3.3.1 Experimental verification of the numerical model

To confirm the predictive accuracy of the established model, the corresponding deposition experiments were carried out. A cathode with 60° cone angle and a 3.2 mm radius float tip provides a good combination. The distance between tungsten electrode tip and workpiece surface was fixed at 4 mm. To avoid heat sinks, the workpieces need to be thermally insulated from the fixtures in VP-GTA welding process. Verifications of the numerical model were carried out by comparing the calculated results with the metallographic macro-sections. Figure 19 presents the simulated and measured geometry of a single-track deposition by MFACM.

Calculated solidus isotherm 811 K—corresponds to the fusion line obtained in the experiments. It is found that the calculated deposition geometries and dimensions agree well with the experimental data. The average values of deposition width and penetration depth were measured to be 8.65 and 1.43 mm, respectively, which is in consistent with the calculated results. The average relative errors of the deposition width and height are never exceeded 5.7 and 11.5%. The fidelity of the models will be verified to better predict the surface evolution in MFCAM process and eliminate the deposition defects based on the understanding of its mechanism.

Figure 18. Calculation model for FCAM process.


## Heat and Mass Transfer of Additive Manufacturing Processes for Metals DOI: http://dx.doi.org/10.5772/intechopen.84889

#### Table 5.

Thermo-physical material properties of 2024 aluminum alloy and calculation data used in the simulation.

#### Figure 19.

Comparison between computed thermal profile (left) and experimentally determined (right) cross-section of a single-track deposition, all dimensions are in mm.

### 3.3.2 Thermophysical phenomena in MFCAM

During the fused-coating deposition process, the zone where the heat source is characterized by the temperature local sharp increase and being heated up to the state of melting, as shown in Figure 20. We need to how this evolutionary sequence of deposited layers. The development of deposition shape and the heat transfer and

#### Figure 20.

Solid fraction distribution and the changes of surface morphology of the melt in four stages during single-track deposition. (a) formation of a shallow molten pool, (b) extrusion process of the melt, (c) extremely changes of surface morphology of the melt, and (d) relatively steady state.

flow characteristics of liquid metal were illustrated. The shape evolution of each single layer can be divided into four stages: (i) formation and growth of a shallow molten pool, (ii) extrusion process of the melt, (iii) extremely changes of surface morphology of the melt, and (iv) relatively steady state.

The shallow molten pool on a moving metal substrate is firstly created in the initial stage. The source of heat in welding is enough to melt the metal. According to the practical conditions, the welding torch needs to be tilted from the nominal center of the workpiece surface, which will slightly change the thermal flow field characteristics of the molten pool.

During the second stage of MFCAM process the hot liquid Al alloy will be extruded continuously toward the shallow molten pool. Meanwhile, the heat flux of the tilted welding arc still melts the base metal. Thus, the local thermal contact resistance of deposition regions can be reduced greatly, which is beneficial to the spreading of the melt. The corresponding thermal and flow fields in this stage is shown in Figure 21.

In the third stage, the extruded liquid alloy and the melt in the shallow molten pool trend to spontaneous fusion once the extruded liquid metal meets the surface of the molten pool. The free surface of the melt will experience extremely complex deformations. Figure 22 illustrates the topology changes in the free surface, surface depression in the molten pool and the temperature distributions in the melt.

It can be seen from Figure 22a that there is a higher temperature region between the welding arc and fused-coating head. The teardrop-shaped gouging region is also found in the front of the molten pool. The causes of these phenomena can be attributed to the fact that it is relative difficult for the liquid metal located in the higher temperature region to dissipate heat because of the heat input by welding arc, at the same time, the melt continuously obtains latent heat from the being extruded liquid metal. The end-face of the coating head and the shallow molten pool will produce an obviously restriction effect to the spreading melt. As shown in Figure 22b, the local fluctuations of the melt's surface and solidification front are

Heat and Mass Transfer of Additive Manufacturing Processes for Metals DOI: http://dx.doi.org/10.5772/intechopen.84889

Figure 21.

Side views of (a) temperature distributions, and (b) solid fraction distributions in the second stage.

Figure 22.

Topology changes in the free surface, surface depression in the molten pool and the temperature distributions in the melt. (a) t = 0.355 s, (b) 0.405 s, (c) 0.68 s, and (d) 0.75 s.

always found, this could be attributed to the way in which a pulsed-current mode was adopted.

As time goes on, the arious forces go erning the fluctuation of molten pool surface and a macroscopic heat balance tend to equilibrium, therefore, the thermocapillary-dri en deposited shape becomes to be continuous and stable, the 3D shape of the stable deposited layer can be obser ed in Figure 23. The calculated temperature field and elocity magnitude could be illustrated by Figure 23a. The dimensions associated to the solidified deposited layers will be characterized by H and W.

Figure 23. (a) 3D shape of a single-track deposited layer and (b) surface velocity contour.

The calculated results show that the shape of deposited layers is flat and regular from the global aspect although there is a local fluctuation of melt's surface and solidification front. It is also found that a maximum liquid metal speed with 0.6 m/s is obtained near the center of the shallow molten pool.

## 3.3.3 Influence of melt flow rate

Figure 24 shows the influence of melt flow rate on the morphology characteristics of single-track deposits. The gap between substrate and fused-coating head is fixed at 1.8 mm, and the substrate moving speed is 6 mm/s.

As illustrated from Figure 24, an approximate linear increase in the deposition height was observed as the melt flow rate increase from 20 to 120 ml/min. However, in the respects of the deposition width, there obviously exist many highly nonlinear relationships. Therefore, the formation mechanism based on the thermal flow dynamics needs to be recognized. The principle of minimum enthalpy may be utilized to explain this phenomenon. The principle is described as follows: the thermocapillary-driven flow always has a characteristic to keep the minimum enthalpy value by automatically changing its size and adjusting its enthalpy. In

Figure 24. Variation of the morphology characteristics of deposited single tracks with melt flow rate.

Heat and Mass Transfer of Additive Manufacturing Processes for Metals DOI: http://dx.doi.org/10.5772/intechopen.84889

Figure 25. Influence of the gap height on the morphology characteristics of single-track deposits.

MFCAM process, the area of the absorbed arc energy is heated alternatively. The residual heat in the material will be continuously accumulated from pulse to pulse. The heat accumulation, as a function of processing parameters, is proportional to the duration of arc heat input.

When the substrate moving speed and arc heat input keep invariant, the local high-temperature range and the molten pool size should remain unchanged. However, the local melting/solidification behaviors will be partially changed with the spontaneous fusion of extruded liquid metal and the melt in the molten pool. As the melt flow rate increases, heat sink effect on the penetration depth becomes more significantly since the additional heat energy from the being extruded melt is partially transferred to the molten pool, thus the additional thermal energy actually prolongs solidification time and makes the penetration depth increase.

#### 3.3.4 Influence of the gap height

On the other hand, the influence of the gap height between substrate and fusedcoating head on the morphology characteristics of single-track deposits is also discussed, as shown in Figure 25. The range of melt volume flow rate is 30–70 mm3 /s. The substrate moves at a constant speed of 5 mm/s.

As illustrated in Figure 25, it was proved that the deposition height increases with the increase in the gap height, but the reverse is true in the deposition width. The reason for this phenomenon can be explained by researching the morphology of free surface and the thermal-flow characteristics around the fused-coating head, two main causes are pointed out: (i) liquid metal state under the condition of being squeezed, (ii) the adhesion properties of the melt around the fused coating head.

Squeezed flow behaviors of the liquid metal within a narrower gap plays a dominant role in MFCAM process, the penetration may be deeper than that in a simple thermocapillary shear flow because of the presence of pressure gradients. In this situation, the flow direction of the melt transforms from the rear toward the lateral, thus the deposition widths achieve actually increase. Meantime, it is also found that a larger gap easily leads to a higher and narrower deposition layer. This is attributed to the fact that as the gap is sufficiently large, the thermocapillary force will become not evidence, while the gravity effect and the adhesion between liquid metal and fused-coating head become remarkable.
