# A Comparative Study of the CMT+P Process on 316L Stainless Steel Additive Manufacturing
## 4 Results and Analysis
### Microstructure  
Take sample #2 as an example, the metallographic microstructure of different areas of the as-deposited sample are shown in Figure 4. It can be seen from the figure that the deposition process of MIG welding layer by layer makes the microstructure of the formed parts show periodic layer distribution, and the adjacent deposition layers are metallurgical combined as shown in Figure 4b. The latter layer remelted the grains of the previous layer, which resulted in the epitaxial growth of grains at the fusion line between layers. In the bottom region, the rapid heat dissipation effect of the substrate made the austenite grains in this region small and the overall growth direction of the grains perpendicular to the deposition direction, with obvious consistency as shown in Figure 4a, the growth morphology of grains observed is consistent with the results in reference [26]. However, austenite gradually formed coarse columnar crystals in the middle region as shown in Figure 4c, which was mainly due to the non-equilibrium solidification of the alloy with the rapid solidification during the deposition process, and because of the heat accumulation of the workpiece, the cooling speed of the droplet after the transition into the  
Figure 4: Microstructure of different regions of the sample #2: (**a**) bottom region, (**b**) lower region close to substrate, (**c**) middle region, (**d**) side wall region and (**e**) upper region. molten pool became slower, the dendrites had enough time to grow up and grow into the liquid phase to form columnar crystals. With the increase of heat accumulation, the dendrite size became larger, the secondary dendrite arm began to appear in the upper part as shown in Figure 4e, and the dendrite arm spacing increased significantly. However, the growth of secondary dendrite length was limited by the primary coarse dendrites around it, which made it unable to grow to the side indefinitely. In general, austinite content was constantly varied along the building direction on cross-section. Originally, in the bottom region, a certain amount of primary ferrite formed in austinite matrix due to sub-rapid solidification during the deposition of the first few layers near the substrate, then primary ferrite continuously dissolved into austenite matrix accompanied by the growth process of austenite grains due to processive heat accumulation during arc-welding deposition process, which resulted in the increase of austenite phase content in the middle and top region of as-deposited wall. In the side wall region depicted in Figure 4d, the strong heat dissipation effect of the surrounding air promoted the formation of fine equiaxed grains in the outermost layer of the workpiece, and the grains gradually formed coarse dendrites towards the center of the specimen until they met with the dendrites in the center of the specimen, thus inhibiting the growth. On the whole, due to the continuous growth of grains caused by the continuous heat accumulation, the average grain size gradually increased from about 10.5 m in the bottom region to about 25.3 m in the middle region and then to about 43.5 m in the upper region. The metallographic micrograph of the middle region of each sample is shown in Figure 5, from which the austenite and ferrite structures can be observed. According to the ternary phase diagram of Fe-Cr-Ni, the normal temperature structure of 316L after equilibrium solidification is uniform and single austenite phase, whereas when the temperature gradient is large, the cooling speed is fast, and the heating process is repeated, which probably cause the formation of non-equilibrium phase easily in the structure [27]. During MIG deposition, due to the heat effect of the latter layer on the former layer, some ferrite dissolved in austenite, and the remaining ferrite became reticular and vermicular [28].