Showing papers on "Aluminium alloy published in 1999"

Low-temperature superplasticity in nanostructured nickel and metal alloys

Abstract: Superplasticity — the ability of a material to sustain large plastic deformation — has been demonstrated in a number of metallic, intermetallic and ceramic systems. Conditions considered necessary for superplasticity1 are a stable fine-grained microstructure and a temperature higher than 0.5 T m (where T m is the melting point of the matrix). Superplastic behaviour is of industrial interest, as it forms the basis of a fabrication method that canbeused to produce components having complex shapes from materials that are hard to machine, such as metal matrix composites and intermetallics. Use of superplastic forming may become even more widespread if lower deformation temperatures can be attained. Here we present observations of low-temperature superplasticity in nanocrystalline nickel, a nanocrystalline aluminium alloy (1420-Al), and nanocrystalline nickel aluminide (Ni3Al). The nanocrystalline nickel was found to be superplastic ata temperature 470 °C below that previously attained2: this corresponds to 0.36T m, the lowest normalized superplastic temperature reported for any crystalline material. The nanocrystalline Ni3Al was found to be superplastic at a temperature 450 °C below the superplastic temperature in the microcrystalline regime3.

Thickness effects on the mechanical properties of micro-arc discharge oxide coatings on aluminium alloys

Abstract: Weight-saving materials are becoming increasingly important, especially in the automotive and aerospace industries. Design engineers would thus like to make more extensive use of light metals such as aluminium, titanium, magnesium and their alloys; however, these materials tend to have poor wear resistance. Previous treatments and coatings applied to aluminium alloys, for example by traditional processes such as hard anodising and thermal spraying, have suffered from the low load support from the underlying material and/or insufficient adhesion, which reduces their durability. Also, although TiN-, CrN- or DLC-coated aluminium alloys (using various PVD methods) can achieve a high surface hardness, in practice they often exhibit poor performance under mechanical loading, since the coatings are usually too thin to protect the substrate from the contact conditions. In the work reported here, a plasma electrolysis technique known as micro-arc discharge oxidation (MDO) was investigated; thick and hard oxide ceramic layers were fabricated on BS Al-6082 aluminium alloy by this method. The phase composition and microstructure of the MDO coatings were investigated by XRD, SEM and EDX analyses. A number of adhesion and tribological sliding and impact wear tests were also performed. It was found that Al–Si–O coatings with a hardness of up to 2400 HV and with excellent wear resistance and load support could be formed. The thickness of the coatings significantly influenced the mechanical properties. In terms of tribological performance, the thicker coatings performed best in sliding, scratch and impact tests whilst thin coatings were also surprisingly effective in both impact and low-load sliding. Coatings of intermediate thickness provided relatively poor performance in all tribological tests.

Characterisation of electron beam welded aluminium alloys

Abstract: Electron beam (EB) welding was performed on three different aluminium alloys, namely alloys 2024, 5005, and 6061 (plate thickness 5 mm except alloy 5005 which was 3 mm in thickness), to establish the local microstructure–property relationships that would satisfy the service requirements for an electron beam welded aluminium alloy component with weld zone strength undermatching. Microstructural characterisation of the weld metals was carried out by optical and scanning electron microscopy. A very low level of porosity was observed in all EB welds owing to surface cleaning before welding and the vacuum environment of the EB welding process. Extensive microhardness measurements were also conducted in the weld regions of the joints. Global tensile properties and fracture toughness properties (in terms of crack tip opening displacement, CTOD) of the EB joints were determined at room temperature. The effects of strength mismatch and local microstructure on fracture toughness of the EB joints are discuss...

Friction welding process of 5052 aluminium alloy to 304 stainless steel

Abstract: Type 5052 aluminium alloy was joined to type 304 austenitic stainless steel via a continuous drive friction welding process. The joint strength increased, and then decreased after reaching a maximum value, with increasing friction time. Joint strength depended on the size and shape of the tensile testpiece. Friction weldability could be estimated by electrical resistmetry. The process of friction welding between the aluminium alloy and the stainless steel is proposed to evolve as follows: welding progresses from the outer to the inner region; an unbonded region is retained at the centre of the weld interface with shorter friction time; longer friction time causes the formation of an intermetallic reaction layer at the weld interface; and the reaction layer grows as the friction time increases. When the thickness of the reaction layer increased above a critical value, the joint was brittle and fractured at the weld interface. The joint was sound when there was no unbonded region and a thin reaction...

After Concorde: evaluation of creep resistant Al–Cu–Mg–Ag alloys

Abstract: A European consortium is evaluating materials for the construction of a new supersonic transport aircraft that may replace Concorde. Current designs propose to use an aluminium alloy for the fusela...

The Effect of Water Vapor on the Oxidation of Alloys that Develop Alumina Scales for Protection

Abstract: Cyclic oxidation tests have been performed at 1100 °C in wet and dry air on a number of alloys and coatings that form α-Al2O3 scales upon exposure to oxidizing conditions. The alloys that were investigated included PWA 1480, PWA 1484, CMSX 4, diffusion aluminide coatings on PWA 1480 and PWA 1484, and Co-24Cr-10.5Al-0.3Y. In cases where some cracking and spalling of the alumina scales occurred in dry air, the presence of water vapor caused the degradation rate to be increased by a factor of 2. When no cracking or spalling of the alumina occurred in dry air, as was the case for low sulfur alloys, water vapor had no effect on the oxidation behavior. It is proposed that water vapor causes stress corrosion cracking at the Al2O3-alloy interface during cyclic oxidation.

A microtexture based analysis of the surface roughening behaviour of an aluminium alloy during tensile deformation

Abstract: The development of surface roughening during uniaxial tensile deformation, with particular attention to the development of parallel ridges and valleys (ribbed profile), has been investigated for a commercial aluminium alloy. A comparison has been made between two samples which exhibit different propensities for roughening and a detailed microstructural characterisation of these samples has been performed in order to explain the differences. Microstructural characterisation techniques included polarised light microscopy, bulk texture and microtexture. In addition, yield properties were calculated from the microtexture data using polycrystal plasticity analysis. The results show that the differences in the roughening behaviour can be attributed to spatial differences in texture at the specimen surface whereby R-component colonies in a cube matrix produce differential straining, which eventually leads to strain localisation through the specimen thickness to produce a ribbed profile.

Effect of scandium and titanium–boron on grain refinement and hot cracking of aluminium alloy 7108

Abstract: The effect of scandium and titanium–boron (Tibor) additions on the solidification behaviour of castings and welds of aluminium alloy 7108 has been investigated. A circular patch test was adopted to evaluate the effects of these elements on the hot cracking suscepti bility of welds made on cast coupons treated with different grain refiner additions. It was observed that grain size, as well as cracking susceptibility, decreased with increasing amounts of scandium and that hot cracking was completely eliminated at scandium additions above 0·25 wt-%. A more pronounced grain refining effect in welds was observed with Tibor and, in addition, no hot cracking was observed with Tibor additions as low as 0·02 wt-%Ti (0·004 wt-%B). Castings, however, were more effectively grain refined with scandium, achieving a finer grain size than with Tibor.

Effect of various parameters on the surface roughness of an aluminium alloy burnished with a spherical surfaced polycrystalline diamond tool

Abstract: The effect of various parameters on the surface roughness of an aluminium alloy burnished with a spherical surfaced polycrystalline diamond tool are studied experimentally with a theoretical analysis. Problems in selecting the optimum burnishing parameters and some burnishing mechanisms are discussed. With suitable parameters employed, the new no-chip finishing process developed can eliminate or reduce the cutting marks left on the workpiece surface by diamond cutting tools, with its surface roughness reduced to Ra=0.026 μm from the original 0.5 μm.

Fatigue performance of a cast aluminium alloy Al-7Si-Mg with surface defects

Abstract: Fatigue performance has been studied in a cast Al-7Si-Mg alloy with regard to various casting defects, particularly surface or subsurface defects Fatigue tests were carried out under four-point bending to investigate the stress-life relationship and fatigue fracture characteristics in order to understand crack initiation and growth behaviour in conjunction with the examination of surface roughness and porosity Surface hollows were found to control crack initiation of as-cast specimens The fracture surfaces of polished specimens revealed that surface or subsurface shrinkage pores replaced the hollows to act as crack initiators when the rough surface was removed The importance of oxide films in crack initiation was also demonstrated The effects of all these casting defects on fatigue life are discussed

Time-dependent rheological properties of semi-solid metal alloys

Abstract: Semi-solid metal alloys, as used in thixoforming, have a special microstructure of globular grains suspended in a liquid metal matrix. The complex rheological properties are strongly influenced by the local solid fraction, particle shape, particle size and state of agglomeration. There is a high demand for models and software tools allowing the simulation of semi-solid casting processes. The material under investigation is a tin-lead alloy (Sn-15%Pb) which exhibits a similar microstructure to aluminium alloys. The experiments were performed with a concentric cylinder rheometer of the Searle type. Initially, the liquid alloy is cooled down to the semi-solid range under constant shearing and then kept under isothermal conditions for further experimentation. Based on the experimental data, a single-phase model has been derived where the semi-solid alloy is regarded as a homogeneous material with thixotropic properties and the microstructure is characterised by a structural parameter. The model consists of two parts: the equation of state, including a finite yield stress, and a rate equation for the structural parameter. The model equations are employed in numerical software and used for the simulation of characteristic filling cases and the comparison with the conventional filling.

On the use of trace additions of Sn to enhance sintered 2xxx series Al powder alloys

Abstract: The mechanical properties of a typical sintered aluminium alloy (Al-4.4Cu-0.8Si-0.5Mg) have been improved by the simultaneous use of trace additions of Sn, high sintering temperatures and modified heat treatments. Tin increases densification, but the Sn concentration is limited to less than or equal to 0.1wt% because incipient melting occurs during solution treatment at higher Sn levels. A sintering temperature of 620 degrees C increases the liquid volume over that formed at the conventional 590 degrees C sintering temperature. However, the higher sintering temperature results in the formation of an embrittling phase which can be eliminated if solution treatment is incorporated into the sintering cycle (a modified TS heat treatment). These conditions produce a tensile strength of 375 MPa, an increase of nearly 20% over the unmodified alloy. (C) 1999 Elsevier Science S.A. All rights reserved.

The effect of strontium on the microstructure of the aluminium-silicon and aluminium-copper eutectics in the 319 aluminium alloy

Abstract: The influence of strontium on the silicon modification and aluminium-copper eutectic microstructures in the 319 aluminium alloy was examined. The strontium levels considered were between 8 and 96 p...

The Morphology of Thermally Grown α-Al2O3 Scales on Fe-Cr-Al Alloys

Abstract: The morphology and growth kinetics of protectiveα-Al2O3 scales formed duringhigh-temperature oxidation of the Fe-Cr-Al andFe-Cr-Al-Y alloys were studied. Scanning electronmicroscopy observations were focused on the oxidewrinkling phenomenonand formation of interfacial cavities in the course ofoxidation of the yttrium-free alloy. In this paper,emphasis is also placed on the effects of surface preparation and yttrium additions on the scaleand scalealloy interface structures. Several features ofalumina morphology, which have not received sufficientattention previously, are discussed in the context of existing models; some criticalissues are outlined.

Ultimate strength of aluminium alloy outstands in compression: experiments and simplified analysis

Abstract: An experimental investigation was undertaken to study the stability and ultimate strength of aluminium alloy outstands. In the tests, thin-walled cruciform aluminium extrusions were stressed into the post-buckling range in axial compression. The local buckling behaviour of the flanges of a cruciform section is the same as that of an outstand, i.e. a plate element which is free along one unloaded edge and simply supported along the other. Two tempers of alloy AA6082 with significantly different stress–strain curves were tested for five b / t ratios. The experimental ultimate strengths were compared with theoretical predictions, using the classical Stowell theory for plastic buckling in combination with the effective width concept, and with ultimate strength formulae recommended in the literature.

Composite aluminium panel

Abstract: A composite aluminium panel comprising two parallel plates and/or sheets secured to the peaks and troughs of a corrugated aluminium stiffener sheet between the parallel plates and/or sheets, wherein the corrugated aluminium stiffener sheet is made from an aluminium alloy rolled sheet of composition (in weight percent): Mg 1.5-6.0, Mn 0.3-1.4, Zn 0.4-5.0, Fe up to 0.5, Si up to 0.5, Zr up to 0.30, optionally, one or more of (Cr 0.05-0.30, Ti 0.01-0.20, V 0.05-0.25, Ag 0.05-0.40, Cu up to 0.40), others up to 0.05 each, 0.15 total, Al balance, and having in an H-condition or in the O-condition a ratio of PS/UTS in the range of 0.4 to 0.9 and having good roll formability.

Internal Nitridation of Nickel-Base Alloys. Part I. Behavior of Binary and Ternary Alloys of the Ni-Cr-Al-Ti System

Abstract: The internal-nitriding behavior of several modelalloys of the Ni-Cr-Al-Ti system in an oxygen-freenitrogen atmosphere at 800-1100°C was studied.Thermogravimetry as well as various metallographic techniques (SEM and TEM) were used. It wasshown that both the nitrogen solubility and the nitrogendiffusion coefficient are strongly affected by the Crcontent of the Ni alloy. Hence, in Ni-Cr-Ti alloys a higher chromium content leads to an increaseddepth of the internal precipitation of TiN. Nitridationof the alloying element Cr takes place only at highconcentrations of Cr. In general, the nitridation rate was found to obey Wagner's parabolic ratelaw of internal oxidation. Changes in the parabolic rateconstant with alloy composition can be understood bymeans of thermodynamic calculations in combination with microstructural observations.

Method for the manufacturing of an aluminium-magnesium-lithium alloy product

Abstract: Method for manufacturing of an aluminium-magnesium-lithium product, comprising the steps of subsequently: (a) providing an aluminium alloy consisting of (in weight %): Mg 3.0-6.0, Li 0.4-3.0, Zn up to 2.0, Mn up to 1.0, Ag up to 0.5, Fe up to 0.3, Si to 0.3, Cu up to 0.3, 0.02-0.5 selected from the group consisting of (Sc 0.010-0.40, Hf 0.010-0.25, Ti 0.010-0.25, V 0.010-0.30, Nd 0.010-0.20, Zr 0.020-0.25, Cr 0.020-0.25, Y 0.005-0.20, Be 0.0002-0.10), balance consisting essentially of aluminium and incidental elements and impurities; (b) casting the aluminium alloy into an ingot; (c) preheating the ingot; (d) hot rolling the preheated ingot to a hot worked intermediate product; (e) cold rolling the hot worked intermediate product to a rolled product in both the length and in the width direction with a total cold rolling reduction of at least 15 %; (f) solution heat treating the cold rolled product in the temperature range of 465 to 565°C for a soaking time in the range of 0.15 to 8 hours; (g) cooling the solution heat treated product from the solution heat treatment temperature to below 150°C with a cooling rate of at least 0.2°C/sec; (h) ageing the cooled product to provide a sheet or thin plate product having a minimum yield strength of 260 MPa or more and a minimum tensile strength of 400 MPa or more in at least the L- and LT-direction, a minimum yield strength of 230 MPa or more and a minimum tensile strength of 380 MPa or more in the 45° to the L-direction, and further having a minimum T-L fracture toughness KCO of 80 MPa.√ or more for 400 mm wide CCT-panels.

Diffusion bonding of investment cast γ-TiAl

Abstract: Intensive alloy development studies on intermetallic gamma (γ) based alloys in recent years has led to the development of several aerospace engine components using advanced γ-TiAl based alloys by ingot and powder metallurgical routes. These materials are of great interest to the aerospace industry owing to their very low density and good high temperature properties. Further application of this material will require the development of successful joining and cost effective fabrication methods. Joining of this intermetallic alloy by fusion joining processes, however, requires very careful process controlling, i.e. low cooling rates and very high preheat temperatures. On the other hand, solid state joining processes, particularly diffusion bonding, brazing, and friction welding, can readily be used to join this material. In the present work, successful application of solid state diffusion bonding to weld investment cast γ-TiAl alloys has been demonstrated. A series of diffusion bonds were produced without using an interlayer at temperatures ranging from 950 to 1100°C with different pressure levels and holding times. Bonds have been characterized using optical and scanning electron microscopy. Defect-free bonds were achieved for all the conditions studied. The bond qualities were assessed by shear testing at room temperature. Reasonable shear strength levels were obtained by bonding at 1000 and 1100°C for 3 h at pressures of 20 and 40 MPa, respectively. The bonds were also post bond heat treated at 1430°C for 30 min, which improved the bond quality in all cases.