Cellular metallic foams are a new class of engineering materials for both structural and functional purposes. These lightweight materials have not only the characteristics of metals, such as weldability, electric conductivity, ductility, but also the characteristics of cellular foams, such as energy absorption, acoustic damping, electromagnetic shielding, fluid infiltration, low thermal conductivity. This makes their properties so interesting that exciting new applications are expected in the near future. With the rapid development of Mg and its alloys, cellular Mg foams have drawn increasing interest. However, various difficulties remain unsolved in making Mg-based foams. Therefore it is of great significance to study and design a fabrication process that is simple, of high output and low cost, and is capable of producing large sized Mg foams.
This study designs and investigates a melt foaming process to fabricate Mg foams based on traditional foaming principles and processes. Large sized Mg foams with uniform cell structure and application potentials are fabricated. The effects of processing parameters on the foam structure, and the mechanism for liquid metal foam stability during the foaming process are also investigated.
The experiments show that MgCO3 is a kind of effective blowing agent for foaming of Mg alloys. By using protective atmosphere and flame retardant, the Mg melt is prevented from severe oxidation. Based on these results, the melt foaming process to fabricate Mg foams is designed, and specific processing parameters are initially set.
Foam porosity increases with increasing amount of MgCO3 addition. When the amount of MgCO3 addition is 3wt% for a given granularity, the foam porosity reaches the maximum. The mean cell diameter of the foam increases with increased granularity of MgCO3 powder. The holding temperature must coordinate with the melting behavior of the alloy. The Mg alloy for foaming must have as wide a melting range as possible to yield foams with uniform cell structure. Longer stirring time and longer foaming time both result in higher foam porosity. The primarily solidified phase in the melt precursor has a great influence on the liquid foam stability during the foaming process. Maximum stability is achieved when the primary solid fraction in the melt precursor is no more than 20%. The thin solid reaction layer generated on the gas/melt interface by reactions between CO2 and Mg melt also plays an important role in stabilizing the liquid metal foam during the foaming process.
The optimal parameters in the foaming process are determined for the foams with optimized cell structure. Mg foams are fabricated by the melt foaming process with porosities in the range from 67% to 82%, and uniform cell structure along the longitudinal section of the foam. The density of the foam generally increases from its bottom up to top, and the cell size can be adjusted to some degree.
Compression tests of the foams show three stages: the linear elastic stage within an initially small strain, a long plateau stage with fluctuating flow stress to large strain, and finally a densification stage. The plateau stress of the foam with smaller cell size is higher than that with larger cell size, thus higher energy absorption capacity. The compressive strength is 2.15 MPa~10.63 MPa and Young’s modulus is 0.58 GPa~1.64 GPa for the Mg foam. The compressive strength and Young’s modulus both decrease with the increase in foam porosity.
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