MODELLING FLUID FLOW AND HEAT TRANSFER PHENOMENA IN KEYHOLING STAGE OF PLASMA ARC WELDING

Because of its high gas velocity and heat input,plasma arc welding(PAW) can penetrate thicker workpieces with a single pass because PAW can operate in the keyhole mode. Compared with electron beam and laser beam welding,keyhole PAW is more cost effective and more tolerant of joint preparation,so that it is widely used in manufacturing structures with medium thickness. However,the keyhole establishment and sustainment during the initial stage of PAW process, i.e.,the keyholing process,has a critical effect on the process stability and the weld quality.Thus, modelling and simulating of the keyholing process and its influence on fluid flow and heat transfer in keyhole PAW process is of great significance to completely understand the process mechanism.With considering the interaction between weld pool and keyhole,a three dimensional transient model of fluid flow and heat transfer in weld pool is developed for numerical analysis of keyholing process in PAW.The volume of fluid method(VOF) is used to track the keyhole shape and size.The latent heat and momentum sink due to solidifying and melting are dealt with by enthalpy-porosity technique. Considering the larger ratio of PAW weld depth to width,a combined volumetric heat source model is established,and one of its distribution parameters is adjusted dynamically with the variation of keyhole depth.The evolution of fluid flow and thermal field in weld pool,and the keyholing process are quantitatively analyzed on the stainless steel plates of thickness 8 mm.The feature of fluid flow in weld pool is revealed.The predicted keyhole size at bottom side of workpiece and fusion line at transverse cross-section of welds agree with the experimentally measured results.