Characterization of processed materials by electrical currents: development of equipment and applications

Dissertação para obtenção do Grau de Mestre em Engenharia Mecânica

Structural characterization of welded or processed materials is critical for a correct assessment of their properties. Microstructural analysis by optical microscopy provides information to identify the distinctive zones, their extent and the major transformations induced. For structural analysis, hardness is the simplest property to measure, providing information on the existence of ductile or brittle zones in the material. Furthermore, it can be qualitatively correlated to yield strength. However, these techniques are time consuming and destructive and, therefore, there is need for new, dedicated, preferably non-destructive, characterization methods. The same changes in properties and structure, due to welding and processing, also alter the electrical conductivity of materials. In fact, studies have demonstrated a relation between microstructure and conductivity of friction stir processed Al alloys, relating decreases in this property to the increase in the barriers to electron mobility due to the refined grain of the dynamically recrystallized zone, while the opposite occurs in heat affected zones were grain growth exists. The aim of this thesis was to apply eddy currents electrical conductivity characterization techniques to a wider range of materials, and develop equipment for an alternative electrical current evaluation method, namely, four-point probe potential drop measurements. A prototype for an automated four-point probe electrical conductivity characterization equipment was designed, built and successfully tested. Electrical conductivity characterization techniques, by eddy currents, have been applied to different metallic materials(AISI 1020, AISI 304, AZ31, Ti Grade 5, Ti Grade 2, Lead and Copper) processed by solid-state (Friction Stir Processing (FSP)) and fusion (Metal Active Gas (MAG) and Tungsten Inhert Gas (TIG)) to evaluate the effects on their electrical conductivity profiles and compare them with the hardness profiles and the microstructural features. Results show that the electrical conductivity variations that occur due to welding and processing can be related to the hardness and the microstructure of materials and, depending on the material, this relation can be direct or inverse.