Appl. Phys. A (2017) 123:660 DOI 10.1007/s00339-017-1275-9

Surface enhanced 316L/SiC nano-composite coatings via laser cladding and following cold-swaging process Yuhang Li1 • Shiyou Gao1,2

Received: 23 June 2017 / Accepted: 19 September 2017  Springer-Verlag GmbH Germany 2017

Abstract Cold-swaging is one of a cold deformation processes, and ceramic-reinforcement nano-composite coatings can effectively improve the performance of metal matrix surface. Therefore, the two processes are innovatively combined to further improve the surface properties of the metal matrix in this paper. The microstructure and surface properties of the laser cladding 316L ? 10 wt% SiC nano-composite coatings were examined through designed experiments after cold-swaging by self-developed hydraulic machine. Furthermore, the coatings were compared with those without cold-swaging coatings at the same time. The result shows that the cold-swaging process can further enhance the tensile strength, micro-hardness and the wear resistance of the composite coating. This study can be used as a reference for further strengthening of laser cladding nano-composite coatings in future research.

1 Introduction As particle reinforced metal matrix composite (PRMMC) coatings, ceramic-reinforcement has drawn much attention of various industries to meet the increasing demand on component life [1–4]. Many surface coating techniques have been attempted to produce MMC coatings, such as gas

& Shiyou Gao [email protected] 1

College of Mechanical Engineering, Yanshan University, Qinhuangdao 066004, People’s Republic of China

2

Key Laboratory of Advanced Forging & Stamping Technology and Science (Yanshan University), Ministry of Education of China, Qinhuangdao 066004, People’s Republic of China

tungsten arc cladding [5], plasma spraying [6] and laser cladding [2, 7]. For purpose of enhancing the surface properties of metal alloys, laser cladding is a flexible technique to modify the surface properties, which is widely employed to fabricate coatings on the metal matrix. As previously reported, the previous studies on laser cladding of metal alloys are focused on the performance characterization of the ceramics or intermetallics reinforced metal matrix. In order to enhance the surface properties of metal matrix by laser cladding, the addition of ceramic powder SiC is a good method. Excellent achievements have been obtained by Li et al. [8], Weng et al. [9], Song et al. [10] and Majumdar et al. [11]. In particular, Duan et al. [12] demonstrated experiment to investigate the reinforcement mechanism and wear resistance of 316L/SiC by laser cladding on Q235 substrate. The results showed that 316L cladding layer with 10 wt% nano-SiC was appropriate, and produced new strengthening phase M7C3 and FeSi. Furthermore, the coating hardness reached 527Hv increased by 132% compared with the pure 316L coatings. Majumdar et al. [13] added 20 and 5 wt% SiC particles in the 316L powder, and the defect-free cladding layer was obtained by laser cladding. The experimental results demonstrated that the micro-hardness and wear resistance of the cladding layer were improved significantly. However, with the rapid development of science and technology, the progress of industry, laser cladding surface techniques cannot satisfy the mechanical properties. Due to the inadequate fusion regions between adjacent clad inclusions, entrapment of oxidized powder particles, gas porosities and micro-segregation at inter-dendritic/grain boundaries, it can’t be widely used in multifarious industry [14]. From published studies on post-treatment of the coating, Abusuilik [15] investigated three coating systems to clarify

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the effect of different methods of pre-, intermediate and posttreatment on coating properties and performance. Wang et al. [16] prepared a-Al2O3 coating and researched the effect of electron irradiation and subsequent annealing process on the surface morphology, composition, phases and corrosion resistance of the coating. Xing et al. [17] modified the lowcarbon nano-crystallite bainite via laser remelting and following isothermal transformation. Although the properties of the coatings were improved, the microstructure parameters of the coatings did not show plastic deformation after post-treatment. In recent years, the severe plastic deformation (SPD) has been proposed to improve the mechanical properties by many scholars producing ultrafine-grained structure [18–21]. Xi et al. [22] made experimental research on laser rapid forming (LRF) GH4169 alloy combined with consecutive point-mode forging (CPF) and the GH4169 alloy appeared plastic deformation during the forming process. The tensile property of the CPF-LRF GH4169 alloy was superior to the forge standards, and the average recrystal grain size of GH4169 alloy was about 12.8 lm. Therefore, a cold-swaging process was developed in this paper, in which simple standard 316L/SiC composite coatings specimen made of laser cladding was pressed by self-developed hydraulic machine and the structure of plastic deformation is achieved. In order to ensure that the substrate would not be destroyed and reduce defects like fractures or folds, the challenge in this process consists mainly in the control of pressure distance and anvil width ratio. Hence, the forming properties of the nano-composite coatings must be mastered and a good pressure distance in the process is required. The microstructure and mechanical properties of the laser cladding nano-composite coatings were investigated after cold-swaging in this paper.

and mechanical properties of AISI 316L steel are listed in Tables 1 and 2, respectively. The substrate was cleaned with acetone to remove grease and polished with emery paper (220 mesh) at the same time. The powder mixtures were prepared by 316L and SiC powders, and they were prepared by mechanically mixing for four hours. The chemical composition of the 316L powders was also presented in Table 1, and the particle size of the powders was about 70–120 lm. At the same time, 10 wt% SiC ceramics (B 90 nm) were added to 316L powders. This combination was selected based on literature [12]. The scanning electron micrograph (SEM, Hitachi S4800) of the alloy composite powder is illustrated in Fig. 1. Under the action of mechanical mixing, the spherical 316L particles were adhered by a small amount of nano-SiC particles, and they formed irregular composite powder aggregates with the milling time reaching a certain extent. 2.2 Laser process parameters Laser cladding was conducted with a 2.0 kw cross-current CO2 laser with nitrogen protection. Cladding powders were placed on the substrate surface with a thickness of 1.0–1.2 mm. The output power, scanning velocity and diameter of the laser beam were 1.5 kW, 5 mm/s and 3.0 mm, respectively. Multi-track double layers were deposited on the substrate surface with 20% overlap ratio. Figure 2 shows the schematic of the laser cladding process.

2 Materials and methods 2.1 Materials The substrate material is commercial AISI 316L steel (size: 80 mm 9 60 mm 9 8 mm). The chemical composition

Table 1 Chemical composition of AISI 316L steel (wt%) Elements

Cr

Ni

Mo

Si

Mn

Fe

Compositions

17.0

12.0

2.5

B 1.0

B 2.0

Bal

Fig. 1 SEM morphology of 316L ? 10 wt% SiC composite powder

Table 2 Mechanical properties of AISI 316L steel Tensile strength (MPa)

Yield strength (MPa)

Hardness (Hv)

Density (g/cm3)

Elongation (%)

Specific heat capacity (20 C) J/(g*K)

C480

C177

180–200

7.98

C40

0.502

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fractures or folds to the least amount, the pressure distance was set to 1.4 ± 0.1 mm. Furthermore, the pressure head presses the specimen from one end to the other end, which is defined as one-time cold-swaging procedure. The laser cladding composite coatings specimens were experienced 0, 1, 2 and 3 times cold-swaging procedure, respectively. Five specimens were conducted in every times coldswaging procedure, and the results were averaged. The dimensions of specimen after 0, 1, 2 and 3 times coldswaging are presented in Table 3, and the thickness of coating after 0, 1, 2 and 3 times cold-swaging was 4, 2.40, 1.34 and 0.94 mm, respectively. 2.4 Microstructure property and characterization Fig. 2 Schematic of the laser cladding process

2.3 Cold-swaging process After laser cladding process, the cold-swaging was conducted. Cold-swaging is one of a cold deformation processes, which can reduce cross-sectional area and increase length of the specimen. The microstructure can be optimized, and the complete metal flow line can be saved. Therefore, the cold-swaging process used self-developed hydraulic machine (maximum loading force: 80 KN) and pressure head (upper anvil size: length 9 width = 20 9 8 mm; lower anvil is metal plate), and full anvil coldswaging (anvil width ratio = 1) was chosen to press specimen. The pressure and velocity of anvil were 20 KN and 1 mm/min, respectively. The schematic of the coldswaging process is illustrated in Fig. 3. In order to keep the substrate from being destroyed and reduce defects like

After cold-swaging process, metallographic specimens were observed using a conventional optical microscope (OM). The phases of the polished coatings were identified by an X-ray diffractometer (XRD, Phillips Rayons-X). Micro-hardness was measured on a Vickers micro-hardness testing machine under 500 g load and 15 s dwell time. Uniaxial tensile test was conducted on Zwick Z100/SN5A electronic universal testing machine (loading rate: 1 mm/ min), in which five measurements were conducted, and the results were averaged. The detailed microstructure of fractured appearance was observed by scanning electron microscopy SEM. Dry sliding wear test was performed on the CETR UMT-3 block-on-ring apparatus at room temperature without lubrication. The quenched and tempered GCr15 steel (4 mm in diameter) with a macro-hardness of 60 HRC was used as the ring material, and the nominal chemical composition of GCr15 steel is listed in Table 4.

Fig. 3 Schematic of the cold-swaging procedure: a self-developed hydraulic machine; b the composite coating specimens with additions of 10 wt% SiC; c partial enlarged drawing; d detailed specimen size; e the one-time cold-swaging procedure

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Table 3 Dimensions of specimen after 0, 1, 2 and 3 times cold-swaging Categories

0 times

1 times

2 times

3 times

Length 9 width 9 height/mm3

25 9 8 9 8

25 9 8.20 9 6.60

25 9 8.42 9 5.34

25.28 9 8.64 9 4.56

Table 4 Nominal chemical composition of GCr15 steel (wt%)

Elements

C

Si

Mn

Cr

Mo

P

S

Fe

Compositions

0.95–0.15

0.15–0.35

0.25–0.45

1.40–1.65

B 0.10

B 0.025

B 0.025

Bal

The test specimens were machined to blocks with a size of 25 mm 9 8 mm 9 5 mm and were polished with abrasive paper before the wear the test. The wear conditions included the following: a normal load of 20 N, a sliding distance of 20 mm, a sliding speed of 65 mm/min and a sliding time of 60 min. After the wear test, the SEM was employed to study the worn surface of the composite coating. The wear weight losses were calculated using a precision analytical balance with an accuracy of 0.01 mg.

3 Results and discussion 3.1 Microstructure characteristics Figure 4 shows the OM microstructure of different coldswaging times at the top of the composite coatings. An equiaxed crystal was observed after 0 times cold-swaging (Fig. 4a), and this phenomenon could be due to rapid heating and solidification during laser cladding [23]. In Fig. 4a, the morphology composite coatings specimen with additions of 10 wt% SiC is similar with literature [13]. The mean size of the grain is 14.6 lm (Fig. 4a). It can be seen that the composite coatings exhibit a continuous and symmetrical morphology (shown in Fig. 4b, c). Furthermore, after 1 times cold-swaging, the shape of the grains is elongated and squashed along with the contour of specimens. The mean size of the grain is 12.1 lm. Furthermore, when the coatings underwent 2 times cold-swaging, the shape and size of the grains become tinier than 1 times and a denser microstructure without cracks and folds was obtained. The mean size of the grain is 9.7 lm. However, the composite coatings exhibit a discontinuous and obvious crack when the coatings were experienced 3 times coldswaging procedure (Fig. 4d). Under action of external force, the 3 times cold-swaging will result in a larger plastic deformation than the other cold-swaging times at the composite coating. The coating is unable to withstand greater deformation and appears crack. Therefore, 3 times cold-swaging is not a better choice to enhance the nanocomposite coatings at this paper.

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In order to better compare the microstructure after coldswaging and research whether the cold-swaging could impact on the substrate, the cross-sectional OM microstructure of the composite coatings at the middle and bottom is displayed in Fig. 5. In Fig. 5a, a-1 showed a dense microstructure without cracks and porosities. A planar crystal was found at Fig. 5a, and a cellular dendritic structure was detected in Fig. 5a-1. This phenomenon is dependent on the ratio of temperature gradient to solidification rate (G/R) [2, 7]. At the same time, a good metallurgical bonding is formed between the substrate and the cladding (Fig. 5a-1). It can be seen in Fig. 5b, the deformation microstructure and the un-deformation microstructure are evenly separated, and the deformation does not occur in the substrate due to the degree of deformation not great. However, the deformation microstructure is about 3/4 of the whole image (Fig. 5c), and the remaining 1/4 parts also exhibit relatively small plastic deformation. As shown in Fig. 5c-1, the microstructure of the coating at the bottom is relatively small and dense, and the heat-affected zone (HAZ) shows a relatively small streamline compared Fig. 5a-1, b-1. However, the substrate is not destroyed by cold-swaging and a typical 316L austenitic stainless steel microstructure without defects is demonstrated (shown in Fig. 5c-1). This phenomenon is probably because the external force has no effect on the substrate, eventually. Therefore, 2 times cold-swaging is an appropriate choice to change the internal structure of the coating. 3.2 Micro-hardness and XRD patterns It is generally known that micro-hardness is an effective means to reflect the internal properties of materials. Therefore, Fig. 6 depicts the micro-hardness distribution of the coatings with different cold-swaging times. At the same time, XRD patterns about the coatings with different coldswaging times are shown in Fig. 7. The micro-hardness test in 0 times specimens showed the ladder-like distribution at the cross section, and the cladding layer showed high average hardness values (520 Hv) because of the low degree of supercooling on the surface. On the other hand, it

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Fig. 4 OM microstructure at the top of composite coatings after cold-swaging: a 0 times; b 1 times; c 2 times; d 3 times

Fig. 5 Cross-sectional OM microstructure of the composite coatings: a, b and c are the middle of the 0 times, 1 times and 2 times, respectively; a-1, b-1, and c-1 are the bottom of the 0 times, 1 times and 2 times, respectively

could be mainly due to the dispersive distribution of SiC ceramic in the composite coatings and the Fe–Cr matrix was separated by hard reinforcements. With the increase in the cold-swaging times, the coatings showed relatively high average hardness values. The maximum average micro-hardness is achieved at 583 Hv

on 2 times cold-swaging, and the micro-hardness is increased by 40–60 Hv than the 0 times. In addition, the micro-hardness curves of 2 times in Fig. 6 can be seen that the micro-hardness is also enhanced at HAZ region, and the micro-hardness is increased by 20–90 Hv than the other times. This phenomenon is consistent with the results

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Fig. 6 Micro-hardness of substrate and composite coatings

Fig. 7 Composite coatings XRD patterns: a 0 times; b 1 times; c 2 times

obtained in Fig. 5c-1. The reason for this result is cold deformation caused deformation strengthening [24, 25]. Due to plasticity is an important property of materials, the cold deformation appears in specimen after cold-swaging, and the emergence of twin and slip probably play a huge role in this procedure. Furthermore, the XRD patterns showed that there are not new phases appearing after undergoing cold-swaging (shown in Fig. 7). The phases of coating are mainly contained by c-CrFeNi, M7C3, FeSi, and the nano-SiC has been dissolved. Therefore, based on the plastic deformation in Figs. 4 and 5, it can be concluded that refined grains and new strengthening phase M7C3, FeSi resulted in the strengthening effect caused by the Hall–Petch mechanism. 3.3 Tensile properties Tensile specimens size reference literature [2]. Whereafter, the tensile specimens were prepared by wire-cut electrical

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discharge machining and rubbed with fine sandpaper. Figure 8a illustrates the stress–strain curves of the composite coatings after cold-swaging, and the results showed that the mechanical properties are improved significantly using cold-swaging strategies. However, due to the laser cladding coatings properties and the cold-swaging process, there is no obvious yielding phenomenon in Fig. 8a. The elastic modulus, ultimate tensile strength and elongation of the coatings are 156.46 GPa, 1529.98 MPa and 9.24%, respectively, when the coatings underwent 2 times coldswaging. In comparison with the 0 times cold-swaging coating, the elastic modulus and ultimate tensile strength were increased by 35.1 and 53.1%, respectively. However, elongation of the coating was decreased by 37.4%. As a result of the existence of second-phase particles in PRMMC coatings, the inhomogeneous deformation generated a stress field at the interface between the particle and the substrate after undergoing cold-swaging. It is well known that the onset of inhomogeneous deformation is governed by the Conside´re criterion [26]. At the same time, the coating appeared cold hardening and diminished strain hardening rate resulting in a lower uniform elongation. It is noteworthy that both higher strength and lower ductility are realized by cold-swaging but no post-annealing processes. Besides, nano-SiC homogeneously mixed with metal particles, thereby refining grains in the composite coating. At room temperature, grain refinement increased the percentage of the grain boundary after cold-swaging. The boundaries acted as strong obstacles to dislocation motion, which led to a high degree of strain hardening and improved tensile strength [27]. Grain refinement of coating can also improve the tensile strength and elastic modulus. Furthermore, the undissolved broken carbide will further penetrate into the coating interior after cold-swaging, which can improve the brittleness of coatings and make the elongation downgrade at the same time. In order to study the fracture structure, a complete analysis with SEM was performed. Observed carefully on Fig. 8b, c and d can be found no obvious plastic deformation, and there are many small planes of glance strongly. The phases M7C3 and FeSi are existence in the coating at the same time. Therefore, they are brittle fracture. With the increase in cold-swaging times, the tearing ridge and crack become large and obvious, the large cleavage plane disappeared, and many small dimples appeared. The radial region of the fracture surface exhibits typical quasi-cleavage fracture (Fig. 8c, d). As a result of the existence of secondphase particles in PRMMC coatings, it will lead to dislocation accumulation within the coatings. This phenomenon can cause the grain boundary carbides of the coatings to crack, and the crack will extend to nearby the second phase under the external force action at the same time [28]. All of these will lead to quasi-cleavage fracture, ultimately.

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Fig. 8 Tensile stress–strain curves and SEM fracture morphology of the composite coatings: a tensile stress–strain curves; b 0 times; c 1 times; d 2 times

3.4 Wear resistance Figure 9 illustrates the result of friction and wear test of the composite coatings with different cold-swaging times. With the increase in the deformation, the wear surface become more and more smoother (Fig. 9a). Due to the 0

times coatings with relatively low micro-hardness, it is 11% less than that of specimen with 2 times cold-swaging, which will easily cause to shear deformation on the edge of wear scar and show a high tendency to layered peeling. The abrasive particles of the friction pairs can plow deeply into the coating surface and cause obvious micro-plowing

Fig. 9 Friction and wear test: a-1, a-2, a-3 surface morphologies of 0 times, 1 times and 2 times, respectively; b friction coefficient; c wear mass loss

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grooves or layered peeling (Fig. 9a-1). On the other hand, the coatings were subjected to cold-swaging. It appears relatively higher hardness and leads to better wear resistance, which corresponds to the micro-hardness results discussed previously. Therefore, there appeared a large number of smooth regions after 2 times cold-swaging, and layered peeling is basically disappeared. Besides, as a result of the internal grain is compacted after cold-swaging, the abrasive particles will be also reduced properly (Fig. 9a-3). Furthermore, as can be seen in Fig. 9b, c, the 0 times exhibits a highest friction coefficient curve and weight loss because of its relatively lower surface hardness. The friction coefficient curve and weight loss of the coatings are obvious decreased via cold-swaging. The friction coefficient and mass loss of 2 times cold-swaging are approximate 0.47 and 3.82 mg, respectively. In comparison with the 0 times, the friction coefficient and mass loss are reduced by about 37.8 and 11.6%, respectively. The wear resistance of the coatings after cold- swaging is found to be closely related to strength and fracture toughness at the same time. In the process of friction and wear, surface of the coatings is not immediately destroyed, because the work hardening is produced by cold- swaging and the fracture toughness of the coating is also improved. Therefore, the optimum coordination of hardness and fracture toughness is a key factor to high wear resistance. In order to better exhibit the wear mechanism of the composite coating after cold-swaging, the wear mass loss of GCr15 steel is given in Fig. 10. It can be seen that the average mass loss of GCr15 steel is increased with the increase in the cold-swaging times. The average mass loss of GCr15 steel after 0 times, 1 times and 2 times coldswaging is approximate 2.06, 2.75 and 3.32 mg, respectively. Consequently, this phenomenon further illustrates that with the increase in cold-swaging times, the wear resistance of the composite coatings is further enhanced.

Fig. 10 Wear mass loss of GCr15 steel

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4 Conclusion In this paper, a laser cladding surface strengthening process and following a cold-swaging process are innovatively combined to further improve the surface properties of the metal matrix. Experimental results showed that 2 times cold-swaging procedure is a better choice to further improve laser cladding coating properties. After 2 times cold-swaging, the shape and size of the grains in composite coatings become tinier than 1 times and 0 times. The mean size of the grain is 9.7 lm. The maximum micro-hardness is achieved at 583 Hv, and the micro-hardness is increased by 40–60 Hv than 0 times. The micro-hardness is also further enhanced by 20–90 Hv at HAZ region than the other cold-swaging times at the same time. The elastic modulus, tensile strength and elongation of the coatings were 156.46 GPa, 1529.98 MPa and 9.24%, respectively. In comparison with the 0 times cold-swaging, the friction coefficient and weight loss are reduced by about 37.8 and 11.6%, respectively, and the wear surface is smoother in micromorphology, when the coating was underwent 2 times cold-swaging. Furthermore, in order to further understand the properties and characteristics of the cold-swaging microstructure, an in-depth study on twins and dislocations after cold-swaging is necessary in the future. Whether the cold-swaging process is effective for a high content of SiC, such as 20 and 30 wt%, also requires a systematic research in the future. Acknowledgements This work was supported by the National Natural Science Foundations (Grant No. 51375425), P. R. China.

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