Version 1.0, August 31, 2001, Copyright, Hugh Jack 1993-2001

42.5 TITANIUM WELDING

 

· Titanium as a metal

  1. - above 885°C the material undergoes beta phase transition to body centered cubic arrangements
  2. - melts at 1800°C
  3. - resistance to corrosion
  4. - high affinity for carbon
  5. - 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,

  1. - a very clean environment with no contaminants or other materials.
  2. - no drafts
  3. - 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,

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

 

· The torch is,

  1. - 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.
  2. - 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,

  1. - 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. 1. degassing
  2. 2. brushing with stainless steel
  3. 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.

 

 

42.5.1 Practice Problems