Multiscale Synergistic Strengthening-Toughening Mechanisms in Lanthanum Oxide-Modified Coiled Tubing Welding Wire Deposited Metal

Abstract

With the increasingly demanding service conditions of coiled tubing, its welded joints require superior synergistic strength-toughness properties to meet comprehensive mechanical performance requirements. This study achieved synergistic optimization of strength and toughness in deposited metal via lanthanum microalloying technology and elucidated microstructural evolution mechanisms and fracture failure mechanisms via multi-scale characterization techniques. The results demonstrate that lanthanum oxide addition effectively modifies inclusion characteristics, inducing phase transformation from O-Mn-Si-Al-Ti to O-Mn-Si-Al-Ti-S-La, with average particle size significantly decreased from 0.19 μm to 0.12 μm. The deposited metal microstructure comprises lath bainite and granular bainite. The addition of 0.5 wt.% lanthanum oxide results in significant microstructural refinement: average grain size decreases from 1.16 ± 1.18 μm to 1.02 ± 1.00 μm, while granular bainite volume fraction decreases from 8.6% to 4.7%. The microstructural optimization also enhances mechanical properties substantially: yield strength increases from 628 ± 14 MPa to 673 ± 12 MPa, and impact toughness improves from 160 ± 6 J to 189 ± 6 J. Mechanistic analysis revealed that proper addition of lanthanum (0.5 wt.%) promotes grain refinement via heterogeneous nucleation and modifies inclusion morphology, effectively inhibiting crack initiation. However, excessive addition (1.0 wt.%) induces inclusion clustering, forming stress concentration sites that degrade mechanical properties.

1. Introduction

Coiled tubing (CT), as a novel oil and gas pipeline material, offers significant advantages such as extended length, low cost, and short operational cycles [1,2]. It is widely utilized in various applications, including drilling, well completion, workover, sand control, well flushing, heavy oil recovery, and shale gas development, demonstrating broad application prospects [3,4,5]. Welding constitutes a critical process during CT manufacturing and field operations, exerting substantial impacts on product performance and service life. Particularly under actual service conditions where CT sustains complex loading scenarios involving tensile stresses and impact forces, stringent requirements are imposed on the comprehensive performance of welded joints [4,6].

Current optimization strategies for welded joint performance primarily focus on two key aspects: chemical composition refinement and morphology control of non-metallic inclusions [7]. Given the inherent difficulty in completely eliminating inclusion formation through welding processes, performance enhancement through morphology regulation of inclusions has emerged as a more viable technical approach. Research demonstrates that while inclusions in steel inherently compromise matrix continuity, their detrimental effects can be mitigated when particle sizes are controlled below 1 μm [7]. Notably, such micro-scale inclusions can activate the oxide metallurgy effect and serve as heterogeneous nucleation sites to effectively refine the size of microstructure grains at the weld joint, thereby enhancing welded joint performance [8,9,10]. However, conventional oxide metallurgy methods frequently encounter challenges of significant inclusion size variations and non-uniform distributions, often failing to meet practical requirements. This necessitates the introduction of additional alloying elements to optimize both inclusion size distribution and spatial arrangement.

Jiang et al. [8] demonstrated that inclusions ranging from 0.6 to 0.8 μm are more effective than those sized 0.3 to 0.5 μm in promoting the nucleation of acicular ferrite in steel, thereby enhancing the comprehensive material properties. Chen et al. [11] introduced the rare earth element cerium (Ce) into self-shielded welding wires, where the addition of Ce-containing inclusions strengthened heterogeneous nucleation. This process achieved grain refinement and significantly improved the impact toughness of welded joints. Yu et al. [12] further revealed that Ce-induced inclusions elevated impact toughness by facilitating acicular ferrite nucleation in the weld zone. Comprehensive analysis indicates that variations in inclusion size and distribution patterns exert a substantial influence on the microstructural evolution and mechanical properties of deposited metal.

Rare earth (RE) metals exhibit unique chemical activity during the steelmaking process due to their special electronic configurations, and their deoxidation and desulfurization products can effectively regulate the morphology and distribution of inclusions. The formation of high-melting-point composite inclusions can reduce the hazard coefficient of sulfides and promote the transformation of brittle inclusions in the original matrix to ductile phases, effectively reducing the degree of stress concentration. Zheng et al. [13] added Ce to transform inclusions from CaS and MnS to CeAlO₃-CaS-MnS composite inclusions. Zhang et al. [14] introduced Ce to transform inclusions from Al₂O₃ and Al₂O₃ + MnS to Ce-Al-O, Ce-O, and Ce-O-S composite inclusions. Similar results have also been obtained by other scholars [15,16,17].

In the regulation of welded joint properties, lanthanum (La) and Ce exhibit similar inclusion modification mechanisms [13,14,15,16,17,18,19,20,21,22,23]. Both can alter the composition and morphological distribution of inclusions, thereby enhancing the toughness of the welded joint. The latest market quotations indicate that lanthanide RE elements have a significant price advantage compared to their homologous Ce. Zhang et al. [18] added lanthanum oxide (La₂O₃) powder during the laser powder bed fusion, resulting in a 34.0% reduction in grain size and a 13.3% increase in tensile strength of the specimens. Parshin et al. [19] electroplated a LaF₃ coating on the surface of the welding wire, which increased the tensile strength of the joint by 4% and the impact toughness by 20%. Although the above studies have confirmed that La can also improve inclusions, its influence on microstructure optimization and fracture mechanisms is currently not clear [20,21,22,23]. Therefore, studying the impact of La on the microstructure and fracture behavior during the welding process is of great significance for the practical engineering application of CT.

In this study, three types of metal-cored welding wires with different La contents were designed, and the deposited metal was subsequently fabricated via welding. The influence mechanism of La on the microstructure and mechanical properties of the deposited metal during the welding process was comprehensively investigated. At the same time, the effect of La on the composition and distribution of inclusions was also studied. Based on this, the influence of inclusions and microstructure on fracture behavior was investigated. The results of this study will provide novel insight and theoretical support for the design of welding materials for CT.

2. Materials and Methods

2.1. Materials

In this study, three types of 1.2 mm diameter metal-cored welding wires were designed. The mass fractions of La₂O₃ in the flux were 0% wt.%, 0.5% wt.%, and 1.0% wt.%, corresponding to 0 La, 0.5 La, and 1.0 La samples, respectively. The wire fill rate was 15%. Gas Tungsten Arc Welding (GTAW, Fronius International GmbH, Graz, Austria) was used to perform multi-layer and multi-pass overlay welding on the surface of Q355 steel base material (BM) to fabricate the deposited metal. Before welding, the base plate surface was thoroughly cleaned to remove impurities such as scale and oil. High-purity argon (purity ≥ 99.99%, Tianjin Liufang Industrial Gas Distribution Co., Ltd., Tianjin, China) was used as the shielding gas with a flow rate of 18 L/min. The welding current was 200 A, the voltage was 14–16 V, and the speed was 120 mm/min. The interlayer temperature was maintained at 130 ± 10 °C. The metallographic samples of the three deposited metals were polished with sandpaper and then tested for composition via a DF-100 optical emission spectrometer (Shandong Dongyi Optoelectronic Instrument Co., Ltd., Yantai, China). The cylindrical specimens with a diameter and height of 5 mm were tested with an ON-3000 pulsed infrared thermal conductivity oxygen and nitrogen analyzer (Steel Research Institute NAC Testing Technology Co., Ltd., Beijing, China). The results are shown in

Table 1. Composition of base metal and three deposited metals (wt.%).

No.	C	Si	Mn	Ni+ Cr+ Mo	Ti	La	Al	S	P	O(ppm)	Fe
BM	0.140	0.183	0.463	0.117	-	-	-	0.0068	0.016	-	Bal.
0 La	0.050	0.338	1.41	1.51	0.041	0	0.036	0.011	0.015	158	Bal.
0.5 La	0.052	0.349	1.51	1.58	0.037	0.0074	0.031	0.008	0.015	123	Bal.
1.0 La	0.053	0.327	1.50	1.50	0.036	0.0094	0.033	0.010	0.016	118	Bal.

.

2.2. Microstructure Characterization

To explore the microstructural characteristics of the deposited metal, the full cross-section of the weld seam was taken as the metallographic observation specimen (as shown in Figure 1). After grinding and polishing the specimen, it was subjected to etching treatment with a nitric acid-alcohol solution with a volume fraction of 4% (nitric acid and alcohol, Tianjin Jiangtian Chemical Technology Co., Ltd., Tianjin, China). Subsequently, the microstructure was observed using an optical microscope (OM, ZEISS Scope. A1, Carl Zeiss AG, Oberkochen, Germany) and a scanning electron microscope (SEM, JEOL JSM-7800F, 15 kV, JEOL Ltd., Tokyo, Japan) equipped with an energy dispersive spectrometer (EDS, JEOL Ltd., Tokyo, Japan).

Furthermore, after grinding and electropolishing the specimen (using a 5% perchloric acid-alcohol solution) (perchloric acid and alcohol, Tianjin Jiangtian Chemical Technology Co., Ltd., Tianjin, China), electron backscattered diffraction (EBSD, JSM-7800F, 20 kV, with a step of 0.22 μm, JEOL Ltd., Tokyo, Japan) was used to study its crystallographic characteristics. The EBSD data obtained from the test was analyzed with OIM software (TSL OIM 7.2). In addition, in order to explore the characteristics of the grain size of the deposited metal, 10 SEM images were randomly captured. Photoshop software (Adobe Photoshop 2023) was used for phase identification and marking, and then Image-Pro Plus software (Image-Pro Plus 6.0) was utilized for quantitative analysis to accurately determine the phase fraction of the microstructure. When measuring the phase fraction of the microstructure, different phases were visually marked with different colors, and then statistical calculations were performed using Image-Pro Plus software. At the same time, in order to investigate the proportion of microstructures and the morphology of inclusions for the metallographic specimens of three groups of deposited metal, after grinding and polishing, the backscattered electron (BSE, JEOL Ltd., Tokyo, Japan) mode of SEM was used for observation. Twenty micrographs were randomly taken for each specimen, and Image-Pro Plus software was used to conduct quantitative analysis on the size and distribution form of inclusions.

2.3. Mechanical Properties Testing

The mechanical properties of the deposited metal were measured by uniaxial tensile tests and Charpy impact tests, and the specific sampling locations and dimensions are shown in Figure 1. In accordance with the ISO 6892-1:2019 standard [24], tensile tests were conducted using a universal testing machine (MTS E45.105, MTS Systems Corporation, Minneapolis, MN, USA) at room temperature (25 °C). In accordance with the ISO 148-1:2016 standard [25], impact tests were carried out using a pendulum impact testing machine (SANS ZBC2752-ED, MTS Systems Corporation, Minneapolis, MN, USA) at 0 °C (ISO 3183:2019 [26]). To ensure the stability of the data obtained, both tensile and impact tests were conducted three times, and the average values were selected. After the tests, the tensile and impact fractures were observed using SEM. In addition, in order to investigate the fracture behavior during the impact process, the impact fractures were nickel-plated, and then the crack propagation paths near the nickel-plated layer were observed using SEM.

3. Results and Discussion

3.1. Effect of La₂O₃ on Microstructure

The microstructure of the deposited metal was characterized at multiple scales using OM and SEM, as shown in Figure 2. The hardness test of the cover layer of the three samples was carried out. Ten test points were selected for each sample. The average hardness values were 306 ± 9 HV10, 312 ± 8 HV10, and 300 ± 10 HV10, respectively. The microstructure primarily consisted of lath bainite (LB) and a minor fraction of granular bainite (GB). Quantitative statistical results show that in the 0 La specimen without La₂O₃ addition, the area fraction of LB in the microstructure was 91.4%, while GB accounted for 8.6%. For the 0.5 La specimen, the proportion of LB increased and GB decreased, with LB occupying 95.3% and GB 4.7%. In the 1.0 La sample, the microstructural distribution changed again, showing LB at 91.7% and GB at 8.3%.

The crystallographic characterization of the deposited metal was carried out using electron backscatter diffraction (EBSD), and the results are shown in Figure 3. From the inverse pole figures (IPF, Figure 3a–c), it can be seen that the orientation difference between the grains in the 0.5 La sample was the largest, and some LB with a network-like distribution appears. In contrast, the other two groups have a smaller grain orientation difference and no obvious network-like structure. The grain size statistical results (Figure 3j) show that the average grain sizes of the 0 La, 0.5 La, and 1.0 La samples are 1.16 ± 1.18 μm, 1.02 ± 1.00 μm, and 1.08 ± 1.25 μm, respectively. In addition, from the grain boundary distribution map (Figure 3d–f) and the statistical results (Figure 3k), it can be seen that with the increase of La₂O₃, the proportion of high-angle grain boundaries increases from 54.9% to 60.1% and 57.3%.

The Kernel Average Misorientation (KAM), a vital parameter for characterizing the degree of local lattice distortion in materials, can directly reflect the dislocation density and plastic deformation behavior in the microstructure [27]. Generally, the KAM value is positively correlated with the dislocation density. Regions with high KAM values correspond to dense dislocation tangles. These regions, due to the accumulation of lattice distortion energy, can trigger significant work hardening effects, thereby enhancing the material’s yield strength. However, the plastic deformation ability is also reduced because the movement of dislocations is restricted. Moreover, continuous high KAM value regions may form microstrain concentration bands, which can easily become preferential paths for crack nucleation and propagation, significantly reducing the material’s impact resistance. Quantitative calculation results show that the KAM values of the three groups of deposited metals are 0.95, 0.83, and 0.90, respectively.

Analysis of the KAM maps of the three deposited metals (from Figure 3g–i) revealed that the average KAM value of the 0.5 La sample was the lowest, indicating a decrease in the average dislocation density. In its KAM map, lath bainite with low dislocation density was distributed in an interlaced pattern. This lath bainite, by dividing high-strain regions and promoting the uniform distribution of dislocations, effectively suppressed local strain concentration. Under impact load, these low-KAM-value laths could induce dislocation slip and grain-boundary coordinated deformation, causing the crack tip to blunt or deflect, thereby absorbing more fracture energy and improving impact toughness.

3.2. Effect of La₂O₃ on Inclusions

As can be seen from Table 1, with the increase of La₂O₃ content in the welding wire, the amount of La transferred to the deposited metal also increased accordingly. The size and distribution form of inclusions were quantitatively analyzed, and the results are shown in Figure 4. It can be seen from the figure that the addition of La₂O₃ has a significant impact on the size and distribution of inclusions. With the increase of La₂O₃ addition (0 to 0.5% to 1.0 wt.%), the average diameter of inclusions showed a non-linear trend of first decreasing and then increasing, while the number density showed a trend of first increasing and then decreasing. Specifically, when no La₂O₃ was added, the size of inclusions varied greatly, and there was a certain degree of segregation. The average size of inclusions in the deposited metal was 0.19 μm, and the number density was 9356/mm². When the addition was 0.5%, the inclusions were significantly refined and distributed uniformly. The average size decreased to 0.12 μm, and the number density increased to 11,286/mm². When the addition was further increased to 1.0%, the inclusions became coarser (average size 0.16 μm) and were accompanied by local aggregation, with a number density of 10,385/mm².

SEM was used to conduct Energy Dispersive Spectroscopy (EDS) elemental analysis of inclusions in the three deposited metals, and the results are shown in

Table 2. Inclusion composition in deposited metal (wt.%).

No.	O	Al	Si	S	Ti	La	Mn	Fe
0 La	11.78	13.24	3.02	0.29	6.21	0	4.32	61.14
0.5 La	10.79	14.23	1.21	1.34	4.90	4.13	2.86	60.54
1.0 La	9.98	13.13	1.71	1.69	6.20	6.31	2.49	58.49

. It can be seen that after the addition of La₂O₃, the chemical composition of inclusions changed from O-Mn-Si-Al-Ti to O-Mn-Si-Al-Ti-S-La. Combining the above analysis, it can be seen that the addition of La₂O₃ not only changed the micro-morphology of inclusions but also significantly affected the chemical composition of inclusions.

The La₂O₃ powder underwent a series of metallurgical reactions in the molten pool. The La₂O₃ decomposes into [La] and [O] in the molten pool²³. Due to the high chemical reactivity of La, the decomposed La would further react with oxygen and sulfur in the molten pool to form La₂O₃ and La₂S₃ (Equations (1)–(3)).

$$\mathrm{L}{\mathrm{a}}_2{\mathrm{O}}_3=2[\mathrm{L}\mathrm{a}]+3[\mathrm{O}]$$

(1)

$$2[\mathrm{L}\mathrm{a}]+3[\mathrm{O}]=\mathrm{L}{\mathrm{a}}_2{\mathrm{O}}_3$$

(2)

$$2[\mathrm{L}\mathrm{a}]+3[\mathrm{S}]=\mathrm{L}{\mathrm{a}}_2{\mathrm{S}}_3$$

(3)

Another significant advantage of RE is that they can react with oxygen and sulfur simultaneously to form RE₂O₂S-type oxy-sulfides (Equation (5)). When the [O]/[S] ratio in the molten pool is maintained within an optimal range, the [La] and La₂O₃ present in the molten pool undergo secondary reactions with oxides such as Al₂O₃ and SiO₂, as well as sulfides like [S], MnS, and FeS, in a secondary reaction (Equations (4)–(9)) [19,28], achieving synergistic deoxidation and desulfurization, and significantly improving the cleanliness of the weld. Due to the relatively fast cooling rate of the molten pool, the generated oxygen and sulfide compounds do not have time to float up completely and partially remain in the weld as inclusions.

$$2[\mathrm{L}\mathrm{a}]+2[\mathrm{O}]+[\mathrm{S}]=\mathrm{L}{\mathrm{a}}_2{\mathrm{O}}_2\mathrm{S}$$

(4)

$$\mathrm{L}{\mathrm{a}}_2{\mathrm{O}}_3+[\mathrm{S}]+[\mathrm{C}]=\mathrm{L}{\mathrm{a}}_2{\mathrm{O}}_2\mathrm{S}+\mathrm{C}\mathrm{O}$$

(5)

$$[\mathrm{L}\mathrm{a}]+\mathrm{A}{\mathrm{l}}_2{\mathrm{O}}_3=\mathrm{L}\mathrm{a}\mathrm{A}\mathrm{l}{\mathrm{O}}_3+[\mathrm{A}\mathrm{l}]$$

(6)

$$\mathrm{L}\mathrm{a}\mathrm{A}\mathrm{l}{\mathrm{O}}_3+[\mathrm{L}\mathrm{a}]+[\mathrm{S}]=[\mathrm{A}\mathrm{l}]+[\mathrm{O}]+\mathrm{L}{\mathrm{a}}_2{\mathrm{O}}_2\mathrm{S}$$

(7)

$$3\mathrm{F}\mathrm{e}\mathrm{O}+3\mathrm{F}\mathrm{e}\mathrm{S}+4[\mathrm{L}\mathrm{a}]=\mathrm{L}{\mathrm{a}}_2{\mathrm{O}}_3+\mathrm{L}{\mathrm{a}}_2{\mathrm{S}}_3+6\mathrm{F}\mathrm{e}$$

(8)

$$6\mathrm{M}\mathrm{n}\mathrm{S}+3\mathrm{S}\mathrm{i}{\mathrm{O}}_2+8[\mathrm{L}\mathrm{a}]=2\mathrm{L}{\mathrm{a}}_2{\mathrm{O}}_3+2\mathrm{L}{\mathrm{a}}_2{\mathrm{S}}_3+6\mathrm{M}\mathrm{n}+3\mathrm{S}\mathrm{i}$$

(9)

After modification with La₂O₃, the harmful impurities originally enriched at grain boundaries were transformed into thermodynamically stable La-based compounds, enhancing the cleanliness of the deposited metal. The size variation in the modified inclusions was reduced, and their number density increased. An appropriate amount of fine and dispersed inclusions can serve as effective heterogeneous nucleation sites, promoting the nucleation of lath bainite with low dislocation density and achieving grain refinement and microstructure optimization. However, when an excessive amount of La₂O₃ was added, the spacing between the reaction products was too small. During the growth process, they would engulf nearby inclusions, resulting in larger-sized inclusions. This not only reduced the density of effective nucleation sites but also weakened the grain boundary pinning effect and the strengthening effect of the second phase, leading to grain coarsening. In summary, inclusions with suitable size and number density can help improve the microstructure and comprehensive mechanical properties of the deposited metal.

3.3. Effect of La₂O₃ on Mechanical Properties

3.3.1. Tensile Properties

The mechanical properties test results and tensile curves of the three deposited metals are shown in

Table 3. Mechanical property results.

No.	Rp0.2/MPa	Rm/MPa	Elongation to Failure	Impact Energy/J (0 °C)
0 La	628 ± 14	723 ± 13	23.3 ± 1.5%	160 ± 6 J
0.5 La	673 ± 12	772 ± 12	23.1 ± 1.2%	189 ± 6 J
1.0 La	607 ± 13	739 ± 14	24.4 ± 1.3%	166 ± 6 J

and Figure 5. It can be seen that when a small amount of La₂O₃ was added, the yield strength of the deposited metal increased from 628 ± 14 MPa to 673 ± 12 MPa, and the tensile strength increased from 723 ± 13 MPa to 772 ± 12 MPa. However, excessive La₂O₃ reduced the yield strength and tensile strength to 607 ± 13 MPa and 739 ± 14 MPa, respectively. The elongation of the three deposited metals showed no significant change, all being around 23%, indicating good ductility.

Figure 6 shows the tensile fracture morphologies of the three deposited metals. As can be seen from Figure 6a–c, the macroscopic morphologies of the fracture surfaces of the three deposited metals all consist of a fibrous region and a shear lip zone. Quantitative calculations of these two regions show that the fibrous region area fractions for the 0 La, 0.5 La, and 1.0 La samples are 28.8%, 41.3%, and 36.8%, respectively. A larger fibrous region area fraction indicates a longer duration of the necking stage and a lower crack propagation rate in the deposited metal. Figure 6b–f1 show the microscopic morphologies of the fibrous region fractures. It can be seen that all three are ductile fractures, with a large number of dimples distributed on the fracture surfaces. In the 0 La sample without the addition of La₂O₃, the size distribution of the dimples is relatively uneven, and some dimples have larger-sized inclusions at the bottom. In the 0.5 La sample, the size difference of the dimples is significantly reduced, and the size of the inclusions at the bottom of the dimples is also smaller. However, excessive addition (1.0 La) results in too small a spacing between inclusions, causing some inclusions to merge and grow into larger-sized inclusions. When the spacing is slightly larger but not at an appropriate distance, the inclusions will not merge but will tend to aggregate, which is clearly reflected in the fracture morphology (Figure 6(f1)). The size difference of the dimples is large, and some dimples have larger-sized inclusions at the bottom. The aggregation of small inclusions triggers the connection of dimples, forming large and shallow dimples. Large and shallow dimples have a lower ability to absorb energy during crack propagation, leading to a decrease in the tensile properties of the material.

As can be seen from Section 3.1, the microstructure of the deposited metal was refined after the addition of La₂O₃. According to the Hall-Petch formula (Equation (10)), the smaller the grain size, the higher the strength, and it can be well-matched with toughness [29,30,31]. After the addition of 0.5% La₂O₃, the average grain size of the microstructure decreased from 1.16 ± 1.18 μm to 1.02 ± 1.00 μm, and the strength provided by fine grain strengthening increased by 6.6%.

$${\mathrm{\sigma }}_s={\mathrm{\sigma }}_0+k_y{d}^{-1/2}$$

(10)

In the formula, ${\mathrm{\sigma }}_{\mathrm{s}}$ represents the yield strength of the material; ${\mathrm{\sigma }}_0$ represents the strength of a single crystal; $k_y$ is a constant related to the material itself; $d$ is the grain diameter.

To further investigate the effect of La on the strengthening and toughening mechanism of deposited metal, microstructural observations were conducted on cross-sections near the tensile fracture surfaces, as shown in Figure 7. Results indicate that all three deposited metals exhibited significant plastic deformation characteristics during tensile testing, demonstrating excellent ductility and toughness. Since the hardness of inclusions is much higher than that of the matrix, they cannot deform with the matrix during plastic deformation and will detach to form cavities. Secondary cracks and voids formed by inclusion detachment were observed in all specimen sections, but the number of voids in the 0.5 La specimen was significantly lower than those of the other two specimens.

In the tensile performance of metallic materials, the size of inclusions significantly affects microscopic stress distribution. As the inclusion size decreases, their role as stress concentrators weakens, reducing the likelihood of crack initiation. Typically, the stress concentration factor is positively correlated with defect size. In the 0.5 La specimen, finely dispersed small inclusions effectively reduce localized stress peaks under tensile loading, suppressing crack nucleation. Additionally, the spatial distribution of these inclusions disrupts the continuity of crack propagation paths, requiring higher energy consumption during crack extension, which macroscopically improves tensile and yield strengths. When inclusions are larger or unevenly distributed (Figure 6(d1,f1)), low interfacial bonding strength between inclusions and the matrix enhances localized stress fields, promoting crack nucleation and unstable rapid propagation. This leads to fracture at lower strain levels and reduced yield and tensile strengths. Optimizing stress field uniformity by controlling inclusion size and distribution represents an effective approach to synergistically enhance material strength and toughness.

3.3.2. Impact Toughness

As can be seen from Table 3, the impact performance first increased and then decreased after the addition of La₂O₃. When the addition was 0.5%, the impact absorption energy at 0 °C reached a maximum value, increasing from 160 J ± 6 J to 189 J ± 6 J. The improvement in impact energy indicates that the impact toughness of the deposited metal was enhanced after the addition of La₂O₃.

Figure 8 shows the tensile fracture morphologies of the three deposited metals. It can be seen that all three fractures consisted of dimples, and no cleavage fracture morphology was observed. In the 0 La sample without the addition of La₂O₃, there were a large number of secondary cracks in the fracture, with a length of 153.6 μm. In the 0.5 La sample, the number of secondary cracks was significantly reduced, and the length was shortened to 52.3 μm. In the 1.0 La sample with excessive addition, the length of secondary cracks increased to 123.1 μm, but it was still better than the 0 La sample. This indicates that La₂O₃ inhibits crack initiation and propagation by controlling the distribution of inclusions, but excessive addition weakens this effect.

In order to reveal the crack initiation and propagation mechanisms of different deposited metals under multi-scale structures, the longitudinal sections of the impact fracture surfaces of the three deposited metals were observed, and the crack characteristics in the initiation and propagation zones were systematically characterized, as shown in Figure 9. It can be seen from the figure that the crack initiation zones of the three deposited metals all underwent significant plastic deformation. However, in the 0.5 La sample, the diameter and number of cavities formed by the detachment of inclusions were significantly smaller than those in the other two groups. During the crack initiation stage, a stress field would form at the tip of the V-notched impact specimen, leading to a plastic deformation zone. During the plastic deformation, inclusions failed to deform cooperatively with the matrix and detached to form voids. Therefore, the size and distribution of inclusions have a significant impact on crack initiation.

As can be seen from the crack propagation paths in the fracture surfaces, the crack propagation path in the 0.5 La deposited metal was the most tortuous, and it had the highest impact energy. In addition, inclusions, cavities, and secondary cracks were present in all three deposited metals, and the secondary cracks were mainly located in GB. Among them, the 0.5 La deposited metal had the fewest secondary cracks in the crack propagation path. GB is a hard and brittle high-carbon particle, which can easily cause stress concentration in the matrix. During the fracture process, GB not only serves as a crack initiation source but also as a low-energy propagation channel for cracks, which has an adverse effect on the impact toughness of the deposited metal. Therefore, the crack propagation mechanisms of the three deposited metals were jointly controlled by inclusions and GB.

4. Conclusions

In this study, three types of metal-cored wires with different compositions were designed. By incorporating La₂O₃, the control of inclusions and microstructure in the deposited metal was achieved, leading to an improvement in the overall mechanical properties of the deposited metal. The following conclusions were drawn:

(1) The addition of La₂O₃ has a significant regulatory effect on the compositional characteristics and morphological evolution of inclusions in the material. It changes the chemical composition of inclusions from O-Mn-Si-Al-Ti to O-Mn-Si-Al-Ti-S-La. The average size of inclusions decreases, and the number density increases. In terms of microstructure, the size of LB grains decreases, and the proportion of GB decreases.

(2) The addition of La₂O₃ in appropriate amounts can effectively enhance the comprehensive mechanical properties of the deposited metal. When the addition was 0.5 wt.%, the yield strength of the deposited metal increased from 628 ± 14 MPa to 673 ± 12 MPa. The impact toughness increased from 160 J ± 6 J to 189 J ± 6 J.

(3) The addition of La₂O₃ enhanced the mechanical properties of the deposited metal. The main reason for this improvement was that it promoted the refinement of inclusions and increased their number density, providing more heterogeneous nucleation sites. The small and dispersed inclusions helped to form finer grains during solidification, thereby improving the mechanical properties via the fine-grain strengthening effect.

(4) When the addition of La₂O₃ increased to 1%, the size variation and distribution of inclusions in the deposited metal began to deteriorate, and the mechanical properties also showed a downward trend. The main reason for this phenomenon was that the excessive La₂O₃ led to an excessive number of inclusions. Some small inclusions merged into larger ones during the growth process, ultimately leading to a decrease in performance.