## 2 Materials and Methods  
The UIT impact head was integrated into the self-developed SLM system as schematically shown in Figure 2a. The gas atomized Ti6 Al4V powder with a size range of 53-106 \(\upmu\)m supplied by AVIC BIAM was used in the experiments. The average particle size of powder is 68 \(\upmu\)m (d10:57 \(\upmu\)m, d90:95 \(\upmu\)m) and the apparent density of powder is 2.48 g/cm\({}^{3}\). Four Ti-6 Al-4V samples about 30 mm \(\times\) 30 mm \(\times\) 2 mm (10 layers) were fabricated, including SLM samples without and with defects, UIT assisted SLM samples without and with defects as shown in Figure 2e.  
A fiber laser (YLR-WC, IPG Photonics Corporation, Oxford, MA, USA) with a maximum power of 500 W in continuous laser mode is used for the equipment. The size of the build envelop is \(\Phi\)200 \(\times\) 100, platform is at room temperature (25 \({}^{\circ}\)C). The working chamber provides a closed environment which is filled with Argon as a protective gas to maintain the content of oxygen below 50 ppm. Because the final purpose of this study is to fabricate large-scale components, the SLM process parameters were chosen to ensure high-efficiency. Here, laser power is 400 W, a spot size is approximately 200 \(\upmu\)m, a laser wavelength is 1074 nm, scanning speed is 50 mm/s, hatch spacing is 0.6 mm, and layer thickness is 200 \(\upmu\)m. The strategy for the fabrication of the cubic specimens is cross-hatching technique. Note that it is difficult to observe the defects in UIT assisted SLM sample, because the density of SLM sample has reached 99.9%. Hence, we compare the SLM sample with defects and UIT assisted SLM sample with defects, in order to study the effect of UIT on the defects. When fabricating SLM sample with defects, the hatch spacing was changed to be 0.8 mm on purpose. During UIT processing, the dimensions of the UIT head is \(\Phi\)5 mm, the output frequency is 20 KHz and output power is 0.8 kW. Moreover, the moving velocity of the impact head was about 50 mm/min, and the SLM samples were treated once by UIT when the surface temperature is lower than 50 \({}^{\circ}\)C after every 2 layers were deposited,  
Figure 1: (**a**) Schematic view of the impact head of ultrasonic impact treatment (UIT); (**b**) Sectional view of the surface region for the sample treated by UIT.  
as shown in Figure 2c. Each impacted layer took about 4 min and the scanning path of UIT head looks like \"Z\", as illustrated in Figure 2d.  
Metallographic specimens were prepared by conventional mechanical polishing method. A mixture of 1 mL HF, 6 mL HNO\({}_{3}\) and 100 mL H\({}_{2}\)O was used as the etching agent. The surface morphology and microstructures of the samples were characterized by optical microscopy (OM) and scanning electron microscopy (SEM). The \(\beta\) grain size was measured using the metallographic image analysis software SISC IAS v8.0. The quantitative measurement was conducted on at least five OM micrographs with a magnification of 200 for each specimen. The residual stress at the top surface was measured by using the hole-drilling method of ASTM standard E837. The diameter of the hole is 1.5 mm and the depth is 2 mm, the residual stress is calculated according to the measured release of the strain during the drilling process.