Desenvolvimento e validação de um sistema GISS-squeeze casting aplicado na liga de alumínio A380

Abstract

Aluminum alloys are widely used in the automotive, aerospace, electronics, and medical industries because of their strength, light weight, and recyclability. To enhance the properties of these alloys, the technology of semisolid metal processing is one of the most widely used and researched methods worldwide. One of the advantages of this process is the ability to reduce surface defects, shrinkage, and porosity compared to conventional casting processes. An innovative alternative has been the use of the Gas-Induced Semisolid (GISS) process, which involves re-melting the material in the semisolid state using inert gas bubbles to stir the material during the solidification process. The GISS-Squeeze Casting process combines the GISS process to obtain the material in the semisolid state, followed by squeeze casting, which provides better mechanical properties compared to conventional compression casting. Studies on the GISS process have primarily focused on the A356 alloy, showing significant improvements in microstructure and mechanical properties. However, the application of the GISS process to the A380 alloy has been less investigated, and the combination of this process with compression casting has been even less analyzed due to the technical challenges that require precise control. This dissertation aims to develop and evaluate, through the analysis of microstructure and mechanical properties, a GISSsqueeze casting system applied to the A380 aluminum alloy. During the GSC process, the alloy was melted and heated to 630°C, and then slowly cooled to 610°C. The porous graphite diffuser was quickly immersed, introducing fine argon gas bubbles at a flow rate of 12 L/min for 15 seconds (approximately 10% solid fraction). The diffuser was then quickly removed, and 69 g of material was poured into a cylindrical H13 steel mold preheated to 200°C. Subsequently, a pressure was applied for 30 seconds, resulting in a conical cup. The samples were cut, polished, and electrolytically etched with a 4% HBF₄ solution for 70 seconds, then analyzed under optical microscopy and subjected to hardness and porosity tests. The results showed that the GISS process is sensitive to changes in operational parameters. Increasing the argon injection time improves the spheroidization of the microstructure up to 25 seconds, achieving a 54% reduction in porosity. However, longer injection times lead to particle coalescence and further reduce porosity. Raising the argon flow rate decreases the grain size and also enhances hardness, making it a key parameter for controlling porosity in the process. The addition of refiners and modifiers such as Al5Ti1B + Sr does not improve the performance of the GISS process in grain refinement. For an initial temperature of 610 °C, a flow rate of 12 L/min, and an injection time of 19 seconds, a grain size reduction of 62.8% is observed compared to the as-cast condition. The use of the GSC process reduced porosity by 61.5% when compared to the SC process in the as-cast condition, by 70.6% with the addition of Al5Ti1B + Sr, and by 78.1% compared to the isolated application of GISS.