TWI Industrial Member Report Summary 895/2008
By G Verhaeghe
The current industrial interest in hybrid laser-arc welding is not surprising when analysing its potential benefits compared with laser or conventional arc welding. For instance, when compared with laser welding, the benefits include increased welding speeds and/or depth of penetration, increased joint gap bridging capabilities and improved weld quality. Some of these advantages have already been demonstrated for a range of applications in a variety of industry sectors, including automotive, shipbuilding and aerospace. Despite this, a true understanding of the interaction between both processes during hybrid welding, which results in the performance advantages over the individual processes, remains outstanding. The hybrid laser-arc process is complex, as optimising the welding conditions means that parameters of both the laser and arc process require consideration. To maximise the welding performance of the hybrid process, it is therefore essential to understand how the combined processes function in unity, creating a process that is much more than the sum of its components. What happens to the arc characteristics when a laser is introduced into the weld pool? And what happens to the laser keyhole? How is its stability affected by the presence of an arc?
Stability of a welding process is essential to achieve good welding performance and welds of acceptable weld quality. As such, the influence of the process conditions on its stability need to be examined. In MIG/MAG welding, the way in which molten metal is transferred from the tip of the consumable electrode into the weld pool, ie metal transfer mode, has a significant influence on the overall performance of the process, which in turn, affects the weld quality, process stability and spatter generation. In laser keyhole welding, the keyhole is inherently unstable, because of changes in vapour pressure inside the keyhole, as the laser beam is traversed through a material. In addition to laser power, speed and focus position, the MIG/MAG arc will also affects the stability of the keyhole during hybrid laser welding. In assessing the stability of the hybrid laser - MIG/MAG welding process, it is therefore important to study the metal transfer (of the filler wire) and the behaviour of both the arc and the keyhole. In this investigation, a high-speed camera and an arc monitor to record the transient arc data (current and voltage) during welding and assessment of the visual weld bead appearance after welding, were used for this purpose.
Research into the fundamental understanding of the hybrid laser-arc process to date, has predominantly focused on the use of CO2 lasers in combination with either TIG or MIG/MAG, because of the widespread use of these lasers in industry. However, since the mid-nineties, fibre-delivered laser power has become available, in the form Nd:YAG lasers, which together with robot manipulation, became very attractive 3D welding applications. This capability has particularly been exploited by the automotive industry, with one European car manufacturer, for instance, using 600 of these lasers (at 4kW output power) at its manufacturing facilities world-wide. For this application, the Nd:YAG lasers are predominantly used on their own, but increasingly also in combination with MIG/MAG to compensate for variations in joint fit-up when welding body-in-white structures. Notwithstanding the fact that the process is already in use in industry, little had been published at the onset of this research work on the fundamentals of the hybrid Nd:YAG laser - MIG/MAG process. For this reason, TWI initiated a study, through the CRP, to gain a better understanding of the interactions between the Nd:YAG laser keyhole and the MIG/MAG arc that determine the stability of the process, to maximise the (industrial) potential of the process over Nd:YAG and MIG/MAG welding. With other fibre-delivered laser sources now also available in the form of Yb-fibre and Yb-YAG disc lasers, at output powers higher than currently available for Nd:YAG lasers and emitting a similar laser wavelength, this research will underpin further work that is envisaged for these new laser sources, increasing the potential of the process over laser and MIG/MAG welding.
To investigate the laser-arc interactions, for different metal transfer modes, and determine the process stabilisation mechanism, for the hybrid Nd:YAG laser - MIG/MAG process.
To determine the influence of processing conditions on hybrid Nd:YAG laser - MIG/MAG process stability, including process configuration, process separation and laser power.