Some aspects of the laser cutting and welding of a Ti–6Al–4V alloy

This thesis firstly contains a review of previous literature concerned with both the laser welding and cutting of titanium alloys. In addition a brief introduction to both laser physics and titanium metallurgy is also included in the literature review section. The experimental part of the thesis can be divided into three sections which are as follows: (1) Measurement of focussed and raw laser beam diameter was carried out using a Laser Beam Analyser. In addition the laser mode used for both welding and cutting were evaluated. (2) 1, 1.7 and 2.7mm thick Ti–6Al–4V alloy was laser welded using both a 400W and a 2kW CO2 laser in the continuous wave mode. The effect of variation in the main laser parameters upon weld morphology, microstructure, mechanical properties and oxygen contamination was evaluated. In addition further. welding studies were carried out using the 400W laser in the pulsed mode. The effect of variation in pulse parameters upon weld bead morphology was studied. It was concluded from the laser welding section that using a 2kW CO2 laser it was possible to produce full penetration welds in up to 2.7mm thick Ti–6Al¬–4V alloy sheet. The use of pulsed welding was found to enhance welding efficiency with a greater weld penetration being achieved than with an equivalent power continuous wave output. (3) Ti–6Al–4V alloy sheet up to 2.7mm thick was laser cut using a 400W CO2 laser. The effect of variation in cutting speed and cutting gas pressure upon surface condition was studied. From detailed SEM examination of laser cuts a mechanism for inert gas assisted laser cutting was postulated. The effect of laser cutting upon the adjacent microstructure and oxygen contamination of the cut edge was also studied. As well as conventional laser cutting a series of laser cuts were also produced using the "Dross Jet". The effect of this device upon oxygen contamination and gross adhesion to the underside of the cut was studied. It was found that by using the dross jet in conjunction with inert gas assisted laser cutting that it was possible to produce cut edges with levels of oxygen contamination equal to that present in a guillotined cut edge. In addition the best laser cut edges were found to be as smooth as a typical guillotined edge. It was concluded that inert gas assisted laser cutting is an excellent process for cutting thin section Ti–6Al–4V alloy. The final section of the thesis aims to compare the experimental results from the laser welding section with results generated by an existing mathematical model. The results show that the model can be used to predict weld penetration depths and HAZ widths to a reasonable degree of accuracy.