# Dimensional Errors Due to Overhanging Features in Laser Powder Bed Fusion Parts Made of Ti-6 Al-4V
## 1 Introduction
They were able to achieve a significant reduction in surface roughness (Ra) by using higher values of laser power under lower values of the feed rate and scan speed [14]. [PERSON] et al. developed a methodology for precise measurement of the focal spot of a Computed tomography (CT) scanner as it is one of the essential factors in determining the uncertainty in CT dimensional measurements of AM parts. Their results showed progress towards defining the optimal process parameters to be used for desired resolutions [15]. L-PBF, down-facing, or down-skin surfaces are present on parts that contain overhanging features that are not printed over solid bulk material, but on lose powder (as seen in Figure 1). It is especially difficult to print these surfaces as they normally show large dimensional errors and require support structures, thereby necessitating extra process steps for support removal and finishing. The main cause of these dimensional deviations is cross formation, which is seen in Figure 1[16]. This is the result of the melting of excess material. This excess melting is caused by the overheating of lose powder, resulting in the formation of large dimensional deviations from the Computer-aided design (CAD) model. The degree of cross formation is dependent on the process parameters and the effect of cross formation on the dimensional deviation and surface topography can vary [13; 17]. A number of studies have investigated overhangs and down-facing surfaces, as well as the accompanying support structures required by overhangs, with the aim of improving the printability and processability of these structures. [PERSON] et al. approached this issue from a mathematical point of view, and proposed a level set-based method to create object-dependent support structures for filament-based 3D printing [18]. In a similar shape optimization study, [PERSON] et al. demonstrated a topology optimization framework that considers overhangs as a constraint during topology optimization and proposes printable designs [19]. Similarly, [PERSON] et al. proposed a density gradient-based approach for determining the overhang constraint and their results showed promise for the inclusion of build orientation into topology optimization in an effective way [20]. [PERSON] et al. conducted an experimental parameter optimization study for down-facing surfaces in L-PBF and compared the results to a thermal simulation model. Their research showed that for inclined surfaces, low powers exhibited better surface qualities, but at an inclination angle of 60\({}^{\circ}\) to the build platform [21]. An alternative approach to improving the quality of overhangs without the use of support structures is achieved through the use of contact-free support structures. [PERSON] et al. have demonstrated and highlighted its benefits in terms of it not requiring or only requiring very little post processing, while effectively reducing overhang distortion and deformation. Their conclusions remain positive for the adoption of the contact-free support structures technique for industrial applications [22]. In subsequent research, [PERSON] et al. proposed two different minimal contact supports, namely a thin wall tooth contact and a non-contact design. They showed promising results in terms of heat dissipation and reduced distortions [23]. [PERSON] et al. developed a novel contactless support structure, specifically a thin blade parallel to the critical down-facing area, to transfer heat away from the melt pool via conduction through the powder bed instead of through direct contact and also conducted numerical modeling in order to understand the parameters that define the optimal distance between the down-facing area and the support structure [24]. [PERSON] et al.