GETTING STARTED WITH TITANIUM

Titanium also offers exceptional strength-to-weight ratios (45 percent lighter, yet more than three times stronger than mild steel). It is extremely corrosion resistant, which reduces its life-cycle costs, increases its service life and allows it to be relatively maintenance free. These characteristics make titanium an ideal metal for aerospace, marine, military, chemical, power generation, medical devices manufacturing and oil and gas extraction applications.

In fact, just about the only thing less-than-ideal about titanium (apart from its high cost) is that it can be very difficult to weld. The following advice will discuss the recommended steps for successfully TIG welding ASTM Grade 5 titanium (Ti-6A1-4V).

KEEP IT CLEAN
The importance of keeping titanium base metal and filler metal clean prior to and during welding cannot be overstated. Because it is a highly reactive metal, titanium will react with body oils, oils from the forming and drawing process, shop dust, paint, dirt, cutting fluids and more during the welding process and can result in weld embrittlement and failure.

Thus the welding environment should be as clean and dust free as possible, and have as little air flow as possible to avoid disrupting the shielding gas. To properly clean the base and filler metal, wear nitrile gloves dedicated to the task and begin by degreasing both components and removing any additional surface contamination by wiping the material with methyl ethyl ketone (MEK) soaked into a clean cloth. The filler metal should then be placed in an air-tight container until ready for use to avoid contamination.

Due to its reactivity, titanium easily forms a very hard oxide layer on its surface (similar to aluminum). This layer provides titanium with its corrosion resistance, but melts at a higher temperature than pure titanium and must be removed from the area to be welded. A die grinder with a carbide deburring tool or carbide file ? set to low grinding speeds and used exclusively for the task ? is needed to remove the layer of titanium oxide. Steel wool and other abrasives can contaminate the metal and lead to weld defects. After removing the oxide layer, once again wipe the area to be welded using MEK or acetone and allow it to fully dry before welding.

FIT TO BE WELDED
Like most aspects of titanium welding, perfection in preparation also applies to part fit-up. Any gap between the two parts to be welded can result in contamination of the weld by the air environment from the underside of the weld.

For this reason, and also to minimize the heat and filler metal needed, do not bevel the edges to create a V-notch joint. Once the parts have been properly formed to a tight fit, use a clamp or tooling to ensure they don?t move prior to striking an arc. Titanium under 1/8 in thick typically does not need to be preheated, but some applications will require a pre- and post-heat to ensure weld integrity.

SHIELD THE POOL
There is little room for shielding gas error when welding titanium ? .005 percent to be exact. The American Welding Society (AWS) recommends measuring shielding gas purity to ensure weld quality. Most titanium applications call for a 100 percent argon shielding gas measured to be at least 99.995 percent pure with no more than 20 parts-per-million (ppm) of oxygen and a dew point greater than ?50 to ?76 degrees Fahrenheit.

Some applications demand 99.999 percent purity. When specifications allow, a 75 percent argon/25 percent helium mixed gas can be used to improve arc stability and increase weld penetration. The gas should be set at 20 cubic ft/hour (cfh) to obtain ideal weld protection.

In addition to shielding the weld pool from contamination from above, back-purging the underside of the weld is also necessary to keep any oxygen from contaminating it from below. Allow enough shielding gas to replace the air environment ten times over in order ensure complete weld protection.

Also important to titanium success is using a TIG torch with a trailing shield. A trailing shield keeps the shielding gas over the weld longer, decreasing the potential for weld contamination while the material is above the 800 deg threshold, below which oxygen can no longer react with the titanium. These can be purchased through a welding distributor, but many companies also fabricate their own trailing shield to match the configuration of a specific application.

Finally, be sure to use a plastic hose to transport the shielding gas. A rubber hose could allow oxygen to mix with the shielding gas and contaminate the weld.

PICKING THE RIGHT FILLER METAL
For most applications, the filler metal should exactly match the titanium being welded. Some applications call for a lower yield strength filler metal in order to improve ductility between the two pieces, but any deviation from an exact match needs to be thoroughly analyzed and tested before to putting it into production. Material less than .010 in thick (.254 mm) does not require filler metal, depending on the joint configuration

EQUIPMENT SELECTION
Although titanium can be successfully welded with lesser equipment, an inverter-based power source with high frequency arc starts, remote amperage control capabilities, a post-flow timer and at least a 250 amp output is recommended for maximizing success. The power source should be set to direct current electrode negative (DCEN), which produces deeper penetration and a narrow weld bead.

Depending on the application, air-cooled and water-cooled torches can both be used to weld titanium. Air-cooled torches work well for applications under 150 amps and those requiring short welds, but they are larger and heavier than water-cooled torches. Lighter and more maneuverable than air-cooled torches, water-cooled torches excel at high-amperage applications and long, continuous welds.

A pointed, 2-percent ceriated (orange colored end) tungsten should be used and sized according to the following schedule: below 90 amps, use a 1/16 in electrode; between 90 and 200 amps, use a 3/32 in electrode; and use a 1/8 in electrode for all applications over 200 amps. A gas lens should also be used to ensure smooth and consistent shielding gas coverage of the weld.

PROPER TECHNIQUE
If all of the foregoing preparation has been carefully completed, striking an arc and successfully welding the two pieces of material together should not be much different than welding stainless steel.

Begin the weld with a freshly cut and ground, contaminant free tungsten, and allow the shielding gas to surround the weld area for a few seconds before striking an arc using the inverter?s high-frequency start feature.
The titanium weld pool is easy to form and sluggish, similar to stainless steel. Use torch and filler metal angles and torch speed similar to that for stainless steel. Be sure to use a dab technique and keep the filler metal within the shielding gas envelope. Excessive heat can cause the weld to crack, so try to minimize heat input as much as possible.

Upon completing the weld, allow the shielding gas to continue covering the weld for 20 to 25 seconds. This prevents the atmosphere from contaminating the weld while it is still above 800 deg F. Some codes require shielding gas to cover the weld until the weld is below 500 deg F, so be sure to check the application?s requirements prior to welding.

The color of the weld once it has cooled indicates the thickness of the resulting oxide layer and whether the shielding gas sufficiently protected the weld from contaminants. Table 1 provides a guide for determining the quality of the weld based on its color, but additional tests, including dye penetrant inspection, hardness testing, x-rays, ultrasonic and destructive tests, should also be used to fully evaluate the weld?s soundness.

NOW GET TO IT!
Titanium can certainly be a finicky metal to weld, but the difficulty primarily lies in properly preparing the worksite, welding equipment and material. Plus, the benefits it offers in terms of corrosion resistance and strength-to-weight ratios make it a great option for a range of applications where other metals simply won?t suffice.

Following these basic procedures and getting some practice in on some scrap material should give any competent TIG welder the resources needed to also be a competent titanium welder.

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Jack Fulcer is the product and marketing manager for Weldcraft, 2741 N. Roemer Road, Appleton, WI 54911, 920-882-6800, Fax: 920-882-6840, www.weldcraft.com.