A new method for refinement of Ti-6Al-4 V prior-β grain structure in the alternating magnetic field assisted narrow gap gas tungsten arc welding (AMF-GTAW) via filler wire oscillation

This study presents a new method for refining Ti-6Al-4 V prior-β grain in the alternating magnetic field assisted narrow gap gas tungsten arc welding (AMF-GTAW) process by stirring the molten pool, which is accomplished directly through the use of an oscillating filler wire. The wire feed rate can be increased from 1.8 m/min to 2.2 m/min by filler wire oscillation in the traditional AMF-GTAW process with minimal heat input. This method greatly increases welding efficiency by efficiently utilizing the energy of the arc and molten pool. Using a high-speed camera, the flow behavior of the molten pool was investigated under different filler wire oscillating amplitudes. The results showed that the oscillation of the filler wire can cause a violent stir in the molten pool. By increasing the oscillating amplitude of filler wire, one can refine the columnar prior-β grain structure and attain a morphology of the upper layer of the welded joint that is nearly equiaxed with crystallographic texture, all while maintaining a fixed heat input. This is because the wire's insertion into the molten pool also creates a negative thermal gradient and the filler wire's high-frequency and high-amplitude oscillation creates an intense convection that prevents randomly oriented nuclei and/or dendrite fragments from remelting and promotes nucleation activation. The oscillating amplitude of 6 mm and the oscillating frequency of 100 Hz are the ideal values for this effect.