Walman Benício de Castro

Current transformers with nanocrystalline alloy toroidal core: analytical, computational and experimental studies

In this paper are presented theoretical analysis and experimental results concerning the performance of toroidal cores used in current transformers. For most problems concerning transformers design, analytical methods are useful, but numerical methods provide a better understanding of the transformers electromagnetic behaviour. Numerical field solutions may be used to determine the electrical equivalent circuit parameters of toroidal core current transformers. Since the exciting current of current transformers alters the ratio and phase angle of primary and secondary currents, it is made as small as possible though the use of high permeability and low loss magnetic material in the construction of the core. According to experimental results presented in this work, in comparison with others soft magnetic materials, nanocrystalline alloys appear as the best material to be used in toroidal core for current transformers.

Deposition of thin film of titanium on ceramic substrate using the discharge for hollow cathode for Al2O3/Al2O3 indirect brazing

Thin films of titanium were deposited onto Al2O3 substrate by hollow cathode discharge method for the formation of a ceramic-ceramic joint using indirect brazing method. An advantage of using this technique is that a relatively small amount of titanium is required for the metallization of the ceramic surface when compared with other conventional methods. Rapidly solidified brazing filler of Cu49Ag45Ce6 in the form of ribbons was used. The thickness of deposited titanium layer and the brazing temperature/time were varied. The quality of the brazed joint was evaluated through the three point bending flexural tests. The brazed joints presented high flexural resistance values up to 176 MPa showing the efficiency of the technique.

Effect of Spun Velocities and Composition on the Microstructure and Transformation Temperatures of TiNi Shape Memory Ribbons

Ti‒50.13Ni and Ti‒49.62Ni (at.%) shape memory alloy ribbons were fabricated by melt-spinning method at different circumferential wheel velocities. The effects of wheel velocity, chemical composition and heat treatments on microstructure and Transformation temperature were investigated. Differences in wheel velocity led to differences in cooling rate and sample dimension, as well as in phase transformation temperatures. Two heat treatment conditions were studied, 350°C for 1h and 350°C for 5h. In the samples produced at high wheel velocity and heat-treated at 350°C for 5h, nanosized Ti-rich precipitates were observed in both chemical compositions. Cross-sectional microstructure was studied by optical microscopy; SEM was used to study the nanometric grains and nano precipitation. The transformation temperatures were analyzed by DSC.

Brazing of Al2O3 with Rapidly Solidified Melt Spun Ni-Cr-P Alloy Ribbons

Melt spun Ni-Cr-P alloy ribbons were used as filler metal to join Al2O3 substrates together using a vacuum brazing process. The influence of the brazing load, brazing temperature and holding time on the shear resistance of brazed joints was evaluated. The experimental results showed that the holding time was the most important parameter. At a brazing temperature of 1060°C, the shear resistance value increased from 6.17 to 93.7 MPa with increasing brazing time from 20 to 100 min, respectively.

INFLUÊNCIA DA TAXA DE RESFRIAMENTO NAS TEMPERATURAS DE TRANSFORMAÇÃO E NAS MICROESTRUTURAS DE LIGAS TiNiCu COM EFEITO DE MEMÓRIA DE FORMA SOLIDIFICADAS RAPIDAMENTE

* Contribuição técnica ao 70o Congresso Anual da ABM – Internacional e ao 15o ENEMET Encontro Nacional de Estudantes de Engenharia Metalúrgica, de Materiais e de Minas, parte integrante da ABM Week, realizada de 17 a 21 de agosto de 2015, Rio de Janeiro, RJ, Brasil. INFLUÊNCIA DA TAXA DE RESFRIAMENTO NAS TEMPERATURAS DE TRANSFORMAÇÃO E NAS MICROESTRUTURAS DE LIGAS TiNiCu COM EFEITO DE MEMÓRIA DE FORMA SOLIDIFICADAS RAPIDAMENTE*

Evaluation of the mechanical properties of ceramic joints using amorphous ribbons as filler metals

The objective of this work is to determine the best parameters for brazing ceramic joints of pre-metallized Al2O3 with Ti by a plasma process using amorphous ribbons of Cu49Ag45Cex alloy as the addition metal. The alloys were prepared in an arc oven, and later processed by melt spinning, varying Cerium content by 4–6%. Brazing took place in a vacuum oven and the following variables were analysed: deposition time of the Ti film and temperature and brazing time, which were related to the flex resistance in three points of the brazed joint. A linear regression equation was obtained, and the interaction between these factors was verified. The metallized ceramic surfaces showed excellent uniformity and the brazed joints demonstrated very good adhesion, achieving flex resistance values of up to176.8 MPa.

Influence of the particle size on phase transformation temperatures of Ni-49at.%Ti shape memory alloy powders

It is important to control the martensitic transformation start temperature (Ms) of Ti–Ni alloys because it determines the temperature range over which the shape memory effect and superelasticity appear. Powder metallurgy (PM) is known to provide the possibility of material-saving and automated fabrication of at least semi-finished products as well as net-shape components for NiTi alloys. In this study powder with different particle sizes was subjected by gas atomization. The evolution of the control the martensitic transformation start temperature (Ms) was studied by differential scanning calorimetry. The effect of the particle size of powders on the transformation temperatures behaviors was discussed.

INFLUÊNCIA DO SUPER-RESFRIAMENTO NAS LIGAS Cu-Al-Ni COM EFEITO DE MEMÓRIA DE FORMA.

In this work, 83%Cu - 13%Al - 4%Ni (percentage in weight) alloy was evaluated in 3 experimental conditions: a) as-melted, b) as casting quenched and c) flux solidified. Effects of the undercooling phase structures were investigated. Compared with the The as-melted alloy presented grains containing precipitate particles inside of the grains, without martensite morphology and the contour of those grains were defined by the segregated particle. The alloy in the as-melted quenched presented grains with internal martensite morphology and contours without precipitate. The flux solidified alloy presented grains with martensite morphology and without precipitate.