## 1 Introduction
Hence it is imperative that the fatigue properties are improved for the next generation of additively manufactured titanium components [33; 37; 38; 39; 40]. In the case of Ti-6 Al-4V, the hexagonal close-packed (HCP) \(\alpha\) phase and trace amounts of a body centered cubic (BCC) \(\beta\) phase are almost entirely replaced with the martensitic \(\alpha^{\prime}\) phase. The poor surface finish and high porosity are also factors that offset the microstructural characteristics on the fatigue performance. The demand for functional AM parts has been rising as high reduction in assembly costs are available coupled with decreases in mass. One example is the fuel nozzles for the General Electric (GE) LEAP engine have been additively manufactured to be 25% lighter while eliminating previous models that required laborious assembly. Its successful design has now been 3D printed more than 30,000 times since its conception [41]. With the increasing demand for metal AM, researchers have begun to develop techniques to improve the fatigue performance by improving the surface finish and by inducing compressive residual stresses at the surface. Shot peening [42], Ultrasonic Nanocrystal Surface Modification (UNSM) [43; 44; 45; 46] and grit blasting [47] are well known examples of beneficial treatments on AM of metals. However, none of these processes were able to address all the following simultaneously: surface roughness, surface porosity, fatigue life and tensile residual stresses. Studies have shown that Ultrasonic Impact Treatment (UIT) improves the surface finish and fatigue properties in the field of post-welding [48]. UIT is a process in which an indenter vibrating at ultrasonic frequencies slides over a surface. This treatment plastically deforms the surface, improving the surface finish while inducing and redistributing residual stress in the part resulting in enhanced fatigue life [48]. UIT devices operate by inducing plastic deformation from the indenter or impact needle by first exciting a transducer by a controlled voltage input. The power source directly controls the oscillations exhibited by the transducer, which sends its frequencies to the sonotrode (ultrasonic horn) [48]. The research of [PERSON]. [PERSON] et al. [49] compared a variety of methods that improve the fatigue life of welded joints. Other similar methods include Hammer Peening, Shot Peening and Tungsten Inert Gas (TIG) dressing. An improvement of 65% was observed in the UIT joints. In [PERSON]. [PERSON] et al. [50] investigated the enhancements that occur in the surface layer of ultrasonic impacted specimens. It was concluded that the compressive residual stresses and work hardening of the surface layer attributed to the improvement in fatigue properties of processed specimens. [PERSON] et al. [51] concluded that at high stress levels, UIT-processed Ti-6 Al-4V has a fatigue life that is twice of the pristine (untreated) samples, and a roughness reduction Ra of 75%. While UIT is widely used in fatigue improvement of welded joints, the aim of this paper is to treat the surface of Ti-6 Al-4V specimens produced by DMLS using UIT to improve roughness, increase hardness and induce surface compressive residual stresses to enhance the fatigue life of the components. It is well known that the resistance to tensile fatigue of a metal will increase by the addition of compressive stress [37]. UIT compares favorably to other surface treatments due to its higher impact forces [48]. Quantifying this level of stress by the cold working of the surface for titanium alloys can prove to be beneficial in future engineering applications, as evaluation of impact forces between the pins and the metal surface of UIT is under researched. Force estimations for other cold hardening treatments such as shot peening have been measured. One method used acoustic emissions for velocities of 30 to 88 m/s; however, this was not able to properly determine the impact force [52].