## 1 Introduction  
Selective laser melting (SLM), as an additive manufacturing technology, is mainly used to fabricate small scale and high-precision components [1; 2; 3; 4]. However, in recent years, large-scale manufacturing has gradually become an important trend of SLM. Until now, the size of the largest available commercial equipment has achieved 800 \(\times\) 400 \(\times\) 500 mm\({}^{3}\) (Concept Laser X line 2000R). It is highly likely that SLM can be used to fabricate large-scale components in the near future, such as large scale aerospace titanium components. Compared with other large-scale additive manufacturing technologies based on powder or wire feeding [5; 6; 7], the predominant advantage of the large-scale SLM lies in the better mechanical properties and higher precision.  
However, three problems still severely restrict the development and application of large-scale SLM. The first one is the residual stress [8], which causes the distortion of components, the difficulty of spreading powder and even the termination of SLM process. The second one is defects, including incomplete fusion defects, and pores, etc. These defects largely reduce the fatigue performance of alloys [9]. Currently, the residual defects are generally eliminated by post hot isostatic pressing. However, for large-scale components, this method will be difficult and costly. The third one is columnar grains. Although the grain sizes of the alloys fabricated by SLM are fine, the columnar morphology results in mechanical anisotropy, and hence restricts its application.  
In order to solve the above problems, a novel method is proposed by combining SLM with ultrasonic impact treatment (UIT) technique. UIT is one kind of well-known surface plastic deformation methods (see Figure 1a), which was originally developed to relax residual stress and improve the fatigue performance of welded structures [10; 11]. It is usually used for welded processing to eliminate residual tensile stress, and is also used for the surface treatment of bulk material to implant compressive stress and form fine grain zone. UIT can provide a number of beneficial effects on the surface region of alloys by introducing compressive plastic deformation. According to the different treatment effects, the surface can be divided into three zones, as shown in Figure 1b [12]: (i) zone of grain refinement at the top surface. Its depth is about 10-30 \(\upmu\)m for titanium alloys [13; 14]. This zone is caused by severe plastic deformation and recrystallization; (ii) zone of plastic deformation with compressive residual stress, but without recrystallization. This zone has a depth of more than 200 \(\upmu\)m for titanium alloys [15]; (iii) zone of stress relaxation with a depth of several millimeters [12]. For SLM, the thickness of the deposited layer is about 30-200 \(\upmu\)m. Therefore, it is hopeful that UIT can effectively reduce the residual stresses, and may eliminate the defects and break the columnar grains. In addition, because the UIT impact head is small, it is suitable to be integrated into large-scale equipment of SLM. Based on these considerations, we explore the feasibility of UIT assisted SLM in this study. A preliminary investigation is undertaken to determine the effect of UIT on the residual stress, defects and grains of Ti-6 Al-4V titanium alloy samples.