Thixoforging of Ultrasound Treated 6060 Aluminum Alloy

Abstract

In the last years researches on thixotropic materials have been developed in order to introduce this new technology in manufacturing processes. For instance, when considering high pressure die-casting, several applications are present in literature mainly related to low melting point alloys (Al and Mg) because of the limited die life experienced when casting higher melting materials. In this case, semi-solid metal forming allows to work at lower temperature with subsequent increase in die life and reduction in production costs, combined with lower porosity level in the casting. On the other hand, in the case of conventional forging, semi-solid processing needs higher performance materials and/or coatings for the mould because of the working temperatures; however, the advantages of obtaining near net shape part in a single step, with reduced machining and finishing costs, make the semi-solid technology competitive. The present paper deals with the thixoforging of aluminum 6061 alloy, whose semi-solid feedstock material was obtained by ultrasound treatment. The application of ultrasonic waves to liquid or solidifying alloys has been already demonstrated to be an effective technique for the obtainment of globular microstructure. Along with a refining effect, ultrasound can also produce a series of beneficial effects, such as hydrogen degassing or oxide and non-metallic inclusion removal, which all improve mechanical properties of the component. The aim of this research was to investigate the influence of process parameters on final forged part quality. The solid fraction percentage as a function of temperature was measured by differential scanning calorimetric analysis. The geometry of the die was properly designed and optimized by FEM simulation in order to be suitable for forging semi-solid material, allowing a comparison with conventional forging process. 14 K-type thermocouples were used for monitoring the temperature of top and bottom dies; an instrumented 100 ton press was also equipped with load cells to acquire the forging force. A deep metallurgical analysis of the forged parts was performed in order to evaluate their mechanical properties and quality.