Deposition behavior, microstructure and mechanical properties of an in-situ micro-forging assisted cold spray enabled additively manufactured Inconel 718 alloy

Materials and Design 155 (2018) 384–395

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Deposition behavior, microstructure and mechanical properties of an in-situ micro-forging assisted cold spray enabled additively manufactured Inconel 718 alloy Xiao-Tao Luo a,⁎, Meng-Lin Yao a, Ninshu Ma b, Makoto Takahashi b, Chang-Jiu Li a,⁎ a b

State Key Laboratory for Mechanical Behavior of Materials, School of Materials Science and Engineering, Xi'an Jiaotong University, Xi'an, Shaanxi 710049, China Joining and Welding Research Institute, Osaka University, Ibaraki, Osaka 567-0047, Japan

H I G H L I G H T S

G R A P H I C A L

A B S T R A C T

• In-situ micro-forging was introduced to cold spray by a facile approach. • Fully dense IN718 deposit was achieved without any contamination. • Increased deposition efficiency induced by oxide scale removal was first detected. • Ultimate strength increases from 96 to 464 MPa due to the micro-forging effect. • Effect of heat treatment on microstructure and tensile behavior was studied.

a r t i c l e

i n f o

Article history: Received 7 April 2018 Received in revised form 21 May 2018 Accepted 15 June 2018 Available online 18 June 2018 Keywords: IN718 alloy Additive manufacturing Cold spray In-situ micro-forging Porosity Mechanical properties

a b s t r a c t Cold spray is capable to additively manufacture oxide-free metallic parts in open air due to its low processing temperature. However, for metals with relatively high hardness such as Inconel 718 superalloy (IN718), it is still a big challenge to get dense deposits by using low-priced nitrogen gas. In this work, In-situ micro-forging (MF) was introduced to cold spray by mechanically mixing big-sized 410 stainless steel (410SS) particles into the IN718 powder so that the deposited IN718 layer can be hammered and plastically deformed by the MF particles during spraying and results in fully dense deposits. As 50 vol% MF particles were mixed into the IN718 powder, the porosity was decreased from 5.6% to 0.26%. Due to the low impact velocity, 410SS particles were not embedded into the IN718 deposits and the possible contamination was avoided. An oxide scale removal induced increment in deposition efficiency was detected for the first time. Due to the lower porosity and enhanced interparticle bonding, a great improvement in ultimate strength from 96 to 464 MPa was achieved by the in-situ MF effect. After heat treated at 1200 °C for 6 h, the sample fractured at a high strength of 1089 MPa and revealed a ductile fracture manner. © 2018 Published by Elsevier Ltd.

1. Introduction ⁎ Corresponding authors. E-mail addresses: [email protected], (X.-T. Luo), [email protected] (C.-J. Li).

https://doi.org/10.1016/j.matdes.2018.06.024 0264-1275/© 2018 Published by Elsevier Ltd.

Additive manufacturing of metallic components has been rapidly evolving since it can be used to fabricate end-use products in aircraft, dental restorations, medical implants, automobiles, and even fashion

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products. There are two categories of additive manufacturing processes available for metals, namely the powder bed fusion and the directed energy deposition which are being developed well in recent years. In these two processes, high energy beam techniques utilize laser, electron beam, plasma arc, gas metal arc etc., as the heat sources to melt/sinter the metals layer by layer [1–3]. Apart from these high-energy-beam based techniques, cold spray has also been of great interest in recent years for additive manufacturing due to its low processing temperature [4–6]. It is worth noting that although the cold sprayed metals show poor ductility at the as-deposited state, post-spray heat treatment has been demonstrated to be an effective approach to recover the intrinsic ductility of the deposited materials [4–6]. In cold spray, micro-sized particles (5–70 μm) are accelerated to supersonic velocities in a De-Laval nozzle by compressed N2, air or He at relatively low processing temperature (b1000 °C and material dependent) and impact onto the substrate. The deposit is formed by accumulated bonding of the particles assisted by high velocity impact induced plastic deformation at completely solid state. Owing to the low processing temperature and high material deposition rate (up to 10 kg/h), cold spray has following advantages vis-a-vis high energy beam based additive manufacturing techniques. I) Metallic materials free of oxidation can be deposited in open air [7–9] so that it is capable to additively manufacture workpieces having large sizes. At the same time, the complex thermal history involving directional heat extraction, repeated melting and rapid solidification in high energy beam based additive manufacturing processes can be avoided [1–3, 7–9]. II) Temperature sensitive materials such as nanocrystalline metals [10], metallic matrix nanocomposites [11], Al and Mg based alloys with low melting temperatures [12, 13] etc. can be built up. III) Composite materials and composite structures can be prepared simply by using mixed powders and changing the feedstock powder materials, respectively, without considering the thermal expansion coefficient mismatch and chemical reactivity among different components [11]. IV) Compressive residual stress is commonly realized in the cold sprayed deposits hence, the tensile residual stress induced cracking or failure associated with deposits made on workpieces using high energy beam additive manufacturing can be avoided [14, 15]. It has been demonstrated that mechanical properties such as hardness, strength, elastic modulus and toughness of the cold spray deposits depend entirely on their microstructure, especially the porosity. Gärtner et al. [16] reported that when the porosity is decreased, a significant improvement in tensile strength of the Cu deposit could be achieved. So far there are mainly two strategies to enhance the plastic deformation, inter-particle bonding and decrease the porosity of the cold sprayed deposits, they are: I) increasing the particle velocity to enhance the driving force for the plastic deformation [7] and II) heating the particles to higher temperatures to decrease the resistance to plastic deformation by thermal softening effect [17]. For relatively soft materials such as Cu, Ni, Ta and stainless steel etc., dense deposits can be achieved by spraying at high gas pressure (up to 5 MPa) and relatively high gas temperature (up to 1000 °C) by using nitrogen as the processing gas [18]. Yin et al. [17] found that the deposition efficiency of Cu powder was greatly improved, and the porosity of the resultant deposit was remarkably decreased by preheating the feedstock particles. However, metals with relatively high hardness such as Inconel alloys and MCrAlY based alloys (M_Ni or Co), require helium as processing gas and it should also be heated to a high temperature (N800 °C) to get dense deposits [19, 20]. The expensive helium gas makes cold sprayed hard metals very difficult to be industrialized. Low cost cold spray approaches capable of depositing high quality hard metallic deposits are desired. In cold spray, the previously deposited layer is inevitably impacted and forged by the following particles. In our previous study involving cold spraying of Ti coating, it was found that such forging effect can in-situ densify the previously deposited layer [21]. However, the in-situ forging effect induced by the sprayed particles alone is very limited due to their low kinetic energy. In our previous study [22], large-sized peening powder was blended into depositing material powders to introduce an extra

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forging to the deposits and the in-situ densification effect was firstly verified in Ti and Ti6Al4V. However, it was found that deposition/embedment of peening particles was observed in Ti deposit and the effect of the enhanced forging on inter-particle bonding quality and mechanical properties was not studied yet. In this work, Inconel 718 (IN718), one of the most widely used superalloy, was selected as a candidate feed-stock material. Enhanced in-situ micro-forging effect is introduced by mechanically mixing portly sized 410 stainless steel particles (N150 μm) into the spraying particles. This attempt was made with an aim that the deposited IN718 particles would be hammered and plastically deformed by the large-sized 410SS particles with much higher kinetic energy than sprayed particles during deposition resulting in a dense deposit. The dependency of deposition behavior of IN718 powder, microstructure and mechanical properties of the deposits on MF particle content were systematically investigated. Effect of the high temperature post-spray heat treatment on the microstructure and mechanical properties was also studied. The methodology adopted in this article can potentially be considered as a low-cost approach to get dense metallic deposit with cold spray having N2 as working gas. 2. Experimental procedure 2.1. Materials Commercial gas atomized IN718 powder (Praxair Surface Technologies) was used as the spray powder. As shown in Fig. 1a and b, the IN718 particles exhibit a spherical morphology with dense microstructure. The particle sizes of the IN718 powders were measured to be from 5 to 33 μm with an average size of 13.5 μm (Fig. 1c). It should be noted that a dendritic type microstructure can be clearly observed from the etched cross section of the as-received IN718 particles shown in Fig. 1b. Since the SEM images was taken under the back scattered electron mode, it is put forth that the brighter contrast suggests a high-volume content of heavy element/elements. Energy dispersive spectrometer (EDS) was carried out to identify the detailed chemical composition differences between the bright dendritic regions and the dark regions. The EDS results listed in Fig. 1d suggests that the bright inter-dendritic regions contain slightly higher Nb as compared to the darker areas. As displayed in Fig. 1e, f and g, 410 Martensitic stainless steel (410SS) powder with a spherical morphology and an average size of ~150 μm was blended into the IN718 powder to introduce the in-situ micro-forging (MF) effect to the deposited IN718 layer during spraying. The material and the size of MF particles were optimized for IN718 deposition according our previous practice [22]. Generally, there are mainly two considerations for the MF particle selection. I) The MF particles should have relatively high hardness so as to effectively forge/deform the deposited IN718 layer and II) embedding/deposition of the MF particle itself should be avoided. In this work, the martensitic 410SS powder has a relatively high hardness of ~340 HV and is not expensive. The large size contributes to both effective MF effect and avoidance of the MF particle deposition. Firstly, the large size ensures a high kinetic energy thus results in a considerable MF effect. On the other hand, the large size makes MF particles have very low impact velocities well below the critical velocity for bonding, so that the MF particle embedment/deposition induced contamination to the IN718 deposit can be also avoided. However, too large MF particles will decrease material building-up speed and also make it very difficult to stability feed the MF particles out, so 410SS particles with an average size of 150 μm were selected as the MF particles for IN718 deposition in the present work. It is also worth noting that the martensitic 410SS is magnetic which allows to collect the rebounded MF particles for reuse. To examine the effect of the insitu micro-forging intensity on deposition behavior of IN718 powders and microstructure of the deposits. 25 vol%, 50 vol% and 75 vol% 410SS particles were mechanically blended into IN718 depositing powders. These raw material powders were sealed in a plastic container and then blended on a drum mixer for 2 h at a rotation speed of 40 rpm. Pure

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Fig. 1. Morphology, cross section and particle size distribution of the IN718 powder (a, b, c, and d) and the stainless-steel 410SS MF powder (e, f and g).

IN718 powders and above-mentioned mixtures were used as the feedstocks for cold spray deposition. The substrate samples were machined out from a 5 mm thick 316 stainless steel plate. The samples were thoroughly grit-blasted with 24 mesh alumina sand grits at a compressed air pressure of 0.8 MPa before deposition. The roughness of the blasted substrate surface was tested to be Ra 6.7 μm.

separately. Pure IN718 powder and pure 410SS powder were separately sprayed at the conditions listed in Table 1 with a powder feed rate of 50 g/min. Although the powder feed rate of each powder was higher than that in deposit preparation, the total powder to nitrogen mass ratio remained the same as that in the IN718 deposit preparation. Since the IN718 and 410SS powders were tested separately, the possible interaction between IN718 particles and 410SS particles within the nitrogen gas flow could not be detected.

2.2. Cold spray deposition An in-house developed cold spray system was employed for IN718 deposition. In this system, a convergent-divergent De-Laval nozzle with a throat diameter of 2 mm, outlet diameter of 6 mm and divergent section length of 100 mm was adopted. Nitrogen was used as the accelerating gas. A relatively low gas pressure of 2.5 MPa achievable for most of the cold spray systems was selected to verify the micro-forging assisted cold spray method. The detailed deposition parameters are listed in Table 1. As illustrated in Fig. 2a, a zig-zags scan strategy was used with a meander offset/step size of 2 mm. The deposits were made by multiple passes of spraying and the thickness of the deposits was from 2 mm to 3.2 mm depending on the MF particles content in the mixed powders. The inflight particle velocities of IN718 powder and the 410SS powder at the standoff distance of 20 mm were measured by a thermal spray particle diagnostic machine containing a laser light source (DPV 2000, Canada). In this test, the velocities of IN718 and 410SS particles were measured

2.3. Heat treatment Heat treatment of the selected as-sprayed IN718 deposits was performed at a relatively high temperature of 1200 °C for 6 h in a vacuum furnace (10−2 Pa). The temperature ramping rate was set to be 10 °C/ min and the total heating-up time is 2 h. After 6 h holding time, the specimens were furnace cooled to ambient temperature. Different from the conventional bulk metals, cold sprayed metallic deposits usually contain some of the mechanically-bonded particles which greatly weaken the mechanical properties [4–6], so the heat treatment in this work was performed at a very high temperature of 1200 °C to enhance the sintering effect and lead to fully metallurgical inter-particle bonding. It is worth noting that all the deposits were removed from the stainless steel substrate by electric spark machining and subsequent hand grinding prior to the heat treatment to avoid the element diffusion from one to the other at elevated temperature during the heat treatment.

Table 1 Detailed cold spray parameters. Gas temperature (°C)

Gas pressure (MPa)

Gun traverse speed (mm·s1)

Standoff distance (mm)

Powder feed rate (g·min−1)

700

2.5

20

20

50

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Fig. 2. Description of the dog-bone tensile test samples; (a) Schematic diagram showing the cold spraying pathway and sampling direction of the tensile specimen; b) geometry and dimensions (unit: mm) of the dog-bone tensile sample and (c) the macro-view of the tensile samples.

2.4. Microstructure characterization The deposition efficiency, which is defined as the proportion of the spraying powder being successfully deposited, of IN718 powder was tested as a function of the 410SS particles. The weight gain of each substrate after cold spray deposition was scaled by an electronic balance (0.1 mg). Field emission scanning electron microscopy (SEM) was used to characterize cross sectional microstructure of the deposits. The porosity of both as-deposited and heat-treated IN718 was measured from the SEM images of the cross sections of the deposits taken in BSE mode followed by image analysis. For each deposit, SEM images were taken at back-scattered electron mode at a magnification of 1000×. Inter-particle bonding and detailed analysis of the lamellar structure were analysed using transmission electron microscopy (TEM). Oxygen element contents of the spray powder and the deposits were tested by an inert gas fusion technique (LECO TC 600 for oxygen). To clearly observe the inter-particle boundaries, pore structures, grain boundaries and phase distributions, some of the polished cross sections were etched in a reagent of 10 ml HNO3, 20 ml HCl, 25 ml distilled water and 10 ml hydrogen peroxide (H2O2).

2.5. Hardness and room temperature tensile tests To assess the effect of the micro-forging and heat treatment on mechanical properties of the cold spray additively manufactured IN718, Vickers microhardness and the room temperature uniaxial tensile tests were carried out. Vickers microhardness indentations were made on the polished cross sections of the as-sprayed and heat-treated specimens with a load of 300 gf with a dwell time of 30 s. An average hardness value was determined based on 10 indentations for each sample. It has been demonstrated that the cold sprayed deposits usually behave anisotropic mechanical properties in the as-sprayed state [23]. Higher strength is usually detected along the in-plane direction than that in the out-plane direction. In this work, the uniaxial tensile test was only performed in the longitudinal direction in-plane. As is shown in Fig. 2a the dog-bone tensile specimens were machined out from the plate along the deposition track direction. Detailed dimensions of the tensile specimen are shown in Fig. 2b. These tensile specimens were machined by the electrical-discharge method and grinded by 800 mesh grit abrasive paper to get a relatively smooth finish of ~Ra 0.7 (Fig. 2c). The uniaxial tensile tests were carried out on both as-deposited and heat-

treated IN718 specimens at 2 mm/min extension rate with an Instron testing machine. The extension was measured by a clip-on extensometer. Samples were pulled apart until fracture occurred in them. The fractured surfaces were then examined through SEM to identify the fracture mechanism of different samples. 3. Results 3.1. Microstructure of the as-deposited IN718 A comparison of the cross sections of the as-deposited IN718 with different content of in-situ 410SS micro-forging (MF) particles is shown in Fig. 3. From the low magnification SEM images it can be clearly observed that the porosity of the deposits decreases gradually with the increasing MF particle content. As the IN718 powder was mechanically mixed with 50 vol% and 75 vol% of the MF particles, visible pores could not be detected anymore. The types of the flaws in the as-deposited IN718 also vary with the increasing MF particle content. From the high magnification SEM images shown in Fig. 3, two types of flaws were observed, namely inter-particulate pores (as marked with the dashed line ellipse) and poorly bonded inter-particulate boundaries (as denoted by the arrows). These flaws occur in the deposit sprayed with pure IN718 powder due to the insufficient plastic deformation of the IN718 particles during deposition. When 25 vol% of MF particles were mixed with the IN718 particles, it results in the in-situ hammering and plastic deformation of deposit, leading to the reduction in the number and the size of the inter-particulate pores. Further enhancements in the plastic deformation of the deposited IN718 deposits were realized by increasing the MF article content to 50 vol% and 75 vol% respectively. In the above-mentioned cases, both the inter-particle pores and poorly bonded inter-particulate boundaries are rarely observed. On the other hand, it is worth noting that even though a large volume of (25 vol% to 75 vol%) 410SS MF particles were mechanically blended into the IN718 feedstock powder, these MF particles were not found to be ingested in the deposits. This can be attributed to the relatively large size of the MF particles which results in low impact velocities, further the IN718 inherently has high hardness which prevents the embedment of MF particles in the deposit. Acceleration of the particles in gas flows are strongly dependent on particle sizes, where larger particles lead to lower velocity due to the higher inertia and vice-versa for smaller particles. In this study, the average velocity of IN718 particle was measured to be 535 m/s while the large-sized MF particles just have a very low

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Fig. 3. Cross sectional microstructure of the cold sprayed IN718 deposits as a function of the MF particle content in the mixed spraying powders.

velocity of 217 m/s [24]. In cold spray, bonding of the particles can be formed by high velocity impact induced adiabatic shear instability (ASI) at inter-particle boundaries [25] and mechanical penetration on the substrate and deposit surfaces [26]. In the former case, a critical velocity is needed to reach the ASI state. At the spraying conditions listed in Table 1, the large-sized MF particle was only accelerated to ~217 m/s which is much lower than the critical velocity for successful deposition of stainless steel via ASI [25]. At relatively low particle velocity range, the penetration of the impacting particles into the substrate or previously deposited layer occurs only when the impacting particle is much harder than the underlying layer/surface [26]. In this work, the 410SS MF powder (340 HV) has a comparable hardness to the IN718 particle (383 HV), hence the penetration of the MF particles into the deposited IN718 layer did not occur. To summarize, the contamination of deposits with MF particles was avoided mainly due to the relatively low impact velocity and intrinsic mechanical properties of MF particles. Besides leading to the formation of a denser deposit, the in-situ micro-forging effect is also capable of refining the dendritic microstructure of the raw IN718 powder (Fig. 4a). When the IN718 particle is deposited by the conventional nitrogen gas aided cold spray without adding MF particles, a continuous dendritic morphology was retained in the deposit, which is similar to the dendrites present in the feedstock powder as denoted by the arrows in Fig. 4b. The dendrites in the feed-stock are retained in the deposit due to the insufficient plastic deformation of IN718 particle during deposition. When 50 vol% of MF particles were used during spraying, the plastic deformation of the deposited IN718 particles were greatly enhanced and it enabled the breakage of dendritic framework in the deposit microstructure (Fig. 4c).

A quantitative measurement of the porosity of the deposits based on the image analysis was performed as displayed in Fig. 5, which shows the porosity as a function of the MF particle content. It can be found that the deposit prepared by pure IN718 powder has a porosity of 5.7%. The porosity decreased remarkably from 5.7% to 1.5% and 0.27% when 25 vol% and 50 vol% MF particles were used respectively. In the case of IN718 deposit sprayed with 75 vol% MF particles mixed into IN718 powders, the porosity decreased to a level of 0.17 vol% which is even lower than the porosity of the deposit sprayed with helium gas [19, 20]. Deposition efficiency, which is defined as the proportion of the sprayed particles that forms the deposit, is also an important criterion to evaluate the effectiveness of cold spray, because it practically determines the cost of the sprayed products. Fig. 6 shows the deposition efficiency of IN718 powder as a function of the MF particle content. It is surprising to find that an increase in deposition efficiency from 23% to 34% was achieved when 25 vol% of the MF particles were mixed into the IN718 powder. With a further increase in the MF particle content namely 50 vol% and 75 vol%, the deposition efficiency was found to get gradually decreased to values nearing 29.5% and 15.1%, respectively. As marked with the dashed red line, the IN718 feedstock powders mixed with 25 vol% and 50 vol% of MF particles revealed higher deposition efficiency than the pure IN718 powder. This fact suggests that insitu MF particles of b50 vol% when mixed with the feedstock and sprayed can facilitate the particle bonding among the deposited IN718 particles. It might be deduced that, in the in-situ MF assisted cold spray, some of the weakly bonded IN718 particles would be removed from the deposit surface due to the impact of the large MF particles

Fig. 4. A comparison of etched cross sections of IN718 powder (a), Conventional cold sprayed IN718 deposit (b) and 50 vol% In-situ MF assisted cold sprayed IN718 deposit (c) showing the micro-forging induced microstructural refinement.

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Fig. 5. Porosity of the deposit as a function of 410SS particle content.

and thus leading to a lower deposition efficiency. The mechanism of the unexpected improvement in deposition efficiency in case deposit formed with 25 vol% of MF powder addition in the feed-stock will be discussed in the discussion section. From the above results, it can be found that the 50 vol% MF particle MF assisted cold sprayed IN718 deposit has both low porosity and relatively high deposition efficiency. Therefore, it was selected to be heat treated. Microstructure and mechanical properties were characterized and compared with those for conventional cold sprayed IN718 deposits in the following sections. 3.2. Microstructure of the heat-treated IN718 deposits To promote formation of the metallurgical inter-particle bonding and to homogenize the elemental distribution, a high temperature heat treatment (1200 °C, 6 h) was applied to the cold sprayed deposits. Fig. 7 presents the etched cross-sectional SEM microstructures of the asdeposited and the heat-treated IN718 alloy prepared by both conventional cold spray and 50 vol% in-situ MF assisted cold spray. For the conventional cold sprayed deposit, the inter-particulate gaps (red arrows in Fig. 7a), a sign of the poor inter-particle bonding are transformed into very tiny discontinuous global pores (b2 μm) located along the inter-

Fig. 6. Deposition efficiency of the IN718 powder as a function of the 410SS particle content.

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particle boundaries after heat treatment as indicated by the solid arrows in Fig. 7b. The dashed ellipses identify the inter-particle pores located at the tri-particle junctions, which have undergone a shape shift from an angular shaped profile to globular or elliptical morphology due to the sintering effect occurring during heat treatment. Although visible inter-particle pores and gaps were rarely detected in the as-prepared 50 vol% MF assisted cold sprayed deposit (Fig. 7c), tiny pores were formed after the high temperature heat treatment (Fig. 7d). As expected the heat-treated MF assisted cold sprayed deposit has much fewer pores than conventional cold sprayed counterpart. In this work, the selected heat treatment temperature is a little bit higher than the solution heat-treatment temperature of IN718 (950–1060 °C) [27]. The significant enhancement in the diffusion and recrystallization occurring during heat treatment makes the dendritic microstructure to dissolve and disappear into the matrix of the deposit. When compared to the normal solution treatment, the specimens in the present work were furnace cooled to the room temperature, which resulted in a relatively low cooling rate vis-a-vis oil/air cooling which are normally adopted in the normal solution heat treatment. The slow cooling results in the formation of δ phase (Ni3Nb) precipitates as denoted by the dashed arrows at the grain boundaries in both cases of heat-treated deposits. After the heat treatment, austenite grains with sizes ranging from approximately 10 to 30 μm can be clearly recognized from Fig. 7b and d. This suggests an evident grain growth as compared with those in the asreceived IN718 powder (Fig. 1b). It is worth noting that although the present heat treatment temperature (1200 °C) is higher than normal solution treatment (950–1060 °C) [27], the yielded austenite grains have much smaller size as compared to those in normal solution heat treated bulk IN718 alloy (50–200 μm [27]). This can be attributed to the ultrafine starting grains in the as-received IN718 powder (Fig. 1b). The porosity of the heat-treated samples was measured from the SEM images and compared with those of the as-sprayed deposits (Fig. 8). For both deposits, an increment in porosity was detected after heat treatment irrespective of their initial porosity values. This finding is totally different from the laser additively manufactured metals, where heat treatment induced porosity variation does not occur [28, 29]. This can be attributed to the nano-sized inter-particle gaps observed in the TEM images as shown in Fig. 9a and b. It was also observed that the crystals adjacent to the inter-particle boundaries in the as-sprayed deposit were greatly refined to nanometer scale (b100 nm) as is shown in Fig. 9c. This was further confirmed by the ring feature of the select area diffraction patterns (Fig. 9d) taken from a 200 nm diameter region as marked with the dashed-line cycle in Fig. 9c. The grain refinement, which is frequently observed in cold sprayed metals, is attributed to the high strain rate plastic deformation induced dynamic recrystallization during particle impact [30]. In the SEM images taken for porosity estimation, these nano-sized inter-particle gaps were invisible due to the relatively low magnification of 1000×. During the heat treatment, the surface energy reduction drives these two-dimensional tiny inter-particle gaps to get spheroidized and cluster them together to form three-dimensional pores as pointed out in Fig. 7b and d. Since these three-dimensional pores are detectable by SEM, an increment in porosity was detected after the heat treatment. In a stark contrast to the metallurgical bonding formed during solidification of the melt pool in laser based additively manufactured metals, the inter-particulate bonding in cold spray deposits can be either metallurgical or mechanical interlocking. Generally, larger plastic strain occurring on the particles during deposition leads to higher proportion of metallurgical bonding [8, 16, 30]. A high ultimate strength of 450 MPa was achieved by Gärtner et al. [16] in helium cold sprayed pure Cu in which the deposited particles were found to have undergone severe plastic deformation supported by the high velocity impact of the particles. In the present study, although helium was not used as the processing gas, the in-situ micro-forging effect has remarkably enhanced the plastic deformation of the deposited IN718 particles, minimized the proportion of poorly bonded particles and thereby leading to the creation of fewer tiny inter-particle gaps. Therefore, the in-

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Fig. 7. Etched cross-sections of the convention cold spray (a and c) and 50 vol% MF assisted cold spray (b and d) deposited IN718 before and after heat treated at 1200 °C for 6 h.

situ MF assisted cold sprayed deposits showed only a slight porosity increment after the high temperature heat treatment. 3.3. Microhardness and tensile behaviors of the IN718 deposit The mechanical properties of the as-deposited and heat-treated IN718 deposit were evaluated in terms of Vickers microhardness and room temperature tensile tests. Fig. 10 presents Vickers microhardness of the as-deposited and heat treated IN718 deposits. As expected, due to much higher strain hardening effect and lower porosity, the in-situ MF assisted cold sprayed deposit shows 25% higher hardness (507 HV) than the conventional cold sprayed one at the as-deposited state. After heat treating at 1200 °C for 6 h, a hardness decrement was measured for both deposits due to the weakened strain hardening by dislocation annihilation and grain growth [31–34]. Furthermore, the present heat treatment is similar to the solution treatment for IN718 and strengthening phases such as γ′ and γ″ were not formed [35] although some δ

Fig. 8. A comparison of porosity of as-deposited and heat treatment IN718 alloy.

phase precipitates marked with the dashed arrows were observed as is shown in Fig. 7b and d. Room temperature tensile behavior of the as-deposited and heattreated IN718 deposits is displayed in Fig. 11. Both as-sprayed deposits show a brittle tensile behavior without an evidence of yielding stage before fracture. The as-sprayed MF assisted cold sprayed deposit attained a much higher strength (~460 MPa) as compared to the conventional cold spray (b100 MPa). After the high temperature heat treatment, the ultimate strength for both deposits were greatly improved. However, the conventional CS deposit still showed very limited elongation of ~0.5% before fracture. For 50% MF assisted cold spray deposits, the strength and the ductility were simultaneously increased after the heat treatment. After undergoing a plastic yielding stage, the tensile bar fractured at a strain of 6%. It is interesting to find that both as-deposited IN718 bars revealed very short linear-elastic regime (Fig. 11b). Similar tensile behavior has been frequently observed in metallic foams [36]. This can be attributed to the relatively poor inter-particle bonding in the assprayed deposits. After heat treatment, the 50% MF assisted cold sprayed IN718 shows similar tensile behavior in the elastic range compared to the conventional bulk and laser additively manufactured counterparts [1–3]. Detailed mechanical properties in terms of elastic modulus, ultimate strength, yield strength and elongation are derived from the tensile curves and summarized in Table 2. Similar to the variation in the trends of strength values, the elastic modulus also increases after the high temperature heat treatment due to the sintering effect. The heat treated in-situ MF cold sprayed deposits shows comparable elastic modulus with the bulk IN718. However, for conventional cold sprayed IN718 bars, even after the high temperature heat treatment, the elastic modulus just increases to 146.7 GPa only, which is equivalent to 70% of the bulk counterparts. For fully dense bulk metals elastic modulus stands for the atomic bond stiffness and is not sensitive to the processing and heat treatment [37]. In cold sprayed metals, the atoms are metallically bonded within the individual particle which forms the deposit. This makes the inter-particle bonding dominate the elastic modulus when the minor porosity contents are not taken into account. In other words, elastic modulus can be used to evaluate the inter-particle bonding of cold sprayed dense deposits [38]. The attainment of elastic

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Fig. 9. TEM observations of 50 vol% MF assisted as-deposited cold sprayed IN718; (a) and (b) bright field TEM images showing the nano-sized inter-particle gaps, (c) a bright field TEM image revealing the ultra-fine grains formed by the enhanced dynamic recrystallization during deposition and (d) the selected area electron diffraction image taken from the area marked with the dashed line cycle.

modulus values to near bulk IN718 suggests that the in-situ MF assisted cold sprayed IN718, the particles have reached 100% metallurgical bonding after the heat treatment. Although the uniaxial tensile behavior of the IN718 deposits prepared with the feedstock powders containing 25 vol% and 75 vol% MF particles was not tested, the variation trend of the strength as a function of the MF particle content is predictable. According to the microstructure (inter-particle bonding and porosity in Figs. 3, 4 and 5, respectively) for the as-sprayed deposits, the highest ultimate strength and ductility is expected for 75 vol% MF IN718 deposit.

Fig. 10. A comparison of microhardness of as-deposited and heat treatment IN718 alloy.

However, when as many as 75 vol% MF particles were mixed into the spraying powders, the depositing material proportion decreases and the deposition efficiency also declines. This will decrease the deposit building-up speed and decrease the efficiency of this technique. It is worth noting that the wrought In718 alloy solution heat treated at relatively high temperatures (1000 to 1060 °C) has lower strength (350 to 430 MPa for yield strength and 700 to 900 MPa for ultimate strength) but much higher elongation (up to 30%) than the heat treated 50% MF IN718 deposit. In the common solution heat treatment for IN718 alloy, rapid quenching is always applied to the samples by water/oil cooling so that precipitates will not be formed. In this work, a vacuum furnace was used to perform the solution heat treatment and fast cooling from high temperature was not allowed. The slow cooling of the samples made the specimens undergo a short period of ageing heat treatment and resulted in some very fine δ phase precipitate as is shown in Fig. 7b and d. This makes the heat treated 50% MF assisted cold sprayed IN718 deposit reveal much higher strength but lower ductility than the solution annealed wrought IN718 alloy. Further study on heat treatment is needed to optimize the mechanical properties especially the ductility. The tensile behaviors were further confirmed by the fracture morphologies shown in Fig. 12. For the as-deposited conventional cold sprayed IN718, irregular shaped granular morphology of the deposited particle can be clearly recognized as seen in Fig. 12a and b indicating that the fracture occurs along the inter-particle boundary and evident metallurgical inter-particle bonding was not formed. Although interparticle debonding still dominate the fracture surfaces of the in-situ MF assisted cold sprayed deposit (Fig. 12c), the traces of trans-particle fracture can be also observed in many regions as denoted by the dashed

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Fig. 11. Tensile curves of the as-deposited and heat-treated IN718 deposit; (b) is close view of (a) in the strain range of 0 to 1% showing the nonlinear tebehhavior of the as-deposited deposits.

ellipses in Fig. 12d. This fact suggests that both metallurgical and mechanical interlocked bonding were formed in the in-situ MF assisted cold sprayed deposit. The MF effect is beneficial for stronger interparticle bonding formation and thus leads to higher elastic modulus and strength values. Even though, the IN718 particles are well compacted the in-situ MF assisted cold sprayed deposit, when tensile load was applied on it, the pre-existing 2D poorly bonded interparticle boundaries act as preferential stress concentration regions and thus they provide a fast-track crack propagating path-way during fracture resulting in relatively low strength and elongation [39]. Even after heat treatment, the inter-particle debonding was found to dominate the fracture mechanism of the conventional cold sprayed IN718 deposit as shown in Fig. 13a. Dimples can be only observed in some regions as denoted by the dashed ellipses in Fig. 13b. The limited metallurgical bonding makes the deposit to show a brittle tensile response and relatively low strength as displayed in Fig. 11. In contrast, the in-situ MF assisted cold sprayed IN718 reveals a ductile fracture as seen in the fracture surface morphology presented in Fig. 13c. Dimples can be observed on the whole fractured surface (Fig. 13a) except few regions around the tiny pores (Fig. 13d) suggesting that full metallurgical inter-particulate bonding was formed due to the sintering effect. Despite the decrement in the hardness, the enhanced inter-particle bonding post heat treatment leads to higher strength and ductility.

Accelerating the particles to higher impact velocity (higher kinetic energy) increases the driving force of plastic deformation [7] in addition, heating the working N2 gas to high temperatures can lead to softening the particles and decreases the deformation resistance [17]. The above-mentioned methods we found to be effective to get denser deposits. In this work, since nitrogen was used as the processing gas, very high particle velocity could not be achieved, but the microforging effect induced by the impact of the large-sized MF particles could greatly enhance the plastic deformation of the deposited particles. The surface morphology of the deposits prepared with mixed powders is shown in Fig. 14, in which the micro-forging induced craters can be clearly recognized. Although the velocity of the MF particle was measured to be only 217 m/s, the kinetic energy of one single MF particle is 201 times higher than a IN718 particle the due its larger volume (Table 3). This high kinetic energy and large volume of MF particles ensures that the deposited IN718 particles can be extensively hammered and forged in large area thus resulting in the formation dense deposits. On the other hand, the low impact velocity and comparable hardness of MF particles compared to the IN718 particles helps circumvent the incorporation of the MF particles into the deposit and thus contaminations are avoided as shown in Fig. 3 [40].

4. Discussion

High deposition efficiency is usually desired since it partially determines the cost of the cold sprayed product. In cold spray, only if the impact velocity of a particle is higher than the critical velocity the particle can be bonded. Hence, increasing the particle impact velocity and decreasing the critical velocity are two main strategies to achieve high deposition efficiency. For a given powder, increasing the gas pressure, gas temperature or using helium gas as the processing are effective approaches to gain higher impact velocity. On the other hand, it has been demonstrated that the critical velocity is remarkably dependent on deformability of the depositing powder. Generally, the softer the powder is, the higher deposition efficiency will be [31]. Since most of the metallic materials become softer at evaluated temperature, preheating the depositing powder to higher temperature prior to impact is another option to get higher deposition efficiency [17]. In our previous study, it was found that the oxide scale on the metallic particle surface will also greatly increase the critical velocity [41]. For most of the metals, as long as they are exposed to the open air atmosphere or even

Based on the above results, it was found that introducing the in-situ MF effect into nitrogen gas based cold spray process can lead to dense IN718 deposit with enhanced inter-particle bonding. At the same time, improved deposition efficiency was achieved when 410SS MF particle content was not N50 vol%. These two merits arising from the in-situ MF effect not only ensures the achievement of high mechanical properties but also reduces the cost of the cold sprayed products. In this section, the densification and deposition efficiency increment mechanisms by achieved the in-situ MF effect were briefly discussed as follows. 4.1. Densification mechanism by the in-situ micro-forging effect It is known that the porosity of cold-sprayed metals relies on the degree of plastic deformation of the sprayed particles during deposition.

4.2. Improved deposition efficiency

Table 2 A summary of the mechanical properties of cold sprayed IN718 alloy derived from the tensile data. Type of deposits

Elastic modulus, E (GPa)

Yield strength, σ0.2 (MPa, at 0.2%)

Ultimate strength, σUTS (MPa)

Elongation (%)

As-deposited 0% MF As-deposited 50% MF Heat-treated 0% MF Heat-treated 50% MF

95.7 172.3 146.8 209.4

Not available Not available Not available 902.7

96.4 463 566.1 1088.7

0.12 0.48 0.56 6.17

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Fig. 12. Fracture surface morphologies of the as-deposited IN718 deposits sprayed by the conventional CS (a and b) and 50 vol% MF assisted CS (c and d); (b) and (d) are close views of (a) and (c), respectively.

inert gas atmosphere, thin oxide scale from several to tens of nanometers depending on the material will be formed on the surface. What is the worse, the micrometer size of the metal particles leads to orders of magnitude increment in specific surface area and results in much higher oxygen content as compared to the large-sized metal parts. On the other hand, when the metal powders are produced by gas atomization, they inevitably undergo a high temperature exposure to the surrounding

atmosphere during the cooling which may also induce surface oxidizing of the particles although inert gases are usually used. Owning to the above two factors, the micrometer sized metal powders usually show much higher oxygen content than the bulk counterparts. In our previous study [41], a significant increase in critical velocity was detected with the increasing oxygen content of the particles since extra energy should be consumed to rupture the oxide scale on the particle surface to allow

Fig. 13. Fracture surface morphologies of the as-deposited and heat-treated IN718 deposits sprayed by the conventional CS (a and b) and 50 vol% MF assisted CS (c and d); (b) and (d) are close views of (a) and (c), respectively.

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Fig. 14. Surface morphology of the 50 vol% MF particle assisted cold sprayed IN718 deposit showing the large-sized 410SS particle impact induced craters; the inset is a close view of a single crater.

fresh metal contact and thereby strong inter-particle bonding. As a result, metallic particles with thick oxide scale on surface are very difficult to be bonded in cold spray. In this work, an unexpected deposition efficiency increment was detected when the MF particle content in the feedstock was lower than 50 vol% as shown in Fig. 6. This happened due to the removal/jettisoning of the nascent oxide film from surface of the deposited particles by the impaction of MF particles. The removal of the nascent oxide film from the surface of the deposited particles could decrease the critical velocity requirement of the sprayed IN718 particles and thus allows more particles to be successfully deposited. In cold spray when the particle impacts the substrate or the previously deposited layer, the oxide scale present on the surfaces of the particles would inhibit contact of fresh metal and thus hinder the successful bonding of the impacting particles [41]. For particles with relatively thick oxide scale, the kinetic energy of the impacting particles would be consumed on fracturing the oxide scale, hence additional kinetic energy is required to establish good contact among the particulates which demands a further increase in the critical velocity and thus the deposition efficiency is decreased under the given spray conditions. In case of pure Cu, when the oxygen element content present in it increases from 0.01 wt% to 0.4 wt%, an increment in critical velocity from 300 to 600 m·s1 was required to get a dense deposit [41]. In this work, effect of the in-situ MF particle content on oxygen content of the deposit was examined as shown in Fig. 15 which presents the oxygen element content of the deposit as a function of MF particle content. A gradual decrease in oxygen element content with increasing MF particle content can be clearly observed. Since the chemical composition inside the deposited particles would not be changed due to the low processing temperature, the oxygen element content decrement of the deposits could only be attributed to the removal of the oxide scale from the surfaces of the deposited particles. It is worth noting that only the oxide scale at the exposed deposited particle surface could be removed by the insitu MF particle impact while the bottom part of the deposited particle is not influenced. Therefore, the decreasing rate of oxygen element content gradually slows down and finally reaches a saturated state. As per

Fig. 15. Oxygen element content of the IN718 deposits as a function of MF particle content in the mixed spraying powders.

the schematic diagram displayed in Fig. 16, the oxide scale removal effect has similar effect on deposition efficiency. Secondly, MF particle erosion will remove the weakly bonded IN718 particles and therefore decreases the deposition efficiency. Finally, the enhanced strain hardening arising from the micro-forging effect was also found to influence the deposition efficiency. As displayed in Fig. 10, the hardness of the in-situ MF assisted cold sprayed deposit shows 25% higher hardness than the conventionally cold sprayed ones. The higher hardness on the surface of previously deposited makes it difficult to be bonded with the successively sprayed particles and thus leads to lower deposition efficiency [42]. On the whole, the deposition efficiency is governed by combination of these three effects. It rose first and then declined with the increasing MF particle content as is illustrated in Fig. 16. 5. Conclusions In this study, an in-situ micro-forging effect was introduced into cold spray process by mechanically mixing large sized 410 stainless steel particles into the spray powder to decrease the porosity of the IN718 deposit by a facile economic way. Effect of micro-forging intensity on the deposition behavior of cold gas sprayed IN718 powder, microstructure and mechanical properties of the as-deposited and heat-treated deposits were investigated. Conclusions were drawn as following:

Table 3 The estimated impacting velocity of IN718 and 410SS MF particles. Particle

Average diameter (μm)

Average velocity (m·s1)

Kinetic energy of single particle (μJ)

IN718 410SS

13.6 148.5

217 535

1.55 312.72

Fig. 16. A schematic diagram showing micro-forging induced effects on deposition efficiency.

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(1) Porosity present in the IN718 deposit was gradually reduced and the inter-particle bonding was improved with the enhanced insitu micro-forging effect. When the large sized 410SS particle content was increased to 25 vol%, 50 vol% and 75 vol%, the porosity decreased from 5.7% to 1.5% 0.21% and 0.13%. Since the 410SS particles had much lower impact velocity and similar hardness as compared with IN718 particles, the mechanical penetration and adiabatic shear instability induced deposition of 410SS particles was not observed in IN718 deposit. (2) An increment in deposition efficiency was achieved when the 410SS micro-forging particle content was b50 vol% due to oxide scale removal on the surface of deposited IN718 particles by micro-forging the particles upon impact. The variation of the deposition efficiency as a function of the micro-forging intensity was dependent on the combination of oxide scale removal effect, strain hardening effect and erosion effect induced by the microforging powder as well. (3) The in-situ micro-forging assisted cold spray deposited IN718 shows much higher elastic modulus and strength than its conventional cold sprayed counterpart due to the enhanced interparticle bonding and decreased porosity. After the high temperature heat treatment at 1200 °C for 6 h, a full metallurgical inter-particulate bonding was formed in the in-situ microforging assisted cold sprayed IN718 due to the sintering effect. A significant improvement in elastic modulus, strength and elongation was achieved and the heat-treated deposit fractured in a ductile manner. Acknowledgments This work was supported by the National Science Fund of China (No. 51401158), China Postdoctoral Science Foundation (No. 2014M550486) and Shaanxi Co-Innovation Projects (2015KTTSGY03-03). Data availability The raw data required to reproduce these findings cannot be shared at this time as the data also forms part of an ongoing study. References [1] T. DebRoy, H.L. Wei, J.S. Zuback, T. Mukherjee, J.W. Elmer, J.O. Milewski, A.M. Beese, A. Wilson-Heid, A. De, W. Zhang, Additive manufacturing of metallic components process, structure and properties, Prog. Mater. Sci. 92 (2018) 112–224. [2] G.D. Goh, S. Agarwala, G.L. Goh, V. Dikshit, S.L. Sing, W.Y. Yeong, Additive manufacturing in unmanned aerial vehicles (UAVs): challenges and potential, Aerosp. Sci. Technol. 63 (2017) 140–151. [3] J.H. Tan, W.L.E. Wong, K.W. Dalgarno, An overview of powder granulometry on feedstock and part performance in the selective laser melting process, Addit. Manuf. 18 (2017) 228–255. [4] W. Li, K. Yang, S. Yin, X. Yang, Y. Xu, R. Lupoi, Solid-state additive manufacturing and repairing by cold spraying: a review, J. Mater. Sci. Technol. 34 (2018) 440–457. [5] H. Assadi, H. Kreye, F. Gartner, T. Klassen, Cold spraying - a materials perspective, Acta Mater. 116 (2016) 382–407. [6] R.N. Raoelison, Ch. Verdy, H. Liao, Cold gas dynamic spray additive manufacturing today: deposit possibilities, technological solutions and viable applications, Mater. Des. 133 (2017) 266–287. [7] X.T. Luo, Y.J. Li, C.X. Li, G.J. Yang, C.J. Li, Effect of spray conditions on deposition behavior and microstructure of cold sprayed Ni coatings sprayed with a porous electrolytic Ni powder, Surf. Coat. Technol. 289 (2016) 85–93. [8] Y.J. Li, X.T. Luo, H. Rashid, C.J. Li, A new approach to prepare fully dense Cu with high conductivities and anti-corrosion performance by cold spray, J. Alloys Compd. 740 (2018) 406–413. [9] S. Bagherifard, S. Monti, M.V. Zuccoli, M. Riccio, M. Guagliano, Cold spray deposition for additive manufacturing of freeform structural components compared to selective laser melting, Mater. Sci. Eng. A 721 (2018) 339–350. [10] J. Liu, X. Zhou, X. Zheng, H. Cui, J. Zhang, Tribological behavior of cold-sprayed nanocrystalline and conventional copper coatings, Appl. Surf. Sci. 258 (2012) 7490–7496. [11] X.T. Luo, C.J. Li, Tailoring the composite interface at lower temperature by the nanoscale interfacial active layer formed in coldsprayed cBN/NiCrAl nanocomposite, Mater. Des. 140 (2018) 387–399.

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