Titanium is not difficult to weld, it is difficult to keep clean while welding it.
Depending on what you are welding, i.e., the thickness and joint details, etc. the best advice is to purge both the weld face (side from which you are welding) and the root side with argon. You need to use a trailing cup on the face side if you are using manual GTAW (TIG) to protect the solidified weld as it cools. The base metal has to be shielded until it cools to about 350 degrees F. Likewise for the root side. I suggest making a trailing cup if you can.
The joint must be cleaned just before welding. Wire brushing with a new stainless steel brush that has been cleaned by with isopropyl alcohol (90% usually works). Flush the joint after wire brushing. Remember to push the brush toward the edge of the joint and do not use a forward/backward motion. Its like using a hacksaw or file, cut on the forward stroke, lift on the back stroke. Allow all the solvent to evaporate before welding.
The welders must wear clean gloves at all times while welding and handling the titanium. Finger prints or sweat are the "kiss of death". Sweat bands are a must. Welding is best performed in an air conditioned room to lessen the likelyhood of the welder working up a sweat. The chlorides in the welders sweat or finger prints, as well as the oils and acids from fingerprints can cause cracking in the weld and adjacent base metal. Likewise, cracking can occur during post weld heat treatment if chlorides from any source are present (think of your cleaning solvents).
Wipe the filler metal with alcohol before using them. Again, think clean! If you need to abrade the filler metal rods before using them, use stainless steel wool, not regular carbon steel wool. You should be able to get stainless steel wool at a marine supply store. Soak the stainless steel wool in alcohol before using it. Must tools, wire brushes, wool, etc. has residual oils on them that must be effectively removed before using them on stainless steel (or aluminum).
Wash the bench top and any fixtures or tooling that will come in contact with the titanium with solvent before using. Think Clean! No chlorinated solvent!
The root side and face side of the weld joint must be purged to reduce the oxygen/nitrogen content to less than 0.5%. Any discoloration of the weld (other than bright silver) is an indication of improper gas coverage while welding and during cool down. Remember, the titanium should be protected with argon until the temperature is down to about 350 degrees F.
Visually examine each weld pass and check for discoloration. Light yellow straw indicates some oxygen contamination, dark straw - more oxygen contamination. Straw or worse; blue, purple, etc. is reason enough to reject the weld. Do not wire brush the weld bead before inspecting it for color. The oxide (discoloration) can be removed easily with a wire brush giving you a false indication all is well. It isn't. The discoloration is an indication the weld is oxidized and embrittled. The entire weld and HAZ must be removed and replaced. Welding over it will not fix the embrittlement! Make sure you use a bright white light source. An incandescent bulb that has a yellow hue will produce false color on the surface of the weld. Also check to see that there are no reflections from nearby surfaces such as the fixturing that can lead to false colors.
