Abstract

To manufacture microparts used in medical and electronic devices, the machining scale must be reduced to the microscale. However, when applying existing plastic forming processes to the machining of microscale parts, the size effect caused by material properties and friction results in variations in product accuracy. To suppress the size effect, tool materials and tool surface treatments suitable for microscale machining must be considered. Using AA6063-T6 billets as test specimens, this study investigated the effects of tool surface properties, such as die surface nanotexture, on micro-extrudability such as extrusion force, product shape, and crystal structure of the product. A cobalt-chromium-molybdenum (CoCrMo) die was used as a new die material suitable for micro-extrusion. To investigate the effects of the die material and die surface nanotexture, AISI H13, CoCrMo, and nanotextured CoCrMo dies were used. The extrusion force increased rapidly with the progression of the stroke for both dies. Compared with the AISI H13 die, the CoCrMo die with nanotexture exhibited considerably lower extrusion force, longer extrusion length, and less adhesion on the die surface. The results of material analysis using electron backscatter diffraction indicated that the nanotextured CoCrMo die improved material flowability and facilitated the application of greater strain. In contrast, the AISI H13 die exhibited lower material flowability and nonuniform strain. Therefore, the tribology between the tool and material was controlled by changing the surface properties of the die to improve the formability.

Introduction

In recent years, the demand for miniature components has rapidly increased in electronics, computers, telecommunications, biotechnology, medicine, and optics. Manufacturing via plastic forming with high productivity has attracted attention for the machining of microscale parts [1,2]. The application of microplastic forming, which is suitable for mass production, to the manufacturing of small parts enables saving space (owing to smaller presses), material resources, and energy used. Moreover, it offers superior industrial advantages compared with micromachining processes such as machining [3,4]. Micro-extrusion is a micromanufacturing process with great industrial potential, such as its application in the manufacture of microgears [510]. However, the application of macroscale processing technologies, such as microscale extrusion, is problematic owing to variations in reproducibility and accuracy. This phenomenon is known as size effect. The size effect occurs when factors such as grain size and surface properties, which cannot be reduced even if the size of the work material is reduced, affect the plastic and friction behaviors [11]. In other words, the size effect on the plastic behavior is caused by the discontinuously formed crystal compositions that need not be considered on a macroscopic scale but appear directly in the product outline. The size and orientation of the crystalline structure differ among workpieces, resulting in variations in the shape of the product after deformation. Engel et al. revealed the tribological effects of smaller product dimensions using double-cup extrusion tests at the microscale [2]. They reported that with decreasing product dimensions, the pockets that held lubricant in the tool-billet contact area decreased, whereas the percentage of true contact surface increased, thereby resulting in higher friction.

Size effects have also been studied in the field of micro-extrusion processing. Cao et al. evaluated the plastic deformation behavior of different grain sizes and orientations during forward micro-extrusion [12,13]. They attempted to determine the friction coefficient from the working force during extrusion; however, the assumption of a constant friction coefficient during processing was incomplete. Moreover, the nonuniform deformation and curvature of the crystals in the extrusion zone were attributed to the coarsening of the crystal grain size. Furthermore, the effectiveness of die coating in stabilizing the frictional behavior during micro-extrusion has been investigated. It has been reported that the deposition of a silicon-containing diamond-like carbon coating is the most effective in micro-extrusion processing [14]. In addition to hard coatings, ultrasonic vibrations can effectively reduce friction during the micro-extrusion process. Chan et al. reported that an ultrasonic micro-extrusion tool is effective for micro-extrusion because ultrasonic vibration reduces the extrusion force, which results in lower energy consumption and better surface properties of the product [1517]. Chan et al. obtained flow stress curves from microscale compression tests for accurate analysis of micro-extrusion and showed that the flow pattern of the material can be accurately analyzed despite its variations depending on the forming method and crystalline state [18,19]. Gu et al. evaluated the thermal stability during the formation of ultrafine- and coarse-grained copper from high-speed micro-extrusion using ultrafine-grained copper [20].

It has been reported that the recrystallization rate of ultrafine-grained copper accelerates at high extrusion rates owing to recrystallization during formation, resulting in lower peak temperatures and stored energy. This suggests that ultrafine-grained materials are not necessarily effective for microforming and that the selection of appropriate grains for the size of the workpiece is critical.

Researchers performed forward and backward micro-extrusions of AA6063-T6 to establish a processing technique for micro-extrusion [2123]. The forward and backward micro-extrusions were performed to produce a forward pin-backward cup shape. The effects of the grain size, die angle, die coating, and lubricant on the micro-extrudability, such as the extrusion force, product shape, and product hardness, were investigated. Consequently, the effect of friction during extrusion was found to increase with a higher extrusion force and backward extrusion length, and the complex changes in forward and backward material flows caused by friction were evaluated from simulations and other data. In addition, the material flow and the effect of friction in the forward extrusion section varied depending on the die angle near the forming section in the forward extrusion. When the die angle of the forming section was perpendicular to the forward extrusion direction, dead metal formed near the forming section, and the material flowed along the dead metal area. The results show that the lubricant contributed to the reduction of friction in the rear section when the lubricant had a high kinematic viscosity. However, when the die angle of the forming section had a tapered angle with respect to the forward extrusion direction, the material flowed along the die angle. This reduced the friction in the forward extrusion section in the case of diamond-like carbon with a low coefficient of friction and lubricants with high kinematic viscosity, thereby reducing the extrusion force. Moreover, the results of these studies indicated that the forward micro-extrusion was more active when the lubricant had a low coefficient of friction in the forward extrusion area. In addition, the friction between the tool and material surface and the crystalline structure affected the extrusion force and product shape in the forward backward micro-extrusion process. In particular, the extrusion force and material flow were dominated by friction, which also had a significant effect on backward extrusion.

To control the effects of friction during forming in micro-extrusion and to establish forming techniques, the contact conditions between the material and tool and the tool surface properties suitable for the working scale must be considered. In microforming, the work material and tool surfaces are rougher than those in the machining scale. It has been shown that the roughening of the surface of both materials relative to the working scale has a significant effect on the friction and forming behavior during forming [11,24]. In cutting tools, the use of micro- and nanoscale textured tools has been shown to be effective in reducing machining forces and improving anti-adhesion and lubrication on the tool surface [2528]. It has been reported that micro- and nanoscale textures on the surface of micropunching tools can reduce the effect of shear strain on the workpiece during the punching process [2931].

In micro-extrusion, it is essential to determine the optimal tool-surface conditions to reduce friction and improve formability. Therefore, to directly reduce the effect of friction in microbackward extrusion, research has been conducted on the reduction and control of the extrusion force by reducing the contact area between the material and tool by adding millimeter- to nanoscale textures to the punch surface [32,33]. Regarding the effect of texture on the punch surface in microbackward extrusion, it was found that as the texture size decreased from the micro- to nanoscale, the extrusion force was reduced, and the adhesion resistance and material flowability to the punch were improved. Moreover, the nanotextured punches were more effective than the microtextured punches. Considering the contact condition between the tool and material in backward micro-extrusion, it is assumed that nanotexturing on the die surface is desirable because the die has a larger contact area with the material.

To reduce friction during micro-extrusion, it is necessary to examine the tool material. In general, tool materials are selected for macroscale processing according to the processing conditions such as the processing temperature and type of material to be processed. However, much research on the tools used in microforming has been conducted using conventional machining tools, and investigations into the selection of tool materials suitable for micro-extrusion are yet to be conducted.

For backward micro-extrusion, a tool material with high strength and excellent tribological properties such as wear resistance must be selected. In this study, we focused on CoCrMo, which is used for artificial joints in the orthopedic field and for orthodontic wires in the dental field, as a new tool material for backward micro-extrusion. CoCrMo exhibits high strength; in addition, it exhibits high wear and corrosion resistances, because of which its application in general industrial use has been suggested [3438].

In this study, to investigate the effects of the die material and die surface nanotexture, AISI H13 and CoCrMo dies, which are normally used in backward micro-extrusion, and a CoCrMo die with a nanotextured surface were used. The nanotextured CoCrMo die was used to stabilize formability by reducing the tool contact area and improving material flowability. The effects of the die material and die surface nanotexture on the backward micro-extrusion formability were observed based on the extrusion force, billet shape after formation, amount of adhesion to the die, and microstructure of the product.

Experimental Method

Figures 1 and 2 show a photograph and schematic, respectively, of the desktop-type micro-extrusion press used in this study [2123,32,33]. The micro-extrusion press was servodriven, and the punch speed and position were controlled by transmitting the torque supplied from the servomotor to the screw shaft. A load cell was installed on a punch holder above the punch to determine the extrusion force.

The billet used was AA6063-T6, which was the same as that used for backward micro-extrusion in previous studies [32,33]. The billet dimensions were 1.7 mm in diameter and 4.0 mm in length. The average grain size of the billet was 23.3 μm, and the Vickers hardness was 33.2 HV.

Figure 3 shows the AISI H13 tools used in this study and a schematic of the backward micro-extrusion [32,33]. The backward micro-extrusion tool comprised a carbide punch (Misumi V30, MISUMI Group Inc., Japan), backward micro-extrusion die, and jig. The jig material was AISI H13 (Proterial Ltd., Japan). The backward micro-extrusion die used was a split die to facilitate the removal of the extruded products after the experiment. The diameter of the container of the die was 1.71 mm, whereas that of the punch was 1.47 mm. Thus, a cup with a thickness of 0.12 mm was formed at the rear. In previous studies, the carbide punches were mirror punches [32,33].

This study aimed to investigate a new die tool material and die surface properties suitable for a backward micro-extrusion die. CoCrMo (COBARION, EIWA Co., Ltd., Iwate, Japan) was used as the new tool material, and Table 1 lists its composition. In addition, the texture size on the punches in previous studies has shown that nanotextures are most suitable for backward micro-extrusion [33]. Therefore, in this study, a nanotexture was applied to the surface of a CoCrMo die. Figure 4 shows a CoCrMo die and a CoCrMo die with a nanotexture. The diameter of the container part of the die was 1.71 mm, which is the same as that of the AISI H13 die. Because the CoCrMo die was 5.0 mm wide, a die holder for the CoCrMo die was fabricated, and the CoCrMo die was fixed. Figure 5 shows the surface nanotexture of the CoCrMo die. The nanotexture was applied via ultrashort pulsed laser processing (Lips Works Co., Ltd., Ota-ku, Tokyo, Japan) [33]. The wavelength, pulse duration, average power, and maximum frequency were 515 mm, 180–190 fs, 8.2 W, and 600 KHz, respectively. The nanotextured CoCrMo die was observed using scanning electron microscopy (SEM; JSM-IT300 LV, JEOL Ltd., Japan) at 10,000 × magnification. The container section was laser-textured with a nanotexture of 2.0 μm depth, 60 deg angle, and 400 nm pitch. Figure 6 shows the die surface roughness of the AISI H13, CoCrMo, and nanotextured CoCrMo dies. The surface roughness of the AISI H13 die was measured using a 3D surface texture measuring instrument (NewViewtM 7300, Zygo Corporation) on the container, which was the backward micro-extrusion area. The surface roughness of the CoCrMo die was an arithmetic mean roughness of Ra 0.607 μm. The surface roughness of the nanotextured CoCrMo die was 0.664 μm.

Extrusion conditions similar to those in previous studies were considered, that is, room temperature, a ram speed of 0.1 mm/s, and a ram stroke of 1.5 mm [32,33]. No lubrication was involved. Backward micro-extrusion experiments were conducted using three types of die: AISI H13, CoCrMo, and nanotextured CoCrMo dies, and the extrusion test was repeated five times for each type to confirm the reproducibility.

An electron probe micro-analyzer (EPMA, JXA-8230, JEOL Ltd., Japan) was used to evaluate the adhesion of aluminum (Al) to the die. Electron backscatter diffraction (EBSD, JEOL-JSM-7001F, JEOL Ltd., Japan) patterns were used to analyze the microstructures of the micro-extruded products. The orientation distribution and size of the grains were analyzed using EBSD to measure the effect on the material when a tool with reduced friction using a CoCrMo die with a nanotexture was used. A measurement of 800 μm × 0.15 mm was obtained from the extrusion tip. The specimens were mirror-polished with an abrasive, and the surface static stress was removed by ion milling. The measurement conditions were an acceleration voltage of 20 kV and an irradiation current of 13 nA. Step sizes of 0.5 and 0.1 μm were used for measurements at the extrusion tip and local measurements, respectively. To determine the work hardening of the micro-extruded products, a micro-Vickers hardness tester (HM-101, Mitutoyo Corporation, Japan) was used to measure hardness under conditions of applied load of 10 gf, application time of 5 s, and indentation distance of 0.2 mm.

Experimental Result

Figure 7 shows the extrusion force–stroke curves for the AISI H13, CoCrMo, and nanotextured CoCrMo dies. The maximum extrusion forces were 3.7, 3.5, and 2.3 kN, respectively. The extrusion forces of the AISI H13 and CoCrMo dies were comparable. Figure 8 shows the maximum extrusion force from the first to fifth cycles of extrusion for both dies. Although the arithmetic mean roughness of all the dies was approximately the same, the extrusion force was significantly reduced by adding nanotexture to the die surface.

Figure 9 shows the cross-sectional shapes after the backward micro-extrusion of the (a) AISI H13, (b) CoCrMo, and (c) nanotextured CoCrMo dies at a 1.5 mm ram stroke and different backward extrusion length (lb) for each product. The lb was 1.81, 1.89, and 2.15 mm for the AISI H13, CoCrMo, and nanotextured CoCrMo dies, respectively. The lb was comparable for all dies; however, it increased with a decrease in the true contact area through texturing, which reduced friction and improved the material flow. The application of nanotexture disrupted the Al adhesion on the die and reduced friction. Al flowed along the die, which was considered to result in smoother plastic flow and longer backward extrusion lengths. A similar phenomenon was observed in the application of nanotexture to punches, and it is believed that a friction reduction effect can be obtained by applying a texture with a small depth to the tool surface in microscale metal forming [33].

Figure 10 shows the product hardness distribution after backward micro-extrusion using the (a) AISI H13 die, (b) CoCrMo die, and (c) CoCrMo die with nanotexture. The product hardness increased significantly compared with that of the 33.2 HV of the billet. In the case of CoCrMo, the work hardening was 65–68 HV at the top, 65–71 HV in the middle, and 65–73 HV at the back, which were smaller than that of the AISI H13 die. The nanotextured CoCrMo die exhibited work-hardening values of 56–62 HV at the top, 62–68 HV in the middle, and 65–71 HV at the back, which were smaller than those of the AISI H13 and CoCrMo dies. In particular, the degree of work hardening at the leading edge of the product was smaller. The lower extrusion force of the nanotextured CoCrMo die also suggested that the nanotexture endowed the material with high flowability from the initial stage of extrusion.

Figure 11 shows the results of the EPMA measurements of Al elemental deposition on the die surface after the first and fifth backward micro-extrusions using the (a) AISI H13, (b) CoCrMo, and (c) nanotextured CoCrMo dies.

The EPAM observation of the Al deposition showed that the deposition extended in the extrusion direction for the AISI H13 die, which showed a larger extent of Al adhesion than the CoCrMo and nanotextured CoCrMo dies, indicating that compared with the AISI H13 die, the CoCrMo and nanotextured CoCrMo dies enables reducing the Al adhesion to the die. It has been shown that the nanotexture imparted to the punch surface by microbackward extrusion breaks up the adhesion into small pieces on the nanoscale [33]. It is believed that the texture disrupts the adhesion of the material flowing into the circumferential container. It is considered that the application of nanotexture makes the pockets into which the material flows smaller and divides up the adhesion into smaller pieces. It is also considered that the frictional force is reduced by disrupting the adhesion, which improves material flowability and reduces the extrusion force. Comparisons of the amounts of Al adhered to the surface of each die in the first and fifth nanotextured revealed that the amount of adhered material increased in the AISI H13 and CoCrMo dies. In particular, a large red area was observed on the fifth AISI H13 die, indicating strong localized deposition. This was attributed to the localized adhesion of Al on the die surface as the number of extrusion cycles increased. However, no localized deposition was observed in the CoCrMo die, which has been reported to have excellent wear resistance [3438], and the deposition on the die and associated wear were less likely to occur than in AISI H13. Thus, this effect was sustained and considered to persist even with an increase in the number of extrusion cycles. The Vickers hardness measurements on the products indicated that the CoCrMo die exhibited less work hardening than the AISI H13 die, suggesting that the application of a die with a higher wear resistance reduced the material adhesion to the die and improved material flowability. In the CoCrMo die with the nanotexture, although certain adhesion to the grooves was observed with increasing number of extrusion cycles, the extent of adhesion was disrupted. In a previous study where nanotexture was applied to punches, it was confirmed that the nanotexture remained on the surface after multiple micro-extrusions [33]. Although the number of extrusion cycles slightly increased the adhesion because of the fragmentation effect of the nanotexture, it is thought that the CoCrMo die with the nanotexture had less adhesion of material on the die surface after the first and fifth extrusion cycles because of the fragmentation of the adhesion.

Compared with the AISI H13 die, the nanotextured CoCrMo die was superior in terms of extrusion force and adhesion to the die. Therefore, EBSD was used to analyze the crystalline state of these products to confirm the material flowability and the crystalline state of the products. Figure 12 shows the inverse pole figure (IPF) map of the product tip using the (a) AISI H13 and (b) nanotextured CoCrMo dies. The IPF map is a measurement method that determines the crystallographic orientation by color, which is defined by the crystal plane. The grains at the tip of the extrudate flowed out without shearing. Toward the rear end of the material, the material was sheared longitudinally, and the grain size was elongated, indicating that the CoCrMo die with nanotexture was sheared, and the grains were elongated longitudinally compared to the AISI H13 die. Therefore, the extrudate length of the CoCrMo die with nanotexture was also longer than that of the CoCrMo die without nanotexture. At the tip of the product, the AISI H13 die exhibited a coarse crystal structure and fine grains owing to the simultaneous dynamic recrystallization. This is attributed to the higher load on the AISI H13 die; therefore, the effect of the strain increased.

Figure 13 shows the results of the kernel average misorientation (KAM) map, which is a measurement method for quantitatively evaluating the internal residual strain of a sample based on its crystal orientation difference information. The AISI H13 die shows several green and red colors with an orientation difference of 3–5 deg or more distributed from the tip to the rear of the material. In contrast, the CoCrMo die with nanotexture exhibited blue color with an azimuthal difference of 0–1 deg at the tip of the product, followed by a large amount of green color with an azimuthal difference of 3–5 deg. This indicates that the nanotextured CoCrMo die had low friction in the early stage of extrusion and that the material flowed easily. Good material flowability in the initial stage of extrusion was considered to have promoted the processing progress via the uniform application of strain in the subsequent forming process. However, the AISI H13 die had high friction, which hindered the material from flowing and is believed to have resulted in a nonuniform accumulation of strain. The nanotextured CoCrMo die also showed a red distribution with an azimuthal difference of 5 deg or greater on the surface where the punch and material come into contact, as shown on the left side of Fig. 13, but the range was smaller than that of the AISI H13 die. Grain deformation was homogenized by applying a nanotexture, and the grain size was refined without the accumulation of local plastic strain. The ability to achieve a low-friction processing environment is expected to improve the processing limits.

Conclusion

In this study, to stabilize the formability by improving the material flowability in backward micro-extrusion, an existing AISI H13 die, CoCrMo die with high strength and excellent wear resistance, and nanotextured CoCrMo die were used. The results obtained were as follows:

  1. The maximum extrusion forces on the AISI H13 and CoCrMo dies were similar. The maximum extrusion force was significantly reduced by adding a nano-texture to the die surface. There was no change in the maximum extrusion force for either die after five repetitions.

  2. The Vickers hardness distributions of the products of the AISI H13, CoCrMo, and nano-textured CoCrMo dies showed that the work hardening of the products was greater for the AISI H13 die and lower for the CoCrMo die. The work hardening was further reduced by applying a nano-texture to the surface of the CoCrMo die.

  3. The evaluation of the die surface adhesion by EPMA confirmed that the use of a CoCrMo die with high wear resistance suppressed Al adhesion to the die surface. The die surface was given a nano-scale texture to break the adhering size into a state suitable for the processing scale.

  4. The IPF and KAM maps obtained via EBSD showed that the material flowability improved and the strain distribution inside the product became uniform via the addition of a nano-scale texture to the die.

Future research will include investigating the effect of nanotexture orientation and applying the results of this study to the formation of biomaterials such as magnesium and titanium.

Acknowledgment

This paper was presented at 6th World Congress on Micro and Nano Manufacturing WCMNM2023. We would like to thank Editage for English language editing.

Data Availability Statement

The authors attest that all data for this study are included in the paper.

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