Javad Hashemi c
Farshi Sina Sabbaghi b
a Department of Mechanical Engineering, Islamic Azad University, North Tehran Branch, Tehran, Iran
b Faculty of Mechanical Engineering, Tarbiat Modares University, Tehran, Iran
c Department of Mechanical Engineering, McMaster University, Hamilton, Canada
⁎ Corresponding author. hamed.hassanpour@iau.ac.ir
06 9 2024
30 9 2024
06 9 2024
10 18 e3733722 6 2024
7 8 2024
2 9 2024
© 2024 The Authors
2024
https://creativecommons.org/licenses/by-nc/4.0/ This is an open access article under the CC BY-NC license (http://creativecommons.org/licenses/by-nc/4.0/).
Tool flank wear, owing to its direct interaction with the machined surface, can have detrimental effects on the workpiece surface integrity. This study investigates the impact of tool flank wear on surface integrity characteristics, particularly white layer thickness (WLT) and chemical corrosion resistance, during high-speed milling of AISI 4340 steel. Twenty-one experiments, ranging in 7 levels of flank wear widths (0–0.6 mm), were carried out under consistent cutting conditions in the presence of a minimum quantity lubrication (MQL) system. The results illustrate that up to a flank wear width of 0.4 mm, there is a modest increase in surface roughness, microhardness, and WLT. However, beyond this threshold, a significant escalation in these parameters is observed. Notably, a wear width of 0.6 mm induces non-uniform material flow, impacting microhardness up to 120 mm beneath the surface and causing a sudden increase in WLT. According to open-circuit potential analysis, the surface's tendency to electrochemical reactions increases slightly as the wear width increases up to 0.5 mm. The electrochemical impedance spectroscopy of the machined surfaces also revealed that utilizing tools worn to 0.4 and 0.6 mm, respectively, led to a decrease in Rcorr values by 35 % and 75 % compared to the specimen machined with a new tool. These insights underscore the critical importance of managing tool wear to maintain surface integrity in high-speed milling operations.

Keywords

Flank wear
Ball nose
High-speed hard milling
MQL
Surface integrity
==== Body
pmc1 Introduction

Surface integrity investigates the essence and characteristics of the machined layer as well as its effect on the functional and mechanical properties of workpiece [1]. Roughness, microhardness, white layer thickness (WLT), and topography are critical elements in determining machined surface integrity. These parameters have a direct influence on workpiece properties, including fatigue strength and corrosion resistance, emphasizing the crucial need to attain an appropriate level of surface integrity during the production cycle [2]. Surface roughness, often the only determining factor for final product certification, influences not only fatigue life but also numerous other characteristics of the workpiece. Furthermore, mechanical material removal processes induce severe plastic deformation, enhancing the microhardness of the workpiece surface and subsurface layer [3]. This intense plastic strain raises the temperature beyond the austenite level, results in metallurgical phase transformations. Following these transformations, a martensitic layer is created on the workpiece surface due to the rapid cooling rate. Research on the surface integrity of milled workpieces has highlighted the presence of a thin, hardened layer on the surface after machining, commonly referred to as the “white layer”. The term originates from the fact that this hardened layer appears white when viewed through an optical microscope after being etched [4]. The brittleness and high hardness of this layer would make it susceptible to crack growth, which adversely affects the workpiece's mechanical properties [5]. In addition, the surface defects stemming from machining processes could be analyzed using the topography concept. Various incidents, including mechanical impact loads, thermal shocks from instant deformation, and existing impurities in the raw material, could influence the tool wear during the cutting process. These inevitable damages would affect the workpiece quality and reduce dimensional accuracy and surface integrity [6,7]. To achieve an acceptable level of integrity, it is essential to use a tool with suitable cutting geometry, along with high thermal and chemical stability throughout the machining process. Thermal and mechanical defects induced by tool flank wear width (VB) are among the major problems in the machining of hardened steels [8]. The schematic of cutting model considering tool flank wear band and formation of thermally and mechanically affected zone is presented in Fig. 1.Fig. 1 Schematics of tool flank wear effect on the machined surface.

Fig. 1

The following review will focus on the primary studies investigating the effects of tool wear on roughness, surface and in-depth hardness, WLT, and topography. Xie et al. [9] studied the effects of tool wear on surface integrity in the machining of AISI 4140 alloy steel, realizing that any increase in material strain rate correlated with higher tool wear as well. Moreover, this rise in strain rate led to elevated cutting energy, resulting in a damaged work. Consequently, they identified tool wear as a major factor contributing to surface defects. Li et al. [10] explored surface integrity of hard steel during dry milling. Their observations showed that work hardening caused by mechanical loading constituted the primary cause for the higher microhardness of machined surface compared to underlying layers. In addition, they reported that the tool flank wear (VB) up to 0.2 mm had minimal impact on the roughness. Peng et al. [11] similarly studied the effect of tool wear on the surface integrity of 718 Inconel. Their experiments involved three different tool wear widths. They found that the tool with the highest wear width exhibited the highest machining forces, while residual stress variations were the least pronounced with the new tool. Additionally, the most micro-structural changes were reported when worn out tools employed. Tan et al. [12] investigated the impact of cutting tool wear on surface integrity in milling process of Inconel 718. They utilized a ball nose end mill and assessed roughness, topography, residual stress, microhardness, and microstructure under four flank wear conditions. Their findings showed that a larger tool flank wear width led to increased surface roughness and deteriorated topography. Wojciechowski et al. [13] conducted a study revealing that using a worn tool can escalate the radial component of the plowing force by 108 times compared to a fresh one. Oliveira et al. [14] explored the influence of coated cemented carbide tool wear on surface integrity during face milling of Inconel 718. They reported a correlation between tool wear and residual stresses, ranging from compressive to tensile stresses, with the latter attributed to thermal effects. Ali Khan et al. [15] conducted a study on tool wear mechanisms and their impact on the machined surface during high-speed hard turning of AISI D2 steel. They found that cutting speed had the most significant effect on tool life, followed by feed rate and depth of cut.

Considering the importance of lubrication in cutting process, some studies have investigated the effect of using minimum quantity lubrication (MQL) methods as an effective method to create a lubrication film, as well as the impact on surface integrity and tool life.

In their comprehensive analysis, Korkmaz et al. [16] compared the effect of different cooling methods including dry, MQL, nanofluids, cryogenic, and hybrid cooling on tool wear, surface roughness, and chip morphology in the sustainable turning of Inconel-601 alloy. They found that cryogenic + nano-MQL results in better tool life. The results underscore the importance of selecting appropriate lubrication techniques to control tool wear and surface integrity. Ross et al. [17] highlighted the role of sustainable cooling/lubrication conditions in improving the tribological and machining characteristics of Monel-400 alloy. The study demonstrated that the combination of CO2 + MQL is the most effective approach which reduced the flank wear up to 55 % compared to other cooling techniques. However, the cryogenic medium outperformed and increased the machined face hardness. In a study conducted by Hassanpour et al. [18], it was found that during high-speed milling (HSM) of Ti6Al4V with MQL, roughness can reach approximately 0.2 μm. Increasing the cutting speed up to 375 m/min raised microhardness, but beyond this, microhardness decreased due to high heat dissipation through the chips. Maruda et al. [19] conducted an evaluation of tool wear during the turning of Ti6Al4V alloy with MQL using Cu nanoparticles of various sizes. Their findings emphasize the role of nanoparticle size in enhancing lubrication properties, which in turn affects tool wear and surface finish. This study underscores the potential for nanoparticles to improve MQL performance, suggesting that similar benefits might be observed in the HSM of AISI 4340 steel. Korkmaz et al. [20] provided a comprehensive review of the use of nanoparticles as lubricants in nano-MQL machining of metallic materials. They highlighted the advantages of nanoparticles in reducing friction and wear, considering the thermophysical and wettability features of hybrid nanofluids. Shah et al. [21] explored eco-friendly lubrication techniques, including electrostatic minimum quantity lubrication (EMQL) and nanoparticles-enhanced EMQL, for improving the machining performance of 15–5 PHSS. The study demonstrated significant improvements in tool life and surface quality with this method, indicating that eco-friendly lubricants can be effective in HSM environments. Maruda et al. [22] investigated the influence of extreme pressure/anti-wear (EP/AW) additives in the MQL method on surface geometrical structure during the turning of 316L steel. The addition of EP/AW additives resulted in better surface finish and reduced tool wear, suggesting that similar additives could be beneficial in HSM of AISI 4340 steel to maintain surface integrity. Korkmaz et al. [23] conducted a prediction and classification study of tool wear in sustainable machining of Bohler steel using different machine learning models. The accurate prediction of tool wear states can help in real-time monitoring and adjustment of machining parameters to maintain surface integrity, which is crucial in HSM.

The literature review highlights a gap in studies focusing on the effects of tool wear on surface integrity characteristics, especially corrosion resistance and white layer thickness. While there have been numerous studies on tool wear in general machining contexts, there is a noticeable paucity of research specifically addressing HSM. High speed cutting presents unique challenges and behaviors due to the elevated cutting speeds and temperatures, which can significantly influence tool wear and, consequently, surface integrity. Moreover, there is a distinct lack of comprehensive studies on the effects of ball nose tool flank wear on surface integrity. Ball nose tools are commonly used in high-precision and complex milling operations, where maintaining surface integrity is crucial. However, the specific impact of flank wear on parameters such as white layer thickness (WLT) and corrosion resistance remains underexplored. Another critical gap is the performance of novel lubrication methods, such as MQL, in the presence of worn tools.

The present study addresses these gaps by systematically investigating the effect of tool flank wear on surface integrity during the HSM of AISI 4340 steel, with a focus on ball nose tools and the application of MQL. Through a series of controlled experiments, the effects of flank wear on surface roughness, texture, microhardness, WLT, and electrochemical corrosion resistance were analyzed. This research provides critical insights into the specific wear thresholds that significantly deteriorate surface integrity and evaluates the performance of dual nozzle MQL under these conditions. These contributions aim to enhance tool wear management strategies and improve the overall efficiency and quality of HSM operations."

2 Materials and methods

In this study, AISI 4340 alloy steel with hardness of 430 ± 5 HV were used as workpiece material and experiments were done on cubic samples with dimensions of 20 × 30 × 40 mm. The chemical composition of the steel is detailed in Table 1. All specimens underwent a process of austenitization at 870 °C, followed by tempering at 425 °C for a duration of 2 h. Subsequently, they were oil quenched to room temperature to achieve the desired hardness [24]. A four-axis CNC milling machine center Mikron HSM 700 with the maximum table feed rate of 20000 mm/min and spindle speed of 42000 rpm was used to perform all experiments. A 16 mm Sandvik Coromant ball nose milling cutter (code: R216-16A20-045) with a 30° helix angle and a functional length of 200 mm was used. The TiC/TiN coated WC inserts (code: R216-16 03 M-M 1025) from Sandvik Coromant, featuring a 12° rake angle, were also utilized. the HSK-A toolholder (code: 930-B40-HD-20-088) was also employed for its high precision, stability, and suitability for high-speed machining applications. The tool overhang was considered to be 40 mm (less than 3 times the tool diameter) [25,26].Table 1 Chemical composition of AISI 4340 steel alloy.

Table 1Element	C	Ni	Cr	Mn	Mo	Si	Fe	
Weight percent (%)	0.38	1.70	0.90	0.69	0.30	0.28	Balance	

As it is known, tool run-out is inevitable during the machining process, and it can have a detrimental effect on the tool life and surface roughness due to the variations in chip geometry, which can be detected by variation in cutting forces. To ensure the effect of tool run-out in an acceptable range, the cutting forces were measured using a Kistler dynamometer during the milling process (Fig. 2). Results showed the total in-plane force had a maximum 3 % difference in two consecutive cutting sections. It should be noted that these conditions were consistently maintained across all milling tests. In addition, tool run-out was measured before experiments using a dial indicator. The maximum total indicator run-out (TIR) was 5 μm.Fig. 2 Experimental setup and MQL nozzles orientation.

Fig. 2

The MQL system was utilized to apply lubrication flow. In this device, high velocity of air flow atomizes the oil and accelerates droplets at the nozzle outlet. The mixed spray of air and oil is then used to lubricate the cutting area during milling process. MQL system was utilized with mineral oil at a flow rate of 240 ml/h and pressure of 6 bars. For better performance, two nozzles positioned at a 45° inclination angle relative to the tool were employed. The experimental setup, workpiece clamping arrangement and the positioning of the MQL nozzles are shown in Fig. 2.

As the maximum tool flank wear width (VBmax) was a focal parameter in this study, intentionally higher levels of tool flank wear were employed to examine the impact of worn tools on surface integrity. Accordingly, the experiments were structured into seven distinct levels of VBmax: 0, 0.1, 0.2, 0.3, 0.4, 0.5, and 0.6 mm, with each level repeated three times. The replicates were conducted to ensure consistency and to account for any variability in the measurements. The flank wear width, as the input parameter, was measured using a SZX16 stereo Olympus optical microscope in conjunction with image analysis software as illustrated in Fig. 3.Fig. 3 Tool flank wear measuring method along with worn insert.

Fig. 3

The cutting parameters used in milling tests are presented in Table 2. When using a ball nose end mill, it is common for only a small portion of the cutter's diameter to be engaged in chip cutting at the effective diameter (Deff) (Eq. (1)) [27]. Therefore, the cutting speed is determined based on the Deff of the ball nose.(1) Deff=2ap(D−ap)

In which ap is the axial depth of cut and D is the tool nominal diameter.Table 2 Cutting parameters used in HSM experiments.

Table 2Spindle speed [rpm]	Cutting speed [m/min]	Feed rate [mm/tooth]	Axial depth of cut [mm]	Radial depth of cut [mm]	Inclination angle [°]	
20000	270	0.06	0.3	2	0	

To achieve the desired tool flank wear, a series of experiments were conducted under constant milling conditions at specific time intervals. The machining parameters used to achieve the desired tool flank wear were consistent with those employed for the subsequent surface integrity analysis (Table 2). All experiments were conducted under down milling conditions. At each interval, the inserts were removed, and the wear widths were measured. Milling processes were halted once the measured wear width reached the desired level. Due to the utilization of two-edged milling cutter, the flank wear was considered as the average of VBmax between two teeth. The tool flank width was accepted for surface integrity analysis within a tolerance of ±0.005 mm. there were no signs of chipping or built-up edge (BUE) on the tools rake face. Ultimately, 21 tests were conducted using the worn inserts. The experiments were conducted under controlled environmental conditions to minimize external factors that could affect measurement accuracy.

2D roughness of the specimens was measured using a portable surface roughness tester (PS1 Portable Mahr tester model) and in order to obtain 3D roughness, Bruker ContourGT-K optical profilometer was used. Roughness is reported based on the linear arithmetic average (Ra) and areal arithmetic average (Sa). Measurements were performed on five different regions on the workpiece surface, with the average being taken as the final roughness of the milled sample. Microhardness indenter Bareiss V-test model was used to measure microhardness according to the Vickers method, applying a 0.5-kg force load for a duration of 4 s. The average microhardness was determined by measuring three different spots on the machined surface, with this value representing the microhardness for each respective test. Hardness variations were investigated both in depth and sub-surface layer of the workpiece. To study in-depth microhardness, the milled samples were cross-sectioned on one side using a wire-EDM machine and subsequently polished. Measurements were performed at four points within the distances of 20, 70, 120 and 170 μm beneath the top surface, respectively (Fig. 4). Topography images of the machined samples were captured using a DP25 Olympus optical microscope with 1000x magnification. Having the workpiece samples sectioned, polished, and etched with a 2 % Nital solution for 10 s, the WLT was also measured using the same microscope. For a more comprehensive examination, topography was assessed using an XL30 Philips scanning electron microscope (SEM).Fig. 4 Cross-section image of microhardness measuring points beneath the milled surface.

Fig. 4

To evaluate the electrochemical characteristic of the machined surfaces, the specimens were subjected to open-circuit potential (OCP) tests and electrochemical impedance spectroscopy (EIS). The experiments were conducted using a 3.5 % sodium chloride solution in laboratory temperature. Electrochemical experiments were carried out in an EG&G flat cell with saturated Calomel reference electrodes and platinum auxiliary electrode, according to ASTM G5 (Fig. 5). EIS measurements were conducted using an IviumStat potentiostat (Ivium Technologies BV, the Netherlands). The impedance data were analyzed using Zview 3.4 equivalent circuit fitting software. Before running the process, each specimen was immersed in the test solution for 1 h and the OCP was determined subsequently.Fig. 5 Measuring method of electrochemical corrosion resistance of milled surfaces.

Fig. 5

To fit the results, the circuit illustrated in Fig. 6, comprising two RC loops in series along with the solution resistance, was employed. In this circuit, Rs represents the electrolyte resistance due to the potential drop between Luggin tip and the surface of work electrode which can be considered as the solution resistance. Rcorr signifies the electrical resistance associated with the charge transfer from the electrolyte into the oxide film formed on the surface layer. Electrical conductivity can be carried out by either electrons or ions. Charge transfer within the electrolyte not only creates a real resistance to the current flow but also, creates a capacitive property, which is indicated here by the constant phase element Coxide. This arises from both the potential variance between the electrode surface and the electrolyte and the electric field present. Roxide also represents the resistance to charge transfer through the oxide film formed on the surface layer. Here, due to the semiconductor nature of the oxide film, current transfer encounters elevated resistance. Cdl is also related to the capacitive effect of this film [28]. In this study, the constant-phase element was utilized instead of an ideal capacitor to achieve a better fit for the equivalent circuit. It worth mentioning that all measurement instruments were calibrated according to manufacturer specifications before use, and calibration checks were performed regularly throughout the experiments.Fig. 6 Equivalent circuit for fitting impedance diagrams.

Fig. 6

3 Results and discussion

As previously mentioned, a total number of 21 tests (7 levels with 3 repeats) were performed using inserts with different flank wear widths. Average amounts of surface roughness, microhardness and WLT for each experiment are presented in Table 3. In the following, each of these outcomes will be analyzed individually. Error assessment was employed to validate the data and ensure the reliability of the experimental results.Table 3 Surface integrity characteristics versus different tool flank wear widths.

Table 3Experiment no.	VBmax
)mm)	Output parameters (avg.)	
Ra (μm)	Sa (μm)	H (HV0.5)	WLT (μm)	
1	0.0	0.46	1.45	457.0	6.6	
2	0.1	0.44	1.53	461.6	7.9	
3	0.2	0.62	1.83	466.6	13.2	
4	0.3	0.72	2.28	475.6	14.8	
5	0.4	0.80	2.54	482.6	16.2	
6	0.5	1.60	3.87	516.2	26.7	
7	0.6	2.65	4.52	540.0	39.0	

3.1 Surface roughness

The average surface roughness variations versus flank wear along with error bars are illustrated in Fig. 7. The results showed that as tool wear increases, surface roughness increases as well. Pavel et al. [29] also claimed the same results during hard turning of 1117 steel. They attributed surface deterioration to the increased engagement of the worn tool with the machined surface, resulting in elevated friction and cutting forces. Wojciechowski et al. [13] found that using a worn tool in ball nose milling increases radial forces significantly, indicating that flank wear has a notable impact on the intensity of the ploughing phenomenon. Consequently, considering the dynamic nature of milling, the material removal process impacts are intensified due to the rise in machining forces and tool vibrations leading to increased surface roughness in milling [30]. Fig. 7 also exhibits an upward trend of surface roughness with a constant slope up to a wear width of 0.4 mm. This reveals a steady milling process up to the wear width of 0.4 mm. As the tool wear reaches 0.6 mm, roughness is significantly increased to 2.65 μm, which is approximately 3.3 times the value observed at a wear width of 0.4 mm. In fact, excessive tool wear leads to the occurrence of the plowing phenomenon at the shear zone, resulting in nonuniform material flow and undesirable roughness [31].Fig. 7 Workpiece surface roughness (Ra) versus tool flank wear width.

Fig. 7

The surface roughness was also measured using the areal average roughness (Sa) to study the spatial effect of ball nose on the surface. Fig. 8 shows the variation of Sa versus tool flank wear between 1.45 and 4.52 μm. Accordingly, the same trend was observed showing the tool wear increases the surface roughness. The difference between Ra and Sa indicates that the machining marks formed during ball end milling are affected by the flank wear and ploughing at the tool tip. In this case, the material under the tool flank face is smeared and consequently the plastic side flow of material affect the height of surface irregularities. The surface alterations can be seen in 3D surface roughness when using a tool with VBmax = 0.4 mm (Fig. 9).Fig. 8 Areal surface roughness (Sa) versus tool flank wear width.

Fig. 8

Fig. 9 3D surface roughness of the ball-end milling with VBmax = 0.4 mm in the scanned area a) 7.4 mm × 7.4 mm, b) analyzed area 1.4 mm × 3.2 mm.

Fig. 9

3.2 Surface and in-depth microhardness

Microhardness variations of the milled surfaces with respect to tool wear width are illustrated in Fig. 10. It is evident that surface hardness increases after the milling process, regardless of the wear width. However, using a worn tool has a greater influence on the microhardness of the surface. The microhardness magnitudes ranged between 457 and 540 HV with changes in tool wear width. Thus, the effect of wear width on microhardness value would be more emphasized.Fig. 10 Surface microhardness variations versus different tool flank wear widths.

Fig. 10

The variations in hardness of the milled surface can be explained by three major mechanisms. Firstly, the severe temperature gradient of the shear zone causes phase transformation, followed by rapid quenching, resulting in an increase in hardness. Secondly, grain refinement and recrystallization occur due to severe plastic deformation induced by the work hardening phenomenon. Lastly, reactions such as oxidation or nitridation between the machined surfaces and the ambient environment contribute to hardness variations [32]. Consequently, the effectiveness of these mechanisms and their interaction with each other determine the final surface hardness. According to what has already been obtained, using a worn tool would increase machining forces as a result of cutting-edge bluntness. In addition, since the tool flank surface is being rubbed on the machined surface, more energy is consumed to cut the material and sliding on the machined surface. As a consequence, surface work hardening is intensified [33]. Furthermore, the elevated friction arising from the increased contact area between the worn-out tool and workpiece results in heightened heat generation within the shear zone. This increased heat is then transferred to the workpiece, leading to elevated temperatures. As a result, surface hardness is increased due to phase transformation and rapid cooling of the workpiece surface.

From one perspective, Fig. 10 shows the hardness difference of 25 HV between surfaces machined by the new tool (VBmax = 0) and the tool with 0.4 mm wear width, representing a 5.5 % increase. Moreover, the tool with 0.6 mm flank wear demonstrates an 83 HV increase in surface hardness compared to the new tool, marking an 18.1 % increase. This indicates the significant effect of tool wear on the surface hardness.

To examine the effect of tool wear on the workpiece surface and subsurface layers, microhardness is measured at depths of 20, 70, 120 and 170 μm beneath the workpiece surface. Fig. 11 illustrates microhardness values corresponding to the mentioned points. It can be observed that as the distance from the surface increases, hardness decreases and eventually reaches the hardness of bulk material. In addition, applying the tools with higher flank wear has a more significant effect on the in-depth microhardness. On the other hand, the least variations in hardness are observed when using the new tool, whereas the highest variations are observed using the tool with a 0.6 mm flank wear width.Fig. 11 In-depth microhardness variations versus tool flank wear width.

Fig. 11

Another important point is the approximately uniform distribution of the hardness resulting from tools with wear widths up to 0.4 mm. Regardless of the wear value of these tools, the hardness of sub-surface layer reaches that of the bulk material at a depth of 70 μm. In fact, beyond this depth, both thermal load effects and work hardening caused by plastic deformation, which are the main reasons for the hardness growth, declined considerably. However, in wear bands of 0.5 and 0.6 mm, microhardness at the depth of 120 μm becomes equivalent to the hardness before machining.

3.3 White layer

The significant amount of retained austenite in the white layer after machining results in the formation of un-tempered martensite on the surface which in turn creates a high surface hardness throughout the machining operation [34]. The white layer plays a crucial role in determining the mechanical properties of machined workpiece, with its thickness serving as a pertinent criterion for analyzing the workpiece surface integrity. Fig. 12 shows the white layer formed in test No. 1 and the thickness measuring method. As shown, WLT was measured at 5 different points using image analysis software and the average value was reported as the thickness value for the white layer.Fig. 12 Image of white layer and thickness measuring method in test No. 1.

Fig. 12

The effect of tool wear on WLT is also shown in Fig. 13. A worn tool can aggravate surface damage in machined workpiece, resulting in the formation of a thicker white layer. This behavior is consistent with the results of a study conducted by Zhang et al. [35] which examined the effects of tool wear on the formation of WLT when hard milling H13 steel. The formation of this layer is a combination of two mechanisms. First, rapid plastic deformation leads to finer grain microstructure. Secondly, metallurgical phase transformation occurs due to the intense temperature gradient [36]. A worn tool, producing higher temperature and pressure on the workpiece surface during the cutting process, induces these mechanisms, resulting in the formation of a thicker white layer.Fig. 13 Variations in WLT versus tool flank wear width.

Fig. 13

Using a new tool yields a white layer with a thickness of 6.6 μm, steadily increasing to 16.2 μm as the tool flank wear width increases to 0.4 mm. However, using a tool with a flank wear exceeding 0.4 mm abruptly leads to a 39 μm WLT, approximately six times thicker than that produced by the new tool. In fact, as it was mentioned before, an excessive amount of tool wear can extremely increase machining forces and heat in the shear zone, thereby influencing the metallurgical properties of surface and sub-surface layer.

3.4 Surface topography

To comprehensively study and analyze Surface topography concept, characteristics such as surface smears, tears, cracks, cavities and feed marks are all to be explored. In this research, captured images from both SEM and optical microscopes are used to study surface topography. Fig. 14 (a) and Fig. 15 (a) illustrate surface topography of milled samples using the new cutting tool. As shown in these images, using new tool results in a smooth, uniform surface topography with fewer surface defects. Fig. 14, Fig. 15 illustrate surface topography obtained using the worn tools (VBmax = 0.6 mm). As previously mentioned, using worn tools results in increased friction, acting as a deterrent against relative movement between the tool and the workpiece, and significantly affecting material flow. The effects of plowing force and material smearing caused by milling with a worn tool is obviously depicted in Fig. 14 (b). Moreover, as the friction increases, shear zone temperature rises as well leading to surface burnings distinguished as small dark spots in Fig. 15 (b). Wojciechowski et al. [37] also revealed that the machining marks formed during ball end milling are affected by the ploughing mechanisms resulting from the tool wear and the low cutting speeds at the tool tip. Additionally, they demonstrated that the variations in irregularity peak height can be attributed to high tool run-out.Fig. 14 Surface topography images by the optical microscope when using a) new tool, and b) 0.6 mm flank wear tool.

Fig. 14

Fig. 15 Surface topography images by the SEM when using a) new tool, and b) 0.6 mm flank wear tool.

Fig. 15

3.5 Electrochemical corrosion resistance

The resistance of machined specimens to electrochemical corrosion is influenced by several factors, including the chemical composition of the machined surface, the thickness of the oxide layer on the surface, the existence of micro and macro cracks, as well as the surface's roughness and texture. This parameter plays a crucial and significant role in the performance of components exposed to dynamic loads during their operational life [38]. As mentioned before, this paper examines the electrochemical properties of the surface through two approaches including; OCP, and EIS.

Fig. 16 illustrates the variations in OCP for machined surfaces with varying tool flank wear widths. It can be seen that using a new tool exhibits the highest (most positive) OCP, whereas the surface machined with a tool wear width of 0.6 mm displays the lowest (most negative) potential. In fact, the OCP is an indicator of the surface's thermodynamic propensity for electrochemical reactions, which is influenced by the roughness, cracks or voids on the surface layer, existence or absence of active compounds, WLT, and etc. Rough surfaces may possess higher surface energy, which can influence the adsorption of corrosive species and impact the initiation and propagation of corrosion cracks. Furthermore, the presence of micro-cracks, surface defects, and grain boundaries, which are often more abundant in using the worn tools, can serve as initiation sites for corrosion. As the wear width increases up to 0.5 mm, the surface's tendency to electrochemical reactions increases slightly (evidenced by a more negative surface potential). However, there is a substantial disparity compared to the corrosion tendency of the specimen machined with a 0.6 mm wear width.Fig. 16 Variations of OCP versus time for milled samples using different worn tools.

Fig. 16

As the OCP provides a qualitative evaluation of the surface's resistance to electrochemical corrosion, the EIS was applied for more detailed examination. The Rcorr values, obtained from EIS using the proposed circuit (Fig. 6) are illustrated in Fig. 17. These values, reflecting the resistance to electrochemical corrosion of machined specimens with varying wear widths, span from 155 to 38 kΩ. In fact, changes in tool wear are associated with variations in impedance values. A reduction in Rcorr suggests an enhancement in the ion transfer across the oxide layer. With these explanations, it is noted that the values of Rcorr for specimens machined with tools up to 0.4 mm wear width differ 35 % compared to the new tools. However, in the surfaces machined with a 0.6 mm worn tool, a noticeable reduction of 75 % in Rcorr was observed. This difference can be explained by the considerable variation in surface roughness when using worn tools with 0.4 mm compared to 0.6 mm widths. Furthermore, excessive wear of the tool (0.6 mm) results in an irregular surface texture (as discussed in section 3.4). These irregularities provide a favorable condition for electrochemical reactions. Furthermore, it can be attributed to the decomposition of the mineral cutting fluid owing to the elevated heat generated during machining with a 0.6 mm worn tool.Fig. 17 Rcorr values obtained from impedance experiment for milled samples using different worn tools.

Fig. 17

4 Conclusions

The present study aimed to survey the high-speed hard milling of AISI 4340 steel under MQL system. Tool flank wear width effects on roughness, topography, surface and in-depth microhardness, WLT, as well as electrochemical corrosion resistance were examined. The summarized findings are as follows.1. In all milling experiments, with the tool flank wear width up to 0.4 mm, the surface roughness remained below 0.8 μm. However, once the tool wear reached 0.6 mm, the roughness significantly increased to 2.65 μm, nearly 3.3 times the roughness value observed at a 0.4 mm wear width.

2. The milled surface hardness was consistently higher than the initial hardness, regardless of cutting conditions, in all experiments. The maximum microhardness was observed when a tool with a flank wear width of 0.6 mm was used, resulting in an 18.1 % increase compared to using the new tool.

3. Using a tool with a flank wear width of 0.6 mm significantly influenced the in-depth microhardness, unlike the new tool. In this case, the hardness at the depth of 120 μm equaled the hardness of the bulk material, whereas wear widths of 0.4 mm and lower altered the in-depth microhardness up to 70 μm.

4. A thicker white layer is formed as the tool wear width increases. For the tool wear width below 0.4 mm, the trend was relatively steady. However, using a tool with a flank wear width of 0.6 mm resulted in a sudden increase in WLT to 39 μm (approximately six times thicker than that formed with the new tool).

5. According to OCP analysis, the surface's tendency to electrochemical reactions increases slightly as the wear width increases up to 0.5 mm. The EIS of the machined surfaces also revealed that utilizing tools worn to 0.4 and 0.6 mm, respectively, led to a decrease in Rcorr values by 35 % and 75 % compared to the specimen machined with a new tool.

Data availability

Data will be made available on request.

CRediT authorship contribution statement

Hamed Hassanpour: Writing – review & editing, Writing – original draft, Validation, Supervision, Resources, Methodology, Investigation. Amir Rasti: Writing – original draft, Validation, Methodology, Investigation, Formal analysis, Data curation. Javad Hashemi Khosrowshahi: Writing – review & editing, Writing – original draft, Visualization, Formal analysis, Conceptualization. Sina Sabbaghi Farshi: Writing – original draft, Visualization, Formal analysis, Data curation.

Declaration of competing interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
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