Adiabatic shearing and performances of milling 304 stainless steel based on cooling strategies of flood, LN₂ and LN₂+LCO₂

Abstract

During high-speed milling, stainless steel is prone to generating adiabatic shear bands in the cutting zone, which affects the cutting quality. The main factors causing adiabatic shear are high temperature and elastoplastic instability. An energy dissipation model for stainless steel milling was established and analyzed based on adiabatic shear. A series of flood and cryogenic cooling milling experiments with liquid nitrogen (LN₂) and LN₂+liquid carbon dioxide (LCO₂) cooling were carried out. Simultaneously, an innovative research about the effect of LN₂+LCO₂ on adiabatic shear was executed. The results show that the energy dissipation is mainly determined by the influence factors such as milling force and shear strength at the same milling parameters. Compared with flood cooling, the influence factors are all increased at LN₂ cryogenic temperature conditions, and the energy dissipation result is also enhanced. At 200 m /min, the temperature in the cutting zone of LN₂+LCO₂ is below 60 °C compared with 400 °C of flood cooling. The formation of serrated chips is related to the brittle cutting characteristic at lower temperature. Meanwhile, the tool wear is obviously inhibited, and the machined surface is improved with a roughness of less than Ra 0.3 μm at v_c = 150 m/min, which is lower than Ra 0.7 μm of flood cooling. Conclusion: due to the instantaneous cold brittleness of LN₂ and lubricating effect of LCO₂, there is difficult to obtain the conditions at the shear zone for generating adiabatic shear bands, and the milling performances are improved at the same time.

Introduction

Stainless steels are a kind of iron-based high-alloy steel with a chromium content of over 12%. Especially, 0Cr18Ni9 (304) stainless steel has excellent corrosion resistance and metallurgical mechanical properties^1,2. And it has been widely used in related fields such as aerospace, marine industry, and automotive industry and so on. However, this kind of material usually has poor machinability. During the cutting process, it is prone to work hardening, which results in severe tool wear. The processing accuracy and surface quality are difficult to meet the usage requirements. In addition, the lower thermal conductivity of stainless steel easily causes high cutting temperatures, severe work hardening, large cutting forces and difficult chip breakage. It also has a large linear expansion coefficient and strong material adhesion, which makes it easy to adhere to the cutting edge, that will result in poor machinability^3,4.

As the typical difficult-to-machine materials, stainless steel and titanium alloys often undergo the adiabatic shear localization under cutting conditions of low speed, small cutting depth and large rake angle. And the chips often appear in a band-like shape. With the increase of cutting speed, the instantaneous high-speed impact between the tool and workpiece causes the cutting higher temperature. There results in the greater thermal softening effect of processed material being than the strain rate hardening effect in a small area, and the instability of the thermal viscoplastic constitutive is caused inevitably. And then the adiabatic shear inevitably is occurred with the serrated-shaped chips spaced by adiabatic shear bands (ASB). This kind of chip makes the cutting process unstable, generates vibration, and affects chip breaking, processing quality, and accelerates tool wear^5,6,7. At the same time, when cutting difficult-to-machine materials at high speed, adiabatic shearing will cause high-frequency vibration for the cutting force, which generates seriously tool wear and the processed surface quality.

Most of literatures are all based on the adiabatic shear theory. There are many reports on serrated-shaped chips considering cutting speed^8,9,10,11, the materials have the properties of the poor thermal physical properties and good thermal conductivity. For the critical condition of the formation of serrated chips, scholars generally believe that the local thermal softening effect at the first deformation zone exceeds the strain strengthening and strain rate strengthening effects^12,13. Meanwhile, the thermal softening effect exceeds the hardening effect of strain and strain rate is a necessary condition for adiabatic shear evolution¹⁴. Therefore, seeking a better cutting cooling strategy is the research direction for reducing and improving adiabatic shear problems.

Many literatures mainly focus on the improvement of machinability by cryogenic cooling methods. Kumar et al.¹⁵ carried out the cutting AISI52100 steel at cryogenic cooling. They found that the thickness of the white layer was decreased by 52–78%, the cutting temperature was dropped by 49%, the roughness was decreased by 29%, and the thin-small serrated-shaped chips were generated. Mozammel et al.¹⁶ could quickly and effectively remove heat, significantly reduce specific energy consumption and temperature, and improve surface quality. Nalbant¹⁷ processed AISI 304 stainless steel under both dry and cryogenic cooling conditions. It was found that cryogenic cooling and cutting speed had an impact on the cutting force. Cryogenic cooling process had a higher processing efficiency than dry one. And the cutting force was increased with the increase of cutting speed.

Some scholars pay more attention to improving the lubrication problems in cryogenic cooling cutting, they introduces LCO₂ into the process of difficult-to machine materials in cryogenic cooling. Cetindag¹⁸ carried out turning tests on AISI 52100 bearing steel at constant cutting speed (200 m/min), feed rate (0.1 mm/r) and cutting depth (0.1 mm). The results were shown that the strategy of CO₂+MQL could improve the surface quality even better, increase the residual compressive stress of the machinated surface, and obtain better surface integrity. Shah¹⁹ analyzed different cooling and lubrication techniques during titanium alloy drilling, no coolant, cryogenic LCO₂ and LN₂ cooling. The results showed that cryogenic LN₂ and LCO₂ were a sustainable cooling and lubrication technology, and that could reduce tool wear and power consumption, and there also enhanced processing productivity and quality. Jamil²⁰ reported that combining dry ice (CO₂-snow) with the least amount of lubrication fluid was the most effective way to dissipate heat and lubrication. When dry ice was added to ethanol-ester oil cutting fluid, the fluid density, specific heat and viscosity were reduced by 20%, 10% and 5% respectively, the thermal conductivity was increased by 15%, and the heat transfer coefficient was significantly increased by 17%. Iqbal et al.²¹ introduced CO₂-snow as a coolant during the processing of the most difficult-to-machine material(β-titanium). Compared with emulsion coolant, the combination of CO₂-snow with the minimum amount of lubricant was found to be the most effective for heat dissipation and lubrication, and it also improved the durability of the cutting tool.

In summary, since adiabatic shearing is mainly caused by the plastic instability resulting from the rapid temperature rise in cutting zone. The research group had carried out investigations about the machinability for difficult-to-machine materials in LN₂ cryogenic cooling, and found the heat dissipation of milling zone is relatively rapid, the temperature rise was reduced slowly, and brittle cutting was occurred with the regular chips. At the same time, considering the advantages of lubrication and reduced friction for LCO₂ cryogenic medium. The novel influence of LN₂+LCO₂ combined cooling on the adiabatic shear characteristics and milling performance will be researched for 304 stainless steel compared with only LN₂ cooling in this paper.

Prediction model of serrated chip

Main shear zone model

During the cutting process, the main shear zone undergoes intense plastic deformation, accompanied by the generation of a large amount of heat. There can be regarded as a strong thermal-force coupling effect. At the same time, there are also effects such as work hardening and thermal softening. To describe its stress–strain relationship, the Johnson–Cook constitutive model is adopted^22,23.

$$\sigma_{f} = \frac{{\sqrt[{}]{3}}}{3}(A + B(\frac{{\sqrt[{}]{3}}}{3}\gamma )^{n} )\left[ {1 + C\ln (\frac{{\dot{\gamma }}}{{\dot{\gamma }_{0} }})} \right]\left[ {1 - (\frac{{T - T_{0} }}{{T_{m} - T_{0} }})^{m} } \right]$$

(1)

where σ_f is the flow stress (MPa), A is the material yield strength under quasi static(MPa), B is the influence coefficient of strain hardening, n is the hardening index, C is the strengthening parameter the strain rate, $\gamma$ is shear strain(MPa), $\dot{\gamma }$ is the shear strain rate, $\dot{\gamma }_{0}$ is the reference shear strain rate, T is the cooling temperature(25 °C and − 190 °C), T₀ is the room temperature(25 °C), T_m is the melting point of material(°C), m is the thermal softening coefficient.

Besides, the material of the workpiece firstly undergoes plastic deformation at the shear surface, and generates shear stress. Due to the influence of cutting speed, an inertial force exists simultaneously with the shear sliding, so its shear stress σ_s can be expressed as

$$\sigma_{s} = \rho (v_{c} \sin \phi )^{2} \gamma + \sigma_{f}$$

(2)

where ρ is the material density (kg/m³), v_c is the cutting speed(m/min), Φ is the shear angle (°).

According to the theory of unequal shear zones proposed by Li et al.²⁴, the strain and strain rate of the shear zone are expressed as

$$\gamma = \left\{ {\begin{array}{*{20}l} {\begin{array}{*{20}l} {\frac{{\dot{\gamma }_{\max } }}{{(q + 1)v_{c} \sin \phi (kh)^{q} }}z_{e}^{q + 1} } & {z_{e} \in \left[ {0,kh} \right]} \\ \end{array} } \\ {\begin{array}{*{20}c} { - \frac{{\dot{\gamma }_{\max } (h - z_{e} )^{q + 1} }}{{(q + 1)v_{c} \sin \phi (1 - k)^{q} h^{q} }} + \frac{{\cos \alpha_{0} }}{{\cos (\varphi - \alpha_{0} )\sin \phi }}} & {z_{e} \in \left[ {kh,h} \right]} \\ \end{array} } \\ \end{array} } \right.$$

(3)

$$\begin{gathered} \dot{\gamma } = \left\{ \begin{gathered} \frac{{\dot{\gamma }_{\max } }}{{(kh)^{q} }}(z_{e} )^{q} \begin{array}{*{20}c} {} & {} & {} & {} \\ \end{array} z_{e} \in \left[ {0,kh} \right] \hfill \\ \frac{{\dot{\gamma }_{\max } }}{{(1 - k)^{q} h^{q} }}(h - z_{e} )^{q} \begin{array}{*{20}c} {} & {} \\ \end{array} z_{e} \in \left[ {kh,h} \right] \hfill \\ \end{gathered} \right. \hfill \\ \hfill \\ \end{gathered}$$

(4)

$$h = \frac{{a_{p} }}{10\sin \phi }$$

(5)

$$k = \frac{{\cos \phi \cos (\phi - \gamma_{0} )}}{10\sin \phi }$$

(6)

where $\dot{\gamma }_{\max }$ is the maximum strain rate in shear zone, h is the thickness of shear zone(m),k is unequal division coefficient, a_p is milling depth(m),γ₀ is the tool rake angle(^o), q is a coefficient that describes the non-uniform distribution characteristic of tangential velocity in the first deformation zone. For the elastoplastic material, that is q = 3 at low-speed cutting and q = 7 for high-speed cutting.

As illuminated in Fig. 1a, the shear-slip zone is firstly generated through the shear-slip boundary CD, and then reaches the terminal line EF and formed chip. And then an element on AB is taken, and a mathematical relationship between the strain rate, strain, stress and temperature of this element is established in the shear zone of CD-EF unit. Based on the parallelism between AB, CD and EF, the shear strain rate in the shear zone can be regarded as the average value of the maximum shear strain velocity zone.

$$\dot{\gamma }_{AB} = \dot{\gamma }_{\max } { = } = \frac{{10(q + 1)v_{c} \sin \phi \cos \gamma_{0} }}{{a_{p} \cdot \cos (\phi - \gamma_{0} )}}$$

(7)

where $\dot{\gamma }_{AB}$ is average strain rate in AB of CD-EF unit.

Fig. 1

Continuous chip cutting model (a) orthogonal cutting (b) oblique angle milling.

At the same time, based on the cutting theory Φ can be expressed by:

$$\phi = \frac{\pi }{4} - \frac{\beta }{2} + \frac{{\gamma_{0} }}{2}$$

(8)

where β is friction angle(^o) which is related to the friction and lubrication problems.

Milling is classified as oblique angle cutting because of helical angle response, and it is different from orthogonal one. Therefore the speed relationship is shown in Fig. 1a and b, which is the cutting speed v_c, chip speed is v_p, the shear speed is v_s, and there is.

$$v_{s} = \frac{{v_{c} \cos \lambda_{s} \cos \phi }}{{\cos \eta_{c} \cos (\phi - \gamma_{0} )}}$$

(9)

where η_c is the chip outflow angle (°), which is basically equal to the tool helical angle β_c, and λ_s is the tool edge inclination angle (°).

Moufki²⁵ adopted a correlation friction coefficient model between the tool and chip, and the coefficient was decreased with cutting temperature. And he studied the influence characteristics of cutting speed on cutting force. It was believed that workpiece material was thermally softened as the cutting speed increases at high temperature, and the friction coefficient between the tool and chip is decreased. The chip flow velocity can be obtained as

$$v_{m} = \frac{{v_{c} \cos \lambda_{s} \sin \phi_{{}} }}{{\cos \eta_{c} \cos (\phi_{{}} - \gamma_{0} )}}$$

(10)

Combining Eqs. (8) and (10), the thermal softening reduces the friction coefficient of tool and chip, which increases the chip flow rate, and there also makes it more likely to cause the formation of band-shaped chips.

The speed incrementing of CD-EF shear unit in x and y directions can be expressed as follows.

$$\Delta \dot{u} = - v_{s} \cos \phi = - \frac{{v_{c} \cos \lambda_{s} \cos^{2} \phi }}{{\cos \eta_{c} \cos (\phi - \gamma_{0} )}}$$

(11)

$$\Delta \dot{v} = v_{s} \sin \phi = \frac{{v_{c} \cos \lambda_{s} \sin \phi \cos \phi }}{{\cos \eta_{c} \cos (\phi - \gamma_{0} )}}$$

(12)

where $\dot{u}$,$\dot{v}$ is the velocities of the CD-EF shear unit in x and y direction.

According to the geometric relationship in the Fig. 1b, the length increments of each effective cutting in the x and y directions can be obtained as

$$\Delta x = - \frac{{60v_{c} \cos \lambda_{s} \cos^{2} \phi }}{{nz\cos \eta_{c} \cos (\phi - \gamma_{0} )}}$$

(13)

$$\Delta y = \frac{{60v_{c} \cos \lambda_{s} \sin \phi \cos \phi }}{{nz\cos \eta_{c} \cos (\phi - \gamma_{0} )}}$$

(14)

where n is the spindle speed(r/min), z is the tooth number of tool.

The shear strain on the shear plane AB can be obtained by the shear strain rate and the shear zone working time. And it is gotten by

$$\begin{aligned} \gamma_{AB} = \dot{\gamma }_{AB} \frac{\Delta x}{{2v_{c} }} & = \frac{{10(q + 1)v_{c} \sin \phi \cos \gamma_{0} }}{{a_{p} \cdot \cos (\phi - \gamma_{0} )}}\frac{{60v_{c} \cos \lambda_{s} \cos \gamma_{0} }}{{2v_{c} nzh\cos \eta_{c} \cos (\phi - \gamma_{0} )}} \\ & \;{ = }\frac{{300(q + 1)v_{c}^{{}} \cos \lambda_{s} \sin \phi \cos^{2} \gamma_{0} }}{{nza_{p}^{2} \cos \eta_{c} \cos^{2} (\phi - \gamma_{0} )}} \\ \end{aligned}$$

(15)

Equation (15) shows that with the increase of speed, the shear strain rate and shear strain all are increased, whilst the energy of the shear band is going to rise. When the energy reaches the limit fracture value of localization adiabatic shearing, the serrated chip will be fracture.

In order to facilitate the calculation of shear strength, a simplified description was adopted to represent the interaction between rate hardening and thermal softening during the formation of the shear zone^26,27,28, thereby achieving the shear stress τ_r of rake face. Since the conventional research focuses on the calculation of shear strength τ_r for orthogonal cutting, the oblique cutting method should be considered for τ_r in milling process²⁹. Based on Fig. 1 and a simplification of calculation, τ_r can be obtained by follows.

$$\tau_{r} = \frac{{(F_{y} \cos \phi - F_{x} \sin \phi )\sin \phi \cos \lambda_{s} }}{{\alpha_{p} \cdot \alpha_{e} \cos \eta_{c} }}$$

(16)

where τ_r is the shear stress of rake face(MPa), a_e is the cutting width(m), F_x is cutting force in x direction and F_y is cutting force in y direction(N), and they can be obtained through measurement tests.

Adiabatic shear model

The phenomenon of adiabatic shear bands involves complex constitutive properties of materials, strain gradient effects, material recrystallization phase transitions and so on. The influence of cryogenic cooling on the crystal structure parameters of materials can easily cause the phase transition of stainless steel which is from austenite to martensite. There results in changes about material’s properties. At the same time, the complex contact geometric structure between tool and workpiece during milling processing will make the analysis of frictional behavior between tool and workpiece complicated. To demonstrate the applicability of the model and facilitate analysis, the friction behavior between tool and workpiece is simplified in this study, and the material phase transformation effect at low temperatures is ignored. There can simplistically describe the coupling effect between rate hardening and thermal softening during the evolution of shear bands.

The temperature rise is an important factor of adiabatic shear during milling processes. It is related to the density of the workpiece, milling speed and continuous milling time. Based on the literatures^27,28, the adiabatic shear basic model based on temperature rise can be obtained by

$$\rho \frac{{\partial v_{c} }}{\partial t}{ = }\frac{{\partial \tau_{r} }}{\partial x}$$

(17)

where v_p(x, t) is the velocity in x long and t time, ρ is density, τ(x, t) is the shear stress which is the same as τ_r in rake face.

$$\frac{\partial T}{{\partial t}}{ = }\frac{{\tau_{r} }}{{\rho {\text{c}}}} \cdot \frac{{\partial v_{c} }}{\partial x} + \xi \cdot \frac{{\partial^{2} T}}{{\partial x^{2} }}$$

(18)

where T(x, t) is the temperature excursion above ambient of the shear band region, ξ is the ratio factor of cutting heat, x is cutting distance, t is working time, c is specific heat and 460 J/(kg °C) for machined workpiece.

Shearing deformation of serrated chip

Due to the significant differences in the formation mechanism between serrated chips and band-shaped chips, the deformation coefficient cannot be used to evaluate the deformation degree of serrated chips. The degree of serrated deformation is usually expressed by the serrated degree. The larger serrated degree value will be attributed to the more severe serrated-shape (Fig. 2).

Fig. 2

Serrated chip cutting model.

For Eq. (18), the $\frac{{\partial^{2} T}}{{\partial x^{2} }}$ can be estimated by the below equation.

$$\frac{{\partial^{2} T}}{{\partial x^{2} }} \approx { - }\frac{{\Delta T_{AB} }}{{2m_{c}^{2} }}$$

(19)

where m_c is ASB thickness(m), $\Delta T_{AB}$ is the temperature increment (°C).

The temperature variation in workpiece is mainly determined by the frictional heat source at the tool and chip interface and the heat source of workpiece’s shear plastic deformation. From the literature³⁰ that ΔT_AB is cutting zone temperature rise.

$$\Delta T_{AB} { = }\frac{{(1{ - }\xi )F_{s} \cos \gamma_{0} }}{{\rho \cdot c \cdot a_{e} \cdot a_{p} \cos (\phi - \gamma_{0} )}}$$

(20)

where F_s is the shear force(N).

Since milling belongs to bevel cutting and is affected by the edge inclination angle λ_s and the chip outflow angle η_c, Eq. (20) is changed to

$$\Delta T_{AB} { = }\frac{{(1{ - }\xi )F_{y} \cos \lambda_{s} \cos \gamma_{0} }}{{\rho \cdot c \cdot a_{e} \cdot a_{p} \cos \eta_{c} \cos (\phi - \gamma_{0} )}}$$

(21)

Equation (19) can be changed and given by

$$\frac{\partial T}{{\partial t}}{ = }\frac{{\tau_{r} }}{\rho c} \cdot \frac{{v_{c} \cos \gamma_{0} }}{{h\cos (\phi - \gamma_{0} )}}{ - }\frac{{\xi \Delta T_{AB} }}{{2m_{c}^{2} }}$$

(22)

In this Eq. (19), ASB thickness m can be deduced and obtained based on literatures²⁶.

$$m_{c} { = }\left( {\frac{{9\rho^{3} \cdot c^{2} \cdot \xi^{3} \cdot \Delta T_{AB} }}{{\tau_{r}^{3} \cdot \dot{\gamma }_{AB} }}} \right)^{1/4}$$

(23)

Adiabatic shear is associated with energy dissipation in shear zone. Meanwhile, the energy dissipation is mainly affected by shear thickness, material strength, the shear strain rate and temperature rise^26,27. So the energy dissipated W_Q in the shear band can be expressed by:

$$Q{ = }m_{c} \cdot \tau_{r} \cdot \dot{\gamma }_{AB} \cdot \Delta T_{AB}$$

(24)

Equations (7), (16), (21) and (23) are substituted into (24), and then simplified, it can be gotten as

$$Q{ = }\left( {\frac{{9\rho^{2} \cdot c \cdot \xi^{3} \cdot F_{y} (1{ - }\xi )(F_{y} \cos \phi - F_{x} \sin \phi )\cos^{2} \lambda_{s} }}{{10(q + 1)v_{c} \alpha_{p} \cdot \alpha^{2}_{e} \cos^{2} \eta_{c} }}} \right)^{1/4} \times \frac{{10F_{y} (q + 1)(1{ - }\xi )v_{c} \sin \phi \cos^{2} \gamma_{0} }}{{a^{2}_{p} \cdot a_{e} \cos^{2} (\phi - \gamma_{0} )}} \times \frac{{\cos \lambda_{s} }}{{\rho \cdot c\cos \eta_{c} }}$$

(25)

Based on Eq. (25), the energy dissipation is mainly affected by the main milling force F_y and the milling speed v_c. Since the milling force is influenced by the cooling temperature, the energy dissipation is affected by the milling force under different milling speeds considering cooling processes. At the same time, when there has ξ = 0.43, v_c = 100m/min, F_y = 187.3N, F_x = 87.6N for LN₂ cooling processed,Φ = 43°, ρ = 7930 kg∙m^-3, c = 460 J/(kg °C), a_p = 10^-2m, a_e = 6 × 10^-2m, γ_AB = 2.4 × 10^–3, q = 7, v_c = 100m/min,γ₀ = 20°, λ_s = 41°, η_c = 16°. W_Q can be calculated as 2.02kJ/m². Therefore, the influence rule of milling speed on energy dissipation can be shown in Fig. 3. The total energy dissipation at cryogenic temperature is always greater than that at flood cooling, especially at higher milling speed, cryogenic cooling is beneficial to energy dissipation during milling process.

Fig. 3

Effect of cutting speed on energy dissipation.

For ductile materials, the work-hardening capacity of the material is increased with the increased milling speed. At the same time, the local temperature in chip shear zone is risen following the work-hardening capacity. And the thermal softening effect of material is intensified. When the work-hardening capacity and the thermal softening effect reach a balanced state, the thermal softening exceeds the accumulated plastic work^3,4,5,6, there will increase the strain hardening rate. The shear deformation is highly localized, and adiabatic shear behavior is occurred.

Under conventional flood cooling, the shear strength is relatively small as well as the energy dissipation value. Moreover, the shear deformation is highly localized due to the uneven heat conduction of the material, and the adiabatic shear behavior is prone to be occurred. At cryogenic cooling, the processed material has the higher shear strength is enhanced (Table 1), and there results in greater energy dissipation.

Table 1 304 stainless steel mechanical performances^31,32.

Experimental details

Experimental material

The 304 stainless steel used in aerospace was adopted for the experimental whose elemental components were illuminated in Table 2. Some workpieces with sizes of 200 mm × 100 mm × 10 mm were fabricated through cutting the 304 sheet. After that, the workpiece surfaces were washed and dried.

Table 2 Elemental composition of 304 stainless steel wt(%).

Experimental equipment

The flood, LN₂ and LN₂+LCO₂ milling tests were conducted using a VMC640m three-axis CNC vertical milling machine, with a maximum spindle speed of 8000 r/min, as shown in Fig. 4a and b. Cryogenic LN₂ and LCO₂ were provided by the self-pressurized tank (DPL-175MP type) produced by Beijing Tianhai Company (as expressed in Fig. 4c), and its pressure regulation range was 0–1.4 MPa. The three-directional milling forces was measured by using a rotary force gauge, and the noise reduction data was analyzed by software, as illuminated in Fig. 4d.

Fig. 4

Cryogenic experiment platform (a) LN₂ milling equipment (b) LN₂+LCO₂ milling equipment (c) Dewar (d) Dynamometer data.

As LN₂ and LCO₂ were all sensitive to temperature and prone to evaporation during transportation, a set of vacuum insulated pipelines was adopted to transport cryogenic mediums. The nozzle temperature and the milling zone temperature could be measured by a contact temperature measuring instrument. After reaching a stable set nozzle temperature, and then a series of cryogenic cooling milling tests would be conducted. In order to capture the temperature conditions and obtain a clear observation of cryogenic cooling process using an infrared thermometer in the cutting area, a fan was usually used to disperse the fog in the experiment. At the same time, in order to reduce the measurement errors caused by foggy conditions during the shooting, multiple shots were taken and the picture with the best shooting effect was selected. Others, German Fuchs JM3 water-soluble cutting fluid was employed for flood cooling cutting.

The 2 blades vertical milling tool bar was made in the Japanese kyocera with TiAlN carbide coating blades(brand for YBG202 which was similar with YG8, and its elemental composition and the geometrical specifications were expressed in Tables 3, 4 and 5) were used to process workpieces, and its diameter was 14mm at milling department.

Table 3 Main phases composition of the tool wt (%).

Table 4 Main elemental composition of the tool wt (%).

Table 5 Tool parameters in detail.

Experimental scheme

Using single factor experimental method, the cutting parameters can obtained in Table 4, and they were cutting speed of v_c = 50, 100,150 and 200 m/min, the cutting depth of a_p = 1 mm, the feed of f = 0.07 mm/r and the milling width of a_e = 10mm, respectively. Others the a_e was the milling width (Table 6). There were 3 cooling strategies which included in flood, LN₂ and LN₂+LCO₂, respectively.

Table 6 Milling parameters.

Temperature measurement strategy

The temperature measurement was carried out by the pre-buried thermocouples and a series of temperature measurement tests were executed for each cooling strategy. Firstly, a hole with a diameter of 0.3mm was positioned on the surface to be fabricated as close as possible to the position of the processed surface. And then the thermal conductive glue was filled into a hole, and it could enhance the heat conduction capacity. A standard 0.2 mm K-type thermocouple was inserted and installed the hole, as shown in Fig. 5a. Meanwhile, the thermocouple was connected with a thermometer. And the thermal response time of the thermocouple was 0.2s. Finally, the detected signal was processed and displayed in the temperature measuring instrument. 5 times experimental repetitions were performed for each milling strategy. The 3D model of temperature measuring was expressed in Fig. 5b, and the thermocouple was buried 0.5 mm below the required milling depth.

Fig. 5

Temperature measurement strategy (a) thermocouple installation (b) temperature measurement model.

For the smallest milling speed 50 m/min, the spindle speed was 1137 r/min, there was 19 r/s with about 8 times milling in the thermal response time 0.2 s of thermocouple. In addition, the feed time in the milling distance of 1 mm was 0.73 s, and there was more than 3 times thermal response for the thermocouple. Therefore, there was suited for the response time of the sensors relative to the high-frequency nature of milling.

Analysis equipment

The SKF TKDT 10 contact thermometer is selected for temperature measurement. Its temperature measurement range was − 200 °C to 1200 °C, with a resolution of 0.1 °C. The TA603B infrared wireless thermometer with a measurement range of − 32 °C to 1380 °C was used to measure the thermal imaging of the cutting zone. Meanwhile, the Kisler9257B three-phase dynamometer, with a measured power of 0.01 N and a natural frequency of 2.34 kHz was adopted to the milling force. And an ultra-deep microscope digital microscope (KEYENCE VHX- 600,Japan) was required to measure sample surface morphology. Besides, a TIME3200 hand-held roughness meter with the highest resolution of 0.001 μm was employed to obtain the machined surface roughness.

Results and discussion

Workpiece surface morphology

Research shows that cryogenic cooling achieves lower tool-chip frictional shear stress, and the use of lubricants can further reduce the tool-chip contact length l_c³³. At LN₂+LCO₂ cryogenic cooling, the tool-chip frictional shear stress and contact length will all be reduced, and it is expressed easily chip breaking. As shown in Table 7, cryogenic cooling enhances the machinability of stainless steel. And the metal has the characteristics of low-temperature hardening and embrittlement, the chip breaking effect is improved, the surface roughness degree of the machined surface is decreased, the chips is become shorter, and chip breaking is easier. At flood cooling milling, the burr phenomenon at the cutting point is very obvious or the cutting is uneven. Meanwhile, the material springback is severe under force, and it is difficult to break the chips. Under cryogenic LN₂ cooling, the amount of burrs begins to be continuously decreased. Although there are still a few burrs in the initial processing area, there are very few burrs in the cutting part. It indicates the chip breaking effect is very obvious under cryogenic cooling strategy with the intervention of LN₂.

Table 7 Workpiece surface morphology considering cooling strategies.

The brittleness of 304 stainless steel is increased and its toughness is decreased at cryogenic temperatures. This is mainly attributed to the fracture toughness of 304 drops significantly with the decrease of temperature. Mainly, the fracture toughness of stainless steel is decreased from room temperature compared with − 190 °C^34,35,36. The 304 stainless steel is more prone to crack propagation and fracture under force, and milling force is more likely to cause it to break chips. Meanwhile, the machined surface under the LN₂+LCO₂ strategy is more uniform, the chip breaking effect is better, and the quality of workpiece’s machined surface is guaranteed.

Chip morphology

Figure 6 shows the chip morphologies obtained under the same cutting parameters and different cooling strategies respectively. At performing lower-speed flood cooling milling, the chip surface is approximately band-like. The tear opening on one side of the chip is round, blunt and irregular, and no adiabatic shear band is observed (v_c = 50m/min). With the increase of milling speed, the serrated surface of the chip is uneven, the distribution of serrated spacing is irregular, and the degree of serrated is not large (v_c = 100m/min). However, there are obvious non-uniform intervals of serrated chips produced by thermal softening at high speeds, and the serrations are sharp and neat, with an increase in tooth depth and a shift in tooth patterns (v_c = 150m/min and v_c = 200m/min).

Fig. 6

Chip morphology of flood cooling.

Under flood cooling, the chips at lower-speed milling are expressed as a band-like shape, but there are squeezing marks in the chip structure (as shown by the blue dotted line in Fig. 7(50 m/min in flood cooling)). When the speed is reached 100 m/min, the serrated chip edge is obviously appeared. But there is irregular with significant height variations and squeezing marks.

Fig. 7

Chip morphology.

Compared with flood cooling, the width and height degree of chip edge are scaled down at cryogenic LN₂ cooling as shown in Fig. 7 (50 m/min in LN₂ cooling), but irregularity for 200 m/min in LN₂ cooling. Besides, under the combined cooling of LN₂+LCO₂, the width and height of the serrated are smaller and more uniform, the gap is still smaller as shown in Fig. 7 (50 m/min in LN₂+LCO₂ cooling) and (200 m/min in LN₂+LCO₂ cooling).

The horizontal cross-sectional image of the chips is shown in Fig. 8. It can be clearly observed that even under low-speed machining at 100 m/min of flood cooling, there are obvious adiabatic shear bands between the serrated-shaped chips, which are presented an extrusion form and accompanied by cracks. However, under cryogenic temperature LN₂ cooling condition at high speed of 200 m/min, adiabatic shear bands is appeared, but no adiabatic shear bands is found when LN₂+LCO₂ is used.

Fig. 8

Horizontal cross-sectional 3D image.

At flood cooling, as the milling speed continues to be increased, the temperature within the shear zone is risen sharply, and the material is shown dynamic recrystallization and refinement of the microstructure^26,27. Compared to flood cooling, it is mainly attributed to the brittle cutting at cryogenic temperatures. The increased speed (lower than 200 m/min) does not lead to organizational changes within the serrated-shaped chips. At this time, the grains are not elongated or broken along the shear direction. However, the microstructure transformation is also occurred during cold quenching takes place under instantaneous cryogenic cooling, but it only causes partial phase transformation of the metal microstructure. It leads to the occurrence of adiabatic shear at higher cutting speeds (higher than 200 m/min). The auxiliary function of LCO₂ improves the defect of cryogenic cooling friction caused by LN₂, alleviates the squeezing between the serrated chips, and makes the thermal shear effect less obvious.

The different shapes of chips reflect the varying degrees of deformation during the cutting process. The material’s toughness failure plays a dominant role at flood cooling milling, which indicates that the material between the serrated chips has undergone intense plastic deformation. Firstly, the stainless steel is prone to generating serrated-shaped chips during high-speed cutting, which causes the high-frequency fluctuation for the cutting force. This results in uneven local force distribution between workpiece and tool. Under the combined effect of strain and heat, thermal (viscoplastic) instability is inescapably occurred. Secondly, the microstructure, hardness and thermal conductivity within 304 materials are unevenly distributed, which results in uneven plastic deformation of the material. At the same position, there exists “dislocation slip” in terms of deformation time and space.

Roughness analysis

For flood cooling, the milling temperature is relatively low at lower speed, and the plastic deformation plays a major role in influencing surface roughness. As the cutting speed is increased, the plastic deformation time is shortened, and the degree of deformation is decreased, as well as the surface roughness, as shown in Fig. 9a. When the milling speed reaches a certain value (150 m/min), the milling temperature rises sharply. The thermal softening effect of the material plays a dominant role, and the material undergoes adiabatic shearing. Meanwhile, the high-frequency vibration is occurred at the tool nose, and the wear rate of tool is accelerated, which seriously affects the roughness of the machined surface. Therefore, the roughness value is increased instead in high milling speed. However, at cryogenic cooling conditions, the surface roughness is decreased with the increase of milling speed even at high speeds, and the surface quality is improved, as illuminated in Fig. 9b.

Fig. 9

Surface roughness (a) flood cooling (b) LN₂ cooling (c) LN₂+LCO₂ cooling.

But at higher speeds, this growth trend is slowed down. It can be known that the thermal softening effect is not obvious, the high-frequency vibration at the tool nose is also weakened, and the wear between tool and workpiece is reduced. At the same time, the surface roughness values are lower significantly compared to LN₂ cooling strategy at the combined cooling condition of LN₂+LCO₂, as shown in Fig. 9c. In addition, the tool wear could significantly be improved for LN₂+LCO₂ cooling strategy from the expression of the tool morphology in Fig. 10. This indicates the introduction of LCO₂ enhances the lubrication effect in milling process, significantly suppresses the high-frequency vibration at tool nose, and it also weakens tool wear.

Fig. 10

The tool morphology (150 m/min) (a) flood cooling (b) LN₂ cooling (c) LN₂+LCO₂ cooling.

Milling temperature

Based on Fig. 11, the milling temperature is relatively high in flood cooling. At 200 m /min, it can exceed 400 °C. As the milling speed is increased, the friction between the tool and chip is intense inevitably, and a large amount of frictional heat is generated at the same time. The generated heat cannot be quickly carried away by the chip and rapidly accumulated in the cutting area. Although the flood can have a certain cooling effect, the milling temperature still rises, which makes the adiabatic shearing phenomenon severe. However, at cryogenic cooling conditions, the temperature is much lower than the former, and it has a slow change with the increase of milling speed. This indicates that the cooling effect of LN₂ is significantly improved compared with flood. At 200 m /min, even if the temperature in the cutting zone of LN₂+LCO₂ is below 60 °C. Cryogenic cooling can significantly inhibit the occurrence of adiabatic shear, while causes brittle cutting of the material. Especially at higher speeds, the sawteeth are numerous and uniform.

Fig. 11

Influence of milling speed on temperature.

As expressed in Fig. 12, the temperature of milling zone can be expressed for 3 cooling strategies based on the infrared wireless thermometer. The cryogenic cooling strategies has obviously lower compared with flood cooling one.

Fig. 12

Influence of cooling strategies on temperature (v_c = 100m/min) (a) flood cooling (b) LN₂ cooling (c) LN₂+LCO₂ cooling.

Periodic milling force analysis

During the milling process of 304 stainless steel, the degree of adiabatic shear is determined not only by the properties of tool and workpiece materials and milling temperature, but also the high load in the contact area. The load causes excessive plastic deformation on the contact surface of tool and workpiece. The serrated shape is more obvious as well as the greater change of milling force. The tool is constantly subjected to alternating load, which makes it very easy to be damaged. At the same time, the load also deteriorates the quality of processed surface and reduces the tool life. In the area near the milling blade, it also needs to withstand relatively high normal pressure as well as higher cutting temperatures. As illustrated in Fig. 13, since the cutting action occurs near the tool nose, the contact surface between tool and workpiece has the greatest pressure. This is mainly attributed to the lower elastic modulus of 304, which causes significant processing springback. The springback subsequently causes more intense friction between the tool rake face and processed surface. This is also an important factor causing adiabatic shear.

Fig. 13

Effect of cutting speed on periodic milling force.

At cryogenic cooling, the elastic modulus is relatively large, so the rebound is small, and the degree of sawteeth is small and regular. From Eq. (16), that the shear stress is increased with the increase of periodic milling force. At the same milling speed, the periodic milling force of cryogenic temperature cooling is greater than that of flood cooling, and the shear strength is also larger, which is corresponded to Table 1. Therefore, it can be found that energy dissipation is large and the degree of adiabatic shear generated is small at cryogenic cooling from Eq. (25). Thus, the processing efficiency can be increased through enhancing the milling speed. Regular sawteeth chips in processing are mainly produced by cryogenic temperature brittle cutting^31,32,36. Meanwhile, the introduction of cryogenic LCO₂ enhances the lubricity of the cutting zone, reduces the periodic milling force, minimizes the vibration during cryogenic cooling cutting of LN₂, and further improves the adiabatic shear.

Model verification

During the formation of serrated chips, the plastic deformation energy accounts for the majority of the energy consumption, and the plastic deformation energy in the first deformation zone can be expressed as follows³⁷.

$$W_{p} { = }\int_{0}^{{\gamma_{AS} }} {\tau_{f} (\gamma_{AS} ,\dot{\gamma }_{AS} ,T)} d\gamma_{AS}$$

(26)

where τ_f is the flow stress (MPa), γ_AS is denotes the equivalent plastic strain,$\dot{\gamma }_{AS}$ is the equivalent plastic strain rate, they have the correspondence relationship with the variable in Eq. (1). And they also can be expressed by

$$\left\{ \begin{aligned} &\gamma_{AS} = \frac{L}{S} = \frac{{G_{s} a_{c} }}{S\sin \phi } \hfill \\ & \dot{\gamma }_{AS} = \frac{{v_{s} }}{S} = \frac{{v_{ch} \sin \gamma_{0} }}{{S\cos (\phi - \gamma_{0} )}} \hfill \\ \end{aligned} \right.$$

(27)

where L is the length of ASB (m), S is the width of ASB (m), G_s is the serrated-shape degree, which can be expressed by G_s = a_h/a_ch, a_h is the height of tooth (m), a_ch is the thickness of chip (m), a_c is the thickness of undeformed chip (m), and v_ch is the chip outflow velocity (m).

The Johnson–Cook constitutive equation is used to describe their material properties during the milling process, and the parameters including stainless steel are shown in Table 8.

Table 8 Parameters of the Johnson–Cook constitutive model of 304 stainless steel^22,38.

The values of L and S can be measured using microscopic methods based on the chip morphology, and the strains γ_AS and the strain rate $\dot{\gamma }_{AS}$ can be obtained, respectively.

Based on Eq. (26), the experimental values can be obtained and combined with the theoretical model for calculation, as shown in Fig. 14. Meanwhile, the theoretical and experimental errors are always controlled within 17%, which indicates this prediction model has definite accuracy. At the same time, the energy values are all increased with the increase of the milling speed, and the value of cryogenic cooling is bigger than flood one, and there was verified by reference³⁷. Since no obvious adiabatic shear is observed during LN₂ cooling low-speed milling, the energy is not calculated as well as the LN₂+LCO₂ cooling process.

Fig. 14

Effect of cutting speed on energy.

The fracture of ASB is a form of energy accumulation and release^39,40,41. The main cause of adiabatic shear is that the accumulated heat during cutting cannot be dissipated in time, which results in thermal plasticization and causing irregular chip breakage at local areas. As the cutting speed gradually is increased to 100 m/min, the unit cutting energy gradually is increased, and it causes the cutting temperature to be risen as well as the plastic deformation of the first deformation. And the energy consumed by the deformation is inevitably increased, and the kinetic energy of the chip gradually is increased, and the crack at the root of the chip extends more severely. Meanwhile, the serration degree of the chip decreased, and the energy consumed in the first deformation zone decreases. Under cryogenic cooling, the heat dissipation conditions are improved, and the temperature in the cutting zone is rapidly reduced. However, in high-speed cutting, due to the lack of lubrication ability of liquid nitrogen, there will still be intense squeezing and friction at local cutting positions. It causes partial isothermal shear. According to references^42,43, under LN₂ cooling at a low temperature, a lower cooling medium temperature can promote the formation of serrated-like features in the chips and facilitate chip fracture. At higher speeds, an adiabatic shear band is more likely to form. Since the intervention of LCO₂ with lubrication ability, the thermal plastic deformation is decreased, the strain is decreased compared to LN₂ cooling, and the energy accumulation is weakened as well as the energy. There effectively improves the severe friction defect, and the adiabatic shear phenomenon is suppressed at high speed.

Conclusions

In this paper, the theoretical model and experimental research on the milling processes of 304 stainless steel based on flood, LN₂ and LN₂+LCO₂ cooling strategies were carried out. The milling adiabatic shear and milling performance were compared in the three cooling strategies. The following conclusions of innovative discovery were drawn from this research:

• (1) Based on the characteristics of oblique angle milling, an adiabatic shear energy dissipation model was established. When there have the same milling parameters, the energy dissipation is mainly determined by the main milling force F_y and shear strength. Compared with flood cooling milling, the periodic milling force and shear strength are increased at LN₂ cryogenic temperature conditions. So the energy dissipation is also enhanced. Therefore, the ability to generate instantaneous adiabatic shear is weaker. At the same time, due to the instantaneous cold brittleness of LN₂ and lubricating effect of LCO₂, there is difficult to obtain the conditions at the shear zone for generating adiabatic shear bands, the adiabatic shear bands are not easy to be formed.

• (2) At flood cooling, when the milling speed is reached 150m/min, the milling temperature has exceeded 300 °C, and the adiabatic shear bands has been formed. There mainly results in serrated chips. And this generates high-temperature and high-frequency oscillation of the periodic milling force, as well as a large temperature gradient between the workpiece and tool. Meanwhile, there is uneven local force distribution. Under the combined action of strain and heat, the thermal instability is inescapability. Besides, the tool is severely worn, and the roughness of the milling surface exceeds Ra 0.7 μm.

• (3) At cryogenic cooling, the chips are all serrated. Especially at high speeds, the serrations are uniform and regular, and no obvious adiabatic shear bands are generated. At 200 m /min, the temperature in the cutting zone of LN₂+LCO₂ is below 60 °C compared with 400 °C of flood cooling. The formation of serrated chips is related to the brittle cutting characteristic of 304 stainless steel at lower temperature. Meanwhile, the tool wear is obviously inhibited, and the machined surface is improved with a roughness of less than Ra 0.3μm.