B.C. Pai

Role of calcium in aluminium based alloys and composites

Abstract The role of calcium in aluminium alloy systems is reviewed, commencing with a survey of binary and ternary Al–Ca(–X) alloy systems. The influences – both beneficial and deleterious – of calcium on the physical, electrical and mechanical properties of aluminium alloys are considered. Beneficial applications of Ca include modifier in Al–Si alloys, iron neutraliser in recycled aluminium alloys with high iron content, scavenger of Sb, P and Bi from secondary alloys, thickening agent in the production of aluminium foams and wetting promoter in the synthesis of aluminium metal matrix composites; as an alloying element, Ca can also impart superplasticity. The interaction of Ca with other elements, high temperature oxidation, and applications of Al–Ca alloys are discussed and avenues for further work recommended.

Liquid Metal Infiltration Processing of Metallic Composites: A Critical Review

Metal matrix composites (MMC) are one of the advanced materials widely used for aerospace, automotive, defense, and general engineering applications. MMC can be tailored to have superior properties such as enhanced high-temperature performance, high specific strength and stiffness, increased wear resistance, better thermal and mechanical fatigue, and creep resistance than those of unreinforced alloys. To fabricate such composites with ideal properties, the processing technique has to ensure high volume fraction of reinforcement incorporation, uniform distribution of the reinforcement, and acceptable adhesion between the matrix and the reinforcing phase without unwanted interfacial reactions which degrades the mechanical properties. A number of processing techniques such as stir casting/vortex method, powder metallurgy, infiltration, casting etc. have been developed to synthesize MMC employing a variety of alloy and the reinforcement’s combinations. Among these, infiltration process is widely used for making MMC with high volume fraction of reinforcements and offers many more advantages compared to other conventional manufacturing processes. The present paper critically reviews the various infiltration techniques used for making the MMC, their process parameters, characteristics, and selected studies carried out worldwide and by authors on the development of metal ceramic composites by squeeze infiltration process.

Enhancement in tribological behaviour of functionally graded SiC reinforced aluminium composites by centrifugal casting

Enhanced tribological and location-specific properties achieved in functionally graded metallic composites make them potential materials for futuristic engineering components. The present investigation aims on the processing of homogenous and functionally graded Al matrix composites reinforced with SiC particles of 23 µm average particle size by gravity and centrifugal casting techniques. The sizes of the primary aluminium and eutectic silicon phases are finer towards the outer periphery due to the higher solidification rate. Functionally graded Al–SiCp composite rings produced by centrifugal casting show higher concentration of SiCp towards outer periphery, followed by a gradient transition region and the particle depleted zone. Particle agglomerates formed due to partially wetted particles associated with voids and gas porosities segregate towards the inner periphery. The higher concentration of reinforcement particles near the outer periphery in FGM enhances the hardness and wear resistance in rotary and reciprocating wear studies. Centrifugal cast alloys show enhanced wear resistance than the gravity cast specimen due to fine grains of primary aluminium and eutectic silicon. The rotary and reciprocating wear test shows the adhesive and abrasive type wear mechanisms.

Squeeze Infiltration Processing of Functionally Graded Aluminum–SiC Metal Ceramic Composites

The objective of the present work is on fabrication of functionally graded SiC/Al composite by direct squeeze infiltration of 6061 aluminum alloy using graded SiC porous preform prepared by inorganic porogen technique. Graded SiC preform is synthesized by varying the concentration of inorganic salt mixture and using Al as the binder. The microstructure analysis indicates the graded distribution of SiC particle and the melt has infiltrated completely throughout the preform to form functionally graded materials. The influence of preform and mold temperature, liquid metal superheat, squeeze pressure, and its rate of application plays major role on solidification microstructures and properties of the composites. The macro porous graded SiC preforms and the composites were characterized using SEM, optical microscopy, and XRD. The major interfacial reaction product is MgAl2O4 spinel which helps in formation of good interface bonding.

Synthesis of Porous SiC Preform and Squeeze Infiltration Processing of Aluminium-SiC Metal Ceramic Composites

Metal-Ceramic Composites with high volume fraction of reinforcement find wide applications in the area of tribology and high temperature resistant components like piston, brake pads, heat shields etc. Most of these components can be made by infiltration processing of porous ceramic preforms. The present investigation is to synthesize porous ceramic preform based on SiC particles using inorganic salt as a pore forming agent and Squeeze infiltration is applied to fabricate the Al-SiC metal-ceramic composites. The direct squeeze infiltration of 6061 aluminum alloy on SiC preform is successfully carried out with the controlled process parameters of initial preform temperature, liquid metal superheat, squeeze pressure and its rate of application, and die temperature. The preform and composites are characterized using XRD, optical microscopy, electron microscopy, and hardness and compression strength. Porous ceramic preform with more than 50% porosity has been fabricated by sodium chloride as pore former. The infiltrated composite have shown uniform and complete infiltration of Aluminium alloy in between SiC particles and posses very high hardness of 147 BHN in as cast condition compared to 57 BHN for the 6061 Al alloy.

The Effect of Yttrium Addition on the Microstructure and Mechanical Properties of Mg Alloys

AbstractAutomotive and aerospace industries shall witness advancement to the next generation if magnesium (Mg) alloys become an integral part of their manufacturing unit primarily because of its light weight. The main limitation which hinders the progress in this direction is the inferior creep properties of Mg alloys. In order to transform this expectation into reality, rare earth (RE) elements are extensively used as alloying elements for improving the room temperature (RT) as well as high temperature (HT) properties. Yttrium (Y) is one of the most extensively used RE elements primarily because of its very high solubility in Mg. Many researchers have studied the influence of Y addition in Mg alloys because of its very high solubility in Mg. However, there is a need to consolidate the work that has been carried out so far which will help in interpolating the future prospects. This review consolidates the work that has been carried out so far in Y addition covering various aspects related to microstructural modifications and RT as well as HT mechanical properties.

Studies on electroless nickel boride coating on boron carbide particles

Abstract Ni–B alloy deposits were successfully formed over alkali surface treated boron carbide by electroless technique and the effects of processing parameters such as reduction bath pH, temperature and post-heat treatment were investigated. Microstructural studies show remarkable changes in structural morphology from mesh-like structure to platelet type Ni–B coating. The main Ni–B phases observed were NiB, Ni2B, Ni3B and Ni4B3. An effective Ni–B alloy coating with better surface characteristics was formed at pH 8 under 75°C reaction bath temperature in the presence of bath stabiliser. The post-heat treatment of Ni–B coated particles under varying bath reaction temperatures at 400°C showed remarkable Ni–B phase transformation from metastable NiB and Ni2B to stable crystalline Ni3B phase. The NiB coated particle shows lower particle–matrix interfacial reaction, better wettability and enhanced dispersion in aluminium composites compared to uncoated particles.

Processing of Functionally Graded Aluminium Matrix Composites by Centrifugal Casting Technique

Functionally graded materials (FGM) possess a gradual transition in the composition or microstructure within a component inducing gradual changes in the functions and properties of the material with varying location. The present investigation is to form functionally graded aluminum composites with varying microstructural features and properties gradation using centrifugal casting by varying the material and process parameters. The ex-situ Al-SiC FGM has shown graded distribution of SiC towards the outer periphery and Al-SiC-Graphite has shown both particle gradations of SiC and Graphite towards inner periphery due to hindered settling phenomena. In in-situ Al-Si and Al-Si-Mg2Si system the particles are distributed towards the inner periphery of the casting. The paper also emphasizes on selected engineering components and tailoring their graded structures and properties by utilization of suitable reinforcement combinations and process parameters are discussed.

Equal Channel Angular Pressing of Aluminum-Alumina In Situ Metal Matrix Composite

The present investigation is on synthesis of in situ Al-alumina composite and to evaluate the effect of equal channel angular pressing on the refinement of the grain structure and enhancement in the hardness and the strength. The billets pressed in as cast condition has shown cracks during first pass. The billets pressed immediately after solution treatment for one pass and followed by ageing treatment immediately after pressing exhibited very high hardness of 125BHN against 95 BHN to that of the T6 condition of 6061 aluminium alloy. The microstructural refinement from 35 µm to 11 µm is obtained in annealed and ECAP 2 pass condition.

Processing of Primary Silicon and Mg2Si Reinforced Hybrid Functionally Graded Aluminum Composites by Centrifugal Casting

In the present investigation, FGMs of mono-dispersed in-situ primary Si and their hybrids with Mg2Si reinforcements have been fabricated by the centrifugal casting process using 390 commercial Al alloy. Hard primary silicon particles are formed during the solidification of the 390 alloy and Mg2Si reinforcements are formed by the addition of varying amount of magnesium into the A390 aluminium alloy. Owing to the difference in density both primary silicon and Mg2Si gets segregated towards the inner periphery during centrifugal casting. The size of the Mg2Si in-situ reinforcement phase is relatively smaller and is distributed in the edges of primary silicon particles and also individually in the matrix. The in-situ Mg2Si and primary silicon can significantly increase the hardness and strength of the inner periphery of the casting. Higher Mg contents have been observed to introduce significant porosity leading to poor castings. Addition of phosphorous to the melt has led to the modification and refinement of primary Si morphology and also helped in the reduction of shrinkage porosity. Maximum hardness of 167 BHN is observed towards the inner periphery of the 390Al-2.5%Mg added in-situ composite.