2018 Volume 59 Issue 12 Pages 1952-1955
A point group mold, whose surface consists of tips of pins arranged to form the desired shape, was newly used to fabricate Al foam with complex shapes. Although dimples formed by the pin tips were clearly observed, the surface of the Al foam retained the shape of the point group mold. No protrusion of the Al foam through the gaps between the pins during foaming was observed. Namely, the surface of the point group mold was transferred to the Al foam.
Aluminum (Al) foam is expected to be used in various industrial fields where lightweight materials are necessary.1,2) The precursor foaming process is one of the popular processes for fabricating Al foam,3,4) where the precursor is solid Al containing blowing agent. For use in industrial fields, Al foams with complex shapes are required. Generally, Al foams with complex shapes are fabricated from a foaming precursor in a mold.5,6) However, the fabrication of molds with complex shapes takes a long time and has a high cost. In addition, the mold is heated together with the precursor during foaming, which requires more heat energy than only foaming the precursor, because the mold should also be heated to slightly below the foaming temperature.
Hangai et al.7,8) demonstrated that the precursor can be foamed by optical heating and that a steel mesh can be used as a mold. Namely, the precursor was directly heated to induce foaming by light passing through the openings of the steel mesh, and the foamed Al could be shaped by the steel mesh with little protrusion of the Al foam through the mesh openings. Therefore, it was demonstrated that the expansion of Al foam was restricted by the wires of the steel mesh, i.e., a mesh was sufficient to restrict the expansion of Al foam owing to the surface tension of the Al foam. Namely, a dense surface is not necessary for a mold used to produce Al foam.9) However, reuse of the steel mesh was difficult.
In this study, the use of a point group mold for foaming was proposed, whose surface consists of tips of pins arranged to form the desired shape. First, free foaming of a precursor was conducted on a mold with a plane surface consisting of the tips of arranging pins. Next, a precursor was foamed on the same plane surface while pressing the foaming Al against the surface. The effect of pressure on the protrusion of the Al foam through the gaps of the pins and on the surface pattern of the obtained Al foam was investigated. Finally, Al foam with a curved surface was fabricated using a mold with a curved surface consisting of a group of tips of arranging pins. From these experiments, the possibility of using a point group mold to fabricate Al foam with a complex shape was demonstrated.
Figure 1 shows the setup of the foaming precursor using the point group mold. SUS304 stainless steel pins, as shown in Fig. 2, with 0.8 mm diameter, 35 mm length, and approximately 1.5 mm tip diameter (BUNKA GAKUEN, Tokyo, Japan) were used. These pins were arranged by inserting them into the holes of a ceramic honeycomb (RZ-400, Shinfuji Burner Co., Ltd., Aichi, Japan) with tips of the pins forming the surface of the mold. The intervals between adjacent pins were 1.5–1.7 mm. A precursor was placed on the tips of the arranged pins. Four halogen lamps were used for the heat treatment of the precursor. The current and voltage of each lamp were 9 A and 180 V, respectively. The distance between the lamps and the top surface of the precursor was 30 mm. If necessary, a steel mesh was placed on top of the precursor to restrict its upward expansion during foaming and to apply downward pressure via the foaming force. The steel mesh was made of SUS304 stainless steel with a sieve mesh size of 1.30 mm, wire diameter of 0.29 mm, and opening ratio of 66.9%. The foaming behavior of the precursor during the heat treatment was recorded by video camera. Three foaming experiments (Samples 1–3) were conducted.
Schematic illustration of foaming precursor on point group mold.
Steel pin used in this study.
Figure 3 shows the fabrication process of the precursors. The friction stir welding (FSW) route10) was employed. First, as shown in Fig. 3(a), two ADC12 (Al–Si–Cu alloy) die-casting plates and a mixture of blowing and stabilization agent powders were laminated. TiH2 (< 45 µm, 1 mass%) and Al2O3 (∼ 1 µm, 5 mass%) were respectively used as the blowing and stabilization agents. Details of the fabrication procedure of ADC12 die-casting plates have been described elsewhere.11) Then, as shown in Fig. 3(b), FSW was conducted on the laminated plates. Multipass FSW, as shown in Fig. 3(c), in which FSW was conducted along several lines,12) and overlapping FSW, as shown in Fig. 3(d), in which FSW was conducted along the same lines as before,13) were employed to obtain a large area of precursor and to thoroughly mix the powder mixture into the Al plates. Precursors with dimensions of 15 × 15 × 6 mm3 were machined from the FSW-stirred zone, as shown in Fig. 3(e).
Schematic illustration of fabrication of ADC12 precursors by FSW route.
Figures 4(a)–(c) show the foaming behavior of Sample 1. Figure 4(a) shows the precursor when the lamps were turned on and heat treatment began. Figure 4(b) shows the precursor shortly after the foaming began, and Fig. 4(c) shows the Al foam shortly before the lamps were turned off and the foaming stopped. Foaming time was 228 s. No protrusion of the Al foam through the gaps between the pins was observed during foaming. The bottom of the Al foam retained the plane surface of the group of pins. Figure 4(d) shows the bottom surface of the obtained Al foam in contact with the pins. Dimples formed by the pin tips were observed but a plane surface matching that of the point group mold was retained. Figure 4(e) shows a cross-sectional X-ray CT image of the obtained Al foam. The precursor was sufficiently foamed even on the point group mold.
(a)–(c) Foaming behavior during free foaming, (d) bottom surface of obtained Al foam in contact with pins and (e) cross-sectional X-ray CT image (Sample 1).
Figures 5(a)–(c) show the foaming behavior of Sample 2. Figure 5(a) shows the precursor when the lamps were turned on and heat treatment began. Figure 5(b) shows the precursor shortly after the foaming began. The precursor was foamed by the light passing through the steel mesh. Figure 5(c) shows the Al foam shortly before the lamps were turned off and the foaming stopped. Foaming time was 138 s. The upward expansion of the foaming was restricted by the steel mesh, consistent with our previous study,7) which may have applied downward pressure against the surface of the point group mold. No protrusion of the Al foam through the gaps between the pins was observed during foaming. The bottom of the Al foam retained the plane surface of the group of pins despite the application of downward pressure. Figure 5(d) shows the bottom surface of the obtained Al foam in contact with the pins. The dimples formed by the pin tips were more pronounced than those of Sample 1, but a plane surface matching the surface of the point group mold was similarly retained. Figure 5(e) shows a cross-sectional X-ray CT image of the obtained Al foam. The precursor was sufficiently foamed on the point group mold even applying downward pressure using the steel mesh.
(a)–(c) Foaming behavior while applying downward pressure, (d) bottom surface of obtained Al foam in contact with pins and (e) cross-sectional X-ray CT image (Sample 2).
Figures 6(a)–(b) show the foaming behavior of Sample 3. Figure 6(a) shows the precursor when the lamps were turned on and heat treatment began, where the surface of the pins was arranged to form a curved surface. Figure 6(b) shows the Al foam shortly before the lamps were turned off and the Al foam was subjected to press forming. Foaming time was 194 s. The surface of the Al foam almost matched that of the group of pins owing to the weight of the Al foam itself and the foaming force, and became curved. No protrusion of the Al foam through the gaps between the pins was observed during foaming. Figure 6(c) shows the Al foam after press forming and solidification. The press forming was carried out using the SUS304 stainless-steel mesh introduced in section 2.1 immediately after the lamps were turned off. The contact of the Al foam on the surface of the point group mold was increased by pressing the Al foam. This resulted in the surface of the Al foam following that of the point group mold. However, no protrusion of the Al foam through the gaps between the pins was observed despite subjecting the Al foam to press forming. Figure 6(d) shows the surface of the Al foam in contact with the pins and Fig. 6(e) shows a cross-sectional X-ray CT image of the obtained Al foam. Although dimples formed by the pin tips were observed and pore structures were not homogeneously distributed, it was found that the curved surface of the point group mold was transferred to the Al foam. The formed Al foam was easily separated from the point group mold and the mold can be used repeatedly. From these results, it is expected that Al foams with complex shapes can be fabricated using point group molds.
(a)–(b) Foaming behavior of precursor placed on curved surface. (c) Al foam after press forming. (d) Bottom surface of obtained Al foam in contact with pins. (e) Cross-sectional X-ray CT image (Sample 3).
A point group mold, whose surface consists of the tips of arranging pins, was newly used to fabricate Al foam with complex shapes. The experimental results led to the following conclusions.
This research is partially supported by the Matching Planner Program from Japan Science and Technology Agency, JST.
