## 1 Introduction  
Additive manufacturing (AM)--the layer-by-layer build-up of parts--has become one of the most promising manufacturing technologies in the past thirty years [1; 2; 3]. Parts that are difficult or expensive to fabricate using conventional material removal processes will favor AM. Popular AM processes for metallic materials include power bed fusion, directed energy deposition, etc. [4]. Selective laser melting (SLM) and electron beam melting (EBM) [5], belonging to first category, are relatively superior in terms of geometrical complexity and accuracy. However, they are currently only suitable for fabricating high-value and low-production parts, restricted by high cost and low efficiency.  
Wire and arc-based additive manufacturing (WAAM), belonging to the second category, has drawn significant interest from both academia and industry in recent years due to its unique cost and efficiency advantages [6; 7; 8]. The low cost is attributed to the easy-to-access wire material and mature welding technologies such as Gas Metal Arc Welding (GMAW). The high efficiency is attributed to its large heat input and high wire feed rate. As reported by [PERSON] et al. [9], its build efficiency can reach up to 50-120 g/min with almost no limitation on the build volume. With these advantages, WAAM is highly competitive in fabricating medium- to large-scale metal parts compared with other AM processes [10], especially for thin-walled structures [11]. Although WAAM has long suffered from low geometry accuracy and poor surface quality resulting from its large heat input, the recent emergence of hybrid manufacturing--integrating additive and subtractive processes into a single setup--may provide a substantial solution to this limitation [12].  
Although WAAM originated from welding, their heat dissipation conditions exhibit obvious differences. For welding, the heat is conducted from the molten pool directly to the substrate (see Figure 1a), while for WAAM the heat is dissipated partly to the substrate through previously deposited layers and partly to the ambient air via convection and radiation (see Figure 1b) [13]. With the wall height increasing, the conductive thermal resistance to the substrate is significantly increased and, therefore, an increasing amount of heat is dissipated to the ambient air. However, such a heat dissipation mechanism is less effective than direct conduction to the substrate, which slows down the solidification of the molten pool and therefore leads to a wider and lower weld bead than the expected one. [PERSON] et al. [14] demonstrated through numerical simulations that the temperature gradient  
and previous forming geometry. It is worth noting that these methods, realized by reducing the heat input, will inevitably sacrifice the deposition rate because the wire feed rate is directly coupled with the heat input. Besides this, real-time adjustment of the heat input relies on experience to a large extent and greatly complicates the build process.  
An alternative solution is to regulate the heat dissipation during WAAM, since variations in both heat input and heat dissipation may affect the solidification of the molten pool and, ultimately, the bead geometry. To control the heat dissipation as needed, an in-process active cooling system based on thermoelectric cooling technology is developed in this study. The relatively poor heat dissipation by means of convection and radiation is replaced with strong heat conduction (see Figure 1c), such that the heat dissipation of the upper layers could reach the same level as that of lower layers, thus eliminating their difference in bead geometry with no need for adjusting the process parameters. That is to say, the conflict between consistent bead geometry and high productivity can be resolved. The rest of this paper will introduce the in-process active cooling system and evaluate its efficacy in regulating the bead geometry during WAAM.