Pradeep Rohatgi

This paper focuses on casting self-healing metal matrix composites with aluminum alloy as a matrix, and Nitinol fiber as reinforcement. The shape memory effect of Nitinol fibers can cause them to shrink and narrow or close the crack in cast matrix material and the bend test restores the shape. Self-healing metal matrix composite samples were cast using (a) semi solid squeeze casting after Rapid Slurry Formation and (b) gravity die casting. Casting procedures, microstructures, and mechanical properties are reported. The cast aluminum-Nitinol fiber composite samples prepared by both techniques were cracked under tensile and bending, both types of samples exhibit a reduction in the width of the crack in the samples upon heating the sample above the transformation temperature of Nitinol. Under certain conditions, the cracks in permanent mold cast alloys reinforced with Nitinol have been completely sealed …

The use of inserts allows for the selective reinforcement of castings to improve mechanical properties, including surface hardness and wear rate. Metal surfaces that undergo wear on account of friction can be strengthened using metal matrix composite (MMC) inserts. An essential factor in the manufacture and application of compound casting involving MMC inserts has been the bond strength between the insert and alloy. This work studied fluxes (CsAlF4 and LiAlF4), preheating, and nickel (Ni) plating to improve the bond strength between A206 and A201-Al 2 SiO 5 fiber MMC. It was observed that Flux 1 (G-2004), Flux 2 (CS-2020), and Ni plating, when used with preheating the insert up to 200 C, led to the formation of a diffusion bond. The effect of fluxes and plating has been discussed, along with the impact of the insert oxide layer on the formation of diffusion bonds. The microstructure formed at the interface …

In this research, the effect of cooling rate on the microstructure, hardness and impact energy of a plastic injection mold made of X210Cr12 steel wasinvestigated. The microstructural studies showed that with increasing cooling rate, the amount of residual austenite (Ar) decreases until it is completely removed.Statistical analysis showed that the size and volume fraction of chromium stabilized with block carbides decreases with increasing cooling rate. As the cooling rate increased, bainites were observed in the microstructure. The hardness decreased via increasing the cooling rate and austenitization time due to the reduced interaction of carbides with dislocations. Martensitic structure prevented a significant reduction in hardness. These factors increased the toughness of the X210Cr12 and led to the ductile failure. Cryogenic treatment modified the structure via distribution of fine carbides into the stable lath martensite. With optimizing the hardness and toughness to withstand the impact of the die, toughness increased to 125 j and hardness decreased to 624 H.V.

Ni-B coatings are promising materials for several applications in electronics, nuclear, automotive, aerospace, petrochemical and food industries owing to their exceptional mechanical and chemical properties such as, high hardness, high wear resistance and good corrosion resistance. However, the literature indicates that there is still significant scope for further improvement in properties of these coatings in order to improve their in-service performance and/or total service life. Considering this fact, a novel Ni-B-Tl2O3 composite coatings has been developed via electrodeposition technique by reinforcing Tl2O3 particles into the Ni-B matrix. The effect of addition of Tl2O3 particles on the morphology, structure and electrochemical properties of Ni-B-Tl2O3 coatings was investigated and compared with that of the Ni-B coatings. A substantial change on surface roughness, thermal properties, crystal structure, mechanical …

In this article, analytical analyses on the coefficient of thermal expansions (CTE) of pure aluminum alloy containing hollow cenosphere particles were performed by considering particle wall thickness and porosity on the CTE of the composite. The particle wall thickness effect was expressed in terms of the ratio of particle wall thickness and its radius (t/R). A theoretical prediction of the CTE of hollow and solid cenosphere particles was made by using modified rule of mixtures. The average CTE of solid cenosphere particle (7.2 × 10–6/°C) was much closer to the CTEs of SiC and Al2O3 compared to CTE of hollow cenosphere particle. The effective CTE of porosity, calculated using the ROM, was around three times higher than the CTE of pure aluminum and one order higher than the CTE of ceramic particles. The higher effective CTE of porosity suggests the importance of controlling pore content to reduce the CTE of …

This paper reviews the synthesis, characterization, healing assessment, and mechanics of NiTi and other shape memory alloy (SMA)-reinforced self-healing metal matrix composites (SHMMCs). Challenges to synthesizing and characterizing the SMA-reinforced SHMMCs and the strategies followed to overcome those challenges are discussed. To design the SMA-reinforced SHMMCs, it is necessary to understand their microstructural evolution during melting and solidification. This requires the knowledge of the thermodynamics of phase diagrams and nonequilibrium solidification, which are presented in this paper for a model self-healing composite system. Healing assessment provides information about the autonomous and multicycle healing capability of synthesized SHMMCs, which ultimately determines their success. Different techniques to assess the degree of healing of SHMMCs are discussed in this paper. Strategies are explored to find the optimum volume fraction of SMA wires needed to yield the matrix and prevent damage to the SMA wires for the most effective healing. Finally, major challenges, knowledge gaps, and future research directions, including the need for autonomous and multicycle healing capability in SMA-reinforced SHMMCs, are outlined.

A surface alloying technique is described to increase the concentration of nickel and chromium on the surface alloyed layer of mild steel (ASTM A216 WCB grade steel) sand casting by coating the mold surface with a slurry containing nickel and chromium. The incorporation of Ni and Cr into the surface alloyed layer can lead to the composition gradients, resulting in different parts of the steel specimen undergoing phase transformation at different temperatures and times during solidification and heat treatment. This time lag in phase transformation in different parts of the surface alloyed sample induces residual stresses in the surface alloyed layer. X-Ray diffraction analysis was used to measure the residual stress of surface alloyed mild steel since this method allows for more accurate measurements of residual stress, unlike the hole drilling technique which requires removing a significant layer thickness. Samples …

Polymer Injection Pyrolysis (PIPs) can be adopted to synthesize in-situ ceramic particles within molten metal by stir-casting process. This paper investigated the effect of pyrolysis temperatures on microstructural and strength properties of in-situ aluminum matrix composites containing 2.5 vol% of SiCNO particles. In-situ composites were synthesized by stir-mixing of cross-linked polysilazane at four different pyrolysis temperatures (675–850°C) at which in-situ pyrolysis occurred and then followed by ultrasonic agitation and squeeze casting process. Microstructural data reveals that grain size and the particle size of SiCNO particles decreases with increasing the temperature of the melt at which polymer was introduced into the melt. The increase in the strength properties of the fabricated composites as compared to pure aluminum is almost 210% for the composites fabricated at 850°C while it is marginally 17% for the …