1*
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The precipitation behavior and hardening of 6061-T6 aluminum alloy was investigated. The experimental study was to determine the effects of artificial ageing on the strength of 6061-T6 aluminium alloy. The precipitation hardening usually undergoes a thermal treatment, which consists of heat treatment, quenching process and artificial ageing. The experimental study was focused on artificial ageing temperature between 175ºC to 420ºC at different period of time. The microstructure variation was analyzed using scanning electron microscope and optical microscope to observe the precipitation before and after the heat treatment. The optimum ageing time and temperature were also determined at the end of this experiment to identify energy reductions and total production cost. The study leads to the conclusion that the optimum aged can be achieved within 175ºC to 195ºC at 2 to 6 hours ageing time.
Keywords: 6061-T6 aluminium alloy, metallographic test, ageing, temperature
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1
Faculty of Mechanical Engineering, Universiti Teknikal Malaysia Melaka, Durian
Tunggal, 76109 Melaka,, Malaysia. Email: cheefai@utem.edu.my
2 Faculty of Engineering, Universiti Putra Malaysia, Serdang, Selangor, Malaysia.
* Corresponding author
Suranaree J. Sci. Technol. XX (X):.....
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Aluminium is ranked after iron and steel in the metal market. The demand of aluminium grows rapidly because it is a unique combination of properties, which make it as one of the most versatile engineering and construction materials. Aluminium alloy become the selection by designers and engineers as the material for different kind of applications.
Aluminium is light in weight, with specific gravity of 2.7; only 30% weight of copper and one-third weight of iron. Except for magnesium, aluminium is the lightest of all common metals. Besides, it has good electrical and thermal conductivity and high reflectivity to both heat and light. It is non-toxic and good in corrosion-resistant under many service conditions. Aluminium provides a wide variety of surface finishes and it can be casted and worked into different form. Aluminium has become prime importance as engineering material with all these outstanding properties.
Generally, strength is further improved by cold working, grain size refining, precipitation hardening and dispersion hardening. In this study, the strength of 6061-T6 aluminium alloy was improved by precipitation hardening. Precipitation hardening is a process that enhances the strength and hardness of some metal alloys by the formation of extremely small uniformly dispersed particle of a second phase within the original phase matrix. The general requirement for precipitation strengthening of supersaturated solid solution involves the formation of finely dispersed (ASM Handbook, 1991). The precipitate particle nucleates and grows; by the diffusion of solute atoms into it from the matrix phase. It is called precipitation because the small particles of the new phase are
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termed “precipitates” (Jastrzebski, 1959). Artificial ageing will be accomplished not only below the equilibrium solvus temperature, but below a meta-stable miscibility gap called
Guinier-Preston (GP) zone solvus line. The paper describes the metallographic analysis of
6061-T6 aluminium alloy under precipitation hardening process.
The basic requirement in a precipitation hardening alloy system is that the solid solubility limit should decreases with the decreasing of temperature. During the precipitation hardening, alloy is first treated with heat at high temperature and subsequently cooled rapidly by quenching it into water or other cooling medium. The process of rapid cooling suppresses the separation of
θ
-phase so that the alloy will be maintained in an unstable supersaturated state at low temperature. However, if the alloy is allowed to ‘age’ for a sufficient of time after quenching, the second phase would precipitates out (Smallman and Bishop, 1995).
Kulkarni and Krishnamurthy (2004) conducted a study the effect of particle size distribution on strength of precipitation-hardened alloys. Ageing of precipitation hardened alloys results in particle coarsening, which in turn affects the strength in alloy.
In this study, the effect of particle size distribution on the strength of precipitationhardened alloys was considered, to better represent real alloy, the particle radii were distributed using Wangner, Lifshitz and Slyozov (WLS) particle size distribution theory.
Lumley et al. (2002) proposed the control of secondary precipitation to improve the performance of aluminium alloy. Beneficial effects of under ageing in enhancing the creep resistance of certain aluminium alloy may be diminished or lost if the alloys are held at close to ambient temperature prior to testing. This problem is associated with undesirable secondary precipitation during the dwell period and may be overcome by
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cooling the alloy slowly from the initial ageing temperature. A method has been successfully exploiting secondary precipitation to improve the mechanical properties of aluminium alloy. It involves interrupting artificial ageing with a low-temperature dwell period.
Eskin (2002) studied the hardening and precipitation in Al-Cu-Mg-Si alloying system. The composition and hardening phase in Al-Cu-Mg-Si alloys, which contain
2.5% - 4.5% Cu were considered with respect to the chemical composition of the supersaturated solid solution. For the first time the composition of the supersaturated solid solution was calculated using Thermocalc software as the equilibrium composition at quenching temperature. The influence of precipitation on the work-hardening behavior of the Aluminium Alloys AA6111 and AA7030 were carried out by Cheng et al. (2003).
Tensile tests were conducted on the aluminium alloy AA6111, after various artificial ageing treatments in order to examine the influence of precipitation state on yield stress and work-hardening behavior. Schiffmann et al. (2004) performed the evolution of precipitates during age-hardening of AW 6016 alloy. Specimens of a sheet of his commercial age-hardening aluminium alloy 6016 were heat treated in order to produce different hardening stages. By neutron small angle scattering (SANS) the precipitation sequence and its development in the nanometer range can be monitored. Variations in the precipitation sequence were observed at different age-hardening temperatures.
Mechanical properties such as hardness and strength has been modified due to the nanostructure precipitation process.
Qiang et al. (2002) performed the influence of ageing conditions on the microstructure and tensile strength of aluminium alloy 6063. Materials in three pre-
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ageing conditions: 288 hour natural ageing i.e. ageing at room temperature, 3 hours natural ageing and 3 hours natural ageing plus 5 hours ageing at 80ºC, were subsequently aged with different temperature of 165ºC, 185ºC and 205ºC from 0.25 hour to 64 hours respectively. Tensile tests were performed after ageing. Microstructure study was performed using transmission electron microscopy (TEM) and atom probe field ion microscopy (APFIM). The results showed that for the materials with same pre-ageing condition, the higher the subsequent ageing temperature, the lower the peak strength with shorter time takes to reach the peak strength.
Kang et al. (2004) performed the effect of applied pressure and heat treatment condition on microstructural characteristics and mechanical properties of the thixoforged
357 aluminum alloy. The characteristics of the microstructure and mechanical properties of thixoforged 357 Al parts with an arbitrary shape were studied for microstructure and mechanical property variations as the result of changes in the applied pressure i.e. 110,
140, and 170 MPa, and the ageing time of heat treatment i.e. 4 hours, 6 hours and 10 hours. Through the experiment of forging with combination of T6 heat treatment, a sample part with good mechanical properties was obtained. For a 6 hours heat treat ageing time, an ultimate tensile strength of 394 MPa was obtained. The percentage elongation for this sample was 10%.
Milan and Bowen (2004) conducted a tensile and fracture toughness properties of
SiCp reinforced aluminium alloys on the effects of particle size; particle volume fraction, and matrix strength. It aimed to evaluate the effects of particle size, particle volume fraction, and matrix strength on the monotonic fracture properties of two different Al
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alloys, namely T1-A12124 and T1-A16061, with the reinforcement of silicon carbide particles (SiCp).
6061-T6 aluminium alloy encompasses a wide range of chemical compositions and hardness. Therefore, various techniques are required for metallographic preparation and examination. The specimen was grounded and polished into a smooth finishing by using abrasive papers and powders. The process involved the removing of material damage parts such as scratch, rough surface and mirror crack. The specimen was undergone solid solution, quenching and ageing process. After that, the specimen was cut into approximately 10 mm. Subsequently, proceed with the mounting, grinding, polishing and cleaning process.
In the mounting process, the specimen was shaped into the preferred size by using mounting die. After that, the specimen was grinded by using silicon carbide abrasive paper. For this experiment, the specimen was cut with a diamond wheel. The specimen was washed thoroughly after each grinding to prevent carryover of abrasive particles to the next stage. Mechanical polishing was accomplished in two steps, i.e. rough polishing and final polishing. For rough polishing, it used a suspension of 600-grit alumina (Al
2
O
3
) powder in distilled water alumina (50g / 500 ml H
2
O) on a billiard cloth fixed to a rotating wheel. Diamond abrasive of 1, 3 or 6 μm was used. After that, specimens were washed thoroughly in order to remove the abrasive. For final polishing, it used a pure,
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heavy grade magnesium oxide (MgO) powder with distilled water on a uniformly textured medium. For the cleaning process, the specimen was immersed in the ultrasonic water bath.
In this section, two types of metallography analysis results are discussed i.e. optical microscope for highest hardness value and scanning electron microscope (SEM) for lowest hardness value.
Microstructure Examination
Specimen ageing at under aged, peak aged, and over aged conditions at 185
°
C (highest hardness) were carried out to analyze its metallography by optical microscope. For half an hour ageing (Figure 1), there were no observable features. This were due to the preaged condition of the samples with the hardness of 80.65 HV.
Figure 1. 6061-T6 aluminium alloy ageing for half an hour at 185 0 C
under 100 x magnification
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As indicated in Figure 2, with the further ageing at 2 hours, dislocation was started to loop around the particles. In this stage, the misfit strain was large; the interface was coherent, or the average particle separation was above a certain critical value; dislocation was unable to cut through the precipitate; instead it was loop around individual partical as shown in Figure 2 and the hardness increased to 112.65 HV.
Figure 2. 6061-T6 aluminium alloy ageing for 2 hours at 185 0 C
under 100 x magnification
Subsequently, 6061-T6 aluminium alloy was aged at 185
°
C for 6 hours (Figure 3).
It showed a fully strengthened condition, such as structure consist of GP zones and β’ precipitates. The highest value of hardness, which was 121.6 HV, was obtained at this point. The interparticle spacing of microstructure was smaller compared to the previous microstructure. According to Orowan’s mechanism , when the sample is at its peak aged, numerous fine precipitate particles will be formed. These particles will cause the degree of complication for the dislocations to pass through the particles to increase.
Figure 3. 6061-T6 aluminium alloy ageing for 6 hours at 185
°
C
under 100 x magnification
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After 10 hours of ageing, the particles started to decrease in number but its’ became bigger (Figure 4). The interparticle spacing also decreased, allowing the dislocation to move easily. The hardness and strength was inclined to 94.1 HV with
304.57 MPa of ultimate tensile strength.
Figure 4. 6061-T6 aluminium alloy ageing for 10 hours at 185
°
C
under 100 x magnification
By using scanning electron microscope (SEM), the microstructure of the lowest hardness value specimens at 420
°
C was examined. As shown in Figure 5, it can be seen that the amounts of the fine precipitate particles decreased rapidly from the beginning of the experiment to half an hour of ageing where the hardness was 41.42 HV.
Figure 5. 6061-T6 aluminium alloy ageing for ½ an hours at 420
°
C
under 4000x magnification
As shown in Figure 6, for the ageing at 2 hours, the majority of the small plateshaped dard precipitate particles were coarsened and started to form at the over aged region. The interparticle spacing also increased and bigger particles were formed.
Figure 6. 6061-T6 aluminium alloy ageing for 2 hours at 420
°
C
under 4000x magnification
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For ageing at 6 hours (Figure 7), the hardness of the inconsistency and the softening of the specimen was declining, simultaneously with the precipitate particles continue to dissolve to form larger and coarse particles. This will eventually cause the hardness and strength of 6061-T6 aluminium alloy to decrease.
Figure 7. 6061-T6 aluminium alloy ageing for 6 hours at 420
°
C
under 4000x magnification
In 10 hours of ageing time at 420
°
C (Figure 8), the number of small plate-shaped dark precipitate particles increased and the intermediate precipitate coarsens the alloy and become weaker in the peak aged condition. However, the peak age could not be determined because the hardness and strength were unable to be increased when longer ageing time was conducted.
Figure 8. 6061-T6 aluminium alloy ageing for 10 hours at 420
°
C under 4000x magnification
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Fracture Behavior
Examination of the fracture surfaces using scanning electron microscope (SEM) showed that the fracture behavior of 6061-T6 aluminium alloy was influenced by ageing tem perature and ageing time. Specimens ageing at 175
°
C and 185
°
C showed that there was a small transition in the fracture behaviour from ductile to a brittle manner in the transformation process from pre-aged to peak aged region. Figure 9 and Figure 10 illustrated the ductile fracture in the sample whereas Figure 11 showed the peak ageing time. The highest particle size was found and the elongation percentage was 11.02%. The strength was not proportional with the elongation; it fulfilled the theory of the ductility where the higher the strength, the lower the elongation was taken.
(a)
(b)
Figure 9. 6061-T6 aluminium alloy ageing for half an hours at 175
°
C
(a) under 3000x magnification
(b) under 6000x magnification
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(a)
(b)
Figure 10. 6061-T6 aluminium alloy ageing for 2 hours at 175
°
C
(a) under 3000x magnification
(b) under 6000x magnification
(a)
(b)
Figure 11. 6061-T6 aluminium alloy ageing for 4 hours at 175
°
C
(a) under 3000x magnification
(b) under 6000x magnification
For 10 hours ageing time at 175
°
C (Figure 12), it reached a point where the precipitate particles started to dissolve to form bigger and coarse particles. It was caused the increment of ductility but the decrement in the strength of particle.
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(a)
(b)
Figure 12. 6061-T6 aluminium alloy ageing for 10 hours at 175
°
C
(a) under 3000x magnification
(b) under 6000x magnification
Figure 13 showed that some large particle exhibited several flat facets of fractured particles and voids within the matrix were larger at 1 hour ageing time with temperature of 185
°
C. The failure mechanism seems to be a combination of particle fracture and ductile failure through the matrix linkage of the fractured particles. Therefore, it was capable to accommodate a higher level of plastic strain between the interparticle spacing and clustering levers. Figure 14 shows the visual result of specimen under 2 hours ageing time at 185
°
C.
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(a)
(b)
Figure 13. 6061-T6 aluminium alloy ageing for 1 hour at 185
°
C
(a) under 3000x magnification
(b) under 6000x magnification
(a)
(b)
Figure 14. 6061-T6 aluminium alloy ageing for 2 hours at 185
°
C
(a) under 3000x magnification
(b) under 6000x magnification
Futhermore, Figure 15 and Figure 16 shows the more brittle fracture since the hardness was the highest, which was 121.6 HV. Depending on the different ageing temperature and ageing time, different trend in toughness was obtained. In addition, there was an optimum particle size for a given particle volume friction/matrix strength in which
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the toughness was minimized. It is suggested that a more complex investigation of the effects of particle size should be carried out by using several temperature and ageing time with different aluminium alloys.
(a)
(b)
Figure 15. 6061-T6 aluminium alloy ageing for 6 hours at 185
°
C
(a) under 3000x magnification
(b) under 6000x magnification
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(a)
(b)
Figure 16. 6061-T6 aluminium alloy ageing for 10 hours at 185
°
C
(a) under 3000x magnification
(b) under 6000x magnification
In the metallographic analysis, it can be concluded that the artificial ageing have positive effect on the mechanical properties of 6061-T6 aluminium alloy. As preliminary research for the evaluation of precipitation hardening on 6061-T6 aluminium alloy, the study identified that the microstructure variation and fracture behavior of the aluminium alloy was subjected to different ageing temperature and time. SEM results has provided importance evidence of the relevance of artificial ageing on the formation of hardenable precipitates of the presence of small plate-shaped dard precipitate particles particle coarsen a little due started of the over aged region at ageing temperature of 420
°
C.
The optimum aged can be produced by using long ageing time at low temperature.
To obtain a better elongation rate for 6061-T6 aluminium alloy, the ageing temperature was increased to 420˚C until over-aged condition. SEM results provided the importance
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of the relevance of artificial ageing on the formable hardenable precipitates where the small plate-shaped dark precipitate particles increased and the intermediate precipitate coalesces and coarsens the alloy and become weaker at temperature of 420
°
C. Based on the results, it was concluded that there is an inconsistency and the softening of the 6061-
T6 aluminium alloy, which makes the ageing at 420
°
C not suitable. Thus, ageing at excessive temperatures must be avoided at any cause.
The authors wish to thank Faculty of Mechanical Engineering, Technical University of
Malaysia Melaka (UTeM) for the support granted for this research.
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