A container voided of air and other matter to provide a controlled atmosphere in which highly reactive metals such as titanium, zirconium and tantalum are welded. Various welding processes, such as electron beam welding, diffusion bonding and welding, and furnace brazing are performed in a vacuum.
A vacuum chamber, sometimes called a “dry box,” is evacuated by a pumping system. The chamber provides a completely sealed enclosure which allows a wide visual range so that all stages of the welding operation can be observed. It may also provide access ports for placing or removing parts without contaminating the atmosphere, and remote control devices to manipulate the weldment. Refinements of the system can allow placement of parts in all positions, multiple glove openings, automatic gas controls, and other accessories and equipment specific to the particular manufacturing requirement.
Applications- The nuclear power and the aerospace industries have applications for controlled-atmosphere welding systems. Extreme corrosion problems encountered in the nuclear industry and the need for materials with low neutron absorption characteristics create numerous applications in which such materials as zirconium and zircaloy-2 must be welded under ideal conditions.
A demand for materials with high strength-to-weight ratios and the capability to retain strength at high temperatures, such as titanium and beryllium, has similarly directed aircraft manufacturers to controlled-atmosphere welding.
A basic controlled atmosphere system is comprised of a vacuum chamber and pumping console, purification trains, power supplies, travel and rotational fixtures, air locks, fully automatic operational controls, and other accessories.
Highly reactive metals readily absorb oxygen, nitrogen and hydrogen when heated to temperatures above 316°C (600”F), which is detrimental to their mechanical and corrosion resistant properties.
For gas tungsten arc welding, the vacuum chamber is a pressure-tight vessel in which the work, GTAW torch, fixtures and power leads can be installed and which can be evacuated to a range of 0.1 to 5 microns, then refilled with helium or argon at atmospheric pressure.
The vacuum chamber atmosphere can be further purified of residual air (due to insufficient pump-down or leakage during refilling) by holding an arc for several minutes on a scrap piece of the alloy to be welded. The operation is halted when weld beads free of discoloration are observed.
Helium, argon, or a mixture of the two may be used as a shielding gas. The most important consideration in the choice between argon and helium as an inert gas for welding is their individual arc characteristics. The normal GTAW welding voltage in an argon atmosphere is from 10 to 12 volts direct current electrode negative (DCEN), and 16 to 20 volts in helium. These values are for similar arc lengths.
For this reason, when all other variables are held constant and equal, power input is greater with helium at the same welding current. It is difficult to strike an arc in helium at less than 30 amperes, whereas an arc can be initiated in argon at 10 amperes. For this rea son, it is generally necessary to use argon on thin materials to prevent excessive penetration or burnthrough.
The most common vacuum chamber is a cylindrical vessel with removable plates for loading, and for access to the operator’s protective gloves extended inside. Specifications for construction of the vessel require that it have the capability to withstand vacuums in the order of lo4 mm of mercury. As an alternate to a steel vessel, rigid or flexible plastic containers have been used successfully.
While normal design procedures for GTAW can be followed, preference should be given those avoiding the use of filler wire to reduce possible contamination from wire surface impurities.