3. Numerical investigation of thermocapillary-induced deposited shape in MFCAM of aluminum alloy

A three-dimensional (3D) numerical model with volume of fluid method is developed for metal fused-coating additive manufacturing (MFCAM) process of aluminum (Al) alloys. It predicts the thermal flow field in the thermocapillaryinduced melt, the surface deformation and solidified deposition geometry during MFCAM in successive depositing passes. Verification of the numerical model was performed by comparing the calculated results with metallography of deposited cross-sections, showing that there is a good qualitative agreement between the two, which indicates that the established numerical model is capable of simulating the complex heat and mass transfer phenomena in the varying polarity gas tungsten arc welding (VP-GTAW) based additive manufacturing. The effects of melt flow rate and the gap height between the substrate and fused-coating head on deposition geometry were studied. The results show that the deposition geometry is closely correlated to the melt flow rate. Increase in melt flow rate will lead to the obvious increase of deposition height, but the reverse is true in the gap height. These detailed physical insights facilitate the prediction of deposition defects in MFCAM of aluminum alloy.

#### 3.1 Principle of MFCAM process

The schematic illustration of MFCAM process is shown in Figure 16. The experimental system is mainly composed of an induction heating, gas protection device, a pressure controller and a movable platform. The deposits of aluminum alloy can be created following the layer-by-layer approach by controlling the synchronization of the movable platform and the extrusion of liquid metal. A programmable multi-axis controller (PMAC) can be used to control the motion of

Figure 16. Schematic diagram of MFCAM process.

Figure 17. Experimental setup for the MFCAM of Al alloys.

movable platform. The high-temperature liquid metal would flow through the inner flow channel of fused-coating head under the combined-action of moving push rod, surface tension and hydrostatic pressure. As the liquid metal contacts with the substrate surface or the previously deposited layers, a local thermocapillaryinduced flow region will be created rapidly. To achieve a metallurgical bonding between the deposited layers, a pulsed variable polarity GTA welding arc was adopted to create a shallow molten pool in front of the thermocapillary-induced flow region. On the other hand, variable polarity GTAW arc can timely remove the aluminum oxide on the deposits. Last but not least, the local solidification conditions and thermocapillary-driven spreading motion of the melt can be tunable by arc heat input and the relative distance between welding arc and fused-coating head.

The corresponding experimental setup for the MFCAM of Al alloys is shown in Figure 17.
