T. Balakrishna Bhat

Effect of process parameters on the densification of 2124 Al–20 vol.% SiCp composites fabricated by explosive compaction

Abstract This paper explores the possibility of producing metal-matrix composites using the explosive compaction method. Attempts have been made to compact powders of 2124 Al and silicon carbide. The effect of the explosive pad thickness and related parameters such as the impact energy imparted to the powder and the ratio of the explosive mass to the powder mass (Em/Pm) on the density of the compacts have been studied. Microstructural variations with respect to the processing conditions have been investigated also. The compacted 2124-20 vol.% SiCp composite specimens have been subjected to hot rolling at 520°C and age hardened. The tensile properties of the composites have also been evaluated.

Dynamic consolidation of aluminium and Al-20 V/o SiCp composite powders

Abstract Aluminium powders and Al-SiCp composite powders have been compacted in the form of cylinders using explosives. The effect of processing parameters such as the explosive pad thickness and the impact energy on the density of the compacts has been investigated. The microstructural variations in the compacts have been studied with respect to the explosive pad thicknes used and pressures imparted on the powder compacts. The hardness variation over the cross section of the Al-20 V/o SiCp composite compacts have been studied with respect to the variation in the thickness of the explosive pads. The optimum processing conditions for the production of compacts of aluminium and Al-20 V/o SiCp systems by dynamic compaction have been defined.

An analysis of pressure sintering by computer simulation

This paper describes the results of computer simulation of pressure sintering using Finite Element Analysis for isostatic as well as uniaxial pressing conditions. An effective pressure concept for obtaining analytical description of the densification rate in terms of porosity and the basic creep properties of the material is also presented. Equations are also derived to take into account the presence of friction at the die walls. Results of computer analysis are used to corroborate the theoretical derivations.

Studies on Aluminum Armour Plates Impacted by Deformable and Non-Deformable Projectiles

Aluminium alloy armour (7017) plates have been impacted by deformable and non-deformable high velocity steel projectiles. The projectiles were fired on the targets at 10 meters distance at 0 0 angle of attack with velocities around 840 m/s. The resulting microstructure, damage pattern and hardness along the path of the penetration in both the cases have been studied and compared. Introduction Aluminium alloy 7017 is one of the commonly used armour materials. It is therefore of interest to study and report how it deforms and changes its structure and hardness during penetration by typical projectiles which may be soft and deformable or hard and non-deformable. A recent work by Kennedy and Murr [1] covers some of these aspects. In that work, tungsten heavy alloy projectiles were used against Aluminium 7039 target plates. The microstructural changes and the hardness variations away from the crater edge and shear band formation along the direction of penetration are studied. Backman et al. [2,3] observed adiabatic shear bands and deformed grains in aluminium 2024 target plates impacted by steel spheres and blunt steel projectiles. Rupert et al. [4] has carried out similar studies on Ti and RHA, and studied the microstructure and hardness variations along the edge of the crater. Material failure at high strain rates as encountered in high velocity impact situations is a complex process. Under dynamic loading failure can occur by a variety of mechanisms depending on the material constitution and state of stress, temperature, rate of loading and a number of other variables [5]. The failure of the material can be brittle or ductile in nature. The material deforms as the projectile penetrates through the plate causing various types of defects like void-formation, nucleation and growth of shear bands, cracking all of which ultimately result in material failure [6]. The study of failure covers a wide range of material responses of both projectile and the target. The depth of penetration of the target material depends mainly upon the resistance offered by the target material, which in turn depends upon the flow factor, yield stress and the extent of flow field. The Waller-Anderson penetration model [7] for the penetration by cylindrical rods gives an expression for the resistance to the penetration offered by the target material when impacted with projectiles with a hemispherical nose. The resistance to penetration is directly proportional to the flow stress of the target material. It also depends on a flow factor, which varies with the shape of the projectile. The flow factor for a projectile with a hemispherical nose is higher than the flow factor for a projectile with a conical nose. The present study covers some aspects of deformation, shear band formation and cracking along the path of penetration by deformable and non-deformable projectiles.

Microstructural characteristics of shock consolidated 2124 Al alloy compacts

Abstract2124 alloy powders have been compacted using explosive compaction. The effect of process parameters like explosive pad thickness and impact energy imparted to the powders on the microstructure and hardness across the cross-section of the compact have been investigated. When the thickness of the explosive pad was increased to 9.5 mm, three distinct microstructures with different hardness values were found across the cross-section of the compact. The size and shape of the θ phase precipitates were different in the fine grained structure when compared with that of the original particles and triple point junctions. Central porosity and pipe formation were observed when the thickness of the explosive pad was increased beyond 14.5 mm. Variation in the microstructure of the compact across the cross-section disappeared when the diameter of the compact was increased from 11 to 25 mm.

Shock synthesis of 2124Al–SiCp composites

Abstract This paper explores the possibility of producing metal matrix composites by using the explosive compaction method. 2124Al alloy reinforced with silicon carbide particulates ranging from 20 to 40vol.% were compacted by this method. The effect of explosive pad thickness and the impact energy imparted to the powder on the density of the compacts were studied. The metal matrix composites of 2124Al–20vol.% SiCp and 2124Al–30vol.% SiCp were further subjected to hot rolling at 520°C and age hardened and the tensile properties of the composites were evaluated.

Inhomogeneous flow and the effective pressure concept in pressure sintering

Many of the models proposed to describe pressure sintering of solids with a randomly distributed porosity have implicit in them the effect of porosity and it is incorrect to incorporate any extra pressure intensification factor. However, usually, pores are segregated into zones which leads to inhomogeneous deformation. A consequence of this inhomogeneity is to enhance densification rates by dislocation creep and particle sliding resulting in an anomalous pressure intensification factor. Some observations on microstructural development during hot isostatic pressing are also made.

Dislocation creep in particle-strengthened systems

A model for climb-controlled creep in two-phase materials is proposed which invokes the presence of a back-stress for providing the necessary mobile dislocations for creep. It is shown that by incorporating this stress in the creep equation it is possible to reduce both the activation energy for creep and the stress exponents to values normally observed in single-phase materials. Creep data on TD nickel and yttriated superalloy when analysed on this basis confirm the applicability of this model.

Strengthening mechanisms in alloys

Metals can be strengthened by methods appropriately combined to meet the operating conditions. In this ‘alloy design’, effort, we are guided by semi-quantitative relations that have been developed over the years and which relate the efficacy of the strengthening methods to the deformation modes. In this paper, the basic concepts are expanded with specific reference to dislocation glide, diffusional creep, grain boundary sliding and high strain rate deformation.

My Tryst with Indigenous Armour Development

Born little after India’s independence, as a child, I used to imagine and feel that I owned the whole great country. My father, Madhav Bhat, was a respected school headmaster who used to get respectful salutations from members of nearly every house situated all along a seven-kilometre-long path of walk to his school through hills and fields. He would reciprocate with appropriate affectionate words without stopping his walk.