Issue 50

I. Papantoniou et alii, Frattura ed Integrità Strutturale, 50 (2019) 497-504; DOI: 10.3221/IGF-ESIS.50.41 498 I NTRODUCTION odern-day research in engineering and material science is focused on developing new composite and hybrid materials for the purpose of producing structural elements of lower density and equal or even higher perform- ances. Cellular and microcellular materials are among a new class of materials and are found in everyday uses for their high stiffness, low specific weight and other properties. Applications range from light-weight construction and pack- aging, to thermal insulation, vibration damping, and chemical filtration [1]. Metallic cellular materials, namely metal foams, merit the use of cellular materials and are becoming a new very promising class of engineering materials. Metal foams offer unique properties, compared to solid metals. Such unique properties are their high strength to weight ratio, high energy absorption capacity, large specific surface, high gas and liquid permeability, and low thermal conductivity [2]. Metal foams serve in a variety of applications, some of which are based on significant mechanical properties (mainly closed-cell foams), while others are based on rheological characteristics and transport processes, made possible by the accessibility of open pores to the ingress and flow of fluid (open-cell foams). The unique properties of foams offer promise in a variety of applications ranging from lightweight construction and impact-energy absorption to various types of acoustic damping and thermal insulation. Areas of applications of those type of materials are the naval industry, aerospace, mech- anical or chemical engineering and can be used as heat exchangers, energy or sound absorbers, filters and implants in medicine. The applications of metal foams depend on their basic characteristics such as relative density, cell structure, wall thickness, strut integrity and cell morphology homogeneity [3, 4]. The most common type of stochastic metallic foam is the aluminium foam which is widely preferred due to its important mechanical and natural properties. Two of aluminium foam main characteristics are recyclability and non-toxicity which are numbered among the many benefits. Low density is the most important virtue of aluminium foam because of its light-weight metal structure [5, 6]. Aluminium foam production routes are classified into four main groups: powder-metallurgy route, foaming of molten metal, metallic deposition, and sputter deposition. Each one gives its own characteristic range of densities, cell sizes and shapes [7]. In powder metallurgy route, metallic powder is mixed with a blowing agent and it is compacted to form a foamable pre- cursor. Then the precursor is heated and formed in a furnace [8, 9]. This production process is not as widely used as the less- expensive molten-metal foaming process, but it also has advantages. The most important advantage of this route is that the precursor can expand in a heated mould and the foam with a complicated shape can be made by mould filling [10]. Worldwide, there is a significant number of ongoing research projects aiming at cheaper and more standardized production of metal foams with high standards, because of their ever-increasing applications [11-13]. The objective of this research is the production of metal foams using powder metallurgy route (with foaming agents) in order to further study and analyze the effect of different parameters in the foam’s final porosity and internal structure. The powder metallurgy foaming process has many parameters that may affect the final result (foaming efficiency and pore morphology). The present research aims to obtain the optimum parameters in order in the next phase of this research to use these parameters to study the effect of introducing different types of reinforcing particles in the aluminium foam matrices. Thus, the main parameters examined were the powder morphology, the compaction pressure and the foaming temperature. E XPERIMENTAL PROCEDURE Materials he powder metallurgy foaming process was applied in our research work. Thus, different type of aluminium powders were applied at the precursor manufacturing process. More specifically, the base materials used were fine aluminium powder (325 mesh, 99.5%), coarse aluminium powder (-40+325 mesh, 99.8%) and aluminium flakes (APS 11 micron, 99.7%). Commercially available titanium hydride powder with particle diameter smaller than 45μm was used as a foaming agent (TiH 2 , -325 mesh, 99%). All the powders were purchased from the Alfa Aesar company. Aluminium Foam Manufacturing Process As already mentioned, in this study three aluminium powders with different particle geometries were used. In all cases, the metallic powder was mixed with mass fraction 0.6 % of TiH 2 powder in a powder mixer for an hour. Ten grams of each mixture was compacted cold, using an uniaxial compaction, in a 25-mm-diameter, lubricated, tool-steel die with pressures in the range from 200 MPa to 1200 MPa to achieve different precursor green densities. The green densities were calculated by assuming that the density of the base metal is 2.7 g/cm 3 . The precursor specimens were later led to a furnace, so as the foaming procedure to take place under high temperatures (Fig.1). For each combination of aluminium powder and com- paction pressure four discrete foaming temperatures were used: 650 o C, 700 o C, 750 o C and 800 o C. M T