In view of the wear and failure requirements of electric winch drum, multi-layer and multi-pass laser cladding was used to symmetrically fill the groove, and the 30CrMnSiA high-strength alloy steel drum was repaired by laser cladding with the same material powder, and the joint structure, hardness, tensile and impact properties were studied and analyzed. The results show that the heat affected zone of the cladding joint has hardened, and the maximum hardness can reach 450 HV0.5. The hardness of the cladding zone is about 380 HV0.5. The average tensile strength (1 176 MPa) and average yield strength (1 049 MPa) of the cladding joint are both 104% of the parent material. The room temperature impact toughness (115.8 J/cm’2) of the 30CrMnSiA additive-30CrMnSiA forging at tensile fracture far exceeds the standard value of the forging (49 J/cm’2), which meets the drum material performance requirements and the repair performance can meet the use requirements.

In the actual use of the electric winch drum, the groove tip and the steel cable will have contact friction, and the groove tip is prone to wear and failure. In order to ensure the size of the drum groove tip and improve its service life, the industry often uses surface modification methods such as cladding and spraying to achieve this. However, the heat input during cladding on the surface of parts is not easy to control, and the spray layer is a mechanical bonding layer with insufficient bonding strength. Laser cladding, as an advanced material surface modification technology, uses high-energy lasers to impact the material surface, so that the thin layer on the surface of the substrate and the cladding material melt together and solidify quickly to form a metallurgical bonding layer. It has the advantages of high energy density, small thermal impact, low dilution rate, good bonding of the cladding layer, and easy automated production. It has been widely used in high-performance surface preparation, failed parts repair and other fields.

The material of the electric winch drum is 30CrMnSiA high-strength alloy steel. At present, there have been many reports on laser cladding repair of high-strength steel, such as melting 1Cr15Ni4Mo3 powder on the surface of 30CrMnSiNi2A steel, and cladding AerMet1000 powder on the surface of 300M steel. The application of laser cladding repair on the surface of 30CrMnSiA high-strength steel is mainly concentrated on 18CrMoA and iron-based alloy powders. There is a softening phenomenon in the heat-affected zone of the matrix, which causes the strength of the cladding layer to be lower than the strength of the matrix. Therefore, how to match the strength and toughness of damaged parts is one of the technical problems that need to be solved in the repair of 30CrMnSiA parts.

Studies have shown that when selecting cladding materials, it is crucial to select alloy powders with the same or similar composition as the matrix for the comprehensive performance of the components. Therefore, this paper takes 30CrMnSiA alloy steel, which is widely used in the aircraft manufacturing industry, as the object, and selects 30CrMnSiA powder with the same composition as the matrix as the cladding material. According to the performance requirements of laser cladding repair of rollers, tensile properties, impact properties, hardness, etc. are used as evaluation indicators of the bonding performance of the cladding layer, and the forming process that meets the performance requirements of the roller is explored to solve the softening problem of the heat-affected zone, and provide theoretical support for the repair and use of parts in the aircraft manufacturing industry.

1 Experimental materials and methods

1.1 Experimental materials and equipment

The experimental substrate is 30CrMnSiA alloy steel with a size of 100 mm×100 mm×10 mm, and the chemical composition is shown in Table 1. The raw material for laser cladding is 30CrMnSiA powder with a size of 15~53 μm, which is dried in a vacuum oven at 100 ℃×1 h before use.

The laser cladding forming equipment uses a six-axis robot laser direct deposition forming equipment, mainly including: TruDisk6002 disc laser, KUKA KR90 six-axis robot, KUKA DKP400 positioner, Medicoat vibrating powder feeder, SPR4800 processing machine, etc.

Before laser cladding, the surface of the substrate is polished with an electric grinding gun and wiped with acetone. Through the preliminary process test, the appearance and structure of the cladding layer were comprehensively evaluated (micro defects such as unfusion and micro cracks are not allowed), and the optimized 30CrMnSiA powder laser cladding parameters are shown in Table 2.

1.2 Test method

The preparation of the tensile mechanical properties test plate is shown in Figure 1, with a 45° groove and a butt gap of 0.5 mm. Multi-layer and multi-pass laser cladding is used to symmetrically fill the groove, that is, after cladding the upper layer, turn over and clad the opposite layer, and fill in turn to reduce residual stress and deformation. According to GB/T228.1-2010, samples are taken from the butt joint in Figure 1, and the sampling position is shown in Figure 2. A total of 3 samples are randomly taken from the upper and lower grooves. The tensile specimen is shown in Figure 3, with a thickness of 2 mm (all in the butt filling area), and the center of the butt test plate is located in the center of the tensile specimen. The room temperature tensile properties of the butt specimens were tested using a Meters CMT5105 universal tensile testing machine, with a tensile rate of 1 mm/min.

The preparation of the room temperature impact mechanical properties test plate is shown in Figure 4a. An 8 mm thick 30CrMn-SiA additive material is clad on a 10 mm thick 30CrMnSiA forging. The test plate size is 140 mm×200 mm×10 mm. The test block is a 10 mm thick 30CrMnSiA forging and an 8 mm thick 30CrMn-SiA additive part. After cladding, annealing (190 ℃×2 h) is performed. According to GB/T229.1-2007, the sampling position and size of the room temperature impact specimen are shown in Figures 4a and 4b respectively. A U-shaped notch is opened, and the notch is located on the bonding line plane of the 5 mm additive material and the 5 mm forging. The room temperature Xiabi impact test is carried out using a PIT452C-2 pendulum impact tester, with a pendulum of 300 J.

The reason why the tensile and impact tests are not done on the same test plate is that the tensile test is mainly used to evaluate the bonding performance after repair and the performance of the repaired part. If the impact specimen is processed and laser clad on the tensile test plate, the substrate thickness is only 1 mm, and the impact performance of the actual repaired part bonding part cannot be objectively evaluated. Half forging and half additive can better solve this problem.

Metallographic specimens were cut from the tensile specimens, corroded with nitric acid alcohol solution, and the organization was observed using a DM4M metallographic microscope. The hardness of the substrate, heat-affected zone, and laser additive zone on the left and right cross sections of the specimens was tested using a Qness Q10M micro-Vickers hardness tester. The test load was 500 N and the loading time was 15 s. The IT500-A field emission scanning electron microscope was used to detect the metallographic structure, tensile and impact fracture morphology. The ATOS Tripe scan type three-dimensional optical scanner was used to compare the dimensions of the roller before and after repair.

2 Test results and discussion

2.1 Organization analysis

The macroscopic morphology of the tensile specimen and the impact specimen is shown in Figure 5. The surface of the specimen is silvery white, smooth and flat, without ripples, and the forming is beautiful.

Figure 6 shows the macroscopic and microscopic structures of the laser cladding 30CrMnSiA tensile specimen. Figure 6a shows the metallographic macroscopic structure, which consists of 30CrMnSiA substrate, heat-affected zone and 30CrMnSiA additive zone. Figures 6b~6d are the enlarged SEM photos of the structures at the positions marked 1, 2 and 3 in Figure 6a. There is a continuous white bright band between the substrate and the additive zone (see Figure 6a), and its microstructure is mainly composed of plane crystals, indicating that the bonding interface is a strong metallurgical bond. 30CrMnSiA joint
There are no defects such as cracks, pores and inclusions. The substrate area is mainly martensite and ferrite, the block structure is ferrite, and the block structure is martensite. The width of the heat-affected zone is about 0.5 mm, and the microstructure does not show obvious changes. It is mainly composed of bundles of laths and a small amount of small particles. The microstructure is martensite and very little ferrite. Under the thermal cycle of multi-layer and multi-pass welding, the martensite decomposes slightly, and the supersaturated carbon atoms precipitate carbides during the cooling process, making the cladding layer microstructure mainly composed of a small amount of hard phase carbide particles distributed on the martensite matrix.

2.2 Microhardness

The cross-sectional hardness distribution curve of the laser cladding 30CrMnSiA tensile specimen is shown in Figure 7. As can be seen from the figure, the heat-affected zone has hardened, the width of the hardened zone is about 0.2 mm, the maximum value is 450 HV0.5, and the width of the heat-affected zone is about 0.5 mm. The hardness of the cladding zone is about 380 HV0.5, which is higher than the hardness of the substrate zone (360 HV0.5). The cladding layer is mainly composed of martensite and a small amount of carbide hard phase, while the substrate contains a small amount of ferrite. The hardness of ferrite is lower than that of martensite, so the hardness of the cladding layer is higher than that of the parent material zone. Figure 8 shows the EDS line scan results of the microhardness measurement position of Si, Cr and Mn elements in the bonding layer. The heat-affected zone is mainly composed of martensite and very small ferrite. The carbide hardness is smaller than martensite. At the same time, the solid solution strengthening effect of alloy elements in ferrite is Mn, Cr, and Si from best to worst. The heat-affected zone has high Mn and Cr contents. Due to the combined effect of martensiticization and solid solution strengthening of alloy elements, hardening occurs in the heat-affected zone.

2.3 Tensile properties

The room temperature tensile properties test results of 30CrMnSiA butt joint specimens and 30CrMnSiA forgings are shown in Figure 9. The average tensile strength and average yield strength of the laser cladding joint are 1176 MPa and 1049 MPa, respectively, which are 104% of the parent material, and the elongation is 10.4%, reaching 75% of the parent material. Studies have shown that tensile strength is proportional to hardness, that is, the area with low hardness undergoes plastic deformation and fractures preferentially during the tensile process. There is no softening phenomenon in the joint, so the butt joint specimens are all broken in the substrate area. Figure 10 is a photo of the room temperature tensile fracture of the laser cladding joint, which also proves this point. At the same time, the cladding layer and the heat-affected zone account for a certain proportion of the tensile specimen, and the hardness is higher than that of the substrate. There are differences in the plastic deformation stage. The plastic deformation before tensile fracture is mainly the deformation of the substrate, so the elongation is lower than that of the simple forging substrate. Analysis shows that the joint strength is higher than the strength of the forging parent material, the tensile performance is good, and the main requirements for the repair performance of the parts are that the strength and impact performance meet the requirements, so the tensile performance after repair meets the requirements of the specimen.

Figures 11a and 11b are the fracture morphologies of the 30CrMnSiA butt joint and forging after tensile fracture, respectively, both of which are substrate fracture morphologies. The substrate fracture of the butt joint has more and deeper dimples than the substrate fracture of the forging, and small particle strengthening phases are distributed in the dimples. The fracture of the 30CrMnSiA forging has a certain tearing phenomenon, and both are ductile fractures.

2.4 Impact properties

Table 3 shows the room temperature impact properties test results of 30CrMnSiA butt joint and substrate. The average impact toughness of 30CrMnSiA additive-30CrMnSiA forgings and 30CrMnSiA forgings are 115.8 J/cm2 and 72 J/cm2 respectively. The impact toughness of the butt joint far exceeds the standard value (49 J/cm2), which is 161% of the forging. This is because under the action of laser multi-layer and multi-pass thermal cycles, the precipitates in the heat-affected zone increase, the alloying effect is enhanced, and it is beneficial to strengthen the structure of the bonding zone. The single-layer multi-pass cladding layer formed has a fine structure in the extremely small thickness area of ​​the upper layer due to the laser rapid heating and cooling process. During the multi-layer and multi-pass cladding, the fine structure of the upper layer is re-melted, and the cladding morphology is finally retained as a fine columnar crystal structure with reduced gradient characteristics and good performance, which is beneficial to improve the uniformity of the joint structure and thus improve the overall performance. At the same time, the organizational factors that affect the toughness of the joint are organizational type and grain size. The grain size of the joint filling area is significantly smaller than that of the substrate, so its impact toughness is higher than that of the forging. Figure 12 is a macroscopic fracture photo of the impact specimen. Under the impact of the pendulum, there is no shedding or peeling in the bonding area of ​​30CrMnSiA additive-30CrMnSiA forging, indicating that the interface between the matrix and the additive area is a dense metallurgical bond.

Figure 13 shows the impact fracture morphology of 30CrMnSiA additive-30CrMnSiA forging and 30CrMnSiA forging. The fracture of the 30CrMnSiA additive-30CrMnSiA forging specimen (see Figure 13a) shows mixed fracture characteristics. The fracture of the substrate on the left shows obvious tearing ridge characteristics and fewer dimples; the fracture of the additive area on the right is relatively flat, composed of a large number of dimples, showing ductile fracture characteristics, and the reduction of tearing ridges indicates enhanced toughness. The fracture characteristics of the 30CrMnSiA forging (see 13b) are similar to those on the left side of Figure 13a. The overall ductile fracture characteristics are still present, but the plasticity has decreased.

2.5 Application Examples

The above process was used to successfully repair the electric winch drum. The size comparison before and after the repair is shown in Figure 14. Except for the size change of the repaired area, no obvious deformation was found in the other areas, indicating that the heat input control during the repair process was relatively uniform and did not cause deformation of the drum groove tip, indicating that the laser cladding repair process is feasible.

3 Conclusions

(1) A dense and defect-free metallurgical bond is formed between the 30CrMnSiA laser cladding additive area and the substrate. The heat affected zone structure is mainly martensite and a small amount of ferrite, and the cladding layer is mainly martensite and a small amount of carbide.

(2) The repair strength and impact toughness of 30CrMnSiA are higher than those of the forging base material. The hardness of the heat affected zone is high and it is no longer a failure area. In summary, the laser cladding process is used to match the strength and toughness of the joint, which can be used to repair the defects of the drum groove tip, and the mechanical properties meet the index requirements.

(3) The electric winch drum using laser cladding technology and specific repair process has small deformation and controllable deformation, which also solves the softening problem of the heat affected zone of the matrix, can extend the life of the parts and save huge costs, but the fatigue performance and durability of the repaired joint need to continue to invest in research and improve the construction of the theoretical system of additive repair.