Laser shock peening Fe-based coatings for enhancing wear and corrosion resistance

Materials and experimental details

U71Mn steel with size of 100 mm × 100 mm × 6 mm cut from the discarded tracks was used as the substrates. The composition of U71Mn steel alloy is listed in Table 1. Before laser cladding, the substrates were ground by 600 #, 800 #, 1200 #, 1500 #, and 2000 # SiC sandpapers to remove the oxide layer, and then polished to obtain smooth surfaces. The samples were then cleaned with acetone and alcohol using an ultrasonic cleaner, respectively.

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Table 1

Nominal chemical components of U71Mn steel used in present work.

Element	C	Si	Mn	P	S	V	Nb	Fe
wt-%	0.65∼0.76	0.15∼0.35	1.10∼1.40	≤0.030	≤0.030	0.030	≤0.010	balance

Fe-based 43X powder (WEL TRADING Co. Ltd, China) with spherical morphology and particle size of 53–150 µm was pre-pasted on the fresh surface of U71Mn steel coupons. The thickness of coating is about 200 µm. The chemical components of Fe-based 43X powder are listed in Table 2. The coated coupons were then dried in an oven for 12 h at a temperature of 55 °C and then transferred on the platform for laser cladding process.

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Table 2

Chemical components of Fe-based 43X powder.

Element	C	Si	Cr	Mn	Ni	Mo	B	Fe
wt-%	∼0.14	∼1.3	∼15	∼0.5	∼4.2	∼1.5	∼0.8	balance

A JHM-1GY-700 pulsed YAG laser was used to carry out laser cladding process. During laser cladding process, pure nitrogen gas was introduced into the melting pool to protect the coating from oxidation. Based on our previous work, laser cladding parameters used in laser cladding are set as frequency of 20 Hz, pulse width of 1.8 ms, diameter of laser beam of 0.3 mm, laser power of 312 W, and current of 300 A. The laser power density I₀ is decided using the following equation [36], (1) where E is laser pulse energy of 5 J, d is the diameter of laser beam of 2.5 mm, and τ is the pulse width of 14 ns. Accordingly, the laser power density used in present work is 3.15 × 10⁹ W m⁻².

A SGR-Extra type pulsed nanosecond YAG laser was used to perform laser shock peening. To investigate the effect of impact times and laser power density on the performances of the coatings, two laser beam diameter corresponding to two power density and three impact times were used based on our orthogonal experiments. The detailed parameters for laser shock peening are listed in Table 3. The laser beam path for LSP is illustrated in Figure 1 using the dots represent the positions at which the stress was measured, and the lines with arrow illustrate the moving direction of laser beam. For convenience, U71Mn steel used for control, the coating before LSP, the coating impacted once with lower power density of 7.3 GW cm⁻², impacted once, twice, and three times with higher power density of 11.4 GW cm⁻² were denoted as Cp-U71Mn, coating A, B, C, D, and E, respectively.

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Figure 1

The laser beam path of laser shock peening. The green dots illustrate the locations to measure residual stress.

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Table 3

The parameters for laser shock peening.

Pulse energy (J)	5
Pulse width (ns)	14
Spot overlap rate (%)	50
Laser diameter (mm)	2, 2.5
Powerdensity (GW cm−2)	7.3, 11.4
Impact times	1, 2, 3

All of the samples were cut into small samples with dimensions of 10 mm × 10 mm × 6 mm for phase examination, microstructure observation, and wear and corrosion tests. Before microstructure observation, the cross-sectional view was obtained, polished, and etched with a certain etchant (HF: HNO₃: H₂O = 1:2:7, volume ratio).

Characterisation methods

X-ray diffraction (XRD) patterns (using TD-3500 X-ray diffractometer with Cu-Kα radiation)) were applied for phase analysis. The diffractometer was operated at voltage of 30 kV, current of 20 mA, scanning speed of 4.67° min⁻¹. 2θ angle ranged from 30° to 90°.

GX71 Optical Microscope (OM) was used to observe the microstructure of the coatings before and after LSP. Surface roughness was measured using OLS3100 Laser Confocal Microscope. The measurement direction is parallel to laser beam scanning direction as the line with red arrow illustrated in Figure 5. Six locations were randomly selected on the surface of the coatings to get the average values and error.

The residual stress was measured using PROTO residual stress tester with Cr-Kα radiation (wavelength of 1.541838 Å). The diameter of X-ray spot was 1 mm. The diffraction plane of (211) and Bragg angle 2θ of 156.41° were used. Four scanning angles corresponding to 0°, 15°, 30°, and 45° were used in present work. Six different locations as illustrated in Figure 1 with green dots were examined to get the average value of the residual stress. The modulus of 210 GPa corresponding to U71Mn steel was used for residual stress testing, because the composition of the coatings used in the present work was similar to that of the U71Mn steel.

Vickers microhardness of the coatings was measured along the cross-sectional depth by a Vickers indenter (Tukon1102) at load of 50 g and a dwell time of 10 s. Five different locations were measured for each sample. The wear resistance of the coatings for each sample before and after LSP against Al₂O₃ balls was carried out by a tribometer (NANOVEA) at a normal load of 10 N. Sliding distance and friction time were 54 and 60 min, respectively. The coefficient of friction (COF) was recorded, the width and depth of the worn scars were measured three times using laser scanning interferometer to calculate the volume wear rate according to Archard equation [37,38]. (2) where k is the wear rate in unit of mm³/(m N), V is the volume of the worn scar in unit of mm³, F_n is the normal load in unit of N, and d is the sliding distance in unit of m.

Corrosion resistance of the coatings was measured by using Potantio dynamic polarisation test that was recorded in 3.5 wt-% sodium chloride solution with PH of 7.2 at room temperature with a scan rate of 0.01 V s⁻¹. Three parallel samples were measured for each group. The samples, the platinum electrode, and the calomel electrode were used as working electrode, the counter electrode, and the reference electrode, respectively. The parameters for electrochemical test are set as sensitivity of 10⁻³, rest time of 2 s, and scanning potential from −2 to 2 V.

Phase identification

XRD patterns of all processed samples are shown in Figure 2. As illustrated in the figure, the constituents of the coatings before and after LSP processing can be characterised as martensite phase α′-Fe corresponding to diffraction peaks at 44.8°, 63.3°, and 80.9° (PDF card # 87-0721), austenite phase γ-Fe corresponding to diffraction peak at 50.8° (PDF card # 52-0513). Besides, typical diffraction peaks for CrFe₄ at 65.0°, and 82.4°, and those for Fe₁₉Ni at 65.0° and 82.3° were also detected, illustrating the reaction between Cr and Fe, Ni and Fe in the melting pool during laser cladding process. It was observed that the peaks corresponding to α′-Fe at 63.3° and 80.9° weakened and disappeared after LSP treatment. Austenite phase γ-Fe founded in all coatings can be attributed to the existence of retained austenite during the martensitic transformation process during laser cladding. The results also agreed with the investigation by Sun and his coauthors [39]. It has been approved that the existence of retained austenite is benefit for improving the wear resistance of materials [40].

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Figure 2

XRD patterns for 43X coatings: A – before LSP, B – impact once with 7.3 GW cm⁻², C–E – impact once, twice, and three times with 11.4 GW cm⁻², respectively.

The penetration depth of Cu radiation in Fe-based 43X alloy could be given by the following equation [41]. (3) where t₀ is the penetration depth, µ_m is a constant, which is approximately 308 for Copper radiation, ρ is the mass density of Fe based 43X alloy, which is 4.42 g cm⁻³.

By substituting the above parameters, we get the penetration depth is approximately 7.35 µm. Actually, due to the significant fluorenscence effect between Copper radiation and Fe element in the coatings, the penetration depth with 66% absorption of incident beam might be less than 1 µm. As shown in Figure 3, the thickness of the coating was about 200 µm. It was noted that only 0.5% (volume proportion) of the treated layer was analyzed. Owing to the temperature field distribution along the depth of the coating during laser cladding process, phase constituents in different depth of the coatings maybe different. However, in the present work, we mainly focused on the influences of microstructure and residual stress of the coatings subjected to LSP on the wear and corrosion resistance. Hence, we did not consider the difference of the phase constituents in depth.

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Figure 3

The cross-sectional microstructure of coatings (a) without LSP, (b) impact once with lower laser power density of 7.3 GW cm⁻², (c)–(e) impact once, twice, and three times with higher laser power density of 11.4 GW cm⁻², respectively.

Microstructure and residual stress

The cross-sectional microstructures of the coating A – coating E were illustrated in Figure 3. No cracks and porosity were observed in all coatings, and a sound metallurgical bonding was formed between the coatings and the substrates in each sample. The depth of each layer subjected to LSP was also indicated in Figure 3. The microstructure of the coatings could be characterised as typical fine dendrite due to higher cooling rate during laser cladding process. The microstructure corresponding to the top, middle, and bottom of the coatings before and after LSP with different laser power density and impact times were illustrated in Figures 3(a–e), respectively. Comparing the microstructure of the coating after LSP with that before LSP (Figure 3(a)), it was evident that after LSP treatment the microstructures in the top of the coatings were refined. Figure 3(b) is the microstructure corresponding to the coating which was impacted once with lower power density of 7.3 GW cm⁻². The deformation depth induced by LSP is about 30 µm. The microstructure in the top of the coating can be characterised as columnar crystal and refined dendrites. The intensity of laser shock wave decreased rapidly due to the absorption and reflection, no obvious deformation was observed in the middle and bottom of the coating. Hence, the characteristic of martensite microstructure was observed in the middle and bottom of the coating. It was clear that the dense needle-like structure was formed at the bottom of the coating due to a higher G/R during laser cladding process. Compared with the coating's impact once with lower laser power density (Figure 3(b)), the coating impact once with higher laser power density of 11.4 GW cm⁻² presented a similar deformation depth of 32 µm as illustrated in Figure 3(c). It demonstrated that the laser power density had slight influence on the microstructure when the coating was just impacted once. On a contrary, both the depth and the microstructure of the coatings impacted twice and three times with laser power density of 11.4 GW cm⁻² changed significantly (Figures 3(d,e)). The deformation depth for the coatings impacted twice and three times was 50 µm and 60 µm, respectively. Meanwhile, the grain in the top of the coatings was refined compared with those in the coatings before LSP and impacted once. The microstructure can be characterised as refined isometric crystal and columnar crystal. The results demonstrated that a higher laser power density and more impact times are favourable for enhancing the microstructure and in turn improving the performances of the laser cladded coating.

The residual stress produced by laser cladding technology is one of the factors that leads to the crack propagation and failure of the coatings. Hence, it is essential to value the stress in the coatings and the influence of LSP on reducing the residual stress. The residual stress produced in the coatings before and after LSP treatment is shown in Figure 4. The residual stress in the coating before LSP is 116.30 ± 7.25 MPa (Figure 4(A)), indicating that a large residual tensile stress was produced during laser cladding process. After impact once with a lower power density, the residual stress decreased to 111.99 ± 8.91 MPa (Figure 4(B)), which is slightly lower than that of the coating without LSP. However, as the power density increased to 11.4 GW cm⁻², the residual stress significantly decreased to 57.45 ± 6.36 MPa (Figure 4(C)) which is approximately half of coating B, illustrating that a higher power density is benefit to relieve stress. Although the residual stress decreased significantly after being impacted once whatever with lower density or higher density, they still presented residual tensile stress. It is well known that the performances of the coatings depend on the value of residual stress. A compressive stress is desired for improving the properties of the coating. The residual stress of the coatings impacted twice and three times with a higher power density of 11.4 GW cm⁻² were shown in Figures 4(D,E). It is evident that the residual tensile stress of the coatings impacted twice and three times decreased to −83.21 ± 7.14 MPa (Figure 4(D)) and even −309.77 ± 8.25 MPa (Figure 4(E)). The negative value means the stress presented as a compressive stress. This illustrated that the residual tensile stress transferred to a compressive stress when the coating was peened with a higher laser power density and more impact times. The transformation of the residual stress can be attributed to the higher laser shock wave pressure produced by higher laser power density and more impact times. The studies in references also proved that the residual stress increased with increasing of laser power density and impact times [42-44].

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Figure 4

Surface residual stress of the coatings before and after LSP with different parameters. The stress was tested along the laser scanning direction as shown in Figure 1.

Surface morphology and surface roughness

The surface morphology and roughness of the coatings before and after LSP are shown in Figure 5. As illustrated in Figures 5(a–e), no cracks were found on the surface of the coatings. No obvious differences in the morphologies for different coatings were observed. For further explanation, the laser scanning confocal microscope was used to measure the roughness of the coatings. Six different locations were examined randomly. The average value and standard deviation of the surface roughness were calculated and illustrated in Figure 5(f).

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Figure 5

Surface morphology of the coatings (a) without LSP, (b) impact once with 7.3 GW cm⁻², (c)–(e) impact once, twice, and three times with 11.4 GW cm⁻², respectively, and (f) surface roughness. The arrow in the figures illustrate the measurement direction for surface roughness.

The surface roughness of 43X coatings before and after LSPwas around 37.6 ± 2.0, 24.5 ± 2.2, 21.9 ± 2.0, 19.3 ± 2.7, and 13.3 ± 2.7 µm, respectively. It should be noted that the finished surface of the laser cladding coatings was not polished before residual stress tests. Because any treatment can destroy the surface roughness and result in additional stress. Hence, the roughness of the coatings is a little higher. The higher surface roughness may lead to the reduction of the residual stress. Hence, the actual residual stresses in the coatings may be larger than the values illustrated in Figure 4. In the present work, we just focused on the influence of laser shock peening on the surface roughness and residual stress transformation. We will study the detailed relation between surface roughness and residual stress in our future work.

It was obvious that the surface roughness corresponding to the coatings after LSP was significantly lower than that of the coating before LSP. Meanwhile, the surface roughness of the coating peened with a lower power density (7.3 GW cm⁻²) is higher than that of the coating peened with a higher laser power density (11.4 GW cm⁻²) as they were impacted once as illustrated in Figure 5(f) B and C. It can be contributed to the stronger deformation due to the higher shock wave pressure produced by a higher laser power density. Comparing C, D, and E in Figure 5(f), it is clear that the surface roughness decreased with increasing of the impact times peened with a higher laser power density of 11.4 GW cm⁻². It can be concluded that a higher laser power density or more impact times result in a higher deformation of the coating, and in turn contribute a lower surface roughness.

Microhardness and wear resistance performance

The cross-sectional microhardness distribution of the coatings before and after LSP treatment was shown in Figure 6(a). The average microhardness for each coating within the depth of 160 µm is 797, 847, 921, 943, and 932 HV, respectively. A significant increase in microhardness was observed after LSP treatment. The high-pressure shock wave caused by a short pulse and a higher power density strengthened the material surface. Moreover, the sample impacted once with a higher power density presented a higher microhardness. For the coatings peened with a higher laser power density, the microhardness increased with increasing of the impact times. The improvement in the microhardness for the coatings can be attributed to the effect of grain refinement due to plastic deformation produced by LSP. The higher microhardness in turn results in a lower wear rate and better wear resistance performance. As shown in Figure 6(b), after LSP treatment, the COF also presented a decreasing trend with increasing of laser power density or impact times. The average COF for U71Mn substrate, 43X coating before LSP, impact once with lower power density, impact once, twice, and three times with higher power density was around 0.24, 0.23, 0.23, 0.20, 0.18, and 0.15 respectively. The reduction in COF might be associated with the surface roughness. As shown in Figure 5(f), the surface roughness decreased with increasing of laser power density and impact times. A smoother surface results in a lower COF and better wear resistance. As illustrated in Figure 6(c), compared with the wear rate for U71Mn steel substrate (9.2 × 10⁻⁵ mm³ N m⁻¹), that for the coatings before and after LSP treatment significantly decreased to 3.6 × 10⁻⁵, 2.4 × 10⁻⁵, 2.4 × 10⁻⁵, 2.2 × 10⁻⁵, and 1.5 × 10⁻⁵ mm³ N m⁻¹, respectively. Especially the coating impacted three times with a higher laser power density exibits the lowest wear rate. The excellent wear resistance of the coatings after LSP can be attributed to the existence of retained austenite as proved by Yang [45].

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Figure 6

Microhardness distribution (a), average COF (b), and volume wear rate (c) for each coating.

Figure 7(a–f) illustrated the morphologies of the worn surfaces of the U71Mn substrate and coating A – coating E, respectively. More damages of micro scratch and a large amount of friction debris and furrows were observed on worn surface of U71Mn steel substrate (Figure 7(a)). The substrate can be mainly characterised as adhesion wear and abrasive wear. The coating B before LSP and coating C impacted once with lower energy were seriously worn as shown in Figure 7(b,c). The furrow was wider and deeper, and serious plastic deformation occurred on both sides of the furrow along the sliding direction which exhibited abrasive wear. As shown in Figure 7(d–f), less wear debris and almost no furrows on the worn surfaces of the coating C–E was observed, which exhibited abrasive wear characteristics.

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Figure 7

Worn morphologies of (a) U71Mn substrate, (b) 43X coating without LSP, (c) LSP coating impact once with laser power density of 7.3 GW cm⁻², (d)–(e) coatings impact once, twice and three times with laser power density of 11.4 GW cm⁻².

Electrochemical corrosion resistance performance

Corrosion is an important factor to cause failure of the tracks. In order to study the corrosion resistance performance of the coatings, electrochemical corrosion testing was conducted in NaCl solution. Potential dynamic (PD) curves and Nyquist diagrams for the coatings were illustrated in Figure 8. Correspondingly, basic electrochemical parameters including E_corr and I_corr were determined, as shown in Table 4. With respect to E_corr, all coatings, except coating E which was impacted three times with a higher laser power density, showed more positive E_corr than CP-U71Mn steel. Comparing coating B with coating C, it is clear that as the laser power density increased, the E_corr moved towards a more positive direction. For coating B, C, and D, it was observed that the coatings impacted once and twice with a higher laser power density presented a more positive E_corr. However, after being impacted three times (coating E), the E_corr moved towards a more negative direction. The E_corr was even more negative than that of the U71Mn control. Regarding to the corrosion current I_corr, coating A, B, C, and D all demonstrated a significant decrease compared with CP-U71Mn. However, the I_corr of coating E was much higher than that of CP-U71Mn and the other coatings. It can be summarised from Tafel curves that the laser cladding Fe-based 43X coating may cause a positive influence on the corrosion susceptibility of U71Mn steel. Moreover, LSP treatment, especially impacted with a higher laser power density, can significantly improve the corrosion resistance performances of the coatings. However, more impact times may cause a negative influence on the corrosion susceptibility of the coating.

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Figure 8

Potential dynamic (PD) curves (a), and Nyquist diagrams (b): A: 43X coating without LSP, B: coating impact once with laser power density of 7.3 GW cm⁻², C–E: coatings impact once, twice and three times with laser power density of 11.4 GW cm⁻².

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Table 4

Electrochemical data of all samples obtained from polarisation curves.

Data	CP-U71Mn	coating A	coating B	coating C	coating D	coating E
Ecorr (V)	−1.12 ± 0.02	−1.10 ± 0.03	−1.07 ± 0.02	−1.03 ± 0.01	−1.00 ± 0.02	−1.17 ± 0.03
Icorr (µA)	13.71 ± 0.15	9.35 ± 0.10	9.08 ± 0.08	8.87 ± 0.18	4.51 ± 0.12	20.53 ± 0.20

With the purpose of providing a detailed characterisation of electrochemical response, EIS measurements were performed. The Nyquist diagrams of all samples were shown in Figure 8(b). All the coatings and CP-U71Mn steel exhibited a characteristic of single capacitive. The Nyquist diagram could provide information about the impedance characteristics of the electrolyte. In general, a larger arc radius of the capacitive reactance illustrates a greater impedance and better corrosion resistance [46]. Except coating E, coating A, B, C, and D all showed a larger circular arc radius than that of CP-U71Mn. The order of the circular arc radius for the samples can be arranged as D > C > B > A > CP-U71Mn > E. The results further proved that the laser cladded 43X coating may improve the corrosion resistance of U71Mn steel. Meanwhile, laser shock peening with a higher laser power density is desirable for enhancing the corrosion resistance of the coatings, but more impact times may cause a negative effect on corrosion resistance performances of the coatings.