# Investigation of Ti65 Powder Spreading Behavior in Multi-Layer Laser Powder Bed Fusion
## 1 Introduction  
Titanium alloys such as TA15 (Ti-6.5 Al-1 Mo-1V-2 Zr), TC4 (Ti-6 Al-4V), and Ti60 (Ti-6 Al-2 Zr-4.8 Sn-1 Mo-0.35 Si-0.85 Nd) exhibit excellent high-temperature strength (up to \(\sim\)600 \({}^{\circ}\)C), low thermal conductivity (\(\sim\)7-12 W/m\(\cdot\)K), and good oxidation resistance in moderate-temperature environments [1, 2, 3]. Ti65 (Ti-Al-Sn-Zr-Mo-Si-Nb-Ta-W-C-Fe), as one of the near-\(\alpha\) titanium alloys [4, 5], demonstrates even better properties, with service temperatures reaching 650 \({}^{\circ}\)C or even 750 \({}^{\circ}\)C. Titanium alloys are considered difficult-to-machine materials [6, 7] due to challenges such as high cutting temperatures, severe tool wear, and material adhesion during machining, primarily resulting from their low thermal conductivity, strain hardening behavior, and high-temperature chemical activity. As the demand for fabricating titanium alloy parts with intricate geometries and internal structures continues to grow, conventional manufacturing methods (e.g., casting, bulk forming, and machining) struggleto meet the stringent requirements [1]. Moreover, conventional processes typically lead to significant material waste, high manufacturing costs, and long production time. By contrast, additive manufacturing (AM) via powder bed fusion using a laser beam (PBF-LB) offers an effective alternative [8; 9; 10] for fabricating titanium alloy components that require high precision, multi-variety customization, small batch sizes, and complex inner structures. Specifically, PBF-LB can achieve final parts with low porosity, reduced surface roughness (_Ra_) (7-20 \(\upmu\)m), and minimal dimensional deviation (0.04-0.2 mm) [7]. However, the forming quality in PBF-LB is determined by various factors, such as processing parameters in powder spreading and the complex interplay of powder material properties during powder melting [11; 12]. Improper control of the influencing factors often results in typical defects such as pores, unfused regions, cracks, and high-density inclusions [13]. As a 10-component near-a titanium alloy, Ti65 possesses distinct physical properties [5]. In addition, the incorporation of high-melting-point elements such as Mo, Nb, and W increases the viscosity of the melt pool [14]. Those distinct properties make the operation parameters (e.g., powder spreading speed, laser power, and scanning speed) of other metals (e.g., 316L stainless steel, Ti-6 Al-4V, W, and Mo [7; 11; 15; 16]) in PBF-LB not applicable to Ti65. Despite T65's promising potential, research efforts to date have largely focused on optimizing its alloy composition, characterizing microstructures, and evaluating mechanical properties [5; 17; 18], with relatively limited work on its PBF-LB manufacturing process. Presently, process development for Ti65 still heavily relies on trial-and-error experimentation, which is costly, time-consuming, and subjected to interlayer cracking and warpage deformation, especially under thermal gradients that accumulate during multi-layer builds. Therefore, there is an urgent need to establish numerical models capable of simulating Ti65 powder behavior during multi-layer PBF-LB. Against this background, a coupled discrete element method-computational fluid dynamics (DEM-CFD) numerical model [19] is utilized to investigate the powder spreading behavior of Ti65 in multi-layer PBF-LB, aiming to address the following critical questions: (1) How does powder bed quality evolves over successive layers? (2) How do solidified regions influence the spreading of new layers and the resulting powder bed quality? (3) What are the underlying mechanisms? Powder bed quality is evaluated using macroscopic properties such as packing density, uniformity, and surface roughness, as well as microscopic properties including particle rearrangement, coordination number, and pore size. The underlying mechanisms are analyzed through particle movement and average contact force.