Showing papers on "Aluminium alloy published in 1997"

Influence of critical surface defects and localized competition between anodic dissolution and hydrogen effects during stress corrosion cracking of a 7050 aluminium alloy

Abstract: Aluminium alloys used in the aeronautical industry, and in particular the high strength Al-Zn-Mg(-Cu) alloys of 7XXX series, can be susceptible to stress corrosion cracking (SCC). In the numerous studies reported in the scientific literature on this subject, two basic mechanisms have been proposed to model SCC: anodic dissolution and hydrogen embrittlement. However, there is currently no consensus on the precise mechanism. By coupling discriminating slow strain rate tests and scanning electron microscopy (SEM) examinations, it is shown that: (1) both anodic dissolution and hydrogen embrittlement operate during the SCC process of a 7050 aluminium alloy stressed in a chloride solution (NaCl 3%); (2) the main role of anodic dissolution is to produce critical defects which promote subsequently localized hydrogen discharge, entry and embrittlement; and (3) the relative influence of these two mechanisms depends on the main parameters that govern cracking, i.e. the microstructure, the electrochemical potential and the strain rate.

Oxidation of TiAl based intermetallics

Abstract: The high temperature oxidation behaviour of the binary and ternary alloys of the Ti–48Al system was studied at different temperatures. The primary objectives of this work were the establishment of the activation energies, the migration tendencies of the alloy species, mechanism of oxidation and chemistry of the oxide scales. The ternary additions were Cr (1.5 at 19%), V (2.2 at%), W (0.2 at%) and Mn (1.4 at%). The addition of ternary additions did not play a significant role in the oxidation behaviour at 704°C. At 815°C the alloys with Cr and V exhibited linear oxidation behaviour with large weight gains while the base Ti–48Al alloys exhibited the best behaviour. At 982°C the Mn-containing alloy was the worst, exhibiting a linear oxidation behaviour while the alloy with V and W and the base alloy with 400 p.p.m. oxygen exhibited the best oxidation behaviour. At 982°C the outermost oxide layer in contact with air is always near stoichiometric TiO2. In all the alloys a layer of porosity is created just below the outer TiO2 layer by the Kirkendall mechanism due to the rapid outward diffusion of Ti atoms. The addition of trivalent atoms like Cr in small amounts appear to be detrimental to the oxidation process as they can generate additional oxygen vacancies while the addition of atoms with valence of 5, such as V, and 6, such as W, appear to have beneficial effect on the oxidation behaviour at 982°C by tying up oxygen vacancies.

Microstructure, tensile deformation and fracture behaviour of aluminium alloy 7055

Abstract: The microstructure, tensile deformation and fracture behaviour of aluminium alloy 7055 were studied. Detailed optical and electron microscopy observations were made to analyse the as-received microstructure of the alloy. Detailed transmission electron microscopy observations revealed the principal strengthening precipitates to be the hexagonal disc-shaped η′ phase of size 2 mm×20 mm and fully coherent with the aluminium alloy matrix, the presence of spheroidal dispersoids, equilibrium grain-boundary η precipitates and narrow precipitate-free zones adjacent to grain-boundary regions. It is shown that microstructural characteristics have a profound influence on tensile deformation and fracture behaviour. Tensile test results reveal the alloy to have uniform strength and ductility in the longitudinal and transverse orientations. Strength marginally decreased with an increase in test temperature but with a concomitant improvement in elongation and reduction in area. No change in macroscopic fracture mode was observed with sample orientation. Fracture, on a microscopic scale, was predominantly ductile comprising microvoid nucleation, growth and coalescence. The tensile deformation and fracture process are discussed in the light of the competing influences of intrinsic microstructural effects, matrix deformation characteristics, test temperature and grain-boundary failure.

Influence of particles on recrystallisation textures and microstructures of aluminium alloy 3103

Abstract: The evolution of recrystallisation textures in the commercial aluminium alloy 3103 has been investigated by means of a three-dimensional orientation distribution function (ODF) analysis. The global texture measurements were supplemented by local texture measurements by means of the electron backscattering pattern (EBSP) technique in SEM and optical microscopy, inparticular grain size measurements. The evolution of recrystallisation textures was determined by the competition between particle stimulated nucleation (PSN) and nucleation from cube bands. Precipitated particles were found to have a higher retarding effect on the nucleation from deformation zones around particles than on the nucleation from cube bands. The result of this is a strong cube texture and a large grain size in cases of a strong precipitation reaction. This phenomenon has also been discussed theoretically in terms of a semiquantitative model.

Formation of a crack-free joint between Ti alloyand Al alloy by using a high-power CO2 laser

Abstract: The present paper aims at producing a crack-free weld between a commercially available Ti alloy (Ti-6 wt% Al-4 wt% V) and a wrought Al alloy (Al-1 wt% Mg-0.9 wt% Si). Ti alloy and Al alloy with a plate thickness of 3 mm are butt welded using a 2.5 kW continuous CO2 laser. The laser power, welding speeds and offset of the laser with respect to the joint are considered as the variable parameters. It is observed that intermetallic compounds (mainly TiAl and Ti3Al) are formed in the fusion zone depending on the amount of Al and Ti melted by the laser. These intermetallic phases are very brittle and the solid-state cracks are formed near the Al side of the interface because of the stress developed after the solidification.

Strain Determination in Thermally Grown Alumina Scales Using Fluorescence Spectroscopy

Abstract: By exploiting the strain dependence of the ruby luminescence line, we have measured room-temperature residual strains in thermally-grown alumina scales. Measurements were made on two alloys Fe-5Cr-28Al and Fe-18Cr 10Al (at.% bal. Fe), oxidized between 300–1300°C. Significantly different levels of strain buildup were observed in scales on these alloys. Results on similar alloys containing a dilute reactive element (RE) are also presented. Scales formed on RE-containing alloys (Zr or Hf) could support significantly higher strains at T ≥ 1000°C. Strain relief associated with spallation thresholds is readily observed. In early-stage oxidation, the evolution of transition phases is monitored using Raman and fluorescence spectroscopies. The fluorescence technique also provides a sensitive probe of early-stage formation of α-Al2O3. It appears that, in the presence of Cr2O3 or Fe2O3, the α-phase of Al2O3 can form at anomalously low temperatures.

Improvement of the oxidation resistance of TiAl alloys by using the chlorine effect

Abstract: Most TiAl alloys lack from their insufficient oxidation resistance in air at temperatures above 800°C. Recently it has been found that small amounts of Cl in the alloy may significantly improve the oxidation resistance. However, the mechanisms responsible for this effect were not yet understood. In the paper oxidation experiments with TiAl alloys are reported where Cl was alloyed to the material by the P/M route or applied to the material surface either by ion implantation or by PVD. The results of the investigations revealed that the Cl-effect is due to the formation of volatile metal chloride species. It is shown that chlorine can have a beneficial effect on oxidation resistance in a certain Cl-range which is quantified by thermodynamic calculations. It is assumed that the Cl-effect offers a significant potential for improvement of the oxidation resistance of technical TiAl alloys.

Corrosion Behavior of Rapidly Solidified Magnesium-Aluminum-Zinc Alloys

Abstract: Rapidly solidified magnesium alloys with 8 at%, 15 at%, and 20 at% Al and 1 at% and 3 at% Zn were fabricated by centrifugal atomization followed by hot extrusion. Microstructure of the alloys was composed of a fine-grain magnesium matrix (0.5 μm) with β-Mg17Al12 precipitates. Electrochemical and weight-loss tests were performed in borate and ASTM D 1384 solution (chloride, carbonate, and sulfate). In both media, corrosion current of the alloys decreased with increases in aluminum or zinc content. In borate solution, a passivating plateau was observed from the corrosion potential (Ecorr) to Ecorr + 1,200 mV. Current density decreased with aluminum and zinc concentrations. Electrochemical behavior of the synthesized matrix and precipitates was characterized. Zinc increased Ecorr of the two phases, with a corresponding decrease of corrosion current. The same trend was noticed for aluminum but with a less dramatic effect. The corrosion mechanism was suggested result from galvanic coupling of the matr...

Effects of solutes on grain refinement of selected wrought aluminium alloys

Abstract: An analysis of the effects of alloy additions on grain refinement in a series of model and commercial aluminium alloy compositions is reported. The data of Birch and Fisher published earlier describing grain refinement in 32 systems, including AA3004, AA5083, AA6063, and AA7050, are considered in terms of the supercooling effect of each alloy addition. A simple model describing grain size in terms of additive supercooling effects of individual alloy additions is proposed which fits the data reasonably well. Deviations from simple additive behaviour are evident in systems where strong intermetallic interactions occur. Interaction coefficients among solutes are invoked to explain these deviations. Individual alloying effects on grain refinement are treated in terms of constitutional supercooling parameters, and behaviour is shown to be similar to that described in earlier studies. Zirconium is shown to have a general poisoning effect in a range of alloy compositions.

Mechanical properties of titanium-5083 aluminium alloy friction joints

Abstract: The influence offriction welding and post-weld heat treatment on the tensile strength and metallurgical properties of the dissimilar friction joints between commercially pure titanium and type 5083 aluminium alloy base materials was investigated. The tensile strength of as welded joints was highest when a high friction pressure was applied in conjunction with afriction time of 0.5 s. There was no evidence of inter metallic formation at the bondline in the as welded joints produced using a friction time of 0.5 s. However, a 200 nm thickness intermetallic layer formed in as welded joints produced using a longer friction time (3 s) and in dissimilar joints post-weld heat treated at 450°C for 3.6 ks. It is suggested that the intermetallic layer comprised a mix of τ-Al (Ti2Mg3AI18), Mg2A13, and Al3 Ti phases. The critical intermetallic layer width in Ti-5083 aluminium alloy joints was much less than that found in dissimilar Ti-304L stainless steel and pure Ti-pure Alfriction welds.

High-spend machining of aluminium using diamond endmills

Abstract: Milling experiments on aluminum alloy were carried out using a laboratory-designed diamond endmill. The machining of aluminium alloy using a conventional tool at a conventional cutting speed generally results in short tool-life, poor surface quality and poor edge finishes because of the formation of built-up edges and burrs. The machining technology of aluminum alloy surfaces with a good surface finish and edge finishing is very important and required in many industries. The experimental results reveal the possibility of machining a mirror-like aluminum alloy without built-up edges and burrs using the designed diamond endmill at high speed.

Influence of Fe-bearing particles and nature of electrolyte on the hard anodizing behaviour of AA 7075 extrusion products

Abstract: The hard anodizing behaviour of aluminium alloy 7075 extrusion products has been examined as functions of variation in the impurity (i.e. Si, Mn and Fe) contents in the alloy and the nature of electrolyte used for the anodizing process. A combination of scanning electron microscopy (SEM), transmission electron microscopy (TEM) and electron probe micro analysis (EPMA) revealed that the chemistry, morphology and the mode of distribution of the Fe-rich constituent particles are important in determining the quality and the growth of the hard anodic oxide film. Using the conventional sulphuric acid—oxalic acid electrolyte, it was established that the Al12(FeMn)3Si-based particles survive the anodizing process, and this results in a steep rise in bath voltage with time as well as a non-uniform growth of the anodic oxide film. The extent of these effects are, however, greatly reduced when the Fe-bearing particles are aligned in a direction parallel to the growth direction of the anodic film. Studies employing a stronger electrolyte that is, sulphuric acid—oxalic acid—hydrochloric acid, on the other hand, revealed that this electrolyte is capable of counteracting the deleterious effects of Al12(FeMn)3Si-based particles by dissolving them during anodizing provided such particles are not too large in size.

The effect of mechanical loading on the contact resistance of coated aluminium

Abstract: Measurements of contact resistance, related to resistance spot welding, were made using pre-treated (coated) and abraded aluminium alloy strip. With conventional domed electrodes, the contact resistance was much larger at the sheet-to-sheet (faying) contact than at the electrode-sheet contact. The effect is believed to be associated with the role of sliding in breaking down contact resistance of sheet with an insulating surface film. When the coating was removed the difference between faying surface and electrode-sheet contact was much smaller. Macroscopic shear stresses are developed by electrode-sheet contact whereas no shear stresses are present at the faying surface. The hypothesis is supported by experiments made with asymmetrical electrode pairs which give rise to varying shear stresses in faying surface contact. Some implications for the control of spot welding of different aluminium surfaces are discussed.

An electron metallographic study of pressure die-cast commercial zinc–aluminium-based alloy ZA27

Abstract: The microstructure of ZA27 pressure die-castings was examined by scanning and transmission electron microscopy after ageing for 5 years at ambient temperatures. Solidification began with the formation of compact aluminium-rich α′ dendrites and tiny rounded α′ particles, followed by the peritectic reaction whereby a zinc-rich β phase formed around the edges of the primary phases. The extremely high cooling rate during solidification reduced the extent of the peritectic reaction so that the liquid became highly enriched with zinc and solidification was completed by eutectic formation of β and η phases, the β joining the peritectic β and the η remaining in the interdendritic regions. On rapid cooling after casting through the eutectoid transformation temperature, the β phase decomposed eutectoidally into well-formed lamellae or semi-particulate irregular particles of α and η, and some lamellar colonies spread into the low-aluminium α′-phase cores of the dendrites to form coarse lamellar products. The bulk of the α′, however, decomposed into a very fine mixture of zinc-rich phases in an aluminium matrix. These structures are consistent with solidification under conditions of high undercooling. Enclosed within the α constituent of the decomposed peritectic and eutectic β phases were small particles of a phase which was identified as the transitional α′m phase containing 30.2%Al or 14.8%Al, with an fcc crystal structure and lattice parameter (at 14.8%Al) of about 0.395 nm. It had a symmetrical cube/cube orientation relationship with the surrounding α phase. This metastable phase was probably stabilized by copper. Copper became concentrated in the eutectic liquid during the first stages of solidification, and was rejected from the liquid in the form of discrete irregular particles, 1–2 μm in diameter, during eutectic solidification. After solidification, copper was also rejected from solid solution in the zinc-rich η phase in the form of a dense precipitation of small particles of 70–120 nm diameter and 2–3 nm thick. Both of these particles were identified as the metastable cph e-phase (CuZn4) with lattice parameters a = 0.274 nm, c = 0.429 nm, and c/a = 1.566.

Prediction of the Rotation Capacity of Aluminium Alloy Beams

Abstract: In this paper the theoretical steps of a semi-empirical method for evaluating the rotation capacity of aluminium alloy members subjected to non-uniform bending are outlined. The approach is represented by the extension to aluminium alloy members of the semi-empirical methods proposed for evaluating the rotation capacity of steel members. The moment-curvature relationship of aluminium alloy members can be conveniently represented by means of a Ramberg-Osgood type relation. This allows, with reference to the classical three-point bending test, the derivation in closed form of the curvature diagram. Furthermore, a closed-form integration of the curvature diagram can be performed, providing a relation for evaluating the rotation corresponding to the occurrence of local buckling. The rotation capacity is then computed. The final ring of this link is represented by the experimental evaluation of the non-dimensional stress corresponding to the attainment of the local buckling. The testing needs for this evaluation are outlined, and both the preliminary test results and the planned activity are presented.

Process model for two step age hardening of 7475 aluminium alloy

Abstract: A model for the age hardening response in 7475 aluminium alloy has been developed that describes the effect of two step aging treatments and the effect of deformation on the kinetics of aging. The model describes the essence of the experimental observations on the alloy subjected to deformation before artificial aging, i.e. the occurrence of the peak stress at shorter times, the acceleration of the kinetics of overaging, and the change in the relative yield stresses between the alloy with and without deformation during aging.

Ultimate compressive strength of plate elements in aluminium: Correlation of finite element analyses and tests

Abstract: A study with the objective of assessing the reliability of non-linear finite element analyses in predictions of the ultimate strength of aluminium plates subjected to in-plane compression is presented. Outstand elements of alloy AA6082 in tempers T4 and T6 and internal elements of alloys 5083 M and 6082 TF were analysed for a range of b/t-ratios. For the latter class both non-welded and welded plates were studied, the non-welded plates having two levels of geometric imperfections. The accuracy of the predictions was evaluated by comparison with existing experimental results. It was found that the overall correlation between the experimental and predicted ultimate compressive strengths was good. The finite element analyses reproduced the main effects of slenderness, stress-strain curve (i.e. alloy and temper), geometric imperfections and welding (i.e. residual stresses and heat-affected zones) that were observed in the experiments