Characteristics of Aluminum alloys 6061 Essay

Aluminum alloys 6061 are commonly used in engineering application such as automotive parts and aircraft structure because of their excellent corrosion resistance, light weight, and good weldability. However, there is still undesired characteristics of this alloy such as no fatigue limit, less fatigue resistance and a large scatter in fatigue life [1] compared to steel [2]. Therefore, fatigue behavior of this alloys has been subjected to numerous studies over the years to improve fatigue material properties [3,4]. For instance, several researchers [5,6] have developed a new 6061-based alloy with an additional content of Mg, and they have successfully confirmed the ability to generate fatigue limit in f.c.c metal via fatigue test and coaxing effect test. However, the effect of excess Mg on fatigue crack propagation behavior in the Al alloys has not been investigated sufficiently.
Generally, the fatigue crack growth can be divided into two stages stage I and stage II. The cracks begin to grow across several grains in the stage I are strongly affected by slip characteristics and material surface condition such as microstructure dimension, surface roughness, and corrosion. Meanwhile, in stage II, a crack tends to propagate perpendicular to the loading direction, and the fatigue crack growth rate (FCGR) depends on the crack growth resistance of the material. An important characteristic of this stage is the presence of fatigue striation formation that represents Mode I fatigue crack growth [7].
The aim of this study was to investigate the influence of excess Mg on fatigue crack propagation behavior in stage II region. The investigation has not limited to the observation of fatigue crack growth on the specimen surface but also included the fractographic examination on the fracture surface. The implications of the results for the fatigue crack propagation and the striation formation were then discussed.
2. Material characterization
The present study is dealing with two types of newly developed precipitation-hardened Al6061-T6 alloys with an additional element of Zirconium (Zr) in both Al alloys and the presence of excess Mg in the latter, in which we denote as Al6061-Zr and Al6061-Zr-Mg. The addition of Zr to Al alloys has exhibited positive effects on microstructure via grain refinement and improvement in the mechanical properties [8]. On the other side, the influence of Mg in Al alloys has been investigated to promote dynamic strain aging (DSA) and to assist non-propagation of a crack in the fatigue limit regime [9]. Details about chemical composition for both Al alloys are listed in Table 1.
The manufacturing processes for both Al alloys were as follows. The ingots of these alloys with a diameter about 155 mm were obtained by a semi-continuous casting method and subjected to a homogenization treatment at 823 K for 14,400 s. The billet was extruded into a bar with a diameter of 23 mm at 773 K. The extruded bar was then solid solution treated at 813 K for 3600 s in an air furnace and immediately water-quenched. Subsequently, T6 aging was carried out at 463 K for 14,400 s.