Materials Science Forum Vols. 830-831

Abstract: Nickel-based superalloy Inconel 625 is widely used in aeronautical, aerospace, chemical, petrochemical and marine applications due to its good mechanical properties, weldability and resistance to high temperature corrosion on prolonged exposure to aggressive environments. It is a solid solution strengthened medium strength superalloy, which contains chromium, molybdenum and niobium as alloying additions. Considering the chemistry and specification requirements of the alloy, it was processed through vacuum induction melting (VIM) process followed by electro slag remelting (ESR) route to obtain alloy with controlled gas and inclusion contents. Homogenisation cycle was selected and was carried out at 1170°C temperature to obtain uniformity in chemistry and microstructure. Chemical homogeneity was confirmed through analysis of samples from top, middle and bottom of the secondary ESR ingot. Hot working range was decided considering the flowability of superalloy and the same was carried out under close monitoring of temperature and with specified amount of reduction per stroke. Intermediate reheating and reduction during forging was noted to be an important aspect so to avoid cracking during forging. Processing parameters were established to obtain forgings of different thicknesses with sound ultrasonic quality. Microstructure analysis revealed single phase austenitic grain structure with ASTM grain size no. 4-7, confirming that material has undergone sufficient amount of mechanical working. Mechanical testing was carried out and the mechanical properties were found to be meeting the requirement. Present paper provides details of melting process selection, thermomechanical processing and characterization of the superalloy to achieve the targeted mechanical properties.

Abstract: Semisolid metal processing (SSMP) is a relatively new technology for production of near net shaped components. This process is carried out at a temperature range between its liquidus and solidus temperature. At thixo-temperatures, the metal exists as a mixture of solid and liquid phases in the slurry form. Thixotropy is a time-dependent shear thinning property where the metal is thick at static conditions but will flow when subjected to shear stress. The present research work focuses to identify the process parameters in SSMP. A mechanical stirring process was employed to obtain the desired microstructure required for SSMP. Process parameters such as stirring time and stirring speed were considered. The effect of addition of grain refiner to the molten alloy on the microstructure was also studied.

Abstract: The calibre of low pressure casting (LPC) process in meeting the demands for improved quality and aesthetics for thin walled components has been widely accepted in casting industries. In spite of numerous advantages, the process is still being used very less to its potential. The basic reason has been the difficulty in determining various processing parameters associated which can be circumvented effectively by numerical simulation. In the present work, the commercial CFD code FLOW 3D^TM, has been used to simulate the mould filling and solidification during the LPC process.

Abstract: Casting simulation packages are used to check a design for its castability. A better starting design should need fewer simulation cycles to arrive at a defect-free component thus cutting computation and manpower costs. Quantitative design of the feeding system is done by an analysis of the solidification pattern of the 3D model of the cast component. A clustering algorithm uses the solidification time/temperature data from the simulation to divide the casting into 3D feeding sections. The sections are created by following hotspots surrounded by areas of decreasing solidification time. Feeders are built by the feeder design module of AutoCAST casting design software. The initial simulation as well as the efficacy of the rigging is tested through the advanced simulation module FLOW+ of AutoCAST X. An industrial case study illustrates the software pipeline in a virtual foundry trial.

Abstract: Centrifugal casting process is one of the potential manufacturing techniques used for producing functionally graded materials viz., composite materials or metallic materials which have high differences of density among constituents. In this process, the fluid flow plays a major role and understanding the complex flow process is a must for the production of defect-free castings. Since the mold spins at a high velocity and the mold wall being opaque, it is impossible to visualise the flow patterns in real time. Hence, in the present work, the commercial CFD code FLOW-3DTM, has been used to simulate the mold filling sequence for a simple hollow cylindrical casting during vertical centrifugal casting process. Effect of various spinning velocities on the fill pattern during vertical centrifugal casting process is being investigated.

Abstract: Nanostructured W₇₀Cr₃₀ powders were produced by mechanical alloying (MA) of elemental tungsten (W), Chromium (Cr) powders in a high energy planetary ball-mill using tungsten carbide as grinding media and toluene as a process control agent. The crystallite size and lattice strain of the nanostructured powders at different milling time (0 h to 10 h) was calculated from X-ray diffraction patterns (XRD). The crystallite size of W in W₇₀Cr₃₀ powder reduced from 100 μm at 0 h to 32.8 nm at 10 h of milling with increase in lattice strain of 0.43% at 10 h of milling. The lattice parameter of tungsten shows initial expansion of lattice upto 0.56% at 5 h of milling and contraction of lattice upto 0.93% at 10 h of milling. The scanning electron microscopy (SEM) micrograph also revealed mixed morphology of elemental W and Cr powders consist of spherical and elongated particles during mechanical alloying (0 h to 10 h). The dark-field transmission electron microscopy (TEM) observations indicated that the crystallite size (~30 nm) of W in W-Cr alloy in the as-milled powder is in good agreement with calculated crystallite size from XRD. Maximum solid solubility of 4.4 at.% Cr in W was found at 10 h of milling. The dislocation density increases from 6.75 (10¹⁶/m²) to 17.56 (10¹⁶/m²) with increase in the milling time from 0 h to 20 h. No cracks in the sintered pellets were visible under scanning electron microscope (SEM). Hardness and Elastic Modulus of sintered W₇₀Cr₃₀ alloy determined by nanoindentation test are less compared to pure W.

Abstract: In this present study Al-5083 alloy powders were prepared from elemental powders using high energy ball milling under optimized milling parameters. Various properties such as crystalline size, particle size and morphology have been studied using X-Ray diffraction analysis and Scanning Electron Microscopy. It was found that Al-5083 alloy was formed and nanocrystalline size particles were achieved. These nanocrystalline Al-5083 alloy powders were consolidated using equal channel angular pressing with and without application of back pressure. Physical and mechanical properties such as density and hardness are studied.

Abstract: This paper reports the development of a cutting tool insert prepared by consolidation, followed by Spark Plasma Sintering (SPS) of TiB₂ (Titanium diboride) particles processed through an in-situ reaction and AlN (Aluminium nitride) manufactured by the direct nitridation process. In-situ TiB₂ particles, formed during reaction of KBF₄ (Potassium tetra Fluoroborate) and K₂TiF₆ (Potassium Fuorotitanate) with Al alloy, are obtained by dissolving the Al-TiB₂ composite in an acidic medium. The extracted TiB₂ (30%) particles are blended with AlN and Al₂O₃ in the weight ratio of 67%-3% and sintered (SPS) at 1440°C with a compaction load of 50 MPa and a total sintering time of 8 minutes. The sintering is carried out in vacuum. The sintered ceramic displays high hardness of nearly 15.5 GPa and extraordinary toughness of 7MPa.m^1/2. The inserts are manufactured according to SNGN (Square Double-sided ceramic) configuration. To study its performance, machining is carried out on hardened steel (EN 24). The developed AlN based inserts show increased wear resistance and provide good surface finish when compared with commercially available ceramic inserts (70%Al₂O₃+30%TiC). Cutting forces are recorded with a Kistler® dynamometer to correlate them with the tool wear. Methods of preparation and comparison of wear resistance and surface finish of the machined material with those pertaining to commercial ceramic inserts are also presented. SEM images are displayed, which support the results.

Abstract: The Metal Matrix Composites (MMCs) especially Aluminum based systems have unique advantages of having superior mechanical, chemical and electrical properties, in addition to light weight and high stiffness. In this work, composites comprising of Aluminum with varied concentrations of Cenospheres as reinforcement was produced by Powder Metallurgy (PM) route. The densification of the composites was effected both by conventional and Microwave (MW) sintering techniques. The microstructures of the sintered samples were observed through scanning electron microscope (SEM) and phases by x ray diffraction technique (XRD), followed by evaluation of tribological parameter namely slide wear behavior and solid particle erosion resistance. The densities and the Brinell hardness values for the samples were also evaluated. The results showed that microwave sintered samples exhibited higher hardness, lower erosion and slide wear loss.

Abstract: Cenospheres are very cheap, and are reasonably strong and thermally stable upto 1200°C. In view of this attempt has been made to use these cenosphere for making Titanium syntactic foams with varying relative densities. Precautions were taken for selecting cold compaction pressure to minimize cenosphere crushing. The sintered samples were then characterized in terms of microstructures primarily to see the extent of cenosphere crushing, distribution of cenosphere, and extent of sintering. The foams made using optimized pressure and sintering parameters, exhibits uniform distribution of cenosphere without any significant crushing. The plateau stress, energy absorption and modulus of these foams are varying with the cenosphere content or the relative density, and these parameters can be engineered by varying cenosphere content in the foam. These foams exhibit considerably higher strength and stiffness than the conventional foam and show the possibility of using them for biomedical and engineering applications.