Forming mechanism of radial V-shaped microgrooves on plate using step punching method

Abstract

The finned surface can increase the specific surface area, which could enhance the boiling efficiency of the working medium to realize the enhancement of heat transfer performance. This research adopted a step punching method to form a radial microgroove structure on the surface of a copper plate. The scanning electron microscopy analysis indicates that when the pressed metal is in the deformation range, it will flow under extrusion and is subject to the reaction force from the metal outside the deformation range, so a micro-finned structure nearby the punching tool is formed. And the deformation area of the formed microgroove is higher than the workpiece’s primitive surface. The relationships of the punching interference length L_i with θ_c, a_c, and α_pt are analyzed; and the influence of forming parameters on the groove surface morphology as well as the relationships among these parameters are discussed. When the punching depth a_c is 0.3 mm and the punching feeding angle θ_c is 2°–4°, the finned surface of the radial V-shaped microgrooves can be obtained.

Introduction

With the increase in the micro/optoelectronic device’s power and integration, the thermal power of per unit volume is also increased causing a rapid rise in heat flux. The traditional heat dissipation through finned metal surface is unable to meet the requirements of heat dissipation, and a new demand for high-efficiency heat transfer technology has been presented. Phase-change heat transfer can realize the highly effective heat transfer by heat absorption and release of the working medium in the vaporization–condensation cycle process, and the heat transfer efficiency could be enhanced over 10 times. A two-phase flow heat transfer thermal module with high heat transfer efficiency can effectively decrease the temperature of high-power heat sources in smaller areas. ¹ Comparing finned surface with common smooth surface structure, both the heat transfer area and heat dissipation efficiency are increased for the former under the same conditions.^2,3 This kind of structure using a finned surface to improve the heat transfer performance is called surface functional structure.

Enhanced boiling structure such as grooves, fins, and porous surface are fabricated using mechanical processing, ⁴ electroplating, ⁵ spraying, ⁶ sintering methods, ⁷ and so on. Mechanical processing methods have attracted more attention due to the efficiency and low cost. Xu et al. ⁸ fabricated the square micro-cavity with micro-channels, the circular micro-cavity with cross keyway, and micro-gear cavity by the micro double-staged laminated object manufacturing (LOM) process. The micro-cavity mold had good surface quality by optimized technological parameters, such as the copper electrode with diameter of 0.5 mm and surface roughness of 0.07 μm. Hafiz and Adrian ⁹ put forward three pairs of pin-fin tubes made of copper, brass, and bronze. The only geometric parameter changed was the pin height, and the testing results showed that copper pin-fin tubes possessed the highest heat transfer enhancement efficiency under the same condition. Dogan et al. ¹⁰ have investigated thermal performance of various shapes of thin fin-arrays on a horizontal base plate. An optimum fin shape yielding the highest average heat transfer coefficient has been determined. Tang and colleagues^11,12 proposed some interesting processing methods of fins, cracks, and microgrooves to improve the heat transfer performance. Lee and Seo ¹³ studied the heat and flow characteristics of temperature-sensitive ferrofluid in the square cavity with and without the magnetic intensity. Das et al. ¹⁴ fabricated some tunnels with circular grooves, rectangular grooves, and round base as the enhanced boiling structure with different processing parameters by controlled spark erosion through a series of repetitive sparks between workpiece and wire. Ben-Mansour and Habib ¹⁵ investigated the natural convection heat transfer from discrete heat sources to nanofluids.

This article proposed a radial microgroove structure and its forming method and analyzed the relationships among the punching interference length L_i, feeding depth a_c, feeding angle θ_c, and cutter edge angle α_pt. At the same time, the forming conditions of radial microgrooves and the influence of punching parameters on the groove surface morphology are also discussed.

Experiments

The article adopted a punching method to form a boiling structure which is composed of the radial microgrooves at the base of finned surface. The workpiece is fixed on the rotating disk and the installation direction of the punching tool is consistent with the workpiece’s axial direction, and the punching tool makes an up-and-down reciprocating movement. When the punching depth a_c is defined, the tool’s downward movement realizes the punching and hence forms a radial microgroove. Then, the tool moves upward and the rotating disk revolves a certain angle. When the tool moves downward again to perform the punching, another microgroove is formed, and the included angle of the two microgrooves is equal to the rotating disk’s revolving angle. The two microgrooves’ ends coincide at the workpiece’s center. This forming process is repeatedly performed until the whole boiling structure with radial V-shaped microgrooves is completed. This structure is beneficial to the flowing of liquid working medium from the evaporation surface’s circumference to the central area. The punching tool is made of high-speed steel with a thickness t_hp = 5 mm, and its cutting edge can be formed by the intersection of the two inclined planes with an included angle α_pt, as shown in Figure 1.

Figure 1.

Forming schematic diagram of radial microgroove and punching tool.

Results and analysis

Interference length analysis

Step punching process can be used to realize the forming of the radial microgroove structure. Punching belongs to a metal plastic forming process and is a green forming method without chips, which uses punching tool’s up-and-down reciprocal movement to make the cutting edge shape the workpiece. When the cutting edge contacts the workpiece’s surface and plows into it gradually, the metal sheared by two inclined extrusions will flow to both sides of the punching tool. When the metal in the deformation range flows under extrusion pressure, it is subject to the reaction force of the metal outside the deformation range, forming a finned structure nearby the punching tool, as shown in Figure 2. Known from the forming process, the shape of radial groove is mainly affected by a_c and θ_c. Thus, the deformation region of the formed microgrooves is higher than workpiece’s original surface, as shown in Figure 2(a).

Figure 2.

SEM of microgroove by punching method: (a) microgroove sectional view away from the center of workpiece and (b) microgroove sectional view near the center of workpiece.

W_vp can be supposed as the groove crown width, D_vp is the groove depth, and a_c is the punching depth. Known from the process of punching, the relationship between groove crown width W_vp and the punching depth a_c is

$$W_{\mathrm{vp}}=2a_c\tan \left(\frac{{\alpha }_{\mathrm{pt}}}{2}\right)$$

(1)

After the first groove is completed, a feeding with certain angle is necessary to perform another punching, and the workpiece center is just the coincident point of radial groove structure. After the tool punches a certain depth, the groove has a certain width. Therefore, the grooves close to the coincident point will interfere mutually, and the interference length is related to the feeding angle θ_c. The feeding angle can be supposed as θ_c, and the relationship between L_i and θ_c is

$$L_i=\frac{a_c\sin {\alpha }_{\mathrm{pt}}}{(1+\cos {\alpha }_{\mathrm{pt}})\sqrt{\frac{1-\cos {\theta }_c}{2}}}$$

(2)

As can be seen, relationships among a_c, α_pt, and L_i under different θ_c are shown in Figure 3. The impact on the length of L_i is consistent with a_c and α_pt. When α_pt increases gradually, L_i will also increase gradually under the same a_c. But the incremental rate of L_i decreases with the decrease in a_c because the greater the included angle of punching tool α_pt, the greater the extrusion pressure between materials near the center of workpiece, which leads to greater L_i. Under the same α_pt condition, L_i increases with the increase in a_c. But the incremental rate of L_i increases with the increase in α_pt. So, the L_i increases with the increase in a_c and α_pt.

Figure 3.

Relationship among a_c, α_pt, and L_i under different θ_c: (a) θ_c = 2° and (b) θ_c = 4°.

The interference length L_i is affected not only by the cutting tool’s parameters but also by the process parameters. Relationships among θ_c, α_pt, and L_i under different a_c are shown in Figure 4. The impact on the length of L_i is adverse from θ_c and α_pt. Under the same α_pt condition, when θ_c increases gradually, L_i will decrease gradually. The incremental rate of L_i also decreases with the increase in θ_c because the spacing of deformation zone increases in the same radius when θ_c increases. So, the L_i decreases with the increase in θ_c under certain α_pt.

Figure 4.

Relationship among θ_c, α_pt, and L_i under different a_c: (a) a_c = 0.1 mm and (b) a_c = 0.3 mm.

Due to the limitation of the workpiece’s thickness, the punching depth a_c cannot be oversized. Considering the punching process and the interference situation, the cutting tool’s included angle α_pt is 30°.

Influence of punching depth a_c on V-shaped microgroove appearance

The structure appearance under different a_c can be shown in Figure 5. In Figure 5(a), the punching depth is 0.2 mm, and the spacing of grooves d_c is 0.4 mm. From the figure, it can be observed that the grooves have no interference, and the forming mechanism of various grooves is consistent with that of a single groove.

Figure 5.

SEM of microgroove under different punching depths: (a) a_c = 0.2 mm, d_c = 0.4 mm; (b) a_c = 0.3 mm, d_c = 0.4 mm; (c) a_c = 0.3 mm, d_c = 0.6 mm; and (d) a_c = 0.5 mm, d_c = 1.0 mm.

In Figure 5(b), the punching depth is 0.3 mm and the spacing of grooves d_c is 0.4 mm. Obviously, the forming of current groove may affect the previous groove because the increased punching depth will reduce the metal thickness between the original surface’s grooves. Another reason is that the copper plate with low hardness will distort easily. When the current groove forms, the metal between the grooves will incline to the previous groove under extrusion action, making the previous groove’s width reduce and the depth-to-width ratio increase. If the spacing is the same and the punching depth increases further, the groove’s inclination will further increase. When the punching depth is the same and the spacing increases to 0.6 mm, the groove’s appearance is as shown in Figure 5(c). At this time, interference will still occur, and the metal between grooves will incline. But with the increase in spacing, the inclination will reduce. When the punching depth increases to 0.5 mm, and the spacing is 1.0 mm, the form between the grooves is consistent with a single groove, as shown in Figure 5(d). As it can be seen, the interference is dependent on both punching depth and groove spacing. So, the final groove appearance is also determined by these two factors.

Influence of feed angle θ_c on V-shaped microgroove appearance

Known from the punching process, the groove’s one end begins at the original point, and the other end approaches the inner wall of the evaporation end surface. The radial groove appearance is shown in Figure 6. Obviously, the farther the groove is from the center, the larger the groove’s spacing is. Because the groove’s spacing is small near the circle center, there is always a mutual influence, which is different from the situation of parallel punching. In parallel punching, the current groove may affect all the grooves. But in radial punching, the current groove only affects the previously formed groove’s section close to the center. As the spacing increases, the influence may reduce gradually. The interference length L_i between two grooves will increase gradually as the θ_c decreases.

Figure 6.

Microgroove appearance under different feeding angles: (a) θ_c = 4° and (b) θ_c = 6°.

When a_c or θ_c is changed, different radial grooves will be obtained. Because the radial groove has the shortest distance for the liquid working medium flowing from the periphery to the center, the θ_c should be selected as smaller as possible. But due to the limitation of practical punching process, if the value of θ_c is too small, the interference length L_i will be too long, causing damage to the groove. Therefore, it is more appropriate that the range of θ_c should be chosen within 2°–4°.

Conclusion

1. A step punching method is adopted, which takes the workpiece’s center as the coincidence point of the boiling structure’s radial microgrooves. The grooves close to the coincidence point may interfere mutually. L_i has a non-linear relationship with θ_c, a_c, and α_pt. The impact on the length of L_i is consistent with a_c and α_pt, and is adverse from θ_c under certain α_pt.

2. The interference length L_i is determined by the punching depth a_c, feeding angle θ_c, and the punching tool’s cutting edge angle a_pt simultaneously. When the current groove is formed, the metal between the grooves will incline to the previous groove under extrusion action, which will lead to reduction of the previous groove's width and the increase of depth-to-width ratio.

3. The different radial V-shaped microgrooves have been formed when the parameters such as punching depth a_c and feeding angle θ_c are changed. The radial groove has the shortest distance for the liquid working medium flowing from the periphery to the center. The smaller the feeding angle is, the longer the interference length will be, which will cause damage to the groove.