Investigation of Experimental and Numerical Analysis on Extrusion Process of Magnesium Alloy Fin Structural Parts

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1 Investigation of Experimental and Numerical Analysis on Extrusion Process of Magnesium Alloy Fin Structural Parts Su-Hai Hsiang, Yi-Wei Lin, and Wen-Hao Chien Department of Mechanical Engineering National Taiwan University of Science and Technology 43 Keelung Road., Sec.4, Taipei, 106, Taiwan, R.O.C. Abstract: - The paper mainly studies the experimental and numerical analysis on the hot extrusion process of AZ31 and AZ61 magnesium alloy fin structural parts with extrusion ratio of For the extruded product, the cross-sectional diameter of its central cylinder is 20mm, and there are 6 fins being 2mm thick each, with an interval of 60 degrees between fins. The study explores the effects of different billet heating temperatures ( ) and extrusion speeds (2-6mm/s) on the forming load and the appearance of finished product. The extruded finished product takes hardness test, and compressive strength test at its fin plate so as to understand the effects of process parameters on its mechanical properties. Besides, the finite element analysis software, DEFORM-3D is used to carry out simulation analysis of extrusion so as to understand the billet deformation behavior, metal flow and speed distribution in the forming process. Finally, the study carries out comparative analysis on the simulative and experimental results in order to prove the accuracy of the simulation analysis. Key-Words: - magnesium alloy,az31,az61, fin structural part, hot extrusion process, finite element simulation 1 Introduction Magnesium alloy has the characteristics of high strength, light weight, energy saving and vibration reduction. Besides, it meets the requirements of environmental protection and recycling for reuse, thus making magnesium alloy become an extremely useful metallic material in different industries. The application scope of magnesium aloy covers people s livelihood, and industries of vehicles, bicycles, aerospace, national defense and 3C electronics. Magnesium alloy has a hexagonal closed-packed (HCP) crystal structure. Since it lacks the slip system that is required by plastic deformation, it has poor elongation properties, and so cannot be easily formed during cold fabrication. The melting point of magnesium alloy is 650 o C, making it suitable for die-casting, semisolid forming and hot fabrication at temperatures of between 300 o C and 400 o C. However, the dominant magnesium alloy forming technology today is die-casting. Moreno et al. [1] explored the microstructural features of MEZ (Mg-2.5RE-0.35Zn-0.3Mn) magnesium alloy after die casting, and found that there are dendrites concentrated in the grain boundary. Huang et al. [2] made AM60B magnesium alloy test rod by die casting. The mechanical properties of die casting magnesium alloy articles are not very favorable. However, the plastic forming and fabrication of magnesium alloy by rolling, forging and extrusion, markedly improved the mechanical properties, quality and conformance rate of the magnesium alloy products. Kim et al. [3] studied the asymmetrical rolling process of Mg-Al-Zn alloy plate, which had the highest yield strength exceeding 300 MPa after rolling, and the greatest elongation rate of 35%. Ho et al. [4] studied the hot forging process of magnesium alloy flanges, and concluded that the optimal forming temperature was between 270 and 320. Hsiang and Lin [5] investigated the influence of process parameters on hot extrusion of AZ31 and AZ61magnesium alloy tubes, and found that strengths of the effects of the factors on the tensile strength of the tube follow the order, heating temperature of billet, initial speed of extrusion, type of lubricant and container temperature. Hsiang et al. [6] applied the artificial neural network (ANN) to study hot extrusion of AZ61 magnesium alloy structural parts, derived that the tensile strength of the structural parts decreases as the heating temperature of the billet increases, and decreases as the extrusion speed increases. Hu et al. [7] employed the recycled AZ91 magnesium alloy under extrusion ratio of 40:1 showed high tensile strength of MPa and higher elongation to failure of 11.32%, compared with those of the cast specimen. Chen et al. [8] investigated the cold extrusion of aluminum billets using three-dimension finite element method(fem). This study uses finite element analysis software, DEFORM-3D to carry out numerical analysis of the extrusion of AZ31 magnesium alloy fin structural parts so as to understand the billet deformation behavior, metal flow and speed distribution in the forming process. The simulative result of extrusion is then compared with the experimental result in order to prove the accuracy of the simulation analysis. Finally, the extruded finished product takes hardness test, and compressive strength test ISBN:

2 at its fin plate so as to understand the effects of process parameters on its mechanical properties. 2 Experimental Scheme 2.1 Extrusion process and billet The forming machine in this study is a 500-ton hotextrusion machine. The pre-extrusion operations include preheating the billet container for 5 hours, applying a lubricant onto the billets and dies, and heating of them in a furnace for 3 hours. Figure 1 shows a schematic diagram of the hot-extrusion process. The raw materials are two magnesium alloys (AZ31 and AZ61). The extrusion billet size is ψ80 mm length 100 mm. A right-angled die, with extrusion ratio of fin structure parts being 8.03, is used for extrusion. The axle center diameter of the finished product is 20mm; and there are 6 fins being 2mm thick each, with an interval of 60 degrees between fins. The cross-sectional dimensions of the extruded finished product are shown in Fig. 2. Fig. 1 schema of hot-extrusion process 2.2 Setting of experimental parameters During the hot extrusion process, billet heating temperature and extrusion speed are the main factors affecting the mechanical properties and extrusion load of the extruded finished product. Therefore, the study explores and analyzes the effects on the formability and mechanical properties of AZ31 and AZ61 magnesium alloy fin structural parts when the billet heating temperature is and the extrusion speed is 2-6mm/s in the extrusion process. The preset experimental process parameters are shown in Table 1. Fig. 2 cross-sectional area of fin structural part 2.3 Test of mechanical properties The compression resistance test made by the study is to know the load borne by the fin plate of AZ31 and AZ61 magnesium alloy fin structural parts. Nevertheless, focusing on the finished product there are not relevant, the study simply cuts a length of 50mm from the finished product as the samplejust referring to the CNS (JIS H4090) standard of tube flattening test - and lets it be clipped between two plates of a 30-ton testing machine. The highest load after flattening is recorded, with the test device shown in Fig. 3, so as to know the strength borne by the fin plate of the fin structural parts. Table 1 Experimental setting of process parameters Experiment number Billet heating temp.( ) Extrusion speed (mm/s) Container temp. ( ) Lubricant 360 Graphite Fig. 3 experimental device of compressive test ISBN:

3 3 Experimental Results and Discussion The study mainly changes two process parameters, billet heating temperature and extrusion speed; and then explores their effects on the mechanical properties of fin structural parts. The finished product acquired from the experiment takes compressive strength test and hardness test. The finite element analysis software, DEFORM-3D is used to carry out simulation analysis of the extrusion of fin structural parts so as to understand the billet deformation behavior, metal flow, billet temperature change, and stress and strain distribution in the forming process. After that, comparative analysis is made between the simulative result and experimental result of extrusion. experiment is shown in Fig.5. As known from Fig. 5, the extrusion load of AZ31 magnesium alloy is slightly lower than AZ61 magnesium alloy. For the same billet, greatest extrusion load can be achieved at the extrusion speed of 2mm/s; and smallest extrusion load can be achieved at the extrusion speed of 6mm/s. This is because as the extrusion speed is faster, the deformation heat is higher, thus leading to lower deformation load. At the same time, it can easily make forming unstable, and cause damage to the finished product. With the same billet heating temperature, the fluidity of AZ31 magnesium alloy is better than AZ61 magnesium alloy. Therefore, when both are at the same extrusion speed, the extrusion load of AZ31 magnesium alloy is lower. 3.1 Exploration of extrusion forming load of fin structural parts Effects of different billet heating temperatures on extrusion load Using the 1st to 4th groups of experimental parameters in Table 1, AZ31 and AZ61 magnesium alloys undergo hot extrusion process of fin structural parts. The billet heating temperatures are 300, 320, 340, and 360, and the extrusion speed is all along the same, 2mm/s. The forming load acquired from the experiment is shown in Fig. 4. As known from Fig. 4, in the extrusion process of AZ31 and AZ61 magnesium alloy fin structural parts, as the billet heating temperature rises, the extrusion load falls accordingly. Nevertheless, at the same extrusion speed, the fluidity of AZ31 magnesium alloy is better than AZ61 magnesium alloy. Therefore, when both of them are at the same extrusion temperature, the extrusion load of AZ31 magnesium alloy is lower than AZ61 magnesium alloy. And the trends of forming load caused by the change of heating temperatures of these two billets are the same Effects of different extrusion speeds on extrusion load When exploring the effects of extrusion speed on extrusion load, the study carries out hot extrusion process of fin structural parts by using the 2nd, 5th, and 6th groups of experimental parameters in Table 1. The extrusion speeds are 2, 4, and 6 mm/s respectively, and the billet heating temperature is 320. The forming load acquired from the Fig. 5 Fig. 4 extrusion load under different heating temperatures extrusion load under different extrusion speeds 3.2 Compressive strength analysis of fin structural part Experiments are carried out by using the experimental parameters in Table 1. The effects of different billet temperatures and different extrusion speeds on the compressive strength of fin plate are shown in Fig. 6 and Fig. 7 respectively. As known from Fig. 6, AZ31 magnesium alloy fin plate has compressive strength available to bear 2695N minimum and 4165N maximum, whereas AZ61 magnesium alloy fin plate has compressive strength available to bear N. When the billet ISBN:

4 heating temperature is 300, the compressive strength of fin plate is highest; and when it is 360, the compressive strength of fin plate is lowest. As the billet heating temperature rises, the compressive strength of fin plate falls. As known from Fig. 7, the compressive strength ranges of AZ31 and AZ61 magnesium alloy are N and N respectively. When the extrusion speed is 2mm/s, the compressive strength of fin plate is highest; and when it is 6mm/s, the compressive strength of fin plate is lowest. It implies that as the extrusion speed increases, the compressive strength of fin plate would decrease. As observed from the above analytic discussion, we can understand that lower billet heating temperature and extrusion speed can be selected to acquire magnesium alloy fin structural parts with higher compressive strength at the fin plate. As clearly shown in Fig. 8, the hardness value of AZ61 fin structural part is higher than AZ31 fin structural part because AZ61 has more aluminum than AZ31 by 3% wt., and has higher resistance to plastic deformation. Therefore, its hardness rises with the increase of aluminum content. Before extrusion process is performed, the Vickers hardness values (H V ) of AZ31 and AZ61 billets are 56 and 62 respectively. When these values are compared with the hardness values shown in Fig. 8, it is known that after the extrusion billets have been extruded to be products, their hardness would be obviously enhanced. AZ61, MT:320 and V:2mm/s AZ31, MT:320 and V:2mm/s Fig. 6 compressive strength of product under different heating temperatures AZ61, MT:320 and V:2mm/s AZ61, MT:360 and V:2mm/s AZ61, MT:320 and V:2mm/s AZ61, MT:320 and V:6mm/s Fig. 8 hardness distribution at different extrusion speeds Fig. 7 compressive strength of product under different extrusion speeds 3.3 Exploration of hardness test of fin structural part The hardness test analysis made by the study is expressed in Vickers hardness value (H V ). The hardness value at the central point on the crosssection of the finished product and the average hardness value of the corresponding point of each fin plate on the 4 concentric circles are taken as the hardness test points. Regarding the effects of billet heating temperature on hardness, taking AZ61 magnesium alloy for example, when the hardness at billet heating temperature 320 is compared with the hardness at billet heating temperature 360, it is observed that the process with higher billet heating temperature can produce finished products with lower hardness. As known from this, the rise of billet heating temperature can soften the billet, leading to the decline of required resistance to plastic deformation. Besides, regarding the effects of extrusion speed on hardness, when comparison of hardness distribution is made between two extrusion speeds of AZ61 magnesium alloy, 2mm/s and ISBN:

5 6mm/s, it is known that higher extrusion speed would produce finished products with lower hardness. This is because higher extrusion speed would increase deformation heat, which not only has softening effect on billet, but also increases instability to the forming of finished product. As a result, the finished product would be easily damaged, and its surface would be easily cracked. As known from this, the increase of extrusion speed would reduce the hardness of finished product. 3.4 Exploration of the free end shape of nonsteady finished product This section compares the simulation and experiment of billet deformation of the free end shape of the extruded finished product in order to explore the accuracy of the simulation of non-steady extrusion. Since the local area of the fin structural part change in shape severely, the metal flow rates of various parts during forming are different. The forming resistance borne by the axle center is smaller, so that the flow rate is faster. However, the deformation at the fin plate and flange is greater, creating greater resistance and slowing down the metal flow. Therefore, the speed difference will create a distance between the free end s axle center and the forming length of fin plate of the extruded finished product. The experimental result is shown in Fig. 9. In the figure, the red outline is the simulated appearance, and the blue outline is the appearance acquired from the experiment. There is a distance created between the free ends in the experiment and simulation of different processes of AZ31 magnesium alloy. As shown in Table 2 and Fig. 10, the value in the table is just the difference between the front end of axial cylinder and the outer flange end of fin plate. As known from the table, when the fin structural part is at the same extrusion speed of 2mm/s, and the billet temperatures are changed to be different, the degree of distance between the ends of the finished product is also different. When the billet heating temperature is 300, the distance between the ends of the finished product is the least. The higher the temperature, the greater the distance faster. As the fin plate and the flange are obstructed, the flow is slower, leading to greater distance caused. Besides, when the billet heating temperature is 320, with extrusion speed changed to be 6mm/s, the distance between the ends of the finished product is the least. As the extrusion speed is reduced, the distance between the ends of the finished product is greater. As observed from the front view diagram, the area of the central cylinder is greater than the single fin plate. Hence, the metal flow is not very much restricted, and the strain is less. Since its metal flow is faster, extrusion forming can be more easily undergone, and cracking or damage would not be easily caused. Therefore, the cross-sectional area of each part is different, which is the main reason for the protruding of the free end. Although the free end has the problem of uneven flow rate, bending basically would not be caused to the extruded finished product because the finished product is a completely symmetrical structure. Fig. 9 (a) Experiment (b) Simulation outline of experimental and simulative free end 300 &2mm/s 320 &2mm/s 340 &2mm/s 2mm/s&320 4mm/s&320 6mm/s&320 Red outline: Experiment result Fig. 10 Blue outline: Simulation result comparison of shapes between simulative and experimental free ends (AZ31) Table 2 Comparison of various distances between free ends (AZ31) Billet heating temp. ( ) (V:2 mm/s) Extrusion speed (mm/s) (BHT:320 ) Experiment Simulation ISBN:

6 4 Conclusion Focusing on the hot extrusion process of AZ31 and AZ61 magnesium alloy fin structural parts, the study explores the effects of different billet heating temperatures and different extrusion speeds on the formability and the mechanical properties of finished product. The finite element software, DEFORM-3D, is used to carry out simulation analysis of extrusion. Finally, the study carries out comparative analysis on the experimental and simulative results in order to prove the accuracy of the simulation of non-steady extrusion. The conclusions drawn are as follows. 1. Within the forming temperature range of hot extrusion of magnesium alloy fin structural parts, lower billet heating temperature can be selected to acquire higher compressive strength for the fin plates. Under the same process conditions, the compressive strength of AZ61 magnesium alloy are both higher than AZ31 magnesium alloy. 2. Within the speed range available for extrusion of magnesium alloy fin structural parts by the equipments, lower extrusion speed can be selected to acquire higher compressive strength for the fin plates. Under the same process conditions, the compressive strengths of AZ61 magnesium alloy are both higher than AZ31 magnesium alloy. 3. Under the same process conditions, higher billet heating temperature or faster extrusion speed would reduce the hardness of billet. 4. The DEFORM is used to carry out simulation analysis of extrusion of magnesium alloy fin structural part. Its axial cylindrical part is more protruding because this part cannot be easily obstructed, making the metal flow here faster. As seen from the simulated speed distribution, the flow of billet goes towards the axle center. However, since the cross-section of parts is designed to be a completely symmetrical shape, no bending would be caused by uneven flow rate. earth alloy, Scripta Materialia, vol.45, 2001, pp [2] Z. Zhang, R. Tremblay and D. Dubé, Microstructure and mechanical properties of ZA104 ( Ca) die-casting magnesium alloys, Materials Science and Engineering A, vol.385, 2004, pp [3] W.J. Kim, J.B. Lee, W.Y. Kim, H.T. Jeong and H.G. Jeong, Microstructure and mechanical properties of Mg Al Zn alloy sheets severely deformed by asymmetrical rolling, Scripta Materialia, vol.56, 2007, pp [4] H.L. Ho, S.H. Hsiang and Z.Y. Huang, Investigation of the formability of flanged parts of magnesium alloy under hot forging process, in The 11 th International Conference on Advances in Materials and Processing Technologies, 2008, AMPT-BF [5] S. H. Hsiang and Y.W. Lin, Investigation of the influence of process parameters on hot extrusion of magnesium alloy tubes, Journal of Materials Processing Technology, vol , 2007, pp [6] S. H. Hsiang, Y.W. Lin and T.C. Lee, Application of artificial neural network (ANN) to hot extrusion of AZ61 magnesium alloy structural parts, Journal of the Chinese Society of Mechanical Engineers, vol.31, 2010, No.1, pp [7] M.L. Hu, Z.S. Ji and X.Y. Chen, Effect of extrusion ratio on microstructure and mechanical properties of AZ91D magnesium alloy recycled from scraps by hot extrusion, Transactions of Nonferrous Metals Society of China, vol.20, 2010, pp [8] D.C. Chen, S.K. Syu, C.H. Wu and S.K. Lin, Investigation into cold extrusion of aluminum billets using three-dimension finite element method, Journal of Materials Processing Technology, vol , 2007, pp Acknowledgment The authors would like to thank the National Science Council, Taiwan, R.O.C., for financially supporting this research under contract No. NSC E MY2. References: [1] I.P. Moreno, T.K. Nandy, J.W. Jones, J.E. Allison and T.M. Pollock, Microstructural characterization of a die-cast magnesium-rare ISBN: