Improvement of the foaming process for 4045 and 6061 aluminium foams by using the Taguchi methodology

Taguchi methodology has been applied to the production process of aluminium foams to investigate the variability detected in several properties (including bulk density, outward appearance and density homogeneity along foaming direction), for foaming tests carried out under identical conditions. The analysis of the process has been performed separately for two different alloys, the 4045 and 6061. The results have allowed finding the main factors that influence those properties. In addition, it has been possible to establish those foaming conditions able to minimize the variability in density, to improve the outward appearance and to obtain a higher homogeneity in density, all at the same time. Different final factors have been found for the two alloys; such differences have been explained in terms of the different viscosity of the aluminium melts as well as the different content of foaming agent.     

Preparation of nickel-coated powders as precursors to reinforce MMCs

The preparation of nickel-coated ceramic particles as precursors for MMC fabrication was studied. Al₂O₃ and SiC powders of three different particle sizes were successfully coated with Ni using an electroless metal plating technique. Uniform and continuous nickel films were deposited on both, alumina and silicon carbide powders, with a final composition ranging from 1.6 to 1.9wt% phosphorus, 18–21wt% of metallic nickel and the balance is ceramic. XRD showed that the Ni-P deposit was predominantly amorphous. However, after heat treatment, the metallic deposits crystallize into Ni and Ni₃P phases, as confirmed by DSC analyses. Preliminary results showed that the use of Ni-coated powders enhances the wettability between the matrix and ceramic phase when processing particulate MMCs by infiltration techniques. The coating promoted easy metal flow through the preform, compared to the non-infiltration behavior of the uncoated counterpart samples.     

Extrusion modelling of 6061 aluminium alloy and particle reinforced MMCs

Abstract

Extrusion modelling was performed for 6061 aluminium alloy and three particle reinforced MMCs (10%Al₂O₃/6061, 15%SiC/6061, 20%Al₂O₃/6061) using constitutive equations previously obtained from torsion test data. In applying the finite element software DEFORM, suitable heat transfer, friction, and velocity boundary conditions were chosen based on a direct extrusion press. Simulations were run for various extrusion conditions and the outputs for the four materials were compared. The simulation results were validated by comparison with real life extrusions and modelling of other researchers. The results showed that an increase in billet temperature, a reduction in ram speed, or a reduction in extrusion ratio had the effect of reducing the ram load. In consequence, extrusion conditions could be selected so that extrusion of the composite was carried out with the same peak ram load as the alloy.     

Hot deformation behaviour of aluminium alloy 6061/SiCp MMCs made by powder metallurgy route

Abstract

The hot deformation behaviour of a 20 vol.-%SiC particle reinforced Al alloy 6061 composite and Al alloy 6061, made via a powder metallurgy (PM) route and treated by a T4 temper, were studied by compression testing over a range of temperatures (300–500°C)and at strain rates of 0.005, 0.05, and 0.09 s^-1. It was observed that the flow stresses of the composites were significantly higher than those of the alloy at lower deformation temperatures. However, the stress–strain curves of both the composite and the alloy showed significant softening during deformation at the lowest strain rate, the softening for the composite being faster than that of the alloy. The activation energy for hot deformation was determined for different strains, using a power law equation, and was found to change significantly with strain for both the alloy and the composite. This phenomenon was explained by the occurrence of dynamic precipitation and coarsening during deformation.     

The reaction sequence in the synthesis of aluminium borate whiskers

The reaction sequences in the synthesis of aluminium borate whiskers from the raw material mixtures (Al₂(SO₄)₃/H₃BO₃/K₂SO₄, K₂SO₄ as a flux) were investigated by differential thermal analysis-thermogravimetric analysis (DTA-TG), X-ray diffraction analysis (XRD), and a direct observation of the reacting mixture. DTA of a mixture with the optimum composition for obtaining well-grown whiskerswith a high yield, and XRD of the same mixtures quenched at various temperatures, showed that the reaction proceeds through formation and decomposition of double salts of aluminium: first tripotassium aluminium sulphate and then monopotassium aluminium sulphate. This reaction sequence and the DTA results of mixtures with different K₂SO₄/(Al-B) ratios were successfully explained by the phase diagram of the Al₂(SO₄)₃ K₂SO₄ system. Direct observation of the reacting mixture confirmed the explanation. The compositions of reacting mixtures were then calculated from the TG data and traced on the phase diagram. The tracing showed that the decomposition of aluminium salts occurred in different physical states, depending on the K₂SO₄/(Al+B) ratio, in solid, solid and then liquid, or liquid phase. This difference in decomposition explained well the effect of the K₂SO₄/(Al+B) ratio on the morphology of the whiskers. The effect of the B/Al ratio in the mixture was similarly explained by the correlated change in the K₂SO₄/Al ratio.Previously known as the Government Industrial Research Institute, Shikoku.     

Early anisotropic expansion of aluminium foam precursors

This work reports a detailed study on the early expansion (before the melting point) of powder metallurgical (PM) aluminium foam precursors and its influence on the intrinsic anisotropy existing in the final cellular structure of PM foams. Hot uniaxial compressed tablets, rectangular and cylindrical extruded profiles and thixocast PM precursors have been considered to evaluate the effect of the processing technique on the early expansion behaviour. An optical device has been used to register, in situ, the dissimilar expansion of the precursors in the three spatial directions. Cellular morphology has been examined by X-ray microtomography and correlated to expansion behaviour. Results demonstrated a high influence of the processing technique and its correlation with powder debonding in preferential directions which, in combination with the early TiH₂ decomposition, generates elongated cracks and causes an anisotropic expansion at early foaming stages. As a consequence, a remaining structural anisotropy is found in the final solid cellular material, even at high porosities. A discussion of the possible factors affecting this early expansion behaviour, complemented with possible solutions to minimize it, is provided in the paper.     

Inorganic aluminium precursors in the synthesis of mullite — an investigation

A variety of inorganic aluminium sols were investigated for their reactivity with colloidal silica to form polycrystalline mullite. Nuclear magnetic resonance (NMR) spectroscopy was used on the precursor aluminium sols to ascertain the co-ordination and reactivity of the species present. Differential thermal and thermogravimetric analysis and X-ray diffraction were used to analyse the products of the fired xerogels. Commercial reactants produced diphasic materials with little or no mullite formed at 1000 °C. A monomeric/dimeric sol produced in house however, showed a sharp exotherm at 920 °C and crystalline mullite formation at this temperature.     

Reduction of disinfection by-products precursors by nanofiltration process

This research is dedicated to determine the rejection ratios of disinfection by-products (DBPs) precursors including resorcinol, phloroglucinol, 3-hydroxybenzoic acid, and tannic acid solution in the presence of calcium by nanofiltration with NF70 membrane. The rejections of these model compounds also were studied at various compositions of a feed solution by changing pH and concentrations of model compounds. It was found that the model compound rejection and membrane permeability increase with pH due to the conformational transformation of ionizable molecules and electric interaction between the model compounds and NF70 membrane. The interactions of model compounds with calcium have no significant effect on model compounds retentions. Because of the complexation of calcium with model compounds, calcium rejection rises with the presence of model compounds and with an increase of pH.     

泡沫铝夹芯板芯材发泡的研究

泡沫铝夹芯板不仅克服了单一泡沫铝材料强度较低的缺点;而且还具有泡沫铝材料的诸多特殊性能,是一种非常有发展潜力的材料之一.通过复合轧制的方法制备了冶金结合的界面的泡沫铝夹芯板.研究表明,早期发泡的孔隙主要以横向方向长扁孔为特征,主要是长扁孔的形成与扩展.通过对泡沫铝芯材在不同的工艺参数下进行发泡得出通过本实验的最佳混料时间为2h,轧制压下率为60%～70%,发泡温度在620～630℃之间,发泡时间在8～10min.     

MMCs with controlled non-uniform distribution of submicrometre Al2O3 particles in 6061 aluminium alloy matrix

Abstract

By employing spherical agglomerate of submicrometre Al₂O₃ particles, a novel composite with controlled non-uniform reinforcement distribution was produced by the squeeze casting technique. The resulting composite has a microstructure in which composite spheres, consisting of Al reinforced by small Al₂O₃ particles, are uniformly distributed in a particle free Al matrix. The response of mechanical properties to the controlled non-uniform distribution of the reinforcement was investigated and the fracture mechanism was discussed in order to understand experimentally the influence of particle clustering on the deformation characteristics. It is demonstrated that a composite with such a controlled microstructure exhibits significant increase in ultimate tensile strength (UTS), elastic modulus, and also a slight increase in ductility compared to composite with a homogeneous microstructure at a given volume fraction. It is proposed that the composite spheres act as units of reinforcement when subjected to deformation and contribute positively to the modulus, UTS, and hardness. The higher ductility is attributed to the extensive ductile deformation of the Al matrix. In addition, the highly restrained deformation observed in the interagglomerate region can also be beneficial to the high strength.     

Damage in MMCs using the GMC: theoretical formulation

In this work the incorporation of damage in the material behavior is investigated. Damage is incorporated into the generalized cells model (GMC), and applied to metal-matrix composites (MMCs). The local incremental damage model of Voyiadjis and Park is used here in order to account for damage in each subcell separately. The resulting micromechanical analysis establishes elasto-plastic constitutive equations that govern the overall behavior of the damaged composite. The elasto-plastic constitutive model is first derived in the undamaged configuration for each constituent of the metal-matrix composite. The plasticity model used here is based on the existence of a yield surface and flow rule. The relationships are then transformed for each constituent to the damaged configuration by applying the local incremental constituent damage tensors. The overall damaged quantities are then obtained by applying the local damage concentration factors obtained by employing the rate of displacement and traction continuity conditions at the interface between subcells and between neighboring repeating cells in the generalized cells model. Examples are solved numerically in order to explore the physical interpretation of the proposed theory for a unit cell composite element.     

Effect of process parameters on bead properties of A359/SiC MMCs welded by laser

Metal matrix composites are well known materials that present several positive features, mainly in terms of mechanical strength to weight ratio. On the other side, MMC’s are extremely difficult to machine (in particular by chip removal processes) and this difficulty has limited a wider diffusion. Welding, together with other joining technologies, could be a solution to limit the chip removal processes in the production of a MMC part, but this is not a problem-free process. This paper presents an experimental study on the laser welding of MMCs using both a CO₂ and a diode laser source. The CO₂ welding is performed in keyhole conditions (deep penetration), whereas the diode laser performs a conduction welding. Results show that the CO₂ welding is greatly affected by the formation of Al₄C₃, that compromises the toughness of the bead. The content of Al₄C₃ can be reduced by acting on the process parameters, but cannot be completely eliminated. Instead, diode laser welding generates a sound bead, with negligible Al₄C₃ formation and of superior quality. Considering also the other positive features of this kind of sources (high efficiency and radiation wavelength, that is, highly absorbed by MMCs), diode laser welding seems to be highly recommendable for MMCs laser welding.     

The wetting of carbon by aluminium and aluminium alloys

The contact angle made by molten aluminium with vitreous carbon was measured by the sessile drop method in vacuum at temperatures up to 1100° C. The effect on wetting behaviour of the oxide layer on the molten metal was highlighted by using two samples of aluminium in different states of oxidation. The investigation involved the variation of certain parameters affecting the stability of the oxide film, e.g. the temperature, additions of Ti, Si, Cr, Be, Ca and Li to aluminium and the time held at a certain temperature. The state of the molten aluminium surface under various experimental conditions was determined indirectly by surface tension measurements.     

The reaction between ruthenium dioxide and aluminium nitride in resistor pastes

Thermodynamic considerations predict that RuO₂ used in resistor pastes for AIN as conductive phase, can be reduced by AIN. In air RuO₂ reacts with AIN only to a very small extent. In the presence of a non-reducible glass the reaction between RuO₂ and AIN is demonstrated by thermoanalytical, mass spectroscopical and XRD methods. The described reaction could be a reason for bubbles in resistor layers on AIN. A crystallizing glass impedes the complete reaction.     

Parameter identification and computational simulation of polyurethane foaming process by finite pointset method

Numerical simulation of the polyurethane foaming process is a valuable method to analyze the molding process at an early stage of product development to shorten time-to-market cycles and cut costs by using fewer prototypes. However, this process involves highly coupled thermo-chemo-rheological modeling and needs adequate model parameters’ identification. A theoretical model including chemical reactions and thermo-rheological coupling of conservation equations was developed. Based on the theoretical model, three-dimensional numerical simulation for mold filling of the polyurethane foam was carried out by using Finite Pointset Method (FPM) to predict flow field, flow front advancement, temperature and density distributions during mold filling. A FOAMAT system was used to monitor foam height rise and reaction temperature on a cylindrical test tube and foam viscosity was measured by using a dynamic rotational rheometer with parallel-plate system. The parameters of the model were identified by an inverse analysis method which consists in determining the parameters by comparing the computed quantities to those measured experimentally. The overall modeling was validated by using short shot foams obtained with a panel mold cavity. Mold filling of an automotive underlay carpet cavity was investigated numerically. Flow front results were successfully compared to short shot foams obtained with the industrial mold cavity.     

Diffusion-based microalloying of aluminium alloys by powder metallurgy and reaction sintering

A diffusion-based technique of microalloying aluminium powder metallurgy products was examined to expand the range of feasible alloying additions. Thermodynamic calculations and diffusion rates for several elements suggested that tin and silver were the most promising; these elements were successfully alloyed into AA 2014 on both a macroscopic and a microscopic scale. The final microstructures were examined using X-ray diffraction, X-ray mapping and energy-dispersive electron probe microanalysis. Silver additions were homogeneous throughout the alloy microstructure, whereas tin was concentrated in intergranular regions only. The results suggested that the technique was viable for a variety of microalloying elements. Also, the extent of alloying was predicted reasonably well using a mathematical mass balance model.     

Simplification of heat treatment process in a tool steel by aluminium addition

A novel concept for simplification of heat treatment process in a tool steel by the addition of aluminium has been proposed in this research. The addition of 1.08?wt-% aluminium leads to an approximately fully pearlitic state, of which the cementite lamellae are largely spheroidised. Excessive addition of 1.58?wt-% aluminium would result in the formation of a large amount of δ-ferrite. These results are mainly attributed to the synthetic effect of aluminium on the driving force of pearlite transformation and the inter-spacing distance between the proeutectoid carbides. The mechanical properties’ analyses show that aluminium has promising potential to substantially simplify the processing method for developing a relative low-cost mould steel without the concomitant mechanical properties’ reductions.     

The UPAL process: a direct method of preparing cast aluminium alloy-graphite particle composites

A direct method of preparing cast aluminium alloy-graphite particle composites using uncoated graphite particles is reported. The method consists of introducing and dispersing uncoated but suitably pretreated graphite particles in aluminium alloy melts, and casting the resulting composite melts in suitable permanent moulds. The optical pretreatment required for the dispersion of the uncoated graphite particles in aluminium alloy melts consists of heating the graphite particles to 400° C in air for 1 h just prior to their dispersion in the melts. The effects of alloying elements such as Si, Cu and Mg on the dispersability of pretreated graphite in molten aluminium have also been reported. It was found that additions of about 0.5% Mg or 5% Si significantly improve the dispersability of graphite particles in aluminium alloy melts as indicated by the high recoveries of graphite in the castings of these composites. It was also possible to disperse upto 3% graphite in LM 13 alloy melts and retain the graphite particles in a well distributed fashion in the castings using the pre-heat-treated graphite particles. The observations in this study have been related to the information presently available on wetting between graphite and molten aluminium in the presence of different elements and our own thermogravimetric analysis studies on graphite particles. Physical and mechanical properties of LM 13-3% graphite composite made using pre-heat-treated graphite powder, were found to be adequate for many applications, including pistons which have been successfully used in internal combustion engines.