1.5 TITANIUM WELDING

 

• Titanium as a metal

- above 885°C the material undergoes beta phase transition to body centered cubic arrangements

- melts at 1800°C

- resistance to corrosion

- high affinity for carbon

- soft and ductile when annealed

 

• Above 260°C titanium absorbs oxygen, nitrogen, and hydrogen. This causes when welding, because in excess they make titanium brittle.

 

• Titanium welding requires,

- a very clean environment with no contaminants or other materials.

- no drafts

- the correct welding equipment

 

• To eliminate unwanted gases and moisture from being absorbed, a gas shield is used on both sides of the weld.

 

• The weld must be shielded until the temperature drops below 427°C.

 

• Gas tungsten arc welding,

- gas is used to cover the tip of the torch, electrode and workpiece.

 

• The torch is,

- a split copper collect holding a tungsten electrode. A nut tightens the collet and holds the electrode. The collet also serves to conduct current to the electrode.

- tubes delivers gas to the torch, and it is channeled to the electrode in such a way as to ensure uniform coverage.

 

• Gas cups are,

- Ceramic, metals or high temperature glass is used to direct the gas about the electrode. The size typically effects the gas consumption.

 

• An optional trailing shield focuses gas on the now welded joint, to allow proper cooling time.

 

 

• The electrode stickout (or electrode extension) is the distance that the electrode protrudes out the end of the collet. A larger stickout is proportional to the energy delivered, and the size of the gascap, and it allows better visibility of the work.

 

• A gas lens can be used to focus/balance the flow of gases, it can be used without a gas cup, or with one to improve gas coverage.

 

• Gas backups are placed on the back of the weld seam, purging is used when the back of the weld is enclosed (eg tubes).

 

• Typical welding parameters,

 

 

• Joints can be prepared by machining. If torch cutting has been used, the edges must be ground to remove the by-products of the cutting torch (typically > 1/16”). After grinding, burrs should be filed off.

 

• Surface cleaning should include,

1. degassing

2. brushing with stainless steel

3. sandblast off heavy scale

 

• Welding can also be done is a sealed chamber flooded with an inert gas. The chamber can have gas evacuated, and then reflooded, or gas flow will eventually exchange air for gas.