Porosity Formation and Control When Laser Welding Titanium Alloys

The development of a laser welding process for titanium alloys which produce welds with acceptably low levels of weld metal porosity, with a high confidence interval, is a key step in encouraging the uptake of this technology for the fabrication of high-performance titanium alloy aerospace components. The advantages offered by this joining technology include high productivity, low heat input and easy robotic automation. This paper reviews recent research performed by TWI Ltd to understand and prevent porosity formation mechanisms when welding titanium alloys with 1µm wavelength laser sources. Three different strategies for porosity control were developed for welding titanium alloys with fibre delivered YAG lasers; a directed gas jet, a modulated laser power, and a dual focus arrangement. Characterisation of the three welding processes using high speed video and optical emission spectroscopy techniques, combined with a thorough assessment of the weld qualities produced, has allowed the effects these strategies have on the process dynamics and the formation of porosity to be determined. These techniques are compared with additional research performed using state-of-the-art Yb-fibre lasers, which are capable of being focussed into power densities exceeding 100kWmm^-2 whilst maintaining an industrially acceptable stand-off distance and depth of focus. These techniques for keyhole laser welding of titanium alloys, combined with research being performed by other research institutes and aerospace companies, should encourage the uptake of keyhole laser welding for fabricating near-net-shape high-performance aerospace components.