TWI Industrial Member Report Summary 900/2008
By A Kostrivas, A Plewka, G B Melton and L S Smith
The production of high quality, welded titanium components can be expensive due to the inherent incompatibility of many welding processes with the material. This has resulted in TIG and electron beam welding being the most applied joining processes. It might be assumed that MIG welding would be more common, but historical difficulties have marginalised this process so that it is only typically used for the lowest quality applications (appliqué armour, for example). Recently, Daido Steel has produced titanium wire that is claimed to result in greater arc stability and reduced porosity and spatter. This has been achieved by the use of a novel wire production process that modifies the wire surface, and the use of optimised pulse parameters.
As with any titanium fusion welding process, porosity can arise from entrapment of shielding gases ('intrinsic' to the process) or from gas bubble formation from absorbed surface contaminants ('extrinsic' to the process). Whilst a more stable arc will reduce spatter and mitigate intrinsic porosity, the cleanliness and preparation method of the mating surfaces and the time lapsed between surface cleaning and welding will have the greatest influence on the extrinsic weld metal porosity of titanium welds.
In order to assess the quality, specifically weld spatter and weld metal porosity, of MIG welded titanium, the present programme considers conventional and novel wire for fabricating commercial purity titanium. Various joint preparation methods were employed in an attempt to differentiate between the intrinsic and extrinsic weld metal porosity and so make a more informed assessment of the 'quality' of the welds. In addition, the fume and ozone generation for the process was measured, to determine compliance with modern health and safety legislation. This is particularly important since prior to this work there was no published information on the fume or ozone generation for titanium welding processes.
- Determine if a novel titanium wire addresses the quality concerns previously associated with MIG welding of titanium, namely weld spatter and porosity, by comparing welding with the novel wire directly with a conventional wire,using different Ar/He shielding gas mixtures.
- Determine the influence of joint face preparation method on porosity and provide greater confidence in distinguishing between intrinsic and extrinsic porosity.
- Produce data for fume and ozone generation for MIG welding of titanium using the novel and conventional titanium wires.