# A Comparative Study of the CMT+P Process on 316L Stainless Steel Additive Manufacturing
## 3 Experimental Methods  
The welding parameters of the welding machine in this experiment was built-in unitary adjustment, therefore, other parameters of the welding process can be adjusted only by adjusting the welding current. The welding voltage and wire feeding speed can automatically match the selected current value to ensure the smooth progress of the deposition process. The linear energy formula of CMT+P technology MIG welding of steel material was as follows:  
\[L=\eta UI/V\]  
where \(L\) represents the line energy during welding (kJ/cm), \(U\) represents the welding voltage (V), \(I\) represents the welding current (A), \(V\) represents the scanning speed (m/s), \(\eta\) represents the welding thermal efficiency, generally equal to 0.9 in MIG welding [23]. In each deposition experiment, the scanning speed of each selected current value in the first layer was set to 30 cm/min, so as to facilitate the full fusion of the deposited layer and the substrate, and the scanning speed of the rest layers was set to 60 cm/min. The shielding gas is 99.999% high-purity argon, and the gas flow rate was 20 L/min, and the system could adjust the arc length automatically. A reciprocating stacking method was adopted for deposition of 30 layers. After each layer of bead with a length of 160 mm was finished, the welding torch was raised by 2.0 mm and then the next layer was scanned until the deposition was completed, however, after the first layer welding was completed, the torch was raised by 2.3 mm. The experimental parameters selected are shown in Table 2. \begin{table}
\begin{tabular}{c c c c c c c c c c} \hline
**Element** & **Cr** & **Ni** & **Mo** & **Mn** & **Si** & **C** & **P** & **S** & **Fe** \\ \hline Welding wire & 18.57 & 11.72 & 2.57 & 1.72 & 0.54 & 0.015 & 0.024 & 0.008 & Bal. \\ Substrate & 16.0–18.0 & 10.0–14.0 & 2.0–3.0 & \(\leq\)2.0 & \(\leq\)0.75 & \(\leq\)0.03 & \(\leq\)0.045 & \(\leq\)0.030 & Bal. \\ \hline \end{tabular}
\end{table} 
Table 1: Chemical elements of ER316L wire and substrate (mass fraction/%). Figure 1: Cold metal transfer plus pulse (CMT+P) process additive manufacturing system. After the deposition, in order to avoid the influence of the two ends of the sample on the characterization performance, a width of about 15 mm at both ends was cut. The effective upright wall with a width of about 130 mm in the middle of the sample was the material property characterization area. The sampling location and the size of the tensile sample were shown in Figure 2.  
The greasy dirt on the surface of each metallographic sample was eliminated using acetone. Firstly, the metallographic sandpaper of #160, #360, #600, #800 were used for rough grinding, and then the # 1500, # 2000, # 3000 sandpaper were used for fine grinding. Followed by polishing diamond polishing agent with particle size of 2.5 um. After that, aqua regia (HCl: HNO\({}_{3}\) = 3:1) was used as the metallurgical sample corrosive agent to corrode each sample for 20 s, and finally they were rinsed and dried with water and alcohol. The microstructure of each metallographic sample was observed with optical metallographic microscope. Micro-Vickers hardness tester HMV-2T was used to test 20 points respectively on the bottom (1-5 layers), middle (13-17 layers) and top (26-30 layers) in the cross-section of the four as-deposited wall, in order to ensure the consistency of experimental tests, the locus of all test points were along the center lines of cross sections, because microstructures changed along the horizontal direction of the