MICROSTRUCTURAL AND MECHANICAL PROPERTIES AL 6061 PROCESSED BY COLD ROLLING AND AGING

An Aluminum alloy 6061was produced by cold rolling and subsequently aging. Cold rolling is used for reducing grain size and aging is used for producing second-phase, thus improving hardness, strength and wear resistance. In the present paper, a process through cold rolling with reduction of 50%, 60% and 70% in thickness then aged at 2000C for 1.800, 3.600, 5.400, 7.200 and 10.800 sec. Microstructural analysis shows that the grain morphology consists of equalized grain for as-received samples and elongated grain for sample after cold roll and aging. X-Ray Diffraction (XRD) analysis showed peaks phases in Al 6061 after cold rolled and aging. Hardness measurements showed that combination between small grain size and presence of second-phase led to a significant increase in the hardness. Maximum hardness of 121 HBN is achieve for sample after cold roll 70% and aged for 5400 second. The wear resistance testing showed that lost particles of 6,209951 x 10-6 mm3/mm for received sample and 4,775436 x 10-6 mm3/mm for sample after aging process. It is showed that high hardness related to increase the wear resistance. It is reasonable that the presence of second phases are function as obstacle for dislocation movement.


EXPERIMENTAL PROCEDURES
The  After recrystallization process is completed, rise new grains replaced all the grain before.

RESULTS AND DISCUSSION
If the temperature is raise, grains will be enlarged by consuming grains with a smaller size, so the size with smaller grains will slowly shrink and then disappear altogether.
Figure 1    The width of the peak in the XRD profile shows no significant difference.As we know, that broadening peak will affect the mechanical properties of the material [ , ].But in this case there are no significant difference.This indicates that the role of the second phase does not significantly affect the mechanical properties of Al-6061.
Aluminum (Al) has been used widely in military, aerospace and automotive industries that required light weight and good corrosion resistance.Aluminum has become the material of choice for aircraft's industry since 1930's [ ]. Generally, the aircraft's industry used Al 2xxx and 7xxx series as a material for aircraft's components.However, since Al 6061 is becoming increasingly widespread applications with many advantages such as good formability, excellent corrosion resistance, good weld ability, and low price, Al 6061 becomes an alternative material for aircraft components, in this case the suitable part is skin of airplane wings [1, , ]. Al 6061 compared to Al 2xxx and 7xxx series is much lower strength.Then, we need to find suitable procedure for improve the mechanical properties of Al 6061.In order to achieve the maximum mechanical properties, required suitable heat treatment, there effective method are cold rolling and aging.Heat treatment is one of the alternative methods that can be used to increase strength without alloying.In this study, the heat treatment used is cold rolling and aging.Cold rolling cause deformation of the material characterized by reduced material thickness.When the deformation occurred, the dislocations movement.The concentration of dislocations becomes dislocation pile-up, thus the mechanical properties increase.Unlike the process cold rolling, strengthening of material through the process of aging, generated through precipitation strengthening.In Al 6061, Mg2Si is precipitate formed by aging.This precipitate has function as barrier of movement of dislocations, which increasing its mechanical properties.When cold rolling was combined with aging, both of them will completing each other.The aim of this work is to determine the effect of cold rolling and the aging process on microstructural and mechanical properties of Al 6061.Despite the cold rolling and aging process of Al 6061 has been studied previously but understanding of these processes is still incomplete because of the compositions, processes and parameters used affect the results obtained.

Figure 1 BFigure 1 .
Figure 1 shows the microstructure of Al 6061: (a) as received, (b) after cold rolled, (c) after the cold rolled and aging.Microstructure of Al 6061 as-received is equaxial, then changed to a elongated grains after cold rolled and back into equaxial after the aging with micron scale grain size.Grains Al 6061 as received is very coarse grain with size about 200 μm and after cold rolled and aging turns into equaxial grains with a size about 36.28 µm.
Figure 1 shows the microstructure of Al 6061 after cold rolled with reduction in thickness: (a) 50%, (b) 60%, and (c) 70%.Items result showed elongated grain and most length at 70% reduction in thickness with average grain size about 34,6 µm.Normally, do not occur refining grain size in Al 6061, because the treatment temperature used below the recrystallization temperature except when the solution treatment.Grains only elongated to the direction roll and after aging, grains changed into equaxial.

Figure 2 .
Figure 2. Microstructure of Al-6061 after cold rolled with reduction in thickness (a) 50%, (b) 60%, (c) 70% Figure 2 shows X-Ray Diffraction (XRD) profiles of Al 6061 after cold rolled and aging with reduction in thickness: (a) 50%, (b) 60%, (c) 70%, for 1800 ( a), 3600 (b), and 5400 sec.(c) is included sample as solution treatment.XRD profiles show that there are three phases in Al 6061 after cold rolled and aging and single phase on Al 6061 as solution.Phases include Al, Mg2Si, and Al2FeSi.Al2Fesi and Mg2Si phases identified as second phase in Al 6061.

Figure 3
Figure 3 shows hardness for Al 6061: (a) as received, as solution treatment, and as rolled; (b) after the cold rolled and aging.

FigureFigure 4 .
Figure 5a shows hardness of Al 6061 samples as received, as solution and as rolled an increased hardness consistently.Hardness of Al 6061 as received, as solution treatment and as rolled (as rolled) are 65 HBN, 76 and 115 HBN respectively.Al 6061 as received has low hardness.However, hardness increased after solution and more after cold rolled.As explained before, Al 6061 as received has a very coarse grain size, but after solution treatment, grains were recrystallized then form a new single phase that has grains finer, so the hardness increases.After Al 6061 processed cold rolled, Al 6061 plastically deformed that cause the grains changed become elongated grains.When Al 6061 plastically deformed by cold rolled, the density of dislocation and lattice strain were increased.Those things that cause an increase in the mechanical properties of Al 6061.It is also stated by previous studies conducted on Al 5083 [ ], Al 6061 [ ] and AA1050 / AA6061 composite [ ].

Figure
Figure 4b shows hardness of Al 6061 after the cold rolled and aging.Unlike Figure 5a, this picture shows no significant hardness increasing after aging.Increased hardness of Al 6061 after aging caused by the presence of a second phase.This is consistent with the XRD profiles.Overall, XRD profiles show that there are other phases than Al.There are Mg2Si and Al2FeSi.This phase called second phase and Mg2Si is precipitate which formed after aging.Precipitate has function improving the mechanical properties of Al 6061 by blocking the movement of dislocations in Al when subjected to deformation.Therefore, Al become stronger [1, 3, , , , ].However, in this study, the hardness does not show significant improvement.There may be a less effective because of the aging resulting precipitates are not spread evenly on Al 6061 or caused by the large size precipitates.The precipitate visible when using Transmission Electron Microscopy (TEM) based on previous studies on Al 6061 [4, , , ]. Hardness increased with percent

Table 1 .
Chemical Composition of Al 6061