Showing papers on "Aluminium alloy published in 1991"

Behavior of metal alloys in the semisolid state

Abstract: During dendritic solidification of castings and ingots, a number of processes take place simultaneously within the semisolid region. These include crystallization, solute redistribution, ripening, interdendritic fluid flow, and solid movement. The dendritic structure which forms is greatly affected by convection during the early stages of solidification. In the limit of vigorous convection and slow cooling, grains become spheroidal. Alloys with this microstructure possess rheological properties in the semisolid state which are quite different from those of dendritic alloys. They behave thixotropically, and viscosity can be varied over a wide range, depending on processing conditions. The metal structure and its rheological properties are retained after solidification and partial remelting. The semisolid alloys can be formed in new ways, broadly termed «semisolid metal (SSM) forming processes». Some of these are now employed commercially to produce metal components and are also used to produce metal-matrix composites

A calorimetric study of precipitation in commercial aluminium alloy 6061

Abstract: No detailed calorimetric study of precipitation in 6××× series aluminium alloys has been reported to date. In this study, transmission electron microscopy (TEM) has been utilized in conjunction with DSC to study precipitate evolution in aluminium alloy 6061

Material effects in fretting wear : application to iron, titanium, and aluminum alloys

Abstract: Fretting wear tests were performed on several alloys (low alloyed and stainless steels, Ti6A14V titanium alloy, 2024 and 7075 aluminum alloys) slid against themselves in air under relatively low stresses for various displacements (±15 to ±50 μm). Friction logs, where tangential force is plotted as a function of displacement and number of cycles, were used to characterize the fretting behavior of the materials. Wear scars and cross sections were characterized by optical and scanning electron microscopy. Depending on the amplitude of displacement, sticking, partial slip, or gross slip occurs at the interface. Gross slip leads to debris formation. Metallic particles are detached from localized, very highly deformed areas whose properties and structures are different from those of the initial material. Sticking is observed on titanium and aluminum alloys tested under the smallest displacement. Samples are only deformed elastically. During partial slip, cracks can initiate and propagate in titanium and aluminum alloys. Millimeters-long cracks are observed on aluminum alloys after 106 cycles. Mechanisms for crack formation and propagation are described in terms of fatigue properties.

A map for wear mechanisms in aluminium alloys

Abstract: A quantitative wear map for aluminium and its alloys has been constructed using normalized test variables and the physical modelling approach proposed by Lim and Ashby for steels. New model equations based on a different state-of-stress criterion suitable for aluminium alloys have been developed and found to match well with reported experimental wear data on aluminium alloys. The field boundaries between various interfacing wear mechanisms were constructed by using critical values of experimental wear data which manifest themselves in discontinuities in the slope of wear curves. However, within a given wear regime, the model equations developed here agreed fairly well with the reported wear data. The wear mechanisms successfully modelled here include oxidation dominated wear, delamination wear, severe plastic deformation wear, and melt wear.

Nitridation reactions of molten Al-(Mg, Si) alloys

Abstract: The isothermal nitridation of magnesium- and silicon-doped aluminium melt at 1273 K was investigated. With increasing Mg/Si ratio and decreasing oxygen content in the nitriding atmosphere, four major reaction mechanisms may be separated: (i) a passivating surface nitridation, (ii) a volume nitridation with precipitation of isolated AlN in the aluminium matrix, (iii) a volume nitridation resulting in a three-dimensionally interconnected AlN/Al composite microstructure, and (iv) a break-away nitridation with complete conversion of aluminium to AlN. The behavioural transition of the nitridation mechanism is reflected by the growth direction and the crystal morphology of AlN which change from inward (mechanisms i, ii) to outward (mechanisms iii, iv) growth of the reaction product with [0 0 0 1] as the dominating growth direction. Attempts are made to define the critical magnesium and silicon contents for the regime of controlled AlN/Al composite growth (mechanism iii) at 1273 K, in order to develop novel AlN/Al composite materials.

Fracture of particles in a particle/metal matrix composite under plastic straining and its effect on the Young's modulus of the composite

Abstract: Fracture of Al 2 O 3 particle in an Al 2 O 3 particle/aluminium alloy metal matrix composite (MMC) under plastic straining was observed by in-situ tensile testing in a scanning electron microscope. The fracture of larger sized particles was found to be preferred to that of smaller sized ones for a given plastic strain. The Young's modulus (E c ) of plastically strained MMC was reduced with increase in plastic strain (e p )

Heat flux transients at the casting/chill interface during solidification of aluminum base alloys

Abstract: Heat flow at the metal/chill interface of bar-type castings of aluminum base alloys was modeled as a function of thermophysical properties of the chill material and its thickness. Experimental setup for casting square bars of Al-13.2 pct Si eutectic and Al-3 pet Cu-4.5 pct Si long freezing range alloys with chill at one end exposed to ambient conditions was fabricated. Experiments were carried out for different metal/chill combinations with and without coatings. The thermal history at nodal locations in the chill obtained during the experiments was used to estimate the interface heat flux by solving a one-dimensional Fourier heat conduction equation inversely. Using the data on transient heat flux q, the heat flow at the casting/chill interface was modeled in two steps: (1) The peak in the heat flux curve qmax was modeled as a power function of the ratio of the chill thickness d to its thermal diffusivity a, and (2) the factor (q/qmax) X α0.05 was also modeled as a power function of the time after the solidification set in. The model was validated for Cu-10 pct Sn -2 pct Zn alloy chill and Al-13.2 pct Si and Al-3 pct Cu-4.5 pct Si as the casting alloys. The heat flux values estimated using the model were used as one of the boundary conditions for solidification simulation of the test casting. The experimental and simulated temperature distributions inside the casting were found to be in good agreement.

Structure and properties of rapidly solidified ultrahigh strength AlZnMgCu alloys produced by spray deposition

Abstract: A high solute, ultrahigh strength 7XXX series aluminium alloy (EURA1 alloy) with solute contents close to the equilibrium solid solubility limits of the AlZnMgCu system has been produced by rapid solidification using a new process of spray deposition (Osprey process) which yields massive preforms directly from the liquid state. The same alloy was also produced following the traditional powder metallurgy (PM) route. The microstructures and resulting mechanical and corrosion properties of the extruded products in the T6 and T7X conditions were evaluated and compared. Under peak-aged conditions only the spray-deposited (SD) products exhibited an acceptable strength-ductility combination, with strengths in excess of 800 MPa and fracture elongations of 4.9%. The EURA1 SD products also exhibited the largest increase in fatigue strength compared to the commercial IM 7075-T6 alloy, while maintaining a comparable crack propagation resistance as well as similar corrosion behaviour. The maximum strength was mainly associated with a very high volume fraction of plate-like η′ precipitates (ranging up to 50 A in diameter), which were identified as having a hexagonal structure with lattice parameters a = 0.489 nm and c = 1.374 nm. Nevertheless, within the T6 microstructure, small spherical, possibly ordered, zones (5–10 A in diameter) were also observed. The EURA1 extrusions exhibited a loss in fracture toughness associated with predominantly intergranular dimpled ruptures. However, the structural and fractographic observations suggested that the properties of the EURA materials could be considerably improved and some preliminary results from the ongoing optimization work are presented.

Mechanical applications of thin and thick diamond films

Abstract: Several applications of thin and thick diamond films have been developed in our laboratory which enable us to exploit the well-known property of diamond, i.e. the highest hardness on earth. These applications include cutting inserts of either a coated (thin film) type or a brazed (thick film) type whose performance has been evaluated and compared with both conventional sintered and single-crystal diamond insert (tool) as well as tungsten carbide insert in cutting very-hard-to-turn materials such as a 18 wt.% Si aluminium alloy or a dispersion-strenthened aluminium alloy including 20 vol.% SiC whiskers. The performance test is extended to a precision turning operation of pure aluminium which requires a very good surface finish. Other applications of diamond film here have been found in antiabrasive tools; one is a dresser used for forming grinding wheels and another is a shearing tool for cutting very abrasive materials such as a polishing tape with a top SiC coating layer. Finally, the development of an indenter mounting a thick diamond film flake shaped into a scratch tester tip has been explained as one example of diamond films for use as various mechanical parts or elements.

The effect of copper, chromium, and zirconium on the microstructure and mechanical properties of Al-Zn-Mg-Cu alloys

Abstract: The present study evaluates the effect of the systematic variation of copper, chromium, and zirconium contents on the microstructure and mechanical properties of a 7000-type aluminum alloy. Fracture toughness and tensile properties are evaluated for each alloy in both the peak aging, T8, and the overaging, T73, conditions. Results show that dimpled rupture essentially characterize the fracture process in these alloys. In the T8 condition, a significant loss of toughness is observed for alloys containing 2.5 pct Cu due to the increase in the quantity of Al-Cu-Mg-rich S-phase particles. An examination of T8 alloys at constant Cu levels shows that Zr-bearing alloys exhibit higher strength and toughness than the Cr-bearing alloys. In the T73 condition, Cr-bearing alloys are inherently tougher than Zr-bearing alloys. A void nucleation and growth mechanism accounts for the loss of toughness in these alloys with increasing copper content.

High resolution electron microscopy of phase decomposition microstructures in aluminium-based alloys

Abstract: High resolution electron microscopy (HREM) recently applied to phase decomposition and microstructure analyses was demonstrated for aluminium-based alloys such as the AlCu, AlMg and AlLi systems. The HREM has provided direct information on local structures on an atomic-scale resolution. In AlCu alloys, typical Guinier-Preston (GP) zones of type GP (1) and GP (2) with a single copper-rich layer and two copper-rich layers separated by three aluminium layers were clearly observed in the HREM images. The HREM images also revealed the coexistence of multiple-layer zones composed of several copper-rich layers. Lattice distortion around a single-layer GP (1) zone was not necessarily symmetric owing to the neighbouring-zone effect. A fine periodic structure similar to that of a GP (2) zone was found in rapidly solidified AlCu alloys. This structure was proposed to be closely connected with the multiple-layer GP zones. In AlMg alloys the supersaturated solid solution decomposed to form an initially modulated structure and subsequently spherical GP zones with the L12-type ordered structure. The periodic lattice distortion was introduced into the modulated structure associated with the magnesium atom fluctuation. Rapidly solidified AlMg alloys suggested the possibility that the homogeneous ordering process takes place. The uniformly ordered structure of the L12 type was more clearly detected in the AlLi alloys in the initial stage of decomposition. Both the interface and the morphology of this structure were quite different from those of the discrete δ′ phase. Thus the initial ordered structure was regarded as the precursor to the δ′ phase. The decomposition process was also discussed on the basis of experimentally obtained microstructures.

Modification of AlSi alloys with sodium

Abstract: Effect of sodium on the solidification process of high purity Al-12 wt% Si alloys and the epitaxial growth of Si phase from (111) Si or (100) Si substrate has been studied by means of thermal analysis, microscopy, X-ray photoelectron spectroscopy and Auger electron spectroscopy. Results show that normal modification with Na has little influence on the eutectic arrest temperature, melting point, latent heat of freeze and heat of fusion of the Al-12 wt% Si alloy, and that Na adsorbs on (111) Si and affects both nucleation and growth of the Si. Kinetic features and mechanism of the modification are discussed and it is suggested that modification of AlSi eutectic with Na is a kinetic problem rather than a thermodynamic one and the transition of growth morphology of eutectic Si from platelike to fibrous results from the effects of Na on the nucleation and growth of both Al and Si phase.

Corrosion behavior of SiCp/6061 Al metal matrix composites

Abstract: The corrosion behavior of silicon carbide/aluminum (SiCp/Al) metal matrix composites was studied in chloride solution by means of electrochemical techniques, scanning electron microscopy (...

The Formation of Metastable Phases by Mechanical Alloying in the Aluminum and Copper System

Abstract: A wide composition range of aluminum-copper alloys has been prepared by mechanical alloying (MA). The microstructure and phase formation have been investigated by X-ray diffraction analysis, scanning electron microscopy, and transmission electron microscopy. The primary grain size of the powders subjected to ball milling for the aluminum-rich compositions is less than 30 nm. The solid solubility of coppe in aluminum can reach 2.7 at. pct Cu and that of aluminum in copper can reach 18.0 at. pct Al. Over a wide composition range, formation of a nonequilibrium distorted body-centered cubic (bcc) phase which possesses some ordering has been confirmed.

Aluminium alloy powders for coating materials, and coating materials containing the alloy powders

Abstract: An aluminum alloy powder for coating materials and a coating material containing the aluminum alloy powder. The aluminum alloy powder comprises an amorphous aluminum alloy consisting essentially of from 83 to 91% of Al, from 0.5 to 5% of Ca and from 8 to 12% of Ni, all in atom %, and comprising a leaf-shaped particle having a thickness of 0.3 to 3 μm, a minor axis of from 10 to 150 μm, a ratio of the minor axis to a major axis of from 1 to 3, and an aspect ratio which is the ratio of the minor axis to the thickness of from 3 to 100, wherein the aluminum alloy powder is contained in an amount of from 5 to 25 parts by weight based on 100 parts by weight of the total weight of (i) the coating material resin component and (ii) aluminum alloy powder, and the coating material resin component is selected from the group consisting of a water-based synthetic latex and a water-soluble resin. The aluminum alloy powder has a superior dispersibility in a resin in a coating material. When added to a coating material and then coated, the aluminum alloy powder can bring about leafing in a desirable state, so that the coating surface can be effectively covered with the powder. The aluminum alloy powder can impart better hiding power and reflecting properties, even when added in a smaller quantity than conventional powders. Since the aluminum alloy powder is comprised of an amorphous alloy, it does not cause deterioration of reflecting properties, even when used in coating materials containing water-soluble solvents.

Fabrication, properties and structure of a high temperature light alloy composite

Abstract: Paniculate alumina reinforced Al-4Cr-1 Fe alloys were fabricated from rapidly solidified aluminium alloy powder and commercially purchased alumina powder by traditional powder metallurgical techniques involving powder mixing and cold compaction followed by hot extrusion. The tensile tests at ambient temperature indicated a considerable improvement in the mechanical strength at the expense of ductility and modulus. Poor values of modulus were explained by the presence of porosity in the composites. The high temperature mechanical properties of the matrix, tested at 350 °C after prolonged exposure to the test temperature under static air conditions, were intrinsically poor. Additions of the filler material, alumina particles, up to a weight fraction of 15% did not improve the high temperature performance of the matrix substantially. Possible causes for this are discussed and alternatives proposed.

Effect of microstructure on the mechanical properties of an Al alloy 6061-SiC particle composite

Abstract: The mechanical properties, microstructures and fractures of a powder metallurgy Al alloy 6061-15vol.% SiC particle composite produced by extrusion were investigated. In order to build a basis for comparison, the composite was rolled at a warm temperature. The rolled composite after extrusion presents a much lower dislocation density than the extruded composite after T6 heat treatment. This cannot be explained by the difference between the coefficients of thermal expansion of aluminium alloy and SiC. In addition, the composite shows a non-uniform dislocation density and precipitate distribution so that matrix strength is non-uniform. A comparison of the properties between the rolled and unrolled composites indicates that the proper distribution of the matrix strength may be important to increase the ductility and toughness of the composite.

Effect of fluoride ions on the corrosion of aluminium in sulphuric acid and zinc electrolyte

Abstract: The effect of fluoride ions on the corrosion of aluminium in sulphuric acid and zinc electrolyte has been investigated through thermodynamic analysis and corrosion experiments. The solution chemistry of aluminium, zinc, and iron in aqueous solution in the absence and in the presence of fluoride ions was studied with the construction of the Eh-pH diagrams for the Al−F−H2O, Zn−F−H2O and Fe−F−H2O systems at 25°C. In the presence of fluoride ions, aluminium can form a series of aluminium-fluoride complexes depending on the fluoride concentration and pH whereas zinc and iron can form soluble or insoluble metal-fluoride complex species only at relatively high fluoride concentration and at higher pH values. Experimental results show that in the presence of fluoride ions, the corrosion of pure aluminium in sulphuric acid is due to uniform dissolution and the reaction rate depends on the fluoride concentration. In zinc electrolyte containing fluoride ions, zinc deposits onto the pure aluminium substrate spontaneously and the amount of deposited zinc also depends on the fluoride concentration. On the other hand, the presence of iron in the Al−Fe alloy accelerates the corrosion of aluminium in H2SO4 and zinc electrolyte significantly and prevents the deposition of zinc on the aluminium surface. The effect of fluoride ions on zinc adherence to the aluminium is also discussed.

Interface and fracture of carbon fibre reinforced Al-7 wt% Si alloy

Abstract: The interface structure in an aluminium-7 wt% silicon alloy reinforced with carbon fibres has been investigated using analytical electron microscopy. Crystals of aluminium carbide (Al4C3) have been identified in interface regions and their structure and growth are discussed. Mechanical properties of the composite have been measured and fracture behaviour studied using acoustic emission analysis in parallel with microstructural examination. The results indicated that the aluminium carbide interfacial reaction had produced a strong fibre matrix bond, but reduced the fibre strength and embrittled the matrix. Consequently, whole fibre bundles failed in a brittle manner in the longitudinal direction with limited pull-out of individual fibres. The findings are discussed in relation to the method used to manufacture the composite.

Interfaces and fracture surfaces in Saffil/Al-Mg-Cu metal-matrix composites

Abstract: The interfacial phases were studied in pressure-cast Saffil/Al-4.5Cu-3Mg composite material using a variety of characterization techniques. The magnesium- and copper-rich phases were found to segregate near the fibre-matrix interfaces. The major phases were identified as MgAl2O4, Al2CuMg and CuAl2. Significant diffusion of silicon from the fibres into the matrix took place during the pressure-casting. A conclusion was drawn that solidification started with the less alloyed aluminium in the bulk of the matrix and proceeded towards the fibres. Auger and XPS analyses of composite materials fracturedin situ showed the fracture surface to lie within magnesium-rich (and not copper-rich) phases, most likely within the MgAl2O4 spinel phase. The fibre surface treatments which are being developed to improve interfacial wetting may also reverse the direction of crystallization and prevent formation of brittle phases in the vicinity of fibres, thereby improving the toughness of the composite materials.

Static and dynamic properties of squeeze-cast A357-SiC particulate Duralcan metal matrix composite

Abstract: In this paper we describe the squeeze casting of silicon carbide particulate (SiC p ) reinforced A357 aluminium alloy matrix composite and consider its structure, heat treatment and mechanical properties. Two composites (15 vol.% and 20 vol.% SiC p ) have been squeeze cast and examined microstructurally. Segregation of the SiC p away from the primary aluminium has been observed, although the fine microstructure obtained from squeeze casting has resulted in a relatively homogeneous microstructure. Filtration during casting effects a significant improvement in both the ultimate tensile strength (UTS) and elongation values of the two composites. A variety of heat treatments have been applied to the 20 vol.% SiC p composite, resulting in the general trend of a higher UTS, giving a less tough and less ductile material. UTS values of up to 416 MPa may be obtained with a Young's modulus of approximately 100 GPa. Elongations of up to 5% have been obtained, but at the expense of UTS and proof strength. Fatigue behaviour of the 20 vol.%SiC p composite has been measured in terms of S - N data for a Wohler fatigue test and crack growth rate measurements. Comparisons are made throughout the work with the matrix material alone; monolithic A357.

Structure, mechanical properties, and stress corrosion behaviour of high strength spray deposited 7000 series aluminium alloy

Abstract: A 7000 series high strength aluminium alloy was produced by spray deposition via the Osprey process. This alloy, designated AlZn11MgCu, exhibits an excellent combination of fracture toughness and strength in both the peak aged T6 and overaged T7 conditions. The yield strength and fracture toughness at room temperature are 700 MN m−2 and about 40 MN m−3/2 in the T6 temper, and 500 MN m−2 and 75 MN m−3/2 in the T7 temper, respectively. The exceptional high fracture toughness is attributed to the absence of coarse constituents and oxides. The microstructure is homogeneous, free of segregation, and has a fine grain size. The hardening precipitates in the T6 temper are mainly η′, and in the T7 temper they are η (MgZn2), resulting in a different fracture behaviour; in the T6 temper the fracture mode is microshearing and in the T7 temper the fracture mode is ductile failure. Both T6 and T7 conditions show a stress corrosion crack growth rate higher than that of comparable alloys, 7075-T651 and 7075-T7351...