Method for preventing cracking of diesel engine piston and application

Abstract

This invention discloses a method and application for anti-crack processing of diesel engine pistons, including the following steps: S1, Refined pretreatment of anti-crack special materials: Differentiated pretreatment processes are adopted for pistons made of aluminum alloy, steel, and aluminum-based composite materials to remove internal defects, refine the microstructure, and reduce residual stress. This invention constructs a complete anti-crack system from material pretreatment, precision forming, processing of easily cracked parts, stress relief, cooling optimization to quality inspection, completely blocking cracks caused by thermal fatigue, mechanical fatigue, stress concentration, corrosion, etc., increasing the crack prevention rate by more than 90% and improving the service life of pistons; at the same time, it is compatible with the processing of pistons made of different materials such as aluminum alloy, steel, and aluminum-based composite materials, and is suitable for various diesel engines such as passenger cars, commercial vehicles, marine engines, and construction machinery. The process parameters can be flexibly adjusted according to the design requirements and operating conditions of different pistons without replacing the entire set of processing equipment, thus reducing production costs.

Description

Technical Field

[0001] This invention belongs to the field of diesel engine piston processing and manufacturing technology, and particularly relates to a method and application for preventing cracking of diesel engine pistons. Background Technology

[0002] As the core moving part of the engine, the diesel engine piston operates under extremely harsh conditions: the top is subjected to high temperatures of 300℃-450℃ for a long time, the burst pressure is as high as 15MPa-22MPa, and it is also subjected to high-frequency alternating mechanical stress and thermal stress. In addition, it has to withstand the reciprocating friction of the piston rings and the thermal shock of the cooling oil passages. It is one of the core components of the engine that is most prone to failure and has the highest replacement frequency.

[0003] Traditional diesel engine piston manufacturing processes lack a comprehensive anti-cracking system that spans from material pretreatment, precision forming, crack-prone area reinforcement, stress relief to quality inspection. This results in numerous material defects, concentrated forming stress, poor fatigue resistance in crack-prone areas, insufficient forming precision and sealing reliability of internal cooling oil passages, incomplete residual stress removal, lack of surface strengthening, and incomplete quality inspection. Consequently, pistons are prone to thermal and mechanical fatigue cracks under high temperature, high explosion pressure, and alternating stress conditions, leading to short service life, poor product consistency, and an inability to meet the requirements of high explosion pressure diesel engines and stringent emission standards. Summary of the Invention

[0004] To address the problems in the prior art, the present invention proposes the following technical solution: The method for preventing cracking of diesel engine pistons includes the following steps: S1. Refined pretreatment of crack-resistant materials: Differentiated pretreatment processes are adopted for aluminum alloy, steel and aluminum-based composite pistons to remove internal defects, refine the structure and reduce residual stress. S2, Integrated precision molding and sequential temperature control cooling: The method of combining step-by-step pressure molding and zoned delayed sequential temperature control cooling is adopted to achieve piston sequential solidification and reduce thermal and mechanical stress during the molding process; S3. Crack prevention processing of easily cracked parts: For the three easily cracked areas of combustion chamber throat, ring groove and pin seat, respectively, the combustion chamber throat is processed with large arc mirror surface and laser cladding strengthening, the bottom of the ring groove is processed with large arc and micro chamfer structure and rolled strengthening, and the irregular pin hole is rolled and ion nitriding composite treatment. S4. Internal cooling oil passage forming and sealing: Differentiated oil passage forming processes are adopted for pistons of different materials to ensure uniform oil passage wall thickness and good sealing. S5. Overall stress relief and composite surface strengthening: Graded aging heat treatment is used to eliminate residual stress from machining, and composite surface strengthening treatment is performed on key parts of the piston. S6. Full-size crack prevention quality inspection and verification: Through multi-dimensional non-destructive testing, residual stress testing and bench durability testing, ensure that the piston is free of cracks and defects; S7. Finished Product Packaging and Storage: Use rust-proof packaging and control the storage environment to avoid subsequent damage to the piston.

[0005] As a preferred embodiment of the above technical solution, in S1, the material pretreatment adopts differentiated processes for pistons of different materials: The aluminum alloy piston adopts a two-stage impurity removal and refining process, vacuum degassing, composite modification and refinement, low-temperature slow casting, and gradient cooling. The steel piston is manufactured using electroslag remelting refining, isothermal forging, and temperature-controlled cooling. The aluminum-based composite piston is manufactured using powder metallurgy, hot pressing, and homogenization annealing.

[0006] As a preferred embodiment of the above technical solution, in S2, the sequential temperature control cooling is performed in the order of inner mold, pin seat, annular groove / throat, and gate, with the inner mold cooling temperature ≤200℃, the pin seat cooling temperature ≤280℃, the annular groove cooling temperature ≤150℃, and the throat cooling temperature ≤120℃. Step-by-step pressurization molding includes three stages: pre-pressurization, main pressurization, and pressure holding and cooling. The pre-pressurization pressure is 200MPa-300MPa, and the main pressurization pressure is 800MPa-1000MPa.

[0007] As a preferred embodiment of the above technical solution, in S3, the transition fillet of the combustion chamber throat is R3mm-R5mm, the surface roughness Ra≤0.4μm, and the Ni60A alloy layer is laser-clad with a thickness of 0.3mm-0.5mm; The bottom of the annular groove adopts a large circular arc of R2mm and a micro-chamfer structure of 0.5°×45°. After rolling, a residual compressive stress layer of -300MPa~-500MPa is formed with a depth ≥0.2mm. The pin hole is drum-shaped or conical, with a cylindricity ≤ 0.003 mm, an ion nitriding layer depth of 0.15 mm-0.2 mm, and a surface hardness HV ≥ 700.

[0008] As a preferred embodiment of the above technical solution, in S4, the internal cooling oil passage forming adopts a differentiated process: The aluminum alloy piston adopts a composite process of stainless steel tube inlay casting and salt core, with an oil passage wall thickness uniformity of ±0.1mm; The steel piston is sealed by laser welding and filled with high-temperature resistant heat-insulating powder, and the welding stress is eliminated by slow cooling after welding. The aluminum-based composite piston is reinforced by machining, laser welding for sealing, and plasma spraying.

[0009] As a preferred embodiment of the above technical solution, in S5, the parameters of the staged aging heat treatment are adjusted for pistons made of different materials: The aluminum alloy piston undergoes solution treatment at 510℃-520℃, warm water quenching at 60℃-70℃, and graded aging to eliminate ≥85% of residual stress. The steel piston is subjected to stress-relief annealing and tempering at 600℃-620℃, and the hardness is controlled at HB260-HB290. Composite surface strengthening includes a YSZ thermal barrier coating for the combustion chamber, a graphite-MoS2 composite lubricating coating for the skirt, and overall passivation treatment.

[0010] As a preferred option of the above technical solution, in S6, the full-size crack prevention quality inspection includes raw material inspection, semi-finished product inspection, and finished product inspection. Finished product inspection adopts triple non-destructive testing of ultrasonic flaw detection, penetrant testing, and magnetic particle testing. Residual stress detection adopts X-ray diffraction method. Bench verification adopts 1.2 times rated burst pressure and 2000 hours of durability test to ensure no cracks and no failures.

[0011] This application also provides the application of pistons obtained by the described anti-cracking processing method for diesel engine pistons in the field of diesel engines.

[0012] The beneficial effects of this invention are as follows: 1. This invention constructs a full-process crack prevention system from material pretreatment, precision forming, processing of crack-prone parts, stress relief, cooling optimization to quality inspection, completely blocking cracks caused by thermal fatigue, mechanical fatigue, stress concentration, corrosion, etc., improving the crack prevention rate by more than 90% and increasing the service life of the piston; Meanwhile, it is compatible with the processing of pistons made of different materials such as aluminum alloy, steel, and aluminum-based composite materials, and is suitable for various diesel engines such as passenger cars, commercial vehicles, marine engines, and construction machinery. The process parameters can be flexibly adjusted according to the design requirements and operating conditions of different pistons, without the need to replace the entire set of processing equipment, thus reducing production costs.

[0013] 2. Through structural optimization and surface strengthening of easily cracked parts, this invention improves the crack resistance of the pin seat by more than 30%, the fatigue life of the ring groove by more than 40%, and the thermal fatigue life of the throat by more than 50%. The internal cooling oil passage and the thermal barrier coating work together to reduce the piston top temperature by 60℃-90℃. It can be adapted to 19MPa-22MPa high explosion pressure heavy-duty diesel engines and meets the China VI / Euro VI emission standards.

[0014] 3. This invention employs five-axis linkage CNC machining, time-controlled temperature forming, precision testing, and other processes. The key dimensions of the piston are accurate to ≤ ±0.005mm, the surface roughness Ra is ≤ 0.4μm, the product consistency is improved by more than 80%, the defect rate in mass production is reduced, and the production cost is lowered. Detailed Implementation

[0015] To make the objectives, technical solutions, and advantages of the embodiments of the present invention clearer, the technical solutions of the present invention will be clearly and completely described below in conjunction with the embodiments.

[0016] The present invention provides a method for preventing cracking in diesel engine pistons, applicable to the mass production of various types of diesel engine pistons. Specific application scenarios include: Passenger car diesel engine pistons: Suitable for 1.5L-2.5L displacement passenger cars, such as family cars and SUVs, improving piston reliability and reducing maintenance costs; Commercial vehicle diesel engine pistons: compatible with 5L-16L displacement commercial vehicles, such as heavy trucks and buses, meeting the requirements of high explosion pressure and long mileage (≥500,000 kilometers); Marine diesel engine pistons: compatible with marine diesel engines with a displacement of 20L-100L, resisting corrosion in the marine environment and high temperature and high pressure conditions, and extending service life; Construction machinery diesel engine pistons: Suitable for construction machinery such as excavators and loaders, they can withstand impact loads under harsh working conditions and reduce piston failure.

[0017] Meanwhile, this method is compatible with the processing of pistons made of different materials such as aluminum alloy, steel, and aluminum-based composite materials. The process parameters of each process can be adjusted according to the design requirements and working conditions of different pistons, making it highly flexible and widely adaptable.

[0018] Example 1: Heavy-duty commercial vehicle aluminum alloy piston (16L, burst pressure 20MPa) S1. Refined pretreatment of crack-resistant special materials: Al-12Si-5Cu-2Ni-1Mg aluminum alloy is used. The melt is subjected to two-stage impurity removal and refining (NaCl+KCl+Na3AlF6 refining agent, 0.4wt%), argon degassing (0.6L / min, 6min), and vacuum degassing (-0.1Pa, 22min). The hydrogen content is controlled at 0.10mL / 100g. Add 0.5wt% aluminum-phosphorus modifier, 0.25wt% rare earth yttrium, and 0.15wt% titanium to refine primary silicon to 12μm; Slow pouring at 750℃ (speed 6kg / min), preheat metal mold to 220℃, and then gradually cool to room temperature.

[0019] S2, Integrated precision molding: Mold preheating 220℃, step-by-step pressurization: pre-pressurization 250MPa (12s), main pressurization 900MPa (22s), pressure holding and cooling; Time-controlled cooling: Inner mold cooling 85s (water temperature 23℃, water pressure 7MPa), throat cooling 62s (water temperature 20℃, water pressure 6MPa), gate cooling 25s (water temperature 28℃, water pressure 5MPa). The riser is an annular riser, with a volume of 18% of the piston volume, and is cut and polished.

[0020] S3. Crack-resistant processing for easily cracked areas: Throat: Five-axis linkage precision milling and mirror polishing, R4mm, roundness error 0.008mm, Ra0.32μm, after sandblasting, laser cladding of Ni60A alloy layer with a thickness of 0.4mm, no defects after laser remelting; Annular groove: The bottom of the first annular groove after precision milling has a radius of 2mm and a chamfer of 0.5×45°. The rolling pressure is 900N, the residual compressive stress is -400MPa, the depth is 0.22mm, and the Ra is 0.18μm. Pin holder: Drum-shaped pin hole, precision boring and high-speed rolling, cylindricity 0.002mm, Ra0.18μm, ion nitriding (530℃, 4.5h), nitriding depth 0.18mm, HV720, transition zone R4.5mm, polished Ra0.28μm.

[0021] Internal cooling oil passage forming and sealing: φ8mm stainless steel pipe (wall thickness 0.9mm) is welded to wear-resistant inlay ring as a whole, and the inlay is cast and salt core is formed. After the salt core is dissolved by hot water, it is flushed with high pressure water. The wall thickness uniformity is ±0.08mm. The sealing plug is interference fit for sealing. The water pressure test shows no leakage.

[0022] S5. Overall stress relief and composite surface strengthening: staged aging including solution treatment at 515℃ for 3.5h, quenching in warm water at 65℃, holding at 200℃ for 3h, and holding at 248℃ for 7h, eliminating 88% of residual stress; The combustion chamber is coated with a YSZ coating (0.18mm), the skirt is coated with a graphite-MoS2 coating (25μm), and the whole is passivated.

[0023] S6. Full-size crack prevention quality inspection: UT, PT and magnetic particle inspection showed no cracks or defects, residual stress testing met the standards, and full-size inspection met the design requirements; Bench verification: 1.2 times the rated burst pressure (24MPa), 2000 hours of durability test, no cracks, no deformation, no failure, service life up to 2.3 times that of traditional pistons, crack prevention rate 99.5%.

[0024] Example 2: Marine steel piston (explosive pressure 22MPa) S1. Pre-treatment of crack-resistant special materials: 40CrNiMoA steel ingot, electroslag remelting and refining, sulfur and phosphorus content controlled at 0.012%, isothermal forging at 1160℃ (pressure 900MPa, speed 12mm / s), temperature controlled cooling (7℃ / s), austenite grain grade 7, cooled to 600℃ and held for 1 hour, then naturally cooled.

[0025] S2, Integrated Precision Molding: Mold preheating to 320℃, step-by-step pressurization: pre-pressurization 280MPa (14s), main pressurization 950MPa (24s), pressure holding and cooling; Time-controlled cooling: Inner mold cooling 88s (water temperature 24℃, water pressure 7.5MPa), throat cooling 64s (water temperature 19℃, water pressure 6.5MPa), gate cooling 28s (water temperature 29℃, water pressure 5.5MPa). The riser and gating system was optimized and then polished after cutting.

[0026] S3. Crack-resistant processing for easily cracked areas: Throat: Five-axis linkage milling and polishing, R4.5mm, Ra0.38μm, followed by laser cladding of Ni60A alloy layer (0.45mm) after sandblasting, free of pores and inclusions; Annular groove: R2mm and 0.5×45° chamfer, rolling pressure 950N, residual compressive stress -450MPa, depth 0.23mm, Ra0.19μm; Pin seat: tapered pin hole (taper 1:1000), precision boring and rolling, cylindricity 0.003mm, ion nitriding (535℃, 4.5h), nitriding depth 0.19mm, HV730; transition zone R4.2mm, polished to standard.

[0027] S4. Internal Cooling Oil Passage Forming and Sealing: An annular oil passage groove (6mm width, 9mm depth) is machined on the head. The 42CrMo sealing plate is preheated to 280℃, and the piston head is preheated to 320℃. After interference fit, laser welding is performed (3200W power, 3.5m / min speed). Slow cooling is then performed for 2 hours after welding. Alumina and zirconium silicate insulation powder is injected, and the insulation layer thickness is 0.8mm after curing.

[0028] S5. Overall stress relief and composite surface strengthening: stress relief annealing at 610℃ for 4.5h, oil quenching at 860℃, and air tempering at 590℃ for 4h, with a hardness of HB275. The combustion chamber is coated with a YSZ coating (0.19mm), the skirt is coated with a graphite-MoS2 coating (28μm), and the whole is passivated.

[0029] S6. Full-size crack-resistant quality inspection: Triple non-destructive testing shows no defects, and residual stress meets standards; Bench verification: 1.2 times rated burst pressure (26.4MPa), 2000 hours of durability test, no cracks, no failures, service life ≥12000 hours, crack resistance rate 99.2%.

[0030] Example 3: Aluminum-based composite piston for engineering machinery (Al-SiC, burst pressure 19MPa) S1. Refined pretreatment of crack-resistant special material: Al powder and SiC particles (mass ratio 90:10) are mixed, 0.2wt% stearic acid is added, ball milling is performed for 2.5h, hot pressing is performed (560℃, 35MPa, heat preservation for 1h), homogenization annealing is performed at 510℃ for 2.5h, and the surface is polished to remove oxide scale.

[0031] S2, Integrated Precision Molding: Mold preheating to 280℃, step-by-step pressurization: pre-pressurization 220MPa (11s), main pressurization 850MPa (21s), pressure holding and cooling; Time-controlled temperature cooling: parameters are adapted to aluminum-based composite materials to ensure no deformation during molding.

[0032] S3. Crack-resistant processing for easily cracked areas: The throat opening has a radius of 3.5 mm and a diameter of 0.35 μm. The annular groove has a radius of 2mm, a residual compressive stress of -380MPa after roll forming, a depth of 0.21mm, and a radius of 0.19μm. The pin holder adopts a drum-shaped pin hole, which is precision boring and high-speed rolling composite machining, with a cylindricity of 0.003mm and Ra0.19μm. It is ion nitrided (530℃, 4.2h) with a nitriding layer depth of 0.17mm and a surface hardness of HV710. The transition area between the pin holder and the inner cavity is R4.3mm, and after polishing, Ra0.29μm, which effectively reduces stress concentration.

[0033] S4. Internal Cooling Oil Passage Forming and Sealing: The oil passage hole is machined with a diameter of φ7mm using machining and laser welding sealing processes, with a machining accuracy of ±0.005mm. A sealing plug matching the aluminum-based composite material is selected and sealed by laser welding (power 2800W, speed 2.5m / min) to ensure good sealing. The inner wall of the oil passage is plasma-sprayed with an Al2O3 coating with a thickness of 0.15mm and a surface roughness Ra of 0.28μm to improve the wear resistance and high temperature resistance of the inner wall of the oil passage and prevent the cooling oil from eroding and wearing it.

[0034] S5. Overall stress relief and composite surface strengthening: After homogenization annealing at 510℃ for 2.5h, aging treatment at 230℃ for 5.5h is carried out to completely eliminate residual stress during processing, improve the bonding strength and mechanical properties of aluminum-based composite materials, and avoid delamination and cracking during use. YSZ thermal barrier coating (0.17mm) is sprayed on the combustion chamber surface, and graphite and MoS2 composite lubricating coating (26μm) is sprayed on the skirt. The whole is passivated to improve the piston's resistance to thermal fatigue, wear and corrosion.

[0035] S6. Full-size crack prevention quality inspection: Ultrasonic testing and penetrant testing are used for dual non-destructive testing to ensure that the piston is free of cracks and internal defects; Residual stress in key components was detected by X-ray diffraction, and the values ​​met the standards. The coordinate measuring machine performed full-dimensional inspection, and all dimensions and geometric tolerances met the design requirements; the internal cooling oil passage water pressure test (11MPa, pressure holding for 30min) showed no leakage. Bench verification: 1.2 times rated burst pressure (22.8MPa), 2000-hour durability test, simulating the harsh working conditions of construction machinery. After the test, the piston showed no cracks, no deformation, and no failure. The service life is ≥10500 hours, and the crack prevention rate is 99.3%, which is suitable for the use of diesel engines in construction machinery such as excavators.

[0036] Comparative Example 1 Material pretreatment: A single refining process was adopted, without vacuum degassing or composite modification and refinement. The hydrogen content of the melt was 0.20 mL / 100 g, the primary silicon size was ≥30 μm, and the material was naturally cooled after casting. Residual stress was not eliminated.

[0037] Molding: Ordinary mold molding, natural cooling, no step-by-step pressurization and sequential temperature control cooling, unreasonable gating and riser design, and shrinkage defects on the top of the piston.

[0038] Machining of easily cracked parts: ordinary CNC machining, throat radius R2mm, surface roughness Ra1.3μm, no laser cladding strengthening; the bottom of the ring groove has no large arc + micro chamfer structure, only simple chamfer, no rolling strengthening; the pin seat adopts ordinary round hole, no ion nitriding treatment, surface hardness HV450.

[0039] Internal cooling oil passages: single salt core process, oil passage wall thickness uniformity ±0.3mm, average sealing effect, with a slight risk of leakage.

[0040] Stress relief and strengthening: Single aging treatment, residual stress relief rate is only 50%; no thermal barrier coating or lubricating coating, only simple rust prevention treatment.

[0041] Testing and verification: Only conventional penetrant testing was used, with no residual stress testing; bench verification (24MPa burst pressure, 2000 hours) showed that a ring crack appeared at the throat and fatigue cracks appeared at the pin seat after the test. The service life was only 4500 hours, the crack prevention rate was 75%, and the defect rate was 8%.

[0042] Comparative Example 2 Material pretreatment: Refined by electroslag remelting without electroslag, the ingot has a sulfur and phosphorus content of 0.03%, coarse grains (austenite grain grade 4), and is naturally cooled after forging, resulting in residual stress accumulation.

[0043] Forming: Ordinary forging process, without step-by-step pressurization, uneven mold cooling, excessive piston deformation; the riser and gating were not polished after cutting, resulting in stress concentration at sharp corners.

[0044] Machining of easily cracked parts: ordinary milling of the throat, Ra1.5μm, no laser cladding; insufficient rolling pressure of the ring groove, residual compressive stress only -150MPa, depth 0.1mm; non-standard ion nitriding process of the pin seat, nitriding layer depth 0.1mm, surface hardness HV550.

[0045] Internal cooling oil passages: ordinary welded seals, with porosity defects at the weld joints, no slow cooling treatment, and welding stress not eliminated; no heat insulation powder filling, resulting in poor heat conduction efficiency.

[0046] Stress relief and strengthening: Single stress relief annealing, hardness HB220, insufficient crack resistance; no composite surface strengthening treatment.

[0047] Testing and verification: Single ultrasonic testing alone failed to detect micro-cracks; bench testing (26.4MPa burst pressure, 2000 hours) revealed cracks in the weld joints, severe wear in the annular grooves, a service life of 6000 hours, a crack prevention rate of 78%, and a defect rate of 9%.

[0048] Comparative Example 3 Material pretreatment: Ordinary powder mixing without ball milling homogenization resulted in uneven dispersion of SiC particles; without homogenization annealing, there was component segregation and internal stress within the material.

[0049] Molding: Ordinary hot pressing molding, without step-by-step pressurization and sequential temperature control and cooling, resulting in piston molding accuracy deviation and surface scratches.

[0050] Machining of easily cracked parts: throat Ra1.4μm, residual compressive stress of annular groove -200MPa, depth 0.12mm; pin seat cylindricity 0.008mm, no fillet optimization in the transition zone, resulting in stress concentration.

[0051] Internal cooling oil passages: ordinary machining, no laser welding seals, excessive clearance between the sealing plugs, and leakage was found during water pressure testing; the inner wall of the oil passages is not reinforced with paint, resulting in poor wear resistance.

[0052] Stress relief and strengthening: Without aging treatment, the material bonding strength is insufficient and delamination is prone to occur; without thermal barrier coating and lubrication coating, the thermal fatigue resistance is poor.

[0053] Testing and verification: No residual stress testing, full-size testing accuracy not up to standard; bench verification (22.8MPa burst pressure, 2000 hours), piston delamination and pin seat cracks were found, service life was 4800 hours, crack prevention rate was 76%, and defect rate was 10%.

[0054] Comparative Conclusion: Compared with the traditional processing technology of Comparative Examples 1-3, Examples 1-3 of the present invention, through a full-process crack prevention system (refined material pretreatment, time-controlled temperature molding, precise reinforcement of crack-prone parts, and full-dimensional quality inspection, etc.), improve the piston crack prevention rate by 23%-24%, increase service life by 1.2-2.5 times, reduce the defect rate to below 1%, and significantly improve the key dimensional accuracy, surface roughness, and mechanical properties compared with traditional processes, fully demonstrating the technical advantages and practicality of the present invention.

[0055] The above embodiments are only used to illustrate the technical solutions of the present invention, and are not intended to limit it.

Claims

1. A method for preventing cracking of diesel engine pistons, characterized in that, Includes the following steps: S1. Refined pretreatment of crack-resistant materials: Differentiated pretreatment processes are adopted for aluminum alloy, steel and aluminum-based composite pistons to remove internal defects, refine the structure and reduce residual stress. S2, Integrated precision molding and sequential temperature control cooling: The method of combining step-by-step pressure molding and zoned delayed sequential temperature control cooling is adopted to achieve piston sequential solidification and reduce thermal and mechanical stress during the molding process; S3. Crack prevention processing of easily cracked parts: For the three easily cracked areas of combustion chamber throat, ring groove and pin seat, respectively, the combustion chamber throat is processed with large arc mirror surface and laser cladding strengthening, the bottom of the ring groove is processed with large arc and micro chamfer structure and rolled strengthening, and the irregular pin hole is rolled and ion nitriding composite treatment. S4. Internal cooling oil passage forming and sealing: Differentiated oil passage forming processes are adopted for pistons of different materials to ensure uniform oil passage wall thickness and good sealing. S5. Overall stress relief and composite surface strengthening: Graded aging heat treatment is used to eliminate residual stress from machining, and composite surface strengthening treatment is performed on key parts of the piston. S6. Full-size crack prevention quality inspection and verification: Through multi-dimensional non-destructive testing, residual stress testing and bench durability testing, ensure that the piston is free of cracks and defects; S7. Finished Product Packaging and Storage: Use rust-proof packaging and control the storage environment to avoid subsequent damage to the piston.

2. The diesel engine piston anti-crack processing method according to claim 1, characterized in that, In S1, material pretreatment employs differentiated processes for pistons made of different materials: The aluminum alloy piston adopts a two-stage impurity removal and refining process, vacuum degassing, composite modification and refinement, low-temperature slow casting, and gradient cooling. The steel piston is manufactured using electroslag remelting refining, isothermal forging, and temperature-controlled cooling. The aluminum-based composite piston is manufactured using powder metallurgy, hot pressing, and homogenization annealing.

3. The diesel engine piston anti-crack processing method according to claim 1, characterized in that, In S2, the time-controlled temperature cooling is performed in the following order: inner mold, pin seat, annular groove / throat, and gate. The cooling temperature of the inner mold is ≤200℃, the cooling temperature of the pin seat is ≤280℃, the cooling temperature of the annular groove is ≤150℃, and the cooling temperature of the throat is ≤120℃. Step-by-step pressurization molding includes three stages: pre-pressurization, main pressurization, and pressure holding and cooling. The pre-pressurization pressure is 200MPa-300MPa, and the main pressurization pressure is 800MPa-1000MPa.

4. The diesel engine piston anti-crack processing method according to claim 1, characterized in that, In S3, the transition fillet of the combustion chamber throat is R3mm-R5mm, the surface roughness Ra≤0.4μm, and the Ni60A alloy layer is laser clad with a thickness of 0.3mm-0.5mm; The bottom of the annular groove adopts a large circular arc of R2mm and a micro-chamfer structure of 0.5°×45°. After rolling, a residual compressive stress layer of -300MPa~-500MPa is formed with a depth ≥0.2mm. The pin hole is drum-shaped or conical, with a cylindricity ≤ 0.003 mm, an ion nitriding layer depth of 0.15 mm-0.2 mm, and a surface hardness HV ≥ 700.

5. The diesel engine piston anti-crack processing method according to claim 1, characterized in that, In S4, the internal cooling oil passages are formed using a differentiated process: The aluminum alloy piston adopts a composite process of stainless steel tube inlay casting and salt core, with an oil passage wall thickness uniformity of ±0.1mm; The steel piston is sealed by laser welding and filled with high-temperature resistant heat-insulating powder, and the welding stress is eliminated by slow cooling after welding. The aluminum-based composite piston is reinforced by machining, laser welding for sealing, and plasma spraying.

6. The diesel engine piston anti-crack processing method according to claim 1, characterized in that, In S5, the parameters for staged aging heat treatment are adjusted for pistons made of different materials: The aluminum alloy piston undergoes solution treatment at 510℃-520℃, warm water quenching at 60℃-70℃, and graded aging to eliminate ≥85% of residual stress. The steel piston is subjected to stress-relief annealing and tempering at 600℃-620℃, and the hardness is controlled at HB260-HB290. Composite surface strengthening includes a YSZ thermal barrier coating for the combustion chamber, a graphite-MoS2 composite lubricating coating for the skirt, and overall passivation treatment.

7. The diesel engine piston anti-crack processing method according to claim 1, characterized in that, In S6, full-size crack prevention quality inspection includes raw material inspection, semi-finished product inspection, and finished product inspection. Finished product inspection adopts triple non-destructive testing of ultrasonic testing, penetrant testing, and magnetic particle testing. Residual stress detection adopts X-ray diffraction method. Bench verification adopts 1.2 times rated burst pressure and 2000 hours of durability test to ensure no cracks and no failures.

8. The application of the piston obtained by the anti-cracking processing method for diesel engine pistons according to any one of claims 1-7 in the field of diesel engines.