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Effect of Building Position on Phase Distribution in Co-Cr-Mo Alloy Additive Manufactured by Electron-Beam Melting
Taiyo TakashimaYuichiro KoizumiYunping LiKenta YamanakaTsuyoshi SaitoAkihiko Chiba
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2016 Volume 57 Issue 12 Pages 2041-2047

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Abstract

Cobalt-chromium-molybdenum (Co-Cr-Mo) alloys are used for biomedical implants such as artificial joints because they have excellent wear and corrosion resistance and biocompatibility. Electron-beam melting (EBM) is a type of additive manufacturing technique for metals. We used EBM to fabricate 20 rods of a Co-Cr-Mo alloy with height of 160 mm arranged in a 4 × 5 matrix and observed the phase constitution in the middle part (at a height of 80 mm) of the rods by scanning electron microscopy-electron backscatter diffraction. We found that the rods in the center part of the matrix consisted of more of the face-centered cubic (γ) phase and less of the hexagonal close-packed (ε) phase than rods in the outer part. This happened because even though each rod was fabricated under the same beam condition, the rods at the center had been exposed to higher temperature than those in the outer part, and less thermal dissipation took place because the neighboring rods were also heated by the electron beam. This difference in the thermal histories should be taken into consideration when many objects are fabricated simultaneously.

 

This Paper was Originally Published in Japanese in J. Jpn. Soc. Powder Powder Metallurgy 63 (2016) 10–16. Figure 4 and Fig. 6 were changed for more clear and appropriate explanation.

1. Introduction

New revolutionary technologies, called such as three-dimensional (3D) printing and additive manufacturing have been attracting attention because of their abilities (i) to fabricate parts with complex shape, which cannot be easily carried out by conventional processes such as molding, plastic forming, and cutting; (ii) to fabricate parts directly from 3D computer-aided design (CAD) data without using molds; and (iii) to fabricate multiple parts simultaneously regardless of whether the parts' shapes and sizes are the same, and so on. The abilities are very beneficial for producing custom-made biomedical implants. Electron-beam melting (EBM) is one of the additive manufacturing techniques for metals, and it is especially known for its ability to fabricate parts with fewer defects and cracks, and with less residual stress13). In the EBM process, the electron beam for fusing metal powder particles is scanned on a thin layer of powder (i.e. powder bed) along a two-dimensional (2D) slice of the 3D CAD model. After that, a new powder layer is formed on the original layer and the electron beam is scanned along the upper 2D slice. This process is then repeated layer by layer until 3D objects are formed. After that, lightly sintered but unmelted powder around the built object is finally removed. This process is similar to selective laser melting (SLM), which uses a laser beam as the heat source. Currently, one of the most important characteristics of the EBM process, which differentiates the products prepared by EBM from those prepared by SLM, is the use of a preheating step. Preheating is the process for heating up the powder layer to a high temperature by the rapidly scanning electron beam before meting the powder. The preheating has effects not only to make it easier to fuse the powder layer by raising the temperature to 50–80% of the melting point, but also to prevent the powder particles from electronic charge-up, which would cause explosive spreading of powder (also known as “smoking”), by increasing the electric conductivity at contact points between the particles4,5). Moreover, the preheating affects the solidification condition (such as the cooling rate and temperature gradient) and, accordingly, the solidification microstructure. The microstructures can also be changed through aging by the preheating of subsequently built layers. In fact, it was found that peculiar microstructures, which have never been formed in conventional processes, result from EBM probably because of its unusual melting and solidification processes6,7). Studies have been conducted on the control of a microstructure by managing its solidification conditions such as the scanning speed of the electron beam, scanning pathway, and energy density8).

Cobalt-chromium-molybdenum (Co-Cr-Mo) alloys have been widely used as implant materials such as artificial hip joints and knee joints because they have excellent mechanical properties9), wear resistance1012), corrosion resistance13), and biocompatibility. As further improvement of these properties is demanded, more research has been conducted1420). During conventional manufacturing processes, Co-Cr-Mo alloys crack easily, resulting in low productivity. EBM additive manufacturing can resolve these problems. Furthermore, EBM additive manufacturing can produce artificial implants in different shapes and sizes for different patients at the same time, so it is an attractive way to manufacture Co-Cr-Mo alloy implants. Many studies2125) are now conducted on these alloys. In a Co-Cr-Mo alloy at equilibrium, the face-centered cubic (fcc, γ, or γ-fcc) phase is stable at temperatures above 800℃, while the hexagonal close-packed (hcp, ε, or ε-hcp) phase is stable at lower temperatures. Sun et al.23) reported that a Co-Cr-Mo alloy fabricated by EBM additive manufacturing has different phase distributions at different vertical positions of the fabricated object. When a Co-Cr-Mo alloy powder is melted by an electron beam and solidified and then quenched to a temperature below the γ–ε transition point, the γ phase is maintained as metastable phase. However, a large portion of the γ phase transform to stable ε phase in the bottom part of the fabricated object since the temperature is held at a temperature where the ε phase is stable for a long period until the fabrication process is completed. Even within the same fabricated object, the higher the vertical position, the shorter the period of time in which the temperature is in a range where the ε phase is stable, and therefore the larger the volume fraction of the γ phase is. For example, for a Co-Cr-Mo alloy rod fabricated by EBM additive manufacturing with a finished height of 85 mm, the top 40-mm section consists of a single γ phase because the top part is held at temperature where ε-phase is stable for only a shorter period of time. The crystal orientations relationship between the γ phase and the ε phase in this alloy are described by the Shoji-Nishiyama relationship, which suggests that the isothermal martensitic transformation is dominant.23) The ε phase formed by the transformation from the γ phase gives rise to great changes in the mechanical properties such as the increased wear resistance while decrease in the ductility15,16); therefore, it is important to control the phase distribution in the EBM-additive-manufactured alloys. However, concerning the phase distribution, the following remains unclear: (I) the repeatability of the height range of the single γ phase region, (II) its dependence on the fabrication conditions, (III) the effect of horizontal position in the building space. It is important to clarify these from the point of view of the reliability of the EBM process and for optimizing the conditions post-fabrication heat-treatment. In this study, we investigated the effects of horizontal position in the building space on the microstructures of Co-Cr-Mo alloy fabricated by EBM.

2. Experimental Procedure

The alloy powder used was a gas-atomized powder with the nominal composition of Co-28Cr-6Mo-0.23C-0.2N (mass%). The powder particles were spherical and had particle size ranging from 45 to 150 μm. The average particle size was 64 μm. The chemical composition of this powder is compared with the ASTM F75 standard in Table 1. The powder contains relatively high concentration of nitrogen within the range of the standard. This high nitrogen concentration made it easier to obtain the metastable γ-fcc phase. This powder was used with the EBM A2X additive manufacturing machine (Arcam, Sweden) to fabricate rods of the Co-Cr-Mo alloy. Twenty cylindrical rods with a diameter of 16 mm and height of 160 mm were fabricated on an SUS304 steel base plate measuring 150 mm × 150 mm. The cylindrical rods were arranged in an array of four lines by five rows, with the central axes of the cylinders positioned so that they were parallel to the building direction. The bottom 10-mm section of each rod was the supporting section, and it was hollow. In addition, cubic lattice structures, measuring 20 mm × 20 mm × 20 mm, were fabricated at the same time for use in another experiment. The lattices were fabricated in an area for one row forming an arrangement of five columns and four stories. The center-to-center distance between the adjacent rods was 27.5 mm, while the center distance between the cubic lattice structure and the rod was 25.6 mm. The rods were assigned numbers 1 through 20, as shown in Fig. 1, and the respective parameters specifying their fabrication conditions are shown in Table 2.

Table 1 Chemical composition (mass%) of the alloy used in the EBM process and ASTM F75 standard.
  Cr Mo Ni Fe C Si Mn W P S N Al Ti B O Co
Powder used 28.3 5.8 0.07 0.23 0.38 0.61 0.2 0.023 Bal.
ASTM F75 27–30 5–7 <0.5 <0.75 <0.35 <1 <1 <0.2 <0.02 <0.01 <0.25 <0.1 <0.1 <0.01   Bal.
Fig. 1

3D-CAD model of 20 cylindrical rods fabricated by EBM. Lattices seen behind the rods were also fabricated simultaneously for other experiment.

Table 2 Parameter of the EBM process.
Condition Preheat temperature 875℃
Accelerating Voltage 60 kV
Hatch Max current 18 mA
Min current 4.5 mA
Scan Speed 210~440 mm/s
Line offset 150 μm
Contour Current (outside) 10 mA
Current (inside) 15 mA
Scan Speed 800~900 mm/s

The scan direction of the electron beam during hatching was controlled by using a mode that randomly selected scanning along the x-axis (the scanning line moved in the y-direction) or along the y-axis (scanning line moved in the x-direction) on every layer, as shown in Fig. 2. The power and scanning speed of the electron beam were controlled automatically by the operating program of the EBM additive manufacturing machine. This was achieved by increasing the current and the scanning speed when the electron beam scanned the central part of the rods while decreasing the current and the scan speed when the electron beam scanned the edge of the rods. The current was controlled within the range of 4.5 to 18 mA, while the scan speed was controlled within the range of 210 to 440 mm/s. The electron beam was controlled in such a manner to ensure that the fusion and solidification conditions were uniform irrespective of the location so that flaws and inhomogeneity resulting from excessive or insufficient fusion can be prevented. Specimens were cut from the middle section of the rods (at a height of 80 mm). Their microstructures were analyzed by a scanning electron microscope equipped with an electron backscatter diffraction (SEM-EBSD) system (Philips XL-30, TSL OIM) to investigate the impact of building position on the microstructure of the rods.

Fig. 2

Scan direction of the electron beam. x-axis scanning and y-axis scanning were randomly selected on every layer.

3. Results and Discussion

The phase maps of the rods obtained by SEM-EBSD analysis are shown in Fig. 3; the maps are arranged in the same manner as the rods fabricated. As shown by the illustration in the lower section of the figure, the map was obtained on a cross section parallel to the building direction (z-axis, also the cylinder's central axis), which is upward in the figure. The observation, however, was performed without identifying the x- or y-axis and as such, axes other than the z-axis are not identified in Fig. 3. Although M23C6 carbides and Cr2N nitrides are formed in this alloy, it is difficult to identify these phases by EBSD. Therefore, the EBSD analysis was conducted focusing on the γ-fcc and ε-hcp matrix phases only. The rods fabricated on the periphery of the arrangement (such as No. 1, 5, 16, or 20) practically consisted of ε phase. The rods fabricated at the center (such as No. 8 or 13) consisted of both the ε and γ phases. The volume fractions of the γ phase in the observed areas of rods No. 6, 7, 8, 9 and 10, arranged along x-axis, and those of rods No. 3, 8, 13, and 18, arranged along y-axis, are plotted against the distance of the rods from the edge of the array in Fig. 4(a) and Fig. 4(b), respectively. Rods No. 8 and 13, which were positioned at the center of the array, had a higher fraction of the γ phase than rods on the periphery of the array. It seems that the higher the heat input, the higher γ phase it is. Since Rod No. 8 was considered to be at the center of the arrangement, including the cubic lattice objects, it was predicted to have the maximum fraction of the γ phase. In reality, however, the maximum volume fraction (0.56) of the γ phase was obtained in Rod No. 13, probably because the heat input and heat capacity of the cubic lattice objects were small and the heat input to the whole objects was centered at Rod No. 13.

Fig. 3

EBSD phase maps of EBM-fabricated Co-Cr-Mo alloy rods. All the maps were obtained at the position of 80 mm in height from baseplate.

Fig. 4

Fraction of ε phase as a function of distance (a) from Rod No.6 in x-axis, and (b) from Rod 3 in y-axis.

Figure 5 depicts the phase map (Fig. 5(a)) and inverse pole figure (IPF) map (Fig. 5(b)), as well as the pole figure of the γ-fcc phase (Fig. 5(c)) and ε-hcp phase (Fig. 5(d)) of Rod No. 8. The carbides and nitride phases are neglected as well as for the EBSD map in Fig. 3. The crystal grains were columnar and elongated in the building direction in the γ phase, and the crystal orientation in the building direction was near the [100] direction. On the other hand, no clear peak was detected in the pole figure of the ε phase. The peaks in the pole figure of the ε phase (0001) partly overlapped with those in the pole figure of the γ phase (111) although there was no clear texture.

Fig. 5

(a) Phase map, (b) IPF map, (c) pole figure for γ phase, and (d) pole figure for ε phase in EBM-fabricated Co-Cr-Mo alloy Rod No.8.

The microstructure of the EBM-built Co-Cr-Mo alloy rods that were vertically built at the same time varied depending on the position of the rod in the arrangement even when comparisons were made at the same height of the rods. This can be explained by the relationship between the phase-transition temperature and the temperature of the specimens during fabrication. Figure 6 is the pseudo-binary phase diagram of the Co-29Cr-6Mo-0.2N-xC (mass%, 0 < x < 0.3) system26), with the horizontal axis representing the C concentration. This phase diagram was prepared using the Thermo-Calc software package from Thermo-Calc, Sweden and the Fe-DATA Version 6 database from Thermotech, UK. The vertical dotted line indicates the C concentration of the alloy used in our study. The γ–ε transformation point (boundary between the γ + Cr2N + M23C6 + σ four phase field and the ε + Cr2N + M23C6 + σ phase field) is located at approximately 930℃ (approximately 1,200 K). According to the phase diagram, Cr2N, M23C6, σ-phase are expected to be formed in equilibrium. In fact, Cr2N and M23C6 were detected in the Co-Cr-Mo alloy with the same composition in both as-EBM-built condition and after subsequent aging at 800℃ for 24 h, while no σ phase was detected18). The formation and dissolution of precipitates in Co-Cr-Mo alloys containing carbon and nitrogen were assessed by Narushima et al.27) The absence of σ-phase agrees with the tendency that the amount of σ-phase decreases with increasing carbon content and is negligibly small for carbon content higher than 0.2 mass%, which were reported by Narushimara et al.27) Figure 7 shows the change in temperature at the center of the lower surface of the base plate during fabrication; this temperature change was measured with a thermocouple. The temperature indicates characteristic changes corresponding to each process of the fabrication, i.e. the (i) bottom-support fabricating process, (ii) fabricating process of the object, and (iii) post-fabrication cooling process. Fabrication began when the temperature of the bottom of the base plate reached 850℃ (point A in Fig. 7). The temperature dropped slightly when the selective fusion in the supporting section began (Point B), but the temperature rose thereafter for about 2 h from the start of rod fabrication, owing to the preheating of each layer and the recurring selective fusion (Range C). As the fabrication progressed, the temperature of the top surface of the powder bed was maintained at a constant value. However, as the distance from the structure's top surface to the base plate increased, the temperature measured on the bottom surface of the base plate continued to decline until fabrication was completed. Each section of the structure remained at a temperature above the γ–ε transformation point for an extremely short time following fusion and solidification, but for the most part, a gradual cooling occurred at temperatures of 900℃ and lower, as indicated by Range D.

Fig. 6

Vertical cross-section of calculated equilibrium phase diagram of Co-29Cr-6Mo-0.2N-xC (mass%, 0 < x < 0.3) system obtained by Thermo-Calc software26).

Fig. 7

Temperature variation beneath the starting plate during EBM process.

Once fabrication was completed, a forced cooling by helium gas was conducted, and the temperatures of each section which was higher at locations closer to the top surface, decreased rapidly as measured in process (iii). At the time the forced cooling started, the temperature was as high as approximately 800℃ even at the lowest section. Therefore, the sections closer to the bottom are held for longer period at the temperature high enough for the γ-to-ε transformation to progress. As a result, the volume fraction of ε phase becomes higher in the lower section, while that of γ phase becomes higher in the upper section. The γ and ε phases coexisted at the center section, and the greater proportion of the γ phase was maintained in the section closer to the upper section. If rods that were fabricated in the same batch shared the same thermal history, then the microstructures of rods at the same vertical position should also be the same. In reality, however, even at the height where the γ and ε phases coexisted in the center rod, the rod on the periphery consisted of practically entirely the ε phase. This means that even at the same height, the thermal history varied depending on the horizontal position in the building space. The following explain the temperature variations with horizontal position.

First, it is possible that the temperature at the center became higher than the temperature on the periphery of the preheated areas, even when preheating was performed with the acceleration voltage, beam current, scan speed, and scanning intervals of the electron beams (line off-set) maintained at constant levels. In addition, it is possible that the macroscopic heat capacity per volume is higher in the consolidated portions than in the unfused portions of the powder bed, and the difference corresponds to the reduction of porosity. Therefore, even if electron beams uniformly irradiate a powder layer, the temperature may still be uneven owing to the shape and position of the fabricated object. In this study, heat was transmitted to the rods at the center from the rods in the periphery as they themselves were heated by electron-beam irradiation during fabrication. This probably caused a rise in the temperature that exceeded the preheating set temperature. In other words, it was more difficult for rods at the center to be cooled down through thermal diffusion because they were surrounded by other rods that had been irradiated by the electron beam in a similar manner. The temperature of the rods at the center rises more readily than those on the periphery that were not surrounded by other rods even when such beams were scanned uniformly on the rods along the same scanning route and with the same level of power. It was predicted that the singular ε phase would be obtained under the intended fabrication conditions in the area at a height of 80 mm because this phase was stable and the temperature was sustained at a level that was high enough for the diffusion transformation (which is a thermally activated process), as well as the isothermal martensitic transformation for a long period of time (15 h or longer). However, the temperature at the center was elevated because of the reasons described above, and it was held at the temperature where γ phase is stable for a long duration. Then, it was held only for relatively short period at the temperature where ε phase is stable in equilibrium, and the progression of a γ-to-ε phase transformation was limited. This is probably responsible for the formation of a mixture of the γ and ε phases. On the other hand, there was little heat transferred to the rods on the periphery from the surrounding rods that were irradiated by electron beams because they were surrounded by smaller number of rods than those in the center of the array. This presumably led to a smaller rise in temperature and as a result, the rods in the periphery consisted almost entirely of the ε phase.

A study conducted by Sun et al.15) confirmed that there was a strong correlation between the {111} plane of the γ phase (hereinafter referred to as {111}γ) and the (0001) plane of the ε phase (hereinafter referred to as the (0001)ε) in the Co-Cr-Mo alloy fabricated by EBM process. This is probably due to the martensitic transformation that maintain the Shoji-Nishiyama orientation relationship denoted by {111}γ//(0001)ε and <110>γ//<11-20>ε. The Co-Cr-Mo alloy fabricated in this study, on the other hand, had no clear correlation between the distribution of the {111}γ plane and the distribution of the (0001)ε plane, as shown by the pole figures in Figs. 5(c) and (d). Furthermore, no correlation was confirmed between the distribution in the direction of <110>γ and the distribution in the direction of <11-20>ε. Figure 8 shows a histogram of the misorientations of neighboring ε phases in the specimen of Rod No. 8. Four types of variants of the ε phase with the (0001)ε plane corresponding to respective {111}γ planes could be formed from a single crystal grain in the γ phase when the γ phase transform into the ε phase in the martensitic manner. The difference in the crystal orientations of the four variants is 70.5°. This means that when the martensitic transformation was dominant, there should be a peak in the vicinity of 70.5°, but that was not the case in our result. This suggests that the γ-to-ε transformation during the EBM process in this study was dominated by the diffusion transformation, which causes no orientation relationship. The difference in the preheating temperatures can be considered to be a reason for the difference in the mode of the phase transformation during the EBM process between this study and Sun et al.'s study23). The preheating temperature in the study conducted by Sun et al. was 750℃, which is lower than the γ–ε transformation temperature by 150℃ or more, while the preheating temperature of this study was 875℃, which is a relatively high temperature and near the transformation temperature. Presumably, for this reason, a mode of transformation in which the diffusion transformation, rather than the martensitic transformation, was dominant.

Fig. 8

Histogram of misorientation angle between adjacent ε phase grains in Rod-No.8.

Based on the above findings, the cause of variations in the microstructure at corresponding positions in Co-Cr-Mo alloy rods of the same size and shape fabricated simultaneously by EBM additive manufacturing, is understood as follows. It was difficult to cool the rods at the center from which heat was not easily dissipated since they were surrounded by rods that were fabricated by high-energy electron-beam irradiation. In contrast, the rods on the periphery were surrounded by powder with lower heat capacity that was irradiated by beams of lower energy for preheating. Therefore, the rods on the periphery were easy to be cooled. The elevation of the temperature was thus inhibited. Such difference in the rods' thermal history is surmised to have led to the difference in their phase constitution. The elimination of the difference in microstructure when fabricating multiple objects at the same time, requires highly sophisticated controls, which can regulate the intensity of the electron beams for preheating or vary the scan speed depending on the targeted location taking into consideration the relationship between thermal history and transformation behavior. However, many technical issues need to be resolved to make such controls possible. For the time being, a more feasible approach would be post-fabrication heat treatment to make the microstructures of each fabricated object more uniform.

Some very interesting studies have been conducted on the control of microstructure through heat treatment and the resulting improvement of the mechanical properties of the Co-Cr-Mo alloys fabricated by EBM additive manufacturing,19,20) It has been demonstrated that crystal grains in Co-Cr-Mo alloys can be refined and oriented isotropically by heat treatment for γ-to-ε transformation followed bt ε-to-γ reverse transformation, and the mechanical properties can be significantly improved. However, microstructures formed by such heat treatments are still under the influence of the initial microstructures, which means that it would be necessary to diligently monitor the impact of uneven microstructures, and to repeat the phase transformations until such inhomogeneity is removed. These efforts are expected to guarantee the EBM-fabricated products with reproducible mechanical properties, even before a fabrication technology, which can prevent the position-dependent microstructures, is developed.

4. Conclusion

An analysis by SEM-EBSD and a tensile test were performed to investigate the microstructures at median heights of twenty Co-28Cr-6Mo-0.23C-0.2N alloy rods that were fabricated simultaneously by EBM additive manufacturing. The investigation focused on the variations in microstructure at different horizontal positions in the EBM fabrication area and the impact of the varying microstructure on the mechanical properties. The findings are summarized as follows.

The microstructures of the rods, which were fabricated simultaneously and under identical conditions, varied with their horizontal position, even when their vertical positions were the same. For rods fabricated on the periphery of the fabricating area, the fraction of the ε phase was greater, while the fraction of the γ phase was relatively high for rods fabricated at the center because the temperature of the central area was elevated beyond the set temperature owing to heating from the surrounding rods, which led to the extended duration in which the temperature was above the γ–ε transformation point. Since the rods on the periphery were hardly affected by heat from surrounding rods, the temperature was maintained for a long period of time within the range where ε phase is stable. While heat treatment can be one method for eliminating the variations in microstructure, it is necessary to take into consideration the difference in thermal histories of the rods, which are affected by both the vertical position and the horizontal position, in order to obtain more uniform microstructure by heat treatment.

Acknowledgments

The authors greatly thank Dr. Shi-Hai Sun (formerly graduate student of Tohoku University, currently assistant professor of Osaka University) for his kind support for conducting this study and preparing for the manuscript. This research was supported by a Grant-in-Aid for Scientific Research from the Japan Society for the Promotion of Science (JSPS) KAKENHI (Grant Numbers: 26289252, 15K14154). A part of this work was performed under the inter-university cooperative research program (Proposal No. 14G0411) of the Cooperative Research and Development Center for Advanced Materials, Institute for Materials Research, Tohoku University.

REFERENCES
 
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