An experimental and analytical study on dome forming of seamless Al tube by spinning process

The thickness distribution of a spin formed dome in an Al pressure vessel was studied experimentally and a simple formulation to predict the thickness distribution of the dome was proposed. The ends of the seamless Al tubes, which are widely used as liners in high pressure FRP composite vessels, were closed by spin forming. The thickness of the boss part of the dome may not be sufficient after forming operations, especially for high pressure vessels in which a deep thread cut is necessary for accommodating a pressure gas valve. Thus, a two-step spin forming of the Al tube end closure was proposed to improve and thicken the boss. First, the boss was deformed to a diameter smaller then the desired one. Later the dome was modified to obtain a greater boss diameter. The second forming provided a greater and better thickness distribution around the boss. Experimental work was carried out to compare the thickness distribution obtained with the conventional process and using the proposed two-step spin forming process. The ends of Al 6061-O tubes were deformed to dome shapes at an elevated temperature using a spinning machine. A simple analytical model, which was based on geometrical shape changes and an assumption of material constancy during forming, was formulated to predict the final thickness distribution of the dome. The results indicate that the proposed two-step spin forming provides a greater boss thickness than that of conventional forming.