Relationship between extrusion characteristics and extrusion variables of wrought aluminum alloy
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https://www.eduzhai.net American Journal of M aterials Science 2013, 3(4): 77-83 DOI: 10.5923/j.materials.20130304.03 Extrusion Characteristics Dependence of Wrought Aluminium Alloy on Extrusion Variables S. O. Adeosun1, Akpan E. I2,*., Gbe nebor O. P.1 1Department of M etallurgical and M aterials Engineering, University of Lagos, Lagos, Nigeria 2Department of M aterials and Production Engineering, Ambrose Alli University, Ekpoma Abstract Effect of in itial temperature and state of wrought alu minu m, die angle and d ie material on the ext rusion characteristics of wrought alu min iu malloy has been studied. Ext rusion dies were constructed with die angles ranging fro m 15 to 90o using both tool steel and mild steel materials. Co ld, hot and annealed billets extrusion responses were investigated by measuring the e xtrusion pressure, linear strain, e xtrusion ratio and surface hardness of e xtrudes. Results show that tool steel die provides superior extrusion response to mild steel die material. The highest linear strain (202 %) was obtained for samp les cold e xtruded using tool steel die with a 90o die angle followed annealed e xt ruded (161 %) sample using tool steel die at the same die angle. Cold extrusion was observed to be easier at higher d ie angles than at lower die angles. Hardness result also indicates that structural change due to severe plastic defo rmation during ext rusion was effective for the improvement of h ard n es s . Keywords Extrusion Pressures, Die Material, Die Angle, Plastic Deformation and Annealed 1. Introduction Extrusion is one of the most important metals forming processes due to its high productivity, lower cost and increased physical properties. Ext rusion of aluminiu m alloys offers a relatively cheap method of p roducing complex shapes in long lengths with high geo metric tolerances. The flexib ility o f the process with respect to alloys that can be extruded and the possible shapes has resulted in widespread use of alu min iu m extrusions in the society today. Aluminiu m ext rusions are used in the building industry (window and door frames, building structures, roofing, curtain walling, etc.), shipping and offshore industry, furniture, and in automotive, aerospace applications and rail v eh icles . There has been considerable interest in the investigation of the effects of d ie geo metry and other extrusion parameters on t h e s t ru ct u re, flo w p at t ern , e xt ru s io n p ress u re and mechan ical p ro pert ies o f shap ed sect io ns[2-3]. Sev ere Plastic Defo rmation (SPD) processes have been receiving increasing attention as one method to develop fine-g rained microstructures. Determination of the required load is one of the important objectives in metal forming studies. When the necessary force for performing the operation is min imized, the capacity of the forming machine and the relevant costs * Corresponding author: email@example.com (Akpan E. I) Published online at https://www.eduzhai.net Copyright © 2013 Scientific & Academic Publishing. All Rights Reserved can be reduced and the die life pro longed. Geo metry and profile of an extrusion d ie constitute very important aspect of die design and determines the extent of redundant work done during the deformat ion. Although redundant work cannot be totally avoided in ext rusion process, it can be min imized by choosing a die profile that min imizes the extrusion pressure . The inﬂuence of die half-angle and reduction ratio on extrusion force has been studied for the hydrostatic ext rusion process and the data obtained can be used for the forward conventional ext rusion process. Experimental investigations have been made to achieve the effect of die reduction ratio, die angle and loading rate on the quality of cold ext ruded parts, extrusion pressures and flow patterns for both lead and alu minu m. Prev ious research has shown that extrusion die geo metry, frict ional conditions at the die billet interface and thermal g radients within the billet greatly influence metal flow in cold ext rusion. Another investigation reported the effects of die geometry and other extrusion parameter on the structure, flow pattern, ext rusion pressure and mechanical p roperties of shaped extrusion. In addition, 300, 450 and 600 die angles has been used to study the effect of die angles on the surface fin ish and hardness of cold ext rusion of alu minu m AA 6351 and AA1100. These and many other works has presented the need to investigate fully the characteristics of extrusion at various die angles and their effect on emerging properties. On the other hand the interaction between the die and surrounding plays a major role in initiat ion of soldering, micro -cracking, crack propagation and catastrophic failure, 78 S. O. Adeosun et al.: Extrusion Characteristics Dependence of Wrought Aluminium Alloy on Extrusion Variables especially in high temperature working environ ment which results in premature failure such as heat and gross cracking, erosive wear, solder and corrosion. To serve such working environ ment, the selection of material becomes crucial. The chosen material must be able to withstand high mechanica l and therma l loading, good resistance to wear and failure. Bjork et al. states that the wear resistance of extrusion die play a major ro le in terms of technological and economical aspect because it affects both the surface fin ish and dimension of the ext rudate. Tool steel is the material widely used to serve such working environ ment and signifies a family of highly alloyed steels that can be hardened and tempered with guaranteed hardening characteristic to provide improved strength and wear resistance. However, mild steel which is cheap and affordable has not been investigated as the working environ ment in relat ion to the conventional tool steel die materia l. In this study the authors investigate the effect of extrusion die angle, ext rusion conditions (cold, hot and annealed) and die material on the ext rusion characteristics (ext rusion pressure, extrusion ratio, surface hardness of extrudates and linear strain) of wrought aluminiu m alloy using experiments designed by the authors. The major objective of this research is to experimentally investigate how changes in die angle and die materia l affect e xt rusion force, e xtrusion ratio and linear strain. Results are purely experimental and does not depend on any model hence we will only present results as they were acquired fro m the experiments. Extrusion dies were constructed with 15, 30, 45, 60, 75, and 90o using both tool steel and mild steel materials for the study. Some b illets were extruded cold while some were extruded hot and some annealed before extrusion. 2. Experimental methods Materials Table 1. Chemical composition of wrought aluminium Element Composition % Element Composition % Al 98.94 Ti 0.015 Fe 0.682 Mg 0.002 Si 0.132 Pb 0.007 21.5mm Cu 0.046 Sn 0.005 Zn 0.057 19.5mm 26.5mm Fi gure 1. Sket ch of billet s prepared for extrusion Wrought aluminiu m alloy billets of co mposition shown in Table 1 were obtained fro m Alu miniu m Rolling Mills Ota, Ogun State Nigeria for the study. Billets of the received alloy were melted and cast into cylindrical shapes of 3 by 30 mm in a sand mould and left to solidify. Cast samples were removed and machined to have a chamfer at the edge to allo w entrance into the extrusion orifice (see Figure 1) Die Design Dies (see Figure 2) with entry angles of 15, 30, 45, 60, 75, and 90 degrees were designed with annealed mild and normalized tool steel materials of compositions shown in Table 2. All mild steel d ies were heated in a furnace to 850℃, held for three hours and allowed to cool in the furnace. All tool steel dies were heated to 750℃, held for three hours and cooled in air. Table 2. Chemical composition of tool and mild steel die Element Mild St eel Tool St eel Element Mild St eel Tool St eel C 0.1195 0.1984 Cr 0.043 0.0055 Si 0.2887 0.4398 Mo 0.0052 0.0046 S 0.0097 0.0101 V 0.0065 0.041 P 0.0099 0.009 Cu 0.0312 0.012 Mn 0.5030 1.3888 Fe 98.9626 97.8735 Ni 0.0207 0.0173 Figure 2. Sketch of the die design Extrusion Process An Avery Dension machine was adapted to exert compressive load on the ram for the extrusion process. The die was fixed into the form tool with a die holder fastened at the edge with three Allen screws (see Figure 3) and the billet inserted through the upper part of the assembly. The extrusion load was read d irectly fro m the machine as the load was applied. A strain gauge was attached to the ram to read the strain rate. Shea butter oil was used as the lubricant for the extrusion. Three types of ext rusion were made including; 1. Hot extrusion 2. Co ld extrusion 3. Annealed ext rusion In hot extrusion the billets were heated in a Muffle reheating furnace to 350℃and transferred to the extrusion assembly for extrusion. In annealed extrusion the billets were heated to 350℃ and allowed to cool in the furnace before transferring it fo r ext rusion. Co ld ext ruded billets were used in as-cast condition at ambient temperature (32℃). A Leco micro hardness tester LM 700AT mach ine was used for the Roc kwe ll hardness test of extruded samples. American Journal of M aterials Science 2013, 3(4): 77-83 79 region whereas sharp rise indicate a turbulent ext rusion progression. At low die angles (15, 30, and 45o) there is ease of extrusion at the beginning of the extrusion process but turbulent extrusion at the end of the ram travel and vice versa for h igh die angles (60, 75, and 90o ). The highest ext rusion pressure (543.9 MPa) occurs when extrusion was done in 75o die fo llo wed by that of 15o die (489.5 MPa) d ifferent fro m that done with tool steel die (see Figure 4). Figure 3. Sketch of the extrusion assembly 3. Results and Discussion Effect of die angle on col d extrusion characteristics The extrusion behaviour of wrought alumin iu m alloys cold ext ruded at different d ie angles is shown in Figure 4. Extrusion in 15o die shows a steady increase in ext rusion pressure with ram d isplacement throughout the ext rusion process to maximu m of 480 MPa before remaining constant. The 15o die shows behaviour different fro m the others indicating the difficu lty in extrusion at the onset of the extrusion process but afterwards became normal after a ram travel of about 19 mm. Th is behaviour was earlier noted by Akbar and Yaseen for AA2014 and AA1015 alloys and may be attributed to the fact that s mall die angle give rise to a dead metal zone formation and process of adhesion with the die wall leading to a higher ram load. Extrusion at die angles of 30, 45, 60, 75, and 90o show sharp rise at the beginning of the ram travel to a peak value and remain steady afterwards. The highest extrusion pressure is seen in 30o die extrudes (499 MPa) whereas that of 45o has the lowest ext rusion pressure (403 MPa) with a low final ram travel (13mm). Experimental results of Onuh et al on the variation of extrusion load with ram d isplacement of alu min iu m and lead show similar behaviour. Figure 5 shows the variation of extrusion pressure with ram displacement of wrought alumin iu m alloy cold ext ruded in mild steel tool dies. Ext rusion in 15, 30, and 45o dies show similar behaviour. The extrusion pressure rose gradually with ram d isplacement in itially (between 1 and 13 mm) but sharply afterwards. For extrusions done in 60, 75, and 90o dies, the pressure rises sharply at start of ram travel but gradually towards the end of the ram travel. A gradual rise in extrusion pressure indicates ease of extrusion within that Extrusion Pressure (MPa) Tool Steel Die 600 500 400 300 200 100 0 1 4 7 10 13 16 19 Ram Displacement (mm) 15 degree 30 degree 45 degree 60 degree 75 degree 90 degree Figure 4. Variation of Extrusion pressure with Ram displacement for cold extrusion in tool steel die Extrusion Pressure (MPa) Mild Steel Die 600 500 400 300 200 100 0 1 4 7 10 13 16 19 Ram Displacement (mm) 15 degree 30 degree 45 degree 60 degree 75 degree 90 degree Figure 5. Variation of Extrusion pressure with Ram displacement for cold extrusion in mild steel die Effect of die angle on hot extrusion characteristics Hot extruded samples show lower ext rusion pressures and low final ram t ravels relative to that of co ld ext ruded samples (see Figure 6 and 7). For ext rusions with tool steel die (see Figure 6) all extrudes have the same behaviour except for that extruded in 15o die. The ext rusion pressure rises steadily to a peak value (407.9 MPa) lower than others with a low final ram displacement of 12 mm. The h ighest extrusion pressure is seen in 60 o and 75 o (453.2 MPa) dies, however that in 75 o die show a preferred final ram d isplacement (17 mm). Extrusions with mild steel die show similar behaviour with that of tool steel die (see Figure 7). 80 S. O. Adeosun et al.: Extrusion Characteristics Dependence of Wrought Aluminium Alloy on Extrusion Variables Extrusion Pressure (MPa) Tool Steel Die 500 400 300 200 100 0 1 4 7 10 13 16 Ram Displacement (mm) 15 degree 30 degree 45 degree 60 degree 75 degree 90 degree Figure 6. Variation of Extrusion pressure with Ram displacement for hot extrusion in tool steel die Extrusion Pressure (MPa) Mild Steel Die 500 400 300 200 100 0 1 4 7 10 13 16 19 Ram Displacement (mm) 15 degree 30 degree 45 degree 60 degree 75 degree 90 degree pressures were lower. In the case of mild steel d ie (see Figure 9), extrusions done at die angles of 15 o and 45 o show similar behaviour but different fro m others. Tool Steel Die 600 Extrusion Pressure (MPa) 500 400 300 200 100 0 1 4 7 10 13 16 19 Ram Displacement (mm) 15 degree 30 degree 45 degree 60 degree 75 degree 90 degree Figure 8. Variation of Extrusion pressure with Ram displacement for annealed extrusion in tool steel die Figure 7. Variation of Extrusion pressure with Ram displacement for hot extrusion in mild steel die Effect of die angle on annealed extrudes Extrusion of annealed samples using tool steel die show that extrusion proceeds with ease for all samples except for samples extruded in 15 o die (see Figure 8). Ext rusion characteristics are the same for all these samples with maximu m ext rusion pressure attained for extrusion in 15 o tool steel die (527 MPa) and 90o mild steel d ie (496 MPa). Final ram displacements were low as compared to those of cold and hot extrudes and the maximu m attained ext rusion Figure 9. Variation of Extrusion pressure with Ram displacement for annealed extrusion in mild steel die Maximum Extrusion Pressure (MPa) 600 500 400 300 200 100 0 15 30 45 60 75 90 Die Angle in Degree Tool Steel Die (cold extruded) Mild Steel Die (Cold Extruded) Tool Steel Die (Hot extruded) Mild Steel Die (Hot Extruded) Tool Steel Die (Annealed extruded) Mild Steel Die (Annealed Extruded) Figure 10. Variation of Maximum Extrusion Pressure with Die Angle American Journal of M aterials Science 2013, 3(4): 77-83 81 Extrusion Ratio 4 3.5 3 2.5 2 1.5 1 0.5 0 15 30 45 60 75 90 Die Angle in Degree Tool Steel Die (cold extruded) Mild Steel Die (Cold Extruded) Tool Steel Die (Hot extruded) Mild Steel Die (Hot Extruded) Tool Steel Die (Annealed extruded) Mild Steel Die (Annealed Extruded) Figure 11. Variation of Extrusion Ratio with Die Angle The variation of maximu m ext rusion pressure with die angles is shown in Figure 10. Reduce extrusion pressure occurred at 90o die angle, independent of the initial state of billet. This is consistent with the result of Onuh et al study on the effect of die angle on the ext rusion characteristics where extrusion pressure decreases to a min imu m ext rusion pressure at 90o die angle. Extrusion pressure at this die angle may be due to the least redundancy energy at this particular die angle. Low die angle means large surface area, leading to increased friction at die-b illet interface resulting in larger ram force but at Large die angle there is small surface area leading to decreased friction at the die -billet interface resulting in lo wer ram force requirement. Effect of die angle on the extrusion rati o The ext rusion ratio of a shape is a clear indication of the amount of mechanica l working that will occur as the shape is extruded. The effective strain is a function of the ext rusion ratio while the ext rusion pressure is a function of the strain. The variation of ext rusion ratio with die angle is shown in Figure 11. Ext rusion ratio for co ld ext rusion using tool steel die increased initially to 2.92 at 30o die angle and remains constant at 45, 60, 75o die angles but later increased to a maximu m (3.34) at 90o die angle. Ext rusion of hot and annealed billets using tool steel die show similar behaviour to that of as-cast extrudes. These indicate that large ext rusion ratio corresponds to low ext rusion pressure and high linear strain for tool steel die (see Figures 10 and 12). Extrusions using mild steel die show the same extrusion ratio (2.58) at 90o which is lower than those at other die angles. The lowest extrusion ratio (2.43) is shown by hot ext rudes in 15o tool steel die. Ext rusion pressures for mild steel 90o die are high (see Figure 10) co mpared to others which substantiates the fact that low extrusion ratio leads to lower linear strain and consequently high ext rusion pressure. Effect of die angle and tool type on deformati on Figure 12 shows the variation o f linear strain with e xtrusion die angle for all the e xtrudes. Linear strain tends to increase with increase in d ie angle. The highest linear strain (202 %) was obtained for samples cold extruded using tool steel 90o die fo llo wed by annealed extruded (161 %) samp le using tool steel in same die. Samp les ext ruded with tool steel die have superior linear strain co mpared to that extruded with mild steel die. It is also observed that as-cast billet extrusion at ambient temperature favours high linear strain. This response is followed by annealed ext ruded but least in hot extruded. The maximu m ext rusion pressures (406, 426, 399 MPa for cold, annealed and hot extrudes respectively) at this die angle are lower than that of all other samples, showing that there is ease of flow of materia l at 90o die angle compared to other die angles used in this study (see Figures 4, 6 and 8). Effect of die angle and tool type on micro hardness Figure 13 shows the hardness (HV) variat ion with die angle and type of tool material. It is evident that surface hardness of extrudes does not follow a well-defined trend but shows a zigzag variation with increase in die angle. The highest hardness (61.2 HV) occurs in cold extruded samples using 75o tool steel die. It is a lso clear that samples e xt ruded with tool steel die exh ibit superior hardness compared to those extruded in mild steel die. The lo west hardness value (37.3 HV) occurred in the samp le ext ruded in mild steel d ie at 30o die angle. This result indicates that structural change due to severe plastic deformation during extrusion was effective fo r the imp rovement of hardness. 82 S. O. Adeosun et al.: Extrusion Characteristics Dependence of Wrought Aluminium Alloy on Extrusion Variables Linear Strain (%) 250 200 150 100 50 0 15 30 45 60 75 90 Die Angle in Degree Tool Steel Die (cold extruded) Mild Steel Die (Cold Extruded) Tool Steel Die (Hot extruded) Mild Steel Die (Hot Extruded) Tool Steel Die (Annealed extruded) Mild Steel Die (Annealed Extruded) Figure 12. Variation of Linear Strain with Die Angle 70 Tool Steel Die (cold extruded) 60 Mild Steel Die (Cold Extruded) 50 Tool Steel Die 40 (Hot extruded) Mild Steel Die 30 (Hot Extruded) Tool Steel Die 20 (Annealed extruded) 10 0 15 30 45 60 75 90 Die Angle in Degree H V Figure 13. Variation of Micro-Hardness of extrude with Die Angle 4. Conclusions In this experiment, the effect of die angle has been studied to determine the effect of die angle and tool material on the extrusion characteristics of wrought alu miniu m alloys using cold, hot and annealed billets. The following can be deduced fro m the study; 1. Ext rusion pressure is dependent on extrusion die angle, whereas extrusion at larger die angles (90o) produces higher extrusion pressure, extrusion at lower d ie angles produces less extrusion pressure 2. Extrusion pressures are dependent on the tool material used. Ext rusion with tool steel die exh ibited superior properties to mild steel die . 3. Linear strain show significant dependence on the extrusion die angle and die material. It is observed that cold billet ext rusion favours high linear strain most followed by annealed extruded with hot ext ruded showing inferior strain in this regard. Linear strain as much as 200 % was recorded for some e xtruded samples in 90o tool steel die. 4. Result of surface hardness of extruded alloy does not show a pre-defined trend but indicates that structural change due to severe plastic deformation during extrusion was effective fo r the imp rovement of hardness. 5. 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