High-performance heterogeneously-welded special-shaped combustion chamber piston and preparation method thereof

Abstract

The invention discloses a high-performance heterogeneously-welded special-shaped combustion chamber piston and a preparation method thereof, a special-basin-shaped combustion chamber and an oil duct structure are directly formed through a rolling process, and heterogeneously-connected between a head part and a skirt part of the piston is realized in combination with a nitrogen protection friction welding technology. The piston head is made of low-silicon high-chromium ferrite-based alloy, the skirt is made of quenched and tempered steel, non-quenched and tempered steel or nodular cast iron, and the welding face is designed to be in a wave shape so as to enhance the bonding strength. The preparation process comprises vacuum smelting, multi-pass rolling, precise welding and surface treatment, the combustion efficiency, the lightweight level and the reliability of the piston are remarkably improved, and the piston is suitable for high-load working conditions such as diesel engines and natural gas engines.

Description

Technical Field

[0001] This application relates to the technical field of engine parts, and particularly to a special-shaped combustion chamber piston with high-performance heterogeneous welding and a preparation method thereof. Background Art

[0002] As a core component of an engine, the piston needs to bear alternating mechanical loads and thermal loads, and has been working in a harsh environment of high temperature, high pressure, high speed and poor lubrication for a long time. For new energy internal combustion engines such as hydrogen-fueled and methanol-fueled engines, the shape of the piston combustion chamber directly affects the combustion efficiency, and corrosive gases (such as formic acid) and liquids generated by fuel combustion are more likely to cause piston surface corrosion, fatigue cracking and wear failure.

[0003] Existing special-shaped combustion chamber pistons generally adopt the process of forging the head and skirt from the same material bar, friction welding and then machining the combustion chamber. This process has significant defects: 1. The machining allowance for the special-shaped combustion chamber, oil passage and ring groove of the piston is large, the manufacturing efficiency is low, and the cost is high: moreover, the forging and forming and machining processes of the piston blank are complex, the production beat is slow, and the material utilization rate is low, resulting in high manufacturing costs; 2. The performance bottleneck is prominent: Machining destroys the continuity of the metal streamline in parts such as the combustion chamber and ring groove, resulting in a decrease in corrosion resistance and fatigue strength, and it is difficult to meet the corrosive environment requirements of new energy fuels; 3. The heat efficiency loss is significant: The traditional material has a high thermal conductivity, the heat dissipation of the piston head is serious, and the utilization rate of combustion energy is low, which restricts the improvement of the engine heat efficiency.

[0004] Therefore, breaking through the limitations of the existing process and developing a manufacturing technology for special-shaped combustion chamber pistons with low cost, high corrosion resistance and high reliability has become a key problem to be solved urgently in the field of new energy engines. Summary of the Invention

[0005] The present invention mainly aims at the problems of high cost and insufficient performance of the existing process, and provides a special-shaped combustion chamber piston with high-performance heterogeneous welding and a preparation method thereof.

[0006] The object of the present invention is mainly achieved through the following solutions: On the one hand, the present invention provides a technical solution: A special-shaped combustion chamber piston with high-performance heterogeneous welding, including a piston head and a piston skirt. The piston head is directly formed into a basin-shaped or special basin-shaped combustion chamber through a rolling process, and is connected to the piston skirt through friction welding under nitrogen protection; The chemical composition of the piston head material by weight percentage is as follows: C 0.04 - 0.08%, Si 2.95 - 3.25%, P 0 - 0.02%, S 0 - 0.005%, Mn 0.6 - 1.0%, Cr 2.0 - 2.5%, Cu 1.0 - 1.5%, Al 0.01 - 0.03%, V 0.01 - 0.03%, Ti 0.02 - 0.05%, N 0.011 - 0.019%, and the balance is Fe; and the matrix structure of the piston head material is ferrite + pearlite, where ferrite is greater than 90% and pearlite is not more than 10%, with a hardness of 180 - 240 HBW, a tensile strength of ≥450 Mpa, a yield strength of ≥300 Mpa, and an elongation greater than 30%; The piston skirt material is 38MnVS6 non - quenched and tempered steel, 42CrMo quenched and tempered steel, or ductile iron.

[0007] On the other hand, the present invention also provides a technical solution: a preparation method of a special - shaped combustion chamber piston with high - performance heterogeneous welding, including the following steps: S1. Preparation of the piston head blank: Accurately weigh each raw material and weigh it according to the proportion. Through vacuum furnace smelting, LF over - refining plus titanium process, and VD vacuum treatment, the melting temperature is controlled at 1650 - 1750 °C, and then a continuous casting method is used to form a piston head slab with a thickness greater than 200 mm; S2. Rolling treatment of the piston head slab: Heat the piston head slab with residual temperature to 1200 - 1280 °C, and perform multi - pass rolling treatment, including cutting the head and tail after rough rolling, cutting the head during medium rolling, pre - finishing rolling, and finishing rolling. The rolling ratio is not less than 1:8. After rolling, controlled - temperature cooling is carried out and flame cleaning is performed. After finishing rolling, the thickness of the piston head slab is reduced to 20 - 50 mm; S3. Forming of the piston head combustion chamber and oil channels: Distribute special patterns on the rough rolling, medium rolling, pre - finishing rolling, and finishing rolling rolls of the piston head. Among them, the surface of the upper roll is distributed with patterns corresponding to the shape of the piston's different - shaped combustion chamber and piston valve pits, and the surface of the lower roll is distributed with patterns corresponding to the shape of half of the piston's oil channels and the upper wall plus the bottom of the piston's first ring groove; after rolling, multiple rows of basin - shaped combustion chambers + valve pits are formed on the upper surface of the piston head slab, and multiple rows of half - oil channels + the upper wall plus the bottom of the first ring groove are formed at the corresponding positions on the lower surface; S4. Cutting and forming of the piston head: Use stamping or laser cutting methods on the rolled piston head slab to form a piston head with a different - shaped combustion chamber and half - oil channels; S5. Preparation of the piston skirt: The piston skirt is prepared by forging, casting, or machining processes; S6. Heterogeneous welding of the piston head and piston skirt. The heterogeneous welding of the piston head and piston skirt is carried out by using nitrogen-protected friction welding technology. After welding, stress-relieving annealing treatment is performed, and then the semi-finished product is finely processed to form a piston semi-finished product with a pin hole and an outer circle having a special profile. S7. Surface treatment. After fine processing, the piston semi-finished product is phosphated and a nano-coating is screen-printed on the surface of the skirt to form a finished piston.

[0008] Preferably, in S3, a heterogeneous pot-shaped combustion chamber and the shape of the piston valve pit are pre-rolled on the upper surface of the piston head slab. The surface roughness of the rolled pot-shaped combustion chamber and valve pit is less than Rz30, and the surface dimensional tolerance reaches ±0.3 mm.

[0009] Preferably, in S3, a half cooling oil passage and the shape of the upper wall and bottom of the piston first ring groove are pre-rolled at the position corresponding to the combustion chamber on the lower surface of the piston head slab. The surface roughness of the rolled oil passage and ring groove is less than Rz30, and the surface dimensional tolerance reaches ±0.3 mm.

[0010] Preferably, the welding surface of the piston head is formed by rolling, and the roughness of the welding surface of the piston head is less than Rz30, and the surface dimensional tolerance reaches ±0.3 mm.

[0011] Preferably, the welding surface of the piston skirt is formed by machining, and the welding surface of the piston skirt is wavy, with the width of the wave being 2 ± 0.1 mm and the depth of the wave being 1 ± 0.1 mm.

[0012] Preferably, in S6, the nitrogen-protected friction welding technology includes the following steps: S1. Clean the semi-finished products of the piston head and piston skirt to ensure cleanliness, where the maximum particle size of impurities is less than 300 microns and the weight is less than 5 mg. S2. Nitrogen-protected friction welding. During the welding process, the nitrogen flow rate is 8 - 10 L / m³, the pressure is 0.8 - 1 MPa, the purity is above 99.999%. The special second-order friction welding process parameters are: the first-order pressure is 150 ± 10 Bar, the second-order pressure is 40 ± 5 Bar, the rotational speed is 1200 RPM, the pressure holding time is 3 - 5 seconds, and the welding shrinkage is 4 ± 0.3 mm.

[0013] Preferably, the center position of the weld of the piston head and piston skirt is located at the bottom of the piston first ring groove, that is, the upper wall and bottom of the piston first ring groove are formed by extrusion during rolling, and the surface hardness of the upper wall and bottom of the piston first ring groove formed by rolling is greater than 250 HV, and the strength perpendicular to the rolling direction is greater than 500 MPa.

[0014] In summary, compared with the prior art, the present invention has the following beneficial technical effects: (1) The present invention reduces the manufacturing cost, eliminates the traditional forging and machining processes of the special-shaped combustion chamber, directly forms complex structures such as the combustion chamber and oil passage through rolling, improves the production efficiency, and has a high material utilization rate in the rolling process; (2) The present invention improves the corrosion resistance and wear resistance. The low-carbon high-silicon chromium alloy system forms a ferrite-rich matrix, combines with V, Ti, and N elements to generate carbide and nitride strengthening phases, improving the anti-formic acid corrosion ability of the combustion chamber surface. Moreover, the high rolling ratio makes the ferrite grains arrange parallel to the surface, forming a strain-hardened layer and reducing the risk of wear failure; (3) The present invention improves the thermal efficiency and reliability. Silicon elements reduce the thermal conductivity of the material, reduce the heat dissipation from the combustion chamber to the piston head, improve the combustion thermal efficiency of the engine, achieve energy conservation and emission reduction. The nitrogen protection two-stage friction welding process combines with the wavy welding surface of the skirt, effectively compensating for the difference in thermal expansion coefficients between the head and the skirt, and improving the fatigue life; (4) The present invention has process compatibility and market adaptability. The skirt material can be flexibly adapted, compatible with existing mature materials such as 38MnVS6 non-quenched and tempered steel, 42CrMo quenched and tempered steel, and ductile iron, without modifying the existing skirt production line. The welding process is stable, suitable for large-scale industrial production. The special-shaped basin combustion chamber precisely matches the combustion characteristics of hydrogen and methanol fuels, with stable and reliable performance. Brief Description of the Drawings

[0015] Figure 1 is the process flow chart of the piston production process of the present invention; Figure 2 is the schematic diagram of the upper rolling roll with the shape of the piston combustion chamber in the present invention; Figure 3 is the schematic diagram of the upper surface of the piston head slab after rolling in the present invention; Figure 4 is the schematic diagram of the combustion chamber surface of the piston head formed by rolling and cutting the upper rolling roll in the present invention; Figure 5 is the schematic diagram of the lower rolling roll with the shape of the piston oil passage in the present invention; Figure 6 is the schematic diagram of the lower surface of the piston head slab after rolling in the present invention; Figure 7 is the schematic diagram of the oil passage surface of the piston head formed by rolling and cutting the lower rolling roll in the present invention; Figure 8 is the schematic diagram of the semi-finished product after welding of the special-shaped combustion chamber piston in the present invention; Figure 9 is the 500-fold schematic diagram of the metallographic structure of the special-shaped combustion chamber piston head in Example 1 of the present invention; Figure 10 is the 500-fold schematic diagram of the metallographic structure after welding the piston head and the 38MnVS6 non-quenched and tempered steel skirt in Example 1 of the present invention; Figure 11 It is a 500 - fold schematic diagram of the metallographic structure of the piston head with a special - shaped combustion chamber in Embodiment 2 of the present invention; Figure 12 It is a 500 - fold schematic diagram of the metallographic structure after welding the piston head with a 42CrMo quenched and tempered steel skirt in Embodiment 2 of the present invention; Figure 13 It is a 500 - fold schematic diagram of the metallographic structure of the piston head with a special - shaped combustion chamber in Embodiment 3 of the present invention; Figure 14 It is a 500 - fold schematic diagram of the metallographic structure after welding the piston head with a nodular cast iron skirt in Embodiment 3 of the present invention. Detailed implementation manners

[0016] The following will further specifically illustrate the technical solutions of the present invention through specific embodiments and in combination with the accompanying drawings. It should be understood that the implementation of the present invention is not limited to the following embodiments, and any formal modification and / or change made to the present invention will fall within the protection scope of the present invention.

[0017] As Figure 1 shown, the present invention discloses a technical solution, a high - performance heterogeneous - welded special - shaped combustion chamber piston, including a piston head and a piston skirt. The piston head directly forms a shallow - basin - shaped or special - shaped - basin - shaped combustion chamber through a rolling process and is connected to the piston skirt through friction welding under nitrogen protection. The shape of the combustion chamber is generally a shallow - basin shape, and the cross - section of the combustion chamber is various shapes such as a perfect circle, an ellipse, a heart - shape, a petal - shape, etc. The depth of the combustion chamber is generally 1 - 20 mm, and it is particularly suitable for steel pistons of new - energy diesel engines, such as hydrogen engines, methanol engines, etc.; Among them, the material of the piston head, by weight percentage, has the following chemical composition: C 0.04 - 0.08%, Si 2.95 - 3.25%, P 0 - 0.02%, S 0 - 0.005%, Mn 0.6 - 1.0%, Cr 2.0 - 2.5%, Cu 1.0 - 1.5%, Al 0.01 - 0.03%, V 0.01 - 0.03%, Ti 0.02 - 0.05%, N 0.011 - 0.019%, and the balance is Fe; and the matrix structure of the piston - head material is ferrite + pearlite, in which ferrite is greater than 90% and pearlite is not greater than 10%, with a hardness of 180 - 240 HBW, a tensile strength ≥ 450 Mpa, a yield strength ≥ 300 Mpa, and an elongation greater than 30%; The material of the piston skirt is 38MnVS6 non - quenched and tempered steel, 42CrMo quenched and tempered steel or nodular cast iron, among which Figure 1 the skirt part is an existing forged - steel piston, but is not limited to this material.

[0018] The present invention also discloses a technical solution, a preparation method of a high - performance heterogeneous - welded special - shaped combustion chamber piston, including the following steps: S1. Preparation of the piston head blank: Weigh each raw material accurately and proportionally. Through vacuum furnace smelting, LF refining with titanium addition process, and VD vacuum treatment, the melting temperature is controlled at 1650 - 1750 °C, and then a continuous casting method is used to form a piston head slab with a thickness greater than 200 mm. S2. Rolling treatment of the piston head slab: Heat the piston head slab with residual temperature to 1200 - 1280 °C, and perform multiple rolling treatments, including cutting the head and tail after rough rolling, cutting the head during medium rolling, pre-finishing rolling, and finishing rolling. The rolling ratio is not less than 1:8. After rolling, control the temperature for cooling and perform flame cleaning. After finishing rolling, the thickness of the piston head slab is reduced to 20 - 50 mm. S3. Forming the combustion chamber and oil channels of the piston head: As Figures 2 - 7 shown, distribute special patterns on the rough rolling, medium rolling, pre-finishing rolling, and finishing rolling rolls of the piston head. Among them, the surface of the upper roll is distributed with patterns corresponding to the shape of the piston's abnormal-shaped combustion chamber and piston valve pit, and the surface of the lower roll is distributed with patterns corresponding to the shape of half of the piston's oil channel and the upper wall plus the bottom of the piston's first ring groove; after rolling, multiple rows of basin-shaped combustion chambers + valve pits are formed on the upper surface of the piston head slab, and multiple rows of half oil channels + the upper wall of the first ring groove plus the bottom are formed at the corresponding positions on the lower surface. S4. Cutting and forming the piston head: Use stamping or laser cutting methods for the rolled piston head slab to form a piston head with an abnormal-shaped combustion chamber and a half oil channel. S5. Preparation of the piston skirt: The piston skirt is prepared by forging, casting, or machining processes. S6. Heterogeneous welding of the piston head and piston skirt: Use nitrogen protection friction welding technology for heterogeneous welding of the piston head and piston skirt. After welding, perform stress relief annealing treatment, and then perform finish machining on the semi-finished product. As Figure 8 shown, form a piston semi-finished product with a pin hole and an outer circle having a special profile. S7. Surface treatment: After finish machining, the piston semi-finished product is phosphated and a nano-coating is screen-printed on the surface of the skirt to form a finished piston.

[0019] Specifically, in S3, pre-roll the abnormal-shaped combustion chamber and the shape of the piston valve pit on the upper surface of the piston head slab. Among them, the surface roughness of the rolled basin-shaped combustion chamber and valve pit is less than Rz30, and the surface dimensional tolerance reaches ±0.3 mm.

[0020] Specifically, in S3, pre-roll half of the cooling oil channel and the shape of the upper wall and bottom of the piston's first ring groove at the position corresponding to the combustion chamber on the lower surface of the piston head slab. Among them, the surface roughness of the rolled oil channel and ring groove is less than Rz30, and the surface dimensional tolerance reaches ±0.3 mm.

[0021] Specifically, the welding surface of the piston head is formed by rolling, and the roughness of the welding surface of the piston head is less than Rz30, and the dimensional tolerance of the surface reaches ±0.3 mm.

[0022] Specifically, the welding surface of the piston skirt is formed by machining, and the welding surface of the piston skirt is wavy, the width of the wave is 2 ± 0.1 mm, and the depth of the wave is 1 ± 0.1 mm.

[0023] Specifically, in S6, the nitrogen protection friction welding technology includes the following steps: S1. Clean the semi-finished products of the piston head and piston skirt to ensure cleanliness, where the maximum particle size of impurities is less than 300 microns and the weight is less than 5 mg; S2. Nitrogen protection friction welding. During the welding process, the nitrogen flow rate is 8 - 10 L / m³, the pressure is 0.8 - 1 MPa, the purity is above 99.999%, and the special second-order friction welding process parameters are: the first-order pressure is 150 ± 10 Bar, the second-order pressure is 40 ± 5 Bar, the rotation speed is 1200 RPM, the pressure holding time is 3 - 5 seconds, and the welding shortening amount is 4 ± 0.3 mm.

[0024] Specifically, the center position of the weld of the piston head and piston skirt is located at the bottom of the first piston ring groove, that is, the upper wall and the bottom of the first piston ring groove are formed by extrusion during rolling, and the surface hardness of the upper wall and the bottom of the first piston ring groove formed by rolling is greater than 250 HV, and the strength perpendicular to the rolling direction is greater than 500 Mpa.

[0025] The following further specifically describes the present application through specific embodiments: Embodiment 1: A preparation method of a special-shaped combustion chamber piston with high-performance heterogeneous welding includes the following steps: S1. Preparation of the piston head blank. Accurately weigh each raw material and weigh it according to the proportion. Through vacuum furnace smelting, LF over-refining plus titanium process, and VD vacuum treatment, the melting temperature is controlled at 1650 - 1750 °C, and then a piston head slab with a thickness of 210 mm is formed by continuous casting; S2. Rolling treatment of the piston head slab. Heat the piston head slab with residual temperature to 1200 - 1280 °C, and perform multi-pass rolling treatment, including rough rolling and then cutting off the head and tail, medium rolling and cutting the head, pre-finishing rolling, and finishing rolling. The rolling ratio is not less than 1:8. After rolling, control the temperature for cooling and perform flame cleaning. After finishing rolling, the thickness of the piston head slab is thinned to 25 mm; S3. Forming the combustion chamber and oil channels on the piston head. Special patterns are distributed on the rough rolling, intermediate rolling, pre-finishing rolling, and finishing rolling rolls of the piston head. Among them, patterns corresponding to the shape of the piston's abnormal basin-shaped combustion chamber and piston valve pits are distributed on the surface of the upper roll, and patterns corresponding to half of the piston's oil channels and the upper wall plus the bottom of the piston's first ring groove are distributed on the surface of the lower roll. The depth of the combustion chamber is generally 5 mm. After rolling, multiple rows of basin-shaped combustion chambers + valve pits are formed on the upper surface of the piston head slab, with a surface roughness of Rz25 and a surface dimensional tolerance of ±0.3 mm. Multiple rows of half oil channels + the upper wall plus the bottom of the first ring groove are formed at the corresponding positions on the lower surface, with a surface roughness of Rz22 and a surface dimensional tolerance of ±0.3 mm. The welding surface of the piston head is also formed by rolling, with a welding surface roughness of Rz18 and a surface dimensional tolerance of ±0.3 mm. S4. Forming the piston head by cutting. The rolled piston head slab is formed into a piston head with an abnormal basin-shaped combustion chamber and half oil channels by stamping or laser cutting. The matrix structure of the piston head material is ferrite + a small amount of pearlite, where ferrite is 95%, pearlite is 5%, the hardness is 195 HBW, the tensile strength is 500 Mpa, the yield strength is 310 Mpa, and the elongation is 31%. S5. Preparing the piston skirt. The material of the piston skirt is the existing 38MnVS6 non-quenched and tempered steel. The welding surface of the piston skirt is formed by machining, and the welding surface of the skirt is wavy, with the width of the wave being 2 ± 0.1 mm and the depth of the wave being 1 ± 0.1 mm. Since there are differences in the hardness, strength, and shrinkage rate between the skirt material and the head welding surface, adding waves to the skirt welding surface can offset the problem of heterogeneous welding defects caused by the differences between the two. S6. Heterogeneous welding of the piston head and piston skirt. The nitrogen protection friction welding technology is used for the heterogeneous welding of the piston head and piston skirt. This new nitrogen protection special second-order friction welding technology includes the following steps: First step, cleaning the semi-finished products of the piston head and piston skirt to ensure cleanliness, where the maximum particle size of impurities is less than 300 microns and the weight is less than 5 mg. Second step, nitrogen protection friction welding. During the welding process, the nitrogen flow rate is 8 - 10 L / m³, the pressure is 0.8 - 1 MPa, and the purity is above 99.999%. The special second-order friction welding process parameters are: the first-order pressure is 150 ± 10 Bar, the second-order pressure is 40 ± 5 Bar, the rotational speed is 1200 RPM, the pressure holding time is 3 - 5 seconds, and the welding shortening amount is 4 ± 0.3 mm. After welding, stress relief annealing treatment is carried out, and then the semi-finished products are finely processed to form a piston semi-finished product with special-shaped pin holes and outer circles. S7. Surface treatment. After fine processing, the piston semi-finished product is formed into the finished product of the special-shaped combustion chamber piston with high-performance heterogeneous welding after phosphating and screen printing a nano-coating on the skirt surface.

[0026] Verify its better corrosion resistance of the piston combustion chamber and the fire deck surface through the formic acid simulation corrosion test (corrosive solution: pour 100 ml of formic acid into 500 ml of water, add 2.5 g of sodium chloride, dissolve and dilute to 1000 ml, shake well; corrosive corrosion is carried out in the corrosive solution at 80 °C for 60 minutes), that is, it can effectively prevent the corrosion of the piston top and the fire deck by formic acid generated during combustion.

[0027] Such as Figure 9 It is a 500-fold schematic diagram of the metallographic structure of the piston head of the special-shaped combustion chamber in this embodiment. Figure 10 It is a 500-fold schematic diagram of the metallographic structure after welding the piston head and the skirt of 38MnVS6 non-quenched and tempered steel in this embodiment.

[0028] Embodiment 2: A preparation method of a high-performance special-shaped combustion chamber piston with heterogeneous welding includes the following steps: S1. Preparation of the piston head blank: Accurately weigh each raw material and weigh it according to the proportion. Through vacuum furnace smelting, LF refining with titanium addition process, and VD vacuum treatment, the melting temperature is controlled at 1650 - 1750 °C, and then a piston head slab with a thickness of 210 mm is formed by continuous casting. S2. Rolling treatment of the piston head slab: Heat the piston head slab with residual temperature to 1200 - 1280 °C, and carry out multi-pass rolling treatment, including cutting the head and tail after rough rolling, cutting the head in medium rolling, pre-finishing rolling, and finishing rolling. The rolling ratio is not less than 1:8. After rolling, control the temperature for cooling and carry out flame cleaning. After finishing rolling, the thickness of the piston head slab is thinned to 25 mm. S3. Forming of the piston head combustion chamber and oil channels: Distribute special patterns on the rough rolling, medium rolling, pre-finishing rolling, and finishing rolling rolls of the piston head. Among them, the surface of the upper roll is distributed with patterns corresponding to the shape of the piston's special-shaped combustion chamber and piston valve pits, and the surface of the lower roll is distributed with patterns corresponding to the shape of half of the piston oil channels and the upper wall plus the bottom of the piston's first ring groove; the depth of the combustion chamber is generally 5 mm. After rolling, multiple rows of basin-shaped combustion chambers + valve pits are formed on the upper surface of the piston head slab, with a surface roughness Rz25 and a surface dimensional tolerance of ±0.3 mm. Multiple rows of half oil channels + the upper wall plus the bottom of the first ring groove are formed at the corresponding positions on the lower surface, with a surface roughness Rz25 and a surface dimensional tolerance of ±0.3 mm. The welding surface of the piston head is also formed by rolling, with a welding surface roughness Rz18 and a surface dimensional tolerance of ±0.3 mm. S4. Cutting and forming of the piston head: Use stamping or laser cutting methods to form a piston head with a special-shaped combustion chamber and half oil channels from the rolled piston head slab. The matrix structure of the piston head material is ferrite + a small amount of pearlite, among which ferrite is 95%, pearlite is 5%, the hardness is 195 HBW, the tensile strength is 500 Mpa, the yield strength is 310 Mpa, and the elongation is 31%. S5. Preparation of the piston skirt. The material of the piston skirt is the existing quenched and tempered 42CrMo steel. The welding surface of the piston skirt is formed by machining, and the skirt welding surface is wavy. The width of the wave is 2 ± 0.1 mm, and the depth of the wave is 1 ± 0.1 mm. Since there are differences in the hardness, strength, and shrinkage rate between the skirt material and the head welding surface, adding waves to the skirt welding surface can offset the heterogeneous welding defect problems caused by the differences between the two; S6. Heterogeneous welding of the piston head and the piston skirt. The nitrogen protection friction welding technology is used for the heterogeneous welding of the piston head and the piston skirt. This new nitrogen protection special second-order friction welding technology includes the following steps: First step, clean the semi-finished products of the piston head and the piston skirt to ensure cleanliness, where the maximum particle size of impurities is less than 300 microns and the weight is less than 5 mg. Second step, nitrogen protection friction welding. During the welding process, the nitrogen flow rate is 8 - 10 L / m³, the pressure is 0.8 - 1 MPa, and the purity is above 99.999%. The process parameters of the special second-order friction welding are: the first-order pressure is 150 ± 10 Bar, the second-order pressure is 40 ± 5 Bar, the rotation speed is 1200 RPM, the pressure holding time is 3 - 5 seconds, and the welding shortening amount is 4 ± 0.3 mm; After welding, stress relief annealing treatment is carried out, and then the semi-finished products are finely processed to form a piston semi-finished product with a special profile pin hole and outer circle; S7. Surface treatment. After fine processing, the piston semi-finished product forms the finished product of the special-shaped combustion chamber piston with high-performance heterogeneous welding after phosphating and screen printing a nano-coating on the skirt surface.

[0029] Through the formic acid simulation corrosion test (corrosive corrosion solution: 100 mI of formic acid is poured into 500 ml of water, 2.5 g of sodium chloride is added and dissolved, and then diluted to 1000 ml and shaken well; corrosive corrosion is carried out with the corrosive solution at 80 °C for 60 minutes) to verify that the piston combustion chamber and the surface of the fire deck have better corrosion resistance, that is, it can effectively prevent the corrosion of the piston top and the fire deck parts caused by the formic acid generated during combustion.

[0030] As Figure 11 is a 500-fold schematic diagram of the metallographic structure of the special-shaped combustion chamber piston head in this embodiment; Figure 12 is a 500-fold schematic diagram of the metallographic structure after welding the piston head and the 42CrMo quenched and tempered steel skirt in this embodiment.

[0031] Example 3: A preparation method of a special-shaped combustion chamber piston with high-performance heterogeneous welding, including the following steps: S1. Preparation of the piston head blank. Accurately weigh each raw material and weigh it according to the proportion. Through vacuum furnace smelting, LF over-refining plus titanium process, and VD vacuum treatment, the melting temperature is controlled at 1650 - 1750 °C, and then a piston head slab with a thickness of 210 mm is formed by continuous casting; S2. Rolling treatment of the piston head slab: Heat the piston head slab with residual heat to 1200 - 1280 °C and perform multiple rolling treatments, including cutting the head and tail after rough rolling, cutting the head during medium rolling, pre-finishing rolling, and finishing rolling. The rolling ratio is not less than 1:8. After rolling, control the temperature for cooling and perform flame cleaning. After finishing rolling, the thickness of the piston head slab is reduced to 25 mm. S3. Forming the combustion chamber and oil channels in the piston head: Distribute special patterns on the rough rolling, medium rolling, pre-finishing rolling, and finishing rolling rolls of the piston head. Among them, patterns corresponding to the abnormal basin-shaped combustion chamber and piston valve pit shapes are distributed on the surface of the upper roll, and patterns corresponding to half of the piston oil channels and the upper wall plus the bottom of the piston first ring groove are distributed on the surface of the lower roll. The depth of the combustion chamber is generally 5 mm. After rolling, multiple rows of basin-shaped combustion chambers + valve pits are formed on the upper surface of the piston head slab, with a surface roughness of Rz25 and a surface dimensional tolerance of ±0.3 mm. Multiple rows of half oil channels + the upper wall plus the bottom of the first ring groove are formed at the corresponding positions on the lower surface, with a surface roughness of Rz25 and a surface dimensional tolerance of ±0.3 mm. The welding surface of the piston head is also formed by rolling, with a welding surface roughness of Rz18 and a surface dimensional tolerance of ±0.3 mm. S4. Cutting and forming the piston head: Use stamping or laser cutting methods on the rolled piston head slab to form a piston head with an abnormal basin-shaped combustion chamber and half oil channels. The matrix structure of the piston head material is ferrite + a small amount of pearlite, where ferrite is 95% and pearlite is 5%. The hardness is 195 HBW, the tensile strength is 500 Mpa, the yield strength is 310 Mpa, and the elongation is 31%. S5. Preparation of the piston skirt: The material of the piston skirt is the existing ductile iron material with the grade QT400 - 18. The welding surface of the piston skirt is formed by machining, and the welding surface of the skirt is wavy, with the width of the wave being 2 ± 0.1 mm and the depth of the wave being 1 ± 0.1 mm. Due to the differences in hardness, strength, and shrinkage rate between the skirt material and the head welding surface, adding waves to the skirt welding surface can offset the problem of heterogeneous welding defects caused by the differences between the two. S6. Heterogeneous welding of the piston head and piston skirt. The heterogeneous welding of the piston head and piston skirt is carried out by using the nitrogen protection friction welding technology. This new nitrogen protection special second-order friction welding technology includes the following steps: First step, clean the semi-finished products of the piston head and piston skirt to ensure cleanliness, where the maximum particle size of impurities is less than 300 microns and the weight is less than 5 milligrams. Second step, nitrogen protection friction welding. During the welding process, the nitrogen flow rate is 8 - 10 L / m³, the pressure is 0.8 - 1 MPa, and the purity is above 99.999%. The process parameters of the special second-order friction welding are: the first-order pressure is 150 ± 10 Bar, the second-order pressure is 40 ± 5 Bar, the rotational speed is 1200 RPM, the pressure holding time is 3 - 5 seconds, and the welding shrinkage is 4 ± 0.3 mm. After welding, stress relief annealing treatment is carried out, and then the semi-finished products are finely processed to form piston semi-finished products with pin holes and outer circles of special profiles. S7. Surface treatment. After fine processing, the piston semi-finished products are phosphated and a nano-coating is screen-printed on the skirt surface to form the finished product of the new high-performance heterogeneous welding special-shaped combustion chamber piston.

[0032] Verify that its piston combustion chamber and the surface of the fire deck have better corrosion resistance through the formic acid simulation corrosion test (aggressive corrosion solution: pour 100 ml of formic acid into 500 ml of water, add 2.5 g of sodium chloride and dissolve, then dilute to 1000 ml and shake well; the aggressive corrosion is to corrode with the corrosion solution at 80 °C for 60 minutes), that is, it can effectively prevent the corrosion of the piston top and the fire deck part caused by the formic acid generated during combustion.

[0033] As Figure 13 is a 500-fold schematic diagram of the metallographic structure of the special-shaped combustion chamber piston head in this embodiment; Figure 14 is a 500-fold schematic diagram of the metallographic structure after welding the piston head and the ductile iron skirt in this embodiment.

[0034] A new high-performance heterogeneous welding special-shaped combustion chamber piston and its preparation method provided by the present invention provide a low-cost and high-performance solution for the pistons of new energy engines through the collaborative innovation of materials - processes - structures, and have important significance for promoting the commercial application of hydrogen energy and methanol fuel engines.

[0035] The above are all the preferred embodiments of this application. The protection scope of this application is not limited accordingly. Therefore, all equivalent changes made according to the structure, shape, and principle of this application should be covered within the protection scope of this application.

Claims

1. A special-shaped combustion chamber piston with high-performance heterogeneous welding, comprising a piston head and a piston skirt, characterized in that: The piston head directly forms a basin-shaped or abnormal basin-shaped combustion chamber through a rolling process, and is connected to the piston skirt through friction welding under nitrogen protection; The chemical composition of the piston head material by weight percentage is: C 0.04 - 0.08%, Si 2.95 - 3.25%, P 0 - 0.02%, S 0 - 0.005%, Mn 0.6 - 1.0%, Cr 2.0 - 2.5%, Cu 1.0 - 1.5%, Al 0.01 - 0.03%, V 0.01 - 0.03%, Ti 0.02 - 0.05%, N 0.011 - 0.019%, and the balance is Fe; and the matrix structure of the piston head material is ferrite + pearlite, where ferrite is greater than 90% and pearlite is not more than 10%, with a hardness of 180 - 240 HBW, a tensile strength of ≥450 Mpa, a yield strength of ≥300 Mpa, and an elongation greater than 30%; The piston skirt material is 38MnVS6 non-quenched and tempered steel, 42CrMo quenched and tempered steel, or ductile iron.

2. The preparation method of a special-shaped combustion chamber piston with high-performance heterogeneous welding according to claim 1, characterized in that It includes the following steps: S1. Preparation of the piston head blank. Accurately weigh each raw material and weigh them according to the proportion. Through vacuum furnace smelting, LF refining with titanium addition process, and VD vacuum treatment, the melting temperature is controlled at 1650 - 1750 °C, and then a piston head slab with a thickness greater than 200 mm is formed by continuous casting; S2. Rolling treatment of the piston head slab. Heat the piston head slab with residual heat to 1200 - 1280 °C and perform multi-pass rolling treatment, including cutting the head and tail after rough rolling, cutting the head during medium rolling, pre-finishing rolling, and finishing rolling. The rolling ratio is not less than 1:

8. After rolling, control the temperature for cooling and perform flame cleaning. After finishing rolling, the thickness of the piston head slab is reduced to 20 - 50 mm; S3. Forming of the piston head combustion chamber and oil channels. Special patterns are distributed on the rough rolling, medium rolling, pre-finishing rolling, and finishing rolling rolls of the piston head. Among them, patterns corresponding to the shape of the abnormal basin-shaped combustion chamber and the piston valve pit are distributed on the surface of the upper roll, and patterns corresponding to half of the piston oil channels and the upper wall plus the bottom of the piston first ring groove are distributed on the surface of the lower roll; after rolling, multiple rows of basin-shaped combustion chambers + valve pits are formed on the upper surface of the piston head slab, and multiple rows of half oil channels + the upper wall plus the bottom of the first ring groove are formed at the corresponding positions on the lower surface; S4. Cutting and forming of the piston head. Use stamping or laser cutting methods to form a piston head with an abnormal basin-shaped combustion chamber and half an oil channel from the rolled piston head slab; S5. Preparation of the piston skirt. The piston skirt is prepared by forging, casting, or machining processes; S6. Dissimilar welding of the piston head and the piston skirt. Use friction welding technology under nitrogen protection for dissimilar welding of the piston head and the piston skirt. After welding, perform stress relief annealing treatment, and then perform finishing machining on the semi-finished product to form a piston semi-finished product with special profile pin holes and outer circles; S7. Surface treatment. After finishing machining, the piston semi-finished product undergoes phosphating and screen printing of a nano-coating on the skirt surface to form a finished piston.

3. The preparation method of a special-shaped combustion chamber piston with high-performance heterogeneous welding according to claim 2, characterized in that: In S3, a different-shaped basin combustion chamber and the shape of the piston valve pit are pre-rolled on the upper surface of the piston head slab. The surface roughness of the rolled basin combustion chamber and valve pit is less than Rz30, and the surface dimensional tolerance reaches ±0.3 mm.

4. The preparation method of a special-shaped combustion chamber piston with high-performance heterogeneous welding according to claim 2, characterized in that: In S3, a half cooling oil passage, the upper wall and the bottom of the piston first ring groove are pre-rolled at the position corresponding to the combustion chamber on the lower surface of the piston head slab. The surface roughness of the rolled oil passage and ring groove is less than Rz30, and the surface dimensional tolerance reaches ±0.3 mm.

5. The preparation method of a special-shaped combustion chamber piston with high-performance heterogeneous welding according to claim 2, characterized in that: The welding surface of the piston head is formed by rolling, and the roughness of the welding surface of the piston head is less than Rz30, and the surface dimensional tolerance reaches ±0.3 mm.

6. The preparation method of a special-shaped combustion chamber piston with high-performance heterogeneous welding according to claim 5, characterized in that: The welding surface of the piston skirt is formed by machining, and the welding surface of the piston skirt is wavy, the width of the wave is 2 ± 0.1 mm, and the depth of the wave is 1 ± 0.1 mm.

7. The preparation method of a special-shaped combustion chamber piston with high-performance heterogeneous welding according to claim 2, characterized in that, In S6, the nitrogen protection friction welding technology includes the following steps: S1. Clean the semi-finished products of the piston head and piston skirt to ensure cleanliness, where the maximum particle size of impurities is less than 300 microns and the weight is less than 5 mg. S2. Nitrogen protection friction welding. During the welding process, the nitrogen flow rate is 8 - 10 L / m³, the pressure is 0.8 - 1 Mpa, and the purity is above 99.999%. The special second-order friction welding process parameters are: the first-order pressure is 150 ± 10 Bar, the second-order pressure is 40 ± 5 Bar, the rotational speed is 1200 RPM, the pressure holding time is 3 - 5 seconds, and the welding shortening amount is 4 ± 0.3 mm.

8. The preparation method of a special-shaped combustion chamber piston with high-performance heterogeneous welding according to claim 5, characterized in that: The center position of the weld of the piston head and piston skirt is located at the bottom of the piston first ring groove, that is, the upper wall and the bottom of the piston first ring groove are formed by extrusion during rolling, and the surface hardness of the upper wall and the bottom of the piston first ring groove formed by rolling is greater than 250 HV, and the strength perpendicular to the rolling direction is greater than 500 MPa.

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