Biomaterial powders are in high development due to expansion of additive manufacturing (AM) processes. Selective laser melting (SLM) is a particular AM technology, which completely melts a powder bed layer by laser beam. Investigations of appropriated physical properties of feedstock (powder alloy) were the aim of this study. Cobalt-chromium-molybdenum (Co-Cr-Mo) alloy was used to overview of gas-atomized powder properties in different granulometric ranges (D1 12 - 19 μm, D2 20 - 46 μm and D3 76 - 106 μm), as their: physical, chemical properties and thermal analysis. SLM manufactured standard tensile specimens of usually granulometric range powder size provided mechanical, chemical and thermal properties of biocompatible Co-Cr-Mo alloy. The physical properties showed that powders in the range of 20 to 50 μm provide a better flow ability and packed density, which are relevant characteristics to SLM processing. Manufacturing by SLM process provided suitable mechanical properties in the health area, as well as, maintained the biocompatible properties of the Co-Cr-Mo alloy.
Biocompatibility properties of Co-Cr alloys provided to these super alloys a wide range of application in the health area [
The selective laser melting (SLM) is a technology that can manufacture 3D components by laser beam action that completely melts a powder bed layer until the final geometry. Traditional SLM machines have a maximum laser power of 400 W with laser focus diameter of 100 μm can be provided by a source of Ytterbium fiber. SLM process occurs layer by layer using a feedstock as a metal powder that is stored in a container that may be the distributor of powder (deposited by gravity), which turn to uniform the powder layer (between 20 to 100 μm) on the build platform [
A relevant feature in the AM processes which may influence the mechanical properties of parts manufactured is the material characteristics (powder). The metal powder must present specific characteristics of: size, shape, granulometric distribution and physical properties (flowability and packing). Powder bed fusion methods can be employed with different granulometric powders sizes [
The production of metal components by laser melting was promising since it allows the manufacture of customized components and your higher final physical properties when compared to conventional techniques [
The aim of this study is to analyse the physical characteristics of powders in different granulometric ranges and to point out the factors that influence the properties of the powders, in order to reveal the better granulometric range for the SLM process. The study primarily focuses on the analyses physical characteristics of ASTM F75 powders. Powders characteristics as a granulometric size, particle format, flow rate, apparent, tap, theoretical and helium pycnometry densities were evaluated. SLM processing standard specimens to evaluated mechanical properties and thermal stability of biocompatible Co-Cr-Mo alloy.
Co-Cr-Mo alloy gas atomized (H.C Starck®, Germany) was provided by the HighBond® (Indaiatuba, Brazil) in three different granulometric range, respectively by terminologies: D1 (12 - 19 μm), D2 (20 - 46 μm) and D3 (76 - 106 μm). The confirmation of the chemical composition was realised by fluorescence spectrometer X-ray energy dispersive (Shimadzu EDX-720 equipment). This study was based on alloy/powder with certification in a medical device registration and approval in Brazil (ANVISA―National Health Surveillance Agency) for use in health care and meets the ASTM 75 [
Several physical properties of gas-atomized powders were obtained such as flow time, apparent density and tap density, respectively according to ASTM B212 [
To evaluate the internal porosity of powders sample were measured the Helium pycnometry in comparison by the theoretical density. The density by pycnometry considered only the internal porosity (excluding the open porosity) and for this purpose was utilized the Micromeritics equipment (Model Accu PYC 1330 Pycnometer).
The hardness of the powders in relation to the granulometric range from the gas atomization process, micro hardness measurements were performed on Co- Cr-Mo powder samples. The micro hardness tests were performed using a Fischerscope HM2000 equipment with a load of 100,000 mN (micro Newton) for 10 seconds.
The consolidation of SLM samples was carried out by SLM Solutions™ using a SLM® 280 HL machine with a single Ytterbium laser beam (maximum power 400 W). SLM processing used the D2 powder at the distribution size of 20 to 46 µm. The building consolidation of specimens was parallel of laser bean and performed using as parameters: layer thickness of 30 μm and diameter of laser beam of 76 µm. Others consolidation parameters were previously pre-selection by manufacturer.
To evaluate the viability of cell growth in the Co-Cr-Mo alloy after the SLM consolidation processes was performed the cytotoxicity analysis, according to ISO 10993-5 [
The tensile and three-point bending specimens were manufactured in standard dimensions according to ISO 22674-06 [
The dilatometric analysis was performed on SLM samples were analysed in the parallel and transversal building direction (SLM 1―parallel direction and SLM 2―transversal direction), as seen in
The chemical composition by X-ray fluorescence and LECO, as the comparison with the SLM samples manufactured by D2 powder (granulometric size of 20 - 46 μm) given in the
Elements (wt%) | Powder | SLM samples | ASTM F75 |
---|---|---|---|
Co | 63.93 ± 0.16 | 65.38 ± 0.32 | Balance |
Cr | 28.83 ± 0.19 | 27.68± 0.13 | 30.00 - 27.00 ± 0.30 |
Mo | 7.07 ± 0.31 | 6.61 ± 0.16 | 7.00 - 5.00 ± 0.15 |
Fe | 0.17 ± 0.01 | 0.33 ± 0.06 | <0.75 ± 0.03 |
C | 0.03 ± 0.010 | 0.03 ± 0.01 | <0.35 ± 0.020 |
S | 0.0002 ± 0.0001 | 0.0001 ± 0.0001 | <0.010 ± 0.003 |
N2 | 0.0018 ± 0.0001 | 0.1330 ± 0.0015 | <0.250 ± 0.020 |
O2 | 0.0003 ± 0.0001 | 0.0240 ± 0.0010 | - |
Physical Properties | Powder | ||
---|---|---|---|
D1 (12 - 19 µm) | D2 (20 - 45 µm) | D3 (76 - 106 µm) | |
Mean diameter (µm) | 12.76 | 32.36 | 92.81 |
Flow time (s/50g) | - | 15.85 ± 0.11 | 22.89 ± 0.03 |
Apparent density (g/cm3) | 4.12 ± 0.01 | 4.51 ± 0.01 | 4.55 ± <0.01 |
Tap density (g/cm3) | 5.00 ± 0.02 | 5.26 ± 0.05 | 5.09 ± 0.02 |
Theoretical density (g/cm3) | 8.37 | ||
Helium pycnometry density (g/cm3) | 8.28 ± 0.001 | 8.30 ± 0.001 | 8.27 ± 0.001 |
Hardness (HV) | - | 148.33 ± 38.89 | 390.35 ± 40.73 |
are characteristic of the gas atomization process [
The physical properties obtained according with powder metallurgy standardizations given in the
The sample of the ratio D1 powder having a very fine granulometry and was characterize as no flow powder by Hall flowmeter funnel. Comparing the flow time between D2 powder and D3 powder it is observed that D2 powder had better flowability than D3 powder. The poor flowability of D3 powder was influenced by presence of satellites particles and its superior roughness (can be observed in
The apparent density increase was progressive in relation to the granulometric size. This phenomenon is associated with the increase of the specific surface (high friction between the particles) being superior in smaller powders and consequently to hinder the accommodation between the particles [
The Co-Cr-Mo powders characterized by theoretical density related to pycnometer Helium density shows a presence of closed porosity detected. The open porosity is discounted in the pycnometry density. This result considers the presence of closedporosity. The powders samples presented approximately 1.30% of internal porosity. This result was evidenced comparing the pycnometry density in relation to theoretical density is observed a difference that is correlated to internal porosity.
The micro hardness of powders samples showed that the powder D3 is superior to the result of D2 powder. At the case of D1 powder has a very fine size, the micro hardness cannot be measured. The increment of particle hardness stay associated with the granulometry size and the solidification rate in the gas atomization process. The coarse powder during the fall into the cooling column of atomizer has a higher cooling rate than finer particles. Consequentially, the higher solidification rate will be results in superior hardness values [
According to the physical characteristics it was verified that granulometric sizes of 20 to 46 µm (D2 powder) can result in better packing and further powder layer densities around 46% of theoretical density with good flow rate. These characteristics allow better requirements for powder utilized in selective laser melting technology.
The result of the cytotoxicity analysis for the processed samples is shown in
Tensile stress-strains curves obtained from the tensile tests of SLM specimens are showed in
Reference | σYS [MPa] | El [%] | E [GPa] | σUTS [MPa] | σr [MPa] | TRS [MPa] |
---|---|---|---|---|---|---|
SLM Co-Cr-Mo | 731.50 ± 40.31 | 13.73 ± 5.32 | 225.18 ± 14.40 | 1136.95 ± 0.92 | 1127.91 ± 0.15 | 2501.2 ± 9.7 |
ISO 22674 (“Type 5”) | 500 | 2 | 150 | - | - | - |
rupture strength (σr) are summarized in
Analyzing the values at
The thermal analysis curves were obtained from dilatometry (SLM sample) and DSC (D2 powder sample) are presented in
As can be seen at the dilatometric analysis is possible to verify the CTE at temperature of 500˚C of samples (SLM 1 and SLM 2) are respectively 15.0 and 19.5 × 10−6 ˚C−1. At the temperature of 600˚C the coefficient value decreases to the values of 12.5 and 14.5 × 10−6 ˚C−1. The thermal properties are very important for metal materials applied to dental restoration at the temperature range of 500˚C to 600˚C. The applications of dental restorations required a porcelain coating (copings and crowns) that can be support values of CTE similar to the biomaterial Co-Cr-Mo alloy. Therefore, to avoid the possibility of forming cracks or voids during the coating process [
The DSC curve is possible check in the heating curves the presence of events: exothermic approximately at 600˚C, endothermic approximately at 950˚C and the fusion point of Co-Cr-Mo alloy occurs at 1354˚C. This phenomenon is similar to described by [
The first peak (exothermic) occurring approximately at 582˚C stay associated with the phase transformation of FCC (αCo) to HCP (εCo). The second peak (endothermic) occurring approximately at 944˚C stay associated with the dissociation of phase HCP to formation of FCC.
These phase transitions represent different behavior of material dilatation, as can be seen at the CTE curve respectively by the dilatations peaks. Therefore, it can be seen from the thermal results that the stability of the Co-Cr-Mo alloy processed by SLM correlates with the phase transitions of the material. In this way, the thermal properties guaranteed the post processing for coating on dental components avoiding defects.
The particles in the study produced by gas atomization showed spherical particle geometry with satellites and internal porosity. The analysis of the granulometric sizes of powders gave a better understanding of the physical results obtained from some properties of the powders, as can be cited flow hate and densities: apparent, tap and helium pycnometry. In addition, the physical properties evaluated showed that powders in the range of 20 to 46 µm, provide a better packing results. These powder behaviors are of great importance in the additive manufacturing process using the selective laser melting technology.
Additive manufacturing using selective laser melting of biocompatible Co-Cr- Mo alloy at the range 20 to 46 µm allowed obtaining significant mechanical properties qualifying at type 5 of ISO22674 standard. SLM process assures the biocompatibility of Co-Cr-Mo alloy after manufacturing the specimens. In addition, the thermal analysis complemented the properties resulting in an interesting point of view to the thermal expansion of the Co-Cr-Mo alloy in different building orientations that are of great importance to manufacture components used in dental area.
The authors would like to thank to Ms. Amed Belaid-SLM® Solutions for SLM specimens and Ms. Carlos E. Podestá-HighBond® for Co-Cr-Mo powder alloy and collaboration. The authors also thanks to PhD. Andrea C. D. Rodas for your contributions and cytotoxicity analysis. This study was financially support by CNPq.
Mergulhão, M.V. and Das Neves, M.D.M. (2018) Characteristics of Bimetallic Alloy to Additive Manufacturing Using Selective Laser Melting Technology. Journal of Biomaterials and Nanobiotechnology, 9, 89-99. https://doi.org/10.4236/jbnb.2018.91008