10-05-2012, 11:20 AM
realization of light weighting metal foams in modern day foundry
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Introduction:
Metallic foams (‘metfoams’ or cellular metals) are a class of material unfamiliar to mechanical engineers [ 1’,2’]. They are made possible by a range of novel processing techniques, many of which are still under development. At present, metfoams are inadequately characterized. Moreover, process understanding and control are incomplete, resulting in variable properties. Yet, even the present generation of metfoams suggests alluring potential [3,4’,5,6,7’,8,9], as process control and characterization rapidly improve. Metfoams have potential in structures that are both light and stiff, for the efficient absorption of energy, for thermal management and perhaps for acoustic control and other, more specialized, applications. They hold, too, the promise for market penetration in applications where several of these functions can be combined. Implementation relies not just on properties, but on additional attributes: such as low manufacturing cost, environmental durability and fire retardancy.
Such materials have been available for decades [ 10,111, but new opportunities are now emerging for two reasons. Firstly, novel manufacturing approaches have beneficially affected performance and cost [Pl-P3,12,13,14’]. Secondly, higher levels of basic understanding about mechanical, thermal and acoustic properties have been developed [2’,15’,16-191 in conjunction with associated design strategies [2’,3,4’]. These provide an integrated pathway between manufacturing and design. The literature is still sparse. Anyone interested in the field must read the book on ‘Cellular Solids’ by Gibson and Ashby [l’]. This book provides a comprehensive assessment of many types of cellular materials, with evident consequences for metal foams. But, cellular metals also have several unique characteristics and accordingly, this book should be supplemented by other readings. The patent literature is pertinent [Pl-P3], as well as two progress reports [20,21] and the ‘Ultralight Metals Web’ page [ZZ]. There is also a good review on manufacturing methods [lo] (though now outdated). A ‘Cellular Metal Design Manual’ [Z’], with associated software and data bases [S], will be available soon. This manual will embrace a full spectrum of properties, applications, design rules and case studies.
The stress/strain response exhibited by low density cellular metals establishes ‘two aspects of their engineering utility’, as is summarized in Figure 1. Firstly, the high stiffness and yield strength achievable at low density, relative to competing materials/systems, creates an opportunity for ultralight structures, with integrally-bonded dense face sheets [9]. Secondly, large compressive strains achievable at nominally constant stress (before the material compacts) imparts a high energy absorption capacity at force levels of practicable relevance for crash and blast amelioration systems [2’,9].
Open-cell metals constitute a third opportunity. These materials have thermal attributes that enable applications as heat dissipation media and as heat recuperators [6,7’]. The attributes include the high thermal conductivity of the material comprising the borders, in combination with a high internal surface area and propitious fluid transport dynamics, which enable high heat transfer rates that can be used effectively for either the cooling of high power density devices or efficient heat exchange.
Cellular metals incorporated within a structure to form sandwich skins can result in systems that achieve mechanical performance and affordability goals at lower weights than competing concepts [4’] such as rib or waffle stiffened designs. Structural analysis of prototypical systems identifies those sandwich constructions which have explicit weight advantages. Such advantages are found in structures controlled by bending or compression, but not in those dominated by tension. For instance, in aircraft design about half of the structure is limited by its bending or compressive performance.
Metal Foams:
A metal foam is a cellular structure consisting of a solid metal, frequently aluminium, containing a large volume fraction of gas-filled pores. The pores can be sealed (closed-cell foam), or they can form an interconnected network (open-cell foam). The defining characteristic of metal foams is a very high porosity: typically 75–95% of the volume consists of void spaces. The strength of foamed metal possesses a power law relationship to its density; i.e., a 20% dense material is more than twice as strong as a 10% dense material.
Metallic foams typically retain some physical properties of their base material. Foam made from non-flammable metal will remain non-flammable and the foam is generally recyclable back to its base material. Coefficient of thermal expansion will also remain similar while thermal conductivity will likely be reduced.
Solid metallic foams are known for their interesting combinations of physical and mechanical properties such as high stiffness in conjunction with very low specific weight or high compression strengths combined with good energy absorption characteristics. Although interest in these materials is increasing, some confusion exists concerning the term “metallic foam,” which is often used in a general way to describe materials that are not foams in the strictest sense.
To properly identify a metallic foam, one has to distinguish between:
• Cellular metals: the most general term, referring to a metallic body in which any kind of gaseous voids are dispersed. The metallic phase divides space into closed cells which contain the gaseous phase.
• Porous metals: a special type of cellular metal restricted to a certain type of voids. Pores are usually round and isolated from each other.
• (Solid) metal foams: a special class of cellular metals that originate from liquid-metal foams and, therefore, have a restricted morphology. The cells are closed, round, or polyhedral and are separated from each other by thin films.
• Metal sponges: a morphology of a cellular metal, usually with interconnected voids.
Types of Metal Foams:
There are basically two types of metallic foams depending on the type of cell matrix, i.e., pores can be sealed (closed-cell foam), or they can form an interconnected network (open-cell foam).
Open-cell metal foams:
Open celled metal foams are usually replicas using open-celled polyurethane foams as a skeleton and have a wide variety of applications including heat exchangers (compact electronics cooling, cryogen tanks, PCM heat exchangers), energy absorption, flow diffusion and lightweight optics. Due to the high cost of the material it is most typically used in advanced technology aerospace and manufacturing. Extremely fine-scale open-cell foams, with cells too small to be visible to the naked eye, are used as high-temperature filters in the chemical industry.
Metallic foams are nowadays used in the field of compact heat exchangers to increase heat transfer at the cost of an additional pressure drop.[2][3][4] However, their use permits to reduce substantially the physical size of a heat exchanger, and so fabrication costs. To model these materials, most works uses idealized and periodic structures or averaged macroscopic properties.
Closed-cell metal foams:
Closed-cell metal foam was first reported in 1926 by Meller in a French patent where foaming of light metals either by inert gas injection or by blowing agent was suggested.[5] The next two patents on sponge-like metal were issued to Benjamin Sosnik in 1948 and 1951 who applied mercury vapor to blow liquid aluminium.[6][7]
Closed-cell metal foams have been developed since about 1956 by John C. Elliott at Bjorksten Research Laboratories. Although the first prototypes were available in the 50s, commercial production was started only in the 90s by Shinko Wire company in Japan. Metal foams are commonly made by injecting a gas or mixing a foaming agent (frequently TiH2) into molten metal. In order to stabilize the molten metal bubbles, high temperature foaming agent (nano- or micrometer sized solid particles) is required. The size of the pores, or cells, is usually 1 to 8 mm.