Materials Science Forum Vol. 710

Abstract: Al–4B master alloy has been prepared by the reaction of fluoride salt KBF₄ with molten Al and different amounts of this master alloy (0.5, 1, 1.5 and 2 wt%) has been added to pure Mg to study in detail its effect on the grain refinement of pure Mg. Considerable reduction in grain size from 1300 to 210μm is obtained and maximum reduction is observed with 2 wt% master alloy addition. The grain refinement is caused by both the growth restriction effect of Al and the presence of potent AlB₂ nucleant particles. Improvement in mechanical properties is obtained with Al–4B master alloy and is attributed to increase in grain boundary area due to the grain refinement and the particle strengthening due to AlB₂.

Abstract: Al-Cu-Mg (Russian grade V65) alloys are used for riveting applications in aerospace industries due to relatively high shear strength of the order of 25 kg/mm² combined with a high plasticity. The main advantage of using V65 aluminum alloy for rivet application comes from its significantly slower natural aging response, which gives flexibility to carryout riveting operation even after 4 days after solution heat treatment and natural aging at room temperature. This very valuable feature is not found in its counterpart alloy AA2024 where due to rapid aging kinetics of this alloy, the riveting operation must be completed within 30 min of the solution treatment, which is many times impractical. This paper presents the processing of V65 aluminium alloy by Direct Chill (DC) casting followed by subsequent secondary processing through extrusion and forging to realize feedstock for caliber rolling. These ingots were caliber rolled to 12 mm diameter rods at different temperatures and mechanical properties were evaluated in T6 and T4 tempers. Detailed microstructural examination using optical microscopy and fractography of the tensile tested samples using scanning electron microscopy were carriedout to correlate microstructure and mechanical properties.

Abstract: High shear strength of 25 kg/mm2 combined with a high plasticity makes Russian grade V65 aluminum alloys as potential candidate compared to AA2024 for riveting applications in aerospace industries. The lower content of magnesium over the AA 2024 alloy decreases the aging kinetics of this alloy because of the decreased vacancy concentration in the as-quenched alloy which in turn gives flexibility to carryout riveting operation even after 4 days after solution heat treat¬ment and natural aging at room temperature. This paper presents the processing and characterization of V65 aluminium alloy wires processed from hot and warm rolled rods. Mechanical properties of the wires have been evaluated in T6 and T4 tempers and correlated with microstructures. Detailed microstructural examination using optical microscopy and fractography of the tensile tested samples using scanning electron microscopy were carriedout.

Abstract: Al-Cu-Mg (Russian grade V65) alloys are used for riveting applications in aerospace industries due to their relatively high shear strength of the order of 25 kg/mm2 combined with a high plasticity. This paper presents the processing of V65 aluminium alloy rivets from wire rods. It was observed that wire rods which have not been intermittently annealed and having more than 25% cold working prior to heading operation, cracked on the free bulged surface of the head. In view of this, it is recommended that a maximum of 25% of cold work is allowed in the wire rods to be used for rivet fabrication to successfully realize defect free rivets.

Abstract: Molten metal droplet impingement is an integral part of droplet based manufacturing processes like spray casting and sprays coating. In these processes, in a single operation, a liquid metal stream is atomized into fine droplets, which impact on a substrate to form a bulk deposit. The properties of casting or coating strongly depend on the shape of splats formed by individual droplets after impingement and solidification. Therefore, considerable research studies have been carried out to characterize individual droplet impact, usually driven by an interest in a particular process. These studies include extensive modeling of droplet deposition, some of which are supported by experimental studies. Most of these modeling activities have focused on the impingement of a droplet on to a surface to predict quantities such as the extent of maximum spread and the final equilibrium diameter, the rate of heat transfer to the substrate, and the solidification rate. Due to interaction of several complex phenomena, comprehensive modeling of the droplet deposition is a challenging task. The flow of liquid droplet upon impingement is itself a complex phenomenon. Heat transfer and solidification occurring concurrently with the flow adds further to the complexity. Thus the present study aims at the development of a comprehensive mathematical model of impingement of liquid metal droplet upon a substrate to understand the effect of process parameters such as initial temperature of droplet, size of droplet and velocity of droplet.

Abstract: Microstructural evolutions of Al-33wt. % Cu alloy strip, prepared by twin roll strip casting (TRC) at different roll speed (100-500 rpm) were studied in the present investigation. The as cast sample were subjected to Scanning Electron Microscope (SEM) to study the effect of rolling speed on microstructure along the thickness of the strip. The microstructure shows that Al-33 wt. % Cu strip is a layered structure which consists of partly lamellar and wavy structure. The lamellar microstructure consists of alternating layers of well bonded α-Al phase and θ-Al₂Cu phase.

Abstract: Metallic thermal protection system (MTPS) offers significant improvements over the ceramic based TPS for reentry applications. Space shuttle refurbishment time is estimated to be around 17000 man hours between flights. Metallic based TPS can be fabricated easily and provides wide range of design options for TPS. Adaptability and robustness of metallic thermal protection systems offers the potential for reusability. In this work, a unique manufacturing process has been evolved to realize light weight honeycomb panels through corrugation, laser welding and diffusion brazing of faceplates, where in 50 micron thick Inconel718 foil is used for making honeycomb core and 0.2mm thick Inconel718 foil as faceplates. The compression and three point bend test on these panels have shown no debond between faceplates and honeycomb core. 150x150x5mm size honeycomb panels were coated with YSZ and NiCrAlY based Thermal Barrier Coatings (TBC) and high temperature tests have shown thermal resistance of around 570 °C with front wall temperature of 1186 °C and back wall of 533 °C. Also these panels have been characterized for reusability by the testing of same panel at different heat flux levels. Though it is found that honeycomb panel has shown its integrity without debond a certain acceptable level of degradation in coating is observed. Thus Inconel718 based honeycomb panels with TBC coating are proved for use as thermal protection system for reusable launch vehicle systems.

Abstract: High yield strength and good wear resistance of hypereutectic steels in hardened and tempered condition made them attractive to manufacture rotating parts of mechanical systems. However, they suffered with poor corrosion, owing to high carbon content. The need for a material with improved strength, wear resistance and corrosion resistance for bearing application resulted in the design of a new steel having 17 wt.% Cr, up to 0.75 wt.% Mo and 1 wt.% C, which was christened as 440C. This martensitic grade of stainless steel was surface hardened by laser transformation hardening (LTH) technique using Pulsed Nd: YAG laser. Optimised process parameter could result in 300 µm thick hardened layer consisted of martensite, retained austenite and fine carbide with an average hardness of 540 VHN, while it was about 220 VHN in the core. Laser process parameter like energy/power density, pulse width, scanning speed and overlap ratio were responsible in influencing the microstructural constituents, hardness achievable and in turn dictates the wear resistance capability of the material. Experimental results such as temperature distribution, depth of hardening have been verified analytically. A reasonable agreement between the theoretical and experimental measurements was obtained. This paper highlights the details of experimental work.

Abstract: Estimation of cooling rates of gray cast iron casting in the sand mold and its dependency on design and process parameters is one of the keys for achieving best processing conditions to produce quality castings. The estimation of cooling rate involves modeling of fluid flow, heat transfer and solidification of molten metal inside the mold. Prediction of heat transfer has been carried out from filling of mold but the estimation of cooling rate has been carried out after complete filling of the mold. In the present work fluid flow, heat transfer and solidification of molten metal in a sand mold model has been developed on a Pro-Cast 2008 platform. A stepped bar pattern with different thickness has been fabricated to carry out the experiment. Stepped bar pattern has been selected because gray cast iron castings are thickness sensitive as well as different section of castings have different cooling rate. Cooling rates have been determined experimentally by measuring the Dendritic Arm Spacing (DAS) and Secondary Dendritic Arm Spacing (SDAS) from the microstructure of different steps. Results show that the morphology of graphite, dendritic arm spacing and secondary dendritic arm spacing as well as the interlamellar spacing of eutectic structure depend on the casting thickness. These decreases as the thickness of castings decrease because thinner section of casting has higher rate of cooling than the thicker section. The estimated cooling rate matched well with the experimentally measured cooling rate.

Abstract: Investment casting has emerged as the foremost casting process for manufacturing of intricate parts where better dimensional accuracy is required. Use of rapid prototyping polymer patterns is one of the major advancements in investment casting process. Elimination of die making step as required in the traditional wax process makes it quicker and a cost effective process. The direct conversion of 3D CAD data into rapid prototyping pattern decreases development time, chances of costly mistakes, minimizes sustaining engineering changes and extend product lifetime by adding necessary features and eliminating redundant features early in the design.