Magnesium (Mg) is the lightest metal that can be used practically, and it possesses excellent strength and rigidity. Mg has been widely implemented in the medical field because it is a commonly occurring element in the body. Extensive research has been conducted on various Mg applications for in vivo medical components, such as implants and stents [1–4]. Mg medical components present various advantages owing to their degradability in vivo, which reduces the burden on patients. Research is ongoing regarding the corrosion resistance and alloy properties corresponding to in vivo environments. However, Mg has a tight-lattice hexagonal crystal structure, characterized by poor workability owing to the anisotropy of this structure. At room temperature, non-basal slip is challenging, and Mg lacks ductility, making it an unsuitable plastic material [5].

Processing small products that are crucial in vivo, such as medical parts, necessitates manufacturing via plastic formation yielding high productivity [6, 7]. The advantages of micro-plastic processing include saving resource materials and space, energy efficiency through the miniaturization of presses, and industrial desirability [8, 9]. The applicability of microextrusion is expected to further expand in the future, such as in medical and electronic devices, along with the manufacturing of micro-gears [10–13]. When the existing plastic-forming technologies, such as extrusion, are converted to the microscale, they face a problem known as the size effect [14]. The size effect directly corresponds to product variation, which significantly reduces productivity and occurs when the workpiece scale approaches the size of the crystalline structure. However, if the size and orientation of the crystalline structure vary from workpiece to workpiece, the material flow behavior during plastic deformation is affected, resulting in variations in the product shape. As the product dimensions decrease, the surface roughness of the tool and workpiece approaches the product size, increasing the proportion of true contact surfaces, along with the friction effect during forming, which significantly influences the plastic deformation behavior [15].

Several studies have been conducted on the effect of size on material flow and frictional behavior in microextrusion. These studies indicate that the flow stress curves from microcompression tests can be used for accurate material flow analysis [16, 17]. Gu and Davies [18] evaluated the thermal stability to analyze the origin of recrystallization during the formation of ultrafine grains in a high-speed microextrusion of copper. They reported that the recrystallization rate accelerated during the extrusion process at high speeds, decreasing the peak temperature and stored energy. Therefore, ultrafine-grained materials are unsuitable for micro-forming, and an optimal grain selection for the working size is crucial. Cao et al. [19] and Parasiz et al. [20] evaluated the plastic deformation behavior of different grain sizes and orientations in micro-forward extrusion. They reported that a coarse crystalline structure causes non-uniform deformation of the crystals and curvature of the extruded part. The deposition of a silicon-containing diamond-like carbon (DLC-Si) coating is the most effective at stabilizing friction behavior during microextrusion [21]. Other methods for stabilizing the friction behavior and reducing the friction of hard coatings include the application of ultrasonic vibration, which reduces the extrusion load and improves the surface properties of the product [22, 23]. High-strength Mg gears can be produced by the microextrusion of Mg with fine-grained refinement through equal channel angular pressing (ECAP) and extrusion [24]. Grain refinement mitigates the effects of anisotropy due to the tight-lattice hexagonal lattice structure (a drawback of Mg formation) and improves the mechanical properties [25]. Venkatesh and Panigrahi [26] analyzed microextrusion processing using superplasticity in ultrafine-grained Mg alloys and reported that crystalline structure refinement is effective for Mg microextrusion [26]. Additionally, ultrasonic vibration can be applied to suppress the effect of friction during forming to improve room-temperature formability, thereby significantly decreasing the extrusion load and improving formability [27].

In our previous study, a micro forward–backward extrusion process was performed for AA6063-T6 [28–30]. We used forward pin–backward cup geometry in the micro-forward and backward extrusion processes. In the forward–backward extrusion process, the material flow is affected by the size of the crystalline structure and friction [28]. The effect of friction increased with the extrusion force, thereby increasing the backward extrusion length. The die angle of the forward-extrusion section was also analyzed. When the die angle was set at a right angle to the extrusion direction, a dead metal region was formed where no material flow occurred, resulting in material flow along the dead metal region [29]. The backward extrusion length was increased owing to the lubrication in the backward extrusion section. As the backward extrusion length significantly increases the extrusion force, the extrusion force was higher for the high-viscosity lubricant than for the low-viscosity lubricant. The mechanical properties of the workpiece, grain size, and friction dominated the backward extrusion section, which significantly affected the formability because the material flows over the surfaces of the punch and dies during forming [30].

The application of low-friction hard coatings, such as DLC and ultrasonic wave supports, is suitable for controlling or suppressing the effects of friction during microextrusion formation [20, 22, 23, 27, 29]. However, these methods have limitations. For example, the life of the coating varies based on the material and shape. Second, there is an increase in the number of ultrasonic support facilities and optimization of the ultrasonic intensity. Along with the application of low-friction hard coatings and ultrasonic supports, the contact conditions between the material and the tool must be analyzed to control the effects of friction during microextrusion. In microextrusion, the surface roughness of the workpiece and tool is relatively close to the machining scale, which significantly affects the friction and forming behavior [14, 31]. In cutting tools, micro and nanoscale textures are suitable for reducing the machining forces and improving anti-adhesion and lubrication on the tool surface [32–35]. The micro and nanoscale textures on the surface of the micro-punching tools can reduce the effect of shear strain on the workpiece during the punching process [36–38].

Micro-to-nanoscale textures can be applied to the tool surface to reduce the true contact ratio between the tool and the workpiece. We applied millimeter to nanoscale textures to the punch surface to directly reduce the effects of friction in micro-backward extrusion for AA6063-T6 [39, 40]. The micro and nanoscale surface texturing reduced the contact area between the material and tool, thereby presenting a reduced extrusion force, increased adhesion resistance to the tool, and improved material flowability [41].

Reducing the friction during microextrusion is essential for analyzing the tool material. Several studies have been conducted on the conventional processing tools used in micro-forming. However, limited research has been conducted on the selection of tool materials suitable for microextrusion. Studies have focused on cobalt–chromium–molybdenum (CoCrMo) as a tool material suitable for micro-backward extrusion, and the microextrusion formability in micro-backward extrusion with AA6063-T6 has been examined [41]. The results demonstrated that CoCrMo, a material used for artificial joints in orthopedics and orthodontic wires in dentistry, exhibits properties such as high strength, high wear resistance, and high corrosion resistance. CoCrMo is suitable for industrial applications owing to its high wear and corrosion resistance [42–46]. The micro-backward extrusion of AA6063-T6 using a CoCrMo die with a nanotextured surface effectively reduced the tool contact area, stabilizing formability by improving material flowability.

In this study, we analyzed the effects of punch and tool surface texture, including CoCrMo dies, on the microextrudability of Mg. Based on previous studies, micro and nanotextures were applied to the punches to analyze the effects of the punch surface texture [39, 40]. AISI H13 and CoCrMo dies, commonly used in micro-backward extrusion, and nanotextured CoCrMo dies were used to analyze the effects of the die material and surface nanotexture [41]. We analyzed the effects of the punch surface texture, die material, and die surface nanotexture on the micro-backward extrudability based on the extrusion force, billet shape after forming, amount of adhesion to the die, and microstructure analysis of the product. We also discussed the effects of the punch and die surface nanotexture on the micro-backward extrusion.

Figures 1 and 2 illustrate the desktop microextruder used in this study [28–30, 39–41]. The microextruder is a servomotor-driven tabletop screw press that transmits torque directly or amplifies it using a servomotor to the screw shaft. The screw axis is connected to the punch via a load cell to control the forming speed and position. The microextruder, die, punch, other tools, and the billet have similar dimensions as those in the previous study [28–30, 39–41].

Pure Mg was used as the billet (Fig. 3). The dimensions of the billet were φ1.7 mm and 4.0 mm (length) [39–41]. The average grain size of the billet was 55.1 µm, and the Vickers hardness was 39.2 HV.

Figure 4 illustrates the AISI H13 tools used in this study, along with a schematic of the micro-backward extrusion [39–41]. The tools used for the micro-backward extrusion comprised a carbide punch (Misumi V30, MISUMI Group Inc., Japan), micro-backward extrusion die, and a jig. The jig material was AISI H13 (Proterial Ltd., Japan). The micro-backward extrusion die was a split die that facilitated the removal of the extruded product after the experiment. The diameter of the container of the die was 1.71 mm, while that of the punch was 1.47 mm, thus forming a cup with a thickness of 0.12 mm at the back.

In this study, we analyzed the surface properties of punches and dies suitable for micro-backward extrusion tools and the new die tool materials. Figure 5 depicts the punches used in this study [39, 40]. Figure 5 (b) depicts a µm-textured punch, in which a 5 µm groove was dug on the punch surface using abrasive paper with a grain size of 400. Figure 5 (c) depicts the nm-textured punch produced by applying a nanoscale periodic groove structure (Ra = 0.099 µm) to the ground mirror punch depicted in Fig. 5 (a) using an ultrashort-pulsed laser pulse. The nanotexture was applied using ultrashort-pulsed laser processing (Lips Works Co., Ltd., Ota-ku, Tokyo, Japan) [39–41]. The wavelength, pulse duration, average power, and maximum frequency were 515 mm, 180–190 fs, 8.2 W, and 600 kHz, respectively. The grooves were 0.01 µm deep with a pitch of 0.3 µm and were applied up to 3 mm from the punch tip. The nano-grooves were oriented parallel to the punch direction.

CoCrMo (COBARION, EIWA Co., Ltd., Iwate, Japan) was used as the new tool material; Table 1 summarizes the composition of CoCrMo [41]. In an analysis of the die surface properties, the texture size on the punches demonstrated that nanotextures are most suitable for micro-backward extrusion [40]. In this study, a nanotexture was applied to the surface of a CoCrMo die. Figure 6 depicts 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 similar to that of the AISI H13 die [41]. Since the CoCrMo die is 5.0 mm wide, a die holder for the CoCrMo die was fabricated and fixed. The CoCrMo die with nanotexture was observed using a scanning electron microscope (SEM, JSM-IT300LV, JEOL Ltd., Japan) at 10,000 times magnification, as shown in Fig. 7 [41]. The U-shaped groove, i.e., the container, was textured through laser processing to a depth of 2.0 µm, angle of 60˚, and pitch of 400 nm. Figure 8 shows the surface roughness of the AISI H13, CoCrMo, and nanotextured CoCrMo dies [41]. The surface roughness of the AISI H13 die was measured using a 3D surface texture measuring instrument (NewViewtM 7300, Zygo Corporation, USA) on the container, which was the micro-backward extrusion area. The surface roughness of the CoCrMo die was an arithmetic mean roughness of Ra 0.607µm. The surface roughness of the nanotextured CoCrMo die was 0.664 µm.

The extrusion conditions included a ram speed of 0.1 mm/s and ram stroke of 1.0 mm at 10–20℃. No lubrication was used [39–41]. The extrusion test was repeated five times for each billet to verify the reproducibility.

An electron probe microanalyzer (EPMA, JXA-8230, JEOL Ltd., Japan) was used to evaluate the adhesion to the punch and die [39–41]. Electron backscatter diffraction (EBSD, JEOL-JSM-7001F, JEOL Ltd., Japan) patterns were used to analyze the microstructures of the microextruded products [39–41]. The orientation distribution and grain size were analyzed using EBSD to determine their effect on the material using a textured tool with reduced friction. The extrusion tip was 800 µm × 0.15 mm. The specimens were mirror-polished with an abrasive, and the surface static stress was removed through ion milling. The measurement conditions included an acceleration voltage of 20 kV and an irradiation current of 13 nA. A step size of 0.5 µm was used for the measurements at the extrusion tip and 0.1 µm for the local measurements. Vickers hardness measurements were performed to determine the work hardening of the microextruded products [39–41]. A micro-Vickers hardness tester (HM-101, Mitutoyo Corporation, Japan) was used to measure the hardness. The product hardness distribution was obtained at an applied load of 0.1 N, applied time of 5 s, and indentation distance of 0.2 mm.

In this study, we analyzed the effects of tool geometry on micro-backward extrudability using three dies with different surface properties and the effects of the dies and material flow. The study results are summarized as follows: