Thixo-formability of the AA6063 alloy: Conventional production processes versus electromagnetic stirring

Cold-formed AA6063 alloy has an excellent mechanical behaviour, and is used in a wide range of structural applications. In Brazil this alloy is produced in large amounts, due to wide application at low cost. The present work aims to present a new use for this material, allowing the subsequent manufacture of metal parts by thixo-forming. The objectives of this work are (a) to establish a suitable condition for the production of thixo-formable structures using electromagnetic stirring during semi-continuous casting, with and without grain refiner, and (b) to compare this to the same alloy produced by the conventional process. The alloy was supplied by Companhia Brasileira do Aluminio (CBA), and processed in a semi-continuous casting equipment at the Thixo-Forming Laboratory of Unicamp, which has a controllable electromagnetic field from 600 to 2000 W/kg of material, induced into the alloy during solidification. The final alloy structure was characterized in terms of grain size, primary particle size and shape and rheological behaviour for 45% solid fraction. Furthermore, results were compared to the chemical refining (Al–1 wt%Ti–0.5 wt%B), as well as the combination of both production methods. According to the thixo-formability evaluation protocol, the commercially produced AA6063 alloy has an excellent degree of thixo-formability, presenting a very low-apparent viscosity (105 Pa s), despite the difficulty of its processing, i.e., a very narrow semi-solid range (only 23 °C). In fact, the most important feature for thixo-formability is to have raw material presenting a very refined structure, as well as, the most globular structure. In this case, the commercial alloy presented grain size of 86 μm and rheocast quality index 0.60. For commercial processing better equipment and a better temperature control will be indispensable in the commercial development of thixo-forming operations for this type of material.