Enhancement of ductility for Al–Si–Fe alloy by refining β phase with a novel ball-die extrusion
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The present study proposes a novel process for refining a coarse and brittle intermetallic compound of Al9Si2Fe2 (β phase) in Al-12 mass%Si-2 mass%Fe alloy by an extrusion process using a ball-die, i.e., arranged ceramic balls instead of a conventional die. The alloy passes through the gap of the arranged balls, and subsequently by passing through a shape-giving die, the alloy is extruded as a rod. Size and arrangement of the balls are varied and the effect of them on the refinement of the β phase is investigated. Finer β phase was obtained with the larger contact area of the balls and the alloy. By the ball-die extrusion, the β phase was refined especially in the center of the extrudate due to the collision of the alloy extruded from the gaps of the balls. Also, the size of the eutectic Si and the grain size of the Al matrix were slightly refined by the ball-die extrusion. The ball-die extrudate exhibited higher elongation than the one of the conventional extrudate. The maximum elongation of 26% was obtained with the ball-die extrudate.
KeywordsAl–Si–Fe alloy Ball-die extrusion β phase Elongation
Al–Si alloys are widely used for automotive parts because of their light weight, excellent castability, high wear resistance and low thermal expansion coefficient [1, 2]. Recently, consumption of the alloys for the automotive parts have been increasing to reduce the vehicle weight, so as to achieve an increase in fuel efficiency and a reduction in CO2 emission. Moreover, more and more Al alloys have been produced from recycled Al scrap for the resource saving . Therefore, it has been expected that the increase in the usage of the recycled Al alloys in the practical application can mitigate the global warming problems. However, most of the automotive components are produced from wrought alloys, which are produced only from virgin ingot of Al, not from recycled scrap. To produce secondary Al alloys from recycled scrap, it only takes approximately 5% of energy comparing to production of Al alloys from virgin ingot . Thus, even if weight of vehicles is decreased by using Al alloys instead of steel, as far as the wrought alloys produced from virgin ingot are used, still a large amount of energy is required, and the global warming problems remain. Therefore, it is necessary to establish a recycling process for fabrication of wrought alloys.
Although such recycling process producing wrought alloys from wrought alloys has been required, can recycling is the only established process for wrought alloys . One of the reasons why the recycling of wrought alloys has not been promoted is due to the strict limit of the impurity. The upper limit of the impurity is much smaller for the wrought alloys than for the cast alloys. During a recycling process, Al is often contaminated by Fe, which forms a coarse and brittle intermetallic compound such as Al9Si2Fe2 (β phase) in Al–Si alloys . The β phase can be an initiation site of cracking and therefore, it significantly deteriorates mechanical properties of alloys . It has been known that refinement of the β phase is effective to mitigate the deterioration . For example, dramatic reduction in the size of the β phase has been achieved by addition of inoculant elements or the application of ultrasonic vibration to the molten alloy . However, some problems have been remained, e.g., formation of other brittle phases or a narrow range of applicable temperature during solidification. Thus, in the present study, we propose a novel process to refine the coarse and brittle β phase in Al–Si–Fe alloys. We propose an extrusion process which is specialized for refining the β phase by using arranged ceramic balls instead of a conventional extrusion die. During the extrusion process, a metal billet passes through the gaps of the arranged balls, and the metal is extruded as a rod by subsequently passing through a conventional die. The gaps between the balls play a role of a bearing part of a conventional die, and the total contact area between the metal and the balls is much larger than that of a conventional die. Thus, we can expect a high strain on a metal by the ball-die extrusion. In the present study, the effect of the newly developed ball-die extrusion on the refinement of β phase is investigated by changing the size and arrangement of the balls. Moreover, the effect of the refinement on the mechanical properties is also investigated.
An pure Al ingot with a purity of 99.99 mass%, Al-24.5 mass%Si alloy ingot, and pure Fe powder with a purity of 99.0 mass% were cast into a square bar with a cross section of 40 × 40 mm at 993 K to prepare an Al-12 mass%Si-2 mass%Fe alloy billet. The bar was cut into a cuboid of 40 mm in height for extrusion.
To investigate the effect of the ball-die extrusion on the refinement of the β phase and the eutectic Si in the Al–Si–Fe alloy, the microstructures of all the extrudates and the as-cast billets were observed with an optical microscope, and the lengths of the β phase and the eutectic Si were measured. The microstructure of the Al matrix was also observed and analyzed with the Electron BackScatter Diffraction (EBSD).
To investigate the effect of the ball-die extrusion on the mechanical properties, tensile tests were conducted. The specimen was prepared with the wire discharge machine to coincide the tensile direction and the extrusion direction. The thickness and the gage length of the specimen were 1.4 and 5.2 mm, respectively. The tests were conducted at room temperature and at an initial strain rate of 1.0 × 10−3 s−1. The fractured surface was observed with Scanning Electron Microscopy (SEM) to investigate the fracture mechanism.
3 Results and discussion
3.2 Tensile test
The elongations of the conventional extrudate (Fig. 8b), and the small-ball extrudate (Fig. 8c) are 20.5 and 22.7%, respectively. All the tests were conducted three times for each kind of specimen, and standard deviations in elongation of the conventional and the small-ball extrudates were approximately 0.39 and 0.90%. Thus, the differences in elongation between the two specimens, (b) and (c), are slight but significant. Consequently, it has been shown that the ball-die extrusion can improve elongation of alloys more in comparison with the conventional extrusion. The elongation of the large-ball extrudate (Fig. 8d) was 23.4%, which was almost the same as the one of the small-ball extrudate. Moreover, the double-layered extrudate shows the largest elongation of 26.0% among all the extrudates. The standard deviation of all the elongation of the extrudate were less than 1.5%, which is so small that the elongation differences among the extrudates can be considered as significant.
As shown in Sect. 3.1, the average lengths of the β phase in the double-layered, the small-ball, the large-ball, and the conventional extrudates increase in this order. The total elongation increases in reverse order. Thus, it has been confirmed that the elongation clearly increases with the decrease in the length of the β phase. From the results, it has been demonstrated that the length of the β phase has a clear correlation with the elongation of the alloy. However, in the present Al-12 mass%Si-2 mass%Fe alloy, there are other factors which can affect the mechanical properties of the alloy, i.e., the eutectic Si and the grain size of the Al matrix. In the next section, the effect of the ball-die extrusion on the refinement of the eutectic Si and the grain size of the Al matrix are demonstrated.
3.3 Al matrix
3.4 Eutectic Si
It has been known that hard Si particles can deteriorate mechanical properties of materials . Therefore, in the present study, not only the size of the β phase but also the size of the eutectic Si influences on the elongation of the alloy. However, it should be noted that the differences in size of the eutectic Si were small and they were only few micro meters in lengths, unlike that of the β phase. Therefore, the improvement in elongation of the alloy in the present study is reasonable to be considered mainly due to the refinement of the β phase.
The β phase (Al9Si2Fe2) in the Al-12 mass%Si-2 mass%Fe alloy was refined by the newly developed ball-die extrusion. As the surface area of the balls used in the extrusion increases, the imposed strain on the alloy increases, and therefore, finer β phase was obtained. The finest β phase was obtained with the double-layered extrusion.
The ball-die extrudates show improvement in total elongation from the conventional extrudate. The largest elongation of 26.0% was obtained with the double-layered extrudate. As the length of the β phase decreases, the total elongation increases.
The size of the eutectic Si and the grain size of the Al matrix were slightly refined by the ball-die extrusion. The size of the eutectic Si and the grain size of the Al matrix were finer as the contact surface area at the balls and the alloy increases.
The authors would like to thank Prof. Kiyotaka Matsuura and Prof. Munekazu Ohno at Hokkaido University for their valuable comments and discussion. The authors are grateful to Prof. Seiji Miura and Prof. Ken-ichi Ikeda at Hokkaido University for their assistance in the tensile tests. This work was partially supported by Nanotechnology Platform Program of the Ministry of Education, Culture, Sports, Science and Technology (MEXT), Japan.
Compliance with ethical standards
Conflict of interest
The authors have declared that no competing interests exist.
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