Joining tungsten to itself or to other metals should be undertaken with caution and an understanding of the limitations involved. Tungsten can be welded to itself. However, the resulting weldment is always recrystallized and hence brittle. Even the use of a W/Re filler rod does not eliminate the brittleness of the metal adjacent to the heat-affected zone. On the other hand, if a leak-tight high temperature joint is required, welding is the only solution.

For joints that do not have to be leak-tight but require good high temperature mechanical strength, the use of either tungsten or tantalum rivets has proven quite satisfactory. Tantalum rivets, while not as strong as tungsten rivets, are much easier to head. Tungsten rivets take more skill to head, but are the most satisfactory.

If tungsten parts which are not to be exposed to very high temperatures are to be joined, brazing is preferred to welding. As long as the braze material has a melting point below the recrystallization temperature of tungsten (under 2200 °F), embrittlement can be avoided. Copper or silver alloys containing small proportions of nickel or iron are suitable for these applications.

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Inverters Add Key Benefits:

A quick review is in order. About 30 years ago, Miller Electric invented and patented the squarewave AC output and adjustable balance control function found in its line of Syncrowave® TIG welders. This technology made the transition through the zero amperage range faster than a regular sine wave, which improved arc starts and created a more stable arc. With balance control, the operator could change the duration of the AC half cycle, adjusting the electrode negative (EN) from 32 up to 68 percent.

Today, the next generation of this technology - AC TIG inverter - features three advanced squarewave technology capabilities.

First: The AC TIG inverter produces incredibly smooth, stable arcs because the squarewave is driven through the zero point thousands of times faster than a rectifier-based welder. Miller's Dynasty® inverters are so fast that its built-in, high frequency capabilities are used for arc starting only. They also offer Lift-Arc™ starting capabilities to eliminate the use of high frequency completely, even on AC.

Second: Inverter-based welders extend EN balance control. The Dynasty lets operators fine-tune duration times from 50 percent to 90 percent. Making the EN portion of cycle last longer achieves greater penetration; narrows the weld bead; increases travel speeds up to 20 percent; may permit using a smaller diameter tungsten to more precisely direct the heat or make a narrower weld bead; and, reduces the size of the etched zone for improved cosmetics.

Third: Inverter-based welders let operators adjust the welding output frequency, from 20 to 250 Hz in the case of the Dynasty. Conventional welders have a fixed output of 60 Hz. Lowering the frequency produces a broader arc cone, widening the weld bead profile, which is great for build-up work. Increasing frequency above 60 Hz produces a tight, focused arc cone. This pinpoint heat control drives more heat into the weldment for better penetration and a narrower weld bead, which helps when welding in corners, on root passes and fillet welds or anywhere precision welding is required.

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Tungsten can be machined by turning, milling, or drilling. For thick-walled parts, slight preheating of the work to 400 °F is sometimes helpful; otherwise highly chlorinated oil such as trichloroethylene is used. Using a C-2 Type Carbide Tool, a suggested Geometry is:

• Side Angel: 10°
• End Cutting Edge Angle: 3° to 10°
• Back Rake: 0°
• Clearance: 3° to 5°
• Nose Radius: 1/32" to 1/16" Radius
• Cutting Speed: 100 to 200 RPM

The final shaping of tungsten parts is frequently done by grinding. Silicon carbide wheels with a hardness of J to L and a grain size of 100 to 120 have proven useful for most applications. Since tungsten is hard and brittle, surface checking can be a problem unless adequate cooling is used. An oil emulsion type coolant is recommended.

For as long as anyone can remember, the prevailing wisdom was that in order to get the best results, anyone who was TIG welding aluminum, magnesium or other alloys that required an AC arc had to use a pure tungsten electrode.

But not anymore. For the last five years the industry has made a dramatic shift toward inverter-based AC TIG welders that incorporate advanced squarewave technology to improve weld quality, increase travel speed and reduce operating costs.

However, in order to gain the maximum benefits from inverter technology, selecting the right tungsten now becomes an increasingly more critical issue. In the past, pure tungsten was the only choice for AC TIG welding. But that's no longer the case with the latest inverter technology. Today, industry experts recommend against pure tungsten in favor of the new "rare earth" tungstens. By adding elements such as cerium or lanthanum these electrodes outperform pure tungsten.

Tungsten Manufacturer & Supplier: Chinatungsten Online - http://www.chinatungsten.com
Tel.: 86 592 5129696; Fax: 86 592 5129797
Email: sales@chinatungsten.com
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