Showing papers on "Aluminium alloy published in 1998"

Reinforcement coatings and interfaces in aluminium metal matrix composites

Abstract: The interface between the matrix and reinforcement plays a crucial role in determining the properties of metal matrix composites (MMC). Surface treatments and coating of the reinforcement are some of the important techniques by which the interfacial properties can be improved. This review reports the state of art knowledge available on the surface treatments and coating work carried out on reinforcements such as carbon/graphite, silicon carbide (SiC) and alumina (Al2O3) and their effects on the interface, structure and properties of aluminium alloy matrix composites. The metallic coatings improved the wettability of reinforcement but at the same time changed the matrix alloy composition by alloying with the matrix. Ceramic coatings reduce the interfacial reaction by acting as a diffusion barrier between the reinforcement and the matrix. Multilayer coatings have multifunctions, such as wetting agent, diffusion barrier and releaser of thermal residual stress. The roles of reinforcement coating as a means of “in situ hybridising” and “in situ alloying” are described.

Nanocrystal evolution in bulk amorphous Zr57Cu20Al10Ni8Ti5 alloy and its mechanical properties

Abstract: Bulk alloys of composite microstructures of nanocrystals and amorphous phases were produced by annealing the bulk Zr57Cu20Al10Ni8Ti5 amorphous alloy prepared by copper mold casting. The mechanism for the evolution of the nanocrystalline microstructures was discussed with respect to the particular crystallization behaviors of the amorphous alloy. The changes of the microstructures due to the annealing conditions were studied. The mechanical properties of the partially nanocrystallized alloys were studied by compression test at room temperature. The fracture stress and the Young's modulus increase with the crystalline fraction to ≈45% (i.e. corresponding to a heat release of 45% of the total heat of crystallization) and then decrease. The yield stress also increases with the crystalline fraction to ≈45%, but then the deformation shows only an elastic part and no yield point appears with a further increase of the crystalline fraction. The hardness increases linearly with the crystalline fraction. Fracture morphology examination indicates that the fracture mechanism transforms from ductile to brittle nature at a crystalline fraction of about 40–45%.

Liquidus casting of a wrought aluminum alloy 2618 for thixoforming

Abstract: A wrought aluminium alloy 2618 was cast from its liquidus temperature to obtain a fine structure of equiaxed, nondendritic primary grains. In contrast, the structures obtained from casting the alloy from above its liquidus temperature displayed the usual dendritic grains. The primary grains were characterized by image analysis which showed that the average grain size in two dimensions was ∼44 μm and the average circularity factor was ∼18, indicating that the grains were both fine and globular. Some particles were also viewed in three dimensions and this showed mostly round and smooth surfaces although some protrusions could be seen to emerge on some particles. The misorientation angles between adjacent particles were measured by the electron back-scattered diffraction (EBSD) and most were >15°, indicating genuinely separate grains. The grains in the as-cast material remained fine for up to 30 min at 610°C during reheating, although particle coalescence led to eventual coarsening. The liquidus cast billets were thixoformed at 610°C and the microstructures after forming were examined. It is concluded that the liquidus casting is capable of producing thixotropic materials without resorting to stirring, grain refining or severe deformation and reheating.

Microstructural evolution of Al-Fe powder mixtures during high-energy ball milling

Abstract: High-energy ball milling has been performed on FexAl1-x powder mixtures with x=0.75, 0.50, 0.25 and 0.20. X-ray diffraction, Mossbauer spectroscopy and electron microscopy have been used to characterize the samples milled for different times and annealed in a differential scanning calorimeter. It is found that, during milling, there is diffusion of both elements into each other, with a prevalence of iron diffusion into aluminium, at least in the early stages of the process. This behaviour is more pronounced in the aluminium-rich samples. The growth of the Fe(Al) and Al(Fe) solid solutions has been observed for x≥0.5, different from the lower iron concentrations where the Fe(Al) phase has not been detected. The annealing of pre-milled samples favours the formation, depending on the sample composition and on the annealing temperature, of intermetallic phases such as Fe3Al, FeAl, Fe2Al5 and FeAl3. © 1998 Chapman & Hall

Melting and casting of FeAl-based cast alloy

Abstract: The FeAl-based intermetallic alloys are of great interest because of their low density, low raw material cost, and excellent resistance to high-temperature oxidation, sulfidation, carburization, and molten salts. The applications based on these unique properties of FeAl require methods to melt and cast these alloys into complex-shaped castings and centrifugal cast tubes. This paper addresses the melting-related issues and the effect of chemistry on the microstructure and hardness of castings. It is concluded that the use of the Exo-Melt{trademark} process for melting and the proper selection of the aluminum melt stock can result in porosity-free castings. The FeAl alloys can be melted and cast from the virgin and revert stock. A large variation in carbon content of the alloys is possible before the precipitation of graphite flakes occurs. Titanium is a very potent addition to refine the grain size of castings. A range of complex sand castings and two different sizes of centrifugal cast tubes of the alloy have already been cast.

Processing of iron aluminides

Abstract: A processing route comprising of air induction melting (AIM) with protective cover and electroslag remelting (ESR) for production of iron aluminide $(Fe_3Al)$ alloys has been developed. The use of protective cover during AIM results in the minimization of hydrogen gas porosity and a significant reduction in the impurity levels (S, O and N). Attempts have been made to further improve the ductility and hot workability through ESR. The AIM melted with high carbon ingots exhibit excellent tensile properties compared to the low ( 0.074 wt.%) carbon ingots where cracks persist even after ESR. Processing maps were developed for vacuum induction melted as-cast binary and ternary (Cr and Mn) $Fe_3Al$ alloys. Processing maps of $Fe_3Al$ and $Fe_3Al-Cr$ exhibit a dynamic recrystallisation domain at temperatures >1123 K. Dynamic recrystallisation in iron aluminides is controlled by migration of interfaces. In the case of $Fe_3Al-Mn$, dynamic recrystallisation is suppressed due to greater resistance to the migration of interfaces. Instead, large grain superplasticity is observed.

Oxidation and corrosion behaviour of Fe-Cr and Fe-Cr-Al alloys with minor alloying additions

Abstract: The oxidation and corrosion properties of alloys based on Fe–40Cr and Fe–35Cr–5Al were studied using gravimetric and potentiodynamic techniques. The properties were modified by microalloying with ruthenium and rare-earth metals (REM). A high resistance to breakaway oxidation was characteristic of all the high chromium alloys assessed. Good oxidation resistance in the Fe–35Cr–5Al alloys was found to be contingent on (i) the rapid establishment of a stable protective aluminium oxide layer in the early stages, and (ii) the development of a secondary chromium oxide layer for long-term stability. Microalloying with 0.2 wt.% Ru promoted the formation of a chromium-rich layer at the substrate interface. The consequence of this differed in each case. In the Fe–35Cr–5Al alloy, the result was improved oxidation resistance, accompanied by segregation of the Ru to the aluminium oxide layer. In the Fe–40Cr alloy, the Ru addition was associated with an initially higher oxidation rate and an increased tendency to spalling. REMs, added as 0.05 wt.% mischmetal to the Fe–35Cr–5Al alloy, also lowered the oxidation rate, as expected. Relative to the Fe–40Cr composition, the substitution of 5 wt.% Al for 5 wt.% Cr had a negligible effect on the aqueous corrosion resistance in 10% H 2 SO 4 , but led to inferior pitting resistance in a 3.5% NaCl solution. As previously shown with stainless steels based on Fe–40Cr, small additions of ruthenium can also enhance the corrosion and pitting resistance of Fe–35Cr–5Al.

Transient liquid phase bonding of 2124 aluminium metal matrix composite

Abstract: The transient liquid phase (TLP) bonding of particle reinforced aluminium metal matrix composites (MMCs) using copper interlayers often results in the segregation of SiC particles to the bond region, and this has the effect of producing bonds with poor mechanical strengths. In this preliminary study, the TLP bonding of a 2124 aluminium alloy MMC is investigated using nickel interlayers, and the initial results show that good bonds are produced with no effect on the SiC dispersion in the matrix. The absence of segregation is attributed to the high diffusivity of the nickel in the aluminium MMC, which produces rapid isothermal solidification at the bonding temperature. Bond shear tests show that near parent metal strengths are possible when thin nickel interlayers are used, and failure occurs at the MMC/bond interface. When thick interlayers are used, failure is predominately through the centre of the bondline.

Modelling the flow processes of a particle reinforced metal matrix composite during machining

Abstract: This paper reports on a numerical test program to model the micromechanics associated with the machining of a particle-reinforced metal matrix composite (PRMMC) The composite material modelled was a 35% by volume SiC particle-reinforced A356 aluminium alloy. A submodelling approach was adopted in order to analyse the micromechanical problem. Simulation of the metal cutting process was performed using FORGE2, an elasto-visco plastic FEA code. The micromechanical submodelling was performed using ANSYS 5.2, an elastoplastic FEA code. The machining model of the aluminium alloy without the reinforcement and the resulting hydrostatic pressure istribution were used as inputs for the ANSYS micromechanical submodels of the composite. The regions modelled included the primary shear zone, the machined surface, and the chip-tool contact region, in both the sticking and sliding regions along the rake face. All FE models were assumed to be plane strain. The results of the FE submodelling agree favourably with those obtained from machined test pieces when observed under a scanning electron microscope (SEM). Particle clustering has a detrimental effect on the rate of void growth. It was observed that SiC particles under the action of normal loads cause intense normal stresses at the point of contact between the aluminium matrix and the tungsten-carbide particles within the cutting tool. Overstraining of the matrix envelopes the SiC particle in contact with the cutting tool. A coarse grade of diamond tool had a superior wear resistance compared to a fine grade.

Mechanically reinforced and corrosion-resistant sputtered amorphous aluminium alloy coatings

Abstract: Aluminium alloy coatings are potentially interesting candidates for the cathodic protection of construction steels. This paper reviews the main features of Al−TM−(N) metallurgical coatings (TM: transition metal = Cr, Ti) deposited by reactive magnetron sputtering of composite Al−TM targets in Ar−N 2 reactive gas mixtures. In particular, both binary and ternary amorphous coatings always present the best compromise between mechanical properties and corrosion resistance. Generally, they present favourable intrinsic compressive stress associated with good compactness, a microhardness around 6000 MPa, a Young modulus of at least 110 GPa, better ductility and wear resistance than crystallized alloys, while maintaining a corrosion potential around that of iron, and an excellent pitting resistance in a chlorine environment. The thermal stability of such coatings is guaranteed for applications up to 300 °C. Their crystallization at higher temperature yields stable (Al 4 Cr) or metastable (disordered AlTi) phases.

Dispersion-strengthened aluminium alloy

Abstract: A dispersion-strengthened material is described which comprises aluminium or aluminium alloy containing a substantially uniform dispersion of ceramic particles to confer dispersion strengthening which is inherently stable at high working temperatures, the ceramic particles having a diameter of less than 400 nm, and preferably in the range 10 nm to 100 nm. Suitable ceramic dispersoids include Al2O3, TiO2, Al3C4, ZrO2, Si3N4, SiC, SiO2.

Corrosion Inhibition of Rapidly Solidified Mg-3% Zn-15% Al Magnesium Alloy with Sodium Carboxylates

Abstract: The ability of sodium linear-saturated carboxylates to protect magnesium alloys against aqueous corrosion was characterized. Electrochemical measurements of polarization resistance (Rp) an...

Chill zone formation in Al–Li alloy welds

Abstract: The presence of a fine equiaxed zone (also known as the 'chill zone') along the fusion boundaries in Al–Li alloy welds has recently been reported. Preferential failure along the chill zone during tensile testing has also been observed. Such zones have not been reported in other aluminium alloy welds. The chill zone was considered to form as a result of high undercooling during solidification of Al–Li alloys. In another study, Al3(Li,Zr) aided nucleation was proposed to be the mechanism of nucleation of the chill grains. The present work demonstrates that the chill zone forms only under conditions when the base alloy contains Al3Zr particles before welding. The presence of Al3Zr particles within the chill grains has been shown for the first time. The grain structure of the base metal does not have any role in the formation of the chill zone. A chill zone is also observed in a non-Li bearing Al alloy which contains Zr. It is further shown that in pulsed current and oscillating arc welds, the chill ...

Analysis of high temperature flow stress of titanium alloys IMI 550 and Ti-10V-2Fe-3AI during isothermal forging

Abstract: High temperature flow stress data obtained from laboratory scale isothermal forging tests on two titanium alloys, IMI550 (Ti-4Al-4Mo-2Sn-0.5Si) and Ti-10V-2Fe-3Al, have been correlated using a gene...

Effects of various homogenisation treatments on the hot workability of ingot aluminium alloy AA2014

Abstract: AA2014 aluminium alloy has high strength but very low ductility, hence low workability. In order to improve the hot workability of this ingot aluminium alloy, different homogenisation conditions have been tested, such as 400, 440, 480, 500 and 510°C for 4, 8, 16 and 24 h, followed by various cooling and strain rates. The effect of homogenisation temperature, time and cooling rate on the workability has been investigated. Hot tensile and torsion tests have been carried out to show the harmony between each other and determine hot workability behaviour. From the experimental work, the data have been converted into equivalent true stress–strain curves. It was observed that the optimum hot ductility for the alloy occurs at the strain rate of 1×10−3 s−1 for the specimens that were homogenised at 500°C for 24 h, then cooled in the furnace at a rate of 12 K h−1.

In situ synthesis of porous ceramics with a framework structure of aluminium borate whisker

Abstract: New porous ceramics with a framework structure of aluminium borate (9Al2O3 · 2B2O3) whiskers, in which the whiskers are distributed uniformly and randomly, can be synthesized in situ by firing of a green powder compact of a mixture of aluminium hydroxide, boric acid and an additive of nickel oxide above 1100°C. During firing, the whiskers of aluminium borate grow in situ in the compact, and are bonded together by sintering. The porous aluminium borate consists solely of whiskers, has a porosity of 85%–50%, which corresponds to a volume fraction of whiskers of 15%–50%, and a flexural strength of 2.2–56.1 MPa. Because the whiskers are strongly bonded to other whisker(s), the problem of whiskers scattering, that can be an inhalation hazard, is solved. The aluminium alloy matrix composite using this porous aluminium borate as reinforcement was fabricated by the squeeze-casting method. The tensile strength of the composite material with a whisker volume fraction of 20% can be improved by up to about 90% compared with the unreinforced matrix alloy at 350°C. © 1998 Chapman & Hall

Study of through-thickness residual stress by numerical and experimental techniques

Abstract: The quenching process of aluminium alloys is modelled using the finite element method. The study of residual stress field induced by quenching is divided into two: the thermal and mechanica...