稀土元素Gd对Al-Si-Mg铸造合金微观组织和力学性能的影响

为了系统地研究稀土Gd对铸造Al-Si-Mg(A357)合金组织和性能的影响,采用OM,SEM,EPMA,XRD,DSC,TEM及拉伸实验等方法对不同Gd含量A357合金进行研究。结果表明:Gd的添加可以细化A357合金的晶粒并减小二次枝晶间距。此外,Gd可以有效地细化合金中的共晶硅,但是对片状共晶硅的形貌影响不大。晶粒和共晶硅的细化及二次枝晶间距的减小使添加Gd后的A357合金的力学性能有了显著的提高。其中,A357-0.5Gd(质量分数/%)合金热处理态抗拉强度为355MPa,相对于未添加Gd元素的A357合金提高了37MPa。当Gd质量分数为1.0%时,尽管组织得到进一步细化,但是大量粗大Al 2Si 2Gd第二相的形成导致了合金力学性能的下降。同时对Gd的细化机制进行探究,结合TEM分析结果可以推断,Gd变质处理后共晶硅上的孪晶密度并不足以引起共晶硅形貌的转变,使得Gd变质效果较弱。而Gd对共晶硅的细化作用可能与Gd增加成分过冷以及形成纳米相阻碍共晶硅生长有关。     

Preparation of cast aluminium-silica particulate composites

α-quartz particles (average size 53 μm) were dispersed in the matrix of aluminium castings by mechanically stirring these particles in the melts, followed by casting of the melts in suitable permanent moulds. For obtaining good composite castings with more than 2 wt % SiO₂ dispersions, the silica particles should be preheated to 700° C and added while they are hot (>500° C) together with magnesium pieces (3.5 wt %) to the surface of the melts, kept between 720 to 790° C. Microscopy of castings indicated a reaction zone at the outer periphery of the dispersed SiO₂ particles, and the electron microprobe analysis confirmed that the magnesium added to the surface of the melt concentrates around the periphery of the silica particles dispersed in the matrix, apparently imparting some wettability to the particles. The hardness of Al-2.5 wt % SiO₂ particulate composite was found to be four times that of pure aluminium, and abrasion resistance was found to be three times that of pure aluminium under the conditions of the present investigation.     

Factors affecting the damping capacity of cast aluminium-matrix composites

The damping capacity of stir-cast aluminium-matrix composites containing graphite and silicon carbide particles, were studied using a cantilever beam specimen and an HP 5423A Structural Dynamics Analyser. Damping data were determined in the first mode of vibration. Aluminium-matrix composites containing 5–10 vol % graphite particles and 10 vol % silicon carbide particles were prepared by the stir-casting technique and die cast to obtain standard samples (6 mm×25 mm100 mm). Graphite particles were found to be more effective in enhancing the damping capacity of composites compared to silicon carbide particles. The damping capacity of composites increased with the volume percentage of graphite within the range studied. However, no notable improvements in damping capacity were observed by dispersion of silicon carbide in aluminium alloy. The results have been analysed in terms of the effect of size, shape, nature and volume fraction of particles on the damping capacity of the aluminium matrix particulate composites and compared with the damping capacity data available in the literature. The effects of frequency, strain amplitude, temperature and processing on damping capacity of the aluminium matrix composites are reviewed.     

Synthesis of cast metal matrix particulate composites

The present review begins by briefly tracing history in the early days of development of cast metal-matrix composites and also outlines different casting routes for their synthesis. The problems faced by the quality of cast products and their relation to the process variables and characteristics of a given process, constitute the main theme of the review. The development of microstructure has been discussed in view of nucleation behaviour anticipated on the basis of estimated interface energies. The solidification around dispersoids and in regions away from it has been highlighted. Porosity in cast composites (its origin and control in cast components by suitable mould design) has engaged attention because of damage to mechanical properties due to porosity. The chemical reactions at the interface between dispersoid and matrix during processing of certain important systems of composites, have been described and the means of controlling these reactions have been indicated. The review concludes by drawing attention to the potential for application of cast composites in different industrial components and underlines the necessity of research in certain related fields so that industrial application of cast metal-matrix composites will soon become a reality.     

Preparation of cast aluminium alloy-mica particle composites

The optimum conditions for producing cast aluminium alloy-mica particle composites, by stirring mica particles (40 to 120 m) in molten aluminium alloys (above their liquidus temperatures), followed by casting in permanent moulds, are described. Addition of magnesium either as pieces along with mica particles on the surface of the melts or as a previously added alloying element was found to be necessary to disperse appreciable quantities (1.5 to 2 wt.%) of mica particles in the melts and retain them as uniform dispersions in castings under the conditions of present investigation. These castings can be remelted and degassed with nitrogen at least once with the retention of about 80% mica particles in the castings. Electron probe micro-analysis of these cast composites showed that magnesium added to the surface of the melt along with mica has a tendency to segregate around the mica particles, apparently improving the dispersability for mica particles in liquid aluminium alloys.The mechanical properties of the aluminium alloy-mica particle composite decrease with an increase in mica content, however, even at 2.2% the composite has a tensile strength of 14.22 kg mm^–2 with 1.1% elongation, a compression strength of 42.61 kg mm^–2, and an impact strength of 0.30 kgm cm^–2. The properties are adequate for certain bearing applications, and the aluminium-mica composite bearings were found to run under boundary lubrication, semi-dry and dry friction conditions whereas the matrix alloy (without mica) bearings seized or showed stick slip under the same conditions.     

Particle distribution control in cast aluminium alloy-mica composites

A suspension of mica particles (40m diameter and 3.7 thick) obtained in a mechanically stirred Al-4 wt % Cu-1.5 wt % Mg melt was poured and solidified in a variety of moulds under different heat flow configurations. The resulting cast structure showed a non-uniform distribution of dispersed mica particles with mica-depleted and segregated zones due to their flotation before and during solidification. The experimentally observed profiles of mica-free regions deviate significantly from those computed on the basis of Stokes's law and freezing-time computations. In relatively thick castings, segregation of mica could be minimized by using low pouring temperatures and/or side as well as bottom chilling. It was found, however, that thin castings (12.5 mm) could easily be produced with a homogeneous distribution of mica particles.     

Role of magnesium in cast aluminium alloy matrix composites

Wetting between the dispersoid and the matrix alloy is the foremost requirement during the preparation of metal matrix composites (MMC) especially with the casting/liquid metal processing technique. The basic principles involved in improving wetting fall under three categories: (i) increasing the surface energies of the solids, (ii) decreasing the surface tension of the liquid matrix alloy, and (iii) decreasing the solid/liquid interfacial energy at the dispersoid matrix interface. The presence of magnesium, a powerful surfactant as well as a reactive element, in the aluminium alloy matrix seems to fulfil all the above three requirements. The role played by magnesium during the synthesis of aluminium alloy matrix composites with dispersoids such as zircon (ZrSiO₄), zirconia (ZrO₂), titania (TiO₂), silica (SiO₂), graphite, aluminium oxide (Al₂O₃) and silicon carbide (SiC), has been analysed. The important role played by the magnesium during the composite synthesis is the scavenging of the oxygen from the dispersoid surface, thus thinning the gas layer and improving wetting and reaction-aided wetting with the surface of the dispersoid. The combinations of magnesium and aluminium seem to have some synergistic effect on wetting.     

Preparation and properties of cast aluminium-ceramic particle composites

A casting technique for preparing aluminium-alumina, aluminium-illite and aluminium-silicon carbide particle composites has been developed. The method essentially consists of stirring uncoated but suitably heat-treated ceramic particles of sizes varying from 10 to 200 m in molten aluminium alloys (above their liquidus temperature) using the vortex method of dispersion of particles, followed by casting of the composite melts. Recoveries and microscopic distribution of variously pretreated ceramic particles in the castings have been reported. Mechanical properties and wear of these composites have been investigated. Ultimate tensile strength (UTS) and hardness of aluminium increased from 75.50 MN m^–2 and 27 Brinell hardness number (BHN) to 93.15 MN m^–2 and 37 BHN respectively due to additions of 3 wt % alumina particles of 100 m size. As a contrast, the tensile strength of aluminium-11.8 wt % Si alloy decreased from 156.89 MN m^–2 to 122.57 MN m^–2 due to the addition of 3 wt % alumina particles of the same size. Adhesive wear rates of aluminium, aluminium-11.8 wt % Si and aluminium-16 wt % Si alloys decreased from 3.62×10^–8, 1.75×10^–8 and 1.59×10^–8 cm³ cm^–1 to 2.0×10^–8, 0.87×10^–8 and 0.70×10^–8 cm³ cm^–1, respectively, due to the additions of 3 wt % alumina particles.Formerly with the Department of Mechanical Engineering, Indian Institute of Science, Bangalore 560 012, India.     

Ultrasonic velocities in cast aluminium alloy-shell char particle composites

Journal of Materials Science Letters -     

Casting practice and cast metal quality in the UK

Abstract

There is a degree of ambiguity about the term quality when applied to cast products: if can refer to the external engineering form of the casting, covering such parameters as dimensional tolerances and surface finish, or it can indicate some measure of performance expected of the cast material, such as the tensile load capability or fatigue behaviour. Each of these meanings is discussed in this paper in relation to a range of casting processes. The need for higher quality, in each sense, is stressed.

MST/831     

The effect of antimony on the interfaces of cast AlSi-SiC composites

When the matrix alloy of an AlSi7Mg-SiC composite has been modified with antimony, the wetting properties of the system deteriorate. Using transmission electron microscopy and microanalysis it was found that this is probably related to the formation of a film-like amorphous antimony-rich oxide at the matrix-silicon carbide interfaces. Associated with this compound are pores at the interfaces. The film-like compound is generally intermixed with magnesium-aluminium spinel crystallites at the interfaces and with the neighbouring matrix. No antimony-rich phase is found away from the interfaces.     

Limitations of elastic definitions in Al-Si-Mg cast alloys with enhanced plasticity: linear elastic fracture mechanics versus elastic-plastic fracture mechanics

Linear elastic fracture mechanics describes the fracture behavior of materials and components that respond elastically under loading. This approach is valuable and accurate for the continuum analysis of crack growth in brittle and high strength materials; however it introduces increasing inaccuracies for low-strength/high-ductility alloys (particularly low-carbon steels and light metal alloys). In the case of ductile alloys, different degrees of plastic deformation precede and accompany crack initiation and propagation, and a non-linear ductile fracture mechanics approach better characterizes the fatigue and fracture behavior under elastic-plastic conditions.To delineate plasticity effects in upper Region II and Region III of crack growth an analysis comparing linear elastic stress intensity factor ranges (ΔK_el) with crack tip plasticity adjusted linear elastic stress intensity factor ranges (ΔK_pl) is presented. To compute plasticity corrected stress intensity factor ranges (ΔK_pl), a new relationship for plastic zone size determination was developed taking into account effects of plane-strain and plane-stress conditions (“combo plastic zone”). In addition, for the upper part of the fatigue crack growth curve, elastic-plastic (cyclic J based) stress intensity factor ranges (ΔK_J) were computed from load-displacement records and compared to plasticity corrected stress intensity factor ranges (ΔK_pl). A new cyclic J analysis was designed to compute elastic-plastic stress intensity factor ranges (ΔK_J) by determining cumulative plastic damage from load-displacement records captured in load-control (K-control) fatigue crack growth tests. The cyclic J analysis provides the true fatigue crack growth behavior of the material. A methodology to evaluate the lower and upper bound fracture toughness of the material (J_IC and J_max) directly from fatigue crack growth test data (ΔK_FT(JIC) and ΔK_FT(Jmax)) was developed and validated using static fracture toughness test results. The value of ΔK_FT(JIC) (and implicitly J_IC) is determined by comparing the plasticity corrected elastic fatigue crack growth curve with the elastic-plastic fatigue crack growth curve. A most relevant finding is that plasticity adjusted linear elastic stress intensity factor ranges (ΔK_pl) are in remarkably good agreement with cyclic J analysis results (ΔK_J), and provide accurate plasticity corrections up to a ΔK corresponding to J_IC (i.e. ΔK_FT(JIC)). Towards the end of the fatigue crack growth test (above ΔK_FT(JIC)) when plasticity is accompanied by significant tearing, the cyclic J analysis provides a more accurate way to capture the true behavior of the material and determine ΔK_FT(Jmax). A procedure to decouple and partition plasticity and tearing effects on crack growth rates is given.Three cast Al-Si-Mg alloys with different levels of ductility, provided by different Si contents and heat treatments (T61 and T4) are evaluated, and the effects of crack tip plasticity on fatigue crack growth are assessed. Fatigue crack growth tests were conducted at a constant stress ratio, R = 0.1, using compact tension specimens.     

Preparation and properties of cast aluminium alloy-granite particle composites

With a view to develop light weight, low cost and abrasion resistant material cast aluminium alloy composites dispersed with granite particles were prepared and their properties were evaluated. Natural mineral granite was crushed and treated prior to its incorporation in the aluminium alloy. Liquid metallurgy techniques was used to prepare composites involving the following steps: melting of aluminium alloy in graphite crucible, stirring of the melt, addition of granite particles and reactive metal in the melt and pouring the composite melt into permanent moulds. Physical, mechanical, tribological and metallographic properties of composites were studied. It was observed that there was reasonably uniform dispersion of granite particles in the matrix. Hardness and tribological (abrasive wear) properties of the base alloy improved considerably due to addition of the granite particles into it. This clearly indicates that these cast aluminium alloy based composites can be used as wear resistant materials.     

Potential new experiments on fabrication of cast particulate composites in space

Recent developments in fabrication of cast metal ceramic particle composites by liquid metallurgy techniques are outlined. Difficulties encountered in preparing cast composites in the ground environment (including non-uniform distribution and agglomeration of dispersed particles and relatively poor bonding between dispersoids and matrix) and how these can be overcome in a microgravity environment have been discussed. This paper also reviews experiments performed by various space agencies including NASA and ESA on fabrication of composites in space. Some new experiments concerning fabrication of cast composites like dispersion of submicron ceramic particles in molten metals, preparation of cermets with very large volume fractions of ceramic particles and dispersion of flake-type ceramic particles to achieve grain refinement have been proposed.     

Production of cast aluminium-graphite particle composites using a pellet method

Copper- and nickel-coated graphite particles can be successfully introduced into aluminium-base alloy melts as pellets to produce cast aluminium-graphite particle composites. The pellets were made by pressing mixtures of nickel- or copper-coated graphite particles and aluminium powders together at pressures varying between 2 and 20 kg mm^–2. These pellets were dispersed in aluminium alloy melts by plunging and holding them in the melts using a refractory coated mild steel cone, until the pellets disintegrated and the powders were dispersed. The optimum pressure for the preparation of pellets was 2 to 5 kg mm^–2 and the optimum size and percentage of aluminium powder were 400 to 1000m and 35 wt% respectively. Under optimum conditions the recovery of the graphite particles in the castings was as high as 96%, these particles being pushed into the last freezing interdendritic regions. The tensile strength and the hardness of the graphite aluminium alloys made using the pellet method are comparable to those of similar composites made using gas injection or the vortex method. The pellet method however has the advantage of greater reproducibility and flexibility. Dispersion of graphite particles in the matrix of cast aluminium alloys using the pellet method increases their resistance to wear.Formerly with Indian Institute of Science, Bangalore, India.     

Thermal expansion characteristics of cast Cu based metal matrix composites

Like any other metal/alloy, copper and its alloys also soften at elevated temperatures. Reinforcing with ceramic or carbon fibres is one of the suggested solutions to overcome this. Very limited literature is available on Cu based metal matrix composites (MMCs); none of these pertain to liquid phase fabrication. Hence, a systematic investigation was carried out on MMCs based on copper, with alumino-silicate fibres and carbon fibres as reinforcements. The MMCs thus produced exhibit a uniform distribution of reinforcement in the matrix. Coefficient of thermal expansion (CTE) values are lower than that of pure copper.