key: cord-0302992-nk7st632 authors: Wang, Fengyi; Bosque, Hernan Del; Hyder, James; Corliss, Mike; Hung, Wayne Nguyen title: Experimental investigation of porosity distribution in selective laser melted Inconel 718 date: 2020-12-31 journal: Procedia Manufacturing DOI: 10.1016/j.promfg.2020.05.117 sha: babd9dbbf317d654609b5f226f1ec4b2dd1a9a85 doc_id: 302992 cord_uid: nk7st632 Abstract This paper reviews the mechanisms of pore forming in additively manufactured metals and presents the statistical analysis of porosity and its distribution in selective laser melted Inconel 718. Samples were printed at different combinations of laser powers and scanning speeds. Printed samples were sectioned, ground, and polished for porosity study. Imaging techniques captured the pore profile and distribution at different region for comparison: from inside to outside and from bottom to top of a sample. Although a linear energy density of 350 J/m significantly reduced porosity in a sample, choosing different combinations of power and scanning speed for the same energy density produced different results in porosity. Pore density at the periphery of a sample was about 3-20 times denser than that in the interior. Finish machining with at least 200 µm depth of cut was recommended to enhance surface finish, control dimension and tolerance while removing the high density porosity layer. The superalloy Inconel 718 (IN718) has been used in extreme environments in industry due to its excellent mechanical properties, corrosion and thermal resistance. In many applications, an engineering component must perform satisfactorily at high temperature and applied stress. Materials for fatigue /creep resistance must be free of material and process-induced defects. Selective laser melting (SLM) is chosen among many additive manufacturing methods to produce near net shape IN718 components. However, researchers are aware of porosity, poor surface finish, and residual stress as inherent issues for parts produced by SLM. Although residual stress can be removed by thermal stress relieving and rough surface can be corrected with subsequent machining /polishing, some porosity can be eliminated with suitable hot isostatic pressing (HIP) but other types of porosity cannot be closed by HIP'ing. Even though SLM'ed IN718 offers some advantages compared to subtractive methods, there are issues with pores, partially melted powder, shrinkage cavities, slags, and incomplete bonding between layers. Published literature has shown defects such as spherical gas-filled pores trapped in the molten metal, balling phenomenon, partially melted powder particles that adhered on the top surface or at the boundary between deposited layers. These defects --caused by incorrect selection of process parameters such as laser scanning speed, laser power level, focusing/ scanning strategy, wrong powder size and shapes--would act as detrimental stress raisers and finally lead to product failure during its service life. The effect of process parameters on porosity has been investigated by several researchers in both experimental studies and computer simulations. Part density, estimated by Archimedes technique, gives a bulk figure on porosity without information on pore distribution and size. A comprehensive study on the distribution of porosity in SLM'ed IN718 components is yet to be found. This paper (i) compares the The superalloy Inconel 718 (IN718) has been used in extreme environments in industry due to its excellent mechanical properties, corrosion and thermal resistance. In many applications, an engineering component must perform satisfactorily at high temperature and applied stress. Materials for fatigue /creep resistance must be free of material and process-induced defects. Selective laser melting (SLM) is chosen among many additive manufacturing methods to produce near net shape IN718 components. However, researchers are aware of porosity, poor surface finish, and residual stress as inherent issues for parts produced by SLM. Although residual stress can be removed by thermal stress relieving and rough surface can be corrected with subsequent machining /polishing, some porosity can be eliminated with suitable hot isostatic pressing (HIP) but other types of porosity cannot be closed by HIP'ing. Even though SLM'ed IN718 offers some advantages compared to subtractive methods, there are issues with pores, partially melted powder, shrinkage cavities, slags, and incomplete bonding between layers. Published literature has shown defects such as spherical gas-filled pores trapped in the molten metal, balling phenomenon, partially melted powder particles that adhered on the top surface or at the boundary between deposited layers. These defects --caused by incorrect selection of process parameters such as laser scanning speed, laser power level, focusing/ scanning strategy, wrong powder size and shapes--would act as detrimental stress raisers and finally lead to product failure during its service life. The effect of process parameters on porosity has been investigated by several researchers in both experimental studies and computer simulations. Part density, estimated by Archimedes technique, gives a bulk figure on porosity without information on pore distribution and size. A comprehensive study on the distribution of porosity in SLM'ed IN718 components is yet to be found. This paper (i) compares the The superalloy Inconel 718 (IN718) has been used in extreme environments in industry due to its excellent mechanical properties, corrosion and thermal resistance. In many applications, an engineering component must perform satisfactorily at high temperature and applied stress. Materials for fatigue /creep resistance must be free of material and process-induced defects. Selective laser melting (SLM) is chosen among many additive manufacturing methods to produce near net shape IN718 components. However, researchers are aware of porosity, poor surface finish, and residual stress as inherent issues for parts produced by SLM. Although residual stress can be removed by thermal stress relieving and rough surface can be corrected with subsequent machining /polishing, some porosity can be eliminated with suitable hot isostatic pressing (HIP) but other types of porosity cannot be closed by HIP'ing. Even though SLM'ed IN718 offers some advantages compared to subtractive methods, there are issues with pores, partially melted powder, shrinkage cavities, slags, and incomplete bonding between layers. Published literature has shown defects such as spherical gas-filled pores trapped in the molten metal, balling phenomenon, partially melted powder particles that adhered on the top surface or at the boundary between deposited layers. These defects --caused by incorrect selection of process parameters such as laser scanning speed, laser power level, focusing/ scanning strategy, wrong powder size and shapes--would act as detrimental stress raisers and finally lead to product failure during its service life. The effect of process parameters on porosity has been investigated by several researchers in both experimental studies and computer simulations. Part density, estimated by Archimedes technique, gives a bulk figure on porosity without information on pore distribution and size. A comprehensive study on the distribution of porosity in SLM'ed IN718 components is yet to be found. This paper (i) compares the 48th SME North American Manufacturing Research Conference, NAMRC 48 (Cancelled due to COVID-19) statistical distributions and formation mechanisms of porosity in SLM'ed IN718, and (ii) suggests suitable machining parameters to effectively remove the harmful pores below a surface. Inconel 718 (50-55 wt% Ni, 17-21 Cr, 13.25-24.6% Fe, 4.75-5.50 Nb, 2.8-3.3 Mo, 0.65-1.15 Ti, 0.2-0.8 Al, <1.0 Co, <0.08 C) is a superalloy that can retain its mechanical properties at high temperature up to 650°C [1] . Both volume and surface defects in additively manufactured (AM) metallic specimens have been reported in literature. Many researchers have established the effect of process parameters on porosity formation using either equations (1-3): Where: ED: energy density (J/mm 3 ) LED: linear energy density (J/mm) BED: beam energy density (J/mm 3 ) P: beam power (W) A: beam focused area (mm 2 ) V: beam scanning speed (mm/s) h: hatching distance (mm) t: layer thickness (mm) Equation (2) is commonly used when both hatching distance and layer thickness are kept unchanged in an experimental study. Two types of porosity were identified [2, 3] : − The spherical pores (gas-filled metallurgical pores) were smaller than 100 µm. These were evolved from powder consolidation and formed mostly at low scanning speed. − The irregular shaped and larger porosity (keyholes, or connected pores) may contain un-melted powder particles and indicate a lack of fusion due to insufficient laser energy at the melt pool. Although hot isostatic pressing (HIP) can be utilized to close metallurgical pores, HIP'ing can reduce the size of keyholes but may not completely closing them. Bean et al. [3] studied the effect of laser focused shift on quality of additively manufactured IN718. A negative correlation between surface finish and material density was established. By shifting the focusing plane +3 mm (by increasing the distance between workpiece and the laser nozzle), porosity reduced to 0.15% compared to 8.47% when the beam was shifted -3 mm closer to the workpiece. The equations (1-3) link AM metal quality with energy density. A high scanning speed of an energy beam reduced the beam energy density, shortened the interaction between laser and powder, decreased the time for liquid metal to flow, therefore, reduced the chance for gaseous porosity to escape from a molten metal pool. Opened pores on top of AM metal surfaces also formed by rapid solidification of molten liquid flow to fill the region previously occupied by shielding gas [4] . Porosity at outside and inside samples of SLM'ed IN625 were reported. The porosity dropped significantly when the beam energy density increased beyond 30 J/mm 3 . The pores, having circular shape between 10-40 µm, was reported to be about 0.3% at center of a sample; however, the circular porosity was replaced with l2.2% porosity of larger irregular shapes with 150-200 µm average size near the surface [5] . Powder shape also contributed to porosity and its variation. A study on porosity of IN625 produced by direct energy deposition technique was completed. The large powder of 160 µm resulted in 0.513% porosity, but only 0.244% porosity was reported when using the smaller powders with 100 µm average size. Different types of gases were found in the pores. Drilling tests in vacuum chamber was performed and the escaped gas was collected and analyzed with a mass spectrometer. Argon gas was reported during the first drilling test. This inert argon gas that shielded the metal powder during printing was trapped in the pores during the melting and solidification steps. In the subsequent drilling test, the hydrocarbon gases were collected and identified. These gases originated from moist powder particles or generating from carbon and silicon from the powder elements [6] . The irregular shape of water-atomized 316L powder (6-50 µm range, 30 µm average) formed irregular keyholes; by switching to the more uniform spherically-gas atomized powder (3-40 µm size, 20 µm average) resulted in much less porosity at the same SLM parameters [7] . Similar results were also reported by other researchers when using different powder for printing IN718 parts [8] . Both the gas atomized powders, had near spherical shape and containing voids themselves, contributed to more porosity in the product. The "perfectly spherical" powder produced by plasma rotating electrode technique resulted in minimal porosity in the product (Table 1) . Different pore forming mechanisms in 316L stainless steel during selective laser melting were investigated using computer simulation [9, 10] . • A laser beam heated and melted powder particles along a track and formed a v-shaped trench below the beam focal point. The dynamic molten metal moved upward along both sides of the trench surfaces, then collapsed inward when the beam moving away in its scanning path. Depending on the cooling rate and molten metal viscosity, pores can either be trapped in solidifying metal or floating up to form opened pores on the surface. • At the end of a scanning track, a laser turned off and caused molten liquid metal in the trench to collapse and trapped pores at the track ends. • Lateral pores were formed between tracks when powder particles did not melt completely to join the melt pool, or due to shorter melt width relative to a hatching distance. • Let (xy) be the building plane and z be the building direction. The pores in xy-planes were formed by incomplete bonding between layers at fast scanning speed. The xy-planes contained gas-filled pores or opened pores. Porosity of as-built AM metals is inevitable and detrimental to the product quality and performance. Formation mechanisms, and distribution of porosity at interior region and peripheral region near surfaces of SLM'ed IN718 will be presented in the following sections. Inconel 718 blocks were selectively laser printed to 15 mm long x 5 mm wide x 25 mm height with the Renishaw AM250 system. The 15 x 5 mm base surface was parallel to the scanning x-y plane while the 25 mm thickness was along the building zdirection (Fig. 1) . The average diameter of IN718 powder was 50µm. Powder particles were fused together using a YAG laser beam at different power levels and scanning speeds with a hatching distance of 110 µm and 60 µm layer thickness in argon gas ( Table 2 ). The stripe scanning strategy was adopted for the manufacturing of blocks. In this mode the deposition direction was uniform on a layer, but the angle shifted by 67ᵒ from one layer to the next (Fig. 2) . Upon completion, all specimen columns were cut off perpendicularly to the z-axis to separate them from the machine platform using wire-type electrical discharge machining (WEDM). Each sample was then sectioned at the middle and parallel to the sample xy-plane. Each sectional sample was mounted in epoxy, hand ground, and fine polished with diamond paste to 0.5 µm finish. Images of polished surfaces were systematically captured with the measuring microscope Olympus STM6 with 0.1 µm resolution. The images, with about 25% overlapping, were pixel-matched and then stitched to form a larger composite image for analysis. The Image J software was used to convert a color image into 8-bit black-and-while image, then filter out scratches before performing the statistical analysis (Fig. 3) . Areas of keyholes or gas-filled pores were calculated by the ImageJ software in terms of pixels. For calculation purpose, such areas were equated to areas of circles with "equivalent diameter." Figure 4 shows examples of pores and pore distributions at corners of the samples S1-S5. Irregular shaped voids were plentiful in the porous sample S1 that was printed at the lowest level of 67 J/m energy density. The low 200 W laser power melted most of the powder but not all since there were some powder particles partially adhered to the void surfaces. At the fast 3 m/s scanning speed when printing sample S1, the molten metal would cool quickly, increased its viscosity and reduced the fluidity of molten metals to fill the pores. When increasing the energy density to 138 J/m, the porosity was significantly reduced in sample S2. Notice the high density of gas-filled spherical pores at the peripheral region, and irregular shaped keyhole pores in the inner region. The three samples S3, S4, and S5 were fabricated at the same 350 J/m energy density, but at decreasing power and scanning speed ( Table 2 ). Irregular shaped pores were still present in sample S3 due to the relatively fast 1 m/s scanning speed. Larger pores were seen in samples S1 and S5 due to low 200 W laser power, but at a slower scanning speed of sample S5 (0.571 m/s) compared to that for sample S1 (3 m/s). The slower scanning speed allowed longer laser-powder interacting time, and the molten metal could flow and fill in most of the voids. Scanning electron microscopy images revealed microcracks on surface of many spherical pores (Fig. 4A) . Composite images of a typical bottom, middle, and top sections of a sample are shown in Fig. 5 . The outside regions, each with 200 µm width, were cropped off from each composite image. Statistical analysis was performed for inside, outside, top, middle, and bottom sections. The average pore diameters and standard deviations for each section was then calculated. The analysis also included the pore density (number of pore per unit area). Since the sample S1 and its replicate were too porous, they were not being analysed for pore size and pore distribution in subsequent study. Pore size distributions of samples S2-S5 are shown in Figs. 6a-d. the solid lines are for outside pores while the dash lines are for pores inside a sample. Both Figs. 5 and 6 indicate the difference of pore distribution in the inside and outside regions for all samples. The average pore size was about 25 µm and large pores (> 100 µm) were seen in samples S2 and S4, but the mean pore reduced to about 15 µm with most pores were smaller than 80 µm in samples S4 and S5. The mean pore diameters and their respective standard deviations were extracted and plotted in Fig. 7 for comparison. Fig. 7a shows the mean outside pores are always larger than inside pores for bottom, middle, and top sections of all samples. The sample S4 had the smallest and most uniformed pores, i.e, the lowest standard deviation value. As a laser beam reached the end of a track and turned off, the molten metals cooled rapidly and trapped voids at the peripheral zones. In contrast, the inside of a sample was surrounded with hot metals and cooled slower; the resulting molten metal with low viscosity at high temperature could redistribute to voids and tend to reduce the pore size. Although fabricated at the same linear density of 350 J/m, the three samples S3, S4, and S5 had different pore size and pore distribution. The samples S4 (350 J/m, 275 W, 0.786 m/s) had smaller and more uniform pores than other samples except at the outside region. The combination of laser power and scanning speed allowed low viscous molten metals to flow and had sufficient time to reduce the pore sizes. Perhaps the residual stress would be the lowest at these optimal conditions. Figures 8a-d compare the average pore density (number of pores per square millimeter). Define the pore ratio as: Table 3 summarizes the pore ratio for different sections of all samples S2-S5. The sample S2 had the lowest and most uniform pore ratio (about 1.0) for all sections, but its pore density was the highest (Fig. 8a) . Sample S4 had the lowest pore density (Fig. 8c ) among all samples but not the lowest pore ratio. Pore density was the highest at the top section compared to middle and bottom regions. Koutiria et al. reported the pore density of 12.2% at outside region and 0.3% at inside region [5] . The pore ratio, therefore, was about 41 for SLM Inconel 625 in their study. Additively manufactured metals have rough surfaces due to balling effect, partially welded metal powder particles to the surface, spatters, open pores, shrinkage cavities, and possible microcracks. The high pore density near the surface raises a concern for reliability since cracks commonly initiate at the surface of a component subjected to fatigue or creep conditions. Post processing techniques, such as finish machining or polishing, are necessary to improve and control the part surface for dimensional tolerance and surface finish. A machining /polishing step should remove at least 200 µm of SLM'ed IN718 to eliminate the high pore density zones. This study investigated the pore dimensions and their distribution in selective laser melted Inconel 718. It was found that: 1) Porosity was inevitable in additively manufactured metals. They degraded the part quality and reliability of printed components. 2) Imaging technique was used to accurately quantify the pore size and pore distribution of AM metals. 3) Linear energy density affected porosity. Printing at high energy density of 350 J/m significantly reduced the pore density. However, the outside pore density was still 3-20 times higher than that in the interior region. 4) The same energy density can be obtained by varying both laser power and scanning speed. The optimal would be 275 W and 0.786 m/s to minimize pore density. 5) Finish machining /polishing with at least 200 µm depth of cut on SLM'ed IN718 would eliminate the high pore density zone below the part surface. 1) Studying the effect of energy density higher than 350 J/m on porosity and mechanical properties of SLM'ed IN718. 2) Modifying the scanning strategy to minimize pore density and pore ratio in different part orientation. Reducing porosity in AlSi10Mg parts processed by selective laser melting Effect of laser focus shift on surface quality and density of Inconel 718 parts produced via selective laser melting Porosity evolution and its thermodynamic mechanism of randomly packed powder-bed during selective laser melting of Inconel 718 alloy Influence of SLM process parameters on the surface finish, porosity rate and fatigue behavior of as-built Inconel 625 parts Influence of metal powder characteristics on product quality with directed energy deposition of Inconel 625 Densification behavior of gas and water atomized 316L stainless steel powder during selective laser melting Studies of Standard Heat Treatment Effects on Microstructure and Mechanical Properties of Laser Net Shape Manufactured INCONEL 718 Laser powderbed fusion additive manufacturing: Physics of complex melt flow and formation mechanisms of pores, spatter, and denudation zones Mesoscopic simulation model of selective laser melting of stainlesssteel powder Influences of energy density on porosity and microstructure of selective laser melted 17-4PH stainless steel; Solid Freeform Fabrication Symposium 316L Stainless Steel with Gradient Porosity Fabricated by Selective Laser Melting Study of the pore formation on CoCrMo alloys by selective laser melting manufacturing process Effects of Selective Laser Melting additive manufacturing parameters of Inconel 718 on porosity, microstructure and mechanical properties A study of the microstructural evolution during selective laser melting of Ti-6A-4V Fine-structured aluminium products with controllable texture by selective laser melting of prealloyed AlSi10Mg powder