Papers by Author: H. Bolvardi

Abstract: Plasma nitriding is a powerful process for surface modification of different materials. In this study, plasma nitriding is applied on a Nickel-Aluminum composite, coated on ST37 steel. Ni+Al composites were fabricated by electrodeposition process in watts bath containing Al particles. For prediction of electrodeposited Al% during the electroplating and microhardness of coatings after plasma nitriding process artificial neural network (ANN) was used. The numerical results obtained via a neural network model were compared with the experimental results. Agreement between the experimental and numerical results was reasonably good.

Abstract: Bronze alloys are widely used in different tribological applications including sliding components. Bronze-Alumina composites have been attracting the interest of researchers in recent years. Among the most recent surface modifications techniques, plasma spraying has been successfully applied to these coatings. In this investigation, an attempt has been made to analyze the sliding wear response of Bronze-Alumina coats over a range of applied pressures and sliding speeds. It was observed that an increase in applied load causes more weight losses in wear test while increasing the sliding speed led to no further losses at this variables range. Wear processes and mechanisms of these behaviors are discussed in the light of the microstructural observations combined with wear surfaces and debris particles examinations.

Abstract: Over the last decades, numerous investigations have been conducted on Nitinol properties. However, the effects of alloying elements on Ni-rich NiTi alloys have been considered less. In this research, different effects of homogenization time and cooling rate on the behaviors of Ni-42.5wt%Ti-3wt%Cu alloy were evaluated. The mentioned alloy was fabricated by vacuum arc melting method. Three different homogenization times (half, one and two hours) and three cooling media (water, air and furnace) were selected. The microstructure and hardness were examined by means of optical and scanning electron microscope (SEM) equipped with an energy dispersive X-ray (EDX) analysis and hardness test, respectively. According to the microscopic investigations, no significant changes were observed after half an hour. However, results indicate that increasing time of homogenization leads to finer precipitations and a uniform distribution of them. The various cooling environments result in the formation of two types of precipitation phases. It was seen that in the high cooling rate, the majority of precipitations consisted of Ti2(Ni,Cu) while by decreasing cooling rate NiTiCu precipitates appeared too, which affect the hardness.

Abstract: NiTi alloys containing more than 55%wt nickel undergo precipitation of Ni4Ti3, Ni3Ti2, and Ni3Ti phases during various heat treatments which could have a great effect on the chemical composition of the matrix and behavior of alloy. In this investigation, a NiTi alloy with Ti-57.5%wt nickel content, produced by vacuum induction melting in a graphite crucible, were subjected to the homogenization heat treatments in 1100oC and for various time periods (0.5, 1, 2, and 4 hours). The subsequent cooling was conducted in different cooling media (furnace and air) in order to examine the effect of cooling rate. Microstructural investigations show Ni4Ti3 particles with bimodal size distribution in furnace cooling. Differential scanning calorimetry demonstrates the correlation between homogenization time and transformation temperatures of the alloy.