TWI Industrial Member Report Summary 967/2010
By J E Blackburn and C M Allen
Porosity in laser welds is of concern in machined components subject to high stress levels or cyclic stresses. After machining, pores within the welds can break the surface and potentially act as stress concentration sites, shortening fatigue life. Conversely, undercut at the weld face and/or root is of concern if the weld bead is not machined. Porosity in laser welds is primarily a result of unsuitable welding parameters, incorrectly prepared material and inadequate inert gas shielding. If porosity criteria specified in laser welding standards could be met consistently, particularly in aluminium, titanium and nickel alloys which are more prone to porosity, this would lead to the wider use of laser welding in high value components. Recent work (Blackburn and Allen, 2009) has demonstrated that a directed gas jet can be used to significantly reduce porosity levels when neodymium-doped yttrium aluminium garnet (Nd:YAG) laser welding titanium alloys to levels within those required by international standards. Nevertheless, as the gas jet requires very precise alignment to work effectively, it is questionable if this is an industrially robust approach.
This project has concentrated on two other methods for producing high quality laser welds in titanium alloys: laser power modulation and twin spot welding. During the course of this project TWI also took advantage of limited access to a multi-kilowatt, high beam quality Ytterbium (Yb) fibre laser. Trials were carried out on a titanium alloy with this relatively new type of laser and preliminary results are included.
- Develop Nd:YAG laser welding routines for titanium alloys, resulting in butt welds to at least quality Class A of AWS D17.1, without having to use a directed gas jet.
- Assess the potential of using a high beam quality Yb-fibre laser for welding titanium alloys.
- Use high speed video imaging to observe the differences in the keyhole and weld pool behaviour when laser welding with either a twin spot configuration or a modulated laser power.