Showing papers by "Defence Metallurgical Research Laboratory published in 2009"

Friction surfacing: novel technique for metal matrix composite coating on aluminium–silicon alloy

Abstract: The present work pertains to the study of wear and corrosion resistance of surface coating of SiCp reinforced AA 2124 aluminium alloy composite onto the surface of aluminium alloy A356. In order to improve wear and corrosion resistance of A356 Al–Si alloy, SiCp reinforced composite was coated on A356 Al–Si alloy substrate by friction surfacing technique. The coating was characterised by metallography, dry sliding wear and potentiodynamic polarisation testing. The present study revealed that aluminium alloys could be friction surfaced with metal matrix composites. The coating exhibited excellent wear resistance, which is attributed to the formation of strong metallurgical bond with the substrate. The coating was also found to have adequate corrosion resistance. The study assumes special significance as deposition of metal matrix composites utilising friction surfacing is reported for the first time.

Improvement of mechanical properties of Inconel 718 electron beam welds—influence of welding techniques and postweld heat treatment

Abstract: Effects of electron beam oscillation techniques (sinusoidal, square, triangular, ramp, circle, and elliptical) in controlling Nb segregation, Laves formation, and tensile behavior of Inconel 718 electron beam welds were studied. Effects of various postweld heat treatments on tensile behavior were compared. Elliptical beam oscillation technique resulted in less Nb segregation and discontinuous finer Laves in the interdendritic regions compared to that of unoscillated beam weld. Response to aging was better for welds made with elliptical oscillated beam compared with welds made with unoscillated beam. Oscillated beam welds exhibited better mechanical properties than unoscillated beam welds in both solution-treated and aged and directly aged conditions.

Microwave sintering of tungsten

Abstract: This study aims at microwave sintering of pure tungsten powder of as-received grade and tungsten powder activated by high-energy milling (HEM). Both the powder compacts are sintered under identical conditions and their sinterability is compared and analyzed for process optimization. Activated powder responds to microwaves and shows significant densification in a relatively shorter duration. The scope of the present work is to understand the effects of high-energy milling on the sinterability of tungsten in microwave and to make a comparative assessment between as-received (coarser) and activated (finer) tungsten powder in terms of their response to microwave sintering, densification and microstructure.

Hardness and tensile properties of tungsten based heavy alloys prepared by liquid phase sintering technique

Abstract: Several tungsten heavy alloys based on W–Ni–Cu and W–Ni–Fe were made recently at DMRL by liquid phase sintering route. These alloys were heat treated and characterized in terms of density, hardness and tensile properties. Although the densities of W–Ni–Cu based heavy alloys were found to be relatively higher than those of W–Ni–Fe based heavy alloys, the former exhibited inferior tensile properties and hardness values than the latter alloys. In both these heavy alloys, un-melted solid tungsten particles were bonded together by the respective low melting matrix alloy. The strength of the matrix or the bonding strength of the interface (between tungsten particles and matrix) was found to control the nature of final fracture. Poorer matrix strength or interfacial strength was found to initiate the fracture by separation of tungsten particles either by matrix failure or by interface failure. On the other hand, tensile fracture takes place predominantly by cleavage fracture of tungsten particles, if both the matrix and interface are stronger than the tungsten particles. The tensile fractured surfaces were examined under scanning electron microscope (SEM). Failure stress is observed to be lower in case of separation of tungsten particles (due to matrix and interface failure) than that in case of cleavage fracture (transgranular fracture) of tungsten particles. SEM image of fractured surfaces clearly indicated that failure in case of W–Ni–Cu based heavy alloys was due to matrix and/or interface (between tungsten grain and matrix) failure, where as, W–Ni–Fe based heavy alloys were failed predominantly by cleavage fracture of tungsten particles. This was evidenced by poorer tensile properties obtained for W–Ni–Cu based heavy alloys as compared to those of W–Ni–Fe based heavy alloys. The present sets of results indicate that heavy alloys based on W–Ni–Fe and W–Ni–Cu can be strengthened by minor additions of Co, Mo and Fe selectively. Higher strength (>850 MPa) of the heavy alloys are associated with the finer tungsten grain size and cleavage fracture of tungsten particles. Where as, the lower strength is associated with the larger tungsten grain size and interface and/or matrix failure. It is found in this study that as the tungsten grain size decreases, the tensile fracture mode changes gradually from interface failure to matrix failure to tungsten grain cleavage failure.

Synthesis of Silver Nanoparticles using Facile Wet Chemical Route

Abstract: Stable aqueous monodispersed silver nanoparticles were synthesised by reducing silver nitrate using various sugars such as glucose, fructose, lactose, and sucrose at 55-60 oC. A mixture of two stabilising agents, polyvinyl pyrrolidone (PVP) of molecular weight (MW 40, 000) and gelatin plays a decisive part in controlling size and shape of superfine silver nanoparticles. Using PVP, better control of particle size was obtained, whereas a mixture of PVP, and gelatin resulted in sea urchin kind of structure. Effects of several processing parameters such as pH, quantities of alkaline solution, reaction time, concentration of stabilising agent, and the metal/dispersant ratio were investigated. The suspensions were stable for at least one week. Silver nanoparticles can be easily collected by centrifugation on mixing with sufficient amount of acetone. Structural characterisation of synthesised silver nanoparticles was carried out by X-ray diffraction (XRD), which shows that the as-synthesised silver nanoparticles are face-centered cubic crystalline. Chemical characterisations of as-synthesized silver nanoparticles were done using inductively-coupled plasma-optical emission spectrometry (ICP-OES) and LECO gas analysers. The optical properties of silver nanoparticles were monitored using UV-Vis spectrophotometer. Role of stabilising agent in protecting the silver nanoparticles was studied using fourier transform infrared spectroscopy (FTIR). Particle size and distribution were characterised using smallangle X-ray scattering (SAXS), zeta particle size analyser (PSA), scanning electron microscopy, and transmission electron microscopy. The average particle size of the as-synthesised silver nanoparticles was 35 nm.Defence Science Journal, 2009, 59(4), pp.447-455, DOI:http://dx.doi.org/10.14429/dsj.59.1545

Synthesis of tungsten nanoparticles by solvothermal decomposition of tungsten hexacarbonyl

Abstract: Tungsten nanoparticles were prepared by thermal decomposition of tungsten hexacarbonyl [W(CO) 6 ] (2 mmol) at 160 °C in presence of a mixture of (1:1) surfactants, oleic acid (6 mmol) and trioctyl phosphine oxide (TOPO) (6 mmol) under a blanket of Ar gas. The synthesized tungsten nanoparticles without surfactant are flocculated. With increase in concentration of surfactant mixture to 12 mmol each, agglomeration of several tungsten nanoparticles are observed. Characterization of surfactant coated tungsten nanoparticles were carried out using Fourier transform infrared spectroscopy (FTIR). Chemical characterizations of synthesized tungsten nanoparticles were done using X-ray photoelectron spectroscopy (XPS), inductively coupled plasma-optical emission spectrometry (ICP-OES) and LECO gas analyzers. XPS study shows the W 0 oxidation state of tungsten nanopowders. Structural characterization of synthesized tungsten nanoparticles were conducted by X-ray diffraction (XRD), which shows that the as-synthesized tungsten nanoparticles are amorphous in nature and they become body centered cubic crystalline after annealing. Particle size, shape and distribution were characterized using small-angle X-ray scattering (SAXS), environmental scanning electron microscopy (ESEM) and transmission electron microscopy (TEM).

Microstructure and mechanical properties of similar and dissimilar stainless steel electron beam and friction welds

Abstract: Microstructure and mechanical properties of similar and dissimilar welds of austenitic stainless steel (AISI 304), ferritic stainless steel (AISI 430), and duplex stainless steel (AISI 2205) have been studied. Welding processes electron beam welding and friction welding were used. Optical, scanning electron microscopy, and electron probe microscopy were carried out to study the microstructural changes. Residual stress, hardness, tensile strength, and impact toughness testing were conducted to study mechanical behavior. Dissimilar metal electron beam welds of austenitic–ferritic, ferritic–duplex, and austenitic–duplex stainless steel welds contained coarse grains, which are predominantly equiaxed on austenitic, duplex stainless steel side, and they are columnar on the ferritic stainless steel side. Diffusion of elements was significant in electron beam welding and insignificant in friction welds. Austenitic–ferritic stainless steel exhibited tensile residual stress on the ferritic stainless steel side adjacent to the interface, compressive stresses on the austenitic stainless steel side that matches with the delta ferrite microstructure observed in this region. High compressive stresses were noted on duplex stainless steel side interface compared to austenitic stainless side interface. The highest tensile strength was observed in duplex–austenitic stainless steel joints. The impact strength and notch tensile strength of electron beam weldments are higher than the friction weldments. All electron beam and friction welds showed toughness lower than parent metals.

Mechanical properties of Ti3AlX (X = C, N): Ab initio study

Abstract: Elastic properties of the alloys Ti3AlC and T i3AlN are derived from the first-principles total energy calculations based on the full-potential linear muffin-tin Orbital (FP-LMTO) method. From the computed elastic constants, theoretical values of Young's modulus, shear modulus, Poisson's ratio, sound velocities and Debye temperature are evaluated. By analysing the ratio between the bulk and shear moduli, it is found that Ti3AlN is ductile in nature, whose ductility is expected to be greater than that of Ti3Al, whereas T i3AlC is found to be brittle. The site-projected density of states and the charge density plots have been used to analyse the chemical bonding between the Ti6N and T i6C cluster and the surrounding metallic lattice of Al atoms. This further reveals that the strong covalent nature of Ti-C bonds in Ti3AlC, together with the high Young, shear and bulk moduli, make the compound more brittle than Ti3AlN.

Microstructure and properties of high strength aluminium alloys for structural applications

Abstract: This article discusses the fundamental basis of high strength Al alloy design and describes the role of alloying elements, mechanical processing parameters and heat treatments toward the evolution of microstructure that controls the desired properties i.e. strength, fracture toughness, stress corrosion cracking (SCC) resistance, fatigue crack initiation and propagation resistance, and weldability in 7xxx series Al alloys. The beneficial effects of suitable micro/trace alloying elements, and deleterious effects of certain impurity elements on a variety of properties are further discussed within the present context.

Microstructure and mechanical property correlations in AA 6061 aluminium alloy friction stir welds

Abstract: The influence of friction stir welding on the microstructure development and its role on residual stress distribution in the weldment and mechanical properties has been investigated. The study also focused on the impact of post weld heat treatment on the microstructure and mechanical properties as well as on residual stress distribution. The weld nugget region contained fine equiaxed grains as a result of thermo-mechanical working. Hardness survey showed that nugget region is soft due to precipitates dissolution. Weld joint exhibited lower strength as compared to the parent metal. Post weld Solution Treatment and Aging (STA) of longitudinal welds resulted in strength and ductility equivalent to that of parent metal while transverse weld tensile strength and ductility were lower than that of parent metal even after post weld STA. Residual stress distribution profiles across the weld region are asymmetric with respect to weld centerline, with the largest residual; stress gradients occurring on the advancing side of the weld. Within the region inside the shoulder diameter, residual stress is entirely compressive. Welds exhibited tensile residual stresses in post weld STA condition

Elastic modulus of Al–Si/SiC metal matrix composites as a function of volume fraction

Abstract: Aluminum alloy matrix composites have emerged as candidate materials for electronic packaging applications in the field of aerospace semiconductor electronics Composites prepared by the pressureless infiltration technique with high volume fractions in the range 041–070 were studied using ultrasonic velocity measurements For different volume fractions of SiC, the longitudinal velocity and shear velocity were found to be in the range of 7600–9300 m s−1 and 4400–5500 m s−1, respectively The elastic moduli of the composites were determined from ultrasonic velocities and were analysed as a function of the volume fraction of the reinforcement The observed variation is discussed in the context of existing theoretical models for the effective elastic moduli of two-phase systems