The Influences of Process Parameters on the Preparation of Closed-cell Aluminum foam by Friction Stir Processing

In the present investigation, the closed-cell Al foam was fabricated by friction stir processing (FSP) combined with heat treatment. The influences of process parameters on microstructures of closed-cell Al foam precursor were investigated by optical metallographic microscope and scanning electronic microscope (OM/SEM). Fluent CFD software was developed to simulate the temperature field and flow field in friction stir processing. The cupping test values were compared for base metal and different weld passes. The results show that the welding speeds have little effect on the mixing of powder in the stir zone because of the relatively small welding heat input. However, the pore size and pore morphology are highly sensitive to change in the rotating speeds. When the welding speed is 50mm/min and the rotating speed is 2000rpm, the powder ring is continuous and uniform due to sufficient plastic deformation and flow. When the foaming time is 110s, the expansion rate of the whole foam increases rapidly, and the diameter of the pore is uniform. Numerical simulation shows that the welding heat mainly comes from the shoulder of the stirring head and the welding temperature peak appears near the stirring pin. The maximum flow velocity appears at the outer edge of the shaft shoulder in which the aluminum matrix is softened preferentially. When the rotating speed increases to 2000r/min, the velocity of the outermost edge of the shaft shoulder increases by 59.96%, and the maximum temperature at the stirring pin reaches 491℃ which is consistent with the experimental results. The formability of the joint interface is improved. The cupping test values increase with the increase of deformation temperature. Especially the cupping test value of foamed preform is close to that of base metal at 450℃.

Design and Fabrication of Highly Porous Replicated Aluminum Foam Using Double-Granular Space Holder

Porous materials are widely employed in a wide variety of industrial applications due to their advanced functional performance. Porous aluminum is among the most attractive metallic materials. It can be produced using repeatable methods involving a replicated Al foam that also provides porosity control. In this work, a highly porous replicated Al foam was fabricated. First, the model of multifunctional packing density was used and corrected to select the appropriate space holders. Then, Al foam was produced using a double-granular sodium chloride space holder. The obtained results showed a maximum porosity of 65% that was achieved using a mix of coarse, irregular granules with spherical granules of intermediate size.

Aluminum Foams as Permanent Cores in Casting

Their low density and high specific stiffness and impact energy/vibration absorption ability make Al-based metal foams promising materials in applications for which a light weight and energy/vibration absorption abilities are crucial. In view of these properties, Al-based foams can be extremely interesting as cores in cast components in order to improve their performances and simplify their whole technological process. However, both in the scientific literature and in technological application, this topic is still poorly explored. In the present work, Al-based metal foams (Cymat foams and Havel metal foams in the form of rectangular bars) are used in a gravity casting experiment of an Al-Si-Cu-Mg alloy (EN AB-46400). The foams were 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’s external skin highlights that the composition of this external skin is crucial for the obtaining of a good joining between the molten metal and the Al foam core. In fact, the presence of Mg oxides on the foam surface prevents bonding, and maintains a gap between the core and the shell. This point opens up the opportunity to design innovative surface modifications for this external skin as promising strategies for the optimization of cast components with a foam core.

Effect of Strain Rate and Loading Direction on the Mechanical Properties of Ni-Cr-Al Superalloy Foam Fabricated by Powder Alloying Method

The powder-alloyed metallic sheet foam manufacturing process has the advantage of being able to control pore shape, size, and distribution more easily and homogeneously than conventional foam manufacturing processes. The effects of strain rate and tensile direction on the mechanical properties of Ni-Cr-Al superalloy foam fabricated by powder alloying method were investigated. As a result of structural characteristics obtained by X-ray tomography and scanning electron microscopy, average pore sizes were measured to be 2762.4 μm (normal direction), 2709.1 μm (rolling direction, RD), and 2518.4 μm (transverse direction, TD) respectively. The γ-Ni matrix and γ’-Ni3Al (which was evenly distributed in the strut) were identified as the main constituent phases of the Ni-Cr-Al foam used in this study. Tensile tests were conducted with strain rates of 10⁻² ~ 10⁻⁴ s⁻¹ along the rolling and transverse directions. The results showed that the tensile strength in the RD direction was 1.84~2.01 MPa, and in the TD direction was 1.2~1.27 MPa. The elongation in the TD direction was higher (30~36%) than in the RD direction (17~22%). It is noteworthy that the effect of quasi-static strain rate on the tensile strength and elongation was negligible. However, the loading direction was found to change mechanical properties significantly. This study also discussed the deformation behavior of the Ni-Cr-Al superalloy foam through observations of the fracture surface, and realtime observations during tensile tests in different directions.