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Influence of aging parameters on the mechanical properties of 6063 aluminium alloy
Journal of Materials Processing Technology 102 (2000) 234±240
In¯uence of aging parameters on the mechanical properties of 6063 aluminium alloy Ra®q A. Siddiqui*, Hussein A. Abdullah, Khamis R. Al-Belushi Department of Mechanical and Industrial Engineering, Sultan Qaboos University, PO Box 33, Muscat-123, Oman Accepted 17 December 1999
Abstract The 6063 aluminium alloy were given various heat treatments at under-aged, peak-aged and over-aged temperatures. The effect of precipitation on the tensile strength, yield strength, hardness, ductility and number of cycles required to fail the alloy at constant stress was investigated. The variation in time and temperature have improved the mechanical properties of the Al-alloy, whereas the ductility has decreased. The experimental work has revealed that time and temperature play a very important role in the precipitation hardening process of the Al-alloy. The initial increase in the tensile strength, yield strength, hardness and fatigue is due to vacancies assisted diffusion mechanism and formation of high volume fraction of guinier preston (GP) zones, which disturbs the regularity in the lattices. In over-aging of the alloy, the size of the individual particle increases, but the number of particles decreases. This causes few obstacles to the movement of dislocations, therefore, the mechanical properties decreases. The scanning electron microscope (SEM) study of the under-aged alloy have exhibited facet fatigue fracture surface, whereas the peak-aged and over-aged alloy show a mixed mode of fracture, i.e. facet fracture with striation and also intergranular fracture. # 2000 Elsevier Science S.A. All rights reserved. Keywords: 6063 Aluminium alloy; Age hardening; Precipitation hardening; Tensile strength; Yield strength; Fatigue; Ductility
1. Introduction Aluminium does not have good casting or mechanical properties. These properties can be achieved by adding magnesium and silicon to aluminium. The addition of these alloying elements increases the aluminium response to heat treatment due to formation of Mg2Si intermetallic compound, which improves the casting, corrosion resistance property as well as the strength of the alloy. This alloy is named as the 6063 aluminium alloy. Al±Mg±Si alloy is also known as architectural and decorative alloy; because of its easy extrudability property, distinctly superior ®nishing quality and strength. Almost half of all the aluminium extrusions produced in UK are used in building as reported by Helby [1]. Zajac at el. [2] in 1993 investigated the hot deformation behavior of AA 6063 and AA 6005 aluminium alloy. It was found that small amount of manganese signi®cantly helps in homogenizing and transforming the plate like b-AlFeSi * Corresponding author. Tel.: 968-515360; fax: 968-513416. E-mail address: [email protected] (R.A. Siddiqui)
phase to more rounded a-AlFeSi phase, which increases the ductility of the material. The addition of manganese also increases quench sensitivity of the alloy even when the cooling rate is as low as 508C minÿ1. This was con®rmed by Musulin and Celliers [3]. They found that addition of manganese accelerates the transformation of b-AlFeSi phase to a favorable a-AlFeSi phase. Okorafor [4] investigated the corrosion resistance property of 6063 alloy in under-aged, peak-aged and over-aged conditions. The results show that weight loss and rate of weight loss were both function of exposure time and heat treatment temperature. Jiang et al. [5] found that fatigue crack propagation in crystalline material is normally divided into two successive stages. In stage I, crack develops along the active slip plane and it is normal to the direction of applied stress. In stage II, the propagation of the crack begins in under-aged 6063 Al± Mg±Si alloy. The alloy exhibits heterogeneous deformation and slip bands are formed only by one slip system. There is strong tendency for single slip system activation, which can be attributed to the high volume fraction and small size of GP zone and low content of dispersoids.
0924-0136/00/$ ± see front matter # 2000 Elsevier Science S.A. All rights reserved. PII: S 0 9 2 4 - 0 1 3 6 ( 9 9 ) 0 0 4 7 6 - 8
R.A. Siddiqui et al. / Journal of Materials Processing Technology 102 (2000) 234±240
235
Table 1 Chemical analysis of the 6063 aluminium alloy Elements
Aluminium
Silicon
Magnesium
Manganese
Iron
Copper
Wt.%
Balance
0.513%
0.521%
0.067%
0.087%
0.081%
The fatigue crack growth for narrow band Gaussian spectrum loading in 6063 was investigated by Veers et al. [6]. The crack growth rates were determined for constant amplitude loading at stress ratio (R) of 0.09, 0.3, and 0.5, and for a variable amplitude loading simulated to match a narrow-band Gaussian spectrum. Crack opening stress levels measured by this method during constant amplitude loading are found different because of the different heat treatment. The results have shown that the crack growth is from intergranular to transgranular formation. The growth rate of fatigue cracks in prestrain 6063 T6 Alalloy under different stress ratio was studied by Kumar and Garg [7]. It was observed that the growth rate of fatigue crack in the prestrain material was more than that of asreceived material. It was shown by crack opening displacement measurements that crack closure occurred to a lesser extent in prestrain material. Kumar and Garg discussed the increase in yield strength for fatigue crack growth and they found that the fatigue life decreased as the percentage of prestrain increases. The effect of different aging conditions, with different chemical composition and dispersoid contents on fatigue fracture behavior of Al±Mg±Si alloy, was conducted by Jiang et al. [8]. It was found that the dispersoid phase could alter the mode of fatigue fracture by the in¯uence on the deformation uniformity of the alloy. Fatigue analysis for typical materials including 6063 Alalloy system used for vertical axis wind turbine blades was investigated by Van Den Avyle and Sutherland [9]. Two types of data were measured: (a) stress versus number of cycles (S±N curve) and (b) fatigue crack growth rate. The S± N experiment was conducted on 6063 extruded material using 100 bend specimens cycled at ®ne alternating stress amplitudes. The cyclic crack growth rates were measured using three loading rates. Considerable work on precipitation hardening has been carried out. However, in the present investigation the effect of time and temperature on the mechanical and fatigue fracture behavior of the 6063 aluminium alloy was investigated. In the precipitation heat treatment process, the alloy was heated to 793 K and the solid solution formed at this temperature is retained in super-saturated state by quenching rapidly in cold blast of air or in water to avoid any precipitation during quenching. Finally, the alloy was heattreated and the mechanical properties such as tensile strength, hardness, ductility and fatigue fracture behaviour of the alloy were studied. The fractured surfaces of different heat-treated specimens were analysed using scanning electron microscope (SEM).
2. Materials and experimental procedure 6063 aluminium alloy in the form of pro®les were received from Oman National Aluminium Company of Sultanate of Oman. The Al±Mg±Si alloy was prepared in Dubai in the form of billet having the following composition, Table 1. The standard tensile and fatigue specimens were fabricated from the pro®les in the College of Engineering Workshop, Sultan Qaboos University, according to BSS speci®cation. The solution heat treatment to all the specimens was carried out in a furnace by soaking the alloy for 2 h at 7935 K followed by quenching in water at room temperature to preserve the supersaturated solid solution at room temperature. After quenching, the 6063 Al-alloy specimens were kept in a freezer to avoid natural aging of the alloy at room temperature. The arti®cial age hardening of the specimens were carried out in a temperature-controlled furnace. The fatigue fractured surfaces were observed using SEM. 3. Results and discussion The solution heat treated specimens were age hardened at 373, 398, 423, 448, 473 and 498 K for 2, 4, 6, 8, 10, 12, and 14 h; to study the effect of heat treatment on the tensile strength, yield strength, hardness and ductility of 6063 aluminium alloy. The effect of precipitation heat treatment on fatigue fracture behavior was also investigated at 323, 348, 373, 398, 423, 448, 473, 498, 523, 548 and 573 K for different intervals, i.e. 2, 3, 4, 5, and 6 h. The results of the mechanical properties of aged Al-alloy are presented in Figs. 1±3. The variation in tensile strength when exposed to different temperatures for different intervals of time is shown in Fig. 1. It can be observed that as the aging time and temperature increase, a continuous increase in tensile strength is noticed. Further increase in the aging temperature between 473 and 498 K for 10±12 h has reduced the tensile strength of the 6063 aluminium alloy. The effect of time and temperature on the yield strength of the alloy is represented in Fig. 2. The yield strength continues to increase initially with the increase in temperature. Maximum yield strength is observed when the alloy is aged between 10 and 12 h at 473 K. Further heating causes a steady decrease in the yield strength of the material. A continuous and pronounced increase in hardness with the increase in both aging time and temperature is observed
236
R.A. Siddiqui et al. / Journal of Materials Processing Technology 102 (2000) 234±240
Fig. 1. Effect of time and temperature on tensile strength of 6063 Al-alloy.
in Fig. 3. The alloy achieves its maximum hardness at 473 K when aged for 8 h, thereafter, a decrease occurs as the time and temperature progresses. Fig. 4 shows the in¯uence of heat treatment on the ductility of the material. The percentage elongation in 6063 Al-alloy falls gradually with increase in both time and temperature. The over-aged specimens have shown as low as 6% ductility when precipitation hardened at 498 K for 14 h. Since time and temperature plays a very important role in age hardening of 6063 aluminium alloy, therefore a further study on fatigue fracture behaviour of the alloy was conducted and is graphically illustrated in Figs. 5 and 6. The fatigue fracture behavior is greatly accelerated by aging the alloy above the room temperature, because of precipitation hardening at elevated temperatures and the
increase in time. It is evident from the graph that as the aging time increases at constant aging temperature, the number of cycles required to fail at constant stress has increased (Fig. 5). As the alloy is heat treated above 473 K for any constant time, continuous decrease in number of cycles to fail in 6063 alloy is observed. The graph in Fig. 6 is a rotation of Fig. 5 by 428. The results of this work show that in most of the precipitation hardening system, a complex sequence of time dependent and temperature dependent changes are involved. The initial increase in tensile strength, yield strength, hardness and fatigue could be explained by diffusion assisted mechanism, and also by hindrance of dislocation by impurity atoms, i.e. foreign particle of second phase, since the material after quenching from 793 K (solution heat treat-
Fig. 2. Effect of time and temperature on yield strength of 6063 Al-alloy.
R.A. Siddiqui et al. / Journal of Materials Processing Technology 102 (2000) 234±240
237
Fig. 3. Effect of time and temperature on the hardness of 6063 Al-alloy.
ment) will have excessive vacancy concentration. As the aging time and temperature increases, the density of GP zones will also increase. Hence, the degree of irregularity in the lattices will cause an increase in the mechanical properties of the Al-alloy. The strengthening effect of 6063 could also be explained as a result of interference with the motion of dislocation due to the presence of foreign particle of any other phase. Further heat treatment at higher temperatures and time decreases the tensile strength, yield strength and hardness of the alloy. This could be due to coalescence of the precipitates into larger particles and bigger grain size, which will cause fewer obstacles to the movement of dislocation, and also due to annealing out of the defects.
The fatigue fracture surface of the 6063 Al-alloy when aged for 323 K for a period of 2 h is shown in Fig. 7. The SEM photomicrograph shows different portion of fatigue patches separating each other. The increase in magnitude in stress has produced an increase in the striation spacing. The fatigue striation spacing changes signi®cantly over a short distance due to change in local stress conditions as the crack propagates on the inclined surface. The regular striation can also be observed in that region, which are perpendicular to the tensile axis. Fig. 8 shows fatigue fracture behavior in Al-alloy when aged for 6 h at 323 K. The alloy exhibits intergranular cracks with some facets. The faceted fracture appears very close to the under-aged alloy. The intergranular fracture is very
Fig. 4. Effect of time and temperature on the percentage of ductility of 6063 Al-alloy.
238
R.A. Siddiqui et al. / Journal of Materials Processing Technology 102 (2000) 234±240
Fig. 5. Effect of time and temperature on the number of cycles to fail in 6063 Al-alloy.
smooth and is of cleavage type with river pattern, which indicates brittle crack behavior of the alloy. The intergranular cracks are usually formed in soft zones, and leads to the formation of facets. These facets and intergranular fractures are usually found near the fatigue crack initiation regions. Since the size of the GP zone in the under-aged conditions is quite small compared to the GP zones in the peak-aged conditions, therefore, a heterogeneous deformation tendency is observed in the micrograph. The SEM photomicrograph (Fig. 9) represents intergranular fracture in peak-aged aluminium alloy. Various points on the fracture surface show intergranular fracture with some secondary cracks in the middle of the grains. Although some fatigue striation are also observed, but the failure
resemble very much the corrosion fatigue fracture surface. The upper right part of the photomicrograph shows a little cleavage surface, which represents brittleness in the material when the alloy was aged at 473 K for a period of 6 h. The over-aged alloy, Fig. 10, shows striation on the fractured surface of the specimen when precipitation hardened for 6 h at 575 K. The fracture surface presents cleavage fracture, which is the low energy fracture and propagates along the cleavage plane. The fatigue striation also indicates that the propagation of the fatigue crack is due to the stage II. Since the fatigue stress is higher in stage II, therefore, a homogeneous deformation has occurred in the material as shown in the SEM photomicrograph, Fig. 10.
Fig. 6. Effect of time and temperature on the number of cycles to fail in 6063 Al-alloy (428 rotation from the position of Fig. 5).
R.A. Siddiqui et al. / Journal of Materials Processing Technology 102 (2000) 234±240
Fig. 7. Al-alloy aged for 2 h at 323 K showing fatigue striation.
239
Fig. 10. Al-alloy aged for 6 h at 575 K showing striation and cleavage fracture.
4. Conclusion
Fig. 8. Al-alloy aged for 6 h at 323 K showing facets fracture.
Fig. 9. Al-alloy aged for 6 h at 473 K showing intergranular fracture.
A complex sequence of time and temperature dependent changes is responsible in precipitation hardening of 6063 aluminium alloy. The experimental results have revealed that aging between 8 and 10 h at 448 K is the most suitable combination of time and temperature imparting maximum tensile strength, yield strength and hardness to the alloy. Aging at 473 K for a period of 6 h has produced maximum resistance to fatigue fracture behaviour in the alloy. The initial increase in the above mechanical properties, is due to vacancies assisted diffusion mechanism. At room temperature and high temperatures, i.e. under-aged and peak-aged condition the vacancies are highly mobile. These vacancies play a signi®cant role in the formation of GP zones, which are considerably rich in solute atoms. The local segregation of solute atoms produces a distortion of the lattice planes both within the zones and extending for several atomic layers in the matrix. With an increase in number/density of zones, the degree of disturbance of the getting regularity in the lattice increases. Therefore, the mechanical properties as well as the fracture behavior of the alloy are enhanced. The strengthening effect can also be as a result of interference with the motion of dislocation, due to the formation of precipitates in under-aged and peak-aged conditions. The peak-aged alloy has the highest mechanical properties compared to the under-aged and over-aged 6063 aluminium alloy. A decrease in the mechanical properties of the alloy in the over-aging conditions (increase in aging time and temperature) has occurred because of coalescence of the precipitates into larger particles, bigger grain size, and also due to annealing of the defects. This will cause less obstacles to the movement of dislocations and hence the mechanical properties starts to decrease. A similar behavior is also observed in the SEM photomicrographs i.e. the under-aged alloy exhibits facet fatigue fracture and the peak-aged and
240
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over-aged alloy show a mixed node of fracture, i.e. facet fracture with striation intergranular fracture. References [1] M. Helby, Aluminium Extrusion. A Flexible Approach Construction Mater World, Vol. 1, 1993, pp. 101±102. [2] S. Zajac, B. Hutchinson, A. Johanson, L.O. Gullman, R. Lagneborg, Microstructure control and extrudability of Al±Mg±Si alloys, micro alloyed with manganese, J. Physique 3 (7) (1993) 251±254. [3] I. Musulin, O.C. Celliers, Role of manganese in 6063 alloy and effect of quench sensitivity in 6063, in: Proceedings of the TMS Annual Meeting on Light Metal, Vol. 119, Minerals Metal and Material Society (TMS), Warrendale, PA, 1990, pp. 951±954. [4] O.E. Okorafor, Effect of heat treatment on the corrosion resistance of 6063 aluminium alloy, Corrosion Prev. Control 38 (6) (1991) 141± 144.
[5] D.M. Jiang, B.D. Hong, T.C. Lei, Fatigue fracture behavior of an under-aged Al±Si±Mg alloy, Scripta Metall. 24 (1990) 651± 654. [6] P.S. Veers, D.A. Van, A. James, Fatigue crack growth from narrowband gaussian spectrum loading in 6063 aluminium alloy, in: Proceedings of the Conference on Advanced Fatigue Lifetime Predictive Techniques, ASTM Special Technical Publication, San Francsico, CA, April 24, 1990, pp. 191±213. [7] R. Kumar, S.B.L. Garg, Effect of yield stress and stress ratio on fatigue crack closure in 6063-T6 aluminium alloy, Int. J. Pressure Vessels Piping 38 (4) (1989) 293±307. [8] D. Jiang, B. Hong, T. Lei, In¯uence of composition and dispersoid on fatigue fracture behaviour of Al±Mg±Si alloys, Acta Metall. Sinica 26 (5) (1990) A388±A390. [9] J.A. Van Den Avyle, H.J. Sutherland, Fatigue characterization of a VAWT blade material, in: Proceedings of Eighth Symposium of ASME on Wind Energy, Houston, TX, Vol. 7, ASME, New York, January 22±25, 1988, pp. 125±129.
In¯uence of aging parameters on the mechanical properties of 6063 aluminium alloy Ra®q A. Siddiqui*, Hussein A. Abdullah, Khamis R. Al-Belushi Department of Mechanical and Industrial Engineering, Sultan Qaboos University, PO Box 33, Muscat-123, Oman Accepted 17 December 1999
Abstract The 6063 aluminium alloy were given various heat treatments at under-aged, peak-aged and over-aged temperatures. The effect of precipitation on the tensile strength, yield strength, hardness, ductility and number of cycles required to fail the alloy at constant stress was investigated. The variation in time and temperature have improved the mechanical properties of the Al-alloy, whereas the ductility has decreased. The experimental work has revealed that time and temperature play a very important role in the precipitation hardening process of the Al-alloy. The initial increase in the tensile strength, yield strength, hardness and fatigue is due to vacancies assisted diffusion mechanism and formation of high volume fraction of guinier preston (GP) zones, which disturbs the regularity in the lattices. In over-aging of the alloy, the size of the individual particle increases, but the number of particles decreases. This causes few obstacles to the movement of dislocations, therefore, the mechanical properties decreases. The scanning electron microscope (SEM) study of the under-aged alloy have exhibited facet fatigue fracture surface, whereas the peak-aged and over-aged alloy show a mixed mode of fracture, i.e. facet fracture with striation and also intergranular fracture. # 2000 Elsevier Science S.A. All rights reserved. Keywords: 6063 Aluminium alloy; Age hardening; Precipitation hardening; Tensile strength; Yield strength; Fatigue; Ductility
1. Introduction Aluminium does not have good casting or mechanical properties. These properties can be achieved by adding magnesium and silicon to aluminium. The addition of these alloying elements increases the aluminium response to heat treatment due to formation of Mg2Si intermetallic compound, which improves the casting, corrosion resistance property as well as the strength of the alloy. This alloy is named as the 6063 aluminium alloy. Al±Mg±Si alloy is also known as architectural and decorative alloy; because of its easy extrudability property, distinctly superior ®nishing quality and strength. Almost half of all the aluminium extrusions produced in UK are used in building as reported by Helby [1]. Zajac at el. [2] in 1993 investigated the hot deformation behavior of AA 6063 and AA 6005 aluminium alloy. It was found that small amount of manganese signi®cantly helps in homogenizing and transforming the plate like b-AlFeSi * Corresponding author. Tel.: 968-515360; fax: 968-513416. E-mail address: [email protected] (R.A. Siddiqui)
phase to more rounded a-AlFeSi phase, which increases the ductility of the material. The addition of manganese also increases quench sensitivity of the alloy even when the cooling rate is as low as 508C minÿ1. This was con®rmed by Musulin and Celliers [3]. They found that addition of manganese accelerates the transformation of b-AlFeSi phase to a favorable a-AlFeSi phase. Okorafor [4] investigated the corrosion resistance property of 6063 alloy in under-aged, peak-aged and over-aged conditions. The results show that weight loss and rate of weight loss were both function of exposure time and heat treatment temperature. Jiang et al. [5] found that fatigue crack propagation in crystalline material is normally divided into two successive stages. In stage I, crack develops along the active slip plane and it is normal to the direction of applied stress. In stage II, the propagation of the crack begins in under-aged 6063 Al± Mg±Si alloy. The alloy exhibits heterogeneous deformation and slip bands are formed only by one slip system. There is strong tendency for single slip system activation, which can be attributed to the high volume fraction and small size of GP zone and low content of dispersoids.
0924-0136/00/$ ± see front matter # 2000 Elsevier Science S.A. All rights reserved. PII: S 0 9 2 4 - 0 1 3 6 ( 9 9 ) 0 0 4 7 6 - 8
R.A. Siddiqui et al. / Journal of Materials Processing Technology 102 (2000) 234±240
235
Table 1 Chemical analysis of the 6063 aluminium alloy Elements
Aluminium
Silicon
Magnesium
Manganese
Iron
Copper
Wt.%
Balance
0.513%
0.521%
0.067%
0.087%
0.081%
The fatigue crack growth for narrow band Gaussian spectrum loading in 6063 was investigated by Veers et al. [6]. The crack growth rates were determined for constant amplitude loading at stress ratio (R) of 0.09, 0.3, and 0.5, and for a variable amplitude loading simulated to match a narrow-band Gaussian spectrum. Crack opening stress levels measured by this method during constant amplitude loading are found different because of the different heat treatment. The results have shown that the crack growth is from intergranular to transgranular formation. The growth rate of fatigue cracks in prestrain 6063 T6 Alalloy under different stress ratio was studied by Kumar and Garg [7]. It was observed that the growth rate of fatigue crack in the prestrain material was more than that of asreceived material. It was shown by crack opening displacement measurements that crack closure occurred to a lesser extent in prestrain material. Kumar and Garg discussed the increase in yield strength for fatigue crack growth and they found that the fatigue life decreased as the percentage of prestrain increases. The effect of different aging conditions, with different chemical composition and dispersoid contents on fatigue fracture behavior of Al±Mg±Si alloy, was conducted by Jiang et al. [8]. It was found that the dispersoid phase could alter the mode of fatigue fracture by the in¯uence on the deformation uniformity of the alloy. Fatigue analysis for typical materials including 6063 Alalloy system used for vertical axis wind turbine blades was investigated by Van Den Avyle and Sutherland [9]. Two types of data were measured: (a) stress versus number of cycles (S±N curve) and (b) fatigue crack growth rate. The S± N experiment was conducted on 6063 extruded material using 100 bend specimens cycled at ®ne alternating stress amplitudes. The cyclic crack growth rates were measured using three loading rates. Considerable work on precipitation hardening has been carried out. However, in the present investigation the effect of time and temperature on the mechanical and fatigue fracture behavior of the 6063 aluminium alloy was investigated. In the precipitation heat treatment process, the alloy was heated to 793 K and the solid solution formed at this temperature is retained in super-saturated state by quenching rapidly in cold blast of air or in water to avoid any precipitation during quenching. Finally, the alloy was heattreated and the mechanical properties such as tensile strength, hardness, ductility and fatigue fracture behaviour of the alloy were studied. The fractured surfaces of different heat-treated specimens were analysed using scanning electron microscope (SEM).
2. Materials and experimental procedure 6063 aluminium alloy in the form of pro®les were received from Oman National Aluminium Company of Sultanate of Oman. The Al±Mg±Si alloy was prepared in Dubai in the form of billet having the following composition, Table 1. The standard tensile and fatigue specimens were fabricated from the pro®les in the College of Engineering Workshop, Sultan Qaboos University, according to BSS speci®cation. The solution heat treatment to all the specimens was carried out in a furnace by soaking the alloy for 2 h at 7935 K followed by quenching in water at room temperature to preserve the supersaturated solid solution at room temperature. After quenching, the 6063 Al-alloy specimens were kept in a freezer to avoid natural aging of the alloy at room temperature. The arti®cial age hardening of the specimens were carried out in a temperature-controlled furnace. The fatigue fractured surfaces were observed using SEM. 3. Results and discussion The solution heat treated specimens were age hardened at 373, 398, 423, 448, 473 and 498 K for 2, 4, 6, 8, 10, 12, and 14 h; to study the effect of heat treatment on the tensile strength, yield strength, hardness and ductility of 6063 aluminium alloy. The effect of precipitation heat treatment on fatigue fracture behavior was also investigated at 323, 348, 373, 398, 423, 448, 473, 498, 523, 548 and 573 K for different intervals, i.e. 2, 3, 4, 5, and 6 h. The results of the mechanical properties of aged Al-alloy are presented in Figs. 1±3. The variation in tensile strength when exposed to different temperatures for different intervals of time is shown in Fig. 1. It can be observed that as the aging time and temperature increase, a continuous increase in tensile strength is noticed. Further increase in the aging temperature between 473 and 498 K for 10±12 h has reduced the tensile strength of the 6063 aluminium alloy. The effect of time and temperature on the yield strength of the alloy is represented in Fig. 2. The yield strength continues to increase initially with the increase in temperature. Maximum yield strength is observed when the alloy is aged between 10 and 12 h at 473 K. Further heating causes a steady decrease in the yield strength of the material. A continuous and pronounced increase in hardness with the increase in both aging time and temperature is observed
236
R.A. Siddiqui et al. / Journal of Materials Processing Technology 102 (2000) 234±240
Fig. 1. Effect of time and temperature on tensile strength of 6063 Al-alloy.
in Fig. 3. The alloy achieves its maximum hardness at 473 K when aged for 8 h, thereafter, a decrease occurs as the time and temperature progresses. Fig. 4 shows the in¯uence of heat treatment on the ductility of the material. The percentage elongation in 6063 Al-alloy falls gradually with increase in both time and temperature. The over-aged specimens have shown as low as 6% ductility when precipitation hardened at 498 K for 14 h. Since time and temperature plays a very important role in age hardening of 6063 aluminium alloy, therefore a further study on fatigue fracture behaviour of the alloy was conducted and is graphically illustrated in Figs. 5 and 6. The fatigue fracture behavior is greatly accelerated by aging the alloy above the room temperature, because of precipitation hardening at elevated temperatures and the
increase in time. It is evident from the graph that as the aging time increases at constant aging temperature, the number of cycles required to fail at constant stress has increased (Fig. 5). As the alloy is heat treated above 473 K for any constant time, continuous decrease in number of cycles to fail in 6063 alloy is observed. The graph in Fig. 6 is a rotation of Fig. 5 by 428. The results of this work show that in most of the precipitation hardening system, a complex sequence of time dependent and temperature dependent changes are involved. The initial increase in tensile strength, yield strength, hardness and fatigue could be explained by diffusion assisted mechanism, and also by hindrance of dislocation by impurity atoms, i.e. foreign particle of second phase, since the material after quenching from 793 K (solution heat treat-
Fig. 2. Effect of time and temperature on yield strength of 6063 Al-alloy.
R.A. Siddiqui et al. / Journal of Materials Processing Technology 102 (2000) 234±240
237
Fig. 3. Effect of time and temperature on the hardness of 6063 Al-alloy.
ment) will have excessive vacancy concentration. As the aging time and temperature increases, the density of GP zones will also increase. Hence, the degree of irregularity in the lattices will cause an increase in the mechanical properties of the Al-alloy. The strengthening effect of 6063 could also be explained as a result of interference with the motion of dislocation due to the presence of foreign particle of any other phase. Further heat treatment at higher temperatures and time decreases the tensile strength, yield strength and hardness of the alloy. This could be due to coalescence of the precipitates into larger particles and bigger grain size, which will cause fewer obstacles to the movement of dislocation, and also due to annealing out of the defects.
The fatigue fracture surface of the 6063 Al-alloy when aged for 323 K for a period of 2 h is shown in Fig. 7. The SEM photomicrograph shows different portion of fatigue patches separating each other. The increase in magnitude in stress has produced an increase in the striation spacing. The fatigue striation spacing changes signi®cantly over a short distance due to change in local stress conditions as the crack propagates on the inclined surface. The regular striation can also be observed in that region, which are perpendicular to the tensile axis. Fig. 8 shows fatigue fracture behavior in Al-alloy when aged for 6 h at 323 K. The alloy exhibits intergranular cracks with some facets. The faceted fracture appears very close to the under-aged alloy. The intergranular fracture is very
Fig. 4. Effect of time and temperature on the percentage of ductility of 6063 Al-alloy.
238
R.A. Siddiqui et al. / Journal of Materials Processing Technology 102 (2000) 234±240
Fig. 5. Effect of time and temperature on the number of cycles to fail in 6063 Al-alloy.
smooth and is of cleavage type with river pattern, which indicates brittle crack behavior of the alloy. The intergranular cracks are usually formed in soft zones, and leads to the formation of facets. These facets and intergranular fractures are usually found near the fatigue crack initiation regions. Since the size of the GP zone in the under-aged conditions is quite small compared to the GP zones in the peak-aged conditions, therefore, a heterogeneous deformation tendency is observed in the micrograph. The SEM photomicrograph (Fig. 9) represents intergranular fracture in peak-aged aluminium alloy. Various points on the fracture surface show intergranular fracture with some secondary cracks in the middle of the grains. Although some fatigue striation are also observed, but the failure
resemble very much the corrosion fatigue fracture surface. The upper right part of the photomicrograph shows a little cleavage surface, which represents brittleness in the material when the alloy was aged at 473 K for a period of 6 h. The over-aged alloy, Fig. 10, shows striation on the fractured surface of the specimen when precipitation hardened for 6 h at 575 K. The fracture surface presents cleavage fracture, which is the low energy fracture and propagates along the cleavage plane. The fatigue striation also indicates that the propagation of the fatigue crack is due to the stage II. Since the fatigue stress is higher in stage II, therefore, a homogeneous deformation has occurred in the material as shown in the SEM photomicrograph, Fig. 10.
Fig. 6. Effect of time and temperature on the number of cycles to fail in 6063 Al-alloy (428 rotation from the position of Fig. 5).
R.A. Siddiqui et al. / Journal of Materials Processing Technology 102 (2000) 234±240
Fig. 7. Al-alloy aged for 2 h at 323 K showing fatigue striation.
239
Fig. 10. Al-alloy aged for 6 h at 575 K showing striation and cleavage fracture.
4. Conclusion
Fig. 8. Al-alloy aged for 6 h at 323 K showing facets fracture.
Fig. 9. Al-alloy aged for 6 h at 473 K showing intergranular fracture.
A complex sequence of time and temperature dependent changes is responsible in precipitation hardening of 6063 aluminium alloy. The experimental results have revealed that aging between 8 and 10 h at 448 K is the most suitable combination of time and temperature imparting maximum tensile strength, yield strength and hardness to the alloy. Aging at 473 K for a period of 6 h has produced maximum resistance to fatigue fracture behaviour in the alloy. The initial increase in the above mechanical properties, is due to vacancies assisted diffusion mechanism. At room temperature and high temperatures, i.e. under-aged and peak-aged condition the vacancies are highly mobile. These vacancies play a signi®cant role in the formation of GP zones, which are considerably rich in solute atoms. The local segregation of solute atoms produces a distortion of the lattice planes both within the zones and extending for several atomic layers in the matrix. With an increase in number/density of zones, the degree of disturbance of the getting regularity in the lattice increases. Therefore, the mechanical properties as well as the fracture behavior of the alloy are enhanced. The strengthening effect can also be as a result of interference with the motion of dislocation, due to the formation of precipitates in under-aged and peak-aged conditions. The peak-aged alloy has the highest mechanical properties compared to the under-aged and over-aged 6063 aluminium alloy. A decrease in the mechanical properties of the alloy in the over-aging conditions (increase in aging time and temperature) has occurred because of coalescence of the precipitates into larger particles, bigger grain size, and also due to annealing of the defects. This will cause less obstacles to the movement of dislocations and hence the mechanical properties starts to decrease. A similar behavior is also observed in the SEM photomicrographs i.e. the under-aged alloy exhibits facet fatigue fracture and the peak-aged and
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