Effect of La₂O₃ on Microstructure and Properties of Laser Cladding SMA Coating on AISI 304 Stainless Steel

Abstract

Known as having a stress self-accommodation characteristic, the laser cladding shape memory alloy (SMA) coatings have been widely used in material failure repair. Nevertheless, their further development is greatly limited by their low microhardness (250 HV_0.2) and corrosion resistance. Benefiting from the capability of refined grain and adjusted microstructure, rare earth oxides play a key role in improving the properties of materials. Herein, to improve the microhardness and anti-corrosion of laser cladding SMA coatings, different amounts of La₂O₃ were doped in SMA coating. The influence of the different La₂O₃ doping amounts on the phases, grain size and microhardness was studied. The anti-corrosion of the SMA/La₂O₃ composite coating was explored in 3.5 wt.% sodium chloride solution. Results showed that the grain of the SMA/La₂O₃ composite coating is significantly refined. When doping with 0.9 wt.%, the refinement rate reaches 19%. Furthermore, based on the Hall–Petch effect, the microhardness of the SMA/La₂O₃ composite coating is increased to 450 HV_0.2. At the same time, the anti-corrosion of the composite coating is enhanced due to the smaller grain size and fewer defects.

1. Introduction

As the conditions of service become more stringent, materials are put forward for higher demands and the problem of material failure becomes more and more severe [1,2]. Based on the advantages of the small heat-affected zone and metallurgical bonding [3,4], laser cladding technology is widely applied in material failure repair [5]. Nevertheless, the residual stress caused by the extremely fast cooling rate will affect the service life. In order to better apply laser cladding technology to material failure repairing, how to further reduce the residual stress has become an urgent problem to be solved [6].

Over the past few decades, extensive studies have been carried out in the field of residual stress release due to its crucial role in practical application [7,8]. Heat treatment was used to reduce the residual stresses by Park [9], and the residual stress was reduced by 50% after a 700 °C treatment. Roehling [10] developed an in situ annealing method to weaken residual stresses in a coating fabricated by LPBF (laser powder bed fusion). The result shows that a residual stress decrease of 90% was achieved in the coating. However, these methods mentioned above have a complex process and high production costs.

Recently, Fe-Mn-Si shape memory alloy (SMA) has attracted more and more attention owing to its stress self-accommodation characteristic. Xu et al. [11] fabricated Fe-Mn-Si-Ni-Cr SMA coating using laser cladding technology, which greatly weakens the residual stress of the coating. Ferretto et al. [12] prepared Fe17Mn5Si10Cr4Ni SMA parts by LPBF for the first time, which further promotes the development of SMA. However, the anti-corrosion and lower microhardness (only 250 HV_0.2) are two key factors that limit its industrial application. To strengthen the performance of SMA coating, PZT, Ti, WC and Nb were used for modification. The corrosion resistance and microhardness were enhanced significantly. However, there is still a certain gap from the practical application [13,14].

Benefiting from their good chemical and physical properties as well as grain refinement, rare earth elements, honored as industrial vitamins, play a vital role in improving the microhardness, wear resistance and anti-corrosion of materials [15,16]. Rare earth elements can purify molten steel, improve inclusions and refine grains, which is an effective method for adjusting the microstructure and enhancing the mechanical properties of steel. Du et al. [17] prepared Y₂O₃-doped, WC-reinforced coating via laser cladding technology. The result showed that the addition of Y₂O₃ promoted the metallurgical bonding between the coating and the substrate, and also improved the weldability and laser absorption rate of Invar alloys. By adding different amounts of La₂O₃, Y₂O₃ and CeO₂, the grains of Ni60 coating were significantly refined and the microhardness was significantly improved [18]. Liu et al. [19] doped different amounts of CeO₂ in TiC/Ti₂Ni coating. The result showed that the microhardness and wear resistance of the composite coating was greatly enhanced. Therefore, rare earth element alloying is an effective method to improve the properties of materials.

To further strengthen the properties of Fe-Mn-Si shape memory alloy composite coating, A FeMnSiCrNiNb/(La₂O₃)_{x (x = 0.3, 0.6, 0.9, 1.2 wt.%)} composite coating was prepared via laser cladding technology. The XRD, Vickers hardness tester, the MFT-5000 Tribometer and electrochemical workstation were applied to explore the phases, microhardness and anti-corrosion of coatings.

2. Materials and Methods

2.1. Experimental Materials and Design

We used AISI 304 stainless steel (SS) as the matrix. The oxide was removed before laser cladding. The composition of AISI 304 SS and experimental powder (50–100 μm) were shown in

Table 1. The composition of matrix and cladding powder.

Coatings	Fe (wt.%)	Ni (wt.%)	Mn (wt.%)	Cr (wt.%)	Si (wt.%)	Nb (wt.%)	La2O3 (wt.%)
304 SS	Bal	8–15	≤2.00	18–20	≤1.00	0	0
SMA	49.4	2.85	30.4	3.8	8.55	5	0
SMA/La2O3	49.4	2.85	30.4	3.8	8.55	5	0.3
	49.4	2.85	30.4	3.8	8.55	5	0.6
	49.4	2.85	30.4	3.8	8.55	5	0.9
	49.4	2.85	30.4	3.8	8.55	5	1.2

. The experimental powder was ball-milled (QM-3SP2) for 2 h and then dried. The fiber laser system was used for laser cladding experiment (YLS6000; IPG Photonics Corporation, KUKA, Germany). The parameters: the pre-sintered powder thickness was 1.5 mm, the laser power (P) was 2000 W at a scanning rate of 6 mm/s. An experimental schematic diagram and powder preparation flow chart are shown in Figure 1.

2.2. Sample Characterization

The coatings were cut into cubes by wire electrical discharge machining (7 mm × 7 mm × 7 mm) and polished with a polishing compound (Shanghai Yunbo Testing Technology, Shanghai, China). X-ray diffraction (XRD, DX-2700B, HAOYUAN Instrument, Dandong, China) was applied to investigate the phases, and the 2θ varied from 10° to 90°. The microhardness and wear resistance of the coating were tested using the HV-50 Vickers microhardness tester (AICEYI Opto-electronic Technology, HongKong, China) and the MFT-5000 Tribometer (the reciprocating distance is 4 mm, the test time is 15 min and the load is 10 N). Scanning electron microscopy (SEM, COXEM, EM30, Daejeon, Korea) was used to analyze the surface microstructure. Elemental analysis of the samples was performed using Energy-dispersive X-ray spectroscopy (EDS, Oxford instruments, Oxford, UK).

The polarization plots were measured by the three-electrode system of a electrochemical workstation (CHI660D; CH Company, Ltd. Shanghai, China) in 3.5 wt.% sodium chloride solution (macklin, AR, Shanghai, China) at 25 °C. The Pt electrode and saturated calomel electrode were used as a counter electrode (the area of Pt is 1 cm²) and reference electrode, respectively. Voltage ranged from −2 V to 2 V, scan rate was 0.01 V/s and the sample exposure area was 0.49 cm². The electrochemical impedance spectroscopy (EIS) was used to test the impedance of the coating. The frequency ranged from 10⁰ to 10⁶ HZ and the amplitude was 0.005 V [20]. ZView software (version: 3.1) was used to simulate the equivalent circuit.

3. Results and Discussion

3.1. Phases Analysis

The XRD patterns of La₂O₃ doped with different amounts are shown in Figure 2. There is no obvious difference in the XRD patterns, all of which are composed of an Nb phase, thermoelastic ε-martensite, non-thermoelastic α’-martensite and γ-austenite phases. The presence of thermoelastic ε-martensite proves that γ-austenite transformation occurs in the SMA coating, which is the characteristic of SMAs [21,22]. Furthermore, thermoelastic ε-martensite is also key to the shape memory effect. Meanwhile, the γ-austenites of the composite coatings are all shifted to high angles, which indicates that the interplanar spacing d decreases based on the Bragg equation (2dsinθ = nλ).

In addition, Figure 3 exhibits a lot of white area on the surface (white circle), and EDS analysis indicates that the Nb phase is precipitated in this area, which is consistent with the XRD pattern [23].

3.2. Microstructure

Figure 4a shows that a large number of columnar crystals were observed in SMA coatings. After doping with La₂O₃, the grains of the coating were significantly refined, mainly forming cellular crystals, as shown in Figure 4b,e. This is because the rare earth oxides act as heterogeneous nucleation sites, increase the probability of heterogeneous nucleation, and refine grains [24,25]. Figure 5 shows the average grain size of the coatings. The average grain size of the SMA coating reached 3.72 μm, and the grain was gradually refined with La₂O₃ doping. Especially when doping La₂O₃ with 0.9 wt.%, the average grain size decreased to 3.03 μm, and the refinement rate reached 19%. However, when doped 1.2 wt.%, the grain size increased to 3.12 μm. This is attributed to the segregation of excess La₂O₃ at the grain boundaries, weakening the refining effect and increasing the average grain size [26,27].

In addition, as shown in Figure 6 [28,29], plenty of eutectic microstructures containing Fe, Si, Mn and Nb elements were also formed on the grain boundary of the SMA/La₂O₃ composite coating, as shown in

Table 2. The composition of eutectic microstructures.

Elements	Fe	Mn	Si	Cr	Ni	Nb	C	O	La
wt.%	42.02	20.25	7.83	2.37	2.55	19.65	3.35	1.76	0.22

.

3.3. Microhardness and Wear Resistance

The microhardness changes in the cross-section are shown in Figure 7. After doping with La₂O₃, the microhardness of the SMA/La₂O₃ composite coating was enhanced. When doped with 0.9 wt.%, the microhardness reached 450 HV_0.2. The grain size of the composite coating was obviously refined, and the grain distribution changed from the columnar crystal to the cellular crystal, as shown in Figure 4. Based on the Hall–Petch formula [30,31], the smaller grain size shows the higher grain interface in the unit volume. The grain boundaries possess higher dislocation density due to the atoms being arranged randomly, resulting in the dislocation tangling and the resistance of movement increasing significantly. Thus, the microhardness of the composite coating is strengthened.

$${\sigma }_d={\sigma }_0+k{d}^{-\frac{1}{2}}$$

(1)

where σ₀ (MPa) is internal friction stress; k is a material constant related to the stress concentration required to activate slip dislocations; d (mm) is the average grain size of the material.

The friction coefficient can be used to estimate the anti-wear. The smaller the friction coefficient is, the better the wear resistance of the coating is. Figure 8a shows that the friction coefficient decreased significantly after doping La₂O₃. When doping with 0.9 wt.%, the friction coefficient was the smallest, at only about 0.3. With continued doping, the friction coefficient increased slightly. The friction coefficient and microhardness are consistent with Archard’s tribology theory [32]. Wear resistance is further analyzed by wear volume loss. Figure 8b shows that the wear volume loss is only 0.15 mm³ at 0.9 wt.%, much lower than that of SMA coating. It is worth noting that the microhardness and friction coefficient of the coating are slightly improved when the doping is 1.2 wt.%. Apparently, Figure 5 shows that the average grain size of the SMA/(La₂O₃)_{1.2 wt.%} composite coating reached 3.12 μm, and some columnar crystals appeared (the circle of Figure 6d). According to the Hall–Petch formula, the microhardness and friction coefficient are slightly reduced [33].

As shown in Figure 9. the wear width of SMA coating reaches 956 μm, while the width of SMA/(La₂O₃)_{0.9 wt.%} composite coating is only 867 μm, indicating that the composite coating has better wear resistance. Furthermore, plenty of furrow marks, which are caused by the falling off of particles on the surface and continued wear of the surface, were observed in both SMA coatings, which is called abrasive wear. However, the wear scars’ depth of the SMA coating is significantly deeper than that of the SMA/La₂O₃ composite coating (the arrow of Figure 9b) owing to the lower microhardness. Moreover, a large area of spalling on the surface was observed in Figure 9b, which is typical of adhesion wear, which is a more serious wear form than abrasive wear. In contrast, the wear marks of the SMA/La₂O₃ composite coating are smoother and flatter. Although there is some spalling, the area is smaller compared with the SMA coating [34,35].

3.4. Corrosion Resistance

The electrochemical impedance spectroscopy (EIS) and polarization curve were tested to further characterize the anti-corrosion of the coatings [36]. In general, the smaller the self-corrosion current (I_corr) is, the smaller the corrosion rate is, and the greater the self-corrosion voltage (E_corr) is, the greater the anti-corrosion of the coating is [37,38].

Figure 10a shows the polarization curves of samples doped with different amounts of La₂O₃. After doping La₂O₃, the E_corr of the composite coatings was significantly improved. When doping with 0.6 wt.%, the E_corr of the coating reached −0.64 V, while the I_corr was the smallest, at only 4.556 × 10⁻⁶. With further doping, E_corr showed a reducing trend, as shown in

Table 3. EIS parameters.

wt.%	Ecorr	Icorr	Rate (gram/hr)	Rate (mil/year)
0	−0.81	4.733 × 10−5	3.296 × 10−5	29.52
0.3	−0.67	4.710 × 10−6	3.280 × 10−6	2.937
0.6	−0.64	4.556 × 10−6	3.173 × 10−6	2.842
0.9	−0.65	1.462 × 10−5	1.018 × 10−5	9.118
1.2	−0.72	1.345 × 10−5	9.367 × 10−6	8.388

. Moreover, the corrosion rates (gram/hr and mil/year) of composite coatings were obviously lower than that of the SMA coating. In particular, when doped with 0.6 wt.%, the corrosion rate of the composite coating decreased by an order of magnitude (3.173 × 10⁻⁶/3.296 × 10⁻⁵ and 29.52/2.842) compared to the SMA coating [39].

The Nyquist plots of the coatings reflect the resistance of the coatings. Z′ and Z″ are the real and virtual parts of the impedance, respectively. The |Z| total impedance modulus is shown as Equation (2) [40]. A larger radius shows greater anti-corrosion [41,42]. Obviously, the radius of the SMA/La₂O₃ composite coating is higher than that of the SMA coating. When doped with 0.6 wt.%, the radius of coating resistance reaches the maximum, as shown in Figure 10b. There was no obvious change in radius as doping continued.

$$\left|Z\right|=\sqrt{Z^{\prime 2}+Z^{″2}}$$

(2)

Figure 10c shows the bode plot of the coatings. |Z| can be applied to evaluate the anti-corrosion of the coatings in low frequency. When the doping amount exceeds 0.6 wt.%, the |Z| of the composite coating is higher than that of the SMA coating from high frequency to low frequency, indicating that doping La₂O₃ is an effective method to improve corrosion resistance. Figure 10d shows the frequency–phase plot. Obviously, there is only one wave peak, indicating that coating has only one time constant.

It is generally believed [9,10] that the grain refinement by conventional methods will increase the surface defects in the alloy and reduce the anti-corrosion. On the contrary, the addition of rare earth oxides (La₂O₃) in the alloy can not only refine the grain but also generate inclusions with impurity elements S and Al, which can purify the microstructure and improve the anti-corrosion. The protection rate of the composite coating was determined according to Equation (3) [43].

$$\eta =\frac{i_{corr}^0-i_{corr}}{i_{corr}^0}$$

(3)

where i_corr and i⁰_corr represent the I_corr of the composite coating and original coating, respectively. According to Equation (3), when doping with 0.6 wt.%, the coating protection rate increased by 20%.

In order to more accurately describe the anti-corrosion, the equivalent circuit was used to simulate EIS parameters. As shown in Figure 11, the EIS data are shown in

Table 4. EIS parameters.

Parameters	Rs (Ω·cm2)	Cf (F cm−2)	CPE1 (Ω−1 cm−2 sn)		Rf (Ω·cm2)	Cdl (F cm−2)	CPE2 (Ω−1 cm−2 sn)		Rct (Ω·cm2)
			Q	n			Q	n
0	1.323	2.06 × 10−7	2.34 × 10−7	0.99	15.01	2.33 × 10−3	1.10 × 10−4	0.7557	1603
0.3	1.640	5.94 × 10−8	8.86 × 10−8	0.97	28.09	2.67 × 10−4	2.00 × 10−4	0.6884	9465
0.6	1.505	8.72 × 10−8	1.13 × 10−7	0.98	26.67	4.57 × 10−5	6.65 × 10−5	0.7670	4366
0.9	1.376	9.67 × 10−8	1.62 × 10−7	0.96	25.52	4.55 × 10−5	6.44 × 10−5	0.7953	4010
1.2	1.512	8.50 × 10−8	1.10 × 10−7	0.98	28.63	5.10 × 10−5	7.08 × 10−5	0.7548	5152

. Here, Rct, Rs, Rf and CPE represent the film resistance charge transfer resistance, solution resistance, coating resistance and constant phase element, respectively. The CPE is applied to compensate the non-uniformity of the system instead of capacitors. The CPE is defined using the equation of Z_CPE = [Y(jω)n]⁻¹, where n represents the dimensionless index, ω represents the frequency and Y represents the CPE constant. If n = 1, the CPE is the equivalent capacitor; if n = 0, the CPE is the equivalent resistance [44].

The capacitance (C) of the coating is calculated by resistance R and CPE (Q), respectively [45]:

$$C=\frac{{\left(QR\right)}^{\frac{1}{n}}}{R}$$

(4)

Rs of all coatings are lower 2 Ω, which will not affect the results. Benefiting from the grain refinement, the Cdl of the composite coating decreased by 1–2 orders of magnitude, and the Rct increased by 3 to 6 times compared with SMA coating. In particular, the SMA/(La₂O₃)_{0.9 wt.%} composite coating processes the minimum Cdl and maximum n value, proving that the corrosion of the composite coating was more difficult. When doping with 1.2 wt.%, the Cdl gradually increased and n decreased. This is consistent with the previous results.

In summary, the anti-corrosion of the SMA/La₂O₃ composite coating is higher than that of the SMA coating.

4. Conclusions

In this paper, an SMA/La₂O₃ composite coating was prepared on AISI 304 SS using laser cladding technology so as to improve the microhardness and anti-corrosion of an SMA coating. The result shows that the phases of all coatings are composed of an Nb phase, thermoelastic ε-martensite, non-thermoelastic α’-martensite and γ-austenite phases. The presence of thermoelastic ε-martensite proves that γ-austenite transformation occurs in the SMA coatings, which is the characteristic of SMAs, and it is also the key to the shape memory effect. The advantage of using rare earth oxide as a heterogeneous nucleation site can increase the probability of heterogeneous nucleation and refine the grain. Based on the Hall–Petch formula, The smaller the grain size, the higher the microhardness. When doping with 0.9 wt.%, the average grain size of the composite coating decreases to only 3.03 μm, and the refinement rate reaches 19%, which greatly improves the microhardness and anti-wear of the coating. Due to the low microhardness of the SMA coating, the surface has a large area of peeling, and the wear pit is larger and deeper after friction testing, corresponding to abrasive wear and adhesive wear. On the contrary, the surface of the composite coating has only abrasive wear, and the wear scars are more shallow.

The E_corr and I_corr of the composite coating are significantly improved; this is attributed to the addition of La₂O₃ in the alloy which can not only refine the grain but also generate inclusions with impurity elements S and Al, which can purify the microstructure and improve the anti-corrosion. Compared with the SMA coating, the Cdl of the composite coating decreases by 1–2 orders of magnitude and the Rct increases by 3–6 times, indicating that the corrosion of the coating is more difficult.