Effect of laser shock peening on corrosion resistance of 316L stainless steel laser welded joint

Effect of laser shock peening on corrosion resistance of 316L stainless steel laser welded joint

Surface & Coatings Technology 378 (2019) 124824 Contents lists available at ScienceDirect Surface & Coatings Technology journal homepage: www.elsevi...

4MB Sizes 1 Downloads 73 Views

Surface & Coatings Technology 378 (2019) 124824

Contents lists available at ScienceDirect

Surface & Coatings Technology journal homepage: www.elsevier.com/locate/surfcoat

Effect of laser shock peening on corrosion resistance of 316L stainless steel laser welded joint

T

Dongwei Liu, Yan Shi , Jia Liu, Long Wen ⁎

School of Mechanical and Electric Engineering, Changchun University of Science and Technology, Changchun 130022, PR China National Base of International Science and Technology Cooperation for Optics, Changchun 130022, PR China

ARTICLE INFO

ABSTRACT

Keywords: Corrosion resistance Stainless steel Laser shock peening Laser welding

This paper describes the testing of laser shock peening (LSP) on the corrosion resistance of a 316L stainless steel laser welded joint. The objective of this research is to explore the changes in corrosion resistance when a laser shocks weldments under the same laser line energy (50 J/mm). The research shows that LSP can increase the corrosion resistance of weldments, produce grain refinement, generate high magnitude compressive residual stress and increase microhardness. Moreover, this research also provides theoretical evidence for optimizing parameters which have the same line energy.

1. Introduction

the enhancement of the corrosion resistance and mechanical properties on various materials. These technologies include shot peening (SP) [22–24] and supersonic fine particles bombarding (SFPB) [25]. However, these methods have rigid contact with workpiece. For instance, Klotz et al. [24] pointed out that shot peening on surface of Inconel 718 may create irregularities that can be detrimental to fatigue, and leads to residual stress relaxation. Zhang et al. [25] indicated that large amounts of micro-cracks were introduced into the surface layer of lowcarbon steel by SFPB, and SFPB can increase the surface roughness. Therefore, SP and SFPB have obvious shortcomings in surface modification. Nowadays, laser shock peening (LSP) [26,27], ultrasonic shot peening (USSP) [28] and surface mechanical attrition treatment (SMAT) [29] are the main technologies in the field of material surface modification. However, the manufacturing technology and testing method of USSP is not particularly advanced [30] and SMAT may decrease the wear resistance in a way because of lower toughness due to severe plastic deformation [31]. LSP can result in residual compressive stresses of several hundred MPa [32,33] and induces nanostructuring of grain [34] when the laser interacts with the materials. Residual compressive stresses and nanostructuring of grain help to improve the surface properties such as the hardness [35], wear resistance [36,37], and the corrosion resistance [38]. For example, Wei et al. [39] concluded that LSP can produce grain refinement, generate high magnitude compressive residual stress and increase surface micro-hardness. Moreover, LSP treated sample exists a higher corrosion resistance due to the formation of the compact passive film with less defects.

It is known that 316L stainless steel (316LSS) has many excellent properties such as toughness [1], superior heat resistance [2], and good corrosion resistance [3–5]. Therefore, it is utilized widely in various fields [6–8]. Although it has a good corrosion resistance owing to the dense passive film produced by chromium atoms [9], it may be vulnerable to pitting and crevice corrosion [10]. In recent years, various innovative welding technologies have emerged. For instance, laser welding [11] and electron beam welding [12] are widely used in metal manufacturing industry. Laser welding (LW) is famous for its concentrated energy as well as smaller material distortion under a certain heat input [13]. Moreover, LW is a stable and efficient welding technique because of its better beam quality [14–16]. However, the corrosion resistance of weldments is not satisfactory, even when LW is used. For instance, the corrosion resistance of a 316LSS welded joint was investigated by Han et al. [17]. They emphasized that there are many defects in the weldments which have a negative effect on the passive film, and the corrosion resistance decreases. There are similar cases which were investigated by a number of investigators [18–20]. On the other hand, Wang et al. [21] pointed out that the existence of residual tensile stress in the weld zone (WZ) and heat affected zone (HAZ) after LW was harmful to the corrosion resistance of weldments. Thus, the decrease in corrosion resistance of weldments has been demonstrated by a number of scholars. Currently, some surface modification technologies have resulted in

⁎ Corresponding author at: School of Mechanical Engineering, Changchun University of Science and Technology, No.7089 Weixing Road, Changchun 130022, PR China. E-mail address: [email protected] (Y. Shi).

https://doi.org/10.1016/j.surfcoat.2019.07.048 Received 11 March 2019; Received in revised form 22 July 2019; Accepted 23 July 2019 Available online 23 July 2019 0257-8972/ © 2019 Elsevier B.V. All rights reserved.

Surface & Coatings Technology 378 (2019) 124824

D. Liu, et al.

Meanwhile, Prabhakaran et al. [40] observed that LSP improved microstructure and mechanical properties (such as residual stress, microhardness, and ultimate tensile strength). The results coincide with those of others [41,42]. However, there are relatively few studies devoted to explore the changes in corrosion resistance when a laser shocks weldments under the same laser line energy. This paper elaborates on the effect of LSP on the corrosion resistance of 316LSS laser-welded joints. The objective of this research is to explore the changes in corrosion resistance when a laser shocks weldments under the same laser line energy. The experiments for revealing the corrosion mechanism include a residual stress evaluation, microhardness test, microstructure analysis, and electrochemical corrosion test. Then we investigated the internal connecting links between them. In a certain sense, this paper offers significant guidance in choosing the parameters of an LW procedure which has the same line energy after LSP according to the changes in corrosion resistance. At the same time, this investigation can also be regarded as an extension of publications by Shadangi et al. [43] and Kalainathan et al. [44].

Table 2 Mechanical and physical properties of 316L stainless steel.

2. Material and methods

residual tensile stress resulting from melting through the absorptive layer, which is ablated (vaporization) under the thermal effects produced by the laser irradiation. On the other hand, the confining layer prevents an irregular and massive plasma spread, and then concentrates it on the target of the surfaces to impart residual compressive stresses of several hundred MPa up to the subsurface by the detonating wave formed by the metal plasma. In other words, residual compressive stresses generated after LSP are believed to be caused by the enormous peak pressure arising from the detonation wave. We intercepted the specimens with and without LSP impacts using the WEDM after accomplishing the above tests, which are prepared into samples for the following series of experiments by grinding and polishing the individual specimens with SiC paper and a slurry of alumina powder. Among them, the metallographic specimen is etched by adopting 5% Nital. A schematic diagram of the LSP process is displayed in Fig. 1.

Tensile strength (MPa)

Yield strength (MPa)

Elongation (%)

Thermal conductivity (J/kg·K)

≥480

≥175

≥40

16.3

Table 3 Optimized laser welding (LW) process parameters.

2.1. Materials The chemical composition and properties of 316LSS are listed in Tables 1 and 2, respectively. Test specimens of 200 × 50 mm were cut by applying electrospark wire-electrode cutting (WEDM) at a 0.6-mm thickness. Then, in order to eliminate the contaminants that adhere to the surface of the specimens, they were grounded mechanically using 400-grit SiC papers and wiped with industrial alcohol, followed by drying with an air dryer. Finally, the specimens were placed in an insulation furnace and subjected to annealing stress to relieve the residual stress inherited from various processing methods The annealing temperature was about 400 °C. 2.2. Laser welding The mechanically grounded specimens were laser welded by an automatic welding system composed of two parts: an HL4006D solidstate laser and a KR30/HA six-axis linkage robot. According to a preliminary test, we chose the optimized laser welding process parameters for weldments under the same liner energy, which is indicated in Table 3. Then, the weldments were grounded slightly using SiC papers of grit sizes from #1200 up to #1500, and they were cleaned with industrial alcohol in order to float uneven surfaces. The width of the weldments was approximately 1 mm.

Numbering

Laser power (kW)

Welding speed (m/s)

Shielding gas

Protective gas flow (L/min)

Defocusing (mm)

LW-1 LW-2 LW-3 LW-4 LW-5

1.5 2.0 2.5 3.0 3.5

0.03 0.04 0.05 0.06 0.07

Argon and helium

30

0

2.4. Residual stress The residual stress on the surface of the weldments with and without LSP impacts was measured by an X-Stress Robot using X-ray diffractometry (XRD). The working voltage and current were 30 kV and 6.7 mA, respectively. The source of the X-ray (the wavelength is in a range of approximately 0.05 nm to 0.1 nm) was Mn Kα radiation, and the measuring diffraction plane was α phase (311) peak at 10 different angles. The error of allowable measurement of the X-ray diffractometer was 20 MPa. In this research, to guarantee the veracity of the experimental data, the measurements were repeated three times for each condition in order to obtain the average value.

2.3. Laser shock peening The specimens for laser shock peening of the target surface belonging to the five sets of weldments were fabricated using a laser shock system consisting of a high-power Nd:YAG laser and five-axis numerical control worktable. The selected LSP process parameters are listed in Table 4. The LSP experiment was conducted at room temperature (25 °C) and the entire length of laser scanning on the specimens was about 70 mm. It should be emphasized that 0.1-mm aluminum foil and clear flowing water were used as the absorptive layer and confining layer, respectively. On the one hand, the function of the absorptive layer is to vaporize to form a mass of plasma when the laser irradiates the target surface. Simultaneously, the treated surfaces avoid the

2.5. Microhardness The cross section (perpendicular to the LSP direction) of the microhardness with and without LSP impacts was determined using an MH-60 micro-hardness tester. The loading time and the load mass were set to 10 s and 200 g, respectively. The micro indentation was used for microhardness evaluation. And in order to eliminate indentation size effect as much as possible, the test force (load mass) was maximized within the allowable range.

Table 1 Chemical composition of 316L stainless steel (wt%). Composition

C

Si

Mn

P

S

Ni

Cr

Mo

Fe

Percent (wt%)

≤0.03

≤1.0

≤2.0

≤0.045

≤0.03

12–16

16–18

1.8–2.5

Balance

2

Surface & Coatings Technology 378 (2019) 124824

D. Liu, et al.

Table 4 Optimized Laser shock peening (LP) process parameters. Numbering

Wavelength (nm)

Pulse duration (ns)

Pulse energy (J)

Spot size (mm)

Overlapping rate (%)

Laser Beam mode

LP-1 LP-2 LP-3 LP-4 LP-5

1064

15

9

3

50

Gaussian beam

on the surface ranges from tensile stress (32–130 MPa and 5–54 MPa corresponding to a and b, respectively) to compressive stress (−114 to −220 MPa and −115 to −292 MPa corresponding to a and b, respectively) without and with LSP impacts. Regardless of directions, all of LSP specimens (LP-1, LP-2, LP-3, LP-4, and LP-5) introduced a high magnitude of surface compressive stress (several hundred MPa), while the LW specimens (LW-1, LW-2, LW-3, LW-4, and LW-5) imparted tensile stress. The residual compressive stress presented on the as-received specimen may arise from the prior material processing. Moreover, it is shown in Fig. 3 that the same specimen has different residual stress (including residual tensile stress and compressive stress) values in different directions (along and perpendicular to the LW movement). The average value of the residual tensile stress of the five specimens without LSP is about 28.4 MPa in a direction perpendicular to the direction of the LW movement. The stress in the direction of the LW movement is approximately 93.4 MPa. Similarly, the residual compressive stress has the same phenomenon, namely, the average value of the residual compressive stress is about 161.8 MPa and 179.4 MPa along and perpendicular to the laser movement, respectively. Obviously, the average residual compressive stress is higher in the direction perpendicular to the laser movement than the parallel direction. This is in agreement with an observation by Majumdar et al. [45]. They thought that the different distributions of residual stress in the two distinct directions can be defined by the anisotropy. With regard to the residual tensile stress after LW, Окерблом [46] put forward the theory of residual plastic deformation under one-dimensional conditions in the 1950s. This theory is assumed that the weld zone plays a part in the original specimen all the time, and that the weld zone undergoes a rapid heating and cooling process, eventually, the process contribute to the residual compressive plastic strains after welding. Based on this foundation, Wang and Lu [47] regarded the welding process as a model with rigid constraints at both ends of a rod. They analyzed the changes of strain in the weld zone under a welding thermal cycle (0 → Tm → 0), where Tm is the heating temperature. A schematic diagram of the model put forward by J.H. Wang is shown in Fig. 4. This diagram includes (a) an actinometrical drawing, (b) planform, and (c) rigid constraints at both ends of a rod evolved from the front elevation. The derivation process (0 → Tm → 0) can be seen as follows:

Fig. 1. Schematic diagram of laser shock peening process.

2.6. Microstructural analysis A scanning electron microscope (SEM) and optical microscope (OM) were used to observe the microstructure and corrosion morphology after the electrochemical corrosion tests, respectively. The models of the SEM and OM were JSM-6510F and LeiCaDM2700M, respectively. In this study, a microstructural analysis was conducted by taking a cross section of the specimen (perpendicular to the LSP direction). 2.7. Electrochemical corrosion test Potentiodynamic polarization tests were developed at room temperature using an electrochemical workstation (Zahner) in 3.5 wt% NaCl solution. The solution was blown by argon gas to eliminate oxygen at 3 min before the experiment. The working electrode (specimen), counter electrode (platinum mesh), and reference electrode (saturated calomel electrode) made up a three-electrode system. The potentiodynamic polarization was tested and plotted at a scanning rate of 5 mV/s from −2 to 2 V. 2.8. Experimental work scheme

=

This paper makes a detailed analysis of residual stress, microstructure, microhardness and electrochemical corrosion for the prepared specimens (with and without LSP). And then, the intrinsic relationship between them was revealed. Experimental work scheme is shown in Fig. 2.

Tm +

c

Tm +

T

+

e

=0

(1)

where ε is the variation of strain in the rod; α is the coefficient of linear expansion; αTm is the strain caused by the thermal effect; εc is the compressive plastic strain in the rod during the heating process; εT is the tensile plastic strain in the rod during the cooling process; and εe is the elastic strain (the resilience of the material is restored at room temperature). Meanwhile, ε is constant owing to the rigid constraints. It is noteworthy that the first two items in Formula (1) are the vector sum of the strain produced by the heating process, and the last three items are the vector sum of the strain produced by the cooling process. Finally, this can be seen in Formula (2):

3. Results and discussion 3.1. Analysis of surface residual stress A thorough investigation of the residual stress carried out on the surface was developed using an X-ray diffractometer. Fig. 3(a) and (b) show bar charts in the direction along and perpendicular to the LSP movement. These charts describe the variation of residual stress on the surface of the as-received specimens, weldments, and the weldments with LSP impacts. The results indicate that the residual stress developed

e

=

(

c

+

T)

(2)

Fig. 4 shows that the absolute value of εc is larger than that of εT. Thus, εc + εT is a negative value, and εe is a positive value. The rod cannot be constricted because of the restraint, so residual tensile stress 3

Surface & Coatings Technology 378 (2019) 124824

D. Liu, et al.

Fig. 2. Experimental work scheme.

mechanism of interaction between the shock wave and materials, many researchers discussed this topic in recent years. For instance, Ballard and Fabbro et al. [51,52] put forward the relationship between the surface plastic strain εp and peak pressure p. This relationship can be noted in Fig. 5. It is easy to see that the plastic strain, that is, the compressive strain, increases linearly when p is between 1 HEL and 2 HEL, where HEL is the Hugoniot Elastic Limit. A metal's HEL is related to the yield strength according to Johnson and Rohde [53]:

HEL =

1 1

v 2v

s

(3)

where v is the Poisson's ratio and σs is the yield strength. In this research, the Poisson's ratio and yield strength of 316LSS are 0.31 and 175 MPa, respectively, thus, the HEL of 316LSS is approximately 318 MPa. Thus, we conclude that the generating of residual compressive stress on the surface in this study is a result of the performance of the residual compressive plastic strain. In addition, the model of mechanism in generating residual compressive stress was also reported by Peyre et al. [54]. They found that the model is divided into two parts, and they are shown in Fig. 6(a) and (b), during the interaction and after the switchoff of the laser pulse, respectively. From region B in Fig. 6(a), it can be seen that the surface of the impact zones are subjected to compressive strain along the direction of the shock wave propagation, while tensile stretching (shock wave propagation makes the grains have a tendency to stretch in the plane parallel to the surface) is generated during the interaction between the shock wave and the materials. Ultimately, as soon as the peak pressure unloads, the elastic strain of the surrounding material (region C) is restored. Apparently, the reaction between regions B and C indicates the formation of residual compressive stress. The residual compressive stress can enhance the mechanical property [21], and the fatigue life [55] is also advanced. On the other hand, it is remarkable that the residual compressive stress is of great importance for improving the corrosion resistance [56], and it will be further discussed below.

Fig. 3. Residual stress of (a) along to the LSP movement; (b) perpendicular to the LSP movement.

is created in the weld zone. Ramkumar et al. [48] indicated that the existence of the residual tensile stress in the weld zone is one of the most important factors that contribute to failure. Thus, it is clear that residual tensile stress is disadvantageous to the service life of parts in the engineering field. Surprisingly, it is worth mentioning that a high magnitude of compressive stress was produced on surfaces when specimens were LSP'ed. The results coincide with those of others [40,41]. The mechanism for generation of residual compressive stress on the surface involves a shock wave and it is produced by the absorptive layer (aluminum foil) that evaporates swiftly under the thermal effects when LSP imparts an enormous peak pressure on the surface. This is supported by Liao et al. [49] and Liu and Hill [50]. With regard to the

3.2. Microstructural analysis A research using the microstructural analysis of the outermost layer of the specimens with and without LSP impacts was performed using SEM. From Fig. 7, it can be seen that the grains are indeed refined after LSP. Evidently, there is a series of larger grains (regions A, B, C, and G) from the specimens of LW than those of LSP that has the relatively smaller grains (regions D, E, F, and J). Studies by Chen et al. [57] put forward that grains were refined after LSP. However, the various grains from specimens of LW basically follow a tendency to coarsen (regions C and H) first and then become refined (regions I and J). In other words, 4

Surface & Coatings Technology 378 (2019) 124824

D. Liu, et al.

Fig. 4. Schematic of (a) axonometrical drawing; (b) planform; (c) rigid constraints at both ends of a rod evolved from front elevation [42].

Combined with the results of previous studies [26,41,55], we assumed that there may be some connections between the grain refinement and dislocation. The appearance of dislocation is because of the elastic and plastic deformation mechanism in the grain (Both of elastic and plastic deformation are operated spontaneously under the external forces). In order to reduce the different orientations among adjacent grains, the slip system groups (high-density configuration) in different orientations separate them by dislocation walls (grain boundaries) with an aggravation of the degree of plastic deformation. Then, the grains are broken into distinct regions (cell blocks) by dislocation walls. When the plastic deformation reaches a certain extent, different slip system groups separated by dislocation walls are broken into various new blocks again in the regions in which they are located. Thus, the grain size will decrease as the number of grain boundaries increases. Grain refining can enhance the surface performance, including the properties of mechanical [21,59] and corrosion resistance [39]. In summary, grain refining is supposed to be the key role in the field of laser surface modification.

Fig. 5. Relationship between the surface plastic strain and peak pressure [46,47,49].

the uniformity and sizes of grains of the specimens LP-4 and LP-5 are satisfactory relative to the other three groups. It should be pointed out that this result is different from L. Chen and L. Zhang et al. Furthermore, Zhang et al. [26], Kalainathan et al. [44] and Peyre et al. [58] pointed out that the plastic deformation brought about the appearance of a slip dislocation inside the crystal grains. This phenomenon may be the product of the spontaneous operation of the coordination deformation mechanism of elastic and plastic in the grain. However, we have not found the plastic deformation in grains temporary duo to the experimental conditions. The investigations on this will be underway in future.

3.3. Microhardness To develop a depth profile of the microhardness with and without LSP, electrolytic polishing was performed to remove 1000 μm from the top of the surface. A definitive understanding of the numerical distribution of microhardness is seen in Fig. 8. In general, it is easy to see that the microhardness declines with an increase in distance.

Fig. 6. Model of the mechanism in generating residual compressive stress of (a) during the interaction between the shock wave and the materials; (b) after the switchoff of the laser pulse [49]. 5

Surface & Coatings Technology 378 (2019) 124824

D. Liu, et al.

Fig. 7. SEM micrograph of the outermost layer of (a) LW-1; (b) LP-1; (c) LW-2; (d) LP-2; (e) LW-3; (f) LP-3; (g) LW-4; (h) LP-4; (i) LW-5; (j) LP-5. 6

Surface & Coatings Technology 378 (2019) 124824

D. Liu, et al.

Fig. 8. Microhardness profile of the specimens with and without LSP from surface towards interior. (a) LW-1 and LP-1; (b) LW-2 and LP-2; (c) LW-3 and LP-3; (d) LW4 and LP-4; (e) LW-5 and LP-5.

Simultaneously, the values of the microhardness of specimens with LSP impacts (LP-1, LP-2, LP-3, LP-4, and LP-5) are much larger than those of the laser welds (LW-1, LW-2, LW-3, LW-4, and LW-5). In addition, turning points in line charts play an important role because they represents the depth of surface modification in LSP. As can be noted from Fig. 8, the most evident turning range is between 100 μm to 200 μm, and then all of the values of microhardness (with and without LSP impacts) goes up and down (they tend to reach a stable

value) with an increase in depth. Thus, we may think that the depth of the surface modification area in LSP is in a range of 100 μm to 200 μm. This result is coincide with the studies by Dai et al. [36] and Liu et al. [60]. The grain size of the surface can be refined by LSP (it was discussed in Section 3.2). Therefore, the relationship between the average size of the grain and the yield strength can be determined by the Hall-Petch formula [21,61,62]: 7

Surface & Coatings Technology 378 (2019) 124824

D. Liu, et al.

3.4. Electrochemical corrosion tests The electrochemical polarization curves and corrosion morphology in 3.5 wt% NaCl solution at room temperature were determined using the electrochemical workstation and SEM, respectively. From Fig. 10, it can be seen that there is an obvious passivation zone (region B) in the anode. No matter whether the specimens have or do not have LSP impacts, the passivation film was formed on the surface of the specimens in the course of electrochemical corrosion. On the whole, the breakdown potential of the passivation film (points C and D) of the specimens with LSP impacts is larger than that of the specimens with LW. The same results were observed by Garcia et al. [66] in 2008. They pointed out that the pitting potential is of great significance to the corrosion resistance. Moreover, an interesting observation to compare the corrosion resistance between the specimens with and without LSP impacts more intuitively is displayed in Table 5. The values for the self-corrosion electrical current density (icorr) of the specimens with LSP impacts (LP1, LP-2, LP-3, LP-4, and LP-5) are smaller than those of the specimens without LSP impacts (LW-1, LW-2, LW-3, LW-4, and LW-5). It is illustrated that the corrosion resistance is improved with LSP impacts. Combined with Table 5 and Fig. 3, residual stress has a great influence on the icorr. According to the relationship between the icorr and corrosion rate, it can be described with the following formula [67]:

Fig. 9. Relationship between the hardness and residual stress during annealing at different temperatures [59]. y

=

0

+ ky d

1/2

or

Hv = Hv0 + k d

1/2

(4)

where σy and Hv are the yield strength and the microhardness of the surface, respectively; σ0 and Hv0 are the internal resistance of the lattice and the microhardness of the material, respectively; d is the average size of the grain (there is the inhomogeneity of plastic deformation in grains with and without LSP because of the different orientations, therefore, grain sizes need to be defined as average sizes) [63]; ky and k are constants that have a relationship with the types of crystal lattice and elastic modulus and the distribution of dislocations. Obviously, the microhardness of the surface increase with a decrease in the average size of the grains. Interestingly, Fu et al. [64] reviewed the relationship between the hardness and residual stress during annealing at different temperatures. This is noted in Fig. 9. They found that the hardness increases with an increase in the residual compressive stress when the temperature is certain. This is not a unique instance of this research. Meanwhile, Sun et al. [65] pointed out that the hardness increases when the surface imparts a residual compressive stress. Apparently, this is the reason that surface imparts a cold-work hardening because the hardness increases. This is consistent with the observation from Hoppius et al. [41]. However, although the residual compressive stress is of great benefit to the material surface, laser power has a critical threshold (2.5HEL in Fig. 3) on the surface of the material. Once the laser power exceeds the specific threshold, the residual compressive stress on the surface will generate the stress relaxation, and then the surface of the modified area will be converted into the subsurface. This is disadvantageous to the corrosion resistance of the surface of material.

(5)

icorr = nFv

where n is the number of metal ions; F is the amount of charge carried by metal ion of 1 mol; and v is the corrosion rate. Apparently, icorr is proportional to v. In other words, residual stress has a great influence on the v. Thus, from Table 5 and Fig. 3, the residual compressive stress (with LSP impacts) can decrease the corrosion rate. On the contrary, the residual tensile stress can increase the corrosion rate. From Fig. 11, it can be seen that the pitting appeared in the specimens with and without LSP impacts, moreover, the specimens with LSP impacts have good corrosion resistance. In region A of image (a), the severe pitting attacked the surface and then clubbed together, at last, it prolonged the area of aggression. However, in region C of image (b), the specimens with LSP impacts deteriorated by slight pitting attack. The results coincide with Prabhakaran et al. [68]. In addition, it can be seen that the corrosion mechanism of 316LSS is a typical intergranular corrosion through images (b) and (k). From images (a)–(j), it can be seen that there is pitting (regions A, E, K, N, S and C, G, M, P, U corresponding to the specimens with and without LSP impacts, respectively) and tearings (regions B, F, J, O, R and D, H, L, Q, T corresponding to the specimens with and without LSP impacts, respectively) on the surface. Furthermore, region I contains the corrosion products. However, we found that the areas of pitting and tearings are larger and deeper on the specimens without LSP than those of with LSP impacts on the whole. Thus, it is illustrated that the specimens with LSP impacts are more resistant to corrosion, and this meant a lesser material dissolution. However, the above phenomenon can only qualitatively compare the corrosion resistance. That is to say, it is impossible to determine which group is the most resistant to corrosion. Significantly, Trdan and Grum [69] further determined quantificationally the highest resistance to corrosion by calculating the protective efficiency. Thus, we can also calculate the protective efficiency to accomplish this purpose. The protective efficiency can be determined by the formula:

=

Icorr , lw

Icorr , lp

Icorr , lw

× 100%

(6)

where η is the protective efficiency; Icorr, lw is the self-corrosion current of the LW; and Icorr, lp is the self-corrosion current of the LSP. The results of the calculations are listed in Table 6. The highest protective efficiency is observed in the fourth group (17.708%), followed by the fifth group (12.875%). Therefore, the corrosion resistance

Fig. 10. Polarization curves of the specimens of LW and LP. 8

Surface & Coatings Technology 378 (2019) 124824

D. Liu, et al.

Table 5 Data from average self-corrosion current (Icorr), three tests of self-corrosion current (Icorr1, Icorr2, and Icorr3), average self-corrosion electrical current density (icorr) and mixed potential (Ecorr) of LW (laser welding) and LP (laser shock peening). LW

Icorr1 (μA) Icorr2 (μA) Icorr3 (μA) Icorr (μA) icorr (μA/cm2) Ecorr (V)

LP

LW-1

LW-2

LW-3

LW-4

LW-5

LP-1

LP-2

LP-3

LP-4

LP-5

6.084 2.548 4.565 4.399 6.284 −0.933

5.815 4.166 4.476 4.819 6.884 −0.973

3.495 5.937 2.907 4.113 5.876 −0.972

3.823 5.115 6.038 4.992 7.131 −1.028

1.314 2.185 3.095 2.198 3.140 −0.996

5.861 3.865 2.355 4.027 5.753 −0.933

6.588 3.728 2.770 4.362 6.231 −0.984

1.715 6.381 3.466 3.854 5.506 −0.972

3.361 3.686 5.277 4.108 5.869 −1.033

1.030 2.690 2.025 1.915 2.736 −0.925

is best when the laser power and laser welding speed are both higher for the same line energy (50 J/mm) when LSP is performed after LW. There is no doubt that this plays an important role in the selection of the laser parameters. Moreover, the corrosion resistances of LP-4 and LP-5 are indeed better than those of the other three groups with regard to images (h) and (j) according to the characteristics of the pitting pits and

tearings compared with the others. In addition, from the Fig. 3, the larger residual compressive stresses in the two directions are also generated on the surfaces of LP-4 and LP-5. Similarly, from the Fig. 7, the uniformity and sizes (This is a trend of changing, and there are no specific grain sizes in this paper) of grains of the specimens LP-4 and LP5 are satisfied relative to the other three groups. Obviously, the effects

Fig. 11. SEM and OM micrograph of (a) LW-1; (b) LP-1; (c) LW-2; (d) LP-2; (e) LW-3; (f) LP-3; (g) LW-4; (h) LP-4; (i) LW-5; (j) LP-5; (k) local metallographic structure of LW-1. 9

Surface & Coatings Technology 378 (2019) 124824

D. Liu, et al.

Fig. 11. (continued)

Table 6 Data from the protective efficiency.

η (%)

1

2

3

4

5

8.456

9.483

6.297

17.708

12.875

of the residual stress and microstructure on the corrosion resistance are prodigious. Recently, considerable research efforts on the relationship between the residual stress and corrosion resistance have been published. Okorokova et al. [70] pointed out that compressive residual stresses improve the corrosion fatigue life in the high-cycle fatigue area. Therefore, it can be seen that the residual compressive stress plays an important role in corrosion resistance.

Fig. 12. Relationship between the Icorr and gs−0.5 [66].

10

Surface & Coatings Technology 378 (2019) 124824

D. Liu, et al.

On the other hand, many investigators have explored the relationship between the microstructure and corrosion resistance. It is worth noting that Ralston et al. [71] identified a curve that can fit the relationship between the self-corrosion electrical current density (Icorr) and the size of the grain (gs). The relationship can be written as follows:

Icorr = (a) + (b) gs

protective efficiency, LP-4 and LP-5 have higher corrosion resistance than the others. 5. The residual stress and microstructure have a significant effect on the corrosion resistance of a 316L stainless steel laser-welded joint. Based on this investigation, the corrosion resistance of LW-4 (P = 3.0 kw, v = 0.06 m/s) and LW-5 (P = 3.5 kw, v = 0.07 m/s) after LSP are better than the others, and this research is of significance for laser processing.

(7)

where a is likely to be a function of the environment; b is a material constant; gs is the grain size; and α = 0.5 (simulation of random grain structures). When gs decreases, Icorr decreases to a certain extent as in Fig. 12. Obviously, the relationship found by Ralston et al. is consistent with our observations. According to this systematic research, various factors (residual stress, microstructure, and microhardness) result in the corrosion resistance with an increase. The residual compressive stress can refine the grain size after LSP, and the increase of microhardness (a cold-work hardening) is the result of grain refinement. Moreover, both of the high magnitude compressive residual stress and refined grains decrease selfcorrosion electrical current density, then the corrosion rate decreases and the protective efficiency in solution increase. Furthermore, the most significant finding of this study is that the corrosion resistance is best when the laser power and laser welding speed are both higher (that is, P = 3.0 kW, v = 0.06 m/s and P = 3.5 kW, v = 0.07 m/s) for the same line energy (50 J/mm) when LSP is performed after LW. This is significant in the field of laser processing.

Declaration of Competing Interest The authors declare that they have no competing. Acknowledgement The authors are grateful for (This work was supported by) the financial aids from the Scientific and Technological Planning Project of Jilin Province (Grant No. 20170204065GX and No. 20180201063GX), the National Key Research and Development Program of China (Grant No. 2017YFB1104601) and the Equipment pre-research field Foundation of China (Grant No. 61409230509). Ultimately, this research is supported by Changchun University of Science and Technology. Thus, we thank him for providing the experimental equipment. Data availability statement

4. Conclusion

All data included in this study are available upon request by contacting with the corresponding author.

Laser shock peening on a 316L stainless steel surface develops a higher residual compressive stress and microhardness. It also modifies the size of the grain. Finally, weldments with LSP impact are more resistant to corrosion than pure laser welds. In this study, the following specific conclusions can be drawn:

References [1] A. Sharifnabi, M.H. Fathi, B.E. Yekta, et al., The structural and bio-corrosion barrier performance of Mg-substituted fluorapatite coating on 316L stainless steel human body implant, Appl. Surf. Sci. 288 (2014) 331–340. [2] L.E. Rnnar, A. Koptyug, J. Olsén, K. Saeidi, Z. Shen, Hierarchical structures of stainless steel 316L manufactured by Electron Beam Melting, Addit. Manuf. 17 (2017) 106–112. [3] I. Gurappa, Characterization of different materials for corrosion resistance under simulated body fluid conditions, Mater. Charact. 49 (2002) 73–79. [4] L.Q. Guo, X.M. Zhao, M. Li, W.J. Zhang, Y. Bai, Annealing effects on the microstructure and magnetic domain structures of duplex stainless steel studied by in situ technique, Appl. Surf. Sci. 259 (2012) 213–218. [5] A. Moteshakker, I. Danaee, Microstructure and corrosion resistance of dissimilar weld-joints between duplex stainless steel 2205 and austenitic stainless steel 316L, J. Mater. Sci. Technol. 32 (2016) 282–290. [6] H. Luo, C.F. Dong, K. Xiao, X.G. Li, Characterization of passive film on 2205 duplex stainless steel in sodium thiosulphate solution, Appl. Surf. Sci. 258 (2011) 631–639. [7] G.T.G. Iii, V. Livescu, P.A. Rigg, C.P. Trujillo, C.M. Cady, Structure/property (constitutive and spallation response) of additively manufactured 316L stainless steel, Acta Mater. 138 (2017) 140–149. [8] J. Marder, B. Rath, S. Obenschain, International thermonuclear experimental reactor, Adv. Mater. Process. 166 (2008) 39–41. [9] M. Sabzi, S.M. Dezfuli, Drastic improvement in mechanical properties and weldability of 316L stainless steel weld joints by using electromagnetic vibration during GTAW process, J. Manuf. Process. 33 (2018) 74–85. [10] E.K. Brooks, R.P. Brooks, M.T. Ehrensberger, Effects of simulated inflammation on the corrosion of 316L stainless steel, Mater. Sci. Eng. C Mater. Biol. Appl. 71 (2017) 200–205. [11] K.M. Hong, Y.C. Shin, Prospects of laser welding technology in the automotive industry: a review, J. Mater. Process. Technol. 245 (2017) 46–69. [12] X.W. Xia, J.F. Wu, Z.H. Liu, et al., Study of microstructure difference properties of electron beam welds with beam oscillation of 50mm 316L in CFETR, Fusion Eng. Des. 138 (2019) 339–346. [13] B. Dhakal, S. Swaroop, Review: laser shock peening as post welding treatment technique, J. Manuf. Process. 32 (2018) 721–733. [14] L. Quintino, A. Costa, R. Miranda, D. Yapp, V. Kumar, C.J. Kong, Welding with high power fiber lasers-a preliminary study, Mater. Des. 28 (2007) 1231–1237. [15] T. Kovacs, Laser welding process specification base on welding theories, Proc. Manuf. 22 (2018) 147–153. [16] S. Kirk, W. Suder, K. Keogh, T. Tremethick, A. Loving, Laser welding of fusion relevant steels for the European DEMO, Nucl. Eng. Des. (2018). [17] L. Han, G. Lin, Z. Wang, H. Zhang, F. Li, L. You, Study on corrosion resistance of 316L stainless steel welded joint, Rare Metal Mater. Eng. 39 (2010) 393–396. [18] H. Tanaka, Effect of repeated weld-repairs on microstructure, texture, impact properties and corrosion properties of AISI 304L stainless steel, Eng. Fail. Anal. 21

1. Regardless of the direction of laser scanning during laser welding, a residual tensile stress is produced on the surface of the weldments. In contrast, a residual compressive stress is generated by LSP. Moreover, the larger residual compressive stresses in the two directions are also generated on the surfaces of LP-4 and LP-5. 2. There are a series of larger grains which have an inhomogeneous distribution in the specimens that were laser welded. However, the grains are refined after LSP because of the spontaneous operation of the coordination deformation mechanism of elastic and plastic when the surfaces are subjected to the enormous impact force (the essence of the LSP). On the other hand, the various sizes of grains from specimens without LSP impacts have a tendency to coarsen first, and then are refined with a gradual increase in the laser power and laser welding speed. Finally, the latter two groups of parameters (P = 3.0 kW, v = 0.06 m/s and P = 3.5 kW, v = 0.07 m/s) are satisfied by the microstructure. 3. After LSP of the surface of the weldments, the microhardness is remarkably improved, and the hardness value of the modified zone is ranges from approximately 180 HV to 200 HV. With regard to the depth direction, the depth of the modified area is in the range of 100 μm and 200 μm. In addition, based on the Hall-Petch formula, the hardness and yield strength increase with a decrease in the size of the grains. Moreover, a phenomenon of cold hardening appears on the surface. 4. The specimens with LSP impacts are more resistant to corrosion than those with LW in light of the self-corrosion electrical current density (icorr). Through an analysis of the polarization curves, it can be seen that a passivation film is formed on the surface of the specimens in the course of electrochemical corrosion according to the obvious passivation zone no matter whether the specimens have LSP impacts. There is a conspicuous secondary passivation area on the LP5. From the perspective of the corrosion morphology and the 11

Surface & Coatings Technology 378 (2019) 124824

D. Liu, et al.

[44] S. Kalainathan, S. Sathyajith, S. Swaroop, Effect of laser shot peening without coating on the surface properties and corrosion behavior of 316 L steel, Opt. Lasers Eng. 50 (2012) 1740–1745. [45] J.D. Majumdar, E.L. Gurevich, R. Kumari, A. Ostendorf, Investigation on femtosecond laser irradiation assisted shock peening of medium carbon (0.4% C) steel, Appl. Surf. Sci. 364 (2016) 133–140. [46] Н.О. Окерблом, Welding Deformation and Stress, China Machine Press, China, 1958. [47] J.H. Wang, H. Lu, Some Discussions on Principle of Causing and Relieving Welding Residual Stress, vol. 23, Transactions of The China Welding Institution, 2002, pp. 75–79. [48] K.D. Ramkumar, P.S.G. Kumar, V.R. Krishna, et al., Influence of laser peening on the tensile strength and impact toughness of dissimilar welds of Inconel 625 and UNS S32205, Mater. Sci. Eng. A 676 (2016) 88–99. [49] Y. Liao, C. Ye, G.J. Cheng, A review: warm laser shock peening and related laser processing technique, Opt. Laser Technol. 78 (2016) 15–24. [50] K.K. Liu, M.R. Hill, The effects of laser peening and shot peening on fretting fatigue in Ti-6Al-4V coupons, Tribol. Int. 42 (2009) 1250–1262. [51] P. Ballard, J. Fournier, R. Fabbro, J. Frelat, Residual stresses induced by laser shocks, J. Phys. IV 1 (1991) 487–494. [52] R. Fabbro, J. Fournier, P. Ballard, D. Devaux, J. Virmont, Physical study of laserproduced plasma in confined geometry, J. Appl. Phys. 68 (1990) 775–784. [53] J.N. Johnson, R.W. Rohde, Dynamic deformation twinning in shock-loaded Iron, J. Appl. Phys. (1971). [54] P. Peyre, R. Fabbro, P. Metrien, H.P. Lieurade, Laser shock processing of aluminium alloys: application to high cycle fatigue behaviour, Mater. Sci. Eng. A 210 (1996) 102–113. [55] B.K. Pant, A.H.V. Pavan, R.V. Prakash, M. Kamaraj, Effect of laser peening and shot peening on fatigue striations during FCGR study of Ti6Al4V, Int. J. Fatigue 93 (2016) 38–50. [56] Hashemi Azar, R. Yazdi, Mahboobeh, The effect of shot peening on fatigue and corrosion behavior of 316L stainless steel in Ringer's solution, Surf. Coat. Technol. 204 (2010) 3546–3551. [57] L. Chen, X.D. Ren, W.F. Zhou, Z.P. Tong, S. Adu-Gyamfi, Y.X. Ye, Y.P. Ren, Evolution of microstructure and grain refinement mechanism of pure nickel induced by laser shock peening, Mater. Sci. Eng. A 728 (2018) 20–29. [58] P. Peyre, X. Scherpereel, L. Berthe, et al., Surface modifications induced in 316L steel by laser peening and shot-peening. Influence on pitting corrosion resistance, Mater. Sci. Eng. A 280 (2000) 294–302. [59] X.L. Wei, X. Ling, M. Zhang, Influence of surface modifications by laser shock processing on the acid chloride stress corrosion cracking susceptibility of AISI 304 stainless steel, Eng. Fail. Anal. 91 (2018) 165–171. [60] L. Liu, J.J. Wang, J.Z. Zhou, Effects of laser shock peening on mechanical behaviors and microstructural evolution of brass, Vacuum 148 (2018) 178–183. [61] E.O. Hall, Proc. Phys. Soc. London, Sect. B 64 (1951) 747. [62] N.J. Petch, J. Iron Steel Inst. Jpn. 174 (1953) 25. [63] Y.N. Yu, Principles of Metallography, Metallurgical Industry Press, China, 2003. [64] P. Fu, R.Q. Chu, Z.J. Xu, G.J. Ding, C.H. Jiang, Relation of hardness with FWHM and residual stress of GCr15 steel after shot peening, Appl. Surf. Sci. 431 (2018) 165–169. [65] Y. Sun, D. Zhang, L.J. Wu, Q.M. Wang, Analysis of the influence of material residual stress on hardness test, J. East China Univ. Sci. Technol. 38 (2012) 652–656. [66] C. Garcia, F. Martin, P.D. Tiedra, Y. Blanco, M. Lopez, Pitting corrosion of welded joints of austenitic stainless steels studied by using an electrochemical minicell, Corros. Sci. 50 (2008) 1184–1194. [67] C.N. Cao, Principle of Corrosion Electrochemistry, Chemical Industry Press, China, 2008. [68] S. Prabhakaran, Aniket Kulkarni, G. Vasanth, et al., Laser shock peening without coating induced residual stress distribution, wettability characteristics and enhanced pitting corrosion resistance of austenitic stainless steel, Appl. Surf. Sci. 428 (2018) 17–30. [69] U. Trdan, J. Grum, Evaluation of corrosion resistance of AA6082-T651 aluminium alloy after laser shock peening by means of cyclic polarisation and ElS methods, Corros. Sci. 59 (2012) 324–333. [70] V. Okorokova, M. Morgantini, Y. Gorash, T. Comlekci, D. Mackenzie, R.V. Rijswick, Corrosion fatigue of low carbon steel under compressive residual stress field, Proc. Eng. 213 (2018) 674–681. [71] K.D. Ralston, N. Birbilis, C.H.J. Davies, Revealing the relationship between grain size and corrosion rate of metals, Scr. Mater. 63 (2010) 1201–1204.

(2012) 9–20. [19] R. Sánchez-Tovar, M.T. Monta?és, J. García-Antón, Effects of microplasma arc AISI 316L welds on the corrosion behaviour of pipelines in LiBr cooling systems, Corros. Sci. 73 (2013) 365–374. [20] T. Hemmingsen, H. Hovdan, P. Sanni, N.O. Aagotnes, The influence of electrolyte reduction potential on weld corrosion, Electrochim. Acta 47 (2003) 3949–3955. [21] J.T. Wang, Y.K. Zhang, J.F. Chen, J.Y. Zhou, M.Z. Ge, Y.L. Lu, X.L. Li, Effects of laser shock peening on stress corrosion behavior of 7075 aluminum alloy laser welded joints, Mater. Sci. Eng. A 647 (2015) 7–14. [22] T. Wang, J. Yu, B. Dong, Surface nanocrystallization induced by shot peening and its effect on corrosion resistance of 1Cr18Ni9Ti stainless steel, Surf. Coat. Technol. 200 (2006) 4777–4781. [23] M. Kobayashi, T. Matsui, Y. Murakami, Mechanism of creation of compressive residual stress by shot peening, Int. J. Fatigue 20 (1998) 351–357. [24] T. Klotz, D. Delbergue, P. Bocher, M. Lévesque, M. Brochu, Surface characteristics and fatigue behavior of shot peened Inconel 718, Int. J. Fatigue 110 (2018) 10–21. [25] L.Y. Zhang, A.B. Ma, J.H. Jiang, et al., Electrochemical corrosion properties of the surface layer produced by supersonic fine-particles bombarding on low-carbon steel, Surf. Coat. Technol. 232 (2013) 412–418. [26] L. Zhang, K.Y. Luo, J.Z. Lu, Y.K. Zhang, F.Z. Dai, J.W. Zhong, Effects of laser shock processing with different shocked paths on mechanical properties of laser welded ANSI 304 stainless steel joint, Mater. Sci. Eng. A 528 (2011) 4652–4657. [27] J.Z. Lu, L. Zhang, A.X. Feng, Y.F. Jiang, G.G. Cheng, Effects of laser shock processing on mechanical properties of Fe–Ni alloy, Mater. Des. 30 (2009) 3673–3678. [28] V. Pandeya, J.K. Singh, K. Chattopadhyay, et al., Optimization of USSP duration for enhanced corrosion resistance of AA7075, Ultrason 91 (2019) 180–192. [29] P. Maurel, L. Weiss, P. Bocher, et al., Oxide dependent wear mechanisms of titanium against a steel counterface: influence of SMAT nanostructured surface, Wear 430431 (2019) 245–255. [30] L.L. Yan, The Technology and Application of Ultrasonic Shot Peening, vol. 6, Manufacturing Technology and Machine Tools, 2010, pp. 29–31. [31] S.A. Kumar, S.G.S. Raman, T.S.N.S. Narayanan, et al., Fretting wear behaviour of surface mechanical attrition treated alloy 718, Surf. Coat. Technol. 206 (2012) 4425–4432. [32] Y.K. Zhang, J. You, J.Z. Lu, C.Y. Cui, Y.F. Jiang, X.D. Ren, Effects of laser shock processing on stress corrosion cracking susceptibility of AZ31B magnesium alloy, Surf. Coat. Technol. 204 (2010) 3947–3953. [33] M. Kattoura, S.R. Mannava, Q. Dong, V.K. Vasudevan, Effect of laser shock peening on residual stress, microstructure and fatigue behavior of ATI 718Plus alloy, Int. J. Fatigue 102 (2017) 121–134. [34] S.H. Luo, W.F. He, L.C. Zhou, et al., Aluminizing mechanism on a nickel-based alloy with surface nanostructure produced by laser shock peening and its effect on fatigue strength, Surf. Coat. Technol. 342 (2018) 29–36. [35] A.M. Mostafa, M.F. Hameed, S.S. Obayya, Effect of laser shock peening on the hardness of AL-7075 alloy, J. King. Saud. Univ. Sci. (2017). [36] F.Z. Dai, J. Geng, W.S. Tan, X.D. Ren, J.Z. Lu, S. Huang, Friction and wear on laser textured Ti6Al4V surface subjected to laser shock peening with contacting foil, Opt. Laser Technol. 103 (2018) 142–150. [37] U. Sánchez-Santana, C. Rubio-González, G. GomezRosas, J.L. Ocaña, C. Molpeceres, J. Porro, M. Morales, Wear and friction of 6061-T6 aluminum alloy treated by laser shock processing, Wear 260 (2006) 847–854. [38] P. Peyre, C. Carboni, P. Forget, G. Beranger, C. Lemaitre, D. Stuart, Influence of thermal and mechanical surface modifications induced by laser shock processing on the initiation of corrosion pits in 316L stainless steel, J. Mater. Sci. 42 (2007) 6866–6877. [39] X.L. Wei, C. Zhang, X. Ling, Effects of laser shock processing on corrosion resistance of AISI 304 stainless steel in acid chloride solution, J. Alloys Compd. 723 (2017) 237–242. [40] S. Prabhakaran, H.G. Prashantha Kumar, S. Kalainathan, et al., Laser shock peening modified surface texturing, microstructure and mechanical properties of graphene dispersion strengthened aluminium nanocomposites, Surf. Interfaces. 14 (2019) 127–137. [41] J.S. Hoppius, L.M. Kukreja, M. Knyazeva, et al., On femtosecond laser shock peening of stainless steel AISI 316, Appl. Surf. Sci. 435 (2018) 1120–1124. [42] W. Guo, R.J. Sun, B.W. Song, et al., Laser shock peening of laser additive manufactured Ti6Al4V titanium alloy, Surf. Coat. Technol. 349 (2018) 503–510. [43] Y. Shadangi, K. Chattopadhyay, S.B. Rai, V. Singh, Effect of LASER shock peening on microstructure, mechanical properties and corrosion behavior of interstitial free stee, Surf. Coat. Technol. 280 (2015) 216–224.

12