E. S. Dwarakadasa

Effect of matrix strength on the mechanical properties of Al-Zn-Mg/SiCp composites

The mechanical properties of Al-Zn-Mg alloy reinforced with SiCP composites prepared by solidification route were studied by altering the matrix strength with different heat treatments. With respect to the control alloy, the composites have shown similar ageing behaviour in terms of microhardness data at 135 degrees C. It was shown that although composites exhibited enhanced modulus values, the strengthening was found to be dependent on the damage that is occurring during straining. Thus the initial matrix strength plays an important role in determining the strengthening. Consequently, compression data had shown a different trend compared to tension

Effect of hydrogen in aluminium and aluminium alloys: A review

Susceptibility of aluminium and its alloys towards hydrogen embrittlement has been well established. Still a lot of confusion exists on the question of transport of hydrogen and its possible role in stress corrosion cracking. This paper reviews some of the fundamental properties of hydrogen in aluminium and its alloys and its effect on mechanical properties. The importance of hydrogen embrittlement over anodic dissolution to explain the stress corrosion cracking mechanism of these alloys is also examined in considerable detail. The various experimental findings concerning the link between hydrogen embrittlement and stress corrosion cracking are also discussed.

Effect of heat treatment environment on Li depletion and on mechanical properties in Al-Li alloy sheets

The loss of lithium during solution heat treatment of various commercial Al-Li alloy sheets as a function of the heat treatment environment is investigated. The thickness of the soft layer formed due to solutionizing at 530‡C for 30 min in laboratory air, argon and salt bath environments is determined by microhardness measurements. Li loss from the surfaces is interpreted with the help of normalized microhardness profiles. Results showed that prior Li-depleted layer does not hinder further loss of Li from the surfaces and the heat treatment environment has relatively little effect on the loss of Li. The loss of tensile strength due to the Li depletion in the peak-aged condition is found to be more pronounced in 1 mm thick sheet than in 1.5 mm or 2 mm thick sheets. Possible reasons for loss of strength in 8090 series alloys after the heat treatment are discussed. Explanations for the observed ductile fracture features in Li-depleted layers is offered.

Effect of hydrogen charging on the mechanical properties of medium strength aluminium alloys 2091 and 2014

Cathodic hydrogen charging in 3·5% NaCl solution altered the mechanical properties of 2091-T351 (Al-Cu-Li-Mg-Zr) determined by a slow (10−3/s) strain rate tensile testing technique. UTS and YS decreased in the case of 2091-T351 and 2014-T6(Al-Cu-Mn-Si-Mg) with increase in charging current density. Elongation showed a decrease with increase in charging current density for both the alloys. However, elongation occurring throughout the gauge length in uncharged specimens changed over to localized deformation, thus increasing the reduction in area in charged specimens. A transition in fracture mode from surface (brittle) to the core (ductile) was observed. The presence of hydrogen increased the hardness, mostly indicative of solution strengthening and it decreased with depth confirming the existence of hydrogen concentration gradient. The effects were similar in 2014-T6, but to a slightly smaller extent.

Effect of corrosive medium on fatigue crack growth behaviour and fracture in high martensite dual phase steel

Fatigue crack growth (FCG) behaviour in both near-threshold and higher stress intensity range (ΔK) in intercritically annealed dual-phase (DP) steel containing martensite between 32% and 76% in ferrite has been studied in 3·5% NaCl solution. It is shown that the amount of martensite content in dual phase steel has a significant effect on threshold (ΔKth) values and FCG rates. Higher content of martensite in ferrite leads to higher threshold values and lower FCG rates. Further, ΔKth is much higher in 3·5% NaCl solution as compared to that in laboratory air. Fractography studies reveal that in the near-threshold region, fracture surfaces are characterized mainly by intergranular cracking in corrosive (3·5% NaCl solution) environment. Higher threshold values in 3·5% NaCl solution is attributed to the higher crack closure induced by rougher fracture surface and by the strong wedge effects of corrosion products.

Influence of hydrogen charging on mechanical properties of gas tungsten arc weldments of aluminium-lithium alloy 8090

The effect of hydrogen charging on the mechanical properties of gas tungsten arc welds (GTAW) of aluminium-lithium alloy 8090 (2 mm thick rolled sheets) was studied using cathodic hydrogen charging. To stimulate an increased amount of hydrogen into welds, the charging current density was increased through a galvanostatic circuit. The deleterious effect of hydrogen on ductility is documented in terms of degradation in tensile ductility (reduction in area and elongation-to-failure). Microscopic analysis was performed to characterize the microstructure and grain morphology of the weldments. Hardness measurements revealed an increase in hardness of the charged welds over the uncharged counterpart. Scanning electron microscopy observations of uncharged welds revealed a mixed mode failure with predominantly ductile rupture. Although, the charged welds exhibited a near similar mode of failure to that of the uncharged welds, extensive planar slip deformation was observed near the outer surface of the uncharged welds. The change in fracture mode from the outer surface to the central portion of the charged welds is attributed to intrinsic differences in hydrogen densities. An attempt has been made to rationalize the role of hydrogen on tensile properties and quasi-static fracture behaviour of the GTAW welds.

Distribution of lead in lead-containing aluminium alloys obtained by liquid phase co-spray forming technique

Liquid phase co-spray forming (LPCSF) technique was employed to produce Al-Pb and Al-Si-Pb alloys to show that it is possible, using this technique, to distribute lead into very fine-sized particles in Al/Al alloy matrix at low melt temperatures. Microstructural studies were carried out to explore the mechanisms governing lead distribution in the matrix of the alloys during processing. Results showed that, regardless of the alloy compositions and experimental conditions, the microstructures of the preforms exhibited great similarity, i.e. less uniform distribution of Pb particles in the base region, and uniform distribution of fine Pb particles in the equiaxed region. During LPCSF process, the behaviour of Pb droplets was similar to that of ceramic particles, except that the shape and size of liquid Pb phase varied corresponding to local solidification condition.

Fracture behaviour of Al-Zn-Mg/SiCp composites

Al-Zn-Mg alloys reinforced with different volume fractions of SiC particulates were prepared by a liquid-metallurgy technique. The mechanical properties in uniaxial tension and compression were evaluated, and fractographic observations were made on the fracture surfaces. The distribution of SiC was quite uniform in the extruded condition, and the mechanical-property data show that the composite properties were inferior to those of the control alloy; they essentially showed a decreasing trend with increasing volume fractions. These observations can be explained in terms of the particle distribution, the porosity and the interfacial characteristics.

Microstructural evolution in liquid metal processed Al-alloy/SiCp composites

Al-Zn-Mg/SiCP composites processed by a liquid metal processing (stir casting) technique have been microstructurally characterised in the as-cast and extruded conditions. Uniform distribution of SiCP is observed with few defects, such as particle clusters, which are due to partial wetting and associated gas porosity. The constituent particles are associated with SiCP although their composition remains unaffected compared with the control alloy. Hot extrusion of the composite using a shear type die showed banding of particles in the extruded direction with 9 vol. % composite. Such defects however, are not predominant in 18% SiCP extruded composites. The presence of Mg2Si is detected at the particle matrix interface as well as in the matrix.

Assessing Formability of Sheet Metals through Advanced Tensile and Laser Speckle Analysis

The ability of a metal to resist strain localisation and hence reduction in local thickness, is a most important forming property upon stretching. The uniform strain represents in this regard a critical factor to describe stretching ability - especially when the material under consideration exhibits negative strain rate sensitivity and dynamic strain ageing (DSA). A newly developed Laser Speckle Technique (LST), e.g. see [1], was used in-situ during tensile testing with two extensometers. The applied technique facilitates quantitative information on the propagating plasticity (i.e. the so-called PLC bands) known to take place during deformation where DSA is active. The band velocity (V-band), and the bandwidth (W-band) were monitored upon increasing accumulated strain. The knowledge obtained with the LST was useful for understanding the underlying mechanisms for the formability limit when DSA and negative strain rate sensitivity operate. The goal was to understand the relationship between PLC/DSA phenomena and the formability limit physically manifested as shear band formation. Two principally different alloys were used to discover alloying effects.