Statistical Analysis of Low Porosity Laser Welding of Ti Alloys using a Directed Gas Jet

By:  Jon Blackburn1, Chris Allen2, Paul Hilton2, Lin Li1

1 Laser Processing Research Centre, The University of Manchester, Manchester, M60 1QD, UK

2TWI Ltd, Granta Park, Abington, Cambridge, CB21 6AL, UK

The properties of titanium alloys are ideal for the service requirements of many aerospace applications. Recent fiscal and environmental pressures are stimulating the demand for lighter, more fuel efficient aircraft in the commercial aerospace industry. A direct result of this is an increase in the utilisation of titanium alloys, and the demand for titanium products is predicted to more than double over the next six years. Current manufacturing methods used to produce titanium components for the aerospace industry, such as forging, casting and machining, have buy-to-fly ratios as high as 20:1 and reduce the economic benefit of utilising titanium alloys because of their relatively high cost when compared to aluminium alloys and structural steels. This ratio can be reduced significantly by using a welding process to manufacture near net shape components.

Keyhole laser welding is a high-energy density process that produces deep penetration with a relatively low heat input. Compared to electron beam and friction welding, it is a relatively flexible process as it can be performed out of vacuum and the near infra-red laser beams of modern solid-state lasers can be easily delivered through an optical fibre. However, the formation of porosity in the weld metal is of concern when laser keyhole welding, and can occur when soluble gases dissolved in the weldpool, such as hydrogen, are rejected upon weld solidification, and when insoluble gases and/or metal vapour become trapped in the weldpool and there is insufficient time for the gas to escape before solidification. The weld quality criteria required for welded aerospace components are known to be particularly stringent and the porosity volume fraction in the weld metal is of primary concern, if welds are machined, as any consequently surface breaking pores can then act as initiators for fatigue cracks and significantly reduce the fatigue resistance of the welded joint.

Experimental work has been performed using a fibre delivered Nd:YAG laser beam to weld 3.25mm thickness Ti-6Al-4V and Ti-2.5Cu. It was observed that directing a jet of argon gas near the laser-material interaction point could significantly reduce the porosity in the weld metal. The optimum process parameters of the directed argon jet, with regards to minimising the internal porosity content and achieving an acceptable weld profile, were determined using two statistically designed and analysed sets of experiments. Several full-penetration, butt welds were produced in the titanium alloys that had internal porosity contents much lower than those stipulated in the most stringent aerospace welding standards. The directed gas jet also eliminated the undercut at the face and root of the weld. High speed imaging and spectroscopic analysis of the welding process have shown that, once correctly set-up, the directed inert gas jet disperses the formation of excited metal vapour above the keyhole and also significantly changes the hydrodynamic behaviour of the weld pool.

Figure 1: Ti-2.5Cu butt welds made without (left) and with (right) an optimised directed argon gas jet.

The above brief overview was extracted from its original abstract and paper presented at The International Congress on Applications of Lasers & Electro-Optics (ICALEO) in Orlando, FL. To order a copy of the complete proceedings from this conference click here