Effect of die coating on Forming of Micro-parts by Forward-Backward Extrusion of 6063 Aluminum Alloy

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1 IWMF214, 9 th INTERNATIONAL WORKSHOP ON MICROFACTORIES OCTOBER -8, 214, HONOLULU, U.S.A. / 1 Effect of die coating on Forming of Micro-parts by Forward-Backward Extrusion of 663 Aluminum Alloy Norio Takatsuji 1, Kuniaki Dohda 2 and Tatsuya Funazuka 3,# 1 Graduate School of Science and Engineering for Research, University of Toyama, Japan, takatsuji@eng.u-toyama.ac.jp 2 Department of Mechanical Engineering, Northwestern University, USA, dohda.kuni@northwestern.edu 3 Graduate School of Science and Engineering for Research, University of Toyama, Japan, m @eme.u-toyama.ac.jp # Corresponding Author / m @eme.u-toyama.ac.jp.319, Gohuku, Toyama city, Toyama, Japan, Tel.: , FAX: KEYWORDS : micro-extrusion, tribology, die angle, die coating The recent trend towards miniaturization of products and technology has boosted a strong demand for such metallic micro-parts with micro features and high tolerances. Conventional forming technologies, such as extrusion and drawing, have encountered new challenges at the micro-scale level due to the size effects that tends to be predominant at this scale level. Friction is one of the predominant factors exercising strong effects in micro-forming. Previous studies varied grain size of the test pieces in order to examine size effects in micro-extrusion. In addition, the effects on the extrusion load, forming shape, as well as hardness of different grain sizes, die coatings and lubricants were compared. DLC coating has proven effective as a die coating. Increasing grain size was effect with lubricants having high viscosity. In this study, the effects of different die properties is compared and examined. 1. INTRODUCTION The rapid advancement of miniaturization technology in recent years has p roduced several novel products and applications they have made a significant impact on a variety of fields such as electronics, biotechnology, and high-precision optics. The field of micro-forming is one of miniaturization technologies receiving a lot of attentions from industrial and academic communities recently. Micro-forming is a branch of manufacturing techniques that fabricate metallic micro-parts, i.e., parts with at least two characteristic dimensions in the sub-millimeter range, such as connector pins for electronics, lead frames for IC chips and micro-gears. Apart from feasibility, micro-forming also focuses on the suitability of the manufacturing process for mass production. Therefore, research on plastic forming has been very intense [1,2]. Conventional forming techniques, such as extrusion and drawing, have encountered new challenges at the micro-scale due to the influence of size effects that tend to be predominant at this level. One of the most important aspects of size effects was observed in the behavior of friction at the micro-scale level. Crystal orientation of materials, lubricants and die coatings are other important factors that also exert strong influences on the friction behavior. This paper focuses on the frictional behavior observed at various grain sizes during micro-extrusion with various lubricants. A novel experimental setup consisting of forming assembly and a loading stage has been developed to obtain force-displacement responses for the extrusion of aluminium pins in the micro/meso scale range (from.1 mm to 1 mm). The paper also investigates the effects exercised by the difference of the angle of the die. 2. EXPERIMENTAL METHOD 2.1. MICRO-EXTRUSION SETUP Fig. 1 shows the micro-extrusion machine used in this study. The die was segmented to facilitate pin removal after extrusion. The punch diameter (φ) was 1.47 mm. Fig.1 (c-1) shows a conical die having a bearing angle of 6. Fig.1 (c-2) shows a flat die having a bearing angle of 9. The container diameter (φ1) was 1.71 mm and the bearing diameter (φ2) was 17

2 Vickers hardness / HV Vickers hardness / HV 2 / OCTOBER -8, 214, HONOLULU, U.S.A. IWMF214, 9 th INTERNATIONAL WORKSHOP ON MICROFACTORIES 1.9 mm. The type of forming process utilized here was a forward-backward extrusion. Test pieces were inserted into the micro die which was accommodated in the mold shown in Fig.1 (a). The punch, the displacement sensor, and the load cell were located inside the main body of the micro press as located in Fig.1 (d). The experiment was carried out at room temperature with a ram speed of.1 mm/s and a forming stroke of 3. mm. The load and the displacement values in extrusion direction obtained during experiment were sent to the controller of the micro press system for data acquisition. Fig.2 Microstructures of the billets Fig.1 Micro-extrusion experimental setup 2.2. MATERIAL In this study, aluminum alloy (A663) was used as the test material. The specimen billets fabricated to 1.7 mm in diameter and 6 mm in length by hot extrusion and machining were non-annealed and heat-treated at 688K for 3 to 48 hours. Fig.2 and Fig.3 shows the microstructures and the Vickers hardness (HV) of the billets that annealed at 688K for, 3, and 48 hours. For the non-annealed billet, a fine grain structure formed by extrusion was observed, and its measured HV was about. In the billet annealed at 688K, a grain growth took place during heat treatment, the approximate grain size of 6 to 1 μm was observed. The HV value was about regardless of the annealing time Extrusion temperature:723k Condition (1) Condition (2) Surface Center Anneal time / h 6 Extrusion temperature:73k 4 4 Surface Center Condition (3) Anneal time / h Fig.3 Vickers hardness of workpiece billets 171

3 IWMF214, 9 th INTERNATIONAL WORKSHOP ON MICROFACTORIES OCTOBER -8, 214, HONOLULU, U.S.A. / WORK-HARDENING COEFFICIENT AND PLASTIC COEFFICIENT The work hardening coefficient (n) and the plastic coefficient (F) were calculated from the compression test using the micro extrusion press. Their relations are expressed as in eqn.(1). σ = F ε n (1) The work hardening coefficient and the plasticity coefficients of a specimen of three conditions used in the experiment are shown in Table 1. It can be seen that the plasticity coefficient of condition (1) is high. However, there is no significant differences in both conditions (2) and (3). The values of work hardening coefficient of these three conditions are similar. Table.1 Work-hardening and plastic coefficients Table.2 Parameters of die coatings 2.4 DIE COATINGS AND LUBRICANTS The actual experiment was with a die that had done four kinds of coatings of, DLCX-EX,, and. The coatings used are diamond-like carbon which disposed by two different coating methods (method 1 is and method 2 is ), as well as chromium nitride () and titanium nitride (). Experiments with uncoated dies have been conducted for comparison as well. Fig. 4 shows the images of these dies obtained by a digital microscope. The average surface roughness (Ra) values of die coated with,,, were. μm,.9 μm,. μm, and. μm respectively. The uncoated die was.7 μm. The coating parameters of each coating are shown in Table.2, and have low friction coefficient of The coefficient friction of is.4. has the highe st friction coefficient of.. 3. RESURTS AND DISSCUSIONS Fig. shows the extruded 663 aluminum micro-pins by using the forward-backward micro-extrusion experiment. The forward extrusion portion of the pins were bent due to the accuracy difference of the left and right die bearings. As the reason, it seems that the difference of the accuracy of a die bearing right and left has influenced. Since the die was segmented, it was extremely difficult to ensure the accuracy of the die positions during assembly. Fig. Extruded pins obtained after the forward-backward micro-extrusion Fig. 4 Dimensions of conical dies (α= ) 3.1 EFFECTS OF DIE ANGLE Figs.6,7,8 show the force-displacement responses of condition (1) (3) with different conical dies and the flat die. The grain sizes were 46 µm, 64 µm, and 96 µm, respectively, and no lubricant was applied. The results showed that the extrusion forces decreased with decreasing hardness values. In addition, the extrusion forces decreased with increasing grain sizes. From these three Figures, it was found that the extrusion forces reduced with decreasing die angle (9, 6, and, 172

4 Finish extrusion force / kn 4 / OCTOBER -8, 214, HONOLULU, U.S.A. respectively). Fig.9 shows the cross-sectional images of the extruded pins at different grain sizes and die angles. It could be observed that smaller grain sizes produced longer backward extrusion portions. The length of the backward extrusion portion varied with the extrusion force. The higher the extrusion force was required, the smaller the length of the backward extrusion portion. The extrusion force increased with the decreasing grain size, which could be seen from the results of Fig.6, Fig.7, and Fig.8. Fig.1 shows the workpiece sections after the extrusion. As for the posterior region, it is understood that the extrusion to the backward side is long and is similar to the one with small grain size. It is understood that the extrusion of flat die to the backward side is longer than that of conical dies. It is thought that the change of extrusion forces have influenced the length of backward extrusion. When the backward extrusion distance is short, it is expected the decrease of extrusion force. When the backward extrusion distance becomes longer, there will be an increase in extrusion force. Moreover, the difficulty for backward extrusion arises when there is grain size growth. This is because of the thickness of a backward extrusion molding part is 12 μm α = α =6 α = Fig. 6 Force-displacement responses of condition (1) (grain size:46µm) IWMF214, 9 th INTERNATIONAL WORKSHOP ON MICROFACTORIES α = α =6 α = Fig. 7 Force-displacement responses of condition (2) (grain size:64µm) α = α =6 α = Fig. 8 Force-displacement responses of condition (3) (grain size:96µm) Condition 1 Condition 2 Condition 3 Die angle / degree Fig. 9 Effect of die angle to the extrusion force of the uncoated die Fig.1 Cross-sectional images of the extruded pins at different conditions of conical dies 173

5 IWMF214, 9 th INTERNATIONAL WORKSHOP ON MICROFACTORIES OCTOBER -8, 214, HONOLULU, U.S.A. / 3.2 EFFECTS OF DIE COATING Figs show the force-displacement responses during extrusion with different coated conical die (α=, 6 ) and flat die (α=9 ) with condition (1). Figs show the forcedisplacement responses during extrusion with different coated conical die (α=, 6 ) and flat die (α=9 ) in condition (3). The extrusion load lowered most when coated die was used. Moreover, the extrusion load rose most when coated die was used. It may be because the influence of friction decreases when the frictional properties of the coating and load are low. When the frictional properties of coating are high, the influence of friction is thought to be a large increase of the load. When extruding it by using as shown in Figs.12, 13 and 16, using, as shown in Figs.12, 13, the load stopped, and it raised and the extrusion stopped on the way because it had exceeded the limit of the device. From these six Figures, it was found that the extrusion forces reduced with decreasing die angle (9, 6, and, respectively). Figs.17 and 18 shows the cross-sectional images of the extruded pins at different die coating and die angles. It shows that the load becomes small by reducing the die corner by using a low frictional properties coating. Figs.2 and 21 show the workpiece sections after the extrusion with Condition (1) and Condition (3) with a die of, coated die and Uncoated die. Figs.22 and 23 show a backward extrusion of different coatings with conical dies (α=, 6 ) and flat die (α=9 ) in Condition (1) and Condition (3). Figs.24 and show the comparison of the length of a backward extrusion of different coatings with conical dies (α=, 6 ) and flat die (α=9 ) in Condition (1) and Condition (3) It shows the forward side of test piece is long and the backward side is short like using when the extrusion force was low. Moreover, the forward side of test piece is short and the backward side is long like using when the extrusion force was high. Therefore, the load reduces when the die angle is small and when the coating with low frictional properties is used Fig. 11 Force-displacement responses in different coatings of condition (1) (grain size:46µm,α= ) Fig. 12 Force-displacement responses in different coatings of condition (1) (grain size:46µm,α=6 ) Fig. 13 Force-displacement responses in different coatings of condition (1) (grain size:46µm,α=9 ) Fig. 14 Force-displacement responses in different coatings of condition (3) (grain size:96µm,α= ) 174

6 Finish extrusion force / kn Finish extrusion force / kn 6 / OCTOBER -8, 214, HONOLULU, U.S.A. IWMF214, 9 th INTERNATIONAL WORKSHOP ON MICROFACTORIES Fig. 1 Force-displacement responses in different coatings of condition (3) (grain size:96µm,α=6 ) Die angle / degree Fig. 17 Effect of die angle to the extrusion force of different die coatings for condition (1) (grain size:46µm) Fig. 16 Force-displacement responses in different coatings of condition (3) (grain size:96µm,α=9 ) Die angle / degree Fig. 18 Effect of die angle to the extrusion force of different die cotings for condition (3) (grain size:96µm) Fig. 2 Workpiece section after the extrusion with and coated dies as well as uncoated die (Condition (1)) 17

7 IWMF214, 9 th INTERNATIONAL WORKSHOP ON MICROFACTORIES OCTOBER -8, 214, HONOLULU, U.S.A. / 7 Fig. 21 Workpiece section after the extrusion with and coated dies as well as uncoated die (Condition(3)) Fig. 22 Backward specimen configurations in different die angles and coatings (Condition (1)) 176

8 backward extrusion/mm backward extrusion/mm 8 / OCTOBER -8, 214, HONOLULU, U.S.A. 4. Conclusions This paper experimentally investigated the effects of die angles and die coatings on the micro-extrusion of aluminum (663) billets. The major findings of this paper were as follows: (1) Length of the extrusion to the backward for flat die becomes longer than that of conical die. (2) The extrusion load reduced when the die coating has low frictional properties is used. (3) The extrusion load rose when the die coating has high frictional properties. (4) The load increases when length of the extrusion to the backward became long. References [1] J. Cao et al.:asme J. Manuf. Sci. Eng., (24) 642. [2] N. Krishnan, J. Cao & K. Dohda : ASME of IMECE, November (2) Orlando. [3] Male : A method for Determination of the Coefficient of Friction of Metals under Conditions of Bulk Plastic Deformation, 1964 [4] N. Takatsuji, K. Dohda and T. Makino, S. Hosokawa: The Proceedings of the 21 Japanese Spring Conference for the Technology of Plasticity, 249- [] N. Takatsuji, K. Dohda and T. Makino, S. Hosokawa: Proceedings of the 12th International Conference on Aluminium Alloys 21 Yokohama Japan, No.162 [6] Aluminum handbook corporation, Japanese Aluminum Association P11 [7] N. Takatsuji, K. Dohda and T. Makino, S. Hosokawa: International Formu on Micro Manufacturing 21 Gifu Japan, [8] N. Takatsuji, K. Dohda and T. Makino, S. Hosokawa: International Formu on Micro Manufacturing 211 Tokyo Japan, No.37, 169~176 IWMF214, 9 th INTERNATIONAL WORKSHOP ON MICROFACTORIES α = α =6 α =9. Fig. 24 Comparison backward extrusion of die angle in different coatings for condition (1) (grain size:46µm) α = α =6 α =9 Fig. Comparison backward extrusion of die angle in different coatings for condition (3) (grain si Fig. 23 Backward specimen configuration in different die angles and coating (Condition(3)) 177