TWI Industrial Member Report Summary 989/2011
By Jon Blackburn
Background
There is currently a requirement in the aerospace industry for a welding process capable of producing near-net-shape titanium alloy components. Such a welding process would allow significant cost savings to be made by reducing buy-to-fly ratios and increasing productivity. Keyhole laser welding is an inherently flexible fusion welding process capable of producing high aspect ratio welds (weld depth:weld width) with a relatively low heat input compared with arc welding processes. Consequently, laser welding should be considered as a primary candidate for the production of near-net-shape titanium alloy components for aerospace applications. Nevertheless, it must be determined if welds of the required quality can be produced prior to it being considered as a manufacturing technique for near-net-shape welding of titanium alloy components. Techniques (using a highly positioned jet of inert gas, modulating the laser power and using a dual beam focus technique) have been developed at TWI (Blackburn and Allen, 2009, 2010) for producing high quality laser welds in titanium alloys up to 3.25mm in thickness. These techniques were necessary since, at the thicknesses studies (2.0 and 3.25mm), welds which met stringent aerospace standards could not be produced using only a continuous-wave Nd:YAG laser beam (Beam Parameter Product, BPP, of ~23mm.mrad).
Recent preliminary research reported by TWI (Blackburn and Allen, 2010) assessing the quality of autogeneous melt runs produced when welding 3.25mm thickness Ti-6Al-4V with a high brightness Yb-fibre laser (BPP of 1.6mm.mrad), indicated that autogeneous melt runs meeting internal aeroengine subsurface porosity criteria could be produced if the laser beam was focused into a small diameter beam waist and an appropriate welding speed was adopted. The techniques developed for Nd:YAG lasers were not required to achieve this weld quality. However, the preliminary research was a relatively small study. This report details the results of research performed with two Yb-fibre laser sources, both emitting high quality laser beams, to process the most readily employed titanium alloy (Ti-6Al-4V) in thicknesses commonly used in airframe and aeroengine manufacture (2-7mm). The qualities of autogeneous melt runs and butt welds have been assessed in terms of spatter, subsurface porosity content and weld profile, and compared against relevant aerospace welding criteria.
Objectives
- Assess the quality, in terms of spatter, subsurface porosity and profile, of autogeneous butt welds and melt runs produced in up to 7mm thickness Ti-6Al-4V with high beam quality Yb-fibre lasers.
- Compare the qualities against the weld quality criteria required for airframe and aeroengine applications.