The Influences of the Die Half Angle of Taper Die During Cold Extrusion Process

Transcription

1 Arab J Sci Eng (213) 38: DOI 1.17/s RESEARCH ARTICLE - MECHANICAL ENGINEERING The Influences of the Die Half Angle of Taper Die During Cold Process S. Syahrullail C. S. N. Azwadi Y. M. Najib Received: 4 January 211 / Accepted: 8 June 211 / Published online: 9 January 213 King Fahd University of Petroleum and Minerals 212 Abstract In this work, the effective strain distributions of billet extruded with taper dies which have different die half angle were investigated. The extrusion ratio for all experiments was set as two. The experimental works were done using cold-work plane strain extrusion apparatus. The experiments were done at room temperature around C. Three types of taper die with three different die half angles, which are,3, and 6 were prepared. Paraffinic mineral oil with low viscosity was used as test lubricant. The material of billet was annealed with pure A11 aluminium. The experimental results were focused on the extrusion load, tool and workpiece surface roughness, and plastic deformation of billet. The velocity and the effective strain of billet were calculated using a visioplasticity method. From the results, we found that the die shape helps to reduce the forming load, which could contribute in minimizing the energy. Keywords Die half angle Paraffinic mineral oil Visioplasticity Surface roughness List of Symbols ψ i X i V Stream function for the ith line The distance from X = totheith line Ram speed u Velocity in the x- v Velocity in the y- Y The distance from Y i 1 to the Y i+1 X The distance from X i 1 to the X i+1 ε X Strain rate in the x- γ XY Shear strain rate ε Effective strain rate ε Effective strain 1 Introduction S. Syahrullail (B) C. S. N. Azwadi Y. M. Najib Faculty of Mechanical Engineering, Universiti Teknologi Malaysia, 8131 UTM, Skudai, Johor, Malaysia syahruls@fkm.utm.my The forming process is designed to exploit the material property or plasticity, the ability to flow as solid without deterioration of the properties. It is a permanent deformation of

2 122 Arab J Sci Eng (213) 38: a material under tension, compression, shear, or a combination of loads. There are two types of mechanical works where material undergoes plastic deformation which is cold work and hot work [1]. Some applications of hot and cold working are the extrusion, forging, rolling, and drawing. In this paper, cold work extrusion was used in the experimental work. is a compression forming process in which the work metal is forced to flow through a die opening to produce a desired cross-sectional shape. The mechanical extrusion of metals and alloy embraces two methods which are hot and cold extrusion. In hot extrusion, the operation is performed at a suitable elevation. In cold extrusion, the billet is placed in the die at room temperature. The advantages of extrusion include variety of section possible, grain and strength enhancement, close tolerance, and less or no material wastage [2]. Both methods may be done using the direct and indirect process. Most engineering and structural metals may be extruded either hot or cold. However, in the extrusion process, few factors such as die shape, billet s properties, frictional constraint condition, and lubrication condition would influence the process. Dies and tools play an important role to determine the surface quality of final product. At the same time, the deformation behavior, the plastic flow of billet, the extrusion load, and the filling of the die cavity deformed shape in extrusion process are also affected by the die shape. Hence, determining the optimum die half angle is a crucial part in tool and die making process. Besides, die surface finishing conditions also play an important parameter in determining the frictional constraint between the tool and billet surface. The difference in the die geometry would influence the material flow characteristic [3]. The differences are clearly observed in grid pattern deformation and velocity vector. The optimization of tool and die shape could reduce the value of frictional constraint and at the same time shows possibility of the reduction of ductile fracture in workpiece [4]. The right selection of die shape also could reduce forming load and create better flow deformation [5]. From the analysis of cold extrusion of aluminum billets using three-dimensional finite element method, the effective stress value increases as the die semi-angle increases [6]. From the surface quality point of view, it is observed that the surface distortion becomes more severe as the die semi-angle increases, as a result of the higher extrusion force [7]. In this paper, the effect of the die half angle of taper die on the billet extruded with three different taper die (the extrusion ratio was remained as two) were analyzed and compared. The billet s material was pure aluminum, A11. The results show that the die shape helps to reduce forming load, which could contribute in minimizing the energy. (a) Punch Dummy block Taper die and container Outer cover Billet 3 die half angle End plate 8 mm 5 mm 5 mm 15 mm (b) Square grid patterns with fine V-shaped grooves Experimental surface of billet. Two billets combined as a set of workpiece. Fig. 1 a A schematic sketch of the plane strain extrusion apparatus and b the combination of billlets 2 Materials and Method 2.1 Experimental Apparatus Figure 1a shows a schematic sketch of the plane strain extrusion apparatus that was used in the experiments. The main components are the container wall, workpiece (called as billet), and taper die. Figure 1b shows a schematic sketch of the billets that were used in the experiments. The billet material is pure aluminium, A11. The billet shape was produced using an NC wire cut electric discharge machining device. Two similar billets were stacked and used as one billet unit. One side of the contact surface of the combined billets was the plastic flow observation plane used during plane strain extrusion. The observation plane is not affected by the frictional constraint of the parallel side walls. A square grid pattern that measures the material flow during the extrusion process was scribed by the NC milling machine onto the observation plane of the billet. The grid lines were V-shaped grooves with a.5- mm deep,.2-mm wide, and 1.-mm interval length. The billets were annealed before the experiments. Billets were heated to a temperature of 28 C for 75 min and then furnace-cooled until it reaches the room temperature. The experimental surface of the billet (surface that contacts the taper die) has a surface roughness of approximately 2.5 µm. The Vickers hardness of the billet is 38 Hv. 2.2 Taper Die Three types of taper die were used in this experimental works. AsshowninFig.2, taper die type A has die half angle. To prevent damage at the taper die, the radius of 2 mm was added. Taper die type B and type C have 3 and 6 die half angle, respectively. The taper die is made from SKD11 tool steel, and the necessary heat treatments were done before the experiments. The experimental surface of the taper die

3 Arab J Sci Eng (213) 38: from the plane strain extrusion apparatus and the combined billets were separated for the surface roughness measurement and the visioplasticity analysis. 2.5 Visioplasticity Method 3 6 Taper die type A Taper die type B Taper die type C die half angle 3 die half angle 6 die half angle Fig. 2 A schematic sketch of the taper die type A, B and C (surface that contacts the billet) was polished with an abrasive paper and has a surface roughness, Ra, of approximately.1 µm. A specified amount of lubricant was applied to this surface before the experiments. The same type of test lubricant was applied to the other surfaces of the experimental apparatus. The taper die has a Vickers hardness of 65 Hv. 2.3 Test Lubricant The testing lubricant used is the additive-free paraffinic mineral oil, VG95. The properties of the additive-free paraffinic mineral oil are shown in Table Experimental Procedure Lubricants were applied onto the experimental surface of the taper die. The amount of lubricant is approximately 25 mg. The mass measurement was done with a digital balance that has a tolerance of.1 mg. The billets were cleaned with acetone. After that, the taper die and the billet were assembled as shown in Fig. 1. The plane strain extrusion apparatus was assembled and placed onto the hydraulic press machine. The load cell and displacement sensor were used to record the extrusion force and the ram displacement; the data was saved in a computer. The experiments were carried out at room temperature. The extrusion was stopped at a piston stroke of 3 mm, where the extrusion process is in a steady-state condition. The average ram speed is around 6.8 mm/s. After the experiment, the partially-extruded billets were taken out Figure 3a shows the grid lines pattern on the observation plane of the billet after the experiment. Then, billet was cleaned with the acetone and the picture of it was taken. The horizontal grid line, which is parallel to the extrusion, shows the plastic flow pattern of behavior during the steady-state extrusion process. The horizontal lines, which are parallel with the extrusion ratio, were traced with tracing software, according to the coordinate system that is shown in Fig. 3b. In undeform area, the flow line moves in linear line. The velocity at this stage is also constant, which is the velocity of the ram speed. The inlet boundary is the point where the flow line starts to change its due to the deformation process, shown as point A in Fig. 3a. In this stage, the velocity starts to change due to the decrement of the cross-section of the dies area. When the deformation process is finished, the flow line moves in linear distribution again. The outlet boundary is the point where the flow line became linear line after the deformation process, shown as point B in Fig. 3a. After tracing, the digital data were prepared as raw data input for the visioplasticity analysis. Figure 3 also shows some of the variables that were used in the analysis and calculations and the position that was established in the same coordinates system in the observation plane of the billet. The plastic flow velocity in the deformation zone, the effective strain rate, and the effective strain were calculated using equations (1) to(5). Since the analytical calculation procedure is explained in an earlier publication, it is omitted here [8 1]. Flow function ψ i = X i V o (1) Velocity component (velocity in the x-, u; velocity in the y-, v) u = ψ Y, v = ψ X Strain rate component (s-1) ε X = u X, γ XY = u Y + v X (2) (3) Table 1 The properties of the additive-free paraffinic mineral oil Parameter, unit Test method Result Density at 15 C, kg/l ASTM D (9).8725 API gravity ASTM D (9) Kinematic viscosity at 4 C, cst ASTM D

4 124 Arab J Sci Eng (213) 38: force (kn) Taper die type A ( ) 25 2 Taper die type B (3 ) 15 Taper die type C (6 ) Piston stroke (mm) Taper die and container x-axis DBO u (a) v Deformation zone Taper die and container DBI DBO: Deformation boundary at outlet DBI: Deformation boundary at inlet (b) X i i-th flowline y-axis Fig. 3 a Photograph of the grid lines pattern on the observation plane of the billet after the experiment. b Coordinate system that was used in the analysis Effective strain rate (s-1) ε = ε 2 X γ 2 XY (4) Effective strain (time integration value of the effective strain rate along the flow line) ε = εdt (5) In the equations, V is the velocity of the press ram in mm/s, and X i is the distance in mm from the y-coordinate axis (X= ) of the ith flow line in the region where deformation does not occur. Fig. 4 force piston stroke curves 3 Results and Discussion 3.1 Load Figure 4 shows the extrusion force with the piston stroke curves. At the early stage, the graph increases rapidly due to the overwhelming of yield strength of the material from the elastic region to plastic region. For all the experimental conditions, the extrusion force reached a constant level during the extrusion process and that the extrusion process became a steady state condition at a piston stroke of more than 15 mm. At a steady state condition, the extrusion load decreases slowly as the piston stroke increases. This is because the friction gradually decreases as the surface contact between the tools and billet reduces. The extrusion force for billet extruded with 6 taper die (type C) give the lowest extrusion force compared to other types of taper die. While, the highest value of extrusion force is the extrusion with taper die (type A). It is because the contact angle of the taper die gives stress concentration at the die. Stress concentration will increase when the die half angle decrease and at the same time, the extrusion load would increase. 3.2 Surface Roughness The values of the arithmetic mean surface roughness, Ra, along the experimental surface of the billet were measured with a profilometer device. The measured is perpendicular to the extrusion. The experimental surface of the billet is the surface of the billet that contacts the taper die and the container. In the graph, the experimental surface of the billet is labeled as the X-axis. This analysis is used to determine the surface quality of the extruded billet. The distribution of the arithmetic mean surface roughness, Ra, is shown in Fig. 5. The surface roughness, Ra, of the billet that extruded with larger die half angle of taper die

5 Arab J Sci Eng (213) 38: Surface roughness Ra (micron) Taper die area Product area X-axis (mm) Undeform area X-axis (mm) Fig. 5 Surface roughness, Ra, of the experimental surface of billet () is lower than those extruded with smaller die half angle (). Surface quality of the billet extruded with taper die () produced poorest surface quality compared to other experimental conditions, as shown in Fig. 6. It is due to the wedge effect at the exit area of the taper die. For billet extruded with taper die (), it has greater wedge effect which mean lubricant was squeezed to become thin layer [11]. It increases the metal-to-metal contact and create rough billet surface. At the same time, it increased the extrusion load. For billet extruded with 6 taper die (), its wedge effect is lesser and the lubricant thickness remain along the die land reduce the metal-to-metal contact area between billet and taper die. This create billet surface with low surface roughness value Ra and decrease the extrusion load. Another reason is because the taper die () is less tapered, the metal flow of the billet during extrusion process was restrained. As a result, there are more frictional constraint caused by the metal-to-metal contact between tools and billet s surface. From this result, we could estimate that the friction factor for 6 taper die () is the lowest compared to other experimental conditions. The CCD picture shown in Fig. 6 is taken at 4mm point of the extruded part of billet. From observation, there are no severe wear found on the experimental surface of billet. 3.3 Metal Flow Observation Figure 7 shows the photographs of extruded billet for all the experimental conditions. All experimental conditions had a single peak lying close to the central region of the extruded billet indicates that the velocity in the central region is higher compared to the billet edge area, due to the frictional constraint that occurred between the billet and the taper dies. Billets that extruded with taper die type A and B show a Fig. 6 CCD pictures of billet surface of the extruded part ( 4 mm) narrow intense shear region along the edge (area mark as an A) in Fig. 7 [12]. It is because the shear deformation of billet which extruded with taper die type A and B was higher compared to taper die type C, due to the large turning angle (changes in metal flow ) at the taper die exit. The grid patterns cannot be observed clearly in the area marked as A for billet extruded with () and 3 taper die (). However, for billet which extruded with 6 taper die (), the vertical grid line was clearly observed. The hardness of the billet at the points 1, 2, and 3 were measured for each billet and plotted in Fig. 8. Points 1, 2 and 3 were located at the product side of billet where the deformation was finished. Point 1 and 3 is at the edge of the billet and Point 3 is at the center point of the billet s product area.

6 126 Arab J Sci Eng (213) 38: (a) Billet extruded with Taper die type A (º) Sliding velocity (mm/s) X-axis X-axis (mm) (mm) Fig. 9 Distribution of the relative sliding velocity along the experimental surface of the taper die X-axis (mm) (b) Billet extruded with Taper die type B (3º) Effective strain (c) Billet extruded with Taper die type C (6º) Fig. 7 Photographs of billet for each experimental condition X-axis (mm) Fig. 1 Effective strain distribution near the experimental surface of billet 62 Vickers hardness (Hv) points (point 1 and 3) were slightly higher compared to the hardness at the center point (point 2) of the billet. It is due to the increment of the effective strain near the experimental surface of billet as shown in Fig. 1. Billet extruded with taper die type C (6 ) has the highest Vickers hardness at the edge points, followed by taper die type B (3 ) and type A ( ). It is because the longer the die land lengths make the deformation occur more and harden the extruded material [13]. 48 Point 1 Point 2 Point 3 Location Fig. 8 Vickers Hardness (Hv) distribution at the product side of the billet For all the experimental condition, the hardness increased from 38 Hv (before the experiment) to around 52 6 Hv (after the experiment). The Vickers hardness at the edge 3.4 Relative Velocity Figure 9 shows the sliding velocity of the billet along the taper die during the extrusion process. The values of sliding velocity were divided with the ram speed to determine the relative sliding velocity. In the deformation area, range 6 mm, billet which extruded with the 6 taper die has the higher relative sliding velocity, while those extruded with

7 Arab J Sci Eng (213) 38: Fig. 11 Effective strain distributions for all the experimental conditions Taper die type A (a) Taper die type B (b) Taper die type C (c) taper die have the lowest relative sliding velocity. Previously, the 6 taper die shows lower extrusion force. This would decrease the tendency of lubricant to be squeezed out during the extrusion process. The existence of lubricant layer helps to reduce metal-to-metal contact and increase the velocity. At the same time, billet which extruded with 6 taper die has low shear stress because the material need less angle to change the flow. 3.5 Effective Strain The mutual comparison of the effective strain distribution near the experimental surface of billet was shown in Fig. 1. Billet extruded with Taper Die has the highest effective strain, due the longest die land lengths it has. In this research, the increment of the effective strain increases the hardness of the extruded material, a good agreement from the previous finding [14,15]. Figure 11 shows the effective strain distribution in the deformation area. From the figure, for all the experimental conditions, the distributions of effective strain were condensed at the taper die area. 4 Conclusion The influences of the die half angle of taper die during cold extrusion process were investigated with cold-work plane strain extrusion apparatus. Billet extruded with larger die half angle shows reduction in extrusion force, smaller surface roughness value, and increase of relative sliding velocity. For all experimental conditions, the effective strains were condensed at the taper die area (deformation area). Acknowledgments The authors would like to thank the Mechanical Engineering Faculty of Universiti Teknologi Malaysia for their cooperation during the preparation of this study. The authors would also want to thank the Ministry of Higher Education and Ministry of Science, Technology and Innovation of Malaysia for the financial support. References 1. Altan, T.; Soo-Ik, O.; Gegel, H.L.: Metal Forming Fundamentals and Applications. American Society for Metals, Metals Park (1983) 2. Brandt, D.A.; Warner, J.C.: Metallurgy Fundamentals. The Goodheart-Willcox Co. Inc., Tinley Park (25) 3. Lin, S.Y.; Lin, F.C.: Influences of the geometrical conditions of die and workpiece on the barrelling formation during forging-extrusion process. J. Mater. Process. Technol. 14, (23) 4. Giardini, C.; Ceretti, E.; Maccarini, G.: Formability in extrusion forging: the influence of die geometry and friction conditions. J. Mater. Process. Technol. 54, (1995) 5. Chun-Yin, W.; Yuan-Chuan, H.: The influence of die shape on the flow deformation of extrusion forging. J. Mater. Process. Technol. 124, (22) 6. Dyi-Cheng, C.; Sheng-Kai, S.; Cing-Hong, W.; Sin-Kai, L.: Investigation into cold extrusion of aluminum billets using three-dimensional finite element method. J. Mater. Process. Technol , (27) 7. Kalpakjian, S.; Nachtman, E.S.: Lubricants and Lubrication in Metalworking Operations. Marcel Dekker, Inc., New York (1985) 8. Syahrullail, S.; Nakanishi, K.; Kamitani, S.: Investigation of the effects of frictional constraint with application of palm olein oil lubricant and paraffin mineral oil lubricant on plastic deformation by plane strain extrusion. J. Jpn. Soc. Tribol. 5(12), (25) 9. Nakanishi, K.; Okamura, S.; Nakamura, T.: Analysis of axisymmetric hot extrusion of aluminum by the visioplastcity method combined with the numerical calculation method for prediciting the flow curves. J. Jpn. Soc. Technol. Plasticity 18(23), (1977) 1. Shabaik, A.; Kobayashi, S.; Thomsen, S.E.: Application of potential flow theory to plane-strain extrusion. J. Eng. Indus. 89, (1967) 11. Kataoka, S.: Tribology in Press Forming. Japan Metal Press Association, Tokyo (22) 12. Onuh, S.O.; Ekoja, M.; Adeyemi, M.B.: Effects of die geometry and extrusion speed on the cold extrusion of aluminium and lead alloys. J. Mater. Process. Technol. 132, (23) 13. Ajiboye, J.S.; Adeyemi, M.B.: Effects of Die Land on the Cold of Lead Alloy, Journal of Materials Processing Technology 171, (26) 14. Fazil, O.S.; Ahmet, D.: Analytical relations between hardness and strain for cold formed parts. J. Mater. Process. Technol. 186, (27) 15. Petrosa, J.; Janiceps, L.: On the evaluation of strain inhomogeneity by hardness measurement of formed products. J. Mater. Process. Technol , 3 35 (23)