FEM Simulation of Surface Micro-Groove Structure Fins Produced by Cryogenic-Temperature Extrusion Machining

Abstract

In the process of metal cutting, a large amount of chips that are difficult to reuse will be produced, resulting in resource waste. As a novel metal forming process, cryogenic-temperature extrusion machining (CT-EM) can directly process chips into usable fins with a surface micro-groove structure, which has the advantage of high efficiency, energy saving and flexibility. In this study, the effects of four parameters (compression ratio λ, rake angle of the tool α, friction coefficient μ and the constraining tool corner radius R) on the effective stress, temperature and formability of micro-groove fins produced by CT-EM and room-temperature extrusion machining (RT-EM) are investigated. The results show that the maximum effective stress and formability of CT-EM are larger than that of RT-EM, which indicates that CT-EM has greater advantages in the preparation of micro-groove fins. At a λ of 0.7, the formability of CT-EM is the best. Reducing the λ and α, or increasing the μ, can improve the forming effect of the fins. CT-EM can produce micro-groove fins with the best formability when λ = 0.7, α = 5°, μ = 0.75 and R = 0.1 mm.

1. Introduction

Metal cutting is one of the main processing technologies of metal materials at present, and is widely used in the aerospace industry, transportation, and other fields [1,2,3]. A large number of chips will be produced in the metal cutting process, causing serious resource waste and environmental pollution. Therefore, the recycling of chips can effectively save resources and reduce costs. Common chip recovery methods, such as remelting [4,5], can reduce the energy consumption of metal materials in production, but the recovery efficiency is lower, and the pollution is serious. Powder metallurgy [6,7], hot extrusion [8,9], cyclic extrusion [10,11] or some large plastic deformation processes, such as equal channel angle extrusion [12,13] or high pressure torsion [14,15], can convert chips into metal materials with excellent mechanical properties, but the chips need to be collected and processed, which is cumbersome and costly. On the other hand, large-strain extrusion machining [16,17] and ploughing–cutting [18,19] can directly process chips into usable fins during processing, which is simple in operation and low in cost.

Extrusion machining (EM) is also a novel metal cutting technology evolved from the traditional free orthogonal cutting. It can directly process chips into practical fins with surface micro-groove structures through one-step cutting, which has the advantage of high-efficiency energy saving and flexibility. The micro-groove has a complex surface structure that can generate more turbulence and further improve the heat dissipation performance of the fin. With the development of electronic equipment and chips towards miniaturization and high-power density, more stringent requirements are put forward for heat dissipation of the equipment. The surface micro-groove structure fins have an excellent heat dissipation effect and have great research value in the application of microelectronic products and chip recycling.

At present, Deng [20] et al. prepared various structures of pure copper groove strips using a finite element model and proved that the forming extrusion cutting could be adjustable in the production of strips with various groove structures, and the molding rate was more than 80%. The groove strip surface had a rich substructure, meaning a wide range of applications in the field of heat transfer. Zhang [21] et al. studied the influence of tool structure parameters on forming extrusion cutting and found that the increase in compression ratio lead to the reduction in material formability. The constraining tool corner radius is proportional to the main cutting force, and the rake angle of the tool is inversely proportional to the main cutting force. To obtain a better forming effect, the compression ratio and constraining tool corner radius should be set smaller, while the rake tool angle should be set larger. The above authors have provided valuable experience in the preparation and performance research of micro-groove fins at room temperature (RT). However, the research at cryogenic temperature (CT) still needs further exploration, because CT can eliminate the cutting heat, inhibiting the dynamic recovery of the material, which is conducive to improving the mechanical properties of the material.

Yin et al. prepared aluminum alloy chips using cryogenic-temperature large-strain extrusion machining. They found that the surface quality of chips obtained at a cryogenic temperature was better than that at room temperature. Cryogenic temperature could increase the precipitation power and dislocation density of alloys. The hardness of chips could be higher after aging treatment [22]. Arunprasath et al. showed that the ductility and machinability of the 6065 aluminum alloy were improved by cryogenic temperature equal channel angle extrusion, and its hardness and strength were also increased [23]. Xiong et al. demonstrated that the microhardness (253 HV), yield strength (650 MPa) and ultimate tensile strength (685 MPa) of the 7075 aluminum alloy after cryogenic rolling were significantly increased, and the corrosion resistance after aging treatment was better than that of room temperature rolling [24]. Similar conclusions are also found in [25,26]. Therefore, a cryogenic environment is an effective way to improve the machining properties of metal materials. At present, there is no research on the preparation and properties of surface micro-groove fins using cryogenic-temperature extrusion machining (CT-EM).

As an effective chip recycle method, CT-EM is also expected to fabricate surface micro-groove structure fins with better machining properties. In this paper, DEFORM-3D software is used to carry out CT-EM and RT-EM finite element simulation on pure copper, and the effects of the two methods are compared. The influence of CT-EM and RT-EM micro-groove fins on effective stress, temperature, and formability under different λ, α, μ, and R is studied in detail, which provides reference for future research.

2. Establishment of the Finite Element Model

In the process of extrusion machining, the rotating motion of the workpiece was converted into the linear motion of the tool along the main direction of motion to simplify the analysis process. The structure and size of the workpiece and tool are shown in Figure 1, with the workpiece set as a plastic body and the tool as a rigid body. The shape and size of the micro-groove channel of the constraint block are illustrated in Figure 2.

An appropriate number of grid units can improve the accuracy of simulation, save time, and improve the simulation efficiency. A tetrahedral mesh was used to divide the workpiece into 10⁵ fine mesh units, and the mesh was locally refined in the cutting deformation zone, as depicted in Figure 3. The minimum mesh size ratio of the refined zone to the non-refined zone is 0.1. The cutting tool and constraint block were divided into 50,000 mesh units. In the process of simulation, the adaptive mesh partitioning technology could re-divide the distorted mesh, so that the distorted mesh had a higher mesh quality, thus ensuring the smooth progress of the simulation.

In the simulation process, the workpiece was in a static state, so the speed of the left surface (ADEF) of the workpiece, front surface (ABCD) and lower surface (ABGF) were set to 0 m/s in the x, y and z directions. The cutting tool and constraint block moved uniformly along the −X axis at a speed of 0.09 m/s. The heat transfer coefficient between the workpiece and the tool was set to 40 N/s/mm/C, the room temperature was 20 °C, and the convection coefficient in the environment was 0.02 N/s/mm/C. As shown in Figure 3, a local temperature window needed to be added to the cutting area to simulate cryogenic temperature cutting. The temperature inside the window was set to −196 °C and the convection coefficient was 5000 N/s/mm/C. The window moved synchronously with the cutting tool. The material of the cutting tool was WC carbide, and the thermodynamic material attribute parameters of pure copper and the cutting tool can be seen in

Table 1. The thermodynamic material attribute parameters of pure copper and the tool.

Parameter	Material (Cu)	Tool (WC Carbide)
Young’s modulus (GPa)	115	650
Poisson’s ratio	0.33	0.25
Heat transfer (N/s/°C)	430	59
heat capacity (N/mm2/°C)	3.42	15
Emissivity	0.7	0

.

The default constitutive model in DEFORM was adopted for the constitutive model of the materials, and its expression is as follows:

$$f\left(\sigma \right)=\sigma \left(\epsilon ,\dot{\epsilon },T\right)$$

(1)

where σ is the effective flow stress of the material, ε is the effective strain, $\dot{\epsilon }$ is the effective strain rate and T is the temperature. This model is the most common format of the constitutive model. The flow stress is defined as a function of strain, strain rate and temperature by the linear difference calculation method. The friction model is simplified to the shear friction model and the default friction coefficient is 0.6.

The final forming result of the micro-groove fins was the fins with a convex micro-tooth structure on the surface. The formability can be measured by the cross-sectional size of the micro tooth when the cutting reaches a stable state. The mean value of the cross-sectional area of three micro-tooths with a regular shape, which were 1 mm away from the upper surface of the workpiece, was taken as the measurement standard and the cross-sectional area of a micro-tooth was calculated using Image J software. Image J is a public image processing software. The user can first set a scale in the original image, then import the image into the software. After setting the corresponding parameters in the software, the area you want can be calculated.

The control variable method was adopted in the experiment. Only one variable was changed in each experiment and different parameters were studied. The four variables and other parameters are shown in

Table 2. Parameters in experiment (variable are filled in the brackets).

Parameter	Value
Cutting speed, m/s	0.09
Cutting depth td, mm	0.6
Compression ratio (λ)	1.3 (0.7, 0.8, 1, 1.3, 1.5)
Rake angle of the tool (α), °	10° (5°, 10°, 15°, 20°, 25°, 30°)
Friction coefficient (μ)	0.6 (0.15, 0.3, 0.45, 0.7, 0.9)
Constraining tool corner radius (R), mm	0 (0, 0.1, 0.2, 0.3, 0.4)

. Notably, compression ratio is the ratio of t_ch to t_d, where t_ch is the thickness of squeeze thickness, and t_d is the cutting depth.

3. Simulation Results and Analysis

3.1. Influence of λ on Formability of Micro-Groove Fins

3.1.1. Effective Stress

The effective stress distribution of RT-EM and CT-EM under different values of λ is shown in Figure 4. The stress concentration area (red area in Figure 4) is mainly distributed in the first deformation zone and the second deformation zone, and the effective stress at the bottom of the micro-groove is larger than that at the top of the tooth. Because the first deformation zone and the second deformation zone are in direct contact with the tool tip and the rake face of the tool, the extrusion of the tool tip separates the chip from the workpiece, and the rake face of the tool and the constraint block also produce large extrusion and friction on the chip. Therefore, the maximum effective stress is generated in the first deformation zone and the second deformation zone. In addition, the convex part of the constraint block extrudes the fins, which causes the fins to deform and form micro-grooves. The effective stress in this part is also relatively large. With the increase in λ, the extrusion effect of the fins in the extrusion channel is reduced, so the range of more than 150 MPa at room temperature and more than 225 MPa at the cryogenic temperature gradually decreases. The effective stress of the fins is greater at a cryogenic temperature. Because the strength of the material is improved at cryogenic temperature, greater force is required to produce the same deformation.

Figure 5 illustrates the effective stress distribution curves of RT-EM and CT-EM along the DEFG trajectory under different values of λ. It can be seen from Figure 5a,b that the effective stress at the bottom of the micro-groove (E, G) is the maximum, while the effective stress at the top of the tooth (F) is the minimum. This is because the second deformation zone and the bottom of the micro-groove are the most severely extruded areas, and the tooth tip of the micro-groove is not in direct contact with the constraint block, resulting in a small effective stress. Thus, the curve of the stress is approximately in a “W” shape. With the increase in the values of λ, the maximum effective stresses of RT-EM and CT-EM are 722 MPa, 715 MPa, 704 MPa, 682 MPa, 674 MPa, 1138 MPa, 1085 MPa, 1096 MPa, 1058 MPa and 1033 MPa, respectively, which decrease with the increase in λ. The difference of the effective stress between the bottom of the micro-groove and the top of the tooth is 113 MPa, 123 MPa, 96 MPa, 73 MPa, 66 MPa, 220 MPa, 154 MPa, 178 MPa, 134 MPa and 116 MPa. By comparing the effective stress of different values of α, μ and R, at a λ of 0.7, the effective stress and difference of RT-EM and CT-EM are both the largest, indicating that the micro-groove fin is severely restricted in the micro-groove channel, which is the most conducive to improving the formability of the micro-groove fin.

3.1.2. Temperature

Figure 6 shows the temperature distribution of RT-EM and CT-EM under different values of λ. For RT-EM, the temperature of the second deformation zone and the processed zone is the maximum, and the temperature concentration range gradually decreases with the increase in λ. Due to the work completed by the plastic deformation of the material and the work completed by the friction between the fins, the cutting tool and the constraint block generate cutting heat, which causes the temperature to rise. With the increase in λ, the larger the space of the extrusion channel, the faster the heat dissipation, and the lower the temperature. Because the cutting speed is small, the cutting heat generated is also small, and the cooling effect of the CT on the cutting area is better, so that the CT-EM fins can maintain a relatively low temperature.

Figure 7 depicts the temperature variation curves of RT-EM and CT-EM along the DEFG trajectory under different values of λ. It can be seen from Figure 7a,b that the temperature of the second deformation is the maximum at room temperature, and the temperature at the bottom of the micro-groove is maximum at a cryogenic temperature. The maximum temperatures of RT-EM and CT-EM under different values of λ are 123 °C, 120 °C, 117 °C, 111 °C, 108 °C, −170 °C, −172 °C, −167 °C, −165 °C and −169 °C, respectively. The maximum temperature of RT-EM gradually decreases, but compared with other variables, the change range of the temperature is smaller. In addition, when the temperature of CT-EM is kept below −160 °C, the temperature curves under different values of λ almost coincide. This indicates that in the RT-EM process, the smaller the λ, the greater the extrusion pressure on the metal material, the more cutting heat generated by material deformation, and the higher the temperature. The cryogenic temperature can effectively restrain the cutting heat and keep the cutting area in a good cryogenic temperature environment.

3.1.3. Formability

As present in Figure 4, micro-groove fins can be obtained at different values of λ. At a λ of 0.7 and 0.8 at room temperature, and 0.7 at cryogenic temperature, the zone at the top of micro-groove shows an obvious wear phenomenon, but the integrity of the CT-EM fins is better. Figure 8 shows the cross-section diagram of the three-dimensional morphology of the micro-groove fins under different values of λ. The tooth shape of the fins is approximately semi-elliptical, indicating that the material has not completely filled the whole space in the groove of the constrain block. With the increase in λ, the tooth height of the fins decreases gradually and the thickness of the base increases. Because the λ is small, the fins are subjected to greater extrusion and friction, and the material flows more easily into the groove of the constraint block, resulting in a larger height and area of the micro-tooth. However, the material is not enough to fill the entire space, and the tooth shape of the fins is irregular. With the increase in λ, the extrusion effect becomes weaker, less material enters the groove, the tooth area is smaller, and the thickness of the base is greater.

As shown in Figure 9, the cross-sectional areas of RT-EM and CT-EM under different values of λ are 0.527 mm², 0.482 mm², 0.436 mm², 0.367 mm², 0.338 mm², 0.532 mm², 0.498 mm², 0.450 mm², 0.368 mm² and 0.320 mm². The tooth area of RT-EM and CT-EM decreases with the increase in λ, but the area of CT-EM is larger than that of RT-EM as a whole. Some other studies [20,21] also prepared the same micro-groove fins and measured the cross-sectional area. Compared with that, with the increase in λ, the area has the same change trend. At RT, the area is the largest at a λ of 0.7 (0.527 mm, 0.503 mm [20], 0.522 mm [21]), and the area value is very close, indicating that the simulation results are more reasonable. With the decrease in λ, the trend of the area reduction in this paper is slightly larger, which may be caused by different constitutive models of materials. The larger area of CT-EM indicates that CT-EM has more advantages than RT-EM in the preparation of micro-groove fins. Compared with the other three groups of variables, the cross-sectional area corresponding to λ is significantly larger than that of the other three groups, and the area varies greatly between the different values of λ, indicating that λ has the most significant influence on the forming effect of micro-groove fins. To sum up, CT-EM and a reducing λ can produce micro-grooved fins with good formability.

3.2. Influence of α on Formability of Micro-Groove Fins

3.2.1. Effective Stress

The effective stress distribution of RT-EM and CT-EM under different values of α is shown in Figure 10. The stress concentration area is mainly distributed in the first deformation area and the second deformation area. With the increase in α, the range of more than 150 MPa at room temperature and more than 225 MPa at a cryogenic temperature gradually decreases, because the larger the rake angle of the tool, the smaller the fins deformation capacity and the smaller the effective stress. The effective stress at the bottom of the micro-groove is greater than that at the top of the micro-groove tooth and the effective stress of CT-EM is greater.

Figure 11 illustrates the effective stress variation curves of RT-EM and CT-EM along DEFG under different values of α. It can be seen from Figure 11a,b that the effective stress in the second deformation zone and the bottom of the micro-groove is larger than that in the tooth top of the micro-groove. The maximum effective stresses of RT-EM and CT-EM under different values of α are 686 MPa, 682 MPa, 700 MPa, 673 MPa, 672 MPa and 649 MPa, and 1064 MPa, 1058 MPa, 1014 MPa, 991 MPa and 956 MPa. The difference of the effective stress between the bottom and the top of the micro-groove is 82 MPa, 73 MPa, 109 MPa, 83 MPa, 75 MPa and 88 MPa, and 110 MPa, 134 MPa, 130 MPa, 108 MPa, 134 MPa and 116 MPa, respectively. At different values of α, the initial extrusion amount of the material entering the extrusion channel is distinct, resulting in a distinct cutting force on the material, which will have a significant impact on the formability of micro-groove fins. Among the four groups of variables, the greater the α, the smaller the effective stress. At an α of 30°, the effective stress of RT-EM and CT-EM is the smallest, which is not conducive to the forming of micro-grooves.

3.2.2. Temperature

Figure 12 shows the temperature distribution of RT-EM and CT-EM under different values of α. For RT-EM, the temperature in the second deformation zone and the processed zone is higher. At the α of 10°, the temperature concentration area is the largest. In fact, the smaller the α, the greater the deformation and the more heat generated. In the process of EM, the smaller the α, the greater the extrusion force on the material. While the different α makes the groove of the constraint block exert different extrusion forces on the material. Under the joint influence of these two factors, at an α of 10°, the temperature concentration range is the largest. The temperature of CT-EM under different values of α do not change much, only the temperature in the micro-groove decreases gradually at the root of the fins with the increase in α, and remains low in other deformation areas.

Figure 13 depicts the temperature variation curves of RT-EM and CT-EM along the DEFG trajectory under different values of α. The temperature at the bottom of the micro-groove and the second deformation zone is relatively high. The maximum temperatures of RT-EM and CT-EM under different values of α are 115 °C, 112 °C, 102 °C, 97 °C, 91 °C, 87 °C, −166 °C, −165 °C, −168 °C, −180 °C, −176 °C and −182 °C, and the maximum temperature gradually decreases. The temperature of CT-EM fins in the deformation zone is kept below −160 °C and the temperature curves of the different values of α change slightly. The higher the temperature of the metal material during processing, the better the fluidity of materials. Compared with the other three groups of variables, the maximum temperature corresponding to different values of α is lower. At an α of 30°, the temperature of RT-EM and CT-EM is the lowest, which is harmful to improving the formability of fins.

3.2.3. Formability

As presented in Figure 10, the micro-groove fins with a complete morphology can be obtained at different values of α. With the increase in α, the length of the fins gradually increases. Figure 14 shows the cross-section diagram of the three-dimensional morphology of micro-groove fins at different values of α. With the increase in α, the height of the fins decreases. The tooth shape is approximately semi-elliptical, indicating that the material has not filled the entire space in the micro-groove of the constraint block.

As shown in Figure 15, the cross-sectional areas of RT-EM and CT-EM under different values of α are, successively, 0.396 mm², 0.367 mm², 0.341 mm², 0.289 mm², 0.262 mm², 0.169 mm² and 0.395 mm², and 0.368 mm², 0.331 mm², 0.325 mm², 0.296 mm² and 0.220 mm². The area decreases with the increase in α and changes greatly under different values of α. Compared with other studies [20,21], with the increase in the α, the change trend of area at RT is the same, but the maximum area in this paper is slightly smaller, which may be caused by the different constitutive models. When the value of α is less than 15°, the formability of RT-EM and CT-EM is similar. When the value of α is more than 15°, the forming effect of CT-EM is better. At the α of 30° especially, the formability is the worst in all variables. Therefore, it is not suitable to choose a large α.

3.3. Influence of μ on Formability of Micro-Groove Fins

3.3.1. Effective Stress

The effective stress distribution of RT-EM and CT-EM under different values of μ is shown in Figure 16. The effective stress is concentrated in the first deformation zone and the second deformation zone. With the increase in μ, the range of values greater than 150 MPa at room temperature and greater than 225 MPa at a cryogenic temperature gradually increases. When the μ is the same, the effective stress of CT-EM is greater than those of RT-EM and the diffusion range of effective stress is more obvious.

Figure 17 illustrates the effective stress distribution curves of RT-EM and CT-EM along the DEFG trajectory under different values of μ. The effective stress of RT-EM and CT-EM is larger in the second deformation zone and the bottom of the micro-groove. The maximum effective stresses under different values of μ are: 702 MPa, 684 MPa, 674 MPa, 668 MPa, 693 MPa, 996 MPa, 1029 MPa, 1021 MPa, 1030 MPa and 1087 MPa. The effective stress difference between the bottom and tooth top of the micro-groove is 70 MPa, 79 MPa, 54 MPa, 65 MPa, 81 MPa, 116 MPa, 97 MPa, 60 MPa, 110 MPa and 126 MPa. At a μ of 0.9, the difference in the effective stress is the largest, indicating that the friction between the cutting tool and the micro-groove fin is the most severe at that moment, which is advantageous to the accumulation of metal materials.

3.3.2. Temperature

Figure 18 shows the temperature distribution of RT-EM and CT-EM under different values of μ. The maximum temperature of RT-EM is mainly distributed in the second deformation zone and the processed zone. With the increase in μ, the distribution area of maximum temperature gradually expands, because under the same conditions, the greater the friction coefficient, the greater the friction force between the fins and the rake face of the cutting tool, the more work the friction force does, and the higher the temperature. The temperature at the root of the micro-groove fins fabricated by CT-EM increases with the increase in μ. The temperature distribution in other areas varies very little and the temperature in the deformation area is still low.

Figure 19 depicts the temperature variation curves of RT-EM and CT-EM along the DEFG trajectory under different values of μ. As can be seen from Figure 19a,b, the temperature of the fins fabricated by RT-EM and CT-EM decreases first and then increases. Under different values of μ, the maximum temperatures are 93 °C, 98 °C, 102 °C, 123 °C, 129 °C, −180 °C, −171 °C, −176 °C, −176 °C and −175 °C. The maximum temperature increases gradually with the increase in μ. Compared with the other three variables, when the μ is the largest, the temperature at room temperature is the highest and the variation range between the highest temperatures of all the variables is also the largest. During extrusion machining, the higher the temperature, the better the formability. The cutting zone of CT-EM still maintains a good cryogenic temperature.

3.3.3. Formability

As presented in Figure 16, the micro-groove fins with good morphology can be obtained under different values of μ. With the increase in μ, the length of the micro-groove fins decreases gradually. Figure 20 shows the cross-sectional diagram of the three-dimensional morphology of micro-groove fins. The tooth shape is approximately semi-elliptical, and the tooth height increases with the increase in μ. The greater the friction coefficient, the greater the friction force of the fins by the cutting tool and the constraint block, and the slower the material flow speed, resulting in the shorter length and larger height of the fins.

As seen in Figure 21, the cross-sectional areas of RT-EM and CT-EM under different value of μ are: 0.281 mm², 0.315 mm², 0.361 mm², 0.378 mm² and 0.407 mm², and 0.315 mm², 0.337 mm², 0.356 mm², 0.389 mm² and 0.248 mm², respectively. With the increase in μ, the overall trend increases, which indicates that the formability of micro-groove fins can be improved by an increasing μ. When the μ is less than 0.75, the forming effect of CT-EM is better. At a μ of 0.9, the fin produced by CT-EM piles up and deforms in the extrusion channel, resulting in a reduction in the formability. The maximum cross-sectional area of μ is similar to that of α, both of which are one of the factors affecting the formability.

3.4. Influence of R on Formability of Micro-Groove Fins

3.4.1. Effective Stress

The size of R will affect the size of the entrance of the micro-groove channel. This will lead to different initial amounts of metal entering the extrusion channel and affect the friction force between the fins and the cutting tool, thus affecting the final forming effect of the micro-groove. The effective stress distribution of RT-EM and CT-EM under different values of R is shown in Figure 22. The effective stress is concentrated in the first deformation zone and the second deformation zone, and the effective stress at the bottom of the micro-groove is greater than that at the top of the micro-groove tooth. The effective stress concentration range of RT-EM decreases with the increase in R. At an R of 0.4 mm, the effective stress concentration range of CT-EM is obviously larger than that of RT-EM.

Figure 23 illustrate the effective stress curves of RT-EM and CT-EM along the DEFG trajectory under different values of R. It can be seen from Figure 23a,b that the effective stress in the second deformation zone is larger. The maximum effective stresses of RT-EM and CT-EM are 682 MPa, 701 MPa, 695 MPa, 701 MPa, 668 MPa, 1058 MPa, 1094 MPa, 1035 MPa, 1047 MPa and 1062 MPa. The difference of the effective stresses between the bottom and the top of the micro-groove is 73 MPa, 96 MPa, 98 MPa, 105 MPa and 59 MPa, and 134 MPa, 157 MPa, 134 MPa, 95 MPa and 108 MPa, respectively. Compared with the other three variables, the distribution of the effective stress is more concentrated, and the variation range is smaller.

3.4.2. Temperature

Figure 24 shows the temperature distribution of RT-EM and CT-EM under different values of R. The maximum temperature of RT-EM is concentrated in the second deformation, and the range of the temperature above 95 °C is almost unchanged. The temperature distribution of CT-EM at different values of R is similar and the effect of the cryogenic temperature can still be maintained in the cutting zone.

Figure 25 depicts the variation curves of the temperature of RT-EM and CT-EM along the DEFG trajectory under different values of R. According to Figure 25a,b, the temperature of RT-EM in the second deformation zone is the highest, and the temperature of CT-EM in cutting zone is kept below −165 °C. The maximum temperatures of RT-EM and CT-EM are as follows: 112 °C, 109 °C, 109 °C, 108 °C, 107 °C, −166 °C, −172 °C, −182 °C, −174 °C and −173 °C. Compared with the other three variables, the maximum temperatures of RT-EM and CT-EM under different values of R change the least and the temperature curves are also very close. Therefore, the R has little influence on the temperature.

3.4.3. Formability

Figure 22 shows the three-dimensional morphology of surface micro-grooves fins. The micro-groove fins with good morphology can be obtained at different values of R and the morphology of the different fins displays little difference. Figure 26 shows the cross-sectional shape of the three-dimensional morphology of micro-groove fins under different values of R. The tooth shape is approximately semi-elliptical, and the size is very close.

As seen in Figure 27, the cross-sectional areas of the micro-groove fins produced by RT-EM and CT-EM are successively: 0.367 mm², 0.378 mm², 0.375 mm², 0.368 mm², 0.337 mm², 0.368 mm², 0.389 mm², 0.385 mm², 0.385 mm² and 0.353 mm². Compared with the literature [20], the value of the area in this paper is smaller and the change trend is different. However, the change trend is similar to the actual processing value, and when R = 0.1, the actual processing area is also the largest, indicating that the simulation results are more accurate. The area of CT-EM is larger than that of RT-EM, which also proves that CT-EM can improve the forming effect of micro-groove fins. Compared with the other three variables, the variation in the area under different values of R is smaller, indicating that the R has the least influence on the forming effect of micro-groove fins.

4. Conclusions

As a novel chip recovery technology, CT-EM can directly process chips into surface micro-groove fins, which has the advantages of high efficiency, energy saving and easy operation. The micro-groove fins processed by CT-EM have better formability and a good heat-dissipation performance and has broad application prospects in the field of microelectronic devices and chips. The effects of λ, α, μ and R on the fins were studied by comparing the effects in CT-EM and RT-EM and the following conclusions can be drawn:

(1)

CT can eliminate cutting heat and improve the processing performance of materials. Compared with RT-EM, CT-EM fins have a larger effective stress and cross-sectional area and better formability, which shows that CT-EM has more advantages than RT-EM in producing high-performance micro-groove fins.

(2)

With the decrease in λ and α, the effective stress, temperature and area of the fins gradually decrease. At a λ of 0.7, the cross-sectional area of CT-EM fins is the largest and the forming effect is the best. When the α is greater than 15°, the forming effect of CT-EM is better.

(3)

With the increase in μ, the maximum effective stress, temperature and area of CT-EM gradually increase, which is conducive to the formation of micro-groove fins. With the increase in R, the maximum effective stress and maximum temperature of CT-EM decrease slightly. At an R of 0.1 mm, the area of CT-EM is large, but the formability of fins under different R values is relatively close.

(4)

Reducing λ and α, or increasing μ, can improve the forming effect of the fins. CT-EM can produce the micro-groove fin with the best performance when λ = 0.7, α = 5°, μ = 0.75 and R = 0.1 mm.

Herein, only the FEM simulation of CT-EM was carried out. Consequently, experimental verifications will be the focus of our future work; for instance, the mechanical and thermal properties of micro-groove fins and the influence of other processing parameters on the formability of micro-groove fins. In addition, other materials such as Al alloys with good heat dissipation performances will be studied.