Titanium is easy to weld, it's similar to stainless steel as it needs to be back purged and similar to aluminium as the oxide layer must be removed before welding. The following is a simple guide to help understand the basics of welding titanium.
Titanium is an exotic metal sort after for its low weight, high strength and corrosion resistant qualities, widely used in marine, military and aerospace applications. Titanium is almost half the weight of stainless steel, more than twice as strong and the melting point of titanium is around 1670 °C compared to stainless steel which is around 1450 °C.
Titanium is a reactive metal, at room temperature titanium reacts with oxygen to form an outer layer of titanium dioxide, this is what gives titanium its sort after corrosion resistance. This outer oxide layer must be removed before welding as it melts at a far higher temperature than the base metal. The different colours that titanium produces when heated are actually different thickness oxide layers formed when heated titanium reacts with oxygen, nitrogen, hydrogen and carbon. This is done on purpose in some cases, see the example below.
When titanium is welded shielding gas must be used all around the weld to avoid the weld absorbing oxides, becoming contaminated and weakened. Those purples and blues might look pretty but in a weld they are a sign of contamination and a weakened weld. A natural titanium coloured weld is ideal and a sign of little or no contamination. See the example below, in most of the weld the colour is raw titanium with a few purple and blue spots at the end of the welds where the still hot titanium has fallen outside of the shielding cup, the purple and blue are signs of contamination.
Below is a photo of the inside of the same section of titanium where I back purged it with 1 litre per hour (1 LPH) of argon, the welds are clearly contaminated (it's a practice piece so the flow rate was turned right down to save argon), but there was still sufficient inert gas (argon) to stop high levels of contamination where it produces an off white powder, marked in red. These few spots of excessive contamination were made by welding with out back purging.
Steps to take when welding titanium:
Prepare the titanium for welding
Remove the oxide layer (I use a dedicated flap wheel only for cleaning titanium)
Wipe down with acetone or thinners
Maintain sufficient gas coverage
Weld with a wide diameter cup
Back purge the titanium section being welded