Preparation of the blank

After deciding about the thickness of the blank, the next step was to prepare the blank for pressing.
The diameter of the hemisphere to be pressed decided the diameter of the flat plate required for forming. This diameter was further increased by about 10-15mm. The blank was cut by an oxyacetylene gas torch. When the blank is cut by oxyacetylene
gas torch two important points are to be kept in mind,namely, gas saturated layer and minute cracks that develop while cutting. Gas saturated layer increases the hardness and minute cracks can grow during pressing. Hence the blanks were machined till the gas saturated layer was removed and dye penetrant test was carried out.If no cracks are visible the blank is ready for pressing.
Elongation of Ti-6Al-4V increases with temperature.In the temperature range of 850-950 degC, the material will be in the super plastic range. Hence the blanks were heated in a gas fired furnace with excess oxygen to a temperature around 850degC.
When the blank is taken out from the furnace the blank loses temperature rapidly by radiation. When the blank is placed on the die for pressing the blank loses temperature by conduction. As the temperature falls the elongation of the material also falls. Our aim is to see that the blank does not fall below 500degC Hence to reduce conduction loss the top punch and the bottom die are heated to a temperature of 500degC approx.After ensuring that the dies have reached the desired temperature the heated blank was transferred rapidly to the press and the pressing was done without any loss of time. The pressed hemisphere was checked for approximate dimensions and was sent for shot blasting. After shot blasting dye penetrant test and ultrasonic test was carried out detect any cracks before the hemisphere went for machining.