# Control of Porosity in Parts Produced by a Direct Laser Melting Process
## 1 Introduction  
Direct laser melting (DLM), also known as selective laser melting (SLM) or direct metal laser sintering (DMLS), is a rapid prototyping, 3D printing, or additive manufacturing (AM) technique designed to use a high power-density laser to melt and fuse metallic powders together. DLM has been widely used for net shape production, owing to its capacity to fabricate complex freeform geometries. The technology enables a wide spectrum of possibilities for constructing 3D parts for various applications such as automobiles, medical supplies, and mechanical parts. When compared with other traditional techniques, laser processing typically does not require mechanical toolings, and, therefore, exhibits high flexibility [1; 2; 3; 4; 5]. Porous metal parts have also been applied to filters, fluid-permeable components, electronic products, and biomaterials. As porous metal parts may be lightweight and exhibit high gas permeability and heat transfer capabilities, the development of functional products using porosity is very important in numerous industrial fields. However, it is challenging to control the porosity of porous metal parts precisely. Therefore, a quantitative analysis was performed in this study for the active control of porosity during DLM. Porosity is a common issue in laser-processed parts, and several studies have been performed on this topic. Porosity formationis a process-induced phenomenon that is strongly dependent on the processing conditions used to produce a part. It is generally believed that irregular-shaped pores are caused by inadequate fusion or melting [6]. [PERSON] et al. [7] classified laser process windows according to the laser power and scan rate. Several process zones were identified, corresponding to incomplete melting, fully dense material, over-melting, and overheating. A statistical porosity evaluation was performed by using descriptors such as circularity, convexity, and elongation, which were related to the different process window zones. [PERSON] et al. [8] directly observed open pores on the top surfaces of laser-processed Ti-based alloy. The spherical pores were attributed to the incomplete re-melting of certain localized sites on the previous layer as well as the insufficient feeding of molten metal to these sites. [PERSON] et al. [9] suggested that the spherical porosity formation in laser-processed Ti6 Al4V alloy was a result of gas confinement during melting and rapid solidification. [PERSON] et al. [10] attributed porosity to poor flow, a lack of fusion, and incomplete re-melting of laser-processed 316L stainless steel under a relatively low energy input. [PERSON] et al. [11] studied the porosity formation mechanism and pore defect generation using 316L stainless steel laser powder. [PERSON] et al. [12] determined that pore formation and the development of rough surfaces in Ti-6 Al-4V alloy were strongly associated with unstable melt flow and splashing of molten material, based on results obtained from high-speed imaging and modeling. [PERSON] et al. [13] developed a melt flow dynamics model, and successfully used it to explain the morphological development of pores formed under different processing conditions. [PERSON] et al. [14] investigated the effect of porosity on tensile behavior of Ti-6 Al-4V specimens produced by laser engineered net shaping technology. They showed that the porous samples had considerably lower elongation at failure which was a result of stress raisers in the material, which facilitates the propagation of cracks in the sample. [PERSON] et al. [15] also investigated the fatigue behavior of porous Ti-6 Al-4V parts produced by laser engineered net shaping technology. Although the static strength of porous samples and non-porous samples were almost identical, considerably lower fatigue strength was observed for porous samples because stress raising effect facilitates fatigue crack initiation and fatigue crack propagation along the net section of the sample.