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Annealing strategies for enhancing mechanical properties of additively manufactured 316L stainless steel deposited by cold spray
Surface & Coatings Technology 370 (2019) 353–361
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Annealing strategies for enhancing mechanical properties of additively manufactured 316L stainless steel deposited by cold spray ⁎
T
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Shuo Yina, , Jan Cizekb,c, Xingchen Yand, , Rocco Lupoia a
Trinity College Dublin, The University of Dublin, Department of Mechanical and Manufacturing Engineering, Parsons Building, Dublin 2, Ireland Institute of Plasma Physics, The Czech Academy of Sciences, Za Slovankou 1782/3, 182 00 Prague, Czech Republic c Institute of Materials Science and Engineering, Brno University of Technology, Technicka 2896/2, 616 69 Brno, Czech Republic d National Engineering Laboratory for Modern Materials Surface Engineering Technology, The Key Lab of Guangdong for Modern Surface Engineering Technology, Guangdong Institute of New Materials, Guangzhou 510651, PR China b
A R T I C LE I N FO
A B S T R A C T
Keywords: Kinetic spraying (KS) Heat treatment Coating Porosity μCT scanning
Cold spray is a solid-state metal deposition and additive manufacturing (AM) technology. The low processing temperatures in cold spraying result in less favorable properties of the deposits as compared to their fusion-based AM counterparts, particularly for materials with high strength such as 316L stainless steel. Therefore, annealing is generally required for the cold sprayed deposits to improve their mechanical properties. In this paper, three different annealing strategies are systematically investigated in order to optimize the microstructure and mechanical properties of cold sprayed 316L stainless steel deposits: air annealing, vacuum annealing, and hot isostatic pressing (HIP), the latter representing a method widely used for densification of components in fusionbased metal AM. The results indicate that the three annealing treatments improve the mechanical properties of the cold sprayed 316L stainless steel deposits through grain recrystallization and diffusion at oxide-free interparticle interfaces. Such improvement is less pronounced for the air annealing as a formation of oxide inclusions impedes full inter-particle metallurgical bonding. This effect is suppressed in the vacuum annealing, resulting in a significant improvement in the tensile strength and ductility. Despite the significant improvement in the deposits' density, the HIP results in mechanical properties equivalent to those after vacuum annealing. The experimental results suggest that the strengthening of cold sprayed deposits is mainly dominated by the improved inter-particle bonding and particle grain structure rather than through a reduction of porosity.
1. Introduction
transformation can be avoided [5–15]. Importantly, cold spray further exhibits several unique advantages in comparison to powder-bed metal AM processes (such as selective laser melting, electron beam melting), including lower thermal stresses during fabrication, lack of phase transformation, short production time, unlimited component size and capability to repair damaged components. Corrosion-resistant 316L stainless steel (316L in the further text) has been widely used in biomedical and food industries. To date, a number of works has been carried out to study the manufacturing parameters, microstructures and properties of cold sprayed 316L coatings [16–29]. Due to its high strength, it was found that the 316L powders are hard to deform upon their impacts during the cold spray deposition. As a consequence, the as-sprayed 316L coatings frequently exhibited high porosity and relatively poor inter-particle bonding, resulting in their inferior properties as compared to the wrought counterparts [18,20,21,27,28]. Annealing post-treatment has shown to be one of the critical procedures for improving the properties of cold sprayed
Cold spray is an emerging, solid-state additive manufacturing (AM) technology, primarily used for a production of metal-based components [1–4]. Differing from the high-temperature deposition processes, the formation of the cold sprayed deposits relies largely on the particles' kinetic energy at impact rather than on its thermal energy. In the process, high-temperature compressed gases (typically nitrogen or helium) are used to accelerate metal powder feedstock to a high velocity (typically exceeding 300 m/s). The deposition is then achieved through two mechanisms: mechanical interlocking and a localized metallurgical bonding induced by plastic deformation of the particles upon their impacts. Sufficient plastic deformation is hence an imperative for dense and high-quality deposits. Since the feedstock used in cold spray remains in a solid state during the entire deposition process, the inevitable defects commonly encountered in the high-temperature deposition processes such as oxidation, residual thermal stress, or phase ⁎
Corresponding authors. E-mail addresses: [email protected] (S. Yin), [email protected] (X. Yan).
https://doi.org/10.1016/j.surfcoat.2019.04.012 Received 16 January 2019; Received in revised form 21 March 2019; Accepted 3 April 2019 Available online 04 April 2019 0257-8972/ © 2019 Elsevier B.V. All rights reserved.
Surface & Coatings Technology 370 (2019) 353–361
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coatings, particularly effective for materials having high strength, such as the 316L. The studies have demonstrated that the coatings' properties including density [17,19,22,23], elastic modulus [17,19,22], electrical conductivity [22] and corrosion performance [17,23,26] can be significantly improved through mitigation of the poor inter-particle bonding. Aside from the surface modification, cold spray has shown a very promising potential as an AM technology for producing bulk materials in the recent years, too. Naturally, mechanical properties of such cold sprayed bulk deposits received a special attention [30–33]. For such cold sprayed bulks, attempts to improve their properties through posttreatments were carried out [2,4]. However, in the case of 316L, very few relevant studies can be found in literature so far. Huang et al. reported an improvement of both tensile strength and elongation of cold sprayed 316L deposits after vacuum annealing [21]. Mangour et al. reported the same using air annealing [20]. Although these two studies have partially unveiled the importance of the annealing treatment for improving the mechanical properties of 316L deposits, a systematic comparison between various annealing strategies is still missing at the moment. The lack of such information becomes even clearer considering that hot isostatic pressing (HIP), a process regularly used for densification in fusion-based metal AM, has not yet been reported for the cold sprayed 316L bulks. In order to manufacture high-quality cold spray bulk deposits comparable to or even surpassing the fusion-based AM bulks, it is important to understand the underlying mechanisms in various annealing strategies. In this paper, we aimed to systematically study the effect of three selected annealing strategies, air annealing, vacuum annealing and HIP, on the microstructure, porosity and mechanical properties of cold sprayed 316L deposits. Based on the results, the best annealing strategy for 316L bulk deposits is suggested herein. From the application point of view, it is important to note that such annealing in the presented cold spray route does not present an additional step compared to the other AM strategies as these also require samples posttreatments. Hence, the annealing step does not increase the fabrication costs as compared to other AM processes.
Table 1 Annotation and parameters of the three annealing strategies used in this work. Gas
AAT VAT HIPT
Air N/A Argon
Temperature
1000 °C 1000 °C 1000 °C
Pressure
Time
Heating/cooling rate
0.1 MPa N/A 0.1 MPa + 150 MPa
4h 4h 4h+4h
Furnace cooling 10 °C/min 10 °C/min
using an in-house cold spray system (Trinity College Dublin, Ireland) [34]. Nitrogen with the inlet pressure of 3.0 MPa and temperature of 1000 °C was used as the propulsion gas. The substrate standoff distance and nozzle traversal speed were 30 mm and 50 mm/s, respectively. The aluminum alloy substrates remained solid state during the cold spray process and were milled away after the deposition. The final thickness of the cold sprayed 316L deposits exceeded 2.2 mm. The standalone deposits were then post-treated using three annealing strategies: air annealing treatment (AAT), vacuum annealing treatment (VAT), and HIP treatment (HIPT). Table 1 lists the detailed annealing parameters used in this work. The annealing conditions were selected based on the previous work [21]. The HIP-treated samples were vacuum-annealed for 4 h first in order to guarantee sufficient softening and then HIPtreated under 150 MPa pressure for another 4 h. The samples were not encapsulated during the HIP treatment. 2.2. Materials characterization The microstructure of the cold sprayed 316L deposits was studied by SEM cross-section observation. The samples were polished using standard metallographic procedures with the final polishing step applied using 0.06 μm colloidal silica solution. The polished samples were then electrochemically etched using a 2.2 mol/L reagent of nitric acid while applying 2 V direct current to expose the grain structure. Energy dispersive X-ray spectroscopy (EDX) was employed to analyze the oxide formation in the air-annealed deposit. The porosity of the deposits was studied using two different methods, image analysis and tomography scanning. The former is a conventional 2-dimensional (2D) image analysis which calculates the average porosity based on analysis of cross-sectional micrographs taken at different areas of the deposits. This method is widely used for measuring the porosity of cold sprayed coatings and deposits. In this work, five micrographs were used to calculate the average porosity of the cold sprayed 316L deposits (ImageJ software). The latter approach is to use 3-dimensional (3D) μcomputer tomography (μCT) scanning (Nikon XT H 225 ST, Japan) to reconstruct the pores structure within a porous material. This method can measure porosity in a large volume (ranging from micrometers to several millimeters) and can reconstruct the contour and spatial distribution of the pores in a 3D space. Given the device resolution
2. Experimental methodology 2.1. Manufacturing procedure Spherical 316L powder (LPW Technology Limited, UK) with the size range between 15 and 45 μm was used as the feedstock. Fig. 1 shows the morphology and etched cross-section of the 316L powder particles observed by scanning electron microscope (SEM, Carl Zeiss Ultra Plus, Germany). The microstructure is characterized by a multi-crystalline dendritic structure, a consequence of the gas-atomization fabrication process. The powder was deposited onto aluminum alloy substrates
Fig. 1. The 316L powders used in this work. (a) Morphology and (b) etched cross-section. 354
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limitation, the μCT scanning used in this work analyzed only the pores with an equivalent diameter of more than 14 μm. In the μCT analysis, three cubic volumes with dimensions of 0.5 × 0.5 × 0.5 mm3 were selected to calculate the porosity.
(Fig. 2a). After annealing under air condition (Fig. 2b), the pore structure did not exhibit any significant changes. However, unlike the pores, the inter-particle interfaces became more pronounced. Similar to the air annealing, the original inter-particle pores were not eliminated during vacuum annealing (Fig. 2c). Among the three annealing strategies, HIP led to the densest deposits (Fig. 2d). There were no visible pores or inter-particle boundaries, undoubtedly a consequence of the applied 150 MPa pressure and 1000 °C temperature. Fig. 3 shows the inter-particle pores and the corresponding EDX line scans of the 316L deposits before and after the respective annealing treatments. In the as-fabricated state, the pores in the deposit were hollow voids without any materials present. However, after air annealing, new phases formed at the interfacial area, primarily composed of Cr, Mn, O and Si. According to [36,37], the phases are likely to be Mn2CrO4, Cr2O3 and SiO2, i.e., oxides typical for 316L steel exposed to air atmosphere. The oxides were also shown in Fig. 2b and their formation inside the deposit may be caused by oxygen penetrating through the inter-particle boundaries. Upon formation, these oxides can act as barriers to inter-particle bonding, reducing the particle mutual cohesion strength, and, ultimately, leading to inferior mechanical properties [38–40]. In the vacuum-annealed deposit, no interfacial oxidation took place under the vacuum environment. Consequently, the inner pores are hollow voids as in the case of the as-fabricated deposits. In the HIPtreated deposit, the pores were difficult to be found, and thus no EDX data are provided. However, given the oxygen-free character of the HIP process, it could be expected that the outcome would also be devoid of oxygen, as in the case of vacuum annealing. Fig. 4 shows the etched cross-sectional microstructure (grain structure) of the 316L deposits before and after the respective annealing treatments. In the as-fabricated state (Fig. 4a), the inter-particle interfaces and grain boundaries are visible, naturally resembling the
2.3. Mechanical property measurement The tensile strength of the samples cut from the substrate-free coatings was measured using universal tensile testing system at a crosshead speed of 2 mm/min (Instron 3360/8801, UK), following ASTM E8 specification [35]. The tensile specimens were cut into dogbone shape with a gauge length of 25 mm, gauge width of 5 mm and a thickness of 2.2 mm using precise electrical discharge machining. The ultimate tensile strength (UTS) and elongation to break (EL) of each sample group were determined based on the average value of three tensile specimens. The fracture surface morphology was then studied by SEM. 3. Results and discussion 3.1. Microstructure Fig. 2 shows the cross-sectional images of the cold sprayed 316L deposits before and after the respective annealing treatments. It is clear that the annealing treatments have a significant impact on the microstructure of the deposits. As provided in the Introduction section, due to its high strength and hardness, 316L is generally hard to be manufactured into fully dense deposits by cold spray [29]. As an outcome, the microstructure of the as-fabricated deposit is characterized by a presence of inter-particle pores as well as visible inter-splat interfaces due to the insufficient particle plastic deformation upon impact
Fig. 2. Cross-sectional images of the 316L deposits before and after the respective annealing treatments. (a) as-fabricated sample, (b) air-annealed sample, (c) vacuum-annealed sample and (d) HIP-treated sample. The white squares in (a) indicate original as-fabricated porosity. AF represents as-fabricated state, AAT represents air annealing treatment, VAT represents vacuum annealing treatment, and HIPT represents HIP treatment. 355
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Fig. 3. Inter-particle pores and the corresponding EDX line scans. (a) as-fabricated sample, (b) air-annealed sample, and (c) vacuum-annealed sample. AF represents as-fabricated state, AAT represents air annealing treatment, and VAT represents vacuum annealing treatment.
no significant effect, showing porosity levels equivalent to the as-fabricated state. The vacuum annealing led to a slight reduction in the porosity, probably due to the occurrence of diffusion across the interparticle interfaces, triggering closing of the inter-particle interfaces and tiny pores. Among the three annealing strategies, HIP led to the lowest porosity due to the applied 150 MPa pressure and 1000 °C temperature effectively compacting the deposit. The porosity measurement is in agreement with the microstructure observation shown in Fig. 2. From Fig. 5, it is also evident that the trend in the porosities measured by the two methods is the same. However, the methods differ in the absolute values; the porosity measured by 3D μCT analysis was lower than that measured by 2D image analysis. This discrepancy could be caused by the limitation of the μCT scanning, unable to detect pores smaller than 14 μm. To study the characteristics of the pores in more detail, the distribution of equivalent pore diameter of the 316L deposits before and after the respective annealing treatments obtained from 2D image analysis was carried out (Fig. 6). It is clear that the as-fabricated and air-annealed samples exhibited quite similar pore size distribution, with the maximum pore size reaching approximately 70 μm. The vacuumannealed sample showed much narrower size distribution with the
microstructure of the original powder feedstock (Fig. 1b). Interestingly, the etching effect was more prominent at the inter-particle interfaces than at the grain boundaries, indicating a poor inter-particle bond and therefore weak deposit cohesion. Microstructure recrystallization took place after the air annealing, resulting in a grain growth and formation of some annealing twins as shown in Fig. 4b. A partial closing of the inter-particle interface gaps was observed at oxide-free areas, prospectively aiding in a partial improvement of the interfacial bonding [41]. The vacuum annealing resulted in a grain structure partially resembling that after the air annealing, yet with much better diffusion across the inter-particle interfaces due to the absence of the oxide inclusions (Fig. 4c). After the HIP, the deposit possessed a greater average grain size, possibly a consequence of the longer annealing time (8 h for HIPT vs. 4 h for AAT and VAT).
3.2. Porosity The graph in Fig. 5 shows the porosity of the 316L deposits before and after the respective annealing treatments obtained from the two methods, image analysis and μCT. As can be seen, the air annealing had 356
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Fig. 4. Microstructure of the 316L deposits before and after the respective annealing treatments. (a) as-fabricated sample, (b) air-annealed sample, (c) vacuumannealed sample and (d) HIP-treated sample. The formed annealing twins are marked by white arrows. AF represents as-fabricated state, AAT represents air annealing treatment, VAT represents vacuum annealing treatment, and HIPT represents HIP treatment.
Fig. 5. Porosity of the 316L deposits before and after the respective annealing treatments.
Fig. 6. Distribution of the equivalent pore diameters in the 316L deposits before and after the respective annealing treatments obtained from 2D image analysis.
maximum pore size of approximately 35 μm. Among all samples, the HIP-treated samples demonstrated the narrowest pore size range and the smallest pore size (15 μm). The calculated pore size distribution results are consistent with the overall porosity results shown in Fig. 5. For understanding the pore structure within the 316L deposits, the spatial 3D reconstructions of the pore structures are presented in Fig. 7. Clearly, pores with various shapes and sizes were uniformly dispersed within the deposits. Between the as-fabricated and air-annealed deposits, no significant differences were observed. In comparison, the density of the pores in the vacuum-annealed deposit slightly decreased,
indicating the reduction of porosity. For the HIP-treated deposit, the density of the pores was dramatically reduced as compared to the other three cases. These facts further confirm that the HIP treatment is effective in reducing the porosity and pore size of cold sprayed deposits. These pores were dispersed rather sparely and hence not easy to be detected through the 2D cross-sectional image unless the cross-section is cut right through the pores. This fact indicates that 3D μCT reconstruction is more representative than 2D cross-sectional imaging in characterizing the pores distribution in the cold sprayed deposits. 357
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Fig. 7. 3D μCT reconstruction of the pores within a 0.5 × 0.5 × 0.5 mm3 regions in the 316L deposits before and after the respective annealing treatments. AF represents as-fabricated state, AAT represents air annealing treatment, VAT represents vacuum annealing treatment, and HIPT represents HIP treatment.
Considering their respective advantages, a combination of the two methods may actually be the best way to characterize the porosity and pore structure of cold sprayed deposits.
3.3. Mechanical properties Fig. 8 shows the microhardness of the 316L deposits before and after the respective annealing treatments. The as-fabricated sample showed the highest hardness due to the deformation-induced dislocation density increment within the cold sprayed particles during their deposition. The overall hardness of the cold spray as-fabricated deposit reached 255 HV0.5. The measurements exhibited a rather remarkable scatter of almost ± 50 HV0.5. Such data dispersion may arise due to the local inhomogeneities in the structure, predominantly the discussed pore content. During the three annealing processes, recrystallization of the plastically deformed material took place, leading to the elimination of dislocations and a growth of the grains [35]. Consequently, the annealed samples exhibited lower hardness than the as-fabricated deposits, which is consistent with previously published results [20,23]. Given the data scatter, the hardness values of the three annealed samples could not be safely declared as statistically different. Fig. 9 shows the ultimate tensile strength and elongation of the 316L deposits before and after the respective annealing treatments. In the asfabricated state, the tensile strength and elongation are low due to the weak inter-particle bonding and a presence of pores discussed earlier. After the air annealing, the strength and elongation increased due to the occurrence of recrystallization and partially improved inter-particle bonding. The values are, however, still significantly lower than the 316L bulk counterparts (485 MPa and 40%) [42]. Importantly, the
Fig. 9. Ultimate tensile strength and elongation of the 316L deposits before and after the respective annealing treatments.
vacuum annealing resulted in a significant improvement in the tensile strength and elongation, reaching 92% and 41% of the corresponding 316L bulk counterparts values, respectively. This is due to the absence of the oxide inclusions and a considerably enhanced bonding strength at the inter-particle interfaces as discussed in reference to Figs. 2 and 4. The strength and elongation were expected to be enhanced even further after performing the HIP treatment. However, despite the much denser deposits, the UTS value of the HIP-treated samples was comparable (6.6% lower in average, in fact) to that of the vacuum-annealed samples. Although this was a rather surprising result, an identical phenomenon was also observed in literature sources pertaining to selective laser melted (SLM) metal samples: Yan et al. and Hutasoit et al. both reported that vacuum-treated SLM Ti6Al4V samples exhibited UTS equivalent to HIP-treated samples [43,44]. Unfortunately, no clear explanation was provided in the two studies. In the current study, we propose the following hypothesis to partially explain the seeming discrepancy of the lower UTS against significantly lower porosity of the HIP-treated samples. The inter-particle metallurgical bonding in a cold spray deposit originates from a severe plastic deformation of the material and a concurrent fracture and removal of the thin oxide films originally present at the particle surfaces [39,45,46]. The pores in the as-fabricated 316L deposit (marked as white squares in Fig. 2a) were formed due to the insufficient particle plastic deformation during deposition. At such areas, the oxide films were not broken and removed from the particle surface, but rather still remained on the surfaces [47]. Upon performing the HIP treatment, the oxide films were indeed compacted during the pores collapse, but not removed from the interparticle interfaces, thereby hindering the metallurgical bonding as schematized in Fig. 10. In other words, the total effective metallurgical interfacial bonding area was fully comparable in both the vacuum-annealed samples and HIP-treated samples, despite the difference in the
Fig. 8. Microhardness of the 316L deposits before and after the respective annealing treatments. 358
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Fig. 10. Schematic of inter-particle pore evolution during HIP treatment. Red lines indicate the thin oxide films originally present at particle surfaces. Importantly, during HIP treatment, the oxides are not removed from the original pore area. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)
annealing may also break the oxide films on the particle surfaces and further improve the inter-particle metallurgical bonding [49,50]. This observation explains why the strength and ductility of these two sample types were superior to the as-fabricated or air-annealed samples.
porosity content. This phenomenon may explain why the HIP led to UTS values comparable to the vacuum annealing samples. Arguably, this hypothesis may be also applicable to the same phenomenon experienced in SLM technology. Unfortunately, the provided hypothesis cannot 100% explain the statistically significant difference in the values of elongation; the described phenomena related to retained oxide shells after HIP treatment could play a significant role, but would probably not account for the measured ~25% difference in the elongation of the HIP-treated and vacuum-annealed samples (Fig. 9). Even though the true mechanism is yet to be cleared, it is further proposed herein that the second effect contributing to the reduced elongation is the bigger grain size of the HIP-treated samples (cf. Fig. 4c and d). Despite the fact that the HIP treatment could not improve the UTS and elongation values as compared to vacuum annealing, a number of works have confirmed that a HIP treatment can greatly improve fatigue strength of additively manufactured metal samples due to the reduction of porosity [43]. In addition, in most previous cold spray works, people mainly attributed the deposit strengthening after annealing to the reduction of porosity and improved inter-particle interface [20,21]. However, the results obtained in this work reveal that, rather than through a reduction of porosity by post-compaction, the strengthening is mainly dominated by the improved inter-particle bonding and particles' grain structure. To better understand the strengthening mechanism of the different annealing strategies, fractographic images of the tensile specimens before and after the respective annealing treatments are shown in Fig. 11. In the as-fabricated state (Fig. 11a), the fracture surface morphology revealed an inter-particle failure decohesion, clearly indicating that the inter-particle interfaces are the weakest microstructure regions. After the air annealing (Fig. 11b), the fracture surface of the tensile specimens exhibited signs of the inter-particle decohesion, too. However, different from the as-fabricated state, ductile dimples suggesting an enhanced cohesion strength and ductility [48] were partially observed at the surfaces. Importantly, disconnected oxide inclusions can be clearly observed inside the dimples as shown in the magnified inset in Fig. 11b. These correspond to the oxide inclusions discussed with reference to Figs. 2 and 4 and hinder the metallurgical bonding between adjacent particles, explaining why the improvement of the mechanical properties is not as prominent. A similar phenomenon was also observed in heat-treated bond coats for thermally sprayed thermal barrier coatings (TBCs) [49,50]. For the vacuum-annealed and HIP-treated samples (Fig. 11c and d), the fracture surfaces exhibited similar features, mainly characterized by the ductile dimples, this time without the oxide inclusions. In other words, aside from recrystallization, the diffusion through the effective metal-to-metal contact area contributed to a significant increase in cohesion of the adjacent particles. In addition, previous work also reported that high-temperature vacuum
4. Conclusions In this paper, the effect of different annealing strategies comprehending air annealing, vacuum annealing and HIP on the microstructure and mechanical properties of cold sprayed 316L stainless steel deposits was systematically studied. It was found that all three annealing treatments improve the mechanical properties of the deposits through recrystallization and diffusion across oxide-free inter-particle interfaces. There were, however, significant differences among the three strategies. Air annealing improved the tensile strength and ductility of the deposit only slightly due to the formation of oxide inclusions at the inter-particle interfaces, hindering a full cohesion bonding via diffusion. In contrast, vacuum annealing significantly improved the tensile strength and ductility due to the absence of such oxides in the inter-particle interfaces. The tensile strength and ductility after vacuum annealing reached 92% and 41% of the corresponding 316L bulk counterparts values, respectively. Although the deposit porosity was significantly reduced after HIP treatment, the mechanical properties did not surpass the vacuum-annealed samples. This may be caused by the original oxide films retained at the particle surfaces in the original inter-particle pore areas, effectively hindering the intimate metallurgical inter-particle bonding even after the pores collapse in HIP. Note that this hypothesis has not been experimentally proved in the current work, and a further experimental investigation using TEM, AES or positron annihilation spectroscopy are planned in our coming study. In summary, the experimental results obtained in this work suggest that, rather than through a reduction of porosity, the strengthening is mainly dominated by the improved inter-particle bonding and particles' grain structure. Taken overall, based on the findings in this work, it is recommended to use vacuum annealing to improve the mechanical properties of cold sprayed 316L deposit or using HIP if additional density improvement is required.
Acknowledgments Shuo Yin and Rocco Lupoi would like to thank the financial support from Irish Research Council project (GOIPD-2017-912) and European Space Agency (4000112844/14/NL/FE), and also the CRANN Advanced Microscopy Laboratory of Trinity College Dublin for the support in data analysis, Jan Cizek would like to acknowledge Czech Science Foundation project (GACR 17-13573S). 359
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Fig. 11. Fractographic surfaces of the 316L deposits ruptured in tensile testing. (a) As-fabricated sample, (b) air-annealed sample, (c) vacuum-annealed sample and (d) HIP-treated sample. AF represents as-fabricated state, AAT represents air annealing treatment, VAT represents vacuum annealing treatment, and HIPT represents HIP treatment.
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Annealing strategies for enhancing mechanical properties of additively manufactured 316L stainless steel deposited by cold spray ⁎
T
⁎
Shuo Yina, , Jan Cizekb,c, Xingchen Yand, , Rocco Lupoia a
Trinity College Dublin, The University of Dublin, Department of Mechanical and Manufacturing Engineering, Parsons Building, Dublin 2, Ireland Institute of Plasma Physics, The Czech Academy of Sciences, Za Slovankou 1782/3, 182 00 Prague, Czech Republic c Institute of Materials Science and Engineering, Brno University of Technology, Technicka 2896/2, 616 69 Brno, Czech Republic d National Engineering Laboratory for Modern Materials Surface Engineering Technology, The Key Lab of Guangdong for Modern Surface Engineering Technology, Guangdong Institute of New Materials, Guangzhou 510651, PR China b
A R T I C LE I N FO
A B S T R A C T
Keywords: Kinetic spraying (KS) Heat treatment Coating Porosity μCT scanning
Cold spray is a solid-state metal deposition and additive manufacturing (AM) technology. The low processing temperatures in cold spraying result in less favorable properties of the deposits as compared to their fusion-based AM counterparts, particularly for materials with high strength such as 316L stainless steel. Therefore, annealing is generally required for the cold sprayed deposits to improve their mechanical properties. In this paper, three different annealing strategies are systematically investigated in order to optimize the microstructure and mechanical properties of cold sprayed 316L stainless steel deposits: air annealing, vacuum annealing, and hot isostatic pressing (HIP), the latter representing a method widely used for densification of components in fusionbased metal AM. The results indicate that the three annealing treatments improve the mechanical properties of the cold sprayed 316L stainless steel deposits through grain recrystallization and diffusion at oxide-free interparticle interfaces. Such improvement is less pronounced for the air annealing as a formation of oxide inclusions impedes full inter-particle metallurgical bonding. This effect is suppressed in the vacuum annealing, resulting in a significant improvement in the tensile strength and ductility. Despite the significant improvement in the deposits' density, the HIP results in mechanical properties equivalent to those after vacuum annealing. The experimental results suggest that the strengthening of cold sprayed deposits is mainly dominated by the improved inter-particle bonding and particle grain structure rather than through a reduction of porosity.
1. Introduction
transformation can be avoided [5–15]. Importantly, cold spray further exhibits several unique advantages in comparison to powder-bed metal AM processes (such as selective laser melting, electron beam melting), including lower thermal stresses during fabrication, lack of phase transformation, short production time, unlimited component size and capability to repair damaged components. Corrosion-resistant 316L stainless steel (316L in the further text) has been widely used in biomedical and food industries. To date, a number of works has been carried out to study the manufacturing parameters, microstructures and properties of cold sprayed 316L coatings [16–29]. Due to its high strength, it was found that the 316L powders are hard to deform upon their impacts during the cold spray deposition. As a consequence, the as-sprayed 316L coatings frequently exhibited high porosity and relatively poor inter-particle bonding, resulting in their inferior properties as compared to the wrought counterparts [18,20,21,27,28]. Annealing post-treatment has shown to be one of the critical procedures for improving the properties of cold sprayed
Cold spray is an emerging, solid-state additive manufacturing (AM) technology, primarily used for a production of metal-based components [1–4]. Differing from the high-temperature deposition processes, the formation of the cold sprayed deposits relies largely on the particles' kinetic energy at impact rather than on its thermal energy. In the process, high-temperature compressed gases (typically nitrogen or helium) are used to accelerate metal powder feedstock to a high velocity (typically exceeding 300 m/s). The deposition is then achieved through two mechanisms: mechanical interlocking and a localized metallurgical bonding induced by plastic deformation of the particles upon their impacts. Sufficient plastic deformation is hence an imperative for dense and high-quality deposits. Since the feedstock used in cold spray remains in a solid state during the entire deposition process, the inevitable defects commonly encountered in the high-temperature deposition processes such as oxidation, residual thermal stress, or phase ⁎
Corresponding authors. E-mail addresses: [email protected] (S. Yin), [email protected] (X. Yan).
https://doi.org/10.1016/j.surfcoat.2019.04.012 Received 16 January 2019; Received in revised form 21 March 2019; Accepted 3 April 2019 Available online 04 April 2019 0257-8972/ © 2019 Elsevier B.V. All rights reserved.
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coatings, particularly effective for materials having high strength, such as the 316L. The studies have demonstrated that the coatings' properties including density [17,19,22,23], elastic modulus [17,19,22], electrical conductivity [22] and corrosion performance [17,23,26] can be significantly improved through mitigation of the poor inter-particle bonding. Aside from the surface modification, cold spray has shown a very promising potential as an AM technology for producing bulk materials in the recent years, too. Naturally, mechanical properties of such cold sprayed bulk deposits received a special attention [30–33]. For such cold sprayed bulks, attempts to improve their properties through posttreatments were carried out [2,4]. However, in the case of 316L, very few relevant studies can be found in literature so far. Huang et al. reported an improvement of both tensile strength and elongation of cold sprayed 316L deposits after vacuum annealing [21]. Mangour et al. reported the same using air annealing [20]. Although these two studies have partially unveiled the importance of the annealing treatment for improving the mechanical properties of 316L deposits, a systematic comparison between various annealing strategies is still missing at the moment. The lack of such information becomes even clearer considering that hot isostatic pressing (HIP), a process regularly used for densification in fusion-based metal AM, has not yet been reported for the cold sprayed 316L bulks. In order to manufacture high-quality cold spray bulk deposits comparable to or even surpassing the fusion-based AM bulks, it is important to understand the underlying mechanisms in various annealing strategies. In this paper, we aimed to systematically study the effect of three selected annealing strategies, air annealing, vacuum annealing and HIP, on the microstructure, porosity and mechanical properties of cold sprayed 316L deposits. Based on the results, the best annealing strategy for 316L bulk deposits is suggested herein. From the application point of view, it is important to note that such annealing in the presented cold spray route does not present an additional step compared to the other AM strategies as these also require samples posttreatments. Hence, the annealing step does not increase the fabrication costs as compared to other AM processes.
Table 1 Annotation and parameters of the three annealing strategies used in this work. Gas
AAT VAT HIPT
Air N/A Argon
Temperature
1000 °C 1000 °C 1000 °C
Pressure
Time
Heating/cooling rate
0.1 MPa N/A 0.1 MPa + 150 MPa
4h 4h 4h+4h
Furnace cooling 10 °C/min 10 °C/min
using an in-house cold spray system (Trinity College Dublin, Ireland) [34]. Nitrogen with the inlet pressure of 3.0 MPa and temperature of 1000 °C was used as the propulsion gas. The substrate standoff distance and nozzle traversal speed were 30 mm and 50 mm/s, respectively. The aluminum alloy substrates remained solid state during the cold spray process and were milled away after the deposition. The final thickness of the cold sprayed 316L deposits exceeded 2.2 mm. The standalone deposits were then post-treated using three annealing strategies: air annealing treatment (AAT), vacuum annealing treatment (VAT), and HIP treatment (HIPT). Table 1 lists the detailed annealing parameters used in this work. The annealing conditions were selected based on the previous work [21]. The HIP-treated samples were vacuum-annealed for 4 h first in order to guarantee sufficient softening and then HIPtreated under 150 MPa pressure for another 4 h. The samples were not encapsulated during the HIP treatment. 2.2. Materials characterization The microstructure of the cold sprayed 316L deposits was studied by SEM cross-section observation. The samples were polished using standard metallographic procedures with the final polishing step applied using 0.06 μm colloidal silica solution. The polished samples were then electrochemically etched using a 2.2 mol/L reagent of nitric acid while applying 2 V direct current to expose the grain structure. Energy dispersive X-ray spectroscopy (EDX) was employed to analyze the oxide formation in the air-annealed deposit. The porosity of the deposits was studied using two different methods, image analysis and tomography scanning. The former is a conventional 2-dimensional (2D) image analysis which calculates the average porosity based on analysis of cross-sectional micrographs taken at different areas of the deposits. This method is widely used for measuring the porosity of cold sprayed coatings and deposits. In this work, five micrographs were used to calculate the average porosity of the cold sprayed 316L deposits (ImageJ software). The latter approach is to use 3-dimensional (3D) μcomputer tomography (μCT) scanning (Nikon XT H 225 ST, Japan) to reconstruct the pores structure within a porous material. This method can measure porosity in a large volume (ranging from micrometers to several millimeters) and can reconstruct the contour and spatial distribution of the pores in a 3D space. Given the device resolution
2. Experimental methodology 2.1. Manufacturing procedure Spherical 316L powder (LPW Technology Limited, UK) with the size range between 15 and 45 μm was used as the feedstock. Fig. 1 shows the morphology and etched cross-section of the 316L powder particles observed by scanning electron microscope (SEM, Carl Zeiss Ultra Plus, Germany). The microstructure is characterized by a multi-crystalline dendritic structure, a consequence of the gas-atomization fabrication process. The powder was deposited onto aluminum alloy substrates
Fig. 1. The 316L powders used in this work. (a) Morphology and (b) etched cross-section. 354
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limitation, the μCT scanning used in this work analyzed only the pores with an equivalent diameter of more than 14 μm. In the μCT analysis, three cubic volumes with dimensions of 0.5 × 0.5 × 0.5 mm3 were selected to calculate the porosity.
(Fig. 2a). After annealing under air condition (Fig. 2b), the pore structure did not exhibit any significant changes. However, unlike the pores, the inter-particle interfaces became more pronounced. Similar to the air annealing, the original inter-particle pores were not eliminated during vacuum annealing (Fig. 2c). Among the three annealing strategies, HIP led to the densest deposits (Fig. 2d). There were no visible pores or inter-particle boundaries, undoubtedly a consequence of the applied 150 MPa pressure and 1000 °C temperature. Fig. 3 shows the inter-particle pores and the corresponding EDX line scans of the 316L deposits before and after the respective annealing treatments. In the as-fabricated state, the pores in the deposit were hollow voids without any materials present. However, after air annealing, new phases formed at the interfacial area, primarily composed of Cr, Mn, O and Si. According to [36,37], the phases are likely to be Mn2CrO4, Cr2O3 and SiO2, i.e., oxides typical for 316L steel exposed to air atmosphere. The oxides were also shown in Fig. 2b and their formation inside the deposit may be caused by oxygen penetrating through the inter-particle boundaries. Upon formation, these oxides can act as barriers to inter-particle bonding, reducing the particle mutual cohesion strength, and, ultimately, leading to inferior mechanical properties [38–40]. In the vacuum-annealed deposit, no interfacial oxidation took place under the vacuum environment. Consequently, the inner pores are hollow voids as in the case of the as-fabricated deposits. In the HIPtreated deposit, the pores were difficult to be found, and thus no EDX data are provided. However, given the oxygen-free character of the HIP process, it could be expected that the outcome would also be devoid of oxygen, as in the case of vacuum annealing. Fig. 4 shows the etched cross-sectional microstructure (grain structure) of the 316L deposits before and after the respective annealing treatments. In the as-fabricated state (Fig. 4a), the inter-particle interfaces and grain boundaries are visible, naturally resembling the
2.3. Mechanical property measurement The tensile strength of the samples cut from the substrate-free coatings was measured using universal tensile testing system at a crosshead speed of 2 mm/min (Instron 3360/8801, UK), following ASTM E8 specification [35]. The tensile specimens were cut into dogbone shape with a gauge length of 25 mm, gauge width of 5 mm and a thickness of 2.2 mm using precise electrical discharge machining. The ultimate tensile strength (UTS) and elongation to break (EL) of each sample group were determined based on the average value of three tensile specimens. The fracture surface morphology was then studied by SEM. 3. Results and discussion 3.1. Microstructure Fig. 2 shows the cross-sectional images of the cold sprayed 316L deposits before and after the respective annealing treatments. It is clear that the annealing treatments have a significant impact on the microstructure of the deposits. As provided in the Introduction section, due to its high strength and hardness, 316L is generally hard to be manufactured into fully dense deposits by cold spray [29]. As an outcome, the microstructure of the as-fabricated deposit is characterized by a presence of inter-particle pores as well as visible inter-splat interfaces due to the insufficient particle plastic deformation upon impact
Fig. 2. Cross-sectional images of the 316L deposits before and after the respective annealing treatments. (a) as-fabricated sample, (b) air-annealed sample, (c) vacuum-annealed sample and (d) HIP-treated sample. The white squares in (a) indicate original as-fabricated porosity. AF represents as-fabricated state, AAT represents air annealing treatment, VAT represents vacuum annealing treatment, and HIPT represents HIP treatment. 355
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Fig. 3. Inter-particle pores and the corresponding EDX line scans. (a) as-fabricated sample, (b) air-annealed sample, and (c) vacuum-annealed sample. AF represents as-fabricated state, AAT represents air annealing treatment, and VAT represents vacuum annealing treatment.
no significant effect, showing porosity levels equivalent to the as-fabricated state. The vacuum annealing led to a slight reduction in the porosity, probably due to the occurrence of diffusion across the interparticle interfaces, triggering closing of the inter-particle interfaces and tiny pores. Among the three annealing strategies, HIP led to the lowest porosity due to the applied 150 MPa pressure and 1000 °C temperature effectively compacting the deposit. The porosity measurement is in agreement with the microstructure observation shown in Fig. 2. From Fig. 5, it is also evident that the trend in the porosities measured by the two methods is the same. However, the methods differ in the absolute values; the porosity measured by 3D μCT analysis was lower than that measured by 2D image analysis. This discrepancy could be caused by the limitation of the μCT scanning, unable to detect pores smaller than 14 μm. To study the characteristics of the pores in more detail, the distribution of equivalent pore diameter of the 316L deposits before and after the respective annealing treatments obtained from 2D image analysis was carried out (Fig. 6). It is clear that the as-fabricated and air-annealed samples exhibited quite similar pore size distribution, with the maximum pore size reaching approximately 70 μm. The vacuumannealed sample showed much narrower size distribution with the
microstructure of the original powder feedstock (Fig. 1b). Interestingly, the etching effect was more prominent at the inter-particle interfaces than at the grain boundaries, indicating a poor inter-particle bond and therefore weak deposit cohesion. Microstructure recrystallization took place after the air annealing, resulting in a grain growth and formation of some annealing twins as shown in Fig. 4b. A partial closing of the inter-particle interface gaps was observed at oxide-free areas, prospectively aiding in a partial improvement of the interfacial bonding [41]. The vacuum annealing resulted in a grain structure partially resembling that after the air annealing, yet with much better diffusion across the inter-particle interfaces due to the absence of the oxide inclusions (Fig. 4c). After the HIP, the deposit possessed a greater average grain size, possibly a consequence of the longer annealing time (8 h for HIPT vs. 4 h for AAT and VAT).
3.2. Porosity The graph in Fig. 5 shows the porosity of the 316L deposits before and after the respective annealing treatments obtained from the two methods, image analysis and μCT. As can be seen, the air annealing had 356
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Fig. 4. Microstructure of the 316L deposits before and after the respective annealing treatments. (a) as-fabricated sample, (b) air-annealed sample, (c) vacuumannealed sample and (d) HIP-treated sample. The formed annealing twins are marked by white arrows. AF represents as-fabricated state, AAT represents air annealing treatment, VAT represents vacuum annealing treatment, and HIPT represents HIP treatment.
Fig. 5. Porosity of the 316L deposits before and after the respective annealing treatments.
Fig. 6. Distribution of the equivalent pore diameters in the 316L deposits before and after the respective annealing treatments obtained from 2D image analysis.
maximum pore size of approximately 35 μm. Among all samples, the HIP-treated samples demonstrated the narrowest pore size range and the smallest pore size (15 μm). The calculated pore size distribution results are consistent with the overall porosity results shown in Fig. 5. For understanding the pore structure within the 316L deposits, the spatial 3D reconstructions of the pore structures are presented in Fig. 7. Clearly, pores with various shapes and sizes were uniformly dispersed within the deposits. Between the as-fabricated and air-annealed deposits, no significant differences were observed. In comparison, the density of the pores in the vacuum-annealed deposit slightly decreased,
indicating the reduction of porosity. For the HIP-treated deposit, the density of the pores was dramatically reduced as compared to the other three cases. These facts further confirm that the HIP treatment is effective in reducing the porosity and pore size of cold sprayed deposits. These pores were dispersed rather sparely and hence not easy to be detected through the 2D cross-sectional image unless the cross-section is cut right through the pores. This fact indicates that 3D μCT reconstruction is more representative than 2D cross-sectional imaging in characterizing the pores distribution in the cold sprayed deposits. 357
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Fig. 7. 3D μCT reconstruction of the pores within a 0.5 × 0.5 × 0.5 mm3 regions in the 316L deposits before and after the respective annealing treatments. AF represents as-fabricated state, AAT represents air annealing treatment, VAT represents vacuum annealing treatment, and HIPT represents HIP treatment.
Considering their respective advantages, a combination of the two methods may actually be the best way to characterize the porosity and pore structure of cold sprayed deposits.
3.3. Mechanical properties Fig. 8 shows the microhardness of the 316L deposits before and after the respective annealing treatments. The as-fabricated sample showed the highest hardness due to the deformation-induced dislocation density increment within the cold sprayed particles during their deposition. The overall hardness of the cold spray as-fabricated deposit reached 255 HV0.5. The measurements exhibited a rather remarkable scatter of almost ± 50 HV0.5. Such data dispersion may arise due to the local inhomogeneities in the structure, predominantly the discussed pore content. During the three annealing processes, recrystallization of the plastically deformed material took place, leading to the elimination of dislocations and a growth of the grains [35]. Consequently, the annealed samples exhibited lower hardness than the as-fabricated deposits, which is consistent with previously published results [20,23]. Given the data scatter, the hardness values of the three annealed samples could not be safely declared as statistically different. Fig. 9 shows the ultimate tensile strength and elongation of the 316L deposits before and after the respective annealing treatments. In the asfabricated state, the tensile strength and elongation are low due to the weak inter-particle bonding and a presence of pores discussed earlier. After the air annealing, the strength and elongation increased due to the occurrence of recrystallization and partially improved inter-particle bonding. The values are, however, still significantly lower than the 316L bulk counterparts (485 MPa and 40%) [42]. Importantly, the
Fig. 9. Ultimate tensile strength and elongation of the 316L deposits before and after the respective annealing treatments.
vacuum annealing resulted in a significant improvement in the tensile strength and elongation, reaching 92% and 41% of the corresponding 316L bulk counterparts values, respectively. This is due to the absence of the oxide inclusions and a considerably enhanced bonding strength at the inter-particle interfaces as discussed in reference to Figs. 2 and 4. The strength and elongation were expected to be enhanced even further after performing the HIP treatment. However, despite the much denser deposits, the UTS value of the HIP-treated samples was comparable (6.6% lower in average, in fact) to that of the vacuum-annealed samples. Although this was a rather surprising result, an identical phenomenon was also observed in literature sources pertaining to selective laser melted (SLM) metal samples: Yan et al. and Hutasoit et al. both reported that vacuum-treated SLM Ti6Al4V samples exhibited UTS equivalent to HIP-treated samples [43,44]. Unfortunately, no clear explanation was provided in the two studies. In the current study, we propose the following hypothesis to partially explain the seeming discrepancy of the lower UTS against significantly lower porosity of the HIP-treated samples. The inter-particle metallurgical bonding in a cold spray deposit originates from a severe plastic deformation of the material and a concurrent fracture and removal of the thin oxide films originally present at the particle surfaces [39,45,46]. The pores in the as-fabricated 316L deposit (marked as white squares in Fig. 2a) were formed due to the insufficient particle plastic deformation during deposition. At such areas, the oxide films were not broken and removed from the particle surface, but rather still remained on the surfaces [47]. Upon performing the HIP treatment, the oxide films were indeed compacted during the pores collapse, but not removed from the interparticle interfaces, thereby hindering the metallurgical bonding as schematized in Fig. 10. In other words, the total effective metallurgical interfacial bonding area was fully comparable in both the vacuum-annealed samples and HIP-treated samples, despite the difference in the
Fig. 8. Microhardness of the 316L deposits before and after the respective annealing treatments. 358
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Fig. 10. Schematic of inter-particle pore evolution during HIP treatment. Red lines indicate the thin oxide films originally present at particle surfaces. Importantly, during HIP treatment, the oxides are not removed from the original pore area. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)
annealing may also break the oxide films on the particle surfaces and further improve the inter-particle metallurgical bonding [49,50]. This observation explains why the strength and ductility of these two sample types were superior to the as-fabricated or air-annealed samples.
porosity content. This phenomenon may explain why the HIP led to UTS values comparable to the vacuum annealing samples. Arguably, this hypothesis may be also applicable to the same phenomenon experienced in SLM technology. Unfortunately, the provided hypothesis cannot 100% explain the statistically significant difference in the values of elongation; the described phenomena related to retained oxide shells after HIP treatment could play a significant role, but would probably not account for the measured ~25% difference in the elongation of the HIP-treated and vacuum-annealed samples (Fig. 9). Even though the true mechanism is yet to be cleared, it is further proposed herein that the second effect contributing to the reduced elongation is the bigger grain size of the HIP-treated samples (cf. Fig. 4c and d). Despite the fact that the HIP treatment could not improve the UTS and elongation values as compared to vacuum annealing, a number of works have confirmed that a HIP treatment can greatly improve fatigue strength of additively manufactured metal samples due to the reduction of porosity [43]. In addition, in most previous cold spray works, people mainly attributed the deposit strengthening after annealing to the reduction of porosity and improved inter-particle interface [20,21]. However, the results obtained in this work reveal that, rather than through a reduction of porosity by post-compaction, the strengthening is mainly dominated by the improved inter-particle bonding and particles' grain structure. To better understand the strengthening mechanism of the different annealing strategies, fractographic images of the tensile specimens before and after the respective annealing treatments are shown in Fig. 11. In the as-fabricated state (Fig. 11a), the fracture surface morphology revealed an inter-particle failure decohesion, clearly indicating that the inter-particle interfaces are the weakest microstructure regions. After the air annealing (Fig. 11b), the fracture surface of the tensile specimens exhibited signs of the inter-particle decohesion, too. However, different from the as-fabricated state, ductile dimples suggesting an enhanced cohesion strength and ductility [48] were partially observed at the surfaces. Importantly, disconnected oxide inclusions can be clearly observed inside the dimples as shown in the magnified inset in Fig. 11b. These correspond to the oxide inclusions discussed with reference to Figs. 2 and 4 and hinder the metallurgical bonding between adjacent particles, explaining why the improvement of the mechanical properties is not as prominent. A similar phenomenon was also observed in heat-treated bond coats for thermally sprayed thermal barrier coatings (TBCs) [49,50]. For the vacuum-annealed and HIP-treated samples (Fig. 11c and d), the fracture surfaces exhibited similar features, mainly characterized by the ductile dimples, this time without the oxide inclusions. In other words, aside from recrystallization, the diffusion through the effective metal-to-metal contact area contributed to a significant increase in cohesion of the adjacent particles. In addition, previous work also reported that high-temperature vacuum
4. Conclusions In this paper, the effect of different annealing strategies comprehending air annealing, vacuum annealing and HIP on the microstructure and mechanical properties of cold sprayed 316L stainless steel deposits was systematically studied. It was found that all three annealing treatments improve the mechanical properties of the deposits through recrystallization and diffusion across oxide-free inter-particle interfaces. There were, however, significant differences among the three strategies. Air annealing improved the tensile strength and ductility of the deposit only slightly due to the formation of oxide inclusions at the inter-particle interfaces, hindering a full cohesion bonding via diffusion. In contrast, vacuum annealing significantly improved the tensile strength and ductility due to the absence of such oxides in the inter-particle interfaces. The tensile strength and ductility after vacuum annealing reached 92% and 41% of the corresponding 316L bulk counterparts values, respectively. Although the deposit porosity was significantly reduced after HIP treatment, the mechanical properties did not surpass the vacuum-annealed samples. This may be caused by the original oxide films retained at the particle surfaces in the original inter-particle pore areas, effectively hindering the intimate metallurgical inter-particle bonding even after the pores collapse in HIP. Note that this hypothesis has not been experimentally proved in the current work, and a further experimental investigation using TEM, AES or positron annihilation spectroscopy are planned in our coming study. In summary, the experimental results obtained in this work suggest that, rather than through a reduction of porosity, the strengthening is mainly dominated by the improved inter-particle bonding and particles' grain structure. Taken overall, based on the findings in this work, it is recommended to use vacuum annealing to improve the mechanical properties of cold sprayed 316L deposit or using HIP if additional density improvement is required.
Acknowledgments Shuo Yin and Rocco Lupoi would like to thank the financial support from Irish Research Council project (GOIPD-2017-912) and European Space Agency (4000112844/14/NL/FE), and also the CRANN Advanced Microscopy Laboratory of Trinity College Dublin for the support in data analysis, Jan Cizek would like to acknowledge Czech Science Foundation project (GACR 17-13573S). 359
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Fig. 11. Fractographic surfaces of the 316L deposits ruptured in tensile testing. (a) As-fabricated sample, (b) air-annealed sample, (c) vacuum-annealed sample and (d) HIP-treated sample. AF represents as-fabricated state, AAT represents air annealing treatment, VAT represents vacuum annealing treatment, and HIPT represents HIP treatment.
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