A calibration for surface analysis on aluminium alloys by RF-Glow-Discharge Optical Emission Spectrometry

Glow-Discharge Optical Emission Spectrometry (GD-OES) is a powerful technique for the rapid analysis of elements in a solid surface as a function of depth. DC-GD-OES allows depth profiling on electrically conductive surfaces only, and has proven to be difficult for the analysis of insulating layers, such as oxides. However, the technique of radio-frequency (RF) GD-OES has the advantage of being able to depth profile through multiple layers, both conducting and insulating. In this work, a LECO GDS- 850A spectrometer was calibrated for aluminium, oxygen, and other elements, with the RF source installed. A quantitative depth profile for a sample of tempered aluminium alloy 7475 is presented and compared with earlier work[1,2].

Geometrical effects on residual stresses in 7050-T7451 aluminum alloy rods subject to laser shock peening

Laser shock peening (LSP) is an emerging surface treatment technology for metallic materials, which appears to produce more significant compressive residual stresses than those from the conventional shot peening (SP) for fatigue, corrosion and wear resistance, etc. The finite element method has been applied to simulate the laser shock peening treatment to provide the overall numerical assessment of the characteristic physical processes and transformations. However, the previous researchers mostly focused on metallic specimens with simple geometry, e.g. flat surface. The current work investigates geometrical effects of metallic specimens with curved surface on the residual stress fields produced by LSP process using three-dimensional finite element (3-D FEM) analysis and aluminium alloy rods with a middle scalloped section subject to two-sided laser shock peening. Specimens were numerically studied to determine dynamic and residual stress fields with varying laser parameters and geometrical parameters, e.g. laser power intensity and radius of the middle scalloped section. The results showed that the geometrical effects of the curved target surface greatly influenced residual stress fields.

Geometrical effects on residual stresses in 7050-T7451 aluminum alloy rods subject to laser shock peening

Laser shock peening (LSP) is an emerging surface treatment technology for metallic materials, which appears to produce more significant compressive residual stresses than those from the conventional shot peening (SP) for fatigue, corrosion and wear resistance, etc. The finite element method has been applied to simulate the laser shock peening treatment to provide the overall numerical assessment of the characteristic physical processes and transformations. However, the previous researchers mostly focused on metallic specimens with simple geometry, e.g. flat surface. The current work investigates geometrical effects of metallic specimens with curved surface on the residual stress fields produced by LSP process using three-dimensional finite element (3-D FEM) analysis and aluminium alloy rods with a middle scalloped section subject to two-sided laser shock peening. Specimens were numerically studied to determine dynamic and residual stress fields with varying laser parameters and geometrical parameters, e.g. laser power intensity and radius of the middle scalloped section. The results showed that the geometrical effects of the curved target surface greatly influenced residual stress fields.

Deformation behaviour of an ultra-fine grained Al6082 alloy

This work focuses on the effect of strain rate on the deformation behaviour of an ultrafine grained Al alloy 6082 produced by equal channel angular  pressing. The uniform tensile elongation was found to increase with  decreasing strain rate very substantially. This effect is discussed in terms of the mechanisms that control plastic deformation of the alloy.

A comparison of phosphorus and fluorine containing IL lubricants for steel on aluminium

Ionic liquids have been shown to be highly effective lubricants for a steel on aluminium system. This work shows that the chemistry of the anion and cation are critical in achieving maximum wear protection. The performance of the ILs containing a diphenylphosphate (DPP) anion all showed low wear, as did some of the tris(pentafluoroethyl)trifluorophosphate (FAP) and bis(trifluoromethanesulfonyl)amide (NTf2) anion containing ILs. However, in the case of the FAP and NTf2 based systems, a cation dependence was observed, with relatively poor wear resistance obtained in the case of an imidazolium FAP and two pyrrolidinium NTf2 salts, probably due to tribocorrosion caused by the fluorine reaction with the aluminium substrate. The systems exhibiting poor performance generally had a lower viscosity, which also impacts on their tribological properties. Those ILs that exhibited low wear were shown to have formed protective tribofilms on the aluminium alloy surface.

On the role of magnesium and nitrogen in the infiltration of aluminium by aluminium for rapid prototyping applications

Selective laser sintering has been used to fabricate an aluminium alloy powder preform which is subsequently debound and infiltrated with a second aluminium alloy. This represents a new rapid manufacturing system for aluminium that can be used to fabricate large, intricate parts. The base powder is an alloy such as AA6061. The infiltrant is a binary or higher-order eutectic based on either Al-Cu or At-Si. To ensure that infiltration occurs without loss of dimensional precision, it is important that a rigid skeleton forms prior to infiltration. This can be achieved by the partial transformation of the aluminium to aluminium nitride. In order for this to occur throughout the component, magnesium powder must be added to the alumina support powder which surrounds the part in the furnace. The magnesium scavenges the oxygen and thereby creates a microclimate in which aluminium nitride can form. The replacement of the ionocovalent Al2O3 with the covalent AlN on the surface of the aluminium powders also facilitates wetting and thus spontaneous and complete infiltration. (C) 2004 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.

Morphological features of interfacial intermetallics and interfacial reaction rate in Al-11Si-2.5Cu-(0.15/0.60)Fe cast alloy/die steel couples

Soldering reactions are commonly observed during high pressure die casting of aluminium alloys, and involve the formation and growth of interfacial intermetallics between the die and the cast alloy. It is generally believed that close to 1% Fe is necessary in the aluminium alloy to reduce soldering. However, the role of iron in the interfacial reaction has not been studied in detail. In this investigation, reaction couples were formed between H13 tool steel substrates and an Al-11Si-2.5Cu melt containing either 0.15 or 0.60% Fe. Examination revealed distinctly different intermetallic layer morphology. The overall growth and chemistry of the reaction layer and the reaction rate measured by the consumption of the substrate were compared for the two alloy melts. It was demonstrated that a higher iron content reduces the rate of interfacial reaction, consistent with an observed thicker compact ( solid) intermetallic layer. Hence, the difference in reaction rate can be explained by a significant reduction in the diffusion flux due to a thicker compact layer. Finally, the mechanism of the growth of a thicker compact layer in the higher iron melt is proposed, based on the phase relations and diffusion both within and near the interfacial reaction zone. (C) 2004 Kluwer Academic Publishers.

The properties of advanced alumimium alloy systems

2XXX and 7XXX series aluminium alloys have been the accepted materials for airframe construction for many decades. However, only minor improvements in properties have been possible by the development of these alloys since the early 1970's. The constant need to reduce weight in aircraft has therefore led to a resurgence in the research for higher performance aluminium alloys. The reason for this investigation was to evaluate possible alternatives for the existing conventional aluminium alloy 2014 for aircraft wheel applications. Three new technologies in alloy development were considered: a metal matrix composite, an aluminium-lithium alloy and a powder metallurgical alloy. The basic mechanical properties of these advanced materials have already been established to an extent, but their fatigue behaviour has yet to be fully understood. The purpose of this work was to investigate the fatigue properties of the materials concerned, in both air and an aerated 3.5% NaCl solution, and compare these properties to 2014-T6. As well as the basic mechanical properties, fatigue crack propagation data is presented for all of the materials concerned. Additionally, fatigue crack initiation data is presented for the aluminium-lithium alloy and 2014. The D.C. electrical potential method was used to monitor crack growth. Of the materials investigated, the most promising was the aluminium-lithium alloy. However, short transverse properties need to be increased and the commercial cost of the material needs to be decreased before it can be considered as a direct replacement for 2014 for aircraft structural applications. It was considered that the cost of the powder metallurgical alloy would limit its further use. The metal matrix composite material proved to be unsuitable for most ambient temperature applications

The compressive creep and load relaxation properties of a series of high aluminium zinc-based alloys

A new family of commercial zinc alloys designated as ZA8, ZA12, and ZA27 and high damping capacity alloys including Cosmal and Supercosmal and aluminium alloy LM25 were investigated for compressive creep and load relaxation behaviour under a series of temperatures and stresses. A compressive creep machine was designed to test the sand cast hollow cylindrical test specimens of these alloys. For each compressive creep experiment the variation of creep strain was presented in the form of graphs plotted as percentage of creep strain () versus time in seconds (s). In all cases, the curves showed the same general form of the creep curve, i.e. a primary creep stage, followed by a linear steady-state region (secondary creep). In general, it was observed that alloy ZA8 had the least primary creep among the commercial zinc-based alloys and ZA27 the greatest. The extent of primary creep increased with aluminium content to that of ZA27 then declined to Supercosmal. The overall creep strength of ZA27 was generally less than ZA8 and ZA12 but it showed better creep strength than ZA8 and ZA12 at high temperature and high stress. In high damping capacity alloys, Supercosmal had less primary creep and longer secondary creep regions and also had the lowest minimum creep rate among all the tested alloys. LM25 exhibited almost no creep at maximum temperature and stress used in this research work. Total creep elongation was shown to be well correlated using an empirical equation. Stress exponent and activation energies were calculated and found to be consistent with the creep mechanism of dislocation climb. The primary α and β phases in the as-cast structures decomposed to lamellar phases on cooling, with some particulates at dendrite edges and grain boundaries. Further breakdown into particulate bodies occurred during creep testing, and zinc bands developed at the highest test temperature of 160°C. The results of load relaxation testing showed that initially load loss proceeded rapidly and then deminished gradually with time. Load loss increased with temperature and almost all the curves approximated to a logarithmic decay of preload with time. ZA alloys exhibited almost the same load loss at lower temperature, but at 120°C ZA27 improved its relative performance with the passage of time. High damping capacity alloys and LM25 had much better resistance to load loss than ZA alloys and LM25 was found to be the best against load loss among these alloys. A preliminary equation was derived to correlate the retained load with time and temperature.

A temporary connection for an aluminium space frame

This thesis examines theoretically and experimentally the behaviour of a temporary end plate connection for an aluminium space frame structure, subjected to static loading conditions. Theoretical weld failure criterions are derived from basic fundamentals for both tensile and shear fillet welds. Direct account of weld penetration is taken by incorporating it into a more exact poposed weld model. Theoretical relationships between weld penetration and weld failure loads, failure planes and failure lengths are derived. Also, the variation in strength between tensile and shear fillet welds is shown to be dependent upon the extent of weld penetration achieved/ The proposed tensile weld failure theory is extended to predict the theoretical failure of the welds in the end plate space frame connection. A finite element analysis is conducted to verify the assumptions made for this theory. Experimental hardness and tensile tests are conducted to substantiate the extent and severity of the heat affected zone in aluminium alloy 6082-T6. Simple transverse and longitudinal fillet welded specimens of the same alloy, are tested to failure. These results together with those of other authors are compared to the theoretical predictions made by the proposed weld failure theories and by those made using Kamtekar's and Kato and Morita's failure equations, the -formula and BS 8118. Experimental tests are also conducted on the temporary space frame connection. The maximum stresses and displacements recorded are checked against results obtained from a finite element analysis of the connection. Failure predictions made by the proposed extended weld failure theory, are compared against the experimental results.

Electrodeposition on aluminium and its alloys

Aluminium alloys S1C, NS4, HE9, LM25 and the 'difficult' zinc containing U.S. specification alloy used for automobile bumpers (X-7046), have been successfully electroplated using pretreatments which utilized either conventional immersion, elevated temperature or electrolytic modified alloy zincate (M.A.Z.) deposits. Satisfactory adhesion in excess of 7•5 KN m -I was only achieved on X-7046 using an electrolytic M.A.Z. pretreatment. The limitations of simple zincate solutions were demonstrated. Growth of deposits ~as monitored using a weight loss technique and the morphology of the various deposits studied using scanning electron microscopy. The characteristics of a specific alloy and processing sequence selected had a significant influence on the growth and morphology of the N.A.Z. deposi t. These all affected subsequent adhesion of electrodeposited nickel. The advantages of double-dip sequences were confirmed. Superior adhesion was associated with a uniform, thin, fine grained M.A.Z. deposit which exhibited rapid and complete surface coverage of the aluminium alloy. The presence of this preferred type deposit did not guarantee adhesion because a certain degree of etching was essential. For a satisfactory combination of alloy and M.A.Z. pretreatment, there was a specific optimum film weight per unit area which resulted in maximum adhesion. An ideal film weight of 0•06 :!: 0•01 mg cm-2was determined for S1C. Different film weights were required for the other alloys due to variations in surface topography caused by pretreatment. S1C was the easiest alloy on which to achieve high bond strength. Peel adhesion was not directly related to tensile strength of the alloy. The highest adhesion value was obtained on S1C which had the lowest strength of the alloys studied. The characteristics of the failure surfaces after peeling depended on alloy type, adhesion level and pretreatment employed. Plated aluminium alloys exhibited excellent corrosion resistance when appropriately pretreated. The M.A.Z. layer was not preferentially attacked. There was a threshold value of adhesion below which corrosion performance ~a8 poor. Alloy type, pretreatment and coating system influenced corrosion performance. Microporous chromium gave better corrosion protection than decorative chromium.

Control of soundness and mechanical properties in aluminium-silicon-magnesium alloys

Previously, specifications for mechanical properties of casting alloys were based on separately cast test bars. This practice provided consistently reproducible results; thus, any change in conditions was reflected in changes in the mechanical properties of the test coupons. These test specimens, however, did not necessarily reflect the actual mechanical properties of the castings they were supposed to represent'. Factors such as section thickness and casting configuration affect the solidification rate and soundness of the casting thereby raising or lowering its mechanical properties in comparison with separately cast test specimens. In the work now reported, casting shapes were developed to investigate the variations of section thickness, chemical analysis and heat treatment on the mechanical properties of a high strength Aluminium alloy under varying chilling conditions. In addition, an insight was sought into the behaviour of chills under more practical conditions. Finally, it was demonstrated that additional information could be derived from the radiographs which form an essential part of the quality control of premium quality castings. As a result of the work, it is now possible to select analysis and chilling conditions to optimize the as cast and the heat treated mechanical properties of Aluminum 7% Silicon 0.3% Magnesium alloy.

The effect of frequency and microstructure on corrosion fatigue crack propagation in high strength aluminium alloys

Fatigue crack growth in high strength aluminium alloy 7150 commercial plate material has been studied in both laboratory air and acidified aqueous salt solution. The aggressive aqueous environment enhanced fatigue crack growth rates by up to an order in magnitude compared to laboratory air. The enhancement in fatigue crack growth rate was accompanied by evidence of embrittlement in the crack path, involving both brittle intergranular and transgranular failure modes. Both the enhancement of fatigue crack growth rates and the extent of intergranular growth modes are dependent on cyclic frequency which, along with the absence of a similar frequency effect in a spray-formed version of the material with a significantly different grain structure, supports a mechanism of grain boundary hydrogen diffusion for intergranular corrosion fatigue crack growth. The convergence of corrosion fatigue crack growth rates at high ΔK in both spray-formed and conventional plate materials coincides with the operation of identical transgranular corrosion fatigue modes dependent on strain-controlled hydrogen diffusion ahead of the crack tip. © 1997 Acta Metallurgica Inc.