Numerical and Experimental Study of the Acoustic Performance of Functionally Graded Metallic Foam

Open-cell metal foams can be used as efficient sound absorbers to overcome the limitations of conventional fibrous foams for high-temperature applications. Its sound absorption efficiency is highly dependent on the microstructure design. The previous study has shown that sound absorption is generally more effective when the pores are smaller. This is because smaller pores exhibit stronger friction-induced damping and thus higher visco-thermal loss to effectively suppress sound waves travelling into the material. Traditional absorbing materials with homogenous porous content only provide monotonous acoustic characteristic and limited effective frequencies. Among recent development of acoustic metamaterials, functionally graded sound absorber shows great potential as the graded architectured acoustic system has the potential to generate unusual or extraordinary acoustic property compared to a homogenous medium. Advance in this field is however hampered by the available manufacturing method. Traditional fabrication techniques such as gas-forming or melting can hardly create the desired microstructure with spatial variation. To overcome this challenge, a template replication method is proposed to produce metallic foams with functionally graded porosity. The template process uses polymer foam as a template, coat metal slurry on the struct of the polymer foam, then burn out the polymer foam and sinter the metal slurry into the metallic foam. The produced metallic foam has open-cellular structures similar to the polymer foam. In another word, the pore size, pore structure and porosity of the metallic foam can be tuned and controlled via the polymer template. Using this method, IN625 foams with reducing pore size and porosity have been successfully manufactured via using polymer templates with a similar change in pore size and porosity and reported by us. The sound-absorbing coefficient of IN625 foams can achieve as high as 90% over a wide range of frequency with the smallest pore and porosity can be produced and in 50mm thickness. Using a stack of polymer foams in different pore size and porosity as templates, after the sintering process, metallic foams in different layers can be sintered together into one piece of functionally graded foam. Therefore, the template replication method can be applied to manufacture functional graded metallic foams. In this work, the sound absorption behaviour of open-cell metallic foams comprised of functionally graded layers with different pore size and porosity is investigated. The sound absorption coefficient of sample materials is measured using impedance tube at room temperature, and numerical study is carried out using finite element analysis. The classical Delany-Bazley model is employed to model the rigid foam layers, with their flow resistivity parameters being derived from the microscale morphology. Good correlation between simulation and experiment is obtained, and the effects of foam combinations are discussed. The proposed numerical model can be utilized as a design tool to optimize functionally graded metal foams for various potential applications.