Aluminum foams as permanent cores in casting

Low density, high specific stiffness and impact energy/vibration absorption ability make Al-based metal foams as promising materials in application for which lightweight, and energy/vibration absorption are crucial. The scientific literature documents an increasing interest in this topic (published papers rise from 100/year to 600/year in the last 20 years) and also industrial applications are emerging. In this context Al-based foams can be extremely interesting as cores in cast components in order to improve their properties and simplify their technological processes (no removal/recycling of traditional sand cores). However, both in the scientific literature and in technological application, this topic is still poorly explored (few paper/year, less than 10 patents). The published works include few details and characterizations and almost no solutions are discussed for the overcoming of criticism. In this context the present research considers and compare different foams, analyze both the foams and the cast objects, individuates main issues and proposes new strategies for their overcoming.

In the present work,Al-based metal foams, (Cymat foams and Havel Metal Foams in the form of bars of rectangular section), are inserted in gravity casting experiment of the Al-Si-Cu-Mg alloy. The foams have been fully characterized before and after insertion in casting: (porosity, cell wall and external skin thickness, microstructure, infiltration degree and the quality of the interface between the foam core and the dense cast shell have been investigated by means of optical microscopy and Scanning Electron Microscopy equipped with Energy Dispersive Spectroscopy (SEM-EDS)).

The analyses evidenced that a continuous and thick external skin protect the foam from infiltration by molten metal preserving the initial porosity and insert shape. A detailed analysis of the foam external skin (absent in the published literature) highlight that the composition of this external skin is crucial for the obtainment of a good joining between the molten metal and the Al-foam core. In fact, the presence of Mg-oxides on the foam surface prevent the bonding and maintain a gap between the core and the shell. This point opens the opportunity to design innovative surface modifications of this external skin as promising strategies for the optimization of cast component with a foam core.