Advanced Materials Research Vol. 1114

Abstract: The properties of ultra-fine grained materials are superior to those of corresponding conventional coarse grained materials, being significantly improved as a result of grain refinement. Equal channel angular pressing (ECAP) is an efficient method for modifying the microstructure by refining grain size via severe plastic deformation (SPD) in producing ultra-fine grained materials (UFG) and nanomaterials (NM). The grain sizes produced by ECAP processing are typically in the submicrometer range and this leads to high strength at ambient temperatures. ECAP is performed by pressing test samples through a die containing two channels, equal in cross-section and intersecting at a certain angle. The billet experiences simple shear deformation at the intersection, without any precipitous change in the cross-section area because the die prevents lateral expansion and therefore the billet can be pressed more than once and it can be rotated around its pressing axis during subsequent passes. After ECAP significant grain refinement occurs together with dislocation strengthening, resulting in a considerable enhancement in the strength of the alloys. A commercial AlMgSi alloy (AA6063) was investigated in this study. The specimens were processed for a number of passes up to nine, using a die channel angle of 110°, applying the ECAP route B_C. After ECAP, samples were cut from each specimen and prepared for metallographic analysis. The microstructure of the ECAP-ed and as-received material was investigated using optical (OLYMPUS – BX60M) and SEM microscopy (TESCAN VEGA II – XMU). It was determined that for the as-received material the microstructure shows a rough appearance, with large grains of dendritic or seaweed aspect and with a secondary phase at grain boundaries (continuous casting structure). For the ECAP processed samples, the microstructure shows a finished aspect, with refined, elongated grains, also with crumbled and uniformly distributed second phase particles after a typical ECAP texture.

Abstract: Four compositions of some usually commercial dental alloys were investigated in order to determine the mechanical characteristics and fractographic analysis of tensile and bending tests surfaces. A correlation between chemical composition (either molybdenum or molybdenum and chromium contents) and mechanical characteristics (longitudinal modulus, tensile strength and elongation) were finally done.

Abstract: Circular saw blades are used exclusively for cut-off work, ranging from small manual feed operations, up to very large power fed saws commonly used for sectioning stock as it comes from a rolling mill or other manufacturing processes for long products. The teeth profile, as well as the tooth configuration are of fundamental importance for the blade performances; through a combination of blade rigidity and grinding wheel condition a good quality surface finish is attained for tools of commercial standard. The materials used for the production of circular saw blades are ranging from high speed steel to cemented carbides. In particular, cemented carbides, being characterized by high hardness and strength, are used in applications where materials with high wear resistance and toughness are required. The main constituents of cemented carbides are tungsten carbide and cobalt. Tungsten carbide imparts the alloys the necessary strength and wear resistance, whereas cobalt contributes to the toughness and ductility of the alloys. The WC-Co alloys are tailored for specific applications by the proper choice of tungsten carbide grain size and the cobalt content. The grain size of the tungsten carbide in WC-Co varies from about 40 µm to around 0.3 µm, the cobalt content from 3 to 30 wt%. The coarse grained hardmetals are mainly used in mining applications, the smallest grain size being about 3 µm and the minimum cobalt content 6 wt%. The grain size of tungsten carbide in the metal cutting industry, as well as for universal applications lies in the range of 1-2 µm. However, with the advent of near net shape manufacturing and thin walled components, the use of submicron carbide is growing, since their high compressive strength and abrasive wear resistance can be used to produce tools with a sharp cutting edge and a large positive rake angle.In this invited paper, a general overview on the actual trends in the choice of the best material when cutting special alloys will be presented and discussed. Based on the recent and past literature some examples of their up-to-date application, such as circular saws used to cut stainless steels and some high strength alloys, are talk over.

Abstract: The main aim of this research was to study the sintering behavior in hydrogen atmosphere of mixed powders of triuranium octoxide (U₃O₈) powder in dioxide uranium (UO₂) powders. By cold pressing the green pellets of mixed powders of triuranium octoxide in hyper stoichiometric uranium dioxide has density over 40 % UO₂ theoretical density (TD). The sintered pellets had a height diameter ratio (H / D) over one and oxygen uranium ratio (O/U) near stoichiometry of uranium dioxide (UO₂). The densities of sintered pellets were up to 97% of uranium dioxide theoretical density. The proposed treatment of sintering are function of green pellets density. Increasing the proportion of triuranium octoxide leads to a decrease in the density of the sinter pellets. The pellets made by 100 % of triuranium octoxide not resist to proposed sintering treatment. The metallographic analysis show uniform grains for sinter pellets made by the 30 % mixture of triuranium octoxide in uranium dioxide. This UO₂ sinter pellets are without surface defects.

Abstract: The SmCo₅ has three sintering stages using spheres (0.6 – 1.0 mm diameter) at temperatures between 1030 and 1200 °C. During the first stage the neck radius increases as X² ~ t for less than 1200 °C temperature, the exponent is 3 at 1200 °C. Interdiffusion is sintering mechanism for exponent of 2. Sm diffuses from inside to the surface, where it is oxidized and oxides fills the neck between the spheres. Co diffuses through the oxides. At 1200 °C the sintering mechanism is evaporation-condensation of Sm. The activation enthalpy of the first stage is 582 kJmol^-1 for temperatures above 1130 °C and 210 kJmol^-1 bellow 1130 °C, respectively. The second stage is characterized by a plateau where the neck growth is arrested. The small pores in the neck and in the sphere surface layer (formed during the first stage) shrink. When these pores disappear a continuous α-Co layer forms and the third stage starts. It is essential growth of the neck formed by dense Co layer. The law of sintering is X⁴ ~ t. The activation enthalpy (276 kJmol^-1 at temperatures above 1130 °C) closes the activation enthalpy of Co self-diffusion. This (together with the exponent of 4) suggests that the Co layer plays a role similar to that of a liquid film. Making slight changes in the chemical composition of the alloys and substituting an argon atmosphere to vacuum have no influence or stages and sintering mechanisms.

Abstract: The paper presents a study about thermal behavior of heat sink - power thyristor assembly. The models of power thyristor and the model of heat sink were modeled in SolidWorks. These models have been used in simulations. We take into consideration different conductor materials for heat sinks and different thermally conductive pastes. A thermal analysis has been performed for different places of power module on heat sink. In our study, we have taken into consideration the influence of the different materials of heat sink in heat dissipations. The results of performed simulations realized with Solid Works are presented in the paper.

Abstract: The aim of the experimental study is to determine thermal conductivity of composites as a function of volume fraction and size of aluminum (Al) particles. High density polyethylene (HDPE) were filled with Al particles that have different particle sizes, 80 nm and 40-80 μm. Nanocomposites were prepared by the melt-mixing technique at various volume fractions (up to 33%). Thermal conductivity of polymer composites has been measured by C-Therm thermal analyzer depending on the modified transient plane source technique. Thermal conductivity of HDPE/Al composites increases by increasing volume fraction of Al in HDPE matrix. It is found that size of Al particles hasn’t significant effect on thermal conductivity, thermal conductivity of HDPE/Al (80 nm) is close to thermal conductivity of HDPE/Al (40-80 μm).

Abstract: We report an investigation on the improvement of magnetic properties in crystalline silicon iron alloys. It was studied the influence of three thermal treatments on the energy losses of non-oriented silicon iron (NO FeSi) sheets, M800-65A industrial grade, at two peak magnetic polarizations 0.5 T and 1 T. First treatment was done at 200 °C for 60 minutes, the second treatment at 550 °C for 30 minutes and the third treatment at 700 °C for 10 minutes. In the case of grain-oriented silicon iron (GO FeSi) sheets, 30Z140 industrial grade, was analyzed the influence of laser scratching on the magnetic properties at peak magnetic polarization 0.25 T. In the case of both steel grades is presented the reduction of the hysteresis, classical and excess losses, that occur after the applied treatments, using the concept of energy loss separation.

Abstract: Low life service of steel dies used in copper extrusion brought us to inspect several discarded copper hot extrusion dies in order to identify failures and failure mechanisms. The investigations performed are mainly hardness tests and microscopic investigations. The profiles extruded are circular, rectangular and hexagonal. It was observed that die design is not at fault; rather the die material is to blame for low life service of the tool. Discarded extrusion dies after copper hot extrusions were investigated. The failure of the dies was first observed by naked eye and the observations recorded. From the dies the ones with most plastic deformations and obvious signs of failure were selected. An improvement in die design could expand the service life, but our main concern is the material: a balance between toughness and hot strength is critical, yet this balance is hard to achieve. Other thermal processing of the die is still in question and investigations are currently being performed. At least an increase of tool service life could be achieved by thermal processing of the material to a state prior nanometric carbide precipitation between martensitic plates.