A.V. Nagasekhar

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.

Finite element study of multipass equal channel angular extrusion/pressing

Equal channel angular extrusion/pressing multipass simulations were carried for two routes, Route A and Route C, by using finite element code Abaqus/Explicit. Realistic parameters like strain hardening behavior of material, friction between the sample and die were considered for simulations. The strain homogeneity and deformation behavior of samples during multipass ECAE with different routes were studied. The deformation behavior of the sample processed through Route A is smooth. Accordingly strain homogeneity of the samples was more of a possibility with Route A than with Route C.

Role of Solute Content on the Intermetallic Structure Development in HPDC Mg-Al Binary Alloys

Scanning electron microscopy has been used to characterize the intermetallic structure development across the tensile cross-section of binary Mg-Al alloys with solute content between ~0.5 and 12 mass%Al. The alloys which contain less than 1 mass%Al exhibited a single phase grain structure. For compositions greater than 1 mass% Al, an eutectic network with a discontinuous distribution of intermetallics across the cross-section became apparent. In alloys with greater than 8.77 mass%Al, the intermetallics form a continuous network over the entire cross-section. The scale of the intermetallics network is finer at the surface and corner regions of the cross section in comparison with the core regions.

Plastic deformation analysis of accumulative back extrusion

Abstract Finite element analysis for the recently developed plastic deformation process, accumulative back extrusion, is carried out to understand the plastic deformation behavior during the processing in terms of geometry, equivalent strain, principal stress, and load history. The simulated results show that the deformation is highly nonuniform, and that it is almost impossible to achieve strain homogeneity by repeating the accumulative back extrusion process. In addition, the folding defects generated in the workpiece with cycles of accumulative back extrusion can cause surface fracture.

Low-strain plasticity in a high pressure die cast Mg–Al alloy

The Kocks?Mecking method was used to compare the strain-hardening behavior at low strains of high pressure die cast Mg-9?mass% Al alloy and gravity cast fine grained pure Mg specimens. The alloy specimens exhibited a rounded flow curve in contrast with the pure metals for which macroscopic yielding occurred at a well-defined stress. Microhardness mapping of the cross-section of an alloy specimen showed a surface layer, or skin, with hardness values ?20 HV above those of the centre or core region. On the assumption that the core strain hardens at the same rate as the pure Mg specimen, it was estimated that ?20% of the alloy specimens cross-section was still elastic when the core reached full plasticity. The micromechanics of the elasto?plastic transition in the alloy specimens are discussed.

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.

The Skin Effect in an Mg-RE High Pressure Die Cast Alloy

Cross-sectional microhardness maps of cast-to-shape flat tensile specimens have been obtained for a binary Mg-3.44 mass% La alloy. Higher microhardness numbers were generally found near the casting surface, at the corners and along the segregation band. The higher hardness values were ascribed to the finer solidification microstructure near the surface and to localized positive macro segregation. The majority of lower hardness numbers was found at the core region. Lower hardness values were ascribed to the coarser grain size prevalent at the core and to dispersed microporosity. The non uniformity of the harder surface layer in both depth and hardness appeared related to local homogeneities in the grain size distribution caused by the scattered presence of large externally solidified grains.

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.

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.

Elastic Behavior of the Percolating Eutectic Structure of a High Pressure Die Cast Magnesium Alloy

The 3D configuration of the (Mg17Al12) β-phase intermetallic microstructure in the AZ91D alloy obtained using dual beam FIB tomography was incorporated into an FEM code and loaded in tension. The structure stiffness is consistent with bending-dominated behavior.