Dragoş Buzdugan

Mechanical Behaviour of Nickel Based Amorphous Alloys during Tensile Test at Different Strain Rates

Nickel based amorphous alloys have been developed in ribbons form by planar flow casting method and were subjected to tensile tests. For this purpose, specimens with calibrated area were obtained by electroerosion cutting method and tested at tensile test with different strain rates. Following the results obtained during the tests, the mechanical properties have been determined and compared and the fracture behaviour has been analyzed by scanning electron microscopy.

Induction Brazing of Copper Parts Using Amorphous Brazing Alloys

Brazing alloys with amorphous structure have excellent technological properties (tensile, wetting), having a self-fluxing character. The brazed joints have good mechanical and corrosion resistance. In this work an amorphous alloy, ribbon form, for brazing of copper pipe components, was elaborated. The brazed joints were analyzed by optical and electronic microscopy. The brazing alloy is self-fluxing and has good tensile and wetting properties. The structure in the brazed joint is monophasic due to the optimization of the process parameters (voltage and brazing time).

Modelling of Resistance Brazing Process Using Factorial Design

The actual state of technological development needs to extend the brazing field of stainless steels and non-ferrous alloys based on Cu, Al, Ni, Co, Ti in leading industries, such as aerospace, energy, chemical, etc. The brazing process is influenced by a large number of parameters, which affects the quality of the joint. This paper presents a mathematical model of resistance brazing process of copper-stainless steel using a factorial experiment. The parameters considered important and which will be analyzed during this experiment are the brazing current, time and force. The output data is given by the quality of brazed joint, represented by shear strength. The objectives of this experiment are establishing the Pareto chart of the effects, the main effects of the parameters, how the response changes during the experiment when each of these factors is changed, and process optimization.

Numerical Model of Thermal Field Developed in Fe67Cr4Mo4Ga4P12B5C4 Bulk Amorphous Alloy Processing

Metallic amorphous materials were developed during 80’s as new materials, with very interesting industrial properties (heat conductivity, magnetic properties, fusion temperature, corrosion resistance, etc.). Technology to obtain these materials, based on very rapid cooling of a melted alloy with glass forming ability, has limitations for the dimensions of the products that can be obtained with amorphous structure (thickness has to be very thin), which can be overpassed by development of bulk amorphous alloys with high glass forming ability and good control of the cooling speed. Numerical modeling of thermal field during ultra-high cooling, developed in researches presented in this paper, allows researchers to estimate the results of applying in reality certain cooling conditions. This model will help developers of bulk amorphous alloys in checking if are ensured conditions to obtain an amorphous alloy with fewer experimental tests, less time and low expenses.

Crystallization of Ni-Cr-Si-B and Ni-Cr-Fe-Si-B Amorphous Alloys

Ni based amorphous alloys with Si and B, which can also, contains Fe and Cr, prepared by rapid solidification, have low melting temperatures. This fact increases their susceptibility to be joined by welding and brazing. The glass forming ability (GFA) is conditioned also by the crystallization delay, due to certain chemical composition of the alloys. The thermal stability of these alloys was revealed by DTA analysis and structural characteristics were investigated by XRD. Applying an annealing at temperatures between 420°C and 540°C, with 30 minutes maintaining time, allowed the investigation of phase occurred during the crystallization and the estimation of the crystalline grains dimensions.

Mechanical Behavior of Fe60Co14Ga2P10B5Si3Al3C3 Bulk Metallic Glass

Development of Fe-based bulk metallic glasses (BMG) with good mechanical and soft magnetic properties has become a major objective in the materials science field. Bulk metallic glasses present an interesting combination of properties. They exhibit very high strength (both in tension and compression), large elastic elongation limit, high hardness, excellent corrosion resistance, and good soft magnetic properties. These properties makes them suitable for many applications like high resistant control cables, pressure vessels, micro-components, pressure sensors, microgears for motors, magnetic cores for power supplies and hard fibers in composite materials. Multi-component Fe60Co14Ga2P10B5Si3Al3C3 bulk metallic glass was synthesized in rod form with a diameter of 1 mm by copper mould casting technique using raw industrial materials. The obtained alloy was analyzed by X-ray diffraction (XRD), differential thermal analysis (DTA) and scanning electron microscopy (SEM) techniques, in order to determine the phase constituent, the thermal stability and the fracture surfaces of as-cast samples. The mechanical behaviour was investigated by microhardness and compression tests. The values recorded for hardness and fracture strength includes this alloy in the category of high resistant materials.

Structural and Mechanical Characterization of Hard Layers Obtained by Plasma Jet Thermal Spraying

In many countries, in the industrial practice, the hard layers deposition is used to increase the hardness of the components active zones. A viable solution to increase the imposed properties consists in the deposition of amorphous layers by thermal spraying. Ferroalloy powders were deposited by plasma jet thermal spraying on aluminium substrates in order to increase the hardness. By plasma jet thermal spraying of Fe-B, Fe-Ni-B and Fe-Cr-Mn-Mo-W-B-P-Si powders on aluminum supports, have been obtained hard layers having the thickness between 72 μm and 86 μm, and granular structures made from α solid solutions, fine particles of specific oxides and complex particles of Fe-B, Fe-B-Si, Fe-Cr, Fe-W-Mo and Fe-P. The measured hardness had the values between 383 and 391 HV1 for the deposited layers made from powders type Fe-B, the values between 410 and 420 HV1 for the deposited layers made from powders type Fe-Ni-B, and the values between 448 and 475 HV1 for the deposited layers made from powders type Fe-Cr-Mn-Mo-W-B-P-Si. No defects, such as cracks or microcracks, have been observed on the hard layers surfaces deposited by plasma jet thermal spraying.

Physical Properties of Magnetorheological Fluid Dampers

This paper presents the preparation of magnetorheological fluids (MRF) starting from iron powder in size of 4-6 µm, silicone oil and a few commercial additives. The structure and magnetic properties of iron powder are evaluated by X-rays diffractions and hysteresis curves. The MRFs were selected through gamma radiation transmission, upon the determination of the sedimentation rate in the gravitational field. The dispersion of MRFs particles is presented using the electron transmission microscopy. The magnetorheological behavior in dynamic conditions was tested in a device specially designed for that purpose

Simulation of Thermal Field in Fe73Cr2Ga4P13Si5C3 Bulk Metallic Glasses

This paper presents some results for numerical simulation of the thermal field developed during rapid cooling of iron base alloy (Fe73Cr2Ga4P13Si5C3), to obtain bulk metallic glasses. Technology of rapid cooling gives the possibility to obtain bulk metallic alloys with an amorphous structure which can be used for industrial applications like magnetic shields. Validity of the numerical model of the cooling process was confirmed by the X-ray diffraction analysis. Once the model being validate, it can be used to optimize the rapid cooling process and casting mold geometry to be able to investigate the possibility to obtain larger amorphous bulk components.

Establishing the Glass Forming Ability of Ferromagnetic Bulk Amorphous Alloys Using a Mathematical Model Based on the Chemical Composition

This paper presents a mathematical model that describes the influence of the chemical composition on the glass forming ability of ferromagnetic alloys. Glass forming ability is given by the difference between the glass transition temperature and the primary crystallization temperature of the alloy. The glass forming ability is better as long this difference has a higher value. These temperatures were determined using differential thermal analysis.