José María Gómez de Salazar

Diffusion Welding of WC-Co (Hardmetal)/ High Strength Steels

The present work shows the microstructural and mechanical results obtained in joints WC-Co / steel 90MnCrV8 (high strength steel). The joints were carried out using diffusion welding processes and soft interlayers Cu-Ni. The binary Cu-Ni alloys were obtained by electrochemical deposition techniques. The microstructural and mechanical changes provoked in the steel, due to heat treatments, were studied in order to know plastic deformation during the joints. The diffusion welds were obtained in an Edwards vacuum furnace (10-3 - 10-4 Pa), three welding temperatures were used, (825 °C, 850 °C and 875 °C) and 30, 45 and 60 minutes, as welding times. The constant pressure was 5 MPa. The welding interface microstructure was studied by scanning electron microscopy (SEM-EDX). In order to determine the mechanical behaviour of all joints were carried out shear tests. The optimum results were obtained at 850 °C / 45 min and 850 °C / 60 min. Nevertheless, the diffusion welding conditions allowed attaining high quality welding interfaces in all joints of WC-Co / 90MnCrV8

Modelling of Voids Closure in the Diffusion Bonding Process

Diffusion bonding is a solid-state welding process that allows the joining of similar or dissimilar heterogeneous materials preventing the weldability problems associated to fusion welding. Modelling of this process has been attempted by several authors like Hill and Wallach [1]. These authors considered that the plastic deformation between two surfaces in contact is one of the most important mechanisms involved in the process. This paper reports the results of diffusion bonding of 1045 steel. Modelling of the process was done using Hill and Wallach [1] parameters to define the surface roughness condition. During preparation for diffusion bonding a series of long parallel ridges, typically with 0.2 to 2 µm high (roughness asperity height) and 30-70 µm width (roughness wavelength) were produced. The initial contact of the asperities on the prepared surfaces created a series of voids with an elliptical shape (infinitely long parallel cylinders with elliptical cross-section). During the diffusion bonding process the shape of the voids changes, becoming smaller, mainly due to the pressure application. Since this is a diffusion controlled process, the use of temperature promotes the deformation process. A model using Finite Element Analysis coupled to a commercial software was developed considering two surfaces, containing an half void, which were brought into contact. The first model was developed with only one elliptical void with two different widths (30 and 70 µm) and 2 µm high. In this model enough pressure was applied to close completely the void. The second model considered three voids placed together to simulate the voids continuity. Finite Element Analysis was developed considering that initial contact between surfaces with asperities creates a series of voids with elliptical shape. In the initial stage of the process, the pressure applied changes the shape of the voids due to plastic deformation increasing the contact area. Microstructures were investigated.

Synthesis of SnO2 by Sol-Gel Method

Nanostructured SnO2 was prepared based on the sol-gel method used in the preparation of crystalline metal oxides. Sol-gel process can be described as a forming network of oxide polycondensation reaction of a molecular precursor in a liquid. Six experiments were carried out. Morphological structures and chemical composition were examined by scanning electron microscopy (SEM) and transmission electron microscopy (TEM) after calcination. It is noted that TEM images show that the spheres consist from nanocrystals, quantitative EDS analysis of the chemical composition shows an absence of the chlorine, which is a desired fact. For structural characterization of the material we used X-Ray Diffraction (XRD). The X-ray diffraction pattern for all samples indicates peaks which are agreeable with standard diffraction pattern of SnO2. The particle size of all samples was in the range of 28-92 nm calculated according to Scherrer equation.