## 3 Results and Discussion
### Effect of Heat Dissipation on the Bead Geometry  
The obtained _RWTH_ in Group III-1, as shown in Figure 4c, is obviously decreased compared with that in Group II under the same process parameters, which clearly illustrates that thermoelectric cooling is more effective than convection and radiation in terms of absorbing heat from the molten pool. It is also noteworthy that when _WFR_ = 3 m/min, the obtained _RWTH_ at upper layers is very close to that at lower layers (in Group I), which implies that the difference in heat dissipation between upper and lower layers is well made up for by thermoelectric cooling. From Figure 5a, we can also see that the two profiles in Group I and Group III-1 tend to overlap. However, for larger _WFR_ ranging from 4 to 5 m/min, the effect of thermoelectric cooling is less significant and the obtained _RWTH_ is still larger than that at lower layers. This is because the cooling power is limited in this case, and a decreasing proportion of the heat is dissipated through the side surfaces with the increase of the heat input. Besides this, the large arc force and strong droplet impingement resulting fromthe large heat input also have great effect on the bead geometry, and could not be eliminated by thermoelectric cooling.  
When the cooling power is increased to the maximum value in Group III-2, the obtained _RWTH_ is further decreased as expected. In this case, the _RWTH_ is much closer to that at lower layers when the _WFR_ is 4 m/min, but still a little larger when the _WFR_ is 5 m/min. If the cooling power is further increased, there is a great possibility to further decrease the _RWTH_ to the same level as that at lower layers. To further validate the above results, the thermal cycling and the molten pool shapes of lower and upper layers are also compared (when _WFR_ = 4 m/min), as displayed in Figures 6 and 7, respectively. The peak temperature is 313 \({}^{\circ}\)C at the lower layer and is 320 \({}^{\circ}\)C at the upper layer with thermoelectric cooling, as seen in Figure 6. The small discrepancy (7 \({}^{\circ}\)C) may reflect that their heat dissipation conditions are quite similar. If there is no thermoelectric cooling, however, this discrepancy is about 102 \({}^{\circ}\)C.  
The molten pool tails at the lower layer and at the upper layer with thermoelectric cooling are also quite similar--like a cone--as seen in Figure 7a,b, respectively. However, the molten pool flows randomly with intensive metallurgical reaction if there is no thermoelectric cooling, which makes it difficult to detect the pool tail as seen in Figure 7c. This further proves that the lower and the upper layers could experience a similar heat dissipation condition with the aid of thermoelectric cooling.  
Figure 6: Thermal cycling comparison (when _WFR_ = 4 m/min).  
Figure 7: Molten pool shape comparison (when _WFR_ = 4 m/min): (**a**) at the lower layer; (**b**) at the upper layer with thermoelectric cooling; (**c**) at the upper layer without thermoelectric cooling.  
From the above results, we can conclude that once the cooling power matches the heat input, the difference in heat dissipation between the upper and lower layers can be eliminated, which contributes to achieving consistent bead geometry from the bottom up with no need to adjust the process parameters, i.e., without sacrificing the deposition rate. When the heat input is 1630 W or 2290 W, the required cooling power is 240 W or 360 W, respectively, which accounts for 14.8% or 15.7% of the corresponding heat input. If this ratio is smaller, the _RWTH_ tends to be larger than that expected; but if larger, the _RWTH_ tends to be smaller. This shows that the ratio of cooling power to heat input should be maintained basically constant. It should be pointed out that more accurate adjustment of the cooling power needs further research with regard to thermal modeling of the WAAM process.