Mechanical properties of fine-grained aluminum alloy produced by friction stir process
Introduction
The friction stir process (FSP) has been actively studied as a new solid state welding technique [1], [2], [3], [4], [5], [6], [7], since its invention at The Welding Institute (TWI) in 1991 [8]. The usefulness of this process as a welding technique has been demonstrated in a number of studies, especially for aluminum alloys [1], [2], [3]. During this process, a large degree of plastic deformation is imparted to a workpiece by the mechanical stirring action of a rotating tool. Previous researchers [4], [5], [6], [7] have reported that this deformation leads to the formation of very fine equiaxed recrystallized grains within the friction stir-processed zone (FZ). The grain sizes were approximately 10–100 times smaller than those in the original workpiece materials [4], [5], [6]. In short, the results of these studies suggest that FSP can cause grain refinement through severe plastic deformation, similar to equal channel angular pressing (ECAP) [9], accumulative roll bonding (ARB) [10] and high-pressure torsion [11].
In this context, FSP has been studied by our group not as a welding technique but as a new grain refinement process [12], [13]. In our previous research [13], 1050 aluminum alloy with ‘submicrocrystalline structure’ was successfully obtained through only a ‘single pass’ of FSP. This result demonstrates that FSP is highly effective at grain refining. On the other hand, the maximum temperature which the FZ reached during processing decreased with the tool rotation speed, so that the grain size within the FZ decreased.
In the present research, 1050 aluminum alloy was processed through only a ‘single pass’ of FSP at various tool rotation speeds. The influence of the tool rotation speed on the mechanical properties, i.e. hardness and tensile strength, of the friction stir-processed (FSPed) 1050 aluminum alloy was then experimentally investigated.
Section snippets
Experimental procedure
The starting materials were monolithic cold-rolled plates of 1050 aluminum alloy with a nominal composition of 0.4 wt.% Fe, 0.25 wt.% Si, 0.05 wt.% Cu, 0.05 wt.% Mn, 0.05 wt.% Mg, 0.05 wt.% Zn, 0.03 wt.% Ti and the balance Al. The dimensions of the workpiece were 45 mm × 100 mm × 5 mm. Fig. 1(a) shows a schematic illustration of a specially designed tool for FSP, which is composed of a cylindrical shoulder and a subconical headpin. This special headpin has a smaller contact area with the workpiece
Microstructure
Fig. 2 shows typical optical micrographs of the cross-section perpendicular to the tool traverse direction of the FSPed specimen. The FZ appears brighter than the unprocessed zone (UZ). The UZ was composed of large grains highly elongated along the rolling direction of the starting material. The cross-section of the FZ exhibited subconical morphology similar to that of the headpin (Fig. 1(a)). In addition, the maximum depth and width of the FZ were about 3 and 7 mm, equal to the headpin height
Conclusions
1050 aluminum alloy was processed through only a ‘single pass’ of the FSP under various tool rotation speeds. The influence of the tool rotation speed on the hardness and tensile strength of the friction stir-processed 1050 aluminum alloy was then experimentally investigated. The following results were obtained.
- 1.
The hardness within the FZ was higher on the AS than on the RS.
- 2.
In the transition zone between the FZ and the UZ, the variation in hardness was more drastic on the AS than on the RS.
- 3.
The
References (18)
- et al.
Mater. Des.
(1997) - et al.
Fusion Eng. Des.
(2000) - et al.
Scr. Mater.
(1997) - et al.
Scr. Mater.
(1997) - et al.
Scr. Mater.
(1998) - et al.
Scr. Mater.
(1998) - et al.
Nanostruct. Mater.
(1998) - et al.
Mater. Sci. Eng. A
(1999) - et al.
Weld J.
(1996)