Microstructure and strength of a squeeze cast aluminium piston alloy composite reinforced with alumina short fibre using Al2O3 binder

Abstract

Alumina short fibre preforms were fabricated using an Al₂O₃ binder and infiltrated with aluminium piston alloy melt by squeeze casting. Al₂O₃ binder is thermodynamically more stable than the conventional SiO₂ binder and reduces the fibre/matrix interfacial reaction. The effects of fibre volume fraction, temperature and heat treatment on the yield strength and tensile strength of the composite were investigated. The Al₂O₃ binder provided a satisfactory interfacial bond between the fibre and the matrix without any interfacial reaction or fibre damage. Aging behaviour was not changed by reinforcement. At every temperature, the composites showed the highest strength with a fibre volume fraction of 18%. The strength of the composite was improved by T6 heat treatment. Examination of the fracture surfaces and calculation of the tensile strength using the rule of mixtures indicated that the 18% fibre reinforced composite had a strong interfacial bond even at high temperatures.     

Large undercooling,rapid solidification,and nucleation mechanism during multistage atomisation

Abstract

The large undercooling and rapid quenching that can occur during multistage atomisation are studied both theoretically and experimentally. Results show that these two effects are closely related to and promoted by each other. The level of undercooling for droplets is dependent on alloy composition, powder particle size, and atomisation condition. The cooling rates of droplets depend heavily on their particle size. Both experiments and theoretical analysis on Sn-Pb alloys reveal that a certain function of undercooling has a linear relationship with the logarithm of the mean powder particle particle size. Based on the present results, a mechanism of nucleation preferred on surface oxide is proposed to give a quantitative interpretation of the experimental observations.     

Processing of Mn–Zn ferrites using mould casting with acrylic thermosetting binder

Abstract

In this paper, a simple manufacturing process for Mn–Zn ferrite powder is described, which can be considered as a modified powder injection moulding process. This method uses acrylic thermosetting resin as the binder. The moulding is carried out at room temperature by directly pouring the slurry (resin and ferrite) in the mould. The mixture is heated at the curing temperature (70°C) of resin to permit polymerisation and cross linking of the polymer. In order to optimise the moulding step, different volume fractions of powder with resin were mixed. The optimal powder load was 50 vol.-%. The best thermal debinding cycle was determined by means of thermo-gravimetric analysis. Sintering was performed according to oxygen partial pressure equilibrium curves at 1330°C for 3 h. Magnetic properties were compared with those obtained by uniaxial compacted parts.     

Determination of rapid heating cycles for binder removal from open pore green ceramic components

Abstract

Rapid heating cycles have been determined for the thermal removal of binder from open pore green ceramic components. The samples are multilayer green bodies with barium titanate as the dielectric, and the binder consists of poly(vinyl butyral) and dioctyl phthalate. The kinetics of binder decomposition, the gas permeability of the green body and the conditions at failure of the green body were determined from a combination of experiments and modelling. These results were then used with an algorithm based on variational calculus to develop successful rapid heating cycles without causing failure of the component.     

Metal injection moulding of bronze using thermoplastic binder based on HDPE

Abstract

In the present work, the injection moulding process of a Cu–10Sn bronze has been studied. Different formulations of binders based on high density polyethylene, paraffin wax and polyethylene glycol have been used. The optimisation of the metallic load is based on torque measurements and rheological studies. The optimum powder loading was 60 vol.-%. The moulding parameters are selected to obtain homogeneous specimens with three different geometries and without distortions. The green parts have an adequate strength for handling. The organic binder was eliminated by thermal debinding under N₂/10%H₂ atmosphere. The debinding process has been designed by means of thermogravimetrical analysis of binder and feedstock and considering the maximum heating rates at which the samples do not present cracks. The specimens were sintered at temperatures between 875 and 950°C in the same reducing atmosphere.     

Master decomposition curve analysis of ethylene vinyl acetate pyrolysis: influence of metal powders

Abstract

Polymer burnout (pyrolysis or delubrication) is a crucial step in sintering die compacted powders. To systematically analyse and design the thermal delubrication step, the master decomposition curve (MDC) has been formulated based on the intrinsic kinetics of polymer pyrolysis. The Kissinger method was used to estimate the activation energy from thermogravimetric analysis (TGA) experiments. The activation energy of poly(ethylene-co-vinyl acetate) (EVA) was determined and an MDC analysis was performed to map the weight loss of the polymer as a function of time and temperature. The developed MDC was used to investigate the effects of powder chemistry, powder shape, and particle size of 316L stainless steel on the decomposition behaviour of EVA. The activation energies for decomposition of EVA decreased in the presence of gas and water atomised 316L stainless steel powders, indicative of a catalytic effect. This effect was more pronounced for the first decomposition step suggesting the possible role of a carboxylate ion – metal transition state complex that promoted decomposition. In addition, the gas atomised 316L stainless steel had a greater effect on lowering the activation energy for decomposition compared to water atomised 316L stainless steel, emphasising the influence of powder surface chemistries. Based on the MDC analysis, the required hold time can be predicted for a given temperature and target binder weight loss. This reduces the experimentation required to optimise the delubrication cycle. Furthermore, when extrapolating to very small particle sizes, this approach is of particular interest for predicting the behaviour of nano-particulate materials.     

Corrosion behaviour of CoW0·013C0·001 in aqueous acidic sulphate solutions containing sodium lauryl sulphate and sodium citrate

Abstract

The corrosion behaviour of the CoW_xC_y binder alloy has been shown to be the key factor in the chemical stability of hardmetals of the WC–Co type. The anodic oxidation of the CoW_0·013C_0·001 alloy was studied in 0·1M H₂SO₄ with and without additions of 20 mM sodium lauryl sulphate or 20 mM tri-sodium citrate which were intended to act as organic corrosion inhibitors. In this investigation, use was made of linear sweep voltammetry, chronoamperometry, and ac impedance measurements (which can convey additional information on anodic activation processes occurring during corrosion). The reaction product morphology and composition were studied by scanning electron microscopy and energy dispersive spectroscopy. The inhibiting effects of the organic additives on the pseudo-passivating mechanisms were described. The precipitation of layers of corrosion products with different protective effectiveness was assessed and correlated with the electrochemical kinetics and with the morphological and chemical characteristics brought about by the presence of the organics. Sodium citrate proved more effective than sodium lauryl sulphate in inhibiting corrosion in the test conditions used in the present study.     

Injection moulding of alumina with partially water soluble binder system and solvent debinding kinetics

Abstract

A partially water soluble binder system was successfully derived and tested for injection moulding of alumina powder. The major binder of the binder system was comprised of poly(2-ethly-2-oxaline) and polyethylene glycol and these constituents formed the water soluble fraction of the system. The rheological properties of the feedstock were investigated systematically over a temperature range of 120 to 160°C and a shear rate range of 100 to 1000 s^?1. Binder removal was accomplished using a two stage process. The water soluble constituents were removed by water leaching. The remaining binder constituents were removed by thermal pyrolysis. The solvent debinding kinetics of the water leaching process were studied as a function of temperature, ranging from 40 to 80°C. Solvent debinding is a two stage process consisting of dissolution and diffusion. In this study, dissolution was the rate limiting step during the first stage of water leaching over a leaching time of 90 min at 40°C. As the process proceeds, it is shown that diffusion becomes the rate limiting step.     

Mechanism of intergranular attack in Zn–Al–Cd sacrificial anodes at elevated temperatures

Abstract

The premature failure of a Zn–0·3 Al–0·03Cd anode at 70°C in sea bed mud has been simulated in laboratory sea water using galvanostatic polarisation. The exposed surface of the anode suffered intense intergranular attack and some dissolution. Within the bulk of the material intergranular attack was observed, but no dissolution. Unpolarised alloys in a variety of environments exhibited the same type of attack; however, it could not be produced on pure zinc. No evidence could be found of segregation or precipitation of aluminium at grain boundaries. Moreover, specimens that had been solution treated to ensure a single phase microstructure suffered from intergranular attack in short term simulation tests. It is concluded that a previously proposed mechanism consisting of grain boundary precipitation of aluminium followed by its preferential dissolution is incorrect. It is proposed from analysis of the fracture morphology and the effect of test conditions that the failure is caused by hydrogen penetration.     

Influence of multisteps thermal control in metal powder injection moulding process

Abstract

In metal injection moulding, the quality of the products depends highly on an effective and suitable debinding process. The improvement of the debinding process thus becomes one of the most important topics in metal injection moulding research. To increase the binder debinding rate and decrease the defects of the products, it is essential to understand the influence of thermal control on the debinding process. The present paper aims to investigate the effect of multisteps thermal control on the debinding rate through experiments. Different from previous researches in which single binder is usually adopted, multicomponents binder is used to reflect the real manufacturing situations in industry. The relationship among viscous force, capillary force and pressure force for different working conditions is discussed in details. Experimental results show that the debinding rate and the final debinding fraction are dominated by particle size in the compact rather than that in the wick. For thick compact with low porosity, the shape of green compact is well maintained and the defect of neck shrinkage is reduced by multisteps thermal control. The neck shrinkage situation can further be improved by using the two component binder.