High Speed Steel tool performance through hybrid texturing followed by shock peening towards sustainable machining

: Surface texturing is a promising sustainable technique to get better the machining performance of cutting tools. Laser surface texturing and micro-EDM are the most commonly used fabrication techniques and the textured tools exhibit better tool performance as compared to conventional tool. However, these processes involve re-solidification of material surface which makes the surface softening and reduction in the surface hardness. In the present work, cutting performance of the High-Speed Steel (HSS) tool is studied by fabricating hybrid (micro-grooves with dimples) textures on its rake face using both laser ablation and micro-EDM methods followed by laser shock peening without ablative coating (LSPWAC) process. Hybrid textured with shock peened HSS tools are used in orthogonal dry turning of 316 L stainless steel. It has been observed that hybrid textures with shock peened HSS tool exhibits higher tool life of up to 255% and 85% than the conventional and the hybrid textured (HT) tools respectively. The results shown that surface texturing followed by shock peening is an innovative method to improve the sustainability in machining process than only surface texturing method. Moreover, analysis of chip is studied by finite element analysis.

Cutting fluids are frequently used in the metal cutting industry to minimise friction and cutting temperature in the machining zone, hence lowering tool wear and cutting forces.Graham et al. [1] reported that machining with the cutting fluids, increases the machining cost up to 15%.Apart from the machining cost, cutting fluids make the environmental pollution while disposal and also causes health hazards [2].Rake face texturing of the cutting tool is most promising technique to reduce friction and cutting temperature in machining zone without use of cutting fluids for sustainability in machining [3].Few researchers have been studied the different types textures and its influence on cutting tool performance [4][5][6].According to Yayun Liu et al. [7], the geometric parameters (groove width, pitch and distance from cutting edge) of the micro-groove textures have a substantial influence on the cutting tool performance.Xu Wanget al. [8] and Yong sheng su et al. [9] fabricated micro-grooves by laser ablation process to improve the tool performance.Researchers performed dry machining experiments on Titanium alloys to study the influence of micro-groove textures on cutting tool performance.The authors noted that micro-textures reduce adhesion and friction between the chip-tool contact, resulting in a decrease in cutting forces and cutting temperature.
Sasi et al. [10] and Singh et al. [11] fabricated micro dimples on HSS tool by laser ablation and micro-EDM (μ-EDM) processes and conducted orthogonal dry turning experiments on Al alloy with dimple textured HSS tool.
Their results shown that textured tools exhibit better performance than the conventional tools.Qian et al. [12] analyzed the effect of micro-textures on tool performance while machining of Titanium (TC 21) alloy by finite element model (FEM).The simulations results were compared with experimental results and it shown that microtextured tool exhibits the better cutting performance than the conventional tool.Kishor Kumar et al. [13] performed finite element analysis to study the stresses developed at the textured tool-chip interaction zone.Their results revealed that the presence of micro-dents on tool rake face creates stress concentration during machining and increases the stresses at the tool-chip interaction zone.Furthermore, improvement in tool performance is observed using hybrid textures by providing superiority in lubricant storage capacity and reduction of tool-chip contact than the homothetic textures [14].Sharma et al. [15] made the hybrid textures such as circular pits along with parallel and perpendicular micro-groove textures on tungsten carbide (WC) tool while the turning of 4340 steel.According to experimental findings, hybrid textured tools perform better than homothetic textured tools.Furthermore, authors observed that perpendicular groves along with circular pits significantly reduce the chip-tool contact area than the other hybrid textured tool.In a minimal quantity lubrication (MQL) environment, Sivaiah [16] examined the machining performance of a dimple-textured tool with a hybrid textured tool (combining linear grooves and dimples).The hybrid textured tool gives superior lubricant storage which leads to lower in cutting temperature and tool wear up to 26% and 31% as compared to traditional tool respectively.Jialin et al. [17] evolved the performance of different tools such as micro-groove textured, micro-pits and hybrid textured (micro-pits and grooves are combined) while machining of AISI 1045 steel and pure iron.Results indicated that the hybrid textured tool performed better than the other tools.
According to earlier research, the Laser ablation and micro-EDM techniques are the most often utilized approaches for fabricating micro-textures on rake surface of the cutting tool.These fabrication methods involves re-solidification of material surface which leads to makes the surface softening and generate tensile residual stresses, this makes lower hardness of the surface [18].Qi et al. [19] created micro-textures on the rake surface of the cutting tool by micro-EDM and discovered significant tensile residual stresses around the texture, which causes tool chipping and shortens the tool life.Costil et al. [20] made micro-dimples on aluminium alloy by laser ablation and observe the low hardness around the dimple edge.Moreover, few researchers [18,21] stated that tensile residual stresses are generated around the dimple due to material expansion during the laser ablation.
Innovative surface-strengthening technique called laser shock peening (LSP) creates compressive residual stresses on a material's surface by using extreme plastic deformation [22].Moreover, this process increases the material's surface hardness by grain refining, which improves the material's tribological properties [23].Dry sliding tests were performed on a variety of materials, including Al, Mg, and Titanium alloys by a few of researchers [24][25][26] to investigate the friction and wear behaviour of shock peened specimens..They found that micro-hardness of the material surface is increased which results in lower friction and wear loss as compared to un-treated specimens.
Yujie Fan et al. [27] performed LSP with ablative coating process on HSS cutting tool and observed an improved tool life up to 40% than the un-treated HSS cutting tool.However, few researchers [28][29][30] performed LSP process without ablative coating process which generates the high compressive residual stresses due to absent of ablative coating.Prasad et al. [31] performed laser shock peening without coating process on HSS cutting tool and observed an improvement in the tool life.
In this present study, the combined effect of hybrid texturing (micro-grooves along with micro-dimples) and laser shock peening without ablative coating (LSPWAC) processes on HSS tool performance is investigated.
Micro-grooves are fabricated on rake face of the tool by micro-Wire Cut EDM (WEDM) and laser assisted surface texturing process is used for fabrication of micro-dimples.Moreover, finite element analysis is used for analysis of chip.

Materials and methods:
High speed steel (HSS) is used as cutting tool material and commercially supplied by Rohit Tools, New Delhi, India.The rake surface of the HSS cutting tool is mechanically polished using silicon carbide (SiC) abrasive sheets with grade of ISO P240 to P2000 before the surface treatments.These tools are then polished using alumina powder having a particle size of 5 μm, and all debris is removed by ultrasonic cleaning for up to 15 minutes in an acetone bath.For dry orthogonal experiments, a hollow cylindrical tube made of AISI 316 L stainless steel with an outside diameter of 52.5 mm and a thickness of 1.75 mm is used as the work piece.

Fabrication of Hybrid textures on rake face of the tool:
Hybrid (micro-groove along with micro-dimple) textures were fabricated on rake face of the tool by micro-Wire Cut EDM (WEDM) and laser assisted surface texturing processes.Figure 1shows the schematic representation of experimental set-up for fabrication of hybrid textures on the HSS tool rake face.Initially micro-grooves were fabricated by micro-EDM process and Table1(a) illustrated the process parameters.The considered size of microgroove textures are 300 µm spacing between the grooves, 250 µm width, and 100 µm groove depth.
After micro EDM process, tool surface has been polished by diamond paste to remove dirt.Afterwards, dimple textures were fabricated using Nd:YAG pulsed laser.The process parameters used in Nd:YAG laser is shown in Table 1(b).The dimple of diameter 150 µm, depth 60 µm and spacing between dimples of 300 µm are considered in the present study.Figure 2 represents the photographic and schematic representation of the hybrid textured HSS tool.

Laser shock peening without ablative coating (LSPWAC) process:
Figure 3 depicts the experimental setup for the LSPWAC process.The LSPWAC process is performed on the rake surface of the hybrid textured tool by using Nd:YAG laser.LSPWAC experiment was carried out at laser pulse energy (E) of 60 mJ, wavelength (λ) of 532 nm, frequency (F) of 5 Hz, pulse duration (τ) of 8 ns and pulse repetition rate (PRR) of 5 Hz.This study uses a single shock and a 50% overlap percentage for the LSPWAC process.
A water layer of thickness 1.5-2 mm is used as confinement layer to create shock wave with high pressure.The ablative coating is not covered on the material surface, so the laser beam directly irradiates on the material surface through the water layer (as shown in Fig. 3).The shock peened hybrid textured tool after the process is shown in Fig. 3 (c).

Experimental conditions and characterization
The experiments are carried out on a precision lathe with variable speed (make: MAGNUM, available at NIT Warangal, India).The tools used in the present study are conventional (CT), hybrid textured (HT) and hybrid textures with shock peened (HTSp).As shown in Fig. 4, the cutting tool is positioned such that the cutting edge is perpendicular to the work piece, and the depth of cut is governed by the tool's longitudinal feed.Cutting conditions considered in this work are cutting speed 35 mm/rev and feed rate0.175mm/rev.Vickers hardness tester (model: Economet VH 1MD), with indentation load of 500 g and holding time of 10 s, was used to measure micro-hardness.
A 3-axis dynamometer (Kistler) is used to measure the cutting force (Fc) and thrust force (Ft), and each turning test was performed three times.The 3D optical microscope is used to measure the flank wear of thetools.SEM (Scanning Electron Microscope; Make: TESCAN -VEGA3 LM) analysis is conducted to observe the chip bottom surface.

Finite Element Analysis (FEA):
A 2-Dimensional Finite element analysis (FEA) was carried out using ABAQUS/Explicit to study the effect of micro-textures on the chip formation at chip-tool interaction.Figures 5 shows the schematic representation of conventional and textured tool with work piece during the cutting operation.In this analysis, the cutting conditions are the same as those used in the experiment.The cutting tool is given as an analytical rigid body and moved into the work piece horizontally at a predetermined pace.
The Johnson-Cook (J-C) constitutive model is used to explain the plasticity behaviour during the machining process and is one of the most widely used constitutive models for predicting flow stress within metallic materials subjected to high strain .The J-C model is expressed by following equations [ 32] ** * First term in Eq. 1 denotes elasto-plastic deformation, second term denotes viscosity deformation, and last term denotes p  is the equivalent plastic strain at the onset of damage functions, d1 to d5 are the material damage parameters.Tables 2 and 3 illustrate the properties of work piece and Johnson-Cook specifications used in the simulation of the cutting process.

Results and Discussion:
The influence of conventional, hybrid textured (HT) and hybrid textures with shock peened (HTSp) tools on the cutting force, chip shapes and tool wear are explained below.

Cutting forces:
The relationship between the cutting forces (Fc and Ft) and the length of the tool-chip interfacial contact (lc) is shown in the following equations [34] cos( ) sin( ) Here, β is the friction angle, α is the rake angle and, c  thickness of the cut and wc is width of the cut.
It can be noticed that cutting forces can be minimized by having lower length of the length of the tool-chip interfacial contact (lc).Length of the tool-chip interfacial contact (lc) can be observed by SEM (scanning electron microscope) images of the chip bottom surface as shown in Fig. 6.When chip flow passes through textured tools, it is seen that deep micro-scratches appear on the bottom surface of the chip.(Fig. 6 b).These micro-scratches show chip-tool interactions, which shorten the length of the tool's interfacial contact with the chip.
The average values of cutting forces (Fc) and thrust forces (Ft) with various HSS tools such as conventional, HT and HTSpd respectively are shown in Fig. 7.In comparison to Conventional tool, HT tool has lower cutting forces due to reduction of length of the tool-chip interfacial contact (lc).The cutting forces (Fc) and thrust forces (Ft) were lowered up to 46.62% and 64.72%, respectively.Moreover, the cutting forces of the HT tool and the HTSpd are marginal difference and it indicates that cutting force reduction is not significantly impacted by the shock peening process.

Chip Shape:
Continuous chips increase the length of the length of the tool-chip interfacial contact (lc), which increases the amount of heat transferred to the cutting tool and shortens its lifespan [35].Whereas, discontinuous chips aid in lowering the amount of heat transferred to textured tools and extends the cutting tool life.Additionally, continuous chips require periodic stops in the process of machining so that they can be removed and so that they don't become entangled [36].The present study observes the chip formation through experimental and finite element analysis during orthogonal turning with conventional and HT tools.Figure 8 represents the photographic images of simulation and experimental results of chip formation by conventional and HT tools.
From Fig. 8, the long and ribbon type of chips are generated with conventional tool and short arc type of chips are generated with HT tool.Textures perform as a secondary cutting edge and obstruct the chip flow which leads to break the chip easily and generate discontinues chips.Moreover, experimentally observed that HTSpd tool also generates discontinuous chips which is similar to HT tool.Therefore, in the present finite element analysis only conventional HT tool is studied.

Tool wear:
Tool life was taken into consideration as one of the following criteria based on ISO 3685.(i) either the maximum or average flank wear land width (ii) notching (iii) nose wear (vi) surface roughness [37].The tool life in this study was defined as the amount of time required to produce 400 μm of average flank wear (Vb) [38].Flank wear of the different tools (conventional, HT and HTSpd respectively) during the turning experiment is shown in Fig. 9.It can seen from Fig. 9, tool life has been improved for HT and HTSpd tools in comparison of conventional tool.However, it can be observed that HT tool exhibits Lower tool life than the HTSpd tool.Due to secondary cutting of chip, the efficiency of HT tool has decreased in relation to machining time.
The effect of secondary cutting of the chip by micro-textures is shown in Fig. 10.It is observed that additional debris produced by secondary cutting of the chips, fills the texture area (refer Fig. 10 (b)) due to which texture efficiency reduces with respect to cutting time [39] and HT tool behaves like as a conventional tool.Because of this HT tool produces continuous chips after five minutes of orthogonal turning operation (refer Fig. 10 (c) and 10 (d)).
In HTSpd tool, shock peening reduces chipping of the tool's cutting edge through the generation of compressive residual stresses (RScom) at the cutting edge [31].The cross-sectional microstructure of untreated and shock peened HSS sample is are observed using scanning electron microscope (SEM) and the images are shown in This grain refinement is attributed to increase in surface hardness and this can observed in Figure 12.It is noticed that, untreated and shock peened samples have average micro-hardness of 874 HV and 935 HV respectively.
The LSPWC process makes the tool rake face harder, which increases abrasion wear resistance [27].Therefore, HTSpd tool exhibits the better tool life and improved nearly 85% than the HT tool and 285% than the conventional tool.

Conclusions:
This study investigates the combined effect of hybrid texturing (micro-grooves and micro-dimples) and laser shock peening without ablative coating (LSPWAC) processes on the performance of HSS tool.The following findings were made: In comparison to conventional, HT and HTSpd tools performs better in terms of minimal cutting forces (Fc, Ft).The cutting forces (Fc) and thrust forces (Ft) were lowered up to 46.62% and 64.72%, respectively.Due to substantial plastic deformation caused by LSPWAC treatment, the grain refinement occurred and depth of grain refining has occurred up to 35 μm.Moreover, average micro-hardness of shock peened HSS sample has improved its hardness up to 935 HV respectively.HTSpd tool exhibits the better tool life and improved nearly 85% than the HT tool and 285% than the Conventional tool.

Declarations
Funding This research has received no external funding.

Conflict of interest
The authors declare no competing interests.

Data availability statement
The authors confirm that the data supporting the findings of this study are available within the article.
B, C, m, and n are the material properties. is the material flow stress,  is the von Mises equivalent plastic strain, * o  is the reference equivalent plastic strain rate, *  is the von Mises equivalent plastic strain, To and Tm are the reference ambient temperature and the material melting temperature respectively.

Fig. 11 .
Fig.11.Due to substantial plastic deformation caused by LSPWAC treatment, the grain refinement occurred.It can be seen that the grain refinement has been occurred up to depth of 35 μm.

Fig. 1 .
Fig.1.Schematic representation of Experimental set-up for (a) micro-EDM (b) laser surface texturing on rake face of the HSS tool.

Fig. 6 .
Fig.6.SEM Images of chip bottom surface during the orthogonal turning by (a) conventional and (b) textured tools.

Fig. 9 .
Fig.9.Flank wear of the conventional, HT and HTSpd tools during the turning operation.

Fig. 10 .
Fig. 10.Photographic representation of chips filled in micro-grooves (a) simulation (b) Experimental work and chips produced by hybrid textured tool at (c) 1 minute and (d) 5 min.

Fig. 11 .
Fig. 11.SEM images for grain distribution for (a) un treated and (b) shock peened samples.

Fig. 12 .
Fig.12.Micro-hardness of the un-treated and shock peened HSS samples in relation to depth from the surface.