Effect of Artificial Aging Temperature on Mechanical Properties of 6061 Aluminum Alloy

Aluminum alloys have been attracted by several engineering sectors due to their excellent strength-weight ratio and corrosion resistant properties. These are categorized into 1, 2, 3, 4, 5, 6, 7and 8xxx on the basis of alloying elements. Among these 6xxx series contains aluminum–magnesium–silicon as alloying elements and are widely used in extruded products and automotive body panels. The major advantages of these alloys are good corrosion resistance, medium strength, low cost, age hardening response no yield point phenomenon and Ludering. 6xxx series alloys generally have lower formability than other aluminum alloys which restrict their utilization for wide applications. Keeping in view of the shortcomings in the set of mechanical properties of 6xxx series the efforts were made to improve the tensile strength and toughness properties through age hardening. In present study heat treatment cycles were studied for 6061 aluminum alloy. Three different age hardening temperatures 160, 200 and 240 o C were selected. The obtained results showed that 17.26, 7.69, and 10.51% improvement in tensile strength, toughness and hardness respectively was achieved with solution treatment at 380 o C followed by an aging 240 o C. Microstructural study revealed that substantial improvements in the mechanical properties of 6061 aluminum alloy under heat treatment were achieved due to precipitation of Mg 2 Si secondary phase.


INTRODUCTION
properties [1][2]. Table 1 shows the capability of aluminum to be alloyed with different metals and nonmetals. Depending on the type of alloying elements, aluminum can be utilized by various sectors. It is reported that worldwide 90% pipes and tubes are manufactured from 6xxx aluminum alloy. This type of aluminum alloy contains Si and Mg as a major alloying element. 6xxx aluminum alloys are heat treatable.
Literature pertaining to 6061 aluminum alloy, hereinafter referred to as 6061-AA, reveals that heat treatments efforts are ongoing to improve the tensile strength in order to make it more efficient and reliable [3][4]. Many others have noted the challenges to improve the strength without compromising on its toughness value through age hardening treatment.
Age hardening of aluminum alloys include two major steps; solution treatment and artificial aging. Solution treatment involves heating of materials above its critical temperatures, soaking for a particular time and rapid quenching. Quenching is carried out to trap the dissolved constituents in solid solution [4][5]. Whereas, artificial aging means re-heating of the solution treated samples in the range of 175-420ºC for various time periods in order to precipitate the secondary phase [6]. Previous research revealed that the selection of appropriate temperature and time in artificial aging is the critical step. Substantial Although much efforts have been paid to improve tensile and hardness properties through age hardening, however little work is reported to evaluate the effect of secondary micro phases on the toughness and bending property.
Due to diverse applicability it is important to design such heat treatment cycle through which it could be possible to improve mechanical properties. Efforts were made to optimize the aging temperature to obtain set of better strength and higher toughness values of 6061-AA.

MATERIALS AND METHOD
Aluminum alloy (6061) was purchased from the local market (Karachi). Chemical composition of the aluminum alloy was determined by using optical emission spark spectrometer (Model: Bruker Q2-ION). The obtained average composition of 6061-AA is given in Table 2, which indicates that Mg and Si are the major alloying elements.
Before heat treatment, standard specimens of tensile, toughness and bend tests were prepared by performing machining operation as per sample specifications as shown in Fig. 1 [11][12][13][14].

RESULTS AND DISCUSSION
Tensile test results of 6061-AA are shown in Fig. 3 [15][16]. It was also validated that age hardened 6061 aluminum alloy was more strengthen as compared to heat treated Al-Ti alloys.
Detailed microstructural examination reported elsewhere, revealed that by solution treatment at 380 o C all the alloying elements were dissolved in aluminum matrix [17][18]. When the solution treated samples were aged at different temperatures the precipitation process of Si are the function of the aging temperature. We noted that at 160 o C aging temperature the SSP entered into GPZ which can be seen in Fig. 5(b). When the aging temperature was increased to 200 and 240 o C the precipitates of secondary phase Mg 2 Si were developed that can be seen in Fig. 5(cd).
It is interesting to note that by the solution treatment the UTS, toughness and hardness of the samples was decreased as compared to received samples. The sole