In the fields of steel, coal mining, molds, etc., wear is one of the main causes of material loss and energy waste. With the rapid development of modern industry, simple steel and metal materials can no longer meet the requirements of use under many harsh working conditions. Ceramic particle reinforced metal matrix composites have the advantages of high strength, high hardness and high wear resistance, and are one of the effective ways to solve the problem of material failure in complex and harsh working conditions. However, due to the difference in thermal expansion coefficients between ceramic particles and matrix materials, poor ceramic/metal interface strength, and high brittleness of reaction products, the plastic toughness of particle reinforced composites is significantly reduced compared to the matrix metal. They are prone to fracture and early failure under impact loads, resulting in the inability to effectively utilize their wear resistance, which greatly limits their application and development. Ye et al. prepared Fe-based composites reinforced with V8C7 of different volume fractions by casting infiltration. With the increase of volume fraction of reinforcing phase, the hardness of the composite increased, while the impact toughness decreased from 8.1J/cm2 to 4.7J/cm2. When the volume fraction of reinforcing phase was lower than 24%, the wear resistance increased with the increase of V8C7 content, while when the volume fraction exceeded 24%, the particle crushing and microcracks caused the wear resistance to decrease. Zhang Ning [7] prepared WC/5CrNiMo composites by composite electrosmelting casting process. With the increase of WC content, the hardness and wear resistance of the composite increased, but the toughness decreased, resulting in the wear resistance under three-body impact wear conditions being much lower than that under two-body friction wear.

From the results of the above studies, ceramic particle reinforced metal matrix composites can significantly improve the hardness of the matrix and improve the wear resistance to a certain extent. However, with the increase of the volume fraction of ceramic particles, the impact toughness of the composite decreased seriously, resulting in particle crushing and even matrix cracking during the wear process, and the wear resistance showed a downward trend. Therefore, how to improve the impact toughness of composite materials without losing their wear resistance has become a research hotspot for particle reinforced metal matrix composites. “Structural toughening” is to separate the composite material into a reinforced particle enriched area (reinforcement area) and a pure matrix area (toughening area) in space through spatial structure design. The toughening area absorbs the impact energy and prevents crack propagation, which can significantly improve the toughness of particle reinforced metal matrix composites and avoid early failure of the composite material. At present, there have been related studies at home and abroad, using structural toughening for casting and infiltration process to prepare high-toughness particle reinforced metal matrix composites. However, there are few reports on the relevant research on combining structural toughening with laser cladding to prepare composite materials. This work adopts the idea of ​​”structural toughening”, takes 20% (volume fraction, the same below) WC/H13 composite material as the reinforcement area and Inconel625 as the toughening area, and prepares WC/H13-Inconel625 structural toughened composite material by multilayer cladding technology, studies the impact toughness and wear properties of the composite material, and analyzes the structural toughening mechanism of the composite material. By using the structural toughening concept and laser cladding technology, the impact toughness and wear resistance of ceramic particle reinforced metal matrix composites are effectively combined.

1 Experimental materials and methods

The experimental substrate material is H13 steel, and its main chemical composition (mass fraction) is: 0.45% C, 0.95% Si, 0.35% Mn, 5.0% Cr, 1.26% Mo, 1.04% V, and the remainder is Fe. The substrate size is 100mm×20mm×20mm, and the surface is repeatedly cleaned with acetone after mechanical grinding and polishing. The cladding material in the reinforcement area is WC and H13 mixed powder, and the particle size is 45-105μm; the cladding material in the toughening area is Inconel625 powder, and the particle size is 45-105μm.

The experiment was completed on a laser processing platform composed of an IPG YSL-4000 fiber laser and a KUKA six-axis robot, and the cladding was carried out by coaxial powder feeding. The laser defocus is 45mm, the spot diameter is 3mm, the laser power is 1300-1700W, the scanning rate is 0.5-0.8m/min, and the powder feeding rate is 9.5-11.5g/min. The laser cladding adopts the method of sequential overlapping multi-layer cladding. Each layer includes 4 WC/H13 cladding layers and 4 Inconel625 cladding layers, with a total of 6 layers. The specific process is shown in Figure 1. The size of the multi-layer sample is 6mm×14mm×60mm.

The metallographic specimens were cut from the cladding layer perpendicular to the laser scanning direction by CNC wire cutting. The specimen cross-section size was 10mm×5mm and the specimens were ground and polished. Since the corrosion resistance of Inconel625 and H13 is too different, Inconel625 was first electrolytically corroded with saturated oxalic acid solution (voltage 6V, corrosion 5s), and then H13 was corroded with 4% nitric acid alcohol solution. The micromorphology of the cladding layer was observed using an AE2000MET optical microscope (OM) and a Sirion 200 scanning electron microscope (SEM). The microhardness of the specimen was tested using a 430SVD Vickers hardness tester. The test method was parallel to the horizontal cross-section direction, with the WC/H13-Inconel625 interface as the center, and a point was taken every 0.3mm from the center of the WC/H13 area and the center of the Inconel625 area, and a load of 0.5kg was applied for 10s.

The Charpy U-shaped notch impact specimens were cut along the cladding layer parallel to the laser scanning direction. The specimen size was 5mm×10mm×55mm, ensuring that all the impact toughness specimens were in the cladding layer area, as shown in Figure 2. The impact test was carried out using a JXB-300 pendulum impact tester. After the experiment, the impact fracture was observed and analyzed using a VHX-1000C ultra-depth microscope and a scanning electron microscope. Wear specimens 1 and 2 were cut parallel to and perpendicular to the laser scanning direction, respectively, as shown in Figure 2. The grinding specimen was a pin with a diameter of 6.3mm, and the material was quenched GCr15 (hardness 60HRC). Traditional 10%WC/H13 composite materials and quenched H13 steel (hardness 53HRC) were used as comparison specimens, and wear block specimens of the same size were made. The pin-block reciprocating friction and wear test was carried out using a UMT-II friction and wear tester. The experimental load was 30N, the upper specimen linear velocity v=0.1m/s, and the wear time was 60min. The weight loss was measured using an electronic balance with an accuracy of 0.0001g.

2 Results and analysis

2.1 Macrostructure and microstructure of structural toughened composite materials
Figure 3 shows the cross-sectional optical microscope morphology of the structural toughened composite material. The reinforcement zone is a WC/H13 composite material. According to software analysis, the WC volume fraction is about 20%; the toughening zone is Inconel625. Due to the effects of overlap rate and dilution rate, WC/H13 and Inconel625 form a tortuous and continuously distributed sandwich structure, which is beneficial to improving the impact toughness of the structural toughened composite material [12]. The reinforcement zone and the toughening zone form a good metallurgical bond at the interface. Due to the advantages of laser cladding such as heat concentration and small heat-affected zone, only a very small number of WC particles sink into the Inconel625 area, ensuring the original good plastic toughness of Inconel625. It should be pointed out that the small amount of WC particles in the center of the toughening zone are the residues in the powder feeding system when cladding WC/H13 powder, and are brought into the molten pool when cladding Inconel625 powder.

Figure 4 shows the microstructure morphology of the structural toughening composite material. During the laser cladding process, WC particles will inevitably be irradiated by the laser to a certain extent, react with the metal melt in the molten pool to form a reaction layer, and partially melt to produce W atoms and C atoms. Due to the short residence time of the laser molten pool, the diffusion distance of W atoms and C atoms is limited, and W-rich and C-rich zones are formed around the particles. When the composition is supercooled and the energy fluctuates, the M6C first crystallization phase precipitates in the liquid metal near the particles. With the consumption of W and C, the liquid metal reaches the eutectic composition and forms a fishbone-shaped eutectic carbide M6C. In the matrix far away from the particles, due to the low content of W and C, it is difficult to form primary carbides. In the final stage of solidification of the molten pool, eutectic carbides M6C are formed at the grain boundaries.

The toughened Inconel625 is a columnar crystal and part of the dendrite structure along the height direction of the cladding layer, with irregular white precipitation phases distributed between the crystals. According to the existing research results, it is inferred that it is an intermetallic compound Laves phase and part of the block carbide. Due to the rapid concentrated cooling of laser heat, the C atoms in the WC/H13 molten pool did not diffuse into the Inconel625 too much, avoiding the formation of a large number of carbide brittle phases in the Inconel625. There is a large Laves phase at the interface. This is because the alloy elements in the H13 steel in the fusion zone diffuse into the Inconel625, resulting in the growth of the Laves phase in the eutectic reaction.

2.2 Microhardness of structural toughened composite materials

The microhardness sampling points and hardness distribution curves of the structural toughened composite materials are shown in Figures 5 and 6. With the WC/H13-Inconel625 interface as the center, the hardness of the toughened Inconel625 in the toughened area does not fluctuate much, with an average value of 230.5HV. The hardness of WC/H13 in the enhanced area gradually increases with the increase of the distance to the interface. The hardness of point E close to the interface is 295HV, while the hardness of point G far from the interface is 402HV. Figure 7 shows the EDS analysis results of the main elements of hardness sampling points A, C, E, and G. It can be seen that the main element contents of points A and C are basically similar, so the hardness values ​​are not much different. Points E and G are located in the WC/H13 area. In theory, their hardness values ​​should be close to those of quenched H13 steel, but in fact, their hardnesses are only 295HV and 402HV respectively. This is because during the laser multi-pass cladding process, due to the effect of the dilution rate, austenite stabilizing elements such as Ni and Mo in the Inconel625 area diffuse into the WC/H13 area, causing the composition of points E and G to change. During the cooling process of laser cladding, more residual austenite is formed, resulting in a hardness lower than the theoretical value. Since point G is farther away from the Inconel625 area than point E, the hardness value of point G is higher than that of point E.

2.3 Impact toughness of structural toughened composites

The impact toughness of structural toughened composites and traditional 10% WC/H13 composites were tested by 5 repeated experiments, as shown in Table 1. The Charpy notched impact energy data fluctuated to a certain extent, especially for materials with smaller impact energy such as 10% WC/H13, but from the overall results, the average impact toughness of the structural toughened composite was 13.8 J/cm2, which is 5.5 times that of the 10% WC/H13 composite.

Figure 8 shows the SEM morphology of the impact specimen fracture. The fracture morphology of the 10% WC/H13 composite is a typical cleavage fracture, with a large number of broken WC particles and carbide skeletons distributed on the fracture, indicating that the weak area in its impact fracture is the interface between WC particles and carbides. The fracture of the structural toughened composite shows that the WC/H13 in the reinforcement area is also a cleavage fracture, while the Inconel625 in the toughening area is distributed with a large number of dimples, which is a tough fracture. Therefore, the comprehensive toughness of the structural toughened composite is greatly improved.

Figure 9 shows the surface 3D morphology of the impact specimen fracture. The fracture of the structural toughened composite material is obviously uneven, and the fracture surface area is much larger than that of the 10% WC/H13 composite material. There are staggered WC/H13 zones and Inconel625 zones in the structural toughened composite material. Since the impact fracture direction is perpendicular to the interface between the reinforcement zone and the toughening zone, when the crack extension front enters the toughening zone from the reinforcement zone, the change in hardness and toughness will cause the crack extension direction to deflect or bifurcate. This deflection and bifurcation increases the total crack area, thereby increasing the total energy absorbed by the crack extension, resulting in an increase in impact energy. At the same time, this structural toughening design can ensure that the workpiece does not produce instantaneous destructive failure under a large impact load.

2.4 Wear properties of structural toughened composites

Figures 10 and 11 are the time-varying graphs of friction coefficient of structural toughened composites and the comparison graphs of wear weight loss. The friction coefficient of structural toughened composites is significantly lower than that of quenched H13 steel and 10% WC/H13. The average friction coefficient of quenched H13 steel is 0.713, and the average friction coefficient of 10% WC/H13 is 0.698, while the average friction coefficients of wear sample 1 (parallel to the laser scanning direction) and wear sample 2 (perpendicular to the laser scanning direction) are 0.550 and 0.586, respectively, which have good friction reduction effects. The wear weight losses of structural toughened composites are 0.0006g and 0.0005g, respectively, reaching the wear resistance level of 10% WC/H13, and the comprehensive wear resistance is 5 times that of quenched H13 steel.

Figures 12 and 13 are the microscopic morphology of the worn sample surface and the EDS analysis results, respectively. Combined with the EDS element analysis results in Table 2, it can be seen that the O element content of all worn surfaces is at a low level, and no serious oxidation wear occurs. There are many pits and wear debris on the surface of the H13 steel wear sample. During the wear process, the contact point on the sample surface produces instantaneous high temperature and softens, resulting in a decrease in local metal strength. Adhesion transfer and lamellar peeling occur under the action of shear deformation force, forming more wear debris and pits, and the wear weight loss is severe. At the same time, due to the local “welding” effect formed by this adhesive wear, a large shear resistance is generated, so the average friction coefficient of H13 steel is relatively large. As the wear progresses, more wear debris is generated on the worn surface, as shown in Figure 12 (a). These tiny wear debris act as “fine balls” on the friction contact surface, converting the sliding friction part into rolling friction, thereby reducing the friction factor of H13 steel in the later stage of wear, as shown in Figure 10. Due to the presence of WC particles, the WC particles of 10%WC/H13 composite material gradually protrude and become the main bearing phase after the base metal is continuously worn during the wear process. On the one hand, the protruding WC particles can play a supporting role, forming a “shadow effect” to protect the base metal; on the other hand, the dispersed WC particles and intergranular carbides can block the continuous “ploughing” of the abrasive particles on the wear surface, playing a wear-resistant effect. However, due to the small volume fraction of WC and the large spacing between WC particles, the base metal in some WC sparse areas is inevitably subjected to grinding, as shown in Figure 12(b). In the middle and late stages of wear, WC particles with extremely high hardness become the main contact bearing phase, and the actual wear contact area decreases accordingly, so the friction coefficient is also reduced.

In the early stage of wear of the structural toughened composite material, the toughened area Inconel625 is severely worn due to its low hardness, and more severe plastic deformation and plowing marks appear on the surface, and the H13 steel matrix in the reinforcement area is also slightly cut. As the wear progresses, the WC particles form a “shadow effect” and play a role of protruding load-bearing. Since the WC volume fraction in the reinforcement area of ​​the structural toughened composite material is high, it can form a denser support effect, so it has a lower friction coefficient. Even if the initial wear loss is severe, the overall wear resistance can still reach the same level as 10% WC/H13. From a macroscopic point of view, during the plane wear process, the entire reinforcement area WC/H13 is also a protruding load-bearing area, forming a “macro shadow effect” during the wear process, forming an “overhead” for the toughened area, and protecting Inconel625 from further wear. Due to the tortuous continuous distribution of the structural toughened composite material, there is a protective effect of the reinforcement area on any wear plane, so the continuous wear resistance of the entire structural toughened composite material can be guaranteed. It should be pointed out that due to the size effect of the wear pin diameter (6.3mm) and the structural toughening spatial structure (the width of the reinforcement zone is about 2mm), the contact area may be too small in the actual wear process so that the reinforcement zone fails to play the role of “macro shadow effect”, resulting in severe local wear. This is also the main reason why the wear resistance of the structural toughening composite sample 1 is slightly lower than that of the 10% WC/H13 composite.

In fact, this structural toughening particle reinforced composite material can be regarded as a “multi-dimensional reinforced” composite material: WC particles as zero-dimensional reinforcements form particle reinforced composite materials in H13, and this “fibrous” and “lamellar” (along the laser scanning direction) composite material as one-dimensional and two-dimensional reinforcements in Inconel625 to form a multi-dimensional reinforced composite material. Under the condition of plane wear with static load, the “multi-dimensional reinforced” composite material can achieve microscopic and macroscopic wear resistance enhancement of the entire wear surface.

Conclusion

(1) Laser cladding technology was used to prepare a structural toughened composite material with WC/H13 as the reinforcement zone and Inconel625 as the toughening zone, forming a sandwich structure with continuous and tortuous distribution of the reinforcement zone and the toughening zone. The reinforcement zone is a 20% WC/H13 composite material with WC particles and carbide M6C generated by the reaction as the main reinforcement phase; the toughening zone is Inconel625 alloy, and the main structure is columnar crystals, dendrites and precipitation phases. The average hardness of Inconel625 is 230.5HV, and the hardness of WC/H13 gradually increases from the strong-tough interface to the center area to 402HV.

(2) The impact energy of the WC/H13-Inconel625 structural toughened composite material is 5.5 times that of the traditional 10% WC/H13 composite material. Since the crack propagation front deflects and bifurcates at the interface between the reinforcement zone and the toughening zone, the fracture surface area of ​​the structural toughened composite material is much larger than that of the traditional WC/H13 composite material, so it can absorb more impact energy and has better impact toughness.

(3) Under dry sliding wear conditions, the wear resistance of the structural toughened composite material reaches the same level as the traditional 10% WC/H13 composite material, which is 5 times that of the quenched H13 steel. The friction coefficient of the structural toughened composite material is 81% of the traditional 10% WC/H13 composite material and 80% of the quenched H13 steel, which has good friction reduction effect and wear resistance. The reinforcement zone of the structural toughened composite material has a “macro shadow effect” and has an “overhead” protection effect on the toughening zone material. A concept of “multi-dimensional reinforcement” composite material is proposed, which relies on the combined action of WC particles and WC/H13 composite layer strips to simultaneously show microscopic and macroscopic comprehensive wear resistance enhancement in plane static load contact wear.