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Prediction and control of porosity in laser welding of non-ferrous metals

Final Report Summary - PCPLAS (Prediction and control of porosity in laser welding of non-ferrous metals)

The PCPlas project aimed to improve the fundamental understanding of porosity formation in the laser welding of non-ferrous materials, of importance to the light-weighting of transport components. To do this, the project:
• Developed numerical process modeling of selected materials and
• Carried out experimental trials to validate the numerical models and study the porosity behavior, in an effort to devise means of reducing porosity contents and improving weld qualities.

The work carried out and the results achieved in the return phase of the PCPlas project mainly include the following four parts:

1) As the existing heat source models are not adaptive to various welding conditions in deep penetration laser welding, a new way was proposed to construct rotational symmetric heat source models with Gaussian distribution, and a concept was developed of clusters of heat resource models. With the new way and concept, a cluster of heat resource models was constructed for various cases of laser welding of titanium alloy Ti-6Al-4V. Results showed that the heat source cluster was more adaptive than conventional models in achieving accurate numerical simulations of the thermal fields, which then could be used as a foundation in analyzing fluid flows and stress-strain during laser welding.

2) With the computational fluid dynamics (CFD) model developed previously (in TWI Ltd, UK, under funding project No. 253487), the correlations were investigated between the fluid flow characteristics and porosity behavior in full penetration laser welding of a titanium alloy. It was found that a turbulence controlled by Reynolds number could result in vortices in weld pool and in turn separations (or voids forming) in molten metals behind the keyhole.

3) High speed camera video imaging was used during actual welding of AA5083 and Ti-6Al-4V to monitor the actual shapes, dimensions, and flow patterns of the keyhole. These observations were used to gain an insight into the actual welding process under different sets of conditions, which could be used to analyse the formation mechanisms of porosity (and also spatters).

4) Porosity was studied for laser welding of titanium alloy Ti-6Al-4V under different welding positions (flat, horizontal, vertical up and vertical down). These trials showed that porosity contents in laser welds were more related to laser power and defocusing distance than welding speed in flat position, while in horizontal position the welding speed was more influential than laser power and defocusing distance. In addition, the flat and vertical up positions would result in less porosities when compared with other welding positions. These porosity levels were closely related to the various fluid flow characteristics under different welding positions.