Solute content and the tensile behavior of high pressure die cast Mg-Al alloys

Binary Mg-Al alloys with varying content of aluminium from 0.5 to 12mass% have been studied. The proof stress increase in two steps whereas the ductility exhibits two correlated stepwise drops, as the aluminium content increases. The first increase in strength, and attendant drop in ductility, is observed between 4 and 5 mass% Al. The second stepwise change is observed between 10 and 12 mass% Al. These effects are connected with well defined changes in the microstructure: at 4 mass% a dispersion of β-phase intermetallic particles appears in the core region and a closed cell structure develops near the surface; at 12 mass% Al, the increased volume fraction of the β- phase intermetallics extends the interconnected network of intermetallics to include the core region as well. The micromechanics of the strengthening and decreased ductility are discussed

Optimal tool angles for equal channel angular extrusion of strain hardening materials by finite element analysis

Equal channel angular extrusion (ECAE) is a novel deformation process capable of imparting a large amount of plastic strain to bulk material through the application of uniform simple shear. ECAE die geometry, material properties and process conditions influence the shear deformation behavior during extrusion that in turn governs the microstructure and mechanical properties of the extruded materials. Finite element analysis, the most appropriate technique was used to analyze the deformation behavior of extruded materials without neglecting important and realistic factors like strain hardening behavior of the material, frictional conditions and speed of the process. In this study the deformation behavior of material, dead zone/corner gap formation and strain homogeneity achieved in the samples during ECAE were studied by using commercial finite element code Abaqus/Explicit. The influence of tool angles, strain hardening behavior of material and friction between the billet and die was considered for simulations. Results showed that the optimal strain homogeneity in the sample with lower dead zone formation, without involving any detrimental effects, can be achieved with channel angle of 90degrees and outer corner angle of 10degrees. (C) 2004 Elsevier B.V. All rights reserved

Equal channel angular extrusion of tubular aluminum alloy specimens—Analysis of extrusion pressures and mechanical properties

The Equal Channel Angular Extrusion (ECAE) process is a promising technique for imparting large plastic deformation to materials without a resultant decrease in cross-sectional area. The die consists of two channels of equal cross section intersecting at an angle; the workpiece is placed in one channel and extruded into the other using a punch. In the present study, the suitability of this technique for processing of tubular specimen geometries has been investigated. Tubular specimens of an aluminum alloy were extruded to three passes through two processing routes using sand as a mandrel. The pressures required for extrusion were measured, and the mechanical properties of the extruded material were evaluated. The low extrusion pressures during ECAE of tubular specimens are due to the movement of the mandrel (sand) along with the specimen (drag friction acts in the same direction as the main punch force). On processing to three passes of ECAE (by inducing a strain of 0.9), the tensile strength, yield strength, and hardness are improved, and elongation to failure (percent) decreased as expected. The process requires low forming loads while ensuring retention of specimen shape. It is also possible to impart further deformation to the specimen using the same die. It is concluded that ECAE is a promising technique for improving properties of tubular specimens

On the development of a pseudo micro-truss intermetallic microstructure in a high pressure die cast AZ91 alloy

The three dimensional features of the intermetallic microstructure that develop across the thickness of a 1 mm thick casting has been studied using SEM and dual beam FIB. The intermetallics form a closely interconnected spatial network which resembles a scaffold or micro-truss structure near the casting surface. The degree of interconnection decreases, and the overall scale of the microstructure is coarser at the core of the casting. The contribution of this pseudo micro-truss structure to the overall strength of the casting is discussed. * © IOP Publishing 201

Section thickness and the skin effect in high pressure die cast Mg-Al Alloys

Microhardness cross sectional maps of cast-to-shape tensile specimens of thickness 1 and 5 mm have been determined for a Mg-I2%AI binary alloy. For the 5 nun thick specimen the hardness was generally higher near the surface and at the comers of the cross-section in comparison with that at the centre. This difference was accounted for by the coarser solidification microstructure and the concentration of porosity at the casting's core. In contrast, no large differences in hardness were found across the section for the I mm thick specimen. The mapping showed a more or less well defined harder surface layer, albeit discontinuous and asymmetrical, for the 5 mm specimen but little differentiation of the skin for the I mm specimen. For both specimens the hardness appeared patchy and non unifonn over the entire cross section. These features of the microhardness maps are largely explained by the non unifonnity of the grain microstructure

Cross-sectional geometry and the intermetallics structure in a high pressure die cast Mg-Al alloy

Specimens of rectangular and circular cross section of a Mg-9Al binary alloy have been tensile tested and the cross section of undeformed specimens examined using scanning electron microscopy. The rectangular cross sections showed three scales in the cellular intermetallics network: coarse at the core, fine at the surface and very fine at the corners, whereas the circular ones showed only two, coarse at the core and fine at the surface. The specimens of rectangular cross section exhibited higher yield strength in comparison to the circular ones. Possible reasons for the observed increased strength of the rectangular sections are discussed

Deformation behavior and strain homogeneity in equal channel angular extrusion/pressing

Equal channel angular extrusion/pressing (ECAE/ECAP) is a novel severe plastic deformation (SPD) technique widely used for fabrication of bulk nanostructured materials, consolidation of powder materials, and property enhancement of tubular materials. The microstructure and mechanical properties of the deformed materials are strongly dependent on the amount of strain induced and strain homogeneity achieved during the ECAE. The knowledge of deformation behavior and strain homogeneity is essential to design a sound ECAE die. Thus, the current study dealt with the influence of channel angle on the deformation behavior and strain homogeneity in ECAE by conducting finite element simulations with Abaqus/Explicit for a range of channel angles along with the consideration of strain hardening of the material and friction. Complicated and smooth deformation behaviors are observed with

3D characterization of intermetallics in a high pressure die cast Mg alloy using focused ion beam tomography

The degree of spatial interconnection of the Mg(17)Al(12) (beta phase) intermetallic in a Mg-9Al-1Zn alloy was assessed through senal sectioning at the centre and near a corner in a casting cross section The three dimensional reconstructions showed that the intermetallics were profusely interconnected forming a scaffold like network over the entire cross section but especially near the casting surface The scale and degree of the interconnection appeared determined by the local concentration of large dendritic grains injected from the shot sleeve The volume fractions of intermetallics obtained through the 3D reconstruction indicated a higher content of p phase at the corner regions in comparison with the core The volume fractions obtained by FIB were consistent with theoretical and experimental values obtained using other techniques (C) 2010 Elsevier Inc All rights reserve