2020 Volume 61 Issue 8 Pages 1703-1706
Precursor foaming is one of the production processes for manufacturing aluminum foam. In this study, the continuous foaming of multiple precursors by combining a conveyor and optical heating with halogen lamps was conducted. First, in a preliminary test, the current of the halogen lamps was adjusted to enable the foaming of a precursor as it moves on the conveyor. Then, multiple precursors were continuously foamed at the adjusted current of the halogen lamps. It was shown that the multiple precursors exhibited similar temperature histories and were sufficiently foamed using the conveyor.
Fig. 4 (a) Foaming behavior of three precursors. The precursors were moved from left to right. The right side of each figure shows the enlarged images of the precursor. (b) T–t relationships corresponding to (a).
Aluminum (Al) foam has low density owing to its numerous pores. It has the potential for application in vehicle components and construction materials, where lightweight materials are required. In these industrial fields, high productivity in production is desired. Precursor foaming is one of the production processes for manufacturing Al foam.1,2) In the process, a foamable precursor is first fabricated by mixing blowing agent powder into Al. The heat treatment of the precursor induces the generation of gases through the decomposition of the blowing agent, resulting in the formation of pores in Al. For example, TiH2 and CaCO3 are used for foaming Al. Recently, the automated foaming of Al foam has been demonstrated using a furnace equipped with a conveyor.3) In this process, the precursor was moved into the furnace by the conveyor. The precursor was foamed as it passed through the furnace, then it was cooled after it left the furnace. The doors of the furnace were closed to decrease the temperature variation. Therefore, although conveyor was used, only one product could be foamed in one cycle. The conveyor was heated at the same time so its thermal resistance was necessary.
Optical heating is an alternative to heating in a furnace. Garcia-Moreno et al. used halogen lamps to rapidly foam a precursor in an X-ray system.4) Kobashi et al. used infrared heater to observe the foaming behavior of Al precursors.5) Optical heating can heat the precursor directly and reduce the thermal damage of the conveyor. In addition, it is expected that the combination of the conveyor and optical heating can easily control the foaming temperature of the precursor with high responsivity by varying the electric power of the lamps, the number of lamps, and the velocity of the conveyor.
In this study, the continuous foaming of multiple precursors by combining a conveyor and optical heating was conducted. First, the number of halogen lamps and the velocity of the conveyor were set constant, and the current of the halogen lamps was adjusted to enable the foaming of one precursor. Then, multiple precursors were continuously foamed at the adjusted current of the halogen lamps. The foaming behavior and temperature of the precursors were observed during their movement on the conveyor.
The foamable precursors were fabricated by the friction stir processing route.6) A1050 commercially pure Al rolled plates were used as the raw material of the precursors. A mixture of a blowing agent (TiH2 powder, <45 µm) and a stabilization agent (Al2O3 powder, ∼1 µm) was mixed into the A1050 plates by friction stir processing. The amounts of blowing and stabilization agents were 1 mass% and 5 mass% relative to the mass of the stirred region, respectively, in accordance with Ref. 6). After the powders were mixed into the A1050 plates by friction stir processing, 5 mm × 5 mm × 5 mm precursors were obtained by the machining of the stirred region.
Figure 1 shows a schematic illustration of foaming a precursor using a conveyor. Three 2 kW halogen lamps with total width of 150 mm were used. The precursor was placed on a ceramic honeycomb, with the initial surface of the precursor 20 mm from the lamps, which was moved at a constant velocity (approximately 1.8 mm/s) by a robot arm. It is well known that the precursor starts foaming above the solidus temperature and vigorously foams above the liquidus temperature.7–9) The solidus and liquidus temperatures of A1050 are 646°C and 657°C, respectively.10) The current I of each lamp was varied from I = 6 A to 8 A. The temperature T was measured by a K-type thermocouple at the center of the precursor, with the thermocouple inserted into a previously drilled hole. Then, the continuous foaming of three precursors was conducted three times at the adjusted current using the same setup as described above.
Schematic illustration of foaming a precursor using conveyor.
Figure 2 shows the foaming behaviors of the precursor at each current of the lamps for different moving times t along with the enlarged images of the precursor. The precursor under the lamps moved from left to right in the figures. Figure 2(a) shows the foaming behavior at I = 6 A. No foaming was observed even after the precursor had passed under the three lamps. Figure 2(b) shows the foaming behavior at I = 7 A. Foaming started as the precursor moved between the second and third lamps (t = 78 s), and sufficient foaming had occurred by the time the precursor reached the third lamp. Figure 2(c) shows the foaming behavior at I = 8 A. Foaming started around the second lamp (t = 55 s), and sufficient foaming had already occurred between the second and third lamps. Heating continued when the precursor moved under the third lamp.
Foaming behaviors of the precursor at each current of the lamps. The precursor was moved from left to right. The right side of each figure shows the enlarged images of the precursor.
Figure 3 shows the corresponding T–t relationships. The ranges of Lamps A, B, and C correspond to those observed when the precursor was moving under the three lamps (cf. Fig. 1). The heating rate of the precursor increased with increasing the current of the lamps. For I = 6 A, the precursor did not reach the liquidus temperature. For I = 7 A, the precursor reached the solidus–liquidus coexistence region and its temperature slightly exceeded the liquidus temperature. For I = 8 A, the precursor reached the solidus–liquidus coexistence region much earlier than in the case of I = 7 A, its temperature significantly exceeded the liquidus temperature, and the foamed precursor remained at a high temperature for a long time. As shown in Fig. 2(c), the foamed precursor started to shrink around t = 82 s owing to excessive heating, leading to the coalescence and collapse of pores. From these results, it was found that I = 7 A was appropriate for sufficient foaming. However, insufficient foaming was observed at I = 7 A in some cases. This is considered to be because the current was adjusted by a dial and some scatter may have occurred. Therefore, we subsequently conducted continuous foaming at I = 7.5 A to ensure sufficient foaming while avoiding excess heating.
T–t relationships corresponding to the foaming behaviors of the precursors shown in Fig. 2.
Figure 4(a) shows the foaming behavior of three precursors at I = 7.5 A. The precursors were moved from left to right and heated one after the other as they passed under the lamps. At t = 58 s, no foaming was observed. At t = 85 s, the first precursor (I) started foaming. At t = 96 s, precursor I finished foaming and the second precursor (II) started foaming. At t = 107 s, precursor II finished foaming and the third precursor (III) started foaming. Therefore, it was shown that multiple precursors can be continuously foamed using the conveyor. Figure 4(b) shows the corresponding T–t relationships. There was some scatter of the T–t relationships among the precursors. This is because T was measured by a thermocouple, which may have moved inside the precursor during heating owing to the softening and foaming of the precursor. However, the temperatures of all three precursors exceeded the liquidus temperature. The porosities of the three Al foams measured by Archimedes’ principle were around 70–80%, which are typical porosities obtained by precursor foaming.11,12) A similar tendency was observed in two other trials. Consequently, it was shown that the multiple precursors exhibited similar temperature histories and were sufficiently foamed using the conveyor.
(a) Foaming behavior of three precursors. The precursors were moved from left to right. The right side of each figure shows the enlarged images of the precursor. (b) T–t relationships corresponding to (a).
In this study, the continuous foaming of multiple precursors was conducted by combining a conveyor and optical heating. The following conclusions were obtained.
This work was financially supported by the Light Metal Education Foundation, Inc.
