Avaliação da tixoconformação e desgaste abrasivo do compósito A380/NbC obtido pelo método stir casting

Abstract

Current engineering demands require materials with specific properties, which are generally not met with conventional materials. This condition leads to the need for the development of new materials in which metallic matrix composite materials stand out. The insertion of ceramic particles in the aluminum matrix contributes to improving mechanical properties, thus seeking to improve wear resistance. In the formulation of complex parts from composites, the aim is to obtain a more refined microstructure with a low level of porosity when compared to one originating from the conventional casting process. For these reasons, the use of the thixoforming technique, that is, the processing of the material in a semi-solid state has been gaining importance. However, the study of the application of this technique in the transformation process of composite materials is still poorly studied. Thus, this dissertation aims to explore the obtaining, characterization, and application of the thixoforming of the composite with the matrix of aluminum alloy A380 reinforced with niobium carbides (NbC). Alloy A380 and NbC were adopted due to the lack of research on composites. A methodology based on the stir casting technique called stir casting was developed to obtain the composite under 4 different conditions. Initially, an alloy without reinforcement was obtained to be used as a basis for comparison, and 3 others contained 5%, 10% and 15% by mass of NbC. In order to obtain a non-dendritic structure, Al-5Ti-1B refiner alloy was added. The temperature of the globularization heat treatment was established as 562 °C under a time of 90 s in all samples in order to obtain 60% solid fraction. Under this globularization condition, later the composites were submitted to the thixoforming process. The images obtained by scanning electron microscopy (SEM) and optical microscopy (OM) helped in the analysis of NbC distribution and metallographic characterization. The technique of analysis via X-ray diffraction (XRD) made it possible to determine the phases formed in the materials. As for obtaining the material, the results showed that the methodology used resulted in composites with a low porosity and good efficiency in the distribution of NbC. To verify the integrity of the material, measurements were performed regarding its porosity and density, performed before and after the use of thixoforming. A reduction in porosity was obtained after the use of thixoforming, thus increasing the density. It was observed that the addition of 10% of NbC promoted a reduction of 62% in the grain size, if compared to the alloy without reinforcement. This microstructural refinement resulted in the improvement of the Rockwell B hardness values (HRB) obtained. There was a 55% increase in hardness after the insertion of 10% NbC, when compared to the unreinforced alloy. Regarding the evaluation of abrasive wear carried out based on the ASTM G65 standard, the results showed a significant reduction in abrasive wear after the progressive insertion of NbC, mainly after thixoforming.