# A Comparative Study of the CMT+P Process on 316L Stainless Steel Additive Manufacturing
## 4 Results and Analysis
### Microstructure
Meanwhile, as shown in the figure, the columnar grains in the middle of the four samples grew vertically upward along the building direction, which further indicated that the grain structure of 316L samples grew along the direction of the maximum temperature gradient. When the linear energy was 2.0 and 2.62 kJ/cm, the main phase was \(\gamma\) - austenite, forming in the middle of the samples #1 and #2, and the formation of \(\delta\) - ferrite, some of them are vermicular, at the austenite grain boundary as shown in Figure 5#1-c and #2-c, and metallographic observation position was shown in the Figure 4c. When the linear energy increased to 2.62 and 2.85 kJ/cm, vermicular ferrites almost disappeared, the austenite columnar crystals in Figure 5#3-c and #4-c were coarse, the secondary dendrite arm was almost invisible, the dendrite gradually changes into cellular crystal, and the content of \(\delta\) ferrites was decreasing. The main reason may be that the higher the linear energy was, the larger the heat input was, which resulted in the grain growth, meanwhile, the middle part of the deposited sample was subjected to repeated heating and the heat dissipation condition was worse than other areas, which promoted the diffusion of solute atoms in the structure, and the primary and secondary dendrite arms of \(\gamma\) austenite grains were easy to coarsen and grow at high temperature. As is well-known, an adequate amount of ferrite present in the austenite matrix can reduce material brittleness, effectively prevent crack initiation thus properly improve mechanical property, meanwhile, grain coarsening caused by high heat input also obviously bring out negative influence on material mechanical property, so, in terms of microstructure, samples #1 and #2 are better than samples #3 and #4. At the same time, Figure 5#1-c depicted that the growth state of grains in sample #1 was more disordered than that in other samples, and the growth process of grains in some regions was interrupted, which resulted in the overall grain size of sample #1 was smaller than that of the other three samples. The fact could be attributed to that during the sample #1 deposition process, grain solidification and growth along the temperature gradient direction were restrained and disturbed in the molten pool with unstable droplet transition, which made the flow field in the molten pool with unstable deposition more disordered than that with stable deposition, resulting in the formation of the special crystal growth morphology. The SEM picture of the top region of sample #4 is shown in Figure 6a. In the figure, the microstructure is mainly composed of austenite. Only a small amount of ferrite was observed on the austenite grain boundary. This may be mainly due to the high temperature formed by the heat accumulation during the continuous arc-welding deposition process, which promoted the ferrite to dissolve into the growing austenite. This observation result is consistent with experimental phenomena in Figure 5. In addition, there are some small lumpy and broken particles marked with arrows and ellipses. Two of the particles were selected for element quantitative analysis by energy spectrum, the result was shown in Figure 6b. The Figure 6b shows that there are a lot of oxygen elements in the two particles, with the percentages of 58.97% and 53.83%, respectively, which proved that although shielding gas was used, oxides could form in the deposited samples during the arc welding deposition process. This may be mainly due to the oxides formed at high temperature had not been cleaned in time between layers during the continuous deposition process. Meanwhile, gas protection region moved away with the movement of the welding gun, which further increased the degree of oxidation of the exposed molten pool under the action of high temperature. Cr and Mn also account for a large proportion in these particles, which was related to the oxidation products of the alloy elements at high temperature.