AlSi Foams Research Articles

Effect of carbon nanotubes content on compressive properties and deformation behaviors of aluminum matrix composite foams

The incorporation of nano carbon reinforcement is a useful strategy to broaden the engineering applications of metal foams. In this work, carbon nanotubes (CNTs) reinforced aluminum (Al) matrix composite foams (CNTs/Al composite foams) with different CNTs content are prepared by combining ball-milling technique with powder metallurgy method. The result shows that the appropriate content of CNTs can reduce the pore size, improve the circularity, and enhance the compressive properties. Especially, the 1.0 wt% CNTs/Al composite foams possess the optimal compressive properties of yield stress (7.90 MPa), plateau stress (8.75 MPa) and energy absorption capacity (5.19 MJ/m3), which have an enhancement of 23.24%, 98.41% and 84.70%, respectively. Composite foams undergo shear deformation throughout the quasi-static compression process, and the failure mode performs as the brittle mode combined with ductile mode. Meanwhile, the incorporation of CNTs can contribute to the inhomogeneous distribution of the Si phase, provide more nucleation sites, and significantly refine the pore size. It is confirmed that the CNTs in composite foams contribute the grain refinement and the formation of micro stacking faults, making the mechanical properties of CNTs/Al composite foams outperform the counterpart of Al-Si foams. Moreover, due to the weak interfacial reaction, the low content of aluminum carbide is formed, which can promote the load transfer between CNTs and Al. The present finding provides a theoretical and practical design strategy to the future preparation of CNTs/Al composite foams.

Research on compression-compression fatigue properties of carbon nanotubes reinforced closed-cell aluminum matrix composite foams by reinforcement content design

As the requirement of compressive fatigue loading on the mechanical properties of aluminium foams increase rapidly, the limited fatigue performance of them significantly restricts their engineering applications. In this study, the compression-compression fatigue properties of carbon nanotubes (CNTs) reinforced closed-cell Al-Si composite foams (C-ASCFs) were investigated. The fatigue strength of C-ASCFs with 0.50 wt% CNTs increased by 54% than that of Al-Si foams, where the CNTs contributed to decrease the crack expansion rate, transfer the fatigue load and hence change the fatigue failure mode, causing a combination of plastic and brittle deformation in them. Meanwhile, the appropriate incorporation of CNTs decreased pore nucleation threshold, optimized the pore morphology, and promoted the formation and refinement of eutectic Si. Such inhibited the probability of fatigue cracks initiation, resulting in the enhancement of fatigue life. This work provides theoretical and practical basis of design strategy of Al-Si composite foams and broadens their application prospect.

Controllable Design of Structural and Mechanical Behaviors of Al–Si Foams by Powder Metallurgy Foaming

Silicon (Si) has a significant influence on the cellular structure and microstructure of the aluminum (Al) foams, which in turn will notably affect their mechanical responses. Herein, the Al–Si foams are fabricated via a novel powder metallurgy foaming approaches with multiple Si contents to investigate the role of Si concentrations on the structural and mechanical properties. The results show that the certain contents of Si can not only delay the expansion process, but also increase the maximum expansion rate to improve the cellular structure. Specifically, the AlSi7 foams process the optimal comprehensive compressive properties of yield stress (4.36 MPa), plateau stress (6.02 MPa), and energy absorption capacity (3.47 MJ m−3) with the relative density of 0.15. However, excessive Si can decrease the expansion rate and deteriorate the cellular structure. Moreover, the deformation mode of the Al foams during compressive process converts from plastic mode to brittle mode by the adding of Si. Generally, the AlSi7 foams possess an excellent trade‐off between toughness and brittleness and exhibit the optimal mechanical responses.

Fabrication and characterization of hypoeutectic open-cell Al-Si foams using gravity die casting and squeeze casting

Gravity die casting and squeeze casting are the techniques used for the fabrication of hypoeutectic open-cell Al-Si foams which are characterized and studied for their energy absorbing quality in compression tests. The effect of different amounts of sodium chloride (NaCl) (up to 56 vol.%) as a space holder in the casting of aluminum foam on the morphology, density, porosity, compressive and energy absorption properties of aluminum foam was studied. The hypoeutectic Al-Si alloy with NaCl particles as a space holder was used to fabricate the aluminum foam using gravity die casting and squeeze casting. The hypoeutectic open-cell Al-Si foams produced by squeeze casting showed smaller pore size, better pore distribution, higher porosity, good compressive strength and greater energy absorption energy compared to that of gravity die casting. The hypoeutectic open-cell Al-Si foams with 44 vol.% NaCl using squeeze casting showed the best properties among all foams due to its moderate and well-distributed porosity.

Welding of metallic AlSi foams and AlSi–SiC composite foams

In this paper, possibilities as well as limitations of aluminium foam welding are defined, and current state of the art methods are presented. In the face of difficulty of achieving joints with TIG welding, the authors’ own innovative methods of preparation of aluminium and composite foams top layer are proposed in order for the foams to be TIG and laser beam welded. Filler consumption was estimated for different variants of edge preparation. Joint structure was examined and tensile strength of the joints was tested. Joints whose strength exceeded the strength of parent material were obtained.

Manufacturing and Characterization of AlSi Foams as Core Materials

AlSi alloys and their foaming properties have been studied in this paper. For adequate comparison it has been necessary to define process parameters and optimal chemical composition of the Al alloys. Such foams have been evaluated in terms of structure and mechanical properties, in particular in the use of foams as cores materials of cylindrical massive skins.

Evaluation of Methods of Soldering AlSi and AlSi-SiC Particle Composite Al Foams

The cellular structure and unique properties of aluminum foams are the reason of their numerous applications and interests in respect of their joining. The paper includes the characterization of the essence of properties and application of aluminum and aluminum composite foams, the limitations, and possibilities of their soldering. The aim of the research is the consideration of methods of soldering AlSi foams and AlSi-SiC composite foams, and the joint structure. EDS and XRD investigations of the AlSi-SiC composite foams’ joints were done. The possibility of soldering AlSi9 foams and AlSi9-SiC composite foams using S-Bond 220 solder was confirmed, and higher tensile strength of the joint than the parent material was also ascertained

Open Cell Al-Si Foams by a Sintering and Dissolution Process

Open cell foams from AlSi12 alloy were successfully fabricated by the Sintering and Dissolution Process, using NaCl as space holder (60 %). The size of the aluminum alloy powder is less than 45 μm, while the space holder powder size is 315-500 μm, 630-800 μm and 800-1250 μm respectively. The appropriate quantities of alloy powder and salt were mixed and cold pressed at 250 MPa. The sintering process was done at 500 °C and 545 °C, in vacuum (10-5torr) for 10, 20 and 30 minutes respectively. The space holder was eliminated by holding the sintered samples in running hot water (70 °C). After the salt was dissolved, the samples were dried and the mass loss was analyzed. Keywords: Aluminum foam, Powder metallurgy, Sintering and dissolution process

Solderability of Alsi Foams and AlSi + SiC Composite Foams and the Joints Properties

Abstract The cellular structure and unique properties of aluminum foams are the reason of problems concerning their cutting and bonding. The content of the paper includes characterization of the essence of properties and application of aluminum foams, limitations and chances of aluminum foams soldering. The aim of the research is consideration of possibilities and problems of soldering AlSi foams and AlSi - SiC composite foams as well as mechanical properties. The possibility of soldering AlSi foams and AlSi - SiC composite foams using ZnAl solders was confirmed and higher tensile strength of the joint than the parent material was ascertained

Semisolid melt squeezing procedure for production of open-cell Al–Si foams

Melt squeezing process in semisolid state was used for the first time for production of open-cell Al–Si foams with improved microstructural and mechanical characteristics. First a given amount of preheated NaCl particles was stirred into molten A356 alloy. Stirring continued during solidification of the slurry until reaching a given solid fraction of primary particles. The resulting mixture was pressed by a perforated piston to squeeze a controlled amount of the residual meltout. Open-cell foams were achieved by leaching the final Al–Si–NaCl composites in water. The suitable values of the NaCl particle size, pressurizing temperature, melt to salt ratio and piston pressure for production of uniform and high integrity foams were obtained to be 3400μm, 605°C, 1.5:1 and 10MPa, respectively. The results showed that the semisolid processing employed could refine and modify the microstructure of the cell ligaments in the final foams. Mechanical properties of the foams such as energy absorption and fracture toughness were also improved by the semisolid processing.

Statistical analysis of pore structure and compression behaviour of Al-Si foams

The evolution of pore morphology of closed cell Al-Si alloy foams during compression was examined using X-ray computed tomography. The stress-strain curves of the foams were obtained. The structural deformation of the pores during compression and the microstructures were analysed. Al-Si alloy foams were found to present brittle behaviour; cracks appeared in cell walls after the elastic deformation stage. As the compression proceeded, the cracks extended and some of the cell walls fractured and a crack developed through the specimen causing failure. As the relative density increased, yield strength and effective elastic modulus of Al-Si foams increased. For φ (volume fraction of solid) between 0.97and 0.2 the measured elastic modulus and yield strength were consistent with existing empirical models for foam materials.

Metal Foaming Studied In Situ by Energy Dispersive X‐Ray Diffraction of Synchrotron Radiation, X‐Ray Radioscopy, and Optical Expandometry

AbstractThree synchronized methods are combined for studying in situ the foaming process of AlSi11 powder precursors containing either as‐received TiH2 or pre‐oxidized TiH2−x. The phase transformations are followed by energy dispersive X‐ray diffraction (ED‐XRD) of synchrotron radiation. The internal structure of the foam is monitored by X‐ray radioscopy and a video camera records the overall foam expansion. Complementary mass spectrometry follows the H2 gas release. Phase transformations of TiH2 and TiH2−x particles inside Al–Si foams are reported in this work for the first time and they are different than the ones observed for loose powders under Ar flow because the solid metal matrix initially retards H2 outgassing, but also because after melting the liquid reacts with TiH2 forming the ternary compounds Ti(AlxSi1−x)2 in the semi‐solid state and Ti(Al1−xSix)3 in the liquid state of the alloy. Further reasons for the larger expansion obtained when using pre‐oxidized TiH2−x are revealed as well. The oxide shell of pre‐oxidized TiH2−x not only shifts the onset of H2 release towards higher temperatures, but it also hinders the reaction between blowing agent and melt, stabilizing the structure of TiH2−x thus preserving its role as a good blowing agent for foaming Al‐based alloys.

Effect of cell shape anisotropy on the compressive behavior of closed-cell aluminum foams

The closed-cell Al–Si foams have been prepared by molten body transitional foaming process using TiH 2 foaming agent. The cell shape anisotropy ratio of specimens with various relative densities was measured. The quasi-static compressive behavior of Al–Si foams in both longitudinal and transverse directions were investigated. The results show that Al–Si foam loaded in the transverse direction exhibits a lower stress drop ratio. The relationship between plastic collapse stress ratio and cell shape anisotropy is in accordance with Gibson and Ashby model. The plastic collapse stress and the energy absorption property of Al–Si foams increase following power law relationship with relative density. Moreover, Al–Si foams exhibit higher plastic collapse stress and the energy absorption property in the longitudinal direction than that in the transverse direction.

Acoustic emission of salt-replicated foams during compression

Deformation on a sample surface was observed and the acoustic emission (AE) response was recorded during compression of salt-replicated AlSi foams of different pore size. During compression, formation of deformation bands with concomitant strain hardening was observed. The wavy character of the AE count rate vs. strain dependence was correlated with the deformation process, e.g. with the formation of deformation bands, and it was found that this wavy character yields information on the localization of the deformation.