Aluminum Foam," ALPORAS ": The Production Process, Properties and Applications

Transcription

1 125 Alminm Foam," ALPORAS ": The Prodction Process, Properties and Applications T. Miyoshi, M. Itoh Shinko Wire Company Ltd., Izmisano, Japan S. Akiyama, A. Kitahara Kysh National Indstrial Research Institte, Tos, Japan Abstract The prodction of foamed alminm has long been considered difficlt to realize becase of sch problems as the low foamability of molten metal, the varying size of celllar strctres, solidification shrinkage and so on. These problems, however, have gradally been solved by researchers and some manfactrers are now prodcing foamed alminm by their own methods. We have been manfactring foamed alminm nder the trade name ALPORAS since 1986 employing a batch casting process. This paper presents the manfactring process, physical properties and some typical applications of ALPORAS. 1. Introdction ALPORAS is an ltra-light material with a closed cell strctre. We have been working on sch a material for many years and scceeded in its practical development. In particlar, we have improved the sond absorption characteristics of ALPORAS and we cold expand the market. We have installed a manfactring plant that is capable of making large sized blocks of foamed alminm with a better qality. The plant employs a batch casting process. With the new plant, we can control the cell size and density according to the applications. 2. Manfactring process of alminm foam ALPORAS is manfactred by a batch casting process (Fig.l ). To make alminm foam from molten alminm, it is necessary to stabilize bbbles in molten alminm. The most important factor for stabilizing the bbbles in the molten alminm is to increase its viscosity and prevent the bbbles from floating. We se 1.5 wt% Ca as a thickening agent. Ca is admixed with molten alminm at 68 C and stirred for 6 mintes in an ambient atmosphere wt% Ca, Pre AI 1.6 wt% TiH 2 68"C Thickening 68QcC Foaming Cooling Foamed block Slicing Fig.1. A manfactring process of ALPORAS J. Banhart, M.F. Ashby, N.A. Fleck: Metal Foams and Poros Metal Strctres. ttit erlag {1999)

2 126 iscosity is represented by the shearing resistance or stirring resistance of a flid, which is measred in torqe. The thickened alminm alloy is pored into a casting mold and stirred with a blowing agent of 1.6 wt% TiH 2 at 68 C. After stirring, the molten material is cred for abot 15 mintes to expand and fill p the mold. Then, the foamed molten material is cooled with a powerfl blower to harden in the casting mold. A cast ALPORAS block measres 45 x 25 x 65 mm and weighs 16 kg. After the alminm foam block is removed from the casting mold, it is sliced into flat plates of varios thicknesses according to the prpose. 2.1 Thickening method For adjsting the viscosity of a molten metal, there is a method to increase the apparent viscosity by sspending and dispersing fine solid-phase particles. Ceramic particles sch as SiC, Si 3 N 4 and Al 2 3, and the solid-phase of an alloy system in the solid-liqid zone can be sed for alminm. In these cases, however, it is difficlt to control the temperatre for keeping the sold-phase ratio at a constant vale. It is reportedly known that several hors of agitation are reqired for evenly wetting and sspending fine powders having a grain size less than 2 m in the case of SiC. The solid phase of an oxide, in contrast, can be evenly dispersed by sing an internal oxidation method sch as agitation in an ambient atmosphere, blowing the air and so on. The addition and sbseqent agitation of an element with a high oxygen affinity, sch as Ca and Mg, facilitates an oxidation process on the srface of the molten metal. This process can generate an oxide (i.e. CaO, MgO, Al 2 3, CaAl 2 4 and so on) in sch a volme as reqired for thickening the molten alminm in a short time. This method tilizes a chemical reaction within a melt and is capable of evenly wetting and dispersing fine solid-phase particles in a liqid [1-2]. Fig. 2 shows the change in torqe (stirring resistance) de to the addition of Ca to molten alminm [1]. While the pre molten alminm does not pick p torqe so mch by stirring, the addition of Ca increases the torqe (viscosity) remarkably..7 8 o.6 bn.5 ::::.4 g ill l"''l\ 2.C.4(" a,,..,"' l.oc a il. I a,oo«'r::l", r:r,. If, _...:.IJ"""""... PreAl Strring time I min Fig. 2. A change in stirring resistance de to the addition of Ca to molten AI 9 'Cf( 8.f' 7 tzl Torqe I kg-cm Fig. 3. Effect of stirring resistance on porosity of the reslting foam Fig. 3 shows the effect of torqe (stirring resistance) on porosity. When a stirring resistance is.25 kg cm, for example, bbbles easily float throgh the molten material and gas is dissipated from the srface. So, the volme of each cell is small (Fig. 4). At.45kg cm, bbbles hardly come to float in the foaming molten material and gas pressre rises so high in its center that the cell membranes collapse. Therefore, there is an appropriate stirring resistance for maximizing the foaming ratio [1].

3 127 Fig. 4. Effects of stirring resistance on foam formation 2.2 Foaming Fig.5 shows a mont of hydrogen gas prodced by the thermal decomposition of 1g of blowing agent, TiH 2, in the molten alminm. The volme was measred nder atmospheric pressre. The volmetric yield of gaseos hydrogen from the decomposing TiH 2 depends on the temperatre: The higher the temperatre, the more and the faster the gaseos hydrogen is released from TiH 2 Most of the gaseos hydrogen bbbles that are released in the first 1 seconds of stirring float throgh the molten material and catch fire. Therefore, the effective blowing gas is only generated after 1 seconds of admixing TiH 2 and stirring N 3 ::c: Ul ;::::l 2 Ul C\1 bl) '"d C) , i- -<>--. 7 C =o...-_ -M- 69 C _._68 C --A C == C --F-- _._63 C Time I sec Fig. 5. Hydrogen gas prodced t the thermal decomposition of TiH 2 in AI 3. Characteristics Typical cep strctres of two different cell sizes of ALPORAS are shown in Fig. 6. Type (a) has smaller cell sizes than the type (b) that is a conventional ALPORAS for sond absorbent. The cell sizes of type (b) are distribted over a range of 1 to 13 mm (Fig. 7(b)) with a mean cell size of 4.5 mm, (Fig. 6 (b)). The mean cell size of type (a) is 3. mm, (Fig. 6(a)). The cell sizes of type (a) are distribted over a range of 1 to 7 mm (Fig. 7(a)).

4 128 (a) A smaller cell type (b) A conventional ALPORAS Fig.6. Typical cell strctres "#. "# >. 15 >. () c: () c: Q) Q) ::I " LL Cell size I mm ::I " Ill I.. LL 1 5 tl Cell size I mm (a) A smaller cell type (b) A conventional ALPORAS Fig. 7. Cell size distribtions Fig.8 shows examples of the distribtion of the apparent density along the width of ALPORAS foamed blocks. The apparent density is high at either side of the block bt gradally decreases toward the center. Becase the foamed molten material keeps expanding ntil it hardens and the nearer center, the later it hardens. The apparent density of the prodct is between g/cm 3, which is approximately 13 times the volme of solid alminm. We can control the cell size and the density to some degrees to meet the intended se ,.3 E.28 "bjj.26, Q) = mrn Width Fig. 8. The distribtion of the apparent density along the width of foamed blocks

5 129 Typical stress-strain crves for type (a) and (b) at a qasi-static strain rate of 1 x 1 o- 3 s- 1 are shown in Fig. 9. The diagram shows a qasi-elastic region at the initial stage, then, followed by a platea region (with nearly constant flow stress). After the platea region, the flow stress rapidly increases becase the specimen densifies. The relative density of both samples is identical (.15). It is clearly observed that the platea stress in type (a), however, is higher than that in type (b). The average vales of energy absorption per nit volme of ALPORAS at a strain of.5 for type (b) and type (a) are evalated as.94 and 1.32 MJ/m 3, respectively. The energy absorption of type (a) is abot 4% higher than that of type (b). It is noted that the enhancement of energy absorption can be achieved by modifying the strctre [3]. as 6 fi 7 ALPORAS Type (a) i 5 Type (b) en UJ a: 4 ti 3 z :i z NOMINAL STRAIN, % Fig. 9. Nomirial stress-strain crves at a qasi-static strain rate of lx1-3 s- 1 CLOSE-CELLED ALUMINUM FOAM (ALPORAS) ) IJ) w a: 4 1- (J) 3 z 2 z DYNAMIC STRAIN RATE: 2.5x1 3 s- 1 \ / QUASI-STATIC STRAIN RATE: 1x1-3 s NOMINAL STRAIN, % Fig. 1. Nominal stress-strain crves at a qasi-static strain rate of 1 X 1 o- 3 s- 1 and at a dynamic strain rate of2.5x1 3 s- 1 Several stress-strain crves at a qasi-static strain rate of 1 x 1 o- 3 s- 1 and at a dynamic strain rate of 2.5x1 3 s- 1 are shown in Fig. 1. The average vales of energy absorption per nit volme of ALPORAS at a strain of.55 for the qasi-static strain rate and the dynamic strain rate are calclated to be 1. and 1.51 MJ/m 3, respectively. The absorption energy at the dynamic strain rate is abot 5% higher than that at the qasi-static strain rate [ 4].

6 13 Fig. II shows tensile stress-strain crves in which the stress sharply decreases after yield and the breaking force becomes very low. After the peak stress, cracks start to propagate from the corners of relatively large cells, leading to an nstable fractre [ 5]. Fig.12 haws the relationship between compressive strength and the porosity of the 8!_ alminm foam [ 6]. With an increase in the b porosity, the strength drops exponentially. The _ ratio of the drop frther increases as the porosity ex,ceeds 7%. This is becase the spherical cells become polyhedron cells when the porosity exceeds 7% (Fig.l3). This is one of the cases of stress concentration that occrs at the defects in the cell walls reslting z from the growth and coalescence of bbbles The tensile stress (Fig.14) and electrical resistance (Fig.15) also have a tendency NOMINAL STRAIN, c /% Fig. 11. Tensile stress-strain crves similar to the compressive strength [6]. 1..c: 1 t;j > ';;} r:n a Porosity I% Porosity 6% Spheroid Porosity 93% Polyhedron Fig. 12 Relationship between Compressive strength and the porosity 1 t':l -..c: bb s:: 1.!:: r:n ';;} s:: Porosity I% T ' I. Fig. 14. Relationship between Tensile strength and the porosity 1 Fig. 13 Strctres of foamed alminm E 1 a - rr;,..., - 1.c : ::l "" s:: 1.g ijj Porosity I% Fig. 15. Effect of porosity ofthe foamed alminm on electric condctivity

7 Applkations Fig. 16 shows the sond absorption coefficient of a rolled ALPORAS measred by the reverberation chamber method in comparison with an nrolled ALPORAS and a glass wool of 4 mm in thickness and 25 kg/m' in density [7]. The rolled ALPORAS has sch a large sond absorbing coefficient that it is eqivalent to the glass wool. Fig.l7 shows an example of an application as a sond absorber: f\lporas fixed to the nder-side of an elevated expressway for noise absorption. A cylindrically bent sond absorbing strctre is laid on the noise reflecting srface of an elevated viadct to absorb the vehicle emissions, ths redcing the noise exposition to the neighborhood residents. E (!) o I:E 1. (!).8 c.g_.6 '- Cl).g -g.4 ::s Cl) '-.2 (!).. ' (!) ;>. 1 1 Freqency I Hz '1 Fig. 16. The sond absorbing coefficient of rolled ALPORAS Fig. 17. Sond absorbing strctre on the nder-side of an elevated viadct ALPORAS has a high impact absorbing fnction owing to its sbstantial deformation capability nder low stress (Figs. 9 and 1). It is sed as an impact cshion for express rolling stocks and vehicles for saving drivers' lives in collisions. Alminm foam has many other applications, inclding the doble layer floor of a room where electronic eqipment is arranged for office atomation, filter material and the microbiological incbation carrier. 5. Smmary ALPORAS is a closed cell type alminm foam, which is manfactred by batch casting where AI is thickened by Ca and blown by TiH 2. The density of a general type is between glee and its mean ce11 size is 4.5 mm in diameter. It has excellent sond absorption and shock absorption capabilities and its main application is as sond absorber.

8 132 References [1] K. Imagawa, H. Ueno, S. Akiyama, A. Kitahara, S. Nagata: Test Report of Agency of Indstrial Science and Technology, 1984, p434. [2] H. Ueno and S. Akiyama: Jornal of Japan Institte of Light Metals ol. 37, No. 1,(1987), p. 42. [3] T. Miyoshi, M. ltoh, T. Mkai, H. Kanahashi, H.Kohz, S. Tanabe, K. Higashi: Scripta Materialia, to be pblished. [4] T. Mkai, H. Kanahashi, T. Miyoshi, M. Mabchi, T. G. Nieh, K. Higashi: Scripta Materialia 4, p. 921, (1999) [5] M. Otska, A. Kojima, M. Itoh, E. Ishii: Proc. Recent Advances in Science and Technologies of Light Metals (RASELM91 ), The Japan Institte of Light Metals, Sendai, 1991, p.999 [6] S. Akiyama, H. Ueno, A. Kitahara: Reprinted from the Reports of the Government, Indstrial Research Institte, K ysh No. 46 March 1991 [7] E. Ishii, M. Itoh, Y Morisawa: Report in Kobe Steel Technical Blletin 1991, o. 41, No.2