After the tip of a certain type of aircraft engine high-pressure turbine blade was worn, it was repaired by laser cladding technology. X-ray flaw detection found that there were longitudinal micro cracks on the joint surface of the cladding layer and the substrate, and the unqualified rate was about 20%. The causes of the cracks were analyzed by means of substrate material composition analysis, metallographic observation, scanning electron microscopy, industrial CT, etc. The results showed that the cracks were thermal cracks caused by excessive stress during the laser cladding process. The crack failure rate can be effectively reduced by increasing preheating and adjusting the laser light mode. After the control measures were implemented, the laser cladding repair of 24 blades was tracked and inspected, and the qualified rate reached more than 99%.
Keywords: turbine blades; laser cladding; thermal cracks; qualified rate

During the long-life service of aircraft engines, turbine blades, as power unit components of the engine, provide power for the engine, and are also subjected to high-temperature gas, hot and cold fatigue and centrifugal force. After long-term use, the blades will have cracks, wear and corrosion
and other damage. If these losses cannot be repaired, a large number of blades will be scrapped. After a certain type of aviation gas turbine engine has gone through a service life cycle, the blade tip of the high-pressure turbine blade is generally severely worn, and the wear amount is mostly between 0.5 and 1.5 mm. Blade tip wear often causes the turbine rotor outer diameter size to exceed the tolerance, the blade tip clearance to increase, the engine performance to decline, and even cause a large number of blades to be scrapped. In order to restore the performance of the blade, it is necessary to use laser cladding repair technology to spray cladding materials on the blade. The cladding material is irradiated by high-density lasers to melt it and the surface layer of the substrate at the same time, and after rapid solidification, it forms a surface cladding layer with extremely low dilution rate and metallurgical bonding with the substrate material, thereby achieving a process method for the substrate size to grow again or the surface material to be wear-resistant and corrosion-resistant. Compared with the currently commonly used plasma arc cladding, oxyacetylene cladding and other technologies, laser cladding technology has the advantages of high degree of automation, high cladding accuracy, small heat-affected zone, high bonding strength between the cladding layer and the substrate, and advantages in restoring the performance of the damaged parts substrate. It has gradually become one of the main methods for repairing aircraft and engine parts.

1 Fault phenomenon The laser cladding repair process of a certain type of blade tip is: blade body coating removal → blade tip grinding and polishing → fluorescent flaw detection inspection → laser cladding repair → visual inspection → blade tip grinding and polishing → fluorescent flaw detection inspection → X-ray flaw detection inspection → delivery and use. During the small-batch laser cladding repair process of a certain type of blade, it was found that the unqualified rate in the X-ray flaw detection process was high, that is, the X-ray flaw detection inspection showed that the cladding layer had cracks perpendicular to the cladding direction. According to statistics, the failure rate reached about 20%, which seriously affected the normal delivery and use of this type of engine.

2 Fault analysis
2.1 Fault situation
The failure situation of 4 blades was statistically analyzed, including visual inspection results, fluorescent flaw detection inspection, etc., and the first two process inspections were qualified. During the X-ray flaw detection process, it was found that cracks existed in the laser cladding joint of some blade tips. The cracks were all located on the joint surface between the blade tip cladding layer and the substrate, extending from the cladding layer to the substrate, with a length of 1.0 ~ 2.0 mm. The specific statistics are shown in Table 1. In order to eliminate the inaccuracy and influencing factors of the X-ray flaw detection results, the defective blades were subjected to industrial CT scanning inspection to further determine the existence of cracks in the laser cladding part of the blade tip. The results showed that the cracks were perpendicular to the joint surface between the cladding layer and the blade substrate, which was consistent with the X-ray flaw detection results.

2.2 Macroscopic inspection
As shown in Figure 1, the cracked part of the faulty blade is located above the joint surface between the blade tip substrate and the cladding layer, cracking along the blade body direction, about 13 mm away from the air inlet edge, and the crack length is about 0.8 mm. It did not crack through the blade tip surface, and the cladding layer near the crack was not damaged. The crack on the end face of the blade tip cladding layer is brittle cracking.

2.3 Scanning electron microscope inspection of the fracture surface
The SEM inspection of the crack fracture surface (Figure 2 ~ 3) shows that the crack fracture surface is relatively smooth, with obvious characteristics of transgranular development, and the fracture surface is brittle fracture characteristics, and no fatigue morphology is found; the surface oxidation is heavy, and the overall cross-section oxidation is uniform; the cross-section energy spectrum detection contains Ni, O, and Al elements, with contents of 56.1%, 34.1%, and 9.8%, respectively. The energy spectrum analysis results show that the middle of the crack is still dominated by oxides and carbides.

2.4 Analysis of blade structure
The blade is a hollow structure without a crown. The inner cavity is directly cast to form the air flow channel. The laser cladding extension part is on the side of the blade basin. The cladding blade tip is a cambered surface with a width of 0.6~0.8 mm. The air intake side is wide and the exhaust side is narrow. In addition, the horizontality of the repaired end surface is not very consistent, that is, the exhaust side is high and the air intake side is low. During the cladding process, the Z axis will fluctuate up and down, affecting the forming effect of the cladding layer. Therefore, it is difficult to laser clad a cladding layer consistent with the blade surface at the blade tip.

2.5 Analysis of blade materials
The Russian grade of the blade base material is ЖС6У, and the domestic grade is K465. The main alloying element content of this nickel-based cast high-temperature alloy is shown in Table 2. The Al and Ti content is high, reaching 7%~9%, which is a difficult-to-weld metal. Therefore, if the process control is not proper during the cladding process, defects such as cracks are very likely to occur. Since the alloy is mainly used to make engine turbine working blades and guide vanes with an operating temperature below 1050℃, the faulty blades are K465 alloy materials, and the number of blades per unit is 80.

2.6 Cladding process analysis
The C600 laser cladding equipment with an IPG laser power of 1000W is used to repair the blade tip wear. The cladding powder is a nickel-based alloy powder that matches the performance of the substrate, with a particle size of 35~150μm, good powder sphericity, uniform particle size, and contains a small amount of satellite powder and special-shaped powder. Before using the powder, take an appropriate amount of powder and put it into a vacuum drying oven for drying. The drying temperature is 120℃ and the insulation time is 2h. Before cladding, the forming chamber was purged to make the water content and oxygen content in the forming chamber less than 1.0 × 10-3. The cladding process parameters were: laser power 450 ~ 480 W; defocus 5.0 mm; spot size 1.0 mm; powder feeding rate 1.0 ~ 1.2 r/min; cladding rate 260 ~ 280 mm/min; Z-axis lifting size 0.3 mm; shielding gas flow rate 8 L/min; restraining gas flow rate 8 L/min; powder carrier gas flow rate 6 L/min; cladding layers 10 ~ 15 layers. The laser light output mode of the equipment is continuous light output, which makes the instantaneous heat input large during single-pass cladding, and there is no preheating requirement before welding in the entire process parameters, which will cause a large temperature gradient difference between the blade substrate and the cladding layer, resulting in a large thermal stress in the blade cladding layer.

2.7 Blade metallographic analysis
2.7.1 Sample preparation Use scrapped turbine blades, and laser clad the blade tip according to the normal process flow and process parameters. The lengthening height is 2.0 ~ 3.0mm. Samples are taken from any cladding part of the blade tip, and the sampling size is about 8mm × 8mm. The sample should include the blade substrate and the cladding part.

2.7.2 Microstructure inspection
For the laser cladding blade, any section is selected for sampling, sample preparation and sample grinding. Then, the blade cladding layer and substrate are corroded by a special etching liquid, namely, CuSO (41.5 g) + concentrated HCl (40 mL) + alcohol (20 mL) for 8 to 10 s. The morphology under an optical microscope is shown in Figure 4. Compared with the blade substrate, the cladding layer is very fine and dense, and no micro cracks are found. As can be seen from Figure 4a, the width of the heat-affected zone varies greatly, indicating that the heat input of the first few cladding layers is very uneven, which may lead to thermal cracks.

2.8 Stress analysis
Through the analysis of the fault condition, blade structure, blade material, cladding process and other aspects, the cracks are mainly caused by uneven heat input during the cladding process and the large temperature difference between the blade and the cladding layer. The crack condition was analyzed from the tip surface of the blade. The crack was located at about 13 mm on the air inlet edge, which is the part with the largest curvature of the blade tip and is directly opposite to the reinforcement rib of the blade inner cavity. This means that this is the position with the largest stress and the largest temperature difference during the cladding process. The temperature difference will cause the thermal expansion between the cladding layer and the substrate to be inconsistent, causing them to affect each other and form internal stress in the joint between the cladding layer and the substrate. If the internal stress exceeds the tensile strength of the blade substrate, cracks will be generated in the cladding part. If this fault is to be solved, it is necessary to further improve and perfect the process to reduce thermal stress and temperature gradient and achieve high-quality repair of the blade tip.

3 Analysis conclusion
1) Fracture analysis shows that the blade tip crack is a brittle crack caused by stress, and the fracture surface is heavily oxidized. According to the crack characteristics, fracture morphology and repair process, the crack is an overload crack caused by stress. The rear end surface of the blade tip needs to be ground. The grinding crack is generated on the surface and the crack direction is longitudinal, indicating that the crack is not generated by working stress. The thermal stress of this type of blade after laser cladding is large, and stress cracks are easily generated, indicating that the crack is a one-time crack.
2) Through the analysis of the blade structure, blade base material and cladding process, it can be seen that: during the laser cladding process, since the melting and solidification behavior of the material are carried out under extremely fast conditions, the blade to be repaired is prone to metallurgical defects such as cracks, pores, inclusions, and poor interlayer bonding during the laser repair process. Therefore, if the process parameters are not properly controlled, micro cracks are easily generated in the cladding area.
3) From the analysis of the metallographic structure of the blade cladding part, it can be seen that the cladding layer structure is very fine and dense compared with the blade matrix, and no micro cracks are found. From the blade structural characteristics and the large difference in laser melting depth and heat affected zone width, it can be seen that the uneven temperature distribution of the laser molten pool and its vicinity is the main reason for the generation of thermal stress and cracks.

4 Process improvement and control
Combined with the conclusion of the fault analysis, the following treatment measures are taken to troubleshoot the fault:

1) During the laser cladding process, add a preheating process, that is, use low power to heat the blade tip twice (without adding powder), and then perform laser cladding on the blade tip to reduce the local temperature gradient of the molten pool and reduce thermal stress.

2) Change the laser light mode of the equipment to the pulse mode, set appropriate frequency, pulse width ratio and other parameters and match them with the cladding parameters, and achieve the best metallurgical bonding with the minimum heat input.

3) The blade tip is polished as smooth and consistent as possible to reduce the impact of size mutation on cladding quality.
After the improvement measures were implemented, the microstructure of the test blade was inspected, as shown in Figure 5.

As can be seen from Figure 5, the width and depth of the heat-affected zone are uniform, and the grain size and direction are more uniform. The factory repaired 24 high-pressure turbine stage I blades in batches. After X-ray inspection, the qualified rate reached more than 99%, and the overall fault situation was under control.