Effect of micro textures on the cutting performance of circular saw blade under fluid lubrication

: In this paper, linear micro textures that parallel to the sawtooth edge were fabricated on the surface of the high speed steel W6Mo5Cr4V2 circular saw blade by laser engraving. Further, cutting performance of micro textured circular saw blade (TCS) and traditional circular saw blade (CS), including sawing arc length, sawing force, sawing temperature, machined surface roughness and wear mechanism, were investigated in sawing 304 stainless steel pipes under the cutting fluid condition. Results showed that the largest sawing arc length and sawing force were occurred on the circular saw blade sawing outward from the inner wall. In addition, TCS circular saw blade exhibited better cutting performance and the mechanisms were found, on the one hand, the effective sawtooth-chip contact length was reduced due to the micro textures fabricated on the sawtooth surface, on the other hand, cutting fluid can be better penetrated into the micro textures and formed stable lubrication film in sawtooth-chip contact interface.


Introduction
In the manufacturing industry, sawing as the starting point of workpiece machining had become one of the important process in the manufacturing. Cutting tools employed in the sawing process was specially, which was characterized by a certain arrangement of the sawtooth fabricated on the cutting edge of the tools. It can be classified into three kinds according to the application and structure of saw blade, which were hack saw blade [1], band saw blade [2,3], and circular saw blade [4,5]. The circular saw blade was widely used in the cutting of various kinds of metal and wood due to the higher machining accuracy and cutting efficiency in comparison with the band saw blade sand hack saw blade. According to the material and parameters of the sawtooth, the circular saw blade can be divided into two categories, one was that the solid saw composed of high speed steel or cemented carbide; the other was that the sawtooth with good hardness and wear resistance inserted or welded into the circular saw blade [6]. The cutting performance， wear characteristic and dynamic stability of the circular saw blade were investigated by numerous scholars [7][8][9][10][11]. Bradbury et al. [7] employed M2 high speed steel as circular saw blade material sawing difficult-to-cut material, and indicated that the sawing characteristic of the workpiece and tools material play a significant role in the sawing process. Alam et al. [10] presented a method to improve the stability of the circular saw blade by controlling workpiece feed speed, and monitored the lateral deflection of the saw blade. It indicated that the lateral deflection of circular saw blade within a desired limit.
Recently, many scholars had proved that micro textures were an effectively method to improve the friction condition of the friction pair surface, and micro textures were widely used in piston rings, tools, seals, wet clutches, and bearings surface [12][13][14][15][16][17][18]. Different types of micro textures engraved on the surfaces of the cutting tools have shown the effect in lowering sliding friction, reducing tool-chip contact length, promoting heat dissipation and relieving adhesive wear [19][20][21][22][23]. Kawasegi et al. [23] fabricated three types of patterns (parallel, perpendicular and cross patterns) on the rake face of the cutting tools. It was found that the textures perpendicular to the chip flow was beneficial to reduce cutting force. Deng et al. [24] produced three rake-face textured tools with different micro structures and found that the elliptical grooves promoted better dry cutting performance than the parallel or linear grooves. In addition, micro textures also used in drilling and milling process [25][26][27][28]. Zhou et al. [27] successfully fabricated micro textures parallel to the cutting edge on the rake face of the milling tools and carry out milling tests with nano-fluids, it obtained that the cutting force, surface roughness and wear rate of the tools were significantly decreased. Ling et al. [28] fabricated rectangular micro textures on the surface of drill tools, and found that the micro textures effectively reduced the adhesion of the workpiece material and improved the tools life. However, very limited research works have been reported concerning micro textures fabricated on the sawtooth surface of the circular saw blade. Thus, the effect of micro textures on the cutting performance of circular saw blade needed to be further studied.
In this paper, four linear micro textures that parallel to the sawtooth edge were fabricated on the surface of the high speed steel W6Mo5Cr4V2 circular saw blade by laser engraving. Then, the depth, width and microstructure of the micro textures were obtained. Sawing 304 stainless steel pipes tests were carried out with the micro textured circular saw blade (TCS) under the cutting fluid condition. The sawing properties, including sawing arc length, sawing forces, sawing temperatures, machined surface roughness and wear of the tools were investigated. This research can provide a good reference for reducing the wear of circular saw blade, improving the machining surface quality and prolonging the tools life. If possible, micro textured circular saw blade will be widely applied industrially in the future.

Specimen preparation
In this paper, High speed steel W6Mo5Cr4V2 was chosen as circular saw blade material from Yongkang Meili Juye Co., Ltd., China, the composition and mechanical properties of circular saw blade were shown in Table 1, As shown in Fig. 1, the diameter of circular saw blade was 200 mm, thickness was 2 mm, teeth were 150, teeth space was 4.2 mm, and the installation hole diameter was 32 mm. Before micro texturing, in order to removed surface dirt, the circular saw blade was put in the alcohol solution ultrasonic cleaning for 15 min, and then drying for 10 min. The micro textures were designed and fabricated on the rake face by Nd: YAG laser equipment (DP-H50, Jinan Xinchu Co., Ltd.,China) and the processing parameters were listed in Table 2. The circular saw blade was fixed on the indexing plate, rotated the indexing by 2.4° degrees after each machining until all the teeth were micro textured. The micro textures of the teeth surface were observed using white light interferometer (Wyko NT9300, Veeco Inc., USA) and scanning electron microscope (SEM; QUANTA FEG 250, FEI Inc., USA).      The 304 stainless steel pipes were installed and fixed on the tool rest and fed along the Y-axis at a certain rate. The entire process of TCS and CS circular saw blades sawing tests were under the emulsified cutting fluids (solcut oil-V600, DOMINO Co., Ltd., China) and the sawing conditions were shown in Table 3.

Sawing arc length
The schematic diagram of sawing force was showed in Fig. 5 (a). In this work, the sawing force was affected by the sawing force per tooth, sawing arc length and teeth space. The sawing force F and sawing arc length L were obtained from the following equations: Where sawing force F was the sum of main force Fz and radial thrust force Fy, P was the sawing force per tooth, L was sawing arc length, and t was the teeth space, R1 was the diameter of circular saw blade, R was external diameter of the workpiece, r was inside diameter of the workpiece, m was wall thickness, λ was the sawing depth.
In the sawing process, the teeth space t and the sawing force of each tooth P were constant, so the sawing force was proportional to the sawing arc length. The variation of sawing arc length was showed in Fig. 5 (b). When the sawing depth increased from 0 to 2 mm, the sawing arc length was increased rapidly, from 0 to about 14 mm. Then, the sawing arc length began to decrease and remain stably about 4 mm with the increasing of sawing depth. Subsequently, the sawing arc length began to increase again, and the sawing arc length reached the maximum at the sawing depth of 28 mm, which was about 16 mm. Finally, the sawing arc length rapidly decreased to 0 mm.

Sawing force
The effect of the sawing speed and feed rate on the sawing force (main force Fz, and radial thrust force

Sawing temperature
During the sawing process, a lot of energy would be involved for chip removal, which would generate considerable amounts of heat. In this work, the influence of sawing speed and feed rate on the sawing temperature for CS and TCS circular saw blade were evaluated under lubrication condition. Fig. 8 illustrated the sawing temperature of CS and TCS circular saw blade under the condition of sawing speed 100 m/min and feed rate 1.2 mm/r. It can be seen that the temperature of the cutting position and the circumferential of the circular saw blade were higher. For TCS circular saw blade, the sawing temperature was decreased by about 6.8 % in comparison with the CS circular saw blade.
The contour map of the sawing temperature of CS and TCS circular saw blade with different sawing speed and feed rate was exhibited in Fig. 9. For the sawing speed of 50, 100 and 150 m/min, the sawing temperature of CS was increased from about 109 to 196 ℃, 120 to 229 ℃ and 137 to 320 ℃, respectively with the increase of feed rate from 0.4 to 1.2 mm/r. For the TCS circular saw blade, the sawing temperature was decreased about 6~14 % under different sawing speed and feed rate in contrast with that of the CS circular saw blade. The emulsified cutting fluids can penetrate into the tool-chip contact area through the micro textures resulted in better lubrication and cooling, and thereby reduced the cutting temperature.

Machined surface roughness
The machined surface roughness Ra of the workpiece was measured and presented in Fig. 10

Morphology of machined surface
The machined surface roughness and burrs were important factors affected the quality of steel pipe machining. The morphology of machined surface by CS and TCS circular saw blade at feed rate 1.2 mm/r and different sawing speed were showed in Fig. 11 and Fig. 12, respectively. For CS circular saw blade, some abrasive marks, burrs and adhesions were formed on the machined surface at the sawing speed 50 m/min ( Fig. 11(a) and (b)). When the sawing speed was 100 m/min, the height of burrs on the inner side machined surface of the steel pipe was higher (Fig. 11(c)). Fig. 11(d) showed the machined surface roughness was increased significantly, and a large number of adhesions and cracks were appeared. For TCS circular saw blade, the number and height of burrs on the inner side machined surface of the steel pipe were reduced compared with CS circular saw blade. When the sawing speed was 50 m/min, some lamellar adhesions and ploughs appeared on the machined surface, and the burrs mainly existed in the inner machined surface of the steel pipe ( Fig. 12(a) and (b)). For the sawing speed 100 m/min, the height of the burrs and the surface roughness were higher in contrast to the low sawing speed (50 m/min), in addition, some cracks and ploughs exposed on the machined surface (Fig. 12(c) and (d)).

Cutter worn characteristics
The worn of the tool surface was mainly caused by the physical and chemical reaction between the chip material and the circular saw blade material. sides had been worn out, and showed an arc shape as shown in Fig. 13 (a) and (b). In addition, the chip was seriously piled up at the corners of the sawtooth, and deformed severely. Fig. 13 (c) illustrated the shape of the chip adhesive on the sawtooth, it can be revealed that the force, extrusion and deformation of the workpiece material removal in the sawing process. The corresponding EDS maps of the sawtooth worn surface were shown in Fig. 13 (d-g). It can be seen that the whole sawtooth worn surface was covered with Ni element because the workpiece material adhesive to the surface during the sawing process. The material of circular saw blade and workpiece didn't contain Na element, however, the emulsified cutting fluid contained Na element. Fig. 13 (f) illustrated Na element was also observed to the sawing zone and played a lubrication and cooling effect during the sawing progress. In addition, oxidation occurred on the sawtooth worn surface (Fig. 13 (g)). As mentioned above, although the emulsified cutting fluid can play a lubrication and cooling role, the severe adhesion and oxidation wear occurred on the sawtooth worn surface. and some adhesions appeared on the side of the micro textures away from the edge (Fig. 14 (b)). The results showed that the cutting fluid can penetrate into the sawtooth-chip friction contact zone through the micro textures, thus reducing the sawing temperature and adhesion of the worn surface. Fig. 14 (c) illustrated the worn and adhesion of the sawtooth corners on the both side were reduced compared to CS.
The corresponding EDS maps of the TCS circular saw blade worn surface were observed and showed in Fig. 14 (d-g). The results showed that the adhesion of the workpiece material also covered whole sawtooth surface, and the serious adhesion occurred on the corners of the sawtooth edge. The distribution of the Na element was observed in Fig.14 (f) further demonstrated that the cutting fluid successfully penetrated into the sawtooth-chip frictional contact zone during the sawing process. Furthermore, Fig.14 (g) illustrated that the oxidative worn occurred in the contact area of the sawtooth surface.

Discussion
The friction and lubricating behavior of the tool-chip interface would affect tool life and machined surface quality. In this work, the micro textures were fabricated on the tool-chip contact interface, and the performance of micro textured circular saw blade (TCS) in sawing 304 stainless steel pipes under different sawing speed and feed rate with fluid lubrication. The friction force (Ff) can be calculated from the following equation [29]: Where aw was the cut width, lf was the tool-chip contact length, and τc was the average shear strength at the Where β was the friction angle and γo was the rake angle.
Eqs. (6), (7) and (8) indicated that Fr, Fz, and Fy were linearly related to the cut width aw, tool-chip contact length lf and average shear strength at the sawtooth-chip interface τc. As shown in Fig. 6 and 7, the sawing force (main sawing force Fz and thrust force Fy) of TCS circular saw blade was reduced compared to CS circular saw blade. It can be attributed two mainly aspects: on the one hand, the tool-chip contact length lf (Fig. 14) of the TCS circular saw blade was decreased in comparison with CS, which was mainly due to the fabrication of micro textures on the sawtooth surface [22,24,30]. And the effective tool-chip contact length lt of TCS circular saw blade can be calculated as: t f t l l nd  (9) where dt was the width of the micro texture and n was the texture quantity in the effective contact zone. On the other hand, in the sawing process of circular saw blade (CS), due to the high pressure and temperature of the sawtooth-chip interface, only little cutting fluid penetrated into the friction and contact zone [22,31].
However, the cutting fluid penetrated into the interface of sawtooth and chip through micro textures and formed a stable lubrication film. Thus average shear strength at the sawtooth-chip interface τc of TCS circular saw blade was reduced in comparison with CS. As mentioned above, the sawing force of TCS circular saw blade was reduced on account of the combined effect of micro textures under the lubrication condition of cutting fluid. Furthermore, the micro textures made the cutting fluid penetrated into the sawtooth-chip interface easier and reduced the effective contact zone, which will effective reduced the sawing force, sawing temperature, machined surface roughness and decreased the sawtooth wear. The pressure in the micro textures was lower than the external pressure and the cutting fluid actively flowed into micro textures, which mainly due to the chip flowed on the sawtooth surface with high speed. Then, the cutting fluid flowed out from the micro-texture, and formed a stable lubrication film on the sawtooth-chip interface as shown in Fig. 15. In consequence, the macro textured circular saw blade (TCS) exhibited a better cutting performance in comparison with traditional circular saw blade (CS). In addition, Fig. 13 and 14 illustrated the wear and adhesion of TCS circular saw blade after sawing 304 stainless steel pipes 15 times were relatively reduced compared to CS.
The cutting performance of micro textured circular saw blade was improved effectively, owing to decreasing the tool-chip contact length, sawing force and sawing temperature, reducing the wear and adhesion of the sawtooth surface, and improving the quality of the machined surface. However, the worn on the corners of the sawtooth edge was seriously, and the sustainability of high speed steel sawtooth need to be improved.

Conclusions
In this paper, linear micro textures that parallel to the sawtooth edge were fabricated on the surface of the high speed steel W6Mo5Cr4V2 circular saw blade. Sawing 304 stainless steel pipes tests were carried out with the micro textured circular saw blade (TCS) under the lubrication condition. The following conclusions can be obtained: (1) The largest sawing arc length and sawing force were occurred on the circular saw blade sawing outward from the inner wall. Then, the second largest sawing arc length and force appeared on the circular saw blade first contact with the inner wall. When the circular saw blade was sawing to middle position of the pipes, the sawing arc length and sawing force were smaller and more stable.
(2) In the sawing process, micro textured circular saw blade (TCS) showed better cutting performance in comparison with traditional circular saw blade (CS) in terms of (i) reduced cutting forces and cutting temperatures, (ii) improved the quality of machined surface and decreased the number of burrs, (iii) reduced the wear and adhesion of sawtooth surface.
(3) The coupling effect mechanisms were found, on the one hand, the effective sawtooth-chip contact length was reduced due to the micro textures fabricated on the sawtooth surface, on the other hand, cutting fluid can be better penetrated into the micro textures and formed stable lubrication film in sawtooth-chip contact interface.

Ethical approval
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