Ageing Characteristics of Sand Cast Al-Mg-Si (6063) Alloy

The effect of variation of ageing temperature and soaking time at the ageing temperature mechanical p roperties of alumin ium 6063 alloy was investigated. Various samples o f the alloy in the as-cast condition were produced, homogenized at 560°C held at the temperature for 2hrs, and cooled in the furnace. They were solution treated at 440°C for 10minutes and quench in water, aged at/soaked for 125°C/3Hrs, 125°C/4Hrs, 125°C/5Hrs, 175°C/3Hrs, 175°C/4Hrs, 175°C/5Hrs, 200°C /3Hrs, 200°C/4Hrs, 200°C/5Hrs, and 225°C/3Hrs, 225°C/4Hrs and 225°C/5Hrs. Tensile and impact test were carried out on the alloy standard samples. It was observed that the Optimum tensile strength is observed in the sample aged at 175°C held for 5hrs, optimum ductility is observed in sample aged at 125°C, held for 4hrs, Optimum toughness is observed in sample aged at 225°C, held 5hrs, closely followed by that of the sample aged at the same temperature, held for 3hrs. From the economic point of view the optimum parameter for obtaining a structural material of the optimum combination of mechanical properties from the sample is ageing temperature of 225°C and soaking time of 3hrs.


Introduction
Aluminiu m (6063) alloy is an alloy belonging to the 6xxx series. This alloy group contains silicon and magnesiu m in appropriate proportions to form magnesiu m silicide, thus making them heat-treatable. They possess good formab ility and corrosion resistance, with mediu m strength. Though the process of age hardening of alumin iu m alloys that contain Mg and Si was first discovered by Alfred Wilm in 1911, [1], this discovery led to searches for other alu miniu m alloy systems that would age harden [2]. These Al-Mg-Si alloys are the most widely used for the production of ext ruded sections. More than 80 percent of the alu minum alloys emp loyed world wide in the manufacturing of ext ruded sections belong to the 6xxx series [3]. In these alloys, Mg and Si co mbine to form the chemical co mpound Mg 2 Si (magnesium-silicide), the primary hardening phase [4].
Most of the added alloying elements to aluminu m alloys are less soluble in the solid phase than in liquid, and then a gap between the liquidus and solidus will exist leading to the segregation of these alloying elements. This segregation is reduced by heating the ingots for an extended period close to the solidus temperature to homogenize the structure [8]. As th e s ize o f th e cas t p ro du ct in creas es th e ch emical co mposit ion th roughout th e th ickn ess will cons iderab ly vary, (macrosegregation). This type of segregation is not much affected by heating but by controlling the manufacturing parameters of the ingot. For examp le, in the Direct-Chill casting process decreasing the ingot thickness, lowering the casting speed and maximizing the superheating temperature will greatly reduce this sort of segregation.

Materials and Methods
The materials used for this work are alu miniu m (6063) alloy obtained fro m 'A lu min iu m Ro lling Mill', Otta, Ogun State, Nigeria. The chemical co mposition of the aluminiu m alloy is given in Table 1. in the as-cast condition. Some quantities of the 6063 alu miniu m sample was melted in a crucible furnace and cast into cylindrical rods of 20 mm d iameter and 200 mm length in a sand mould. Having solidified and allowed to cool, the cast rods were machined into standard tensile and impact strength specimen. 12 test samples were prepared for each of tensile and impact tests (24 samples in all); all the samples were ho mogenized at 560℃ held at the temperature for 2hrs, and cooled in the furnace, solution treated at 440℃ for 10 minutes and quench in water [8,9]. The samples were divided into four groups of six samp les per group (each group to be aged at 125℃/3Hrs, 125℃/4Hrs, 125℃/5Hrs, 175℃/ 3Hrs, 175℃/ 4Hrs, 175℃ /5Hrs, 200℃/ 3Hrs, 200℃/4Hrs, 200℃/ 5Hrs, and 225℃ /3Hrs, 225℃/ 4Hrs and 225℃/5Hrs), each group containing three test samples for each of the tests to be carried out.

Tensile Testing
The tensile tests were performed on various samples using Monsanto tensometer. The fracture load for each sample was noted as well as the diameter at the point of fracture and the final gauge length. The initial d iameter and in itial gauge length for each sample was noted before uniaxial load. Fro m the generated data the ultimate tensile strength and percentage elongation of each sample was calculated.

Impact Test
The impact tests were performed on various sample determine the impact strengths by the "V-notch method using the Honsfield Balance Impact Testing Machine. Prior to mounting on the machine, the test sample is notched to a depth of 2mm with v-shaped hand file. The notched test sample was then mounted on the impact-testing mach ine, which is the operated to apply a (constant) impact force on the test sample. The impact strength (the amount of impact energy the specimen absorbed before y ield ing) was then read off the calibrated scale on the impact testing machine.  Figure 5. the effect of Soaking time on the Impact strength of the samples is clearly seen.

Results and Discussion
It could be seen from Figure 1. that the ultimate tensile strength (UTS) of the samples aged at 125℃ , increased slightly as the soaking t ime increases from 3 to 4 hours, thereafter further increase in the soaking t ime results in sharp reduction in the UTS. For those samples aged at 175℃ and 200 ℃ the UTS remains constant and reduced slightly respectively with increment in the soaking time fro m 3 to 4 hour, further increase in the soaking time results in increase in the UTS. The UTS of those samples aged at 225 ℃ decreased abruptly as the soaking time increased fro m 3 to 4 hours and thereafter increases sharply. This could be attributed to the fact that in silicon-containing alu minum alloys the most common phases are the α-Al 8 Fe 2 Si and β-Al 5 FeSi phases. Depending upon the alloy composition and solidification rate, one or both phases can be present in the microstructure. The β-Al 5 FeSi phase has needle-/ plate-like mo rphology and is considered detrimental to the ductility of the alloy. The α-A l 8 Fe 2 Si phase is believed to be less harmful than the β-phase form as it has more desirab le compacted Chinese script morphology. It is well known that at higher cooling rates, neutralization by the addition of trace elements such as Mn, Be, Sr can change the crystallization of the iron intermetallic to the Ch inese script α-phase form which improves the mechanical p roperties. Also, the presence of a small amount of bery lliu m produces a new Be-Fe phase (Al 8 Fe 2 SiBe) having a Ch inese script morphology or polygon shapes, which is different fro m the β-phase in the alloy samples [10,11]. Fro m Figure 1., samp le aged at 125℃ contain some α-A l 8 Fe 2 Si phase in the matrix of the alloy when it was soaked at the ageing temperature for between 3 and 4 hours, that explained the slight increase in the UTS with in this period. With further increase in the soaking time, all the α-A l 8 Fe 2 Si phase in the matrix of the alloy were transformed into a mixture of β -Al 5 FeSi phase and Mg 2 Si precipitate, wh ich accounts for the sharp reduction in UTS with further increase in the soaking time [10]. For those samples aged at 175 ℃ and 200 ℃ whose UTS remains constant and reduced slightly respectively with increment in the soaking time fro m 3 to 4 hour, this is due to the presence of a mixture of β-A l 5 FeSi phase and Mg 2 Si precipitate, within the soaking period at those temperatures. Increase in the soaking time to 5 hours resulted into transformat ion of the β-Al 5 FeSi phase and Mg 2 Si precipitate to a mixture of a new Be-Fe phase (Al 8 Fe 2 SiBe) and Mg 2 Si precipitate; this accounts for the increase observed in the UTS at this condition. The UTS of those samples aged at 225 ℃ wh ich decreased abruptly as the soaking time increased fro m 3 to 4 hours and thereafter increases sharply, can be exp lained by the explanation given for the behaviour of the samp les aged at 200℃, but because of higher ageing temperature the amount of β-Al 5 FeSi phase is mo re wh ich later transformed to Be -Fe phase as the soaking time is increased to 5 hours [12].   Figure 2. for samp les soaked at the different ageing temperature for 3 and 4 hours, the UTS is almost constant (and the same) as the ageing temperature increased from 125℃ to 175℃, thereafter, the UTS reduced (though at different rates) with increase in the ageing temperature. For samp les soaked at the different ageing temperature for 5 hours, it is obvious that the UTS increases with increase in the ageing temperature, the maximu m UTS is attained at around 175℃, thereafter, increase in the ageing temperature results in reduction in the UTS o f the samples. Moreover, in Figure 3. the effects of various ageing temperature on the ductility of the alloy samp les is clearly seen. As expected, increased hardness will defin itely lead to reduced ductility, [13]: The ductility of the alloy samples soaked for 3 and 4 hours at various ageing temperature reduces with increasing ageing temperature (though at different rates), wh ile the ductility of the samples soaked for 5 hours first increases with the ageing temperature up to about 200 ℃ before reduction in the ductility follows further increase in the ageing temperature.  Figures 4. and 5. relate the effects of ageing temperature and soaking time at the ageing temperature respectively on the impact strength (absorbed energy) of the alloy samples. It could be seen that for samp les soaked for 3 and 5 hours, the impact strength increases with the ageing temperature; this is expected the stiffness of the samples increases with the ageing temperature as exp lained in Figure  3. above. For those samples soaked at the various ageing temperature for 4 hours, the impact strength reduces with increase in ageing temperature. This is also clear in Figure 5.; this is fully exp lained by the exp lanations offered on Figure  1..

Conclusions
Fro m the discussion thus far, it can be concluded that: • Optimu m tensile strength is observed in the sample aged at 175℃ held fo r 5hrs.
• Optimu m ductility is observed in sample aged at 125℃, held for 4hrs.
• Optimu m toughness is observed in sample aged at 225℃, held 5hrs, closely fo llo wed by that of the samp le aged at the same temperature, held fo r 3hrs.
• Fro m the economic point of v iew the optimu m parameter for obtaining a structural material of the optimu m combination of mechanical p roperties fro m the sample is ageing temperature of 225℃ and soaking time of 3hrs.