## 1 Introduction  
Ti-6 Al-4V titanium alloy was widely used in the aerospace and military industries due to its excellent specific strength, high shear modulus, low density and thermal expansion coefficient [1; 2; 3]. However, traditional manufacturing methods of Ti-6 Al-4V titanium alloy, such as forging or cutting, which have the disadvantages of a high buy-to-fly ratio and low manufacturing flexibility, are not economical. The laser additive manufacturing (LAM) process which fabricates Ti-6 Al-4V titanium alloy with a near net shape has no restriction in the component shapes. It has been popular in recent studies [4; 5; 6; 7] and has been widely used to fabricate complex components in aerospace and other industries [1]. The typical grain structure in the fabricated Ti-6 Al-4V titanium alloy samples is coarse columnar prior \(\beta\) grains, which arise due to high temperature gradients and distribute approximately parallel to the building direction [8].  
Wire and powder are the two main material forms of the Ti-6 Al-4V in LAM process [6; 9]. SLM (Selective Laser Melting) is a typical LAM method which uses powder as the feedstock. It has the advantages of high manufacturing capacity and accuracy. Compared with powder, using wire as the feedstock form has the great advantages of high efficiency, low cost and simple handling and storage operation during the manufacturing process [9; 10]. Therefore, it has received more attentions in recent research activities [11; 12; 13; 14; 15; 16].  
LAM with wire feeding can be divided into vertical wire feeding and paraxial wire feeding [9; 17]. LAM with wire feeding evolved from laser wire feed welding, so paraxial wire feeding accounts for the majority of the wire feeding methods, and the schematic drawing of it is indicated in Figure 1a. Although it is easy to implement, there is an anisotropic forming problem in the deposition process due to different wire feed orientations [18]. There are three kinds of wire feed orientations in the deposition process, which are shown in Figure 2. The depositions from front feeding and side feeding have a smoother surface than back feeding [19], and the smoothest surface can be obtained when the wire is positioned at the leading edge of the molten pool in front feeding [20]. Due to the different forming morphologies in different wire feed orientations, different layer thicknesses may appear in the same layer, especially when forming large complex components.  
Vertical wire feeding with an axisymmetric multi-laser source can be realized by exchanging the position of the laser head and wire feeder, as compared with the paraxial wire feeding. The schematic diagram is shown in Figure 1b. It is obvious that the wire feeding orientation will not change no matter what the scanning direction is in the deposition process. Therefore, there will be no anisotropic forming problem in the deposition process, but few studies have been done to characterize the forming ability of vertical wire feeding with an axisymmetric multi-laser source. Thus, the aim of this paper is to investigate the forming ability of vertical wire feeding with an axisymmetric multi-laser source as well as the microstructure and mechanical properties of the fabricated components.  
Figure 1: The schematic of laser additive manufacturing with: (**a**) paraxial wire feeding; (**b**) vertical wire feeding.  
Figure 2: The schematic of wire feed orientations: (**a**) front feeding; (**b**) back feeding; (**c**) side feeding.