Zirconium Bar

Shielding

Zirconium requires inert gas protection (using argon, helium, or mixtures only) of the molten weld to prevent oxygen, nitrogen, and moisture from normal atmospheres from contaminating the weld metal. Zirconium also requires inert gas protection of the solidified weld and adjacent metal surfaces heated during welding to prevent surface oxidation and related degradation.

Primary (torch) and secondary (trailing) shielding. Primary shielding must protect the weld metal until it solidifies. Air contact with the molten weld pool will contaminate the entire volume of molten metal and result in contamination of the weld bead. This is why a large ceramic gas cup and a gas lens are among the most important elements of successful zirconium welding.Avoid excessive gas flow rates, because they can cause turbulence and will waste argon.

Torch shielding is sufficient for applications in which the heat input is low and the torch gas itself can blanket the front and back of the tack, such as short tack welds. Multipass tacks and long or heavy tacks, however, always require backup shielding.

Secondary shielding blankets the solidified weld and adjacent hot metal with inert gas until the surface temperature reaches 600 to 800 degrees F.

Backup shielding. Backup shielding protects the molten root of the weld on the first pass and blankets the solidified weld during subsequent passes until the root surface temperature stays below 600 degrees F. Backup shielding also should be used on the back surface of sheet or plate less than 1/4 in. thick to prevent oxidation.

Gas requirements. Welding-grade argon (99.998 percent purity) usually is chosen for primary, secondary, and backup shielding, as well as for purging. Argon provides excellent arc stability, and because it is heavier than air, it blankets the weld and provides protection.

Helium or argon/helium mixtures sometimes are used for torch shielding if added penetration is desired. Argon and helium mixtures also are occasionally employed for backup shielding and purging, in which helium's low density can effectively purge blind spaces.

Gas purity. Argon usually is purchased to 99.998 percent purity with a guarantee of less than 5 parts per million (PPM) oxygen, moisture, or total hydrocarbons. The dew point at the gas supply should not be more than -60 degrees F (-51 degrees C).

Flowmeters and timers. Each gas flow, including purge flows, must be supplied through a separate flowmeter. On/off valves or prepurge and postpurge timers can reduce gas consumption by automatically shutting off or reducing gas flow between welding cycles. Interlocking gas flows with welding current is desirable so welding cannot be started until all shielding gas has purgedthe torch lines.

Gas hoses. Use clean, clear plastic gas hoses (PVC, vinyl, Teflon®, polypropylene, or high-density polyethylene) to eliminate the air permeability that occurs with rubber. Gas connections must be tight, but barbed connectors without clamps often are adequate. Keep hoses off the floor as much as possible to keep them clean and otherwise protected and to minimize the risk ofpunctures or leaks.

Trailing and backup shielding devices. Shields are designed to prevent velocities at the gas inlet that can entrain air. Using two- or three-stage reduction of gas pressure and flow from the inlet tube, the shields provide a gentle, uniform blanket at the surface.

A shield can be made of copper or stainless steel, soldered or welded to form a light, compact body. It may be attached directly to the welding torch and fits closely around the torch cup, with a flat baffle filling the space between them. The gas inlet is above an internal baffle to eliminate velocities that can entrain air into the device. A packing of coarse stainless or bronze wool, atleast 1 in. thick, provides a gradual reduction of pressure and creates a uniform blanket of gas. The final diffuser can be porous (sintered) bronze, stainless steel, or even a copper plate with a uniform grid of small perforated or drilled holes or both.

A typical shield uses about 7 to 10 cubic feet per hour (CFH) per square inch of surface, so the 1-1/2- by 3-in. shield in Figure 6 would be expected to use 30 to 40 CFH. As with the welding torch, excessive flow rates can cause turbulence or air entrainment. To check a shield's performance, hold it 1/4 to 1/2 in. above a zirconium surface, and then heat the surface from the opposite side andcool it with the shield in position.

High-current applications, particularly with automatic equipment, may require a shield that is 3 to 4 in. wide and 12 or more in. long. While the use of skirts (typically high-temperature tape) around the shield is common, they may not be necessary with a properly designed shield.

Manually positioned backup shielding devices follow the same design principles as trailing shields. They are often contoured to match inside diameters or surfaces.

Copper backup bars typically are used in seam welding machines and some production applications. A typical backup bar is 1 to 2 in. wide and has a gas groove of 1/8 to 3/8 in. wide and deep under the weld groove. A wider groove usually is used for heavier material.

A sealed gas distribution channel inside the bar and along its full length is used to provide uniform pressure and gas distribution along the length. The distribution channel is connected to the gas groove with a series of small orifice holes 4 to 6 in. apart along the full length. The material to be welded must be clamped tightly to the gas groove, preferably with a full-length hold-down.

It is common practice to use tape to seal the weld temporarily while initial purge is established, but usually you can remove the tape a longer distance ahead of the torch because the full-length gas distribution eliminates the problems encountered when a space is purged from just the weld start end.

Maintain a continuous argon flow to sweep away any material released from the zirconium surface by the welding heat and to ensure a constant, slight positive pressure. Argon is a poor heat conductor, so welded parts can become quite hot and may discolor slightly as the zirconium adsorbs minor oxygen remaining in the argon atmosphere.