Micro-extrusion of ECAP processed magnesium alloy for production of high strength magnesium micro-gears

Transcription

1 Scripta Materialia 54 (26) Micro-extrusion of ECAP processed magnesium alloy for production of high strength magnesium micro-gears W.J. Kim *, Y.K. Sa Department of Materials Science and Engineering, Hong-Ik University 72-1, Sangsu-dong, Mapo-gu, Seoul , South Korea Received 18 October 25; received in revised form 2 November 25; accepted 24 November 25 Available online 18 January 26 Abstract Micro-gear extrusion of the fine-grained equal channel angular pressed (ECAPed) AZ31 alloy was successfully performed. High strength gears (yield stress >35 MPa) could be produced by effective grain-refinement through ECAP and texture restoration to the original state before ECAP by subsequent extrusion. Ó 26 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved. Keywords: Extrusion; Equal channel angular pressing (ECAP); Magnesium alloys; Hardness test; Superplasticity 1. Introduction Fabrication of components for microsystems is a key enabling technology for a new generation of miniaturized devices in a wide range of application areas such as optics, medical equipment, semiconductor manufacture, robotics and biotechnology. Usage of metal, ceramics, or metallic glasses in microsystem technology becomes important in future technology because these materials have higher strength and wear resistance than the polymers that are being used most widely. Currently, the microinjection molding method is used to produce micro-components of metals or ceramics. From the viewpoint of production engineering, however, plastic forming processes can offer an advantage in productivity and enable mass production with controlled quality, low cost and reduced processing steps. Saotome and Iwazaki [1] demonstrated that amorphous alloys exhibiting Newtonian viscous flow under special conditions can be used as materials for fabricating various microparts for micro-electro-mechanical systems by plastic working. When dealing with polycrystalline metallic alloys, on the other hand, a small grain size is * Corresponding author. Tel.: ; fax: address: kimwj@wow.hongik.ac.kr (W.J. Kim). important since superplastic flow controlled by grain boundary sliding can be enhanced and has a great advantage in achieving large deformation under low stresses compared to conventional plastic deformation. Magnesium is the lightest metal that can be used for structural application and has superior specific stiffness and strength. Significant grain-refinement by equal channel angular pressing (ECAP) has been demonstrated in Mg alloys [2 7]. During ECAP, the sample undergoes plastic deformation by pure shear, in theory, as it is deformed through the intersecting corner. As it can be deformed repeatedly, very high strain can be accumulated on the same sample. Superplasticity of the ECAPed Mg alloys has been studied by several authors [2,3,7]. The ECAPed Mg alloys were capable of producing both low temperature superplasticity and high-strain rate superplasticity. Superplastic forging ability of the ECAPed AZ31 alloys in a closed die at a relatively low temperature and high-strain rate has been investigated by the current authors [8]. The forged sample shown in Fig. 1 is where a cylinder-shaped ECAPed Mg billet prepared from the AZ31 alloy after 6 passes (13.5 mm diameter 14.5 mm height) was deformed to 2 mm in height at 553 K at an initial strain rate of s 1. An emblem with high surface quality and good replication could be produced /$ - see front matter Ó 26 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved. doi:1.116/j.scriptamat

2 1392 W.J. Kim, Y.K. Sa / Scripta Materialia 54 (26) Fig. 1. A cylinder-shaped ECAPed Mg billet obtained after 6 passes (13.5 mm diameter 14.5 mm height) was forged to an emblem to 2 mm in height in a close die at 553 K at an initial strain rate of s 1. In this paper, micro-extrusion ability of the ECAPed Mg alloy was examined. The present result demonstrates that high strength AZ31 micro-components with yield stress of >35 MPa could be fabricated by controlling grain-refinement and texture. 2. Experimental methods The extruded rod of AZ31 alloy with a diameter of 14.5 mm was cut to pieces with a length of 1 mm. The rod was held at 553 K for 1.8 ks and then pressed with a speed of 4 mm/s through a die made of SKD 61, with an internal angle (U) of 9 between the vertical and horizontal channels and a curvature angle (W) of 3, which was preheated to 533 K. Molybdenum disulphide (MoS 2 ) was used as a lubricant. Repetitive pressings of the same sample were performed to 6 passes by rotating it about the longitudinal axis by 9 in the same direction between consecutive passes (designated as route B c ). After the ECAP process, the sample was cooled to room temperature in air. Tensile specimens of dog-bone geometry with the 5 mm gauge length, 4 mm width, 2 mm thickness, and 2 mm shoulder radius, having the gauge length parallel to the longitudinal axis were extracted from the center portion of the ECAPed Mg by using electro-discharge machining (EDM). For micro-extrusion testing, a forward micro-extrusion machine has been developed. The machine is composed of two parts: the extrusion unit and the container unit. A stepping motor has been employed as a punch driver with 1.8 of resolution capability in the extrusion unit. Extrusion velocity was controlled to move 1 mm as the stepping motor rotates by 36. In Fig. 2, a schematic illustration of the container unit is shown. The container unit is divided into four parts: dummy, specimen, micro-die and die supporter. Micro-extrusion samples with 2 mm in diameter and 6 mm in length were extracted from the center of the ECAPed AZ31. The dimensions of the die machined with a wire electrical discharge machine are as follows: the gear module is.18 mm and the number of teeth is 8, and consequently, diameter of pitch circle is 1.44 mm and diameter of outer tooth is 1.8 mm. The shape of micro-die is shown in Fig. 2. Micro-extrusion testing was conducted at 533 or 573 K at a ram speed of.4 mm/min in a chamber purged by N 2 gas. Boron nitride lubricant was used to reduce the friction between the sample and container as well as between the sample and microdie. The sample was loaded into a container, heated to the testing temperature and then extruded into the gear die after a holding time of 1 min. Testing temperature was controlled to be within ±1 K throughout the test. The Vickers micro-hardness (H V ) was measured on the plane perpendicular to the longitudinal axes, by imposing a load of 1 g for 15 s. 3. Results and discussion Fig. 3 shows the photographs of the materials before ECAP and after ECAP at 553 K by 6 passes. As seen from the figures, the microstructure of the as-extruded material (d = 23.9 lm, where d is the true grain size) has been significantly refined to fine grains after 6 passes (d = 4.1 lm). More uniform distribution of grain size is also resulted. Fig. 4 shows the engineering stress engineering strain curves of the unecaped and ECAPed AZ31 alloys after 6 passes. Comparison of the two materials indicates that there occurs a large drop in yield stress after ECAP despite the large reduction in grain size. According to the texture analysis, EDkh1 1i fiber texture of the unecaped AZ31 alloy where {1} basal planes and h1 1i directions in most grains are distributed parallel to the extrusion direction (ED) has completely changed to ð1 12Þ½ 2 243Š Fig. 2. A schematic diagram for the forward extrusion unit (left) and a gear die made by electro-discharge machining (right).

3 W.J. Kim, Y.K. Sa / Scripta Materialia 54 (26) Fig. 3. Photographs of the AZ31 (a) before ECAP and (b) after ECAP at 553 K by 6 passes. Engineering Stress, σ /MPa unecaped 6 passes (553K) Load (Kgf) AZ31(6 passes) ram speed =.4mm/min holding time =1min T= 533K T= 573K Engineering Strain Displacement (mm) 8 Fig. 4. Engineering stress engineering strain curves of the unecaped and ECAPed AZ31 alloys after 6 passes. Fig. 5. Load vs. displacement measured during micro-extrusion at 533 and 573 K. after 6 passes at 553 K [4]. Schmid factors of three slip directions on the basal and prismatic planes could be calculated based on the major texture components [4,5] and it was found that the basal planes originally aligned parallel to the extrusion direction (fiber texture) rotated to be favorably oriented for slip during ECAP as proved by the increased Schmid factors. Another observation from the curves is that uniform tensile elongation as well as total tensile elongation have been significantly improved after 6 passes, namely, by about three times. This is a consequence of strain-hardening increase after ECAP, and activation of non-basal slip systems and twinning are considered to yield this result. Based on TEM work, Koike et al. [9] proposed that the activities of dynamic recovery as well as non-basal slip systems are important in enhancement of tensile ductility of the ECAPed Mg. Fig. 5 shows the load vs. displacement measured during micro-extrusion. The load increases at the beginning, becomes saturated for a while and then increases again. The load is larger as the extrusion temperature is lower. Fig. 6 shows SEM micrographs of a micro-gear shaft extruded from the ECAPed AZ 31 alloy. Microparts with a height of 5 mm and a diameter of approximately 1.8 mm could be successfully extruded, as shown in Fig. 6(a) and (b). The extruded gear shaft shows good replication of the die dimension with high surface quality. The presence of scratch lines is observed parallel to the extrusion direction at a high magnification (insert in Fig. 6(a)) and this reflects the die-surface condition with high roughness created during the EDM process. The cross-section of the gear shaft extruded at 533 K after chemical etching is shown in Fig. 7. The grain sizes were measured on the cross-section. The tooth area that must have experienced the most severe deformation during micro-extrusion reveals the smallest grain size while the center area that must have experienced the least deformation has the largest grain size. The measured grain size also depended on the extrusion temperature. For 573 K, the grain size at the tooth area is about equal to that of the as-ecaped material and the grain size at the center area is even larger than this. For the lower extrusion temperature of 533 K, the grain sizes at the tooth and center areas are 1.9 lm and 2.6 lm, respectively, which are notably less than those of the as-ecaped material. This result indicates that lower extrusion temperature produces smaller grain size, which is related to low grain growth rate at a low temperature. The result of the grain size measurement is summarized in Fig. 8.

4 1394 W.J. Kim, Y.K. Sa / Scripta Materialia 54 (26) Fig. 6. SEM micrographs of a micro-gear shaft extruded from the ECAPed AZ 31 alloy (a) side view (b) front view. Fig. 7. The cross-section of the gear shaft extruded at 533 K after chemical etching (a) center region and (b) tooth region K Grain size (µm) Hardness (H v ) K As-ECAPed 533K TOOTH 533K CENTER 573K TOOTH 573K CENTER Material and Position Fig. 8. The result of grain-size measurement. The microhardness (H V ) has been measured before and after the micro-extrusion and the comparison is shown in Fig. 9. The hardness differs depending on the position as well as testing temperature. It was higher at the lower extrusion temperature and highest at the tooth area. In order to examine the Hall Petch relationship, the microhardness values of the micro-extruded gears in Fig. 9 were plotted as a function of d 1/2. This is shown in Fig. 1. The microhardness data of the as-ecaped alloy as well as the 65 6 ECAPed alloys annealed at several temperatures in the range between 45 and 75 K for 1.8 ks, with the purpose of increasing the grain size by static grain growth, were also plotted. Both the ECAPed and micro-extruded alloys follow the linear relationship described by Eq. (1): H V ¼ H þ K H d 1=2 As-ECAPed CENTER TOOTH Position Fig. 9. The microhardness (H V ) measurement before and after microextrusion. ð1þ

5 W.J. Kim, Y.K. Sa / Scripta Materialia 54 (26) H v AZ31 extruded under hydrostatic pressure Micro-extruded gears (AZ31) As-ECAP (AZ31) As-ECAP (AZ31) + annealing % Proof Stress, σ y /MPa AZ61 (extruded) AZ31(extruded) Predicted strength at the tooth area (533K) =38 MPa AZ31(extruded) d -1/2 (µm -1/2 ) d -1/2 (µm -1/2 ) (a) (b) x AZ31 extruded under hydrostatic pressure Fig. 1. The microhardness of the micro-extruded gears and the as-ecaped alloys as a function of d 1/2. with K H = 42.2 and H = 4. for the ECAPed alloys, and K H = 5. and H = 47.4 for the micro-extruded alloys, where H and K H are the material constants. This result indicates that the micro-extruded alloys are notably harder than the ECAPed alloys when compared at the same grain size. The texture difference between the two groups is believed to have resulted in the strength difference. The contribution to H V hardness by grain size reduction and texture modification during micro-extrusion can be evaluated based on the two curves in Fig. 1(a). For example, the H V hardness expected at the tooth area with the grain size of 1.9 lm in the micro-extruded gear at 533 K is 7 according to the Hall Petch relation for the ECAPed alloys. This is the value obtained when it is assumed that the H V increase after the micro-extrusion is wholly due to the grain-refinement. The experimentally measured H V value at the tooth area (=84.7) is, however, considerably larger than this value, indicating that another strengthening factor should be taken into considered. Strengthening caused by texture change during micro-extrusion is believed to be the factor since texture is known to be very influential in the strength of the Mg alloys [4,5]. In fact, the contribution of strengthening by grain size reduction (DH V = 6.3) to the total strengthening (DH V = 21) after the micro-extrusion is only 3%. This indicates that the strengthening after the micro-extrusion at 533 K primarily comes from the texture modification. For the case at 573 K, on the other hand, the strengthening seems to come entirely from the texture modification since little grain size reduction is accompanied with the micro-extrusion. It is worthwhile noting that the hardness datum of the AZ31 alloy that was directly extruded after ECAP under hydrostatic pressure [8], and was proven to almost regain the fiber texture after the extrusion, falls onto the curve established based on the data for the micro-extruded gears. This indirectly supports the proposition that texture restoration to fiber texture takes place during the micro-extrusion process. Therefore, it can be concluded that application of micro-extrusion after ECAP to Mg alloys brings out the normal grain size strengthening effect by restoring the texture to the original state before ECAP. The yield stress of the tooth area with the grain size of 1.9 lm (micro-extruded at 533 K), therefore, can be predicted by knowing the relationship between the yield stress and grain size in the extruded AZ31 alloy. According to the Hall Petch relation between yield stress and d 1/2 for the extruded materials shown in Fig. 1(b) [4], a high yield stress of 38 MPa is predicted at 1.9 lm, indicating that a very high strength Mg gear can be produced by combining ECAP and micro-extrusion processes. 4. Conclusion Micro-gear extrusion was performed on ECAPed AZ31 Mg alloys with fine grain size. High strength gears could be produced by using ECAP combined with the extrusion method: ECAP refines the microstructure while direct extrusion restores the ECAP texture to the original state before ECAP. It was found that the increase in hardness during micro-extrusion primarily comes from the texture restoration effect. Acknowledgment This work is supported by 21C Frontier R&D Program Ministry of Science and Technology. References [1] Saotome Y, Iwazaki H. J Mater Process Technol 21;1 3:37. [2] Mabuchi M, Ameyama K, Iwasaki H, Higashi K. Acta Mater 1999;47:247. [3] Lin HK, Huang JC, Langdon TG. Mater Sci Eng A 25;42:25. [4] Kim WJ, Jeong HT. Mater Trans 25;46:251. [5] Kim WJ, Hong SI, Kim YS, Min SH, Jeong HT, Lee JD. Acta Mater 23;51:3293. [6] Yamashita A, Horita Z, Langdon TG. Mater Sci Eng A 21;3:142. [7] Chuvil deev VN, Nieh TG, Gryaznov MY, Kopylov VI, Sysoev AN. J Alloys Compd 24;378:253. [8] Kim WJ, unpublished work. [9] Koike J, Kobayashi T, Mukai T, Watanabe H, Suzuki M, Maruyama K, et al. Acta Mater 23;51:255.