Yuyuan Zhao

A NOVEL SINTERING-DISSOLUTION PROCESS FOR MANUFACTURING Al FOAMS

Al foams have found increasing applications in a wide range of structural and functional products, due to their exceptional mechanical, thermal, acoustic, electrical and chemical properties [1–3]. Al foam structures have densities only fractions of that of a solid structure and therefore have high specific strength and stiffness. They also have excellent properties for impact energy, vibration and sound absorption. Examples of their applications include lightweight panels for building and transport against buckling and impact, non-flammable ceiling and wall panels for thermal and sound insulation. Open cell foams can also be used as heat exchangers, filters and catalyst carriers. The applications of Al foams on a large scale are likely to be in the automotive industry with an aim to improve the vehicle crashworthiness and thus passenger safety. There currently exist a wide range of manufacturing methods [1–3], which can generally be grouped into five categories according to the forms of the precursory Al and the types of the pore-forming agents, namely melt-gas injection, melt-foaming agent, powder-foaming agent, investment casting and melt infiltration. However, the Al foams produced by these methods are either too expensive due to the high production costs or too poor in quality due to poor controllability in pore structure and porosity. As a consequence, the commercial applications of Al foam components are still limited. With a rapidly increasing demand for high quality Al foams, there has been a growing need for developing cost effective manufacture technologies. This paper describes a new sintering-dissolution process (SDP) for manufacturing net-shape, open-cell Al foams, characterises the porosity, microstructure and compressive properties of the foams produced under a range of SDP conditions, and discusses the capabilities of SDP.

Acoustic absorption behaviour of an open-celled aluminium foam

Metal foams, especially close-celled foams, are generally regarded as poor sound absorbers. This paper studies the sound absorption behaviour of the open-celled Al foams manufactured by the infiltration process, and the mechanisms involved. The foams show a significant improvement in sound absorption compared with close-celled Al foams, because of their high flow resistance. The absorption performance can be further enhanced, especially at low frequencies, if the foam panel is backed by an appropriate air gap. Increasing the air-gap depth usually increases both the height and the width of the absorption peak and shifts the peak towards lower frequencies. The foam samples with the smallest pore size exhibit the best absorption capacities when there is no air gap, whereas those with medium pore sizes have the best overall performance when there is an air gap. The typical maximum absorption coefficient, noise reduction coefficient and half-width of the absorption peak are 0.96–0.99, 0.44–0.62 and 1500–3500 Hz, respectively. The sound dissipation mechanisms in the open-celled foams are principally viscous and thermal losses when there is no air-gap backing and predominantly Helmholtz resonant absorption when there is an air-gap backing.

Mechanical Response of Al Matrix Syntactic Foams Produced by Pressure Infiltration Casting

Aluminum matrix syntactic foams with low-cost porous ceramic spheres of diameters between 0.25 and 4 mm have been manufactured by pressure infiltration casting. These syntactic foams were homogeneous in structure and had densities as low as half of the density of the Al matrix. The mechanical response of the four types of syntactic foams with different sphere sizes and densities under static and dynamic conditions have been investigated. The plateau strength and thus the amount of energy absorption of the syntactic foam are largely determined by the volume fraction of Al and to a lesser extent by the mechanical properties of the ceramic spheres in the foam. Compared with the Al foams with similar Al volume fractions, the syntactic foams had better energy absorption capacity, especially under impact conditions.

Correlation of Strength with Hardness and Electrical Conductivity for Aluminium Alloy 7010

The tensile strength, proof strength, hardness and electrical conductivity of Al alloy 7010 under different temper and ageing conditions were investigated with the aim to correlate strength with hardness and electrical conductivity so that the strength of the alloy can be determined nondestructively. Following the solutionising treatment, continuous age hardening was performed on a series of test coupons, taken from a large plate, to produce a wide range of precipitation hardening conditions, which gave rise to progressive variations of strength, hardness and conductivity. The relationship between strength and hardness was found to be reasonably linear, whereas the relationship between hardness and strength with electrical conductivity was non-linear. The ageing conditions and therefore the mechanical properties of the components can be predicted more accurately by the simultaneous combination of hardness and conductivity values.

Effects of processing conditions on powder particle size and morphology in centrifugal atomisation of tin

Abstract This paper investigates the effects of atomiser design and processing parameters on the morphology and size distribution of centrifugally atomised tin powder. Premature solidification of the melt on the atomiser and poor wetting of the atomiser by the melt were found to be the main causes of unsuccessful atomisation. The particle size distributions of the powders follow a lognormal distribution. The median particle size increased with decreasing atomiser rotation speed and with increasing melt flowrate. Cups with a high included angle made significantly finer powders than flat discs under the same operating conditions.

The microstructure of spray-formed Ti-6Al-4V/SiCf metal-matrix composites

Spray‐forming is a possible manufacturing route for the fabrication of Ti alloy fibre‐reinforced metal‐matrix composites (MMCs) because high rates of alloy‐droplet cooling on impact with the fibres prevent excessive fibre‐matrix reaction. Ti–6Al–4V matrix MMC monotapes containing TiB2‐coated SiC fibres have been manufactured by electric‐arc spray‐forming, and the key MMC microstructural characteristics in the as‐sprayed monotapes have been investigated by optical and scanning electron microscopy. Fibre infiltration increases with decreasing spraying distance, decreasing atomizing gas pressure and increasing arc current, because of higher temperatures in the Ti alloy spray droplets on impact with the fibres. Too much binder in the fibre preform leads to poor fibre–matrix contact, while removing the binder leads to the fibres becoming misaligned during spraying.

Fabrication of high melting-point porous metals by lost carbonate sintering process via decomposition route

Porous metals with high melting points can be manufactured by the lost carbonate sintering (LCS) process either via the dissolution route or via the decomposition route. In the current paper, porous copper and steel samples with porosity in the range of 50 to 85 per cent and cell size in the range of 50 to 1000 μm have been produced via the decomposition route. The effectiveness of carbonate loss and the characteristics of the decomposition route have been studied. In comparison with the dissolution route, the decomposition route can be applied to a wider range of conditions and often requires shorter times to achieve a complete carbonate removal. The porous metal samples produced by the decomposition route generally have higher tensile strength and higher flexural strength than those produced by the dissolution route.

Analysis of flow development in centrifugal atomization: Part I. Film thickness of a fully spreading melt

Centrifugal atomization of metal melts is a cost-effective process for powder production and spray deposition. The properties of the as-produced powder and deposit are determined primarily by the characteristics of the atomized droplets, which in turn are largely dependent on the flow development of the melt on the atomizer. This paper develops a model for analysing the flow development of a fully spreading melt on and off the atomizing cup. The model can be used to calculate the velocity and film thickness of the melt as a function of melt volume flow rate, cup rotation speed, cup radius and cup slope angle, as well as to predict the trajectory of the spray off the cup. The model implies that the disintegration of a fully spreading melt takes place in the region just off the cup edge and the film thickness at the cup edge is a critical factor determining the sizes of the resultant droplets. The film thickness at the cup edge is shown to decrease with decreasing volume flow rate, with increasing cup rotation speed, with increasing cup radius and with decreasing cup slope angle.

Stochastic Modelling of Removability of NaCl in Sintering and Dissolution Process to Produce Al Foams

The sintering and dissolution process (SDP) is a novel method for manufacturing Al foams. One concern of the process is the presence of residual NaCl in the as manufactured Al foams under certain circumstances, which may have undesirable effects on the properties of the foams. This paper develops a stochastic model to predict the fraction of entrapped NaCl based on the assumptions that the particles of the Al and NaCl powders used in SDP are spherical and monosized and that they are distributed randomly in the preform. The model predicts that the fraction of entrapped NaCl in the foam decreases with increasing volume fraction of NaCl in the preform and with decreasing NaCl-to-Al particle size ratio. The model predictions are in general agreements with the preliminary experimental measurements. The model provides a basis for the selection of Al and NaCl powders in order to minimise the entrapped NaCl in the foam.

Analysis of flow development in centrifugal atomization: Part II. Disintegration of a non-fully spreading melt

Centrifugal atomization of metal melts is a cost-effective process for powder production and spray deposition. The properties of the as-produced powder and deposit are determined primarily by the characteristics of the atomized droplets, which in turn are largely dependent on the flow development of the melt on the atomizer. This paper develops a model for analysing the flow development of a non-fully spreading melt on the atomizing cup. The model shows that the melt can disintegrate prematurely before reaching the edge of the cup when the dynamic contact angle of the melt exceeds a critical contact angle. The critical contact angle is very small for a flat disc but increases markedly with increasing slope angle of a cup. The critical contact angle also increases with increasing melt flow rate and cup rotation speed. The model gives a good insight into the atomization mechanism and explains well the phenomena observed in centrifugal atomization, including the conditions of the occurrence of the three atomization modes and the existence of an optimum melt flow rate, cup radius, cup slope angle and cup rotation speed for achieving small droplet sizes.