A comparative study of limit diagram of ultrasonic-assisted and conventional hot incremental forming of Ti-6AL-4V alloy

Abstract

Electric hot incremental forming (EHIF) technique is a novel sheet metal forming method for low-formability materials like Mg and Ti-6Al-4V alloys. There are some defects in EHIF including low surface quality, unexpected sheet tear, and oxidation of the fabricated workpieces. These defects limit its widespread use. In this article, longitudinal ultrasonic vibrations of forming tools are used for Ti-6Al-4V sheets in the EHIF process. This technique led to a significant reduction of friction coefficient and improvement of workpieces’ surface quality. Also, a combination of mechanical vibrations and heat reduced the phase transformation temperature of α phase (HCP structure) to the more formable β phase (BBC structure) at 600 °C. It improved Ti-6Al-4V sheets’ formability. In this study, various tensile and compressive strains were created by manufacturing different geometries such as cone, frustum, torispherical, and five-lobe. These strains are used to compare the forming limit diagram of the ultrasonic-assisted and conventional hot incremental forming. The design of the experiment and analysis of variance were employed to identify the optimum condition for creating the largest strains. Also, effective parameters were optimized by response surface methodology. Then major and minor strains for both techniques were compared. Achieved strains in all types of geometries for ultrasonic-assisted technique were higher than those achieved by a conventional process.