This paper focuses on the tailoring of calcium phosphate powders for their use as powder bed selective laser process feedstock. Hydroxyapatite and chlorapatite were used as starting powders for the preparation of different blends through the addition of graphite as a laser absorptance additive. A methodical study was conducted to compare the processing windows of the blends containing different amounts of graphite through the laser patterning of circular samples. It was found that the addition of graphite increases the process window of the powder blends being the powder without additive non processable. Hydroxyapatite showed a clear phase transition (decreased when using higher volumetric energy density) into other calcium phosphate phases while chlorapatite was demonstrated to be thermally stable during the whole process (examined through X-ray diffraction and vibrational spectroscopies). In parallel, the study evaluating the powder blend composed of hydroxyapatite and graphite for the production of solid and complex parts was carried out although it required long printing times. The productivity of the process was improved by modification of printing parameters. Then, a series of solid samples were produced for the analysis of the microstructure and mechanical properties. High interconnected porosity was observed in the samples which could improve the bioactivity of the bioceramic scaffolds. A post-treatment of the parts increased their proportion in the hydroxyapatite phase and their me- chanical properties. These results are expected to contribute to the application of powder bed selective laser processing of calcium phosphates powders toward bone tissue engineering. 1.Introduction The inclusion of ceramics as material feedstock in powder bed fusion (PBF) (ISO/ASTM 52900) additive manufacturing (AM) technologies made the traditional terms to define the process selective laser sintering and selective laser melting inaccurate. The mechanism occurring during the densification of ceramics processed by a laser source is not completely examined. Some materials can experience change in their compositions before sinter/melt (sintering understood as solid state diffusion and not partial melting), and moreover both phenomena can occur as result of increasing the temperature [1]. Recently, the term powder bed selective laser process (PBSLP) has been proposed to avoid a misusing of the previous terms to define the technique [1]. The production of calcium phosphate scaffolds through PBSLP has been largely studied [1]. The printed bioactive scaffolds would serve as the starting point for the growth of new bone for bone replacement applications. Three main strategies have been followed by researchers for the shaping of bioceramic powders through PBSLP. The first strategy consisting of the use of polymeric powders (e.g., polyethylene, poly- ϵ-caprolactone, and poly (L-lactic acid)) filled with calcium phosphate powders has been widely studied [2–5]. The polymer Abbreviations: A, Absorptance; AM, Additive manufacturing; AMCSD, American mineralogist crystal structure database; APS, Atmospheric plasma spray; CIF, Crystallographic information framework; ClA, Chlorapatite; COD, Crystallography open database; CT, X-ray computed tomography; ESEM, Environmental scanning electron microscope; FEG, Field emission gun; FTIR, Fourier-transform infra-red; G, Graphite; HA, Hydroxyapatite; HA_Cyl, Calcium phosphate cylinders; HA_Cyl_PT, Calcium phosphate cylinders after post-treatment.; ICSD, Inorganic crystal structure database; LFD, Large field detector; OA, Oxyapatite; PBF, Powder bed fusion; PBSLP, Powder bed selective laser process; PGT, Princeton gamma-tech; PSD, Particle size distribution; R, Reflectivity; Ra, Roughness average; SEM, Scanning electron microscopy; T, Transmission; TA6V, Titanium alloy with aluminium and vanadium; TTCP, Tetracalcium phosphate; UV–Vis, Ultraviolet; XRD, X-ray diffraction; α-TCP, α-Tricalcium phosphate. *Corresponding author. E-mail address: david.grossin@ensiacet.fr (D. Grossin). Contents lists available at ScienceDirect Additive Manufacturing u{�~zkw! s{yo|kr o>!ÐÐÐ1ow�o �to~1m{y2w{m k�o2knnyk! https://doi.org/10.1016/j.addma.2021.102542 Received 16 September 2021; Received in revised form 13 November 2021; Accepted 29 November 2021 Additive Manufacturing 50 (2022) 102542 2as the main component in the powder feedstock is melted through the laser irradiation giving as result the printed polymer/ceramic composite scaffold. Since these polymers are not removed after the PBSLP process, the time of implantation can have some adverse effects, thus losing some of their properties or affecting negatively to the tissue regeneration [6]. A second strategy is based on the use of a more similar ratio of polymer /ceramic components to print green body (polymer/ceramic composite) scaffolds. These are then submitted to pyrolysis (of the sacrificial polymer) and sintering steps to obtain the final sintered bio- ceramic scaffold free of the polymer [7]. The third strategy is the use of the bioceramic powder directly as the main component in the PBSLP process feedstock [8]. In this case, the material interacting with the laser is the ceramic powder whose response will be highly influenced by the type of laser-equipped in the PBSLP apparatus. The laser beam energy has to be absorbed and trans - formed into heat to trigger the sintering/melting of the powder. Different materials show different absorptance levels depending on the wavelength of the laser used. It is known that ceramic insulators (e.g. calcium phosphates) mainly absorb the light energy emitted by CO2 lasers (λ10,600 nm) however it is not the case when using a fibre laser source (λ1070 nm). The second one is the most common laser-equipped in PBSLP apparatus since it is greatly absorbed by metals, which are the most explored material feedstock in PBSLP technology. For this reason and because of the high cost of the PBSLP equipment there is a significant demand for the development of ceramics adapted to this kind of laser increasing the number of suitable materials feedstock for the same printer. The addition of absorptance additives has been studied as a method to deal with the observed low absorptance by oxide ceramic materials of fibre laser energy needed for their processing. Carbon graphite-based filler has been demonstrated to increase the fibre laser absorptance by adding small amounts in the material feedstock making possible the printing of oxide ceramics [9,10] . The energy input of the process is defined as volumetric energy density (Ed) (Jcm3) and different formulations have been considered for its calculation [1]. Most of the current research on process optimi - sation considers the Eq. (1) for its calculation, which will be used in the present study. However, caution should be given when using Ed since recent works already showed that this equation gives only an approxi - mate estimation and further analyses on the hatch parameters and ma- terial properties should be done for a correct evaluation of the energy density [11]. EdPlaser vscan∙hspace∙tlayer (1) where Plaser is the laser power (W), vscan the scanning speed (cms1), hspace is the hatching distance (cm), and tlayer is the layer thickness (cm). Recently, Leo et al. [12] remarked the importance of the laser defocus (positive and negative sense) for the process since it has a significant effect on the final microstructure by varying the spot size and for hence the Ed. In particular, when working at low laser power (~ 38W), they found that small variations in the defocus value induced significant changes in the part properties. It is then of vital importance the study of the different PBSLP parameters for each material feedstock to find a processing window showing better sintering/melting phenomena of the powder, thus obtaining high-quality parts. The goal of the present work was to evaluate the performance of different graphite-filled powder blends feedstock with calcium phos- phates, such as hydroxyapatite (HA) and chlorapatite (ClA), as main component in a PBSLP process. The use of ClA in addition to HA was explored due to its known higher thermal stability which could be an interesting advantage for processes involving high temperatures [13, 14]. Indeed, the ClA conversion into HA has been demonstrated to be feasible through hydrothermal ion exchange of Cl– by OH– if the appli - cation requires it [15]. Then, the study started with the preparation of calcium phosphate powders feedstock through the addition of different amounts of graphite as an absorptance additive. The processability of the different powder feedstock was tested in a PBSLP process at different printing conditions to observe the influence of the Ed in the final sample properties (e.g. phase transition, surface finishing …). The comparison between the different powders and parameters was possible through the examination of the chemical and structural composition, morphology, and mechan - ical properties of the printed samples. Additionally, a thermal post- treatment was carried out on the solid samples to study its effect on their composition and macro-structural properties. 2.Materials and method 2.1. Materials Hydroxyapatite (HA) microspheres powder was used as the main component for the production of the powder blends that will be used as material feedstock for the PBSLP process. This initial powder was pro- duced by Urodelia (SA Company, Saint-Lys, France) via wet-chemical coprecipitation and a posterior spray-drying process (product refer- ence 300.08.2). More information about the wet-chemical approach followed was offered in a previous work [16]. Moreover, the spray drying process of hydroxyapatite powders was covered previously with a deeper examination of the HA microspheres flowability in a previous work [17]. The spray drying step was crucial to produce the flowable HA powder composed of spherical agglomerates. In addition, pure chlorapatite (ClA) powder was produced through a solid-gas reaction (chlorination) of the initial HA powder in a tubular furnace. The procedure followed was already described in the work of Demnati et al. [18]. This ClA powder was also used as the main component for the preparation of blends employed as PBSLP process feedstock. Different powder blends were produced by mixing graphite as an absorptance additive with HA and ClA powders as main components. TIMREX ® KS 44 Graphite by Imerys is a carbon graphite-based filler that acts as a near-infra-red absorptance additive improving the laser- material interaction of the powders. The preparation of the blends was carried out by pouring the previously weighted amounts of powders (a total of 0.5kg) in a mixing jar, then the powders were mechanically blended using a Turbula shaker mixer at 42rpm for 1h. A list of the acronyms referring to the raw and prepared powder blends is given in Nomenclature Rb Bragg error [%] Rexp Residual exponential distribution [%] Rwp Weighted residual profile [%] Ed Volumetric energy density [Jcm-3] h Hatching distance [cm] k Thermal conductivity [Mm-1K-1] Sig Sigma [%] x10 The 10th percentile [μm] (diameter of a sphere at which 10% of the particles in the sample are smaller) x50 Median particle size [μm] the 50th percentile (diameter of a sphere at which 50% of the particles in the sample are smaller) x90 The 90th percentile [μm] (diameter of a sphere at which 90% of the particles in the sample are smaller) P Laser power [W] t Layer thickness [cm] v Scanning speed [cms-1] P. Navarrete-Segado et al. Additive Manufacturing 50 (2022) 102542 3Table 1. TA6V discs with one side coated by plasma spray with HA (0.1mm coating thickness) were used as a substrate to improve the attachment of the first printing layers to the building platform surface. The dimensions of the discs were 10cm in diameter, 2mm of TA6V thickness. They were produced by the company 2PS (Projection Plasma Systˇeme, Montbazens, France), which is specialised in plasma projection coating of orthopaedic implants. 2.2. Characterisation methods 2.2.1. Chemical and structural analyses Different characterisation techniques were used to confirm the chemical and structural composition of the raw and produced materials. X-ray diffraction (XRD) analyses of both powders and printed samples were performed in a BRUKER ’s X-ray diffractometer D8 Advance system with Cu Kα radiation (wavelength λ0.15406 nm), a nickel filter at 40kV and 40mA, and a LYNXEYE XE-T detector (energy resolution of 0.38 keV). Samples were analysed using a 6mm×2mm slit in a 2θ range between 20•and 90•, a step of 0.03•2θ, and a time per step of 0.2s. Rietveld refinement method was applied to estimate the crystalline phase compositions of the different printed samples containing a multiphase calcium phosphate mixture of hydroxyapatite (HA) Ca10(PO 4)6(OH) 2, alpha-tricalcium phosphate (α-TCP) (Ca3(PO 4)2, tet- racalcium phosphate (TTCP) Ca4(PO 4)2O, and graphite (C) phases. MAUD (Material Analysis Using Diffraction) software (written by Luca Lutterotti [19]) was used for the diffraction analyses. The following methodology was followed to estimate the phase compositions of the multiphase samples. Rietveld refinements were performed in a 2θ range between 21•and 36•. This range was chosen to include the main re- flections of each phase. All samples were refined assuming the possible presence of the four phases, and the crystallographic information frameworks (CIF) of the standard single-phase standards were employed for the refinement (phase/database/code: HA/AMCSD/0002281; Graphite/AMCSD/0000049; α-TCP/COD/2106194 TTCP/ICSD/2631). Seven background points were settled for the calculation of the complete background curve via linear interpolation. Fourier transform infra-red spectroscopy (FTIR) analyses were per- formed to determine the chemical composition of powders using a spectrometer FTIR iS50 and the KBr pellet preparation method. About 9mg of powdered sample was transferred to a clean mortar, then a weighed amount of dry KBr powder (300±5mg) was added and homogenously mixed. A compression at 6000 kg was employed to pro- duce the sample disk used for the analysis. FTIR spectra were recorded in the 4000 –400cm1 wavenumber range using a step width of 0,48 cm1. A Horiba-Jobin Yvon Labram HR 800 Raman spectrometer provided with 633nm laser excitation was used to obtain the Raman spectra of the samples during our study. For reference, standard Raman spectra have been obtained from the RRUFF ™ Project database for pure HA (RRUFF id: R100225, 780nm), and ClA (RRUFF id: R060192, 780nm). Raman spectra from TTCP and α-TCP were obtained through the ana- lyses of reference samples produced in the laboratory. 2.2.2. Morphological and structural properties The particle size distribution (PSD) of the different powders used in this work was analysed in dry conditions with a Mastersizer MS 3000 (Malvern Panalytical ®) laser particle size analyser. For the analyses, the powder was dispersed at a venturi pressure of 0.1bar. Refractive in- dexes for the different powders were taken as 1.63 (HA), 1.65 (ClA), 2.70 (graphite) while the absorption indexes were 0.01 (HA), 0.01 (ClA), 1 (graphite). The morphology of powders was investigated with a scanning electron microscope (SEM) LEO 435 VP (Leica ®) equipped with a Ge detector (Imix-PC, PGT) after metallisation of the ceramic surface with a thin film of silver using a Scancoat Six sputter coater. Printed samples were analysed in a FEI ESEM Quanta ™ 450 FEG, an environmental scanning electron microscope (ESEM) used with a large field detector (LFD) in low vacuum (LowVac) mode without previous metallisation of the surface of the sample. ImageJ was used for the processing of the images [20]. High-resolution X-ray micro-computed tomography (CT) analyses of the HA cylinders were performed in a phoenix nanotom ® equipped with a 180kV/15 W high-power nanofocus X-ray tube (9µm voxel size, resolution). A 3D perspective of the cylinders was reconstructed and it was possible to examine their total porosity. VGStudio MAX 2.1 software was used to reconstruct the data sets and visualise the objects three- dimensionally. The geometrical density of the cylinders was calculated from the mass and dimensions of at least ten samples. For comparison, the density of the parts was also measured by Archimedes ’ method in distilled water as liquid. 2.2.3. Laser absorptance of powders Diffuse reflectance spectra were collected using a double beam spectrophotometer AGILENT ® Cary 5000 UV/Visible/NIR and integra - tion sphere accessory of 110mm. Before the measurements, pre- determined photometry calibration was made using specimens with known reflectance. Experimental data were collected within the 900–1200 nm range with 1nm step and 0.5s integration time. Powders were compacted in the sample holder and were approximatively 3mm thick. The absolute transmittance and reflectance can be measured directly with high accuracy and the absorptance can be obtained from these by a simple calculation: A1RT (2) where A represents absorptance, R is reflectance and T is transmittance (which is considered equal to zero). A predetermined photometry cali- bration was made using specimens with known reflectance. Three different samples (N3) were taken for each blend and were measured obtaining the mean value and the standard deviations (mean±standard deviation) evaluating their homogeneity. 2.2.4. Mechanical properties of HA cylinders The procedure followed for the analysis of the compression modulus and strength of the HA cylinders is described in ASTM E111 – 17 and ISO 13175-3:2012 [21]. The compression strength and modulus of the printed cylinders before and after thermal post-treatment was measured in an MTS Criterion ® Model 42 electromechanical universal test system. An MTS load cell model LSB.203, with a capacity of 2000 N was used to measure the shear force at a crosshead speed of 0.085 mm/s at room temperature. Samples were submitted to a pre-stress of 3N to avoid any irregularity due to the state of the sample surface before starting the compression test. The measurements were stopped manually once the cylinder collapsed. The samples were set in the middle of the plate vertically inverted to the printing orientation. Since it was a preliminary study, a low number of measurement repetitions were performed: three Table 1 Acronyms for different raw materials and blends feedstock used during our study. Powder/blend acronym Description HA Hydroxyapatite microspheres used as raw material. ClA Chlorapatite microspheres used as raw material. G Graphite flakes used as raw material (absorptance additive). HA_5G Powder blend composed of hydroxyapatite and a 5wt% of graphite. HA_10G Powder blend composed of hydroxyapatite and a 10wt% of graphite. ClA_5G Powder blend composed of chlorapatite and a 5wt% of graphite. P. Navarrete-Segado et al. Additive Manufacturing 50 (2022) 102542 4post-treated cylindric samples (N3, mean±standard deviation) and one as-printed. 2.3. PBSLP process PBSLP process of the blend powders was performed in a 3D Systems ProX ® DMP 200. Parts are built up layer by layer sintering/melting the powder particles by a laser which is directed to the powder particles. The laser follows the pre-loaded 3D model building selectively the subse - quent horizontal layers. The manufacturing process is then carried out by joining each new layer to the previous one. The main components present in the manufacturing chamber of the printer are illustrated in Fig. 1. A roller (with 110mm of diameter) which can move in a straight line using a carriage (speed of 400mm/s, accuracy D1mm) and rotate (with speed 3.7 turns per second, accuracy D5•) carry out the actions of layering such as spreading, smoothing, and compacting. The layer thickness of powder can differ depending on the material properties (particle size, flowability …). A scraper enables to carry the powder from the feeding piston to the sintering piston. The feeding piston contains the powder reservoir used for sintering (140×140mm, powder volume: 3.85 L). Parts are manufactured on the TA6V discs coated with HA (which are glued onto the sintering plate which is positioned on plate support on the sintering piston (140×140mm). A collecting tank recovers all non-sintered powders and residues that can be treated for recycling. Since no material is removed, waste is reduced significantly. It is a Class I Laser System. The main optical devices are a fibre laser source (continuous operation, single-mode, power at the collimator output 300W, wavelength in the shortwave infra-red range (approx. 1070 nm), a collimator (outlet beam diameter at 1/e2 15mm), a scanner head, and a flat field F-Theta lens (420 mm focus length). This configuration gives a beam diameter of the focal spot of 38.1 µm. A preliminary trial-and-error study of the pow- ders’ processability allowed us to enclose the parameters range used in the present study. These are described in Table 2. Three different approaches were performed in parallel to study the influence of the printing parameters and feedstock formulation. For each of the approaches different 3D models were used for the printing of the samples, also they were printed with two different laser scanning stra- tegies. These two laser scanning strategies are illustrated in Fig. 2. The three kinds of samples together with the strategies followed for each approach are described below: ≡The first approach, laser-induced circular patterns: The 3D model consisted of a short cylinder of 8mm of diameter and 0.8mm of height. In this case, the laser scan strategy used was a hexagonal (island type) strategy (Fig. 2a). The continuous hexagons had 0.2mm of diameter and overlap between them of 0.1mm. The orientation of the hexagons was rotated 90•between layers. This kind of sample was conceived to study the influence of the printing parameters (or Ed) and evaluate the process window of the different powder blends. This specific sample design allowed us to shorter the printing times and to perform different characterisation techniques (XRD, Raman, SEM …) directly on the substrate. Once the samples were produced, compressed air was used to clean the surface of the disk to remove the raw material particles settled on the samples. ≡The second approach, HA complex shape: In parallel to the previous approach, the printing of two different 3D models with increased shape complexity was evaluated. One model consisted of a cylin- drical scaffold (20×17mm) with controlled internal porosity while the second one was based on a trapezoid with a toothed bottom surface (8×25×13mm). The same hexagon-based laser scan strategy as in the first approach was used (Fig. 2a). However, only one combination of laser parameters was tested (laser power: 30W, scanning speed: 25mm/s, Ed: 240Jcm3) for both parts. The model used for these samples was designed to test the printing feasibility of 3D complex parts with a set of printing parameters. After printing, a homemade endless diamond wire saw was used to detach the scaffold from the substrate while for the trapezoid it was not necessary thanks to its toothed bottom surface. ≡The third approach, HA cylinders: A cylinder of 10mm of diameter and 15mm of height was used as a 3D model (taking into account the dimensions needed for mechanical tests which are described in ISO 13175-3:2012). The Ed and laser parameters combination used for the printing was obtained from the results of the first approach studying the process window (laser power: 36W, scanning speed: 75mm/s, Ed: 96Jcm3). Due to the low building rate of the hexagonal-based laser strategy for the printing of solid parts (second approach), a new laser scan strategy of zig-zag was used to increase it (Fig. 2b) (the building rates obtained for the second and third ap- proaches will be discussed in the next section). Because the Fig. 1.Schematic illustration showing main components present in the manufacturing chamber of a 3D Systems ProX ® DMP 200 printer. Table 2 Fixed and studied parameters for the determination of the processing window of the PBSLP process in the present work. Parameter Value Studied Fibre laser power (W) 30–54 Scan speed (mm/s) 25–125 Laser scan strategy Hexagonal & Zig-Zag (5 laser passes) (90• rotation between layers) Fixed Hatching distance (mm) 0.05 Layer thickness (mm) 0.10 Atmosphere Air Powder compression (%) 300 Laser defocus 3cm downward of the focus plane Printing substrate HA-coated TA6V disc Chamber preheating No P. Navarrete-Segado et al. Additive Manufacturing 50 (2022) 102542 5overlapping of laser passes in the same region seemed to enhance the final result, an overlapping of 5 laser passes per layer over the same irradiated region was used. The orientation of the laser tracks was rotated between both passes and layer steps. The model used for these samples was designed to perform a posterior compression strength test as well as chemical and structural characterisation. After printing, it was possible to detach the HA cylinders from the substrate by hand without modifying their surface. A list of the acronyms referring to the printed samples corresponding to each of the three different approaches followed is given in Table 3. 2.4. Thermal post-treatment A posterior thermal post-treatment was performed at air presence for the HA cylinders to study the changes in their chemical and structural properties as well as in their mechanical properties. The cylinders were placed between two alumina crucibles which were set in the middle of the muffle Nabertherm ® LT 9/13 furnace chamber favouring a more homogeneous heat distribution. The temperature programme consisted of a heating ramp of 2•C/min till a temperature of 1250•C. This tem- perature was kept constant for 2h before starting the cooling phase (1•C/min). 3.Results and discussion 3.1. Characterisation of raw/prepared powders and substrate The PSD and the SEM micrographs of each of the three raw powders (HA, ClA, and graphite) used for the preparation of the PBSLP feedstock materials are illustrated in Fig. 3. It can be observed how the ClA and the HA powders have almost similar properties. It demonstrates that the chlorination of the HA powder to obtain ClA powder does not modify their morphology keeping unaltered the PSD and the spherical shape of the agglomerates. The graphite powder is composed of flakes with smaller median particle size and a more dispersed distribution. The obtained Raman spectra and X-ray diffractograms of the initial ceramic powders are offered in Figs. 4 and 5, respectively. For reference, standard Raman spectra have been obtained for HA (RRUFF id: R100225), and ClA (RRUFF id: R060192) from the RRUFF ™ Project database. The chemical and structural composition of each of the initial powders was confirmed. We can point out the presence of both mono - clinic and hexagonal phases in the ClA diffractogram. This partial phase change could be produced during its synthesis since it is known that ClA undergoes a phase change from hexagonal to monoclinic upon heating at 200–300•C [14]. The study of the powdered material absorptance at a certain laser wavelength is of particular interest for the PBSLP process because it allows one to determine the processing window suitable for sintering or melting. Tolochko et al. [22] carried out an exhaustive study on the laser absorption of different materials stating that the absorption mechanism Fig. 2.Schematic diagram of the two laser scanning strategies used during the three approaches: (a) Hexagonal/island type scanning (1 laser pass per layer) and (b) zig-zag scanning (5 laser passes per layer). In red the pattern followed by the laser scans and the blue regions refer to the overlap between hexagons. (For inter- pretation of the