## 3 Results and Discussion  
The top surface morphologies of SLM sample and UIT assisted SLM sample are presented in Figure 3. Figure 3a shows the multi-track morphology on the surface of the SLM sample. There are a lot of sputtering powder particles on the surface. Comparatively, the surface of UIT assisted SLM sample is more smooth. It is hard to observe beads and spatter as shown in Figure 3b. Obviously, the compressive plastic deformation produced by UIT optimizes the surface morphology, and the smooth surface is helpful to reduce the defects. The average residual stresses of the SLM sample and UIT assisted SLM sample without defects are 176.3 MPa and 49.9 MPa, respectively. This clearly shows that the UIT assisted SLM process has a significant advantage of reducing residual stress, and the residual stress value can be largely reduced. Figure 3: Scanning electron microscopy (SEM) images showing the typical surface morphology of (a) SLM sample; (b) UIT assisted SLM sample. Figure 2: Schematic view of (**a**) UIT assisted selective laser melting (SLM); (**b**) SLM sample and (c) UIT assisted SLM sample; (**d**) scanning path of UIT; (**e**) four samples in this study. The reasons can be simply explained as follows. The rapid solidification and high temperature gradient lead to the generation of residual stresses in SLM fabricated sample. According to the solidification shrinkage and temperature gradient mechanism [8], tensile residual stresses exist in new deposited layers. UIT can introduce the compressive stress into the layer, and hence reduce the tensile stress. Figure 4a shows that there are many large and irregular incomplete fusion defects in the SLM sample, due to the inappropriate hatch spacing [16] (used on purpose to generate defects). However, since UIT is applied after depositing every two layers, some small defects are basically eliminated. Meanwhile, large defects are hammered flat, showing a trend of closure, as shown in Figure 4b. We can clearly see from this comparison that UIT plays a significant role in reducing defects. This is due to the compressive surface plastic deformation caused by UIT. It should be noted that, during the long time SLM additive manufacturing of large-scale components, defects (pores, etc.) are unavoidable. These defects largely reduce the fatigue performance of alloys [9]. However, our above results suggest that UIT assisted SLM may be used to obtain the full density components. Therefore, the costly post hot isostatic pressing method can be avoided. For the SLM sample, long columnar \(\beta\) grains dominate in the sample with the average width about 100 \(\upmu\)m as shown in Figure 5a,b. For the UIT assisted SLM sample, it consists of fine equiaxed grains with the diameter about 20 \(\upmu\)m, and short columnar grains with the width about 50 \(\upmu\)m and height about 250 \(\upmu\)m as shown in Figure 5c,d. Meanwhile, it is interesting to find that the columnar grains and equiaxed grains are distributed alternately, forming a novel bamboo-like morphology (see Figure 5d). Besides, the height of these bamboo-like grains is about 400 \(\upmu\)m, which is identical to the thickness of 2 layers. Considering that the sample was treated once by UIT every 2 layers, this morphology implies that the formation of fine equiaxed grains is associated with UIT. Next, the grain evolution during UIT assisted SLM is proposed as shown in Figure 6. Firstly, during SLM process, there is high temperature gradient due to rapid heat loss through the substrate or previously deposited layers. This results in the epitaxial growth of \(\beta\) grains as schematically shown in Figure 6a [5; 17], and hence leads to the long columnar grain morphology in SLM samples (see Figure 5a). During UIT assisted SLM process, when layer 1 and 2 (counting from the bottom layer) are deposited, the columnar grains are also formed. Then, the UIT is applied. Compressive plastic deformation occurs in the surface region as schematically shown in Figure 6b.