Abstract
This paper aims to develop a new forming technique to manufacture a long semihollow stepped part. Traditionally, hot backward extrusion is used, but this technique is not suitable because it requires a very high forming load to act on the die and punch, especially at the contact between the punch and workpiece. As a result, the service life of the punch is very low. Therefore, a new technique to overcome this problem is needed. A combined bulging-piercing technique was proposed and developed in this research. The main concept of this technique is to bulge the part by upsetting the workpiece between the punch and counterpunch to generate high frictional contact pressure, which will help to restrain the material from sliding down into the die cavity during the piercing step. In other words, this technique utilizes frictional force at the die-workpiece interface to reduce the forming load of the punch. Finite element modeling was employed to investigate and determine a suitable level of the bulging which can reduce the forming load without generating any significantly high force to the counterpunch. Only experiments with the minimum forming load were selected and implemented to validate this concept because other conditions with high load will risk damaging the punch and machine press of the product line. The results show that this technique can reduce the forming load by almost 40% and also control a good concentricity of the part and reduce the wall thickness variation.
