Welding gas bottles

A trailing shield is essential for welding titanium or titanium alloy. In this method of welding certain amount of oxygen and nitrogen will be picked up from air by the weld.The pick up of these gases lowers the fracture toughness of the welds. For aerospace applications welding in open air is not allowed. Hence we used a chamber for welding.The welding chamber is a plexiglass dome mounted on a steel base.The chamber is connected to a vacuum pump which can evacuate the chamber at a fast rate.There are several feed throughs which allow current tobe carried to the welding torch, a welding positioner,an electric lamp etc. and a system of valves for gas inlet,gas outlet and evacuation of the chamber. The welder stands outside and performs welding through a glove port.
The chamber is first evacuated by a vacuum pump to a low pressure and then back filled with high purity argon.This argon filled chamber is again evacuated and high purity argon is filled for the second time.The chamber is maintained at a positive presuure by the flow of a small quantity of gas. The welder stands outside the chamber and manipulates the welding torch through a glove port and carries out welding
The machined hemispheres are sent to the metrology department to check uniformity of thickness and contour. If these are within the permissible limits,they are taken up for welding. Before the spheres are welded they are thoroughly degreased and the weld edges are etched with a solution of HF-HNO3. The welding wires are also degreased and etched with the above etching solutions.
Assembled hemispheres, filler wire and the other tools for welding are placed in the chamber before the start of the evacuation.To remove any residual oxygen and nitrogen in the chamber the welder welds on a scrap titanium plate. The welded titanium plate acts as a getter absorbing the residual gases, if any ,in the chamber. The welding operator standing out side lays the root bead by manipulating the welding torch.When the root bead is finished, the welded sphere is taken out and sent for radiography. If the radiography is cleared the sphere is taken for further welding in the chamber. For higher thickness spheres one more radiography test is performed at the mid thickness of the weld. These tests are done to detect any defects and rectify them and not wait till the sphere is completely welded. Specification allows rectification of the defects only two times. After the welding is completed the job is allowed to cool in the chamber. Then the job is taken out and sent for final radiography.If the bottle has passed the radiography test bottle is sent for hydraulic testing.