**1. Introduction**

As a unique type of structural and functional material, a porous structure has unique advantages in fields including filtration and separation [1], energy absorption [2], heat exchange [3], electromagnetic shielding [4], and artificial implants [5], and is widely used in aerospace, automotive, chemical, and biological medical industries. Depending on the demand, the porous structure can be manufactured from materials including metal [6,7], ceramics [8], and polymers [9]. With the deepening of their application, porous structures with function-oriented design have been widely developed to obtain precise and complex structures, such as porous implants [10]. However, it is difficult to fabricate such structures using the traditional processing method. Additive manufacturing (AM) techniques are attracting increasing attention due to their impressive capability to produce precise parts with a controlled architecture [11]. Using a layer-wise building approach and a direct link with a

computer-aided design (CAD) model, AM has been described as a crucial production technique to achieve a more controlled porous structure [12–15]. A recently developed AM technique, the laser powder bed fusion process (LPBF), usually called selective laser melting (SLM), can directly create a functional and complex metal part, like a Ti-based porous scaffold for bone tissue engineering, bringing a high degree of freedom to design [16,17]. Most researchers have focused on evaluating the function or forming quality of as-designed porous samples fabricated by LPBF using a set of universal process parameters embedded in the commercial equipment [6,18–22]. However, a porous structure is composed of the struts or walls, which usually have small geometric characteristic size, so a set of process parameters with a universal nature may not be optimal for manufacturing porous structures. A small number of researchers have focused on the impact of process parameters on the forming quality of LPBF-built porous structures. For example, Liu et al. [23] investigated the effect of the scan speed on the forming defects, precision, and mechanical properties of biomedical titanium alloy scaffolds fabricated by LPBF. Ahmadi et al. [24] studied the effects of laser power on features including the surface roughness, strut diameter, relative density, hardness, and elastic modulus of the porous structures. In addition, Jamshidinia and Kovacevic found that the thin wall achieved more heat accumulation during the LPBF process, affecting the forming quality [25]. These findings indicate that the small geometric characteristic size may be a factor affecting the forming quality of LPBF-built porous structures.

On the basis of the above statement, the effect of the geometric characteristic size of the objective part on the forming quality of the solid strut is discussed in this work. The energy density was introduced to reveal the combined action of scan speed, laser power, hatching space, and layer thickness [26], which represented the thermal input during the LPBF process. As the most commonly used material in metal implant fields, Ti–6Al–4V alloy powder was selected for the experiment. Taking the geometric characteristic size into consideration, the influence rule of the factor on the forming quality of solid struts fabricated via LPBF using different scan speeds was investigated from the perspective of thermal transmission. The entire experiment was made up of two steps. Firstly, struts with different geometric characteristic size were fabricated using different combined process parameters. Secondly, porous structures with two strut sizes were fabricated with two scan speeds to verify the analysis in the first step.

## **2. Materials and Methods**
