Processing and Stability of LM25/SiCP Composite Foams

A simple method of metal foam production is to introduce a blowing agent (e.g. TiH₂) into an aluminium melt containing foam stabilisers such as oxides (usually Ca-based) and/or particles (e.g. SiC, Al₂O₃). In this work, Al/SiC composites (in-house and commercial Duralcan) [both of them with LM25 matrix (Al?C7Si?C0.3Mg)] containing particles of various sizes and contents were foamed at different temperatures using TiH₂. Foamability is characterised through their expansion and collapse. It is observed that high expansions and good quality foams could be obtained upon manipulating SiC particle size and content. However, irrespective of particle size/vol.% combination, significant effect of foaming temperature is noticed on the fundamental stability of the liquid foam until solidification. Both cell size and foam density varied along the ingot height. The distribution of SiC_P within the cell wall is random with no preferential segregation to gas/metal interface. The evolution of foam, and the role of SiC on foam stability are discussed based on macro and cell wall microstructural results.     

Effect of Sn on the Dehydrogenation Process of TiH2 in Al Foams

The study of the dehydrogenation process of TiH₂ in aluminum foams produced by the powder metallurgy technique is essential to understanding its foaming behavior. Tin was added to the Al foam to modify the dehydrogenation process and stabilize the foam. A gradual decomposition and more retention of hydrogen gas can be achieved with Sn addition resulting in a gradual and larger expansion of the foam.     

Fabrication of Al-3.7?Pct Si-0.18?Pct Mg Foam Strengthened by AlN Particle Dispersion and its Compressive Properties

Al-3.7 pct Si-0.18 pct Mg foams strengthened by AlN particle dispersion were prepared by a melt foaming method, and the effect of foaming temperature on the foaming behavior was investigated. Al-3.7 pct Si-0.18 pct Mg alloy containing AlN particles was prepared by noncompressive infiltration of Al powder compacts with molten Al alloy in nitrogen atmosphere, and it was foamed at different foaming temperatures ranging from 1023 to 1173 K. The porosity of prepared foam decreases and the pore structure becomes homogeneous with increasing foaming temperature. When the foaming temperature is higher than 1123 K, homogeneous pores are formed in the prepared ingot without using oxide particles and metallic calcium granules, which are usually used for stabilizing a foaming process. This stabilization of the foaming at high temperatures is possibly caused by Al₃Ti intermetallic compounds formed at high temperature and AlN particles. Compression tests for the prepared foams revealed that the absorbed energy per unit mass of prepared Al-3.7 pct Si-0.18 pct Mg foam is higher than those of aluminum foams strengthened by alloying or dispersion of reinforcements. It is remarkable that the oscillation in stress, which usually appears in strengthened aluminum foams, does not appear in the plateau stress region of the present Al-3.7 pct Si-0.18 pct Mg foam. The homogeneity in cell walls and pore morphology due to the stabilization of pore formation and growth by AlN and Al₃Ti particles is a possible cause of this smooth plateau stress region.     

Reduced-Pressure Foaming of Aluminum Alloys

We developed a novel process for foaming aluminum and its alloys without using a blowing agent. The process involves a designated apparatus in which molten aluminum and its alloys are first foamed under reduced pressure and then solidified quickly. Foaming was done for pure aluminum (99.99 pct) and AlMg5 alloy not containing stabilizing particles and AlMg5 and AlSi9Mg5 alloys containing 5 vol pct SiO₂ particles. We discuss the foaming mechanism and develop a model for estimating the porosity that can be achieved in this process. The nucleation of pores in foams is also discussed.     

Compressive Deformation Behavior of Highly Porous AA2014-Cenosphere Closed Cell Hybrid Foam Prepared Using CaH<Subscript>2</Subscript> as Foaming Agent: Comparison with Aluminum Foam and Syntactic Foam

Closed Cell AA2014-cenosphere hybrid foams have been prepared through stir-casting technique using varying amount of CaH₂ powder as foaming agent. The cenospheres in hybrid foams created micro-pores in the cell wall and in the plateau region. It reduced the requirement of CaH₂ for foaming by 30–40% by attaining equivalent level of relative density. These foams have been characterized for of microarchitechtural characteristics and mechanical properties like strength, densification strain and energy absorption. The properties of hybrid foams have been compared with those of conventional AA2014-SiC foam and Al-cenosphere syntactic foam. The closed cell AA2014-cenosphere hybrid foam exhibited comparable plateau stress, densification strain and energy absorption characteristics to those of AA2014-SiC foams with same relative density. Empirical relations to correlate plateau stress, densification strain and energy absorption for entire range of porosities have been established.     

Fracture mechanisms of a 2124 aluminum

This study was aimed at investigating the effects of microstructure on the fracture behavior of a 2124 aluminum composite reinforced with SiC whiskers. Particular emphasis was placed on the role of matrix intermetallic particles, inhomogeneous distribution of whiskers, and whisker breakage in the fracture process. Various tests were conducted on the composite to identify the micromechanical processes that were involved in microvoid or microcrack formation. Detailed microstructural analyses showed that the aluminum matrix contained a significant amount of coarse manganese-containing particles of various sizes which could have been formed during composite processing.In situ scanning electron microscope (SEM) fracture study of the crack initiation and propagation processes clearly showed that these coarse particles fractured prior to matrix/whisker decohesion or whisker breakage, suggesting that the manganese-containing par- ticles significantly accelerated crack initiation in the 2124 Al-SiC_w composite. For a better ma- trix alloy for use in the composite, it is suggested that microalloying elements must be monitored to prevent the formation of the coarse intermetallic particles.     

Prediction of Bubble Size Distribution in Aluminium Foam as a Function of %Titanium Hydride Addition

Foaming of liquid aluminium by addition of foaming agent (TiH₂ particles) is numerically simulated using population balance equations. Phenomena such as hydrogen release by the TiH₂ particles, heterogeneous nucleation of bubbles in oxide surface cavities, and diffusion based bubble growth are modelled. A simple mass transfer coefficient, which varies inversely with the bubble size is used to estimate the bubble growth rate. Simulation is performed to study the effect of TiH₂ content on the final bubble size distribution, total number of bubbles and average bubble size. In general, the average properties of the predicted distributions are close to the experimental values, whereas the spread in the bubble size is observed to be considerably narrower for the predicted values. The deviation in the spread of the distributions is attributed to the inverse bubble size dependent growth rate and non-inclusion of bubble coalescence in the model.     

Fabrication of Aluminum Foams with Small Pore Size by Melt Foaming Method

This article introduces an improvement to the fabrication of aluminum foams with small pore size by melt foaming method. Before added to the melt, the foaming agent (titanium hydride) was pretreated in two steps. It firstly went through the traditional pre-oxidation treatment, which delayed the decomposition of titanium hydride and made sure the dispersion stage was controllable. Then such pre-oxidized titanium hydride powder was mixed with copper powder in a planetary ball mill. This treatment can not only increase the number of foaming agent particles and make them easier to disperse in the melt, which helps to increase the number of pores, but also reduce the amount of hydrogen released in the foaming stage. Therefore, the pore size could be decreased. Using such a ball-milled foaming agent in melt foaming method, aluminum foams with small pore size (average size of 1.6 mm) were successfully fabricated.     

Gas Release Behavior of Cu-TiH<Subscript>2</Subscript> Composite Powder and Its Application as a Blowing Agent to Fabricate Aluminum Foams with Low Porosity and Small Pore Size

Compared to traditional pore structure with high porosity (≥ 80 pct) and large pore size (≥ 3 mm), aluminum foams with low porosity (60 to 70 pct) and small pore size (≤ 2 mm) possess higher compressive property and formability. In order to achieve the goal of reducing pore size, Cu-TiH₂ composite powder prepared by ball milling preoxidized TiH₂ with Cu powder was used as a blowing agent. Its gas release behavior was characterized by thermogravimetric analysis and differential scanning calorimetry. The results show that the ball milling treatment can advance the gas release process and slow the gas release rate at the same time. All these changes are favorable to the reduction of porosity and pore size. Such Cu-TiH₂ composite powder provides an alternative way to fabricate aluminum foams with low porosity and small pore size.     

Effect of Foaming Temperature on Bubble Size Distribution of Liquid Aluminium Foam: Modeling and Experimental Studies

The present study examines the effect of foaming temperature on the final foam expansion and the bubble size distribution of liquid aluminium foam through mathematical modeling and validation experiments. The model calculates the rate of hydrogen release from the foaming agent (TiH₂) particles, super saturation of the melt, nucleation and growth of bubbles and finally, evaluates the evolving bubble size distribution using a population balance approach. The model does not consider bubble coalescence and breakage and uses only solute diffusion for bubble growth. The simulation is performed for two conditions; firstly, for pure temperature effects and secondly, for temperature and TiH₂ quantity combined effects. Upon comparison of simulation results with the experiments, following important observations are made; firstly, the predicted total number of bubbles is found to be one order of magnitude higher than the experiments while the predicted average size is one order of magnitude lower. Secondly, the spread of the predicted distributions is observed to be much narrower. These discrepancies are considered to be due to bubble coalescence and coarsening which are not modeled and shown to be strongly influenced by the foaming temperature.     

Microstructural and Mechanical Characterization of Two Aluminium Based In Situ Composite Foams

In situ composites are multiphase materials where the reinforcing phase is synthesized within the matrix during composite fabrication. The present paper deals with the processing, microstructural and mechanical characterization of Al?C7Si?C0.3Mg?C10TiB₂ and Al?C4Cu?C10TiB₂ foams. Composite foams with very low relative density (??_r?=?0.17?C0.37) and foams containing uniform cell sizes were successfully processed. Since the TiB₂ particle sizes are less than 2???m and have a good wetting behaviour, TiB₂ can be very good foam stabilizers. Microstructural characterization of the cell walls showed significant grain refinement since TiB₂ is a grain refiner. Elemental mapping clearly showed TiB₂ particles at inter dendritic boundaries. Compression testing of the processed foams showed some interesting features. Stress?Cstrain curve showed a lot of serrations which indicated brittle fracture of the cell walls and edges. Hence, it is observed that a balance should be attained between the grain refinement of ??-Al grains and the amount of TiB₂ particles to obtain desirable mechanical properties. Energy absorbed by the processed foams was calculated and they were observed to be close to that of the commercially available ALPORAS foams.     

泡沫铝制备与其压缩性能研究

采用粉末致密化发泡（PCF）工艺制备了泡沫纯铝,对制备过程及影响孔结构的因素进行了分析.系统研究了压力、发泡温度、发泡时间、发泡剂含量和粒度对泡沫纯铝结构变化的影响规律,用自行设计的软件FoamScan对孔结构进行了描述.得出了试验条件下的优化工艺参数配置.进行了泡沫铝压缩性能测试,通过理论模型、性能测试数据作图对比的方法获得了孔隙率83%～87%泡沫纯铝的屈服强度表达式.确定了泡沫纯铝的制备工艺、结构、性能的相互关系.     

Microstructural modification of as-cast Al-Si-Mg alloy by friction stir processing

Friction stir processing (FSP) has been applied to cast aluminum alloy A356 plates to enhance the mechanical properties through microstructural refinement and homogenization. The effect of tool geometry and FSP parameters on resultant microstructure and mechanical properties was investigated. The FSP broke up and dispersed the coarse acicular Si particles creating a uniform distribution of Si particles in the aluminum matrix with significant microstructural refinement. Further, FSP healed the casting porosity. These microstructural changes led to a significant improvement in both strength and ductility. Higher tool rotation rate was the most effective parameter to refine coarse Si particles, heal the casting porosity, and consequently increase strength. The effect of tool geometry was complicated and no systematic trend was observed. For a standard pin design, maximum strength was achieved at a tool rotation rate of 900 rpm and traverse speed of 203 mm/min. Post-FSP aging increased strength for materials processed at higher tool rotation rates of 700 to 1100 rpm, but exerted only a marginal effect on samples prepared at the lower rotation rate of 300 rpm. Two-pass FSP with 100 pct overlapping passes resulted in higher strength for both as-FSP and post-FSP aged conditions.     

Fabrication of foamable precursors by powder compression and induction heating process

In the powder compact melting technique, metallic foams are fabricated by heating a precursor, thus initiating cell growth and foam formation. Proper precursor fabrication is very important because the density distribution after foaming and the foamability are determined during the precursor-fabrication process. The fabrication of the precursor has to be performed very carefully because any residual porosity or other defects will lead to poor results in further processing. In order to evaluate the effect of the compaction parameters on the kinetics of the foaming process, a series of experiments were performed. In this study, 6061 aluminum foams having a closed-cell structure were fabricated by the powder compact method and an induction heating process. An induction coil was designed to obtain a uniform temperature distribution over the entire cross-sectional area of the precursor. To establish the foamable precursor fabrication conditions, the effects of process parameters such as titanium hydride content (0.1 to 1.5 wt pct) and the compression pressure of the foamable precursor (50 to 150 kN) on the pore morphology were investigated.     

Modelling study of slag foaming phenomenon

A one‐dimensional model based on mass and momentum conservations has been developed to predict the heights of foams caused by gas injection. In the development of the model, a dimensionless number, N_foam = u_s.[(3C_d)/(4d_bg)]^1/2, has been deduced to characterise the foaming behaviour. According to the model, an increase in this dimensionless number results in an increase in foaming height. The validity of the model has been experimentally examined using silicon oils. The experiments have also shown that foamings can be classified into two types, namely one‐layer foaming and two‐layer foaming. The former type results in much larger height than the latter.     

Microstructural change during superplastic deformation of δ-ferrite/austenite duplex stainless steel

Superplasticity of a 25 pct Cr-6.5 pct Ni-3 pct Mo-0.14 pct N δ/γ duplex stainless steel has been studied with particular emphasis on the microstructural change during deformation. Two large superplastic elongations are obtained at temperatures around 1323 K in δ/γ duplex phase region and 1173 K where σ phase particles precipitate dynamically at a strain rate of ~10^?3 s^?1. During deformation in the higher temperature region, fine Widmanstätten γ particles coarsen and coarse γ grains formed during the prior treatments are broken into spherical particles, resulting in a homogeneous dispersion of γ particles within the σ-ferrite matrix. The dynamic recrystallization of soft σ-ferrite matrix occurs locally in the region where the strain reaches some critical value, and the final microstructure consists of equiaxed σ and γ grains. In the case of lower temperature deformation, a eutectoid decomposition of δ-ferrite into γ and σ phases occurs. The relatively soft γ grains which are severely deformed by hard σ particles recrystallize dynamically, and these processes lead to the γ/σ equiaxed duplex structure. The extremely large superplasticity of this alloy can mainly be explained in terms of the above microstructural change during deformation.     

Twin Roll Casting of Al-Mg Alloy with High Added Impurity Content

The microstructural evolution during twin roll casting (TRC) and downstream processing of AA5754 Al alloy with high added impurity content have been investigated. Strip casts with a high impurity content resulted in coarse α-Al grains and complex secondary phases. The grain size and centerline segregation reduced significantly on the addition of Al-Ti-B grain refiner (GR). Coarse-dendrite arm spacing (DAS) “floating” grains are observed in the impure alloy (IA) with higher volume in the GR strips. Two-dimensional (2D) metallographic analysis of the as-cast strip suggests that secondary phases (Fe-bearing intermetallics and Mg₂Si) are discrete and located at the α-Al cell/grain boundaries, while three-dimensional (3D) analysis of extracted particles revealed that they were intact, well interconnected, and located in interdendritic regions. Homogenizing heat treatment of the cast strip breaks the interconnective networks and modifies the secondary phases to a more equiaxed morphology. During rolling, the equiaxed secondary phases align along the rolling direction. X-ray diffraction (XRD) analysis suggests that α-Al(FeMn)Si and Mg₂Si are the predominant secondary phases that are formed during casting and remain throughout the downstream processing of the GR-IA. The high-impurity sheet processed from TRC resulted in superior strength and ductility over the sheet processed from small book mold ingot casting. The current study has shown that the TRC process can tolerate higher impurity levels and produce formable sheets from the recycled aluminum for structural applications.     

Effect of Microstructure on Cavitation during Hot Deformation of a Fine-Grained Aluminum-Magnesium Alloy as Revealed through Three-Dimensional Characterization

The effect of microstructure on cavitation developed during hot deformation of a fine-grained AA5083 aluminum-magnesium alloy is investigated. Two-point correlation functions and three-dimensional (3-D) microstructure characterization reveal that cavitation depends strongly on the mechanism that controls plastic deformation. Grain-boundary-sliding (GBS) creep produces large, interconnected cavities rapidly during plastic straining. Solute-drag (SD) creep produces isolated cavities with less total volume fraction at a given strain. The 3-D microstructure data reveal adjacency between various microstructural features. Cavities are observed to be preferentially adjacent to large Al₆(Mn,Fe) particles and to Mg-Si particles of all observed sizes. These data suggest that cavities preferentially nucleate at Mg-Si particles and at large Al₆(Mn,Fe) particles. This result may be applied to reduce cavitation in commercial hot-forming operations utilizing aluminum-magnesium alloys.     

泡沫铝及其三明治结构累积叠轧制备

通过累积叠轧法制备泡沫铝.采用称重法研究泡沫铝孔隙结构,利用光学显微镜观察泡沫铝孔隙形貌.发现以TiH₂为发泡介质,当发泡温度660~680℃和发泡时间6~10 min时,利用累积叠轧法制备泡沫铝的孔隙结构特性最好.发泡温度和发泡时间的最佳值与发泡剂用量有关,TiH₂质量分数为1.5%,在670℃发泡8 min,泡沫铝的孔隙率可达到42%,孔径为0.43 mm.以制备的泡沫铝为夹芯,通过轧制复合制备了TC4钛合金/泡沫铝芯和1Cr18Ni9Ti不锈钢/泡沫铝芯三明治板.利用光学显微镜和能谱仪研究了三明治板的界面.面板与芯板间的化合反应形成了界面的反应层,界面实现了冶金结合.      

Manufacturing of copper foams through accumulative roll bonding (ARB) process: structure and damping capacity behavior

Abstract

Closed cell copper foams have been produced through accumulative roll bonding (ARB) using calcium carbonate (CaCO₃) as blowing agent. Effects of temperature, time and number of rolling passes on the final porosity of the foam have been investigated. The foam with highest porosity has been achieved at 1100°C for soaking time of 3 min. Structure of composite has also been studied by optical and electron microscopy. The result shows that increasing the number of rolling passes reduces the size of powder and homogeneously distributes the particles within the copper substrate. By reducing the size of the particles, free surfaces of particles increase and the gas releasing sites in the foams are enhanced. Consequently, the final porosity of the composite is enhanced as well. The closed-pore foams have also been examined by modal analysis. It has been found that higher porosity of the final foams results in higher natural frequency and damping index.