Casting Protocols for the Production of Open Cell Aluminum Foams by the Replication Technique and the Effect on Porosity

Summary

Replication is one of the processing techniques used for the production of porous metal sponges. In this paper one implementation of the method for the production of open celled porous aluminum is shown in detail.

Abstract

Metal foams are interesting materials from both a fundamental understanding and practical applications point of view. Uses have been proposed, and in many cases validated experimentally, for light weight or impact energy absorbing structures, as high surface area heat exchangers or electrodes, as implants to the body, and many more. Although great progress has been made in understanding their structure-properties relationships, the large number of different processing techniques, each producing material with different characteristics and structure, means that understanding of the individual effects of all aspects of structure is not complete. The replication process, where molten metal is infiltrated between grains of a removable preform material, allows a markedly high degree of control and has been used to good effect to elucidate some of these relationships. Nevertheless, the process has many steps that are dependent on individual “know-how”, and this paper aims to provide a detailed description of all stages of one embodiment of this processing method, using materials and equipment that would be relatively easy to set up in a research environment. The goal of this protocol and its variants is to produce metal foams in an effective and simple way, giving the possibility to tailor the outcome of the samples by modifying certain steps within the process. By following this, open cell aluminum foams with pore sizes of 1–2.36 mm diameter and 61% to 77% porosity can be obtained.

Introduction

Metal foams have attracted a large amount of interest and research effort in recent years as shown by the large body of work cited in wide ranging review articles such as Banhart¹, Conde et al.² or more recently Goodall and Mortensen³. Among the methods used for production of the material, the replication process is distinguished by its experimental simplicity and the degree of control over the final foam structure that can be offered. It should be noted that although in the literature such materials are often described as foams (and are here) as they are not produced by bubbles of gas within a liquid they are more appropriately called porous metals or microcellular metals.

The first report of the replication process was in the early 1960s⁴, and it has been developed further at different stages since then, with notable advances by the research group of Mortensen at the Ecole Polytechnique Federale de Lausanne in Switzerland.

The process relies on the casting of the metal around a preform of particles that defines the shape of the porosity in the final material^{2, 5}. After cooling the preform can be removed by solvent leaching or pyrolysis that causes oxidation. A popular use of this technique utilizes NaCl as a space holder to produce aluminum^5-10 or aluminum alloy foams^11-14. NaCl has several advantages such as being readily accessible, non-toxic and can be removed from the foam by dissolution in water. By having a melting point of 801 °C, it can be used with metals that have a melting point lower than this value, most commonly aluminum, but examples also exist of the use with materials such as bulk metallic glasses, by humidifying a mix of liquid palladium-based bulk metallic glass alloy and NaCl granules¹⁵. Substitution of the NaCl with higher melting point materials also permits the production of foams from higher melting point metals¹⁶. This may include other water-soluble materials, or insoluble ones including different types of sand. In this form the process becomes more like conventional sand casting as to remove the sand, high pressure water jets^{17, 18} or different forms of washing¹⁹ or agitating²⁰ are required.

The essential process²¹ proceeds by taking grains of NaCl and placing them in a mold^{4, 22, 23}. The basic method has been used to make aluminum and aluminum alloy foams^24-26 for a wide range of foam behavior investigations. Additional steps have been introduced to further control the density and to increase the interconnectivity of the pores; these include the densification of the preform. To densify the preform, sintering has been employed^{27, 28} and has been used in different experiments since¹³, with the sintering behavior of NaCl based on temperature, granule size and density described by Goodall et al.²⁹. Another method used for this purpose is cold isostatic pressing (CIP)^{5, 30}; this is a faster technique that can achieve a larger spectrum of comparable densities. The procedure can also be performed in the solid state with metal powder and NaCl grains, and is then sometimes called the Sintering and Dissolution Process³¹.

A full survey of the use of the replication technique to date and comparison with other techniques is given in Goodall and Mortensen³.

In this work we report in detail equipment and experimental protocols that have been used for the processing of metal foams by the replication method, and which are relatively easy to implement in a research laboratory setting. It is important to acknowledge that other versions of the equipment, with different capabilities exist in other research groups, and that while the equipment presented here is suitable to process the material, it is not the only version or protocol that can be made to work. In any case, a thorough understanding of any particular method is essential for experimental success.

The precise protocols used are detailed below. The protocol variations (A, B, C and D) have small changes between them, principally intended to alter the density of the foams produced. The porosity has been calculated from measurements of the bulk weight of the samples, their volume and the density of aluminum (2.7 g/cm³). In developing the methods described for aluminum foam production by replication, attempts have been made to reduce the amount of advanced equipment to the smallest possible extent, such that the method is as easy to implement as possible. Other variations that may be used at different stages are discussed later.

Protocol

NOTE: The instructions below are for Protocol A (Figure 1). Modifications for Protocol B, C, and D are listed as well.

1. Aluminum Bar Preparation

1. Place a large piece (500 g – 1 kg) of commercial purity aluminum ingot into a crucible.
2. Place the crucible in a furnace at 800 °C for about 1 hr, until molten.
3. Take the crucible out of the furnace and pour the molten aluminum into a cylindrical mold that is 50 mm in diameter, slightly smaller than the final diameter of the chamber to be used for infiltration (51 mm) giving a gap of about ½ mm.
4. Wait 1 hr for the bar to cool down.
5. Remove the bar from the mold.
6. Using a band saw, cut it into 4 same-size pieces.
7. Sand the edges of each piece to ensure a good fit in the infiltration mold.

2. Furnace Preparation

1. Program the furnace to reach a 740 °C plateau for at least 2 hr.
2. Set the heating rate of the furnace to 20 °C/min.

3. Preform Preparation

NOTE: Depending on the height of the foam aimed for, vary the amount of NaCl to use for infiltration between 100 g and 300 g.

1. Choose the infiltration NaCl to use, with a diameter corresponding to the pore size range required (for example a range between 1.4 mm and 1.7 mm). The material can be obtained from chemical suppliers in high purity, or supermarket bought table salt can be used (such material will have additives such as iodine and anti-caking agents, but these do not in practice influence the process to a significant extent).
2. Select sieves of an appropriate size range and stack on a base container with the smaller opening size at the bottom.
3. From the suppliers’ bag of NaCl, take approximately 500 g and pour it in the stacked sieves.
4. Agitate the sieves, either manually or using a sieve shaker, for 1 min.
5. Discard the NaCl left in the larger aperture size sieve and the bottom container, the NaCl left in the smaller aperture sieve is used for infiltration.
6. Weigh the amount of infiltration NaCl obtained.
7. If the amount is insufficient, repeat steps 3.4 to 3.7.
   NOTE: For Protocols B, C or D, obtain 100 g of fine NaCl (< 500 µm). This creates an extra space in the mold for air trapped in the preform during infiltration in case the air in the preform does not escape the chamber adequately.

4. Mold Preparation

1. Using sandpaper and laboratory paper roll, clean the mold cylinder (Figure 2), taking special consideration for both the top and bottom edges, and keeping the mold free of any noticeable impurities from previous use.
2. Spray the inside of the mold cylinder with boron nitride aerosol spray, creating a thin coat covering the inside of the mold.
   NOTE: This is achieved when the original color of the mold is replaced by a white layer of the spray; it is not necessary to measure its specific concentration.
3. Let the mold cylinder dry for at least 5 min at RT (heating to around 100 °C for up to 1 hr may be applied for further drying if desired).
4. Using fine sandpaper, remove any residue of boron nitride from the edges of the mold cylinder, to improve the seal between the mold cylinder and the mold base.
   NOTE: The next 3 steps are for Protocols A and B; for Protocols C and D cut only one gasket ring for the lid.
5. Cut 2 gasket rings from 1 mm thick graphite sheet (OD = 60 mm, ID = 51 mm), one for the union between the top edge of the mold cylinder and the mold lid leading up to the valve system, the other for the union between the bottom edge of the mold cylinder and the mold base.
6. Place one of the gaskets in the mold base groove.
7. Place the bottom of the mold cylinder into the groove with the gasket.
8. Tap lightly with a mallet on the top of the mold cylinder to secure the bottom to the base groove.
   NOTE: For Protocol B, C, or D, add the following step.
   1. Pour the 100 g of fine NaCl (< 500 µm) into the mold cylinder and flatten the top with an uncut aluminum bar tapping the top of it lightly with the mallet to ensure the fine NaCl is packed to a high density.
      NOTE: For Protocol D add the following step.
   2. Cut 2 circles of soft 2 mm thick ceramic Kaowool blanket the size of the mold diameter (51 mm) and place them on top of the fine NaCl, use the uncut aluminum bar and the mallet to press them against the fine NaCl.
9. Pour the NaCl to be infiltrated into the mold cylinder.
   NOTE: For Protocol D add the following step.
   1. Attach the mold and base to a vibrating table, making sure the mold cylinder does not move from the base groove. Vibrate for 1 min at 50 Hz with a 0.01 m amplitude.
10. Holding the top of the cylinder in place, pick up the base and shake lightly until the NaCl inside the mold forms a flat surface at the top.
11. Place the prepared aluminum bar on top of the NaCl preform.
12. Place a graphite gasket in the groove of the mold lid.
13. By hand screw the 4 stainless steel studs to the base and secure them with 4 sets of stainless steel nuts and washers on the top of the base using a wrench and place the mold lid on top of the mold cylinder through the studs.
14. With a torque wrench set at 16 N·m, screw the 4 sets of steel nuts and washers on the 4 threaded rods screwed into the base and extending up through the lid, where the nuts are tightened to lock the mold lid in place.
15. Attach the top of the lid to the valve system with the gasket, clamp, bolt and butterfly nut.
16. Close all the valves of the system.
17. Open the valve leading to the vacuum pump and the mold (valve 3).
18. Turn on the vacuum pump until the dial gauge of the valve system indicates the lowest pressure possible.
19. Turn off the vacuum pump.
20. If the loss of vacuum in the system is lower than a rate of 50 Torr/sec for the first 10 sec after shutting down the vacuum pump the seal is sufficiently good for infiltration.
21. Leave the lid valve open (valve 3) to keep the system at ambient pressure and close the vacuum pump valve (valve 1).
22. Without detaching the valve system, place the mold in the preheated furnace and wait for 1 hr.

5. Infiltration

1. Close all the valves of the system (Figure 3).
2. Open the valve leading to the argon gas cylinder (valve 2).
3. Open the main valve on the argon gas tank and set the infiltration pressure with the regulator valve (for a range of 1.4 mm to 1.7 mm of NaCl particle size, use a pressure of 3.5 bars).
   NOTE: For Protocol B, an infiltration pressure of 3 bar is used. Use a pressure of 1 bar for Protocols C and D.
4. In a swift manner, open the lid valve (valve 3).
5. After 1 min, remove the mold from the furnace and place it on top of a cooling surface (in this case a copper block).
   NOTE: While cooling, the pressure in the system will change. For the first 5 min of this process, pay close attention to the pressure indicated by the regulator and adjust back to the infiltration pressure if necessary.

6. Sample Extraction

1. After 30 min, when the mold is cool enough to handle with light heat resistant gloves, detach the valve system and place the mold base on a workbench vise. Unscrew the lid from the top of the cylinder.
2. With the lid off, lightly tap the top of the mold cylinder with a mallet in a perpendicular direction to the vise’s grip to loosen the mold cylinder from the base groove.
3. Remove the mold base from the vise and place the mold cylinder in the vise grip.
4. With the mallet tap the remaining aluminum on top of the sample to push it out of the mold cylinder.
5. Using a band saw, cut the bottom part of the foam sample, removing the surplus aluminum.
6. Depending on the height of foam required, cut where desired, close to the top of the sample.
7. Place the infiltrated foam in a beaker with water and a magnetic stirring bar on a stirring hot plate to dissolve the NaCl preform.
8. Set the temperature of the hot plate to 60 °C. Change the water every 10 min until there is no NaCl left in the foam.
   NOTE: To ensure there is no NaCl left in the foam, change the water approximately 10 times. It is also possible to make periodic checks of the sample weight after a brief drying stage. When this ceases to change significantly with further immersion, the NaCl must be completely removed.
9. Finally using an electric air drier remove all the water left in the foam. The foam sample is ready.

Results

In Figure 4 the morphology of the NaCl grains can be seen (angular and spherical), for illustrative purposes. The foams obtained with Protocol A were made using angular shaped grains and the rest were made with the spherical grains. It was found that the use of different shape NaCl grains had no observed effect on the porosity obtained in the samples.

From the results we can determine that samples a, b, and c (made with Protocol A), are on average 63% porous (Figure 5<...

Discussion

The basic method described here has been used in different forms by other researchers. Some of the key variants that allow foams of different types to be created are discussed. In characterizing these foams we have measured the porosity, as this is a quick and easy assessment to make, but characterization of other structural characteristics, such as pore size, specific surface area or strut thickness might be required to obtain a full understanding of foam characteristics for different applications. In practice, for prod...

Materials

List of materials used in this article

Name	Company	Catalog Number	Comments
Salt	Hydrosoft	Granular Salt 25 kg 855754	http://www.travisperkins.co.uk/p/hydrosoft-granular-salt-25kg/855754/3893446
Aluminum	William Rowland	Aluminum Ingots 99.87% pure 25 kg drum	http://www.william-rowland.com/products/high-purity-metals#product-id-1
Crucible	Morgan Advance Materials	Syncarb Crucible	http://www.morganmms.com/crucibles-foundry-products/crucibles/syncarb/
Furnace	Elite Thermal Systems	TLCF10/27-3216CP & 2116 O/T	http://www.elitefurnaces.com/eng/products/furnaces/1200%20Top%20Loading%20Furnaces.php
Bar Mold	The University of Sheffield	Custom Made	Stainless Steel 304, 15 cm height, 5 cm inner diameter, 6 cm outer diameter
Band Saw	Clarke	CBS45MD (6" x 4 1/2") 370W 060710025	http://www.machinemart.co.uk/shop/product/details/cbs45md-41-2in-x-6in-metal-cutting-ban
Sandpaper	Wickes	Specialist wet & dry sandpaper 501885	http://www.wickes.co.uk/Specialist-Wet+Dry-Sandpaper-PK4/p/501885
Sieves	Fisher Scientific	Fisherbrand test sieves 200 mm diamater	http://www.fisher.co.uk/product/brand_listing.php/F/Fisherbrand/Sieve
Balance	Precisa	XB 6200C	http://www.precisa.co.uk/precision_balances.php
Boron Nitride	Kennametal	500 ml spray can	http://www.kennametal.com/content/dam/kennametal/kennametal/common/Resources/Catalogs-Literature/Advanced%20Materials%20and%20Wear%20Components/B-13-03401_ceramic_powders _brochure_EN.pdf
Infiltration Mold, Base and Lid	The University of Sheffield	Custom Made	Stainless Steel 304, 15 cm height, 5.1 cm inner diameter, 6 cm outer diameter
Cylindrical Mold	The University of Sheffield	Custom Made	Low carbon steel 1020, 15 cm height, 5 cm inner diameter, 6 cm outer diameter
Graphite Gasket	Gee Graphite	Geegraf Stainless Steel Reinforced Graphite 1 mm thick	http://www.geegraphite.com/steel_reinforced.html
Mallet	Thor Hammer Co. Ltd.	Round Solid Super Plastic Mallet	http://www.thorhammer.com/Mallets/Round/
Wrench	Kennedy Professional	13 mm Ratchet Combination Wrench KEN5822166K	https://www.cromwell.co.uk/KEN5822166K
Nuts	Matlock	M8 Steel hex full nut galvanized	https://www.cromwell.co.uk/CTL6400068J
Washers	Matlock	M8 Form-A steel washer bzp	https://www.cromwell.co.uk/CTL6451208H
SS Nuts	Matlock	M8 A2 st/st hex full nut	https://www.cromwell.co.uk/CTL6423008F
SS Washers	Matlock	M8 A2 st/st Form-A washer	https://www.cromwell.co.uk/CTL6464008H
Stainless Steel Studding	Cromwell	M8 x 1 Mtr A2 Stainless Steel Studding QFT6397080K	https://www.cromwell.co.uk/QFT6397080K
Valves	Edwards	C33205000 SP16K, Nitrile Diaphragm	https://www.edwardsvacuum.com/Products/View.aspx?sku=C33205000
Fitting Cross	Edwards	C10512412 NW16 Cross Piece Aluminum	https://www.edwardsvacuum.com/Products/C10512412/View.aspx
Fitting T	Edwards	C10512411 NW16 T-Piece Aluminum	https://www.edwardsvacuum.com/Products/C10512411/View.aspx
Vacuum Pump	Edwards	A36310940 E2M18 200-230/380-415V, 3-ph, 50 Hz	http://www.edwardsvacuum.com/Products/View.aspx?sku=A36310940
Dial Gauge	Edwards	D35610000 CG16K, 0-1,040 mbar	http://www.edwardsvacuum.com/Products/View.aspx?sku=D35610000
Argon Gas	BOC	Pureshield Argon Gas	http://www.boconline.co.uk/en/products-and-supply/industrial-gases/inert-gases/pureshield-argon/pureshield-argon.html
Stainless Steel Hose	BOC	Stainless Steel Hose	http://www.boconline.co.uk/en/products-and-supply/speciality-equipment/hoses-and-pigtails/index.html
Regulator	BOC	HP 1500 Series Regulator	http://www.boconline.co.uk/en/products-and-supply/speciality-equipment/regulators/single-stage-regulators/hp1500-series/hp1500-series.html
Copper Block	William Rowland	Copper Ingot 25 kg	http://www.william-rowland.com/products/high-purity-metals#product-id-18
Vise	Record	T84-34 H/Duty Eng Vice 4 1/2" Jaws REC5658326K	https://www.cromwell.co.uk/REC5658326K
Beaker	Fisher Scientific	11567402 - Beaker, squat form, with graduations and spout 800 ml	https://webshop.fishersci.com/insight2_uk/getProduct.do;jsessionid=16D5812 D71B8CB37B475E94281E2BEA 5.ukhigjavappp11?productCode=11567402&resultSet Position=0
Stirring Hot Plate	Corning	Corning stirring hot plate Model 6798-420d	http://www.corning.com/lifesciences/us_canada/en/technical_resources/product_guid/shp/shp.aspx
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Stir Bar	Fisher Scientific	11848862 - PTFE Stir bar + Ring 25x6 mm	https://webshop.fishersci.com/insight2_uk/getProduct.do;jsessionid=16D5812 D71B8CB37B475E94281E2BEA 5.ukhigjavappp11?productCode=11848862&resultSet Position=0
Air dryer	V05	V05 Max Air Turbo Dryer DR-120-GB	http://reviews.boots.com/2111-en_gb/1120627/v05-v05-max-air-turbo-hair-dryer-dr-120-gb-reviews/reviews.htm
Ceramic Sheet	Morgan Advance Materials	Kaowool Blanket 2 mm thick	http://www.morganthermalceramics.com/downloads/datasheets?f[0]=field_type%3A84
Vibrating Table	Peveril Machinery	Pevco Vibrating Table 1.25 m x 0.625 m x 0.6 m	https://peverilmachinery.co.uk/equipment/vibrating-tables