## 2 Materials and Methods  
Experiments were carried out with a cold metal transfer wire + arc additive manufacturing (CMT-WAAM) system, which mainly consists of a Fronius CMT 4000R power supply (Fronius, Pettenbach, Austria) with a wire feeder and a CMT torch (Fronius, Pettenbach, Austria), an ABB robot IRB2400 (ABB, Zurich, Switzerland), a shielding gas system (Fronius, Pettenbach, Austria) and a working table, as shown in Figure 1. CMT process is a modified gas metal arc welding (GMAW) variant based on a controlled dip transfer mode mechanism. It has four different variants, which are standard CMT, CMT-P, CMT-ADV and CMT-P-ADV. As presented in the study of the effects of these different CMT variants on the porosity in the WAAM aluminum thin wall structure [15], adding a pulsing cycle to the standard CMT helps the control of porosity. Compared with the CMT-P process, the CMT-ADV process controls porosity even more efficiently due to its lower heat input, cleaning effect on the tip of the wire during the negative cycle and the smaller microstructure [15]. In this study CMT-P and CMT-ADV were employed to build the samples. Although using the CMT-P-ADV process can provide the most effective porosity control, it is not suitable for building block structures due to its very low heat input, which results in a lack of fusion defects when one pass of bead is deposited next to other beads. The actual arc current and voltage waveforms of these two modes are presented in Figure 2.  
ER2319 wire (1.2 mm in diameter) (VBC, Leicestershire, UK) and 2219-T87 plate (200 mm \(\times\) 200 mm \(\times\) 19 mm in dimension) (CHALCO, Chongqing, China) were chosen as the material and substrate, respectively. The nominal compositions of the material and substrate are listed in Table 1. All substrates were cleaned shortly before being used, by washing in alkaline solution, drying, then finishing and degreasing with acetone afterwards. Figure 1: Cold metal transfer-wire + arc additive manufacturing experimental system. The depositing paths of thin wall samples and block samples are shown in Figure 3. Thin wall samples were deposited layer-by-layer in the vertical direction with only one pass of bead in each layer. On the other hand, the block samples were produced by first laying a number of parallel passes one by one next to each other and then repeating the deposition of the same number of passes on top of the previous layer. In this study, thin wall structures and block structures were built with CMT-P and CMT-ADV, and all the experiments were carried out in an open atmosphere with torch shielding only. Pure argon (99.99%) was used as the shielding gas with a constant flow rate of 25 L/min, and the contact tip to work distance was kept to 15 mm for all the trials. A constant waiting time of 60 s was applied between each layer/pass for cooling the samples below 50 \({}^{\circ}\)C before the deposition of the subsequent layer/pass. This was monitored by using a portable thermal couple measuring at the middle section of the top surface of the newly deposited layer/pass. The detailed parameters of the depositing process and the dimensions of the components are listed in Table 2. The heat input (HI) was calculated using the equation  
\[\text{HI}=\frac{\eta\times\sum\limits_{i=1}^{n}\frac{I_{i}U_{i}}{n}}{\text{ TS}}\]  
where \(I_{i}\) (A) and \(U_{i}\) (V) are arc current and voltage for each sample, and the arc thermal efficiency (n) of CMT is set to 0.9 [16]. TS is the travel speed (mm/min). Some WAAM structures are presented in Figure 4.