Across various industries, additive manufacturing (AM) is witnessing a surge in interest around machine and material development. Collectively, AM describes a vast array of layerwise forming technologies. For metals specifically a subset of AM known as selective laser melting (SLM) has demonstrated utility in the production of one-off components, functional assemblies, and the screening of novel materials and fabrication strategies. Commercial implementation of SLM has intensified the need for sustainable production practices. SLM is a powderbed AM process. This means for a given build volume, only a portion of the raw material is fused. Since the cost of powder is one the highest associated expenses for SLM operations, there is a negative cost implication when unused powder is amor- * Corresponding author. Tel.: 704-687-5086.
E-mail address: steve.schmid@uncc.edu (Steven R. Schmid). tized across production volumes. Used powder can be recycled as waste or incorporated into pressed and sintered powder metal parts, but reuse does not recover the full powder cost and powder metallurgy may not be a readily available option. In response, SLM operators have adopted practices that improve powder utilization, lowering manufacturing costs. A common strategy is powder reuse, where un-consumed powder in the build and reclaimed chamber is sieved and then replaced in the supply hopper for subsequent production runs. Since powder reuse enables a larger percentage of raw material to be converted into valued parts, increased powder efficiencies translate to direct cost savings.
Despite improved powder economics, reuse has been shown to induce measurable changes in powder quality from repeated cycling of metallic feedstocks. Artificially aged Ti-6Al-4V (Ti64), AlSi 10 Mg, and common ferritic alloys processed through commercial AM machines revealed that the average size of reused powder trended larger from finer powder tending to agglomerate and increasing flowability overtime [1, 2] . This finding was supported when reused Ti64 powder was shown to have a reduced volume of smaller-satellite particles, while some
Across various industries, additive manufacturing (AM) is witnessing a surge in interest around machine and material development. Collectively, AM describes a vast array of layerwise forming technologies. For metals specifically a subset of AM known as selective laser melting (SLM) has demonstrated utility in the production of one-off components, functional assemblies, and the screening of novel materials and fabrication strategies. Commercial implementation of SLM has intensified the need for sustainable production practices. SLM is a powderbed AM process. This means for a given build volume, only a portion of the raw material is fused. Since the cost of powder is one the highest associated expenses for SLM operations, there is a negative cost implication when unused powder is amor- * Corresponding author. Tel.: 704-687-5086.
E-mail address: steve.schmid@uncc.edu (Steven R. Schmid). tized across production volumes. Used powder can be recycled as waste or incorporated into pressed and sintered powder metal parts, but reuse does not recover the full powder cost and powder metallurgy may not be a readily available option. In response, SLM operators have adopted practices that improve powder utilization, lowering manufacturing costs. A common strategy is powder reuse, where un-consumed powder in the build and reclaimed chamber is sieved and then replaced in the supply hopper for subsequent production runs. Since powder reuse enables a larger percentage of raw material to be converted into valued parts, increased powder efficiencies translate to direct cost savings.
Despite improved powder economics, reuse has been shown to induce measurable changes in powder quality from repeated cycling of metallic feedstocks. Artificially aged Ti-6Al-4V (Ti64), AlSi 10 Mg, and common ferritic alloys processed through commercial AM machines revealed that the average size of reused powder trended larger from finer powder tending to agglomerate and increasing flowability overtime [1, 2] . This finding was supported when reused Ti64 powder was shown to have a reduced volume of smaller-satellite particles, while some
Across various industries, additive manufacturing (AM) is witnessing a surge in interest around machine and material development. Collectively, AM describes a vast array of layerwise forming technologies. For metals specifically a subset of AM known as selective laser melting (SLM) has demonstrated utility in the production of one-off components, functional assemblies, and the screening of novel materials and fabrication strategies. Commercial implementation of SLM has intensified the need for sustainable production practices. SLM is a powderbed AM process. This means for a given build volume, only a portion of the raw material is fused. Since the cost of powder is one the highest associated expenses for SLM operations, there is a negative cost implication when unused powder is amor- * Corresponding author. Tel.: 704-687-5086.
E-mail address: steve.schmid@uncc.edu (Steven R. Schmid). tized across production volumes. Used powder can be recycled as waste or incorporated into pressed and sintered powder metal parts, but reuse does not recover the full powder cost and powder metallurgy may not be a readily available option. In response, SLM operators have adopted practices that improve powder utilization, lowering manufacturing costs. A common strategy is powder reuse, where un-consumed powder in the build and reclaimed chamber is sieved and then replaced in the supply hopper for subsequent production runs. Since powder reuse enables a larger percentage of raw material to be converted into valued parts, increased powder efficiencies translate to direct cost savings.
Despite improved powder economics, reuse has been shown to induce measurable changes in powder quality from repeated cycling of metallic feedstocks. Artificially aged Ti-6Al-4V (Ti64), AlSi 10 Mg, and common ferritic alloys processed through commercial AM machines revealed that the average size of reused powder trended larger from finer powder tending to agglomerate and increasing flowability overtime [1, 2] . This finding was supported when reused Ti64 powder was shown to have a reduced volume of smaller-satellite particles, while some
Across various industries, additive manufacturing (AM) is witnessing a surge in interest around machine and material development. Collectively, AM describes a vast array of layerwise forming technologies. For metals specifically a subset of AM known as selective laser melting (SLM) has demonstrated utility in the production of one-off components, functional assemblies, and the screening of novel materials and fabrication strategies. Commercial implementation of SLM has intensified the need for sustainable production practices. SLM is a powderbed AM process. This means for a given build volume, only a portion of the raw material is fused. Since the cost of powder is one the highest associated expenses for SLM operations, there is a negative cost implication when unused powder is amor- * Corresponding author. Tel.: 704-687-5086.
E-mail address: steve.schmid@uncc.edu (Steven R. Schmid). tized across production volumes. Used powder can be recycled as waste or incorporated into pressed and sintered powder metal parts, but reuse does not recover the full powder cost and powder metallurgy may not be a readily available option. In response, SLM operators have adopted practices that improve powder utilization, lowering manufacturing costs. A common strategy is powder reuse, where un-consumed powder in the build and reclaimed chamber is sieved and then replaced in the supply hopper for subsequent production runs. Since powder reuse enables a larger percentage of raw material to be converted into valued parts, increased powder efficiencies translate to direct cost savings.
Despite improved powder economics, reuse has been shown to induce measurable changes in powder quality from repeated cycling of metallic feedstocks. Artificially aged Ti-6Al-4V (Ti64), AlSi 10 Mg, and common ferritic alloys processed through commercial AM machines revealed that the average size of reused powder trended larger from finer powder tending to agglomerate and increasing flowability overtime [1, 2] . This finding was supported when reused Ti64 powder was shown to have a reduced volume of smaller-satellite particles, while some
Across various industries, additive manufacturing (AM) is witnessing a surge in interest around machine and material development. Collectively, AM describes a vast array of layerwise forming technologies. For metals specifically a subset of AM known as selective laser melting (SLM) has demonstrated utility in the production of one-off components, functional assemblies, and the screening of novel materials and fabrication strategies. Commercial implementation of SLM has intensified the need for sustainable production practices. SLM is a powderbed AM process. This means for a given build volume, only a portion of the raw material is fused. Since the cost of powder is one the highest associated expenses for SLM operations, there is a negative cost implication when unused powder is amor- * Corresponding author. Tel.: 704-687-5086.
E-mail address: steve.schmid@uncc.edu (Steven R. Schmid). tized across production volumes. Used powder can be recycled as waste or incorporated into pressed and sintered powder metal parts, but reuse does not recover the full powder cost and powder metallurgy may not be a readily available option. In response, SLM operators have adopted practices that improve powder utilization, lowering manufacturing costs. A common strategy is powder reuse, where un-consumed powder in the build and reclaimed chamber is sieved and then replaced in the supply hopper for subsequent production runs. Since powder reuse enables a larger percentage of raw material to be converted into valued parts, increased powder efficiencies translate to direct cost savings.
Despite improved powder economics, reuse has been shown to induce measurable changes in powder quality from repeated cycling of metallic feedstocks. Artificially aged Ti-6Al-4V (Ti64), AlSi 10 Mg, and common ferritic alloys processed through commercial AM machines revealed that the average size of reused powder trended larger from finer powder tending to agglomerate and increasing flowability overtime [1, 2] . This finding was supported when reused Ti64 powder was shown to have a reduced volume of smaller-satellite particles, while some
Across various industries, additive manufacturing (AM) is witnessing a surge in interest around machine and material development. Collectively, AM describes a vast array of layerwise forming technologies. For metals specifically a subset of AM known as selective laser melting (SLM) has demonstrated utility in the production of one-off components, functional assemblies, and the screening of novel materials and fabrication strategies. Commercial implementation of SLM has intensified the need for sustainable production practices. SLM is a powderbed AM process. This means for a given build volume, only a portion of the raw material is fused. Since the cost of powder is one the highest associated expenses for SLM operations, there is a negative cost implication when unused powder is amor- * Corresponding author. Tel.: 704-687-5086.
E-mail address: steve.schmid@uncc.edu (Steven R. Schmid). tized across production volumes. Used powder can be recycled as waste or incorporated into pressed and sintered powder metal parts, but reuse does not recover the full powder cost and powder metallurgy may not be a readily available option. In response, SLM operators have adopted practices that improve powder utilization, lowering manufacturing costs. A common strategy is powder reuse, where un-consumed powder in the build and reclaimed chamber is sieved and then replaced in the supply hopper for subsequent production runs. Since powder reuse enables a larger percentage of raw material to be converted into valued parts, increased powder efficiencies translate to direct cost savings.
Despite improved powder economics, reuse has been shown to induce measurable changes in powder quality from repeated cycling of metallic feedstocks. Artificially aged Ti-6Al-4V (Ti64), AlSi 10 Mg, and common ferritic alloys processed through commercial AM machines revealed that the average size of reused powder trended larger from finer powder tending to agglomerate and increasing flowability overtime [1, 2] . This finding was supported when reused Ti64 powder was shown to have a reduced volume of smaller-satellite particles, while some 2351-9789 c 2019 The Authors. Published by Elsevier B.V. Peer review under the responsibility of the scientific committee of NAMRI/SME. 48th SME North American Manufacturing Research Conference, NAMRC 48 (Cancelled due to larger particles depicted accumulated surface damage [3] Subsequent studies found that SLM nickel-based alloys observe a drift in various elemental compositions as powder was recycled [4] . Given the physical and chemical differences observed for recycled SLM powder, process validation is necessary to evaluate the effects of thermal and mechanical powder recycling on resultant SLM parts, especially for highly regulated industries.
The effect of powder reuse on sintered properties has been investigated for a variety of AM technologies including electron beam and laser-based AM. Static mechanical testing across consecutive builds has revealed that tensile strength for AM titanium and stainless steel (SS) alloys are relatively robust to changes in powder [5, 6, 7, 8] . But though the reuse evaluations of static properties are critical, many applications encounter cyclic loading, where a larger material volume is exposed to low magnitude stress for an extended period. Under these conditions the same material can experience different failure modes based on the spatial location of internal defects and how microdamage accumulates in the microstructure [9] . For orthopedic and dental components, this type of fatigue fracture persists as a primary causes of aseptic loosening and poor clinical outcomes, highlighting the importance of assessing SLM metal under relevant loading conditions [10, 11] .
Accelerated fatigue testing has indeed been examined for a variety of AM metals (Table 1) . Trends indicate that SLM austenitic steels, 15-5PH and 17-4PH, report lower fatigue strength compared to conventional wrought material. AM fatigue studies also highlight the importance of processing history, surface finish, and part print orientation, as horizontal, 0 • , direction is reported to have less severe fatigue conditions for selective laser melted 17-4PH SS compared to the vertical, 90 • , direction [12, 13, 14] .
Another portion of the previous fatigue work has focused on AM Ti-6Al-4V (Ti64), which report lower fatigue strength due to laser-induced anisotropy and weaker grain boundaries within the microstructure, while others demonstrate improved fatigue performance [11, 16, 18, 20, 24, 25, 26] . The varied consensus around AM fatigue, highlights the knowledge gaps in the compendium of AM materials with respect to processing. For instance, incomplete fusion Ti-6Al-4V near the specimen surface was identified as the primary source for crack initiation [19] . Similar flaws anticipated for in nickel-based alloy specimens was then developed into a model that relates AM fatigue strength to microstructural defects and material hardness [27] . Thermal conditioning is another important factor that can either improve or diminish fatigue strength. Post-processing operations need to balance the release of residual stresses with the coarsening of grains, as larger grains can increase anisotropic behavior and have been shown to heighten localized strain along grain boundaries [28] . Surface finish and build direction are also critical to fatigue outcomes, where polished surfaces and a horizontal build direction, relative to the long axis of the specimen, yield higher endurance limits [17, 20, 22, 26] . Research encompassing feedstock recycling and cyclic loading of AM materials demonstrate that understanding SLM-AM has matured, but fatigue outcomes with respect to powder reuse still have not been examined. This paper investigates the fatigue Note: HT = heat treated, AB = as-built, P = polished, M = machined, HIP = hot isostatic pressed, 0 = x − y plane, 90 = z direction.
properties of metal alloys fabricated with recycled feedstocks in a rotating beam configuration. The endurance limit of sintered AM parts fabricated with virgin and reused materials were compared for 316L SS, 17-PH SS, and Ti64. 
Virgin 316L SS, 17-4 PH SS, and Ti64 powder (Electro Optical Systems, EOS GmbH, Germany) was obtained and classified as Reuse 0. A 'Reuse' cycle was defined as the movement 2 from the supply hopper to the build plate, followed by sieving. Reuse 8 powder was generated by cycling all three metallic feedstocks through either a commercial DMLS machine or a custom-built test cell designed to mimic the environment (argon) and layer-wise spreading (Fig. 1) . After all powder batch was cycled, the entire volume of powder was screened through a mesh sieve (63 µm for 316L/Ti64 and 74 µm for 17-4PH prior to being returned to the supply cylinder. The time between reuses was not longer than one day.
Reuse 0 and 8 powders were then placed in a commercial DMLS machine (M290, EOS, Germany) to build cylindrical blanks, 11.4 x 82.0 mm in diameter. All bars were oriented horizontally to significantly reduce the amount of powder necessary to achieve eight reuses. For all three materials optimized machine parameters were identified from a static strength DOE of the tensile coupons (Table 2 ) [29] . 316L and Ti64 builds were stress-relieved at 650 • C for three hours and heat-treated at 800 • C for two hours in an inert atmosphere, while 17-4PH builds received a solution anneal at 1040 • C ± 15 • C held for 30 minutes, followed by a H900 procedure and cooled in air less than 32 • C. All blanks were machined with the hour-glass geometry and a 4.76 mm gauge diameter, d (Fig. 2) . 
All machined specimens were tested at room-temperature on units that applied fully reversed, rotating beam fatigue (RBF) at 83.3 Hz (HT200 System Integrators, Glendale, Arizona). For the study, the endurance limit was determined using the staircase method and Dixon & Mood analysis to estimate the statistical distribution of fatigue strengths for the three metal alloys (Fig. 3) [9] . Also known as the up-and-down method, the stair-case method evaluates samples at a stress amplitude dictated by the survival of the previous specimen, permitting a reduced number of specimens, as low as 5-10, needed to make a reasonable estimate for the endurance limit [30] . The laminar architecture ran parallel to the long axis of each bar such that in the rotating beam configuration the strength within a layer was evaluated in pure bending, as opposed to the bond strength between successive layers. This was considered appropriate since the objective was to establish material-specific, not system-specific reuse behavior.
A combination of equations that account for stress level, stress level increment from the lowest identified stress, and the frequency of the survivals and failures are employed to calculate the average and standard deviation of fatigue stress limit [9] . Each fatigue test is evaluated a variable stress level and after all samples are evaluated, the number of failures and survivals are summed, n i , for each stress level, σ i . The lowest stress, σ o , where no failure is identified, is used to assign an order number to the remaining stress levels. The endurance limit is then calculated from
and d is step size of the tested stress levels. An estimated endurance limit was calculated for each material group. If survival was more common, A n represented the instances of survival and the plus sign was applied. If failure was more common, A n represented the number of failures and the minus sign was applied [9] . The infinite fatigue region for ferrous metal alloys is often characterized as one to ten million cycles [16] . For this study, the cycle limit was set to five million cycles; if a specimen survived this cycle count, then the stress amplitude was raised, and if the sample fractured before this limit, the stress amplitude was lowered. RBF testing was randomized for the three metals to reduce experimental bias.
Though AM is characterized by layer-wise consolidation and not bulk sintering, powder metallurgy density standards can still be applied [17] . Following fatigue testing, the same specimens were used to measure the bulk density of the Reuse 0 and Reuse 8 materials. Using the Archimedes approach, all materials were submerged in an oil with a viscosity of 31.8 cSt at 82 • C (ASTM B962-15). Comparisons were made using the reported theoretical sintered densities (EOS Material Data Sheets). A two-tailed, unpaired student's t-test was employed to determine the effect of 216L and Ti64 density with respect to virgin (n = 5) and reused (n = 5) feedstocks. 17-4PH SS bulk density measurements were examined with a two-way Analysis of Variance (ANOVA) to establish influence of machine settings, default (n = 4) or optimized (n = 4) parameters, in addition to reuse condition. A post-hoc Fisher's Least Significant Difference test was applied as appropriate (α = 0.05 for all tests). Fractography of the fatigued samples was performed on a Keyence Olympus VHX 5000 for high depth of field. Images were coupled this with scanning electron microscopy (SEM) as needed.
The staircase method only requires a modest amount of specimens, therefore the characteristic 'bend' near the infinite fatigue region of the standard S-N, or Wohler's, curves would require samples tested for a larger range of stress amplitudes [30] . Determination of the stair-case method was performed by identifying the lowest stress amplitude each material survived five million cycles and the number of survivals and failures for the subsequent stress intervals. The endurance limit for all materials was determined with the up-and-down method. Results shows that fatigue performance, S e , was highest for 17-4PH SS, followed by Ti64, and then 316L SS (Fig. 4) . Comparing virgin and reused feedstocks, no significant differences were seen in DMLS metal fatigue behavior. A nominal increase of 3-7 MPa was observed in steel fatigue strength from Reuse 0 to Reuse 8, but the difference was not significant. Reused Ti64 Fig. 4 . Summary of mean endurance limits. The endurance limit in rotational bending, defined by the 50% survival of parts subject to five million cycles and the standard deviation. Increased cycles of powder reuse correlated with decreased scatter in the low (10%) and high (90%) scatter seen between the 10% and 90% 316L and Ti-6Al-4V. demonstrated a decrease in fatigue strength, but the difference was within the standard deviation of the range of stress amplitudes tested. Large variance, over 100 MPa, was observed for virgin and reused 17-4PH SS indicating that the range of stress amplitudes tested was greatest for this material. Large scatter seen for DMLS 17-4PH SS is common even for wrought steel and is not unusual for fatigue behavior of hardened materials [30] . Subsequently, the range of 10-and 90-percent survival rate was greatest for 17-4PH, then Ti64, and then 316L.
Given that these values estimate the stress levels for the defined percentage of part survival, the 90-percent survival associated with a lower stress amplitude. Bulk density measurements were evaluated for all virgin and reused specimens. Two-tailed unpaired t-tests revealed no significance difference between the relative density of Reuse 0 and Reuse 8 bars (n = 5) for 316L stainless steel (p = 0.64) and Ti64 (p = 0.06) (Fig. 5) . Though bulk density trended towards significance for Ti64, Reuse 8 specimens demonstrate a nominally higher density compared to Reuse 0 specimens. A two-way ANOVA was applied to the relative density 17-4PH samples to investigate if feedstock treatment or machine parameters had a significant effect. An effect 4 on bulk density was not found from machine settings (p = 0.14) but was a result of reuse condition (p = 0.38). Since no significant interaction was calculated between reuse and machine parameter setting (p = 0.96), bulk densities were aggregated for Reuse 0 and Reuse 8, a Fisher's Least Significant Difference post-test was evaluated (LSD= 0.77). The difference between the average bulk density of 17-4PH Reuse 0 and Reuse 8 was less than 0.04, which determined that significant effect on density with respect to reuse was not seen.
Fractography of virgin 17-4PH parts revealed parallel bands of metallic striations. Regardless of powder type or build parameters used, the initiation site was proximal to the radial circumference of the cross-section (Fig. 6) . Accumulated damage was seen at or below the surface or superficial pores. Characteristic beach markings are evident on 17-4PH surfaces fabricated with optimized and default parameters (Fig. 6a, c) .
Certain specimens exhibited a shear lip opposite the site of initiation (Fig. 6d) . Topography of fractured 316L fatigue samples were similar for virgin and reused specimens. Compared to 17-4PH SS, the surfaces are flat but faceted. Cleavage topography suggests that the directional changes in crack behav- ior occurred along grain boundaries or crystallographic planes [30] . Crack initiation was again shown to have been located at or near the surface of the specimen (Fig. 7) . SEM images of Ti64 revealed the origin of failure to be large pores. For both virgin and reused condition, sub-surface damage propagated to failure after encountering a larger defect closer to the surface of the specimen (Fig. 8 ).
The fatigue limit of DMLS 316L SS has been previously measured between 260-280 MPa, suggesting that the material in the present study had an improvement of 100 MPa [13, 17] . Chemical composition could be a contributing factor to the higher reported fatigue life of 316L SS, as fatigue results from a 316L alloy produced a more similar fatigue life of 360 MPa, 5 identified to be the result of the higher nitrogen content [31] . However, part orientation for DMLS 316L could have also had an influence, as previous work printed 316 SS specimens in the horizontal (0 • ) direction, which typically yields higher fatigue limits ( Table 1) .
The fatigue life of a material has been related to the fracture strength of a material during a single load cycle. This permits a relationship to be generated between a material's static and dynamic strength, which has been experimentally confirmed [30] . For axial loading a general relationship between the endurance limit of steel can be approximated by
where S ut is the ultimate tensile strength. When S ut is calculated from the estimated S e , the results indicated that the difference in axial strength for metal sintered with reused feedstocks is higher by 100 MPa for ferrous materials (316L and 17-4PH SS), but lower in the case of Ti64 (Table 3) . When SUT is compared to reported values for wrought SUT, values calculated from the endurance limit are much higher for both virgin and Reuse 8 material. 
The nature of the stair-case approach dictates that the stress amplitude of each test is dependent on the previous specimen. A direct comparison of the fatigue behavior revealed no significant difference between Reuse 0 17-4PH, but for Reuse 8, fatigue bars printed with the optimized parameters had a lower bulk density compared to those printed with the recommend manufacturer settings. This would suggest that the use of optimized parameter settings identified from static mechanical testing is a viable practice for reuse powder, but might require additional studies for further validation.
The results suggest that DMLS powder can essentially be reused without any penalty on fatigue strength. As can be seen from the print parameters (Table 3) , bed temperature for the steels and titanium powders typically experiences 80 • C, and the powder is always shielded by argon to protect against oxidation during reuse [4] . Overall, the majority of the powder metal experiences a relatively mild environment during cycling. The effects of reuse primarily manifest as a drift towards larger mean particle sizes in a powder, since large particles are pushed into an overfill trough and small powders are preferentially fused [1, 5, 7] . However, this was shown to be a relatively slow effect, and this observation holds here as well. Future experimentation that assesses the peripheral aspects relevant to fatigue would benefit this study. Though bulk density measurements were taken, apparent hardness, chemical analysis, and surface roughness of sintered parts could enhance the context of the reported data. For this study, all bars were machined to the dimensions, resulting in surface finish between polished and asprinted parts. Additional fatigue studies acquired from the analysis of DMLS metals could contribute to the establishment of a surface finish factor for AM parts as a function of reuse.
Powder reuse is a viable strategy for improving additive manufacturing economics. Utilization of powder from previous builds translates into direct cost savings, making AM more competitive and enabling wider use of AM to allow design flexibility and customized product offerings. This practice is commonplace for DMLS metals but requires process validation 6 with respect to overall part performance. Estimates from static strength to fatigue limits are possible, but since there is minimal work in the fatigue performance of AM parts with recycled feedstocks, experimental testing is necessary to benchmark long-term material behavior with respect to powder reuse. Fatigue is an important extension of DMLS performance in commercial applications, especially if AM could be leveraged to mitigate factors related to implant failure. Accelerated lifecycle testing was employed to analyze material behavior in repeated low magnitude loading conditions. The endurance results of this study found that machined, DMLS metallic alloys did not result in a significant reduction on the fatigue behavior.