# A Comparative Study of the CMT+P Process on 316L Stainless Steel Additive Manufacturing
## 4 Results and Analysis
### Tensile Strength  
The strain-stress curves of the horizontal and vertical tensile specimens named H2 and V2 respectively of four as-deposited wall was shown in Figure 11. The tensile strength of vertical tensile specimen was significantly higher than the corresponding horizontal tensile specimen within every deposited wall, while the elongation was just the opposite. Among these, for example, the tensile strength of #1-V2 is 13.22% higher than that of #1-H2, and the value within the other three pairs of specimens (V2 and H2 tensile specimens of #2, #3, and #4) are 7.4%, 7.56%, and 3.22%, respectively. Therefore, the tensile strength of the as-deposited samples showed obvious anisotropy. Meanwhile, as the welding current increased, the ultimate tensile strength values of both V2 and H2 tensile specimens of as-deposited wall represented a gradually declining trend, which could be attributed to the reason that the higher heat input introduced by higher welding current could obviously promote grains growth and sub-grains merging during metal deposition process, thus decreased the grain boundaries and dislocation density, resulting in the decline of ultimate tensile strength of each sample at the macro level. In addition, the ultimate strength of each specimen in the vertical direction is better than that in the horizontal direction, the elongation results show the opposite. Figure 10: Schematic diagram of the relationship between hardness change and microstructure. Figure 9: Microhardness distribution of different parts of as-deposited samples. The average values of the horizontal and vertical yield strength (YSH and YSV), as well as the average values of the horizontal and vertical ultimate tensile strength (UTSH and UTSV) of each as-deposited sample are shown in Figure 12. When the linear energy decreased from 2.85 kJ/cm to 2.0 kJ/cm, the value of YSH went from 217 to 260, an increase of 20.0%, as well as the YSV, UTSH and UTSV, the increase rates of them were 22.6%, 13.8%, and 16.9%, respectively. It can be seen that the changes of the four types of values show a clear correlation with those of line energy. This may be mainly due to the reduction of heat input during the deposition process, which resulted in grain refinement and thus improved the mechanical properties of the alloy. The results were consistent with the trend of microhardness in Figure 9.  
In addition, the figure also shows that the yield strength and tensile strength of sample #1 were significantly higher than those of the other three samples. This may also be related to the disordered grain growth morphology and the large degree of grain boundary disorder in the #1 deposition sample, as shown in Figure 5. The tensile strength of polycrystalline alloy can be improved by smaller grain size and larger tortuosity of grains boundary [18]. While, the average values of the yield strength and the ultimate tensile strength of all four as-deposited samples satisfy the standard requirement for stainless steel forgings: 170 MPa for YS, and 450 MPa for UTS [30], respectively, as shown in the Figure 12, which indicates that in terms of mechanical properties, the single channel multilayer continuous deposition of wire arc-welding with CMT+P process can be used in additive manufacturing of 316L stainless steel. Figure 11: Stress strain curves of the H2 and V2 tensile specimens. Figure 12: Yield and tensile strength of different specimens in horizontal and vertical directions. The tensile strength values obtained in this study, except for the sample #4 (under the highest welding current condition), are much higher than those reported in reference [31] in which the 316L parts are produced by SLM with hot-isostatic pressed, containing the highest UTS of 528 MPa. However, SLM process has much higher requirement than GMAW process on raw material quality for AM, meanwhile, the preparation process of alloy powder for SLM consumes much more time, which makes the cost of SLM process extremely high for AM.