Processing method of anti-fracture piston skirt copper ring

Abstract

The invention discloses a processing method of an anti-fracture piston skirt copper ring, which aims at solving the problems that MB7 alloy used by a marine diesel engine piston skirt copper ring is easy to generate lead emission, segregation and assembly fracture, adopts a composite casting mold of a copper ring molten steel jacket and a resin sand mold core, and is matched with bottom pouring type pouring to form a sequential solidification temperature gradient; a circulating water continuous chilling process is combined, so that the defect of non-uniform solidification of a low-melting-point phase is overcome, and meanwhile, copper matrix grains are refined to improve the ductility; step 1, casting mold preparation; 2, alloy smelting; thirdly, bottom pouring type sequential solidification pouring is conducted; 4, continuous chilling with circulating water; and 5, post-processing and assembling. According to the machining method for the anti-fracture piston skirt copper ring, the machined copper ring can bear 120 MPa circumferential pressure without cracks when being embedded into a piston skirt, the product percent of pass is increased, and the machining method is already applied to production of marine diesel engines in batches and is high in market share.

Description

Technical Field

[0001] This invention relates to the field of manufacturing technology for key components of marine diesel engines, and in particular to a method for machining a fracture-resistant piston skirt copper ring suitable for MB7 lead-tin bronze alloy. Background Technology

[0002] The piston is one of the key components of an engine. The piston skirt mainly guides and transfers heat to the piston, and bears the lateral thrust from the crankshaft and connecting rod mechanism. The piston skirt copper ring is a copper ring-shaped accessory installed on the piston skirt of an internal combustion engine. It is an auxiliary wear-resistant and heat-dissipating structure of the piston. Its core function is to optimize the friction characteristics between the piston and the cylinder wall and improve the piston's heat dissipation efficiency. It is embedded in a special ring groove in the piston skirt and is usually a thin-walled ring structure. The material is mostly bronze. Marine diesel engine piston skirt copper rings usually use Finnish standard MB7 material, which is similar in composition to the domestic ZCuPb20Sn5 alloy. It has excellent anti-friction properties and can significantly improve the service life of the piston skirt. However, in the traditional casting process, this alloy has significant defects. Its casting defects are frequent. Low melting point elements lead and tin are prone to "sweating", "lead leakage" and compositional segregation problems during casting, resulting in a high product scrap rate.

[0003] Based on this, a search revealed that application number CN201310722770.2 relates to a method for remanufacturing a piston skirt and a piston. The method includes a substrate pretreatment step to remove grease and dust from the substrate surface; a fixture installation step, where the fixture exposes only the piston skirt to be sprayed; one or more spraying steps to prepare a coating on the piston skirt surface; and a post-coating processing step, where the piston skirt is ground to meet process specifications. This method uses supersonic plasma spraying technology to prepare a nickel-aluminum composite coating on the piston skirt surface. This invention can remanufacture pistons scrapped due to piston skirt wear, or solve the problem of piston mismatch after cylinder inner wall size enlargement. It allows the piston skirt to be restored or enlarged according to actual needs while improving its surface strength and wear resistance, extending its service life, improving remanufacturability, reducing repair cycle, and saving energy, money, and labor.

[0004] The above-mentioned piston skirt copper ring processing methods have the following shortcomings: the risk of breakage during embedded assembly is high in traditional processing methods, the copper ring matrix has insufficient ductility, and micro-cracks are easily generated under circumferential pressure when embedded into the piston skirt, eventually leading to breakage, which seriously affects the assembly quality and equipment operation safety; traditional casting processes mostly adopt top-pouring and static cooling methods, which cannot solve the problems of uneven solidification of low-melting-point alloys and poor feeding channels, making it difficult to meet the high reliability requirements of marine diesel engines for piston skirt copper rings.

[0005] To address the aforementioned problems, a method for machining a fracture-resistant piston skirt copper ring is proposed. Summary of the Invention

[0006] The purpose of this invention is to provide a method for processing a fracture-resistant piston skirt copper ring. Using this device, the invention solves the problems of high risk of fracture during embedding and assembly in traditional processing methods, insufficient ductility of the copper ring matrix, and the tendency for micro-cracks to form under circumferential pressure during embedding into the piston skirt, ultimately leading to fracture and severely affecting assembly quality and equipment operational safety. Furthermore, traditional casting processes often employ top-pouring and static cooling methods, which cannot solve the problems of uneven solidification of low-melting-point alloys and obstructed feeding channels, making it difficult to meet the high reliability requirements of marine diesel engines for piston skirt copper rings.

[0007] To achieve the above objectives, the present invention provides the following technical solution: a method for processing a fracture-resistant piston skirt copper ring, characterized by comprising the following steps:

[0008] Step 1: Mold preparation. A composite mold is made of a copper ring molten steel jacket and a resin sand core. A circulating water channel is opened on the inner wall of the molten steel jacket, and a bottom pouring gate is reserved in the core.

[0009] Step 2: Alloy smelting. Melt the MB7 alloy raw material to 1150-1200℃, remove the gas, let it stand, and then keep it at that temperature.

[0010] Step 3: Bottom-pouring sequential solidification casting, where molten metal is poured from the bottom of the mold, forming a unidirectional temperature gradient in the lower, upper, and central parts;

[0011] Step 4: Continuous quenching with circulating water. During the solidification stage, quenching with circulating water is used, with the cooling rate controlled at 10-20℃ / min and the quenching time at 1.5-2.5h.

[0012] Step 5: Post-processing and assembly. After machining, the copper ring is embedded into the piston skirt.

[0013] Preferably, the MB7 alloy composition is approximately ZCuPb20Sn5, containing 70%-78% Cu, 18%-22% Pb, 4%-6% Sn by mass percentage, and a total lead-tin content of 25%-35%.

[0014] The core of this design lies in precisely matching the anti-friction properties of the MB7 alloy with its compatibility with the casting process: Cu, as the matrix element, with a content range of 70%-78%, ensures the overall structural strength of the copper ring, preventing insufficient load-bearing capacity due to low copper content, while also preventing excessive copper content from reducing alloy fluidity, ensuring the molten metal can smoothly fill the mold during casting; Pb, as a key anti-friction element, with a content of 18%-22%, forms a continuous lead lubricating film on the working surface of the copper ring, significantly reducing the friction coefficient between the piston skirt and the cylinder wall, but the content is strictly controlled below 22% to prevent excessive Pb from accumulating during solidification. Large lead phase particles are formed, leading to lead oxide defects. Sn's role is to refine the copper matrix grains and improve the alloy hardness. A content of 4%-6% can form Cu3Sn strengthening phase with Cu, avoiding insufficient strengthening effect due to too low Sn content or increased alloy brittleness due to too high Sn content. The total lead-tin content is limited to 25%-35%, which is the optimal range based on extensive experimental verification: below 25%, the friction reduction performance drops significantly, and above 35%, the proportion of low melting point phase is too high, which easily leads to compositional segregation and solidification cracks. This composition range works synergistically with the subsequent bottom-pouring casting and circulating water quenching process to suppress low melting point phase defects from the source.

[0015] Preferably, the molten steel jacket of the mold is made of low-carbon steel with a wall thickness of 15-25mm; the room temperature compressive strength of the resin sand core is ≥3MPa, and the gas generation is ≤10mL / g.

[0016] Preferably, the flow rate of the molten metal in the bottom-pouring casting is 0.4-0.6 m / s, and the temperature gradient difference between the lower and upper parts of the mold during the casting process is 50-80℃.

[0017] Using the above design, a molten metal flow rate of 0.4-0.6 m / s is considered a "stable filling flow rate": when the flow rate is below 0.4 m / s, the molten metal fills too slowly and is prone to premature solidification during pouring, resulting in cold shut defects; when the flow rate is above 0.6 m / s, the molten metal will impact the core, causing core wear or splashing, forming oxide inclusions; this flow rate range allows the molten metal to rise steadily along the inner wall of the mold, avoiding turbulence and air entrapment; a temperature gradient difference of 50-80℃ is key to sequential solidification. The lower part of the mold contacts the molten metal first, while the central part has the lowest temperature, forming a temperature gradient. This gradient difference ensures that the lower molten metal solidifies first, while the upper molten metal acts as a feeding source, continuously replenishing the volume shrinkage during the solidification of the lower part, completely eliminating shrinkage cavities and porosity defects; if the gradient difference is less than 50℃, the sequential solidification effect is not obvious, and the feeding channel is prone to premature closure; if it is greater than 80℃, the large temperature difference between the upper and lower parts is prone to generating thermal stress, leading to cracking of the copper ring.

[0018] Preferably, the circulating water inlet temperature for the circulating water quenching is 20-30℃, and the water flow velocity is 1.2-1.8m / s; during the quenching process, the time for the temperature of the inner wall of the mold to drop from 1000℃ to 300℃ is 40-60min.

[0019] The above design ensures that the inlet water temperature is within the normal temperature range of 20-30℃ for circulating water. This avoids excessively low water temperatures that could lead to large temperature differences between the mold's inner wall and the molten metal, causing thermal shock cracks. Conversely, excessively high water temperatures would result in insufficient cooling efficiency, failing to achieve a cooling rate of 10-20℃ / min. A water flow velocity of 1.2-1.8 m / s ensures turbulent flow within the spiral channel, improving heat exchange efficiency and preventing localized temperature increases and uneven cooling when the flow velocity is too low. Excessively high flow velocities would increase energy consumption and hinder the flow of water into the mold. The cooling effect is improved in one step; the time for the inner wall of the mold to drop from 1000℃ to 300℃ is controlled within 40-60 minutes, corresponding to a cooling rate of 11.7-20℃ / min, which falls exactly within the 10-20℃ / min range specified in the claims; this cooling process can quickly solidify the low-melting-point phase of lead-tin, refine the lead-tin particles to 5-10μm, and distribute them evenly in the copper matrix to avoid segregation; at the same time, it refines the copper matrix grains to 20-30μm, and the ductility is greatly improved compared with traditional static cooling.

[0020] Preferably, in the alloy smelting step, argon gas is used for degassing, with an argon gas flow rate of 0.8-1.2 L / min and a degassing time of 10-15 min; the holding temperature is 1120-1160℃ and the holding time is 20-30 min.

[0021] The above design aims to improve the purity of the molten metal and ensure its compositional uniformity. Argon, as an inert gas, does not react with the molten metal. A flow rate of 0.8-1.2 L / min can form uniform bubbles in the molten metal. During the bubble's ascent, it adsorbs gaseous impurities such as hydrogen and nitrogen, achieving a degassing effect. The degassing time of 10-15 min is based on experimental verification. More than 90% of gaseous impurities can be removed within 10 min; after 15 min, the degassing efficiency improvement is not significant and energy consumption increases. If the flow rate is lower than 0.8 L / min, the bubble density is insufficient, and the degassing... Incomplete gasification; a flow rate above 1.2 L / min, excessively large bubbles can easily lead to molten metal splashing; a holding temperature of 1120-1160℃, lower than the melting temperature, can prevent the molten metal from overheating and causing coarse grains, while ensuring good fluidity; a holding time of 20-30 min allows the alloy components to diffuse fully, ensuring uniform distribution of Cu, Pb, and Sn elements and avoiding segregation defects caused by local component deviations; these holding parameters, when combined with subsequent casting processes, can maintain stable fluidity of the molten metal during casting, preventing poor filling due to excessively rapid temperature drops.

[0022] Preferably, the post-processing steps include turning, grinding and surface polishing, with the outer diameter of the copper ring having a machining accuracy of IT6 grade and a surface roughness Ra≤0.8μm; before embedding and assembly, the inner surface of the copper ring is coated with a molybdenum-based anti-friction coating with a thickness of 5-10μm.

[0023] With the above design, the IT6 grade outer diameter machining accuracy ensures a tight fit between the copper ring and the piston skirt ring groove, avoiding excessive assembly clearance that could cause impact during operation, or excessive clearance that could cause excessive pressure during assembly. A surface roughness Ra≤0.8μm reduces the frictional resistance between the copper ring and the ring groove, preventing stress concentration and cracking during assembly due to surface roughness. The molybdenum-based anti-friction coating has excellent lubrication performance and high-temperature resistance, with a thickness of 5-10μm being the optimal range. When the thickness is less than 5μm, the coating is prone to wear and cannot provide long-term anti-friction effects. When the thickness is greater than 10μm, the coating adhesion decreases, making it prone to peeling off and forming impurities, affecting piston movement. This coating can reduce the friction coefficient between the copper ring and the ring groove from 0.15 to 0.08, further reducing circumferential pressure during assembly and extending the service life of the copper ring.

[0024] Preferably, the piston skirt copper ring prepared by the above method has a copper matrix grain size of 20-30 μm, a room temperature elongation of 15%-18%, and a Brinell hardness of HB80-HB95.

[0025] The above design achieves a grain size of 20-30μm, a direct result of the circulating water chilling process, which refines the grains. Compared to the 45-60μm grains of traditional processes, the refined grains significantly improve matrix ductility and reduce grain boundary defects. The room temperature elongation of 15%-18% represents a 114%-260% increase compared to the 5%-8% of traditional processes. This allows the copper ring to withstand circumferential pressure of 100-130MPa during assembly without cracking, completely solving the problem of assembly breakage. The Brinell hardness of HB80-HB95 balances wear reduction and toughness: when the hardness is below HB80, the copper ring is prone to wear; when it is above HB95, the toughness decreases, and it is prone to brittle fracture. This hardness range, combined with the molybdenum-based wear-reducing coating, can increase the service life of the copper ring to more than 2.5 times that of traditional processes.

[0026] Preferably, the method is applicable to the mass production of piston skirt copper rings for marine low-speed diesel engines with a cylinder diameter of 160-400mm.

[0027] Preferably, the circulating water channel of the molten steel jacket has a spiral structure, a rectangular cross-section, a width of 8-12mm, and a depth of 6-10mm; and the circulating water channel is detachable to facilitate mold maintenance and cleaning.

[0028] Using the above design, the spiral channel allows circulating water to flow evenly along the inner wall of the mold, forming 360° cooling without dead angles, avoiding the problem of uneven local cooling caused by traditional straight channels; compared with the circular cross-section, the rectangular cross-section has a larger heat exchange area (30% larger than the circular cross-section for the same channel width), which can improve cooling efficiency; the channel width of 8-12mm and depth of 6-10mm are designed based on the coordinated optimization of mold wall thickness and water flow velocity: when the width is less than 8mm or the depth is less than 6mm, the water flow resistance is too large and turbulence cannot be formed; when the width is greater than 12mm or the depth is greater than 10mm, the mold wall thickness is insufficient and it is easy to deform;

[0029] The detachable design allows the channel module to be connected to the molten steel jacket with bolts. The channel can be disassembled after the mold is used to clean the scale and impurities inside, preventing the channel from being blocked and affecting the cooling effect. At the same time, if the channel is damaged, it can be replaced separately without replacing the entire molten steel jacket, reducing the mold maintenance cost.

[0030] Compared with the prior art, the beneficial effects of the present invention are as follows:

[0031] 1. This application discloses a method for processing a fracture-resistant piston skirt copper ring, which significantly reduces casting defects. By using bottom-pouring sequential solidification and circulating water quenching, the problems of sweating, lead emission, and segregation in low-melting-point alloys are completely solved, and the product qualification rate is greatly improved.

[0032] 2. This application provides a method for processing a fracture-resistant piston skirt copper ring, which significantly improves fracture resistance. Continuous chilling refines the grains, significantly improving the ductility of the copper matrix. Continuous chilling refines the copper matrix grains to 20-30μm, increasing the elongation from 7% to 15%-18%. There are no circumferential pressure cracks during embedding and assembly, resulting in high assembly reliability.

[0033] 3. The piston skirt copper ring processing method of this application has strong process adaptability. The method is designed for lead-tin bronze alloys such as MB7 and can be directly applied to the mass production of piston skirt copper rings for marine diesel engines. It has high market adaptability and high production efficiency. Attached Figure Description

[0034] Figure 1 This is a schematic diagram of a method for processing a fracture-resistant piston skirt copper ring according to the present invention. Detailed Implementation

[0035] The technical solutions of the embodiments of the present invention will be clearly and completely described below with reference to the accompanying drawings. Obviously, the described embodiments are only some embodiments of the present invention, and not all embodiments. Based on the embodiments of the present invention, all other embodiments obtained by those skilled in the art without creative effort are within the scope of protection of the present invention.

[0036] To further understand the content of this invention, a detailed description of the invention will be provided in conjunction with the accompanying drawings.

[0037] Combination Figure 1 A method for machining a fracture-resistant piston skirt copper ring includes the following steps:

[0038] Step 1: Mold preparation. A composite mold is made of a copper ring molten steel jacket and a resin sand core. A circulating water channel is opened on the inner wall of the molten steel jacket, and a bottom pouring gate is reserved in the core.

[0039] Step 2: Alloy smelting. Melt the MB7 alloy raw material to 1150-1200℃, remove the gas, let it stand, and then keep it at that temperature.

[0040] Step 3: Bottom-pouring sequential solidification casting, where molten metal is poured from the bottom of the mold, forming a unidirectional temperature gradient in the lower, upper, and central parts;

[0041] Step 4: Continuous quenching with circulating water. During the solidification stage, quenching with circulating water is used, with the cooling rate controlled at 10-20℃ / min and the quenching time at 1.5-2.5h.

[0042] Step 5: Post-processing and assembly. After machining, the copper ring is embedded into the piston skirt.

[0043] The composition of MB7 alloy is similar to ZCuPb20Sn5, containing 70%-78% Cu, 18%-22% Pb, and 4%-6% Sn by mass percentage, with a total lead and tin content of 25%-35%.

[0044] The molten steel jacket of the casting mold is made of low-carbon steel with a wall thickness of 15-25mm; the room temperature compressive strength of the resin sand core is ≥3MPa, and the gas emission is ≤10mL / g.

[0045] The flow rate of molten metal in bottom-pouring casting is 0.4-0.6 m / s, and the temperature gradient difference between the lower and upper parts of the mold during the casting process is 50-80℃.

[0046] The circulating water inlet temperature for quenching is 20-30℃, and the water flow velocity is 1.2-1.8m / s. During the quenching process, the temperature of the inner wall of the mold drops from 1000℃ to 300℃ in 40-60 minutes.

[0047] In the alloy smelting step, argon gas is used for degassing, with an argon flow rate of 0.8-1.2 L / min and a degassing time of 10-15 min; the holding temperature is 1120-1160℃ and the holding time is 20-30 min.

[0048] The post-processing steps include turning, grinding and surface polishing. The outer diameter of the copper ring is machined to IT6 grade, and the surface roughness Ra≤0.8μm. Before embedding and assembly, the inner surface of the copper ring is coated with a molybdenum-based anti-friction coating with a thickness of 5-10μm.

[0049] The piston skirt copper ring prepared by the above method has a copper matrix grain size of 20-30μm, a room temperature elongation of 15%-18%, and a Brinell hardness of HB80-HB95.

[0050] The method is applicable to the mass production of piston skirt copper rings for marine low-speed diesel engines with cylinder diameters of 160-400mm.

[0051] The circulating water channel of the molten steel jacket has a spiral structure with a rectangular cross-section, a width of 8-12mm, and a depth of 6-10mm. The circulating water channel is also designed to be detachable, which facilitates mold maintenance and cleaning.

[0052] The present invention will be further described below with reference to embodiments.

[0053] Example 1:

[0054] Mold preparation: Make a steel jacket with an inner diameter of 150mm and a height of 200mm, open a φ8mm spiral circulating water channel on the inner wall, make a resin sand core with a diameter of 100mm, and reserve a φ20mm bottom pouring gate.

[0055] Alloy smelting: 75kg pure copper, 20kg lead ingots and 5kg tin ingots are put into an induction furnace and smelted to 1180℃. Argon gas is introduced for degassing for 12 minutes and then held at the temperature for 20 minutes.

[0056] Bottom-pouring casting: Molten metal is poured from the bottom at a speed of 0.5 m / s, forming a unidirectional temperature gradient of 1100℃ at the bottom and 1050℃ at the top;

[0057] Circulating water quenching: Turn on the circulating water, control the cooling rate at 15℃ / min, and continue quenching for 2 hours until the alloy is completely solidified;

[0058] Post-processing and assembly: After machining, the copper ring has an outer diameter of 148mm and an inner diameter of 102mm. When embedded in the piston skirt, it can withstand a circumferential pressure of 120MPa. No cracks were found after flaw detection.

[0059] Example 2

[0060] Mold preparation: Make a steel jacket with an inner diameter of 200mm and a height of 250mm, open a φ10mm spiral circulating water channel on the inner wall, make a resin sand core with a diameter of 140mm, and reserve a φ25mm bottom pouring gate.

[0061] Alloy smelting: 750 kg of pure copper, 200 kg of lead ingots and 50 kg of tin ingots are put into an intermediate frequency furnace and smelted to 1190°C. Argon gas is introduced for degassing for 15 min and then held at the temperature for 25 min.

[0062] Bottom-pouring casting: Molten metal is poured from the bottom at a speed of 0.6 m / s, forming a unidirectional temperature gradient of 1120℃ at the bottom and 1080℃ at the top;

[0063] Circulating water quenching: Turn on the circulating water, control the cooling rate at 18℃ / min, and continue quenching for 2.5 hours until the alloy is completely solidified;

[0064] Post-processing and assembly: After machining, the copper ring has an outer diameter of 198mm and an inner diameter of 142mm. When embedded in the piston skirt, it can withstand a circumferential pressure of 130MPa. No cracks were found after flaw detection.

[0065] Example 3:

[0066] Mold preparation: A low-carbon steel jacket with an inner diameter of 160mm and a height of 180mm (wall thickness of 15mm) is made, and a spiral detachable circulating water channel with a width of 8mm and a depth of 6mm is opened on the inner wall; the resin sand core has a diameter of 110mm, a room temperature compressive strength of 3.2MPa, a gas generation of 8mL / g, and a φ18mm bottom pouring gate is reserved.

[0067] Alloy smelting: 75kg of pure copper, 20kg of lead ingots and 5kg of tin ingots are put into a medium frequency induction furnace and smelted to 1150℃. Argon gas is introduced at a flow rate of 0.8L / min for 10min to remove gas, and then the mixture is kept at the temperature for 20min.

[0068] Bottom-pouring casting: Molten metal is poured from the bottom at a speed of 0.4 m / s, forming a unidirectional temperature gradient with a lower temperature of 1090℃ and an upper temperature of 1040℃;

[0069] Circulating water quenching: Turn on the circulating water, with an inlet water temperature of 20℃ and a flow rate of 1.2m / s. Control the cooling rate at 10℃ / min and continue quenching for 1.5h until the alloy is completely solidified. It takes 40min for the inner wall of the mold to drop from 1000℃ to 300℃.

[0070] Post-processing and assembly: After turning, the copper ring has an outer diameter of 158mm and an inner diameter of 112mm, with an accuracy of IT6 grade and Ra=0.8μm. The inner surface is coated with a 5μm molybdenum-based anti-friction coating. When embedded in the piston skirt, it can withstand a circumferential pressure of 100MPa and has been tested for cracks.

[0071] Performance testing: Copper matrix grain size 22μm, room temperature elongation 15%, Brinell hardness HB82; casting defect rate 0.3%, assembly defect rate 0.2%;

[0072] Example 4:

[0073] Mold preparation: A low-carbon steel jacket with an inner diameter of 400mm and a height of 350mm is made, and a spiral detachable circulating water channel with a width of 12mm and a depth of 10mm is opened on the inner wall; the resin sand core has a diameter of 280mm, a room temperature compressive strength of 3.5MPa, a gas generation of 7mL / g, and a φ30mm bottom pouring gate is reserved.

[0074] Alloy smelting: 1500kg of pure copper, 400kg of lead ingots and 100kg of tin ingots are put into a medium frequency induction furnace and smelted to 1200℃. Argon gas is introduced at a flow rate of 1.2L / min for 15min to remove gas. The mixture is then held at the temperature for 30min at 1160℃.

[0075] Bottom-pouring casting: Molten metal is poured from the bottom at a speed of 0.6 m / s, forming a forward temperature gradient of 1130℃ at the bottom and 1070℃ at the top, with a gradient difference of 60℃.

[0076] Circulating water quenching: Turn on the circulating water, with an inlet water temperature of 30℃ and a flow rate of 1.8m / s. Control the cooling rate at 20℃ / min and continue quenching for 2.5 hours until the alloy is completely solidified. It takes 60 minutes for the inner wall of the mold to drop from 1000℃ to 300℃.

[0077] Post-processing and assembly: After turning, the copper ring has an outer diameter of 398mm and an inner diameter of 282mm, with an accuracy of IT6 grade and Ra=0.6μm. The inner surface is coated with a 10μm molybdenum-based anti-friction coating. When embedded in the piston skirt, it can withstand a circumferential pressure of 130MPa and no cracks were found after flaw detection.

[0078] Performance testing: Copper matrix grain size 28μm, room temperature elongation 18%, Brinell hardness HB95; casting defect rate 0.2%, assembly defect rate 0.1%;

[0079] Example 5:

[0080] A copper ring of the same specification, with a cylinder diameter of 200mm, was manufactured using existing top-injection casting and static cooling processes, with the following parameters:

[0081] Casting mold: integral resin sand casting mold with top pouring gate.

[0082] Cooling method: Natural static cooling, cooling rate 3-5℃ / min;

[0083] Performance testing: Copper matrix grain size 45μm, room temperature elongation 7%, Brinell hardness HB75; casting defect rate 42%, cracks appear when subjected to 60MPa circumferential pressure during assembly, assembly failure rate 18%.

[0084] In summary: The present invention provides a method for processing a fracture-resistant piston skirt copper ring, comprising the following steps: mold preparation: a composite mold is formed by using a copper ring molten steel jacket and a resin sand core, a spiral circulating water channel is opened on the inner wall of the molten steel jacket, and a bottom-pouring pouring port is reserved in the core, forming an integrated structure of the jacket, core and pouring port;

[0085] Alloy smelting: The MB7 alloy raw materials are put into the medium frequency induction furnace in the proportion of Cu 75%, Pb 20% and Sn 5%, and smelted to 1150-1200℃. Argon gas is introduced for degassing for 10-15 minutes, and the mixture is kept at the temperature for 20 minutes.

[0086] Bottom-pouring sequential solidification casting: The molten metal is steadily filled from the bottom of the mold at a speed of 0.4-0.6 m / s, forming a forward temperature gradient in the lower, upper and central parts, so that the alloy liquid solidifies sequentially from bottom to top, ensuring that the feeding channel is unobstructed throughout the process;

[0087] Continuous cooling with circulating water: Immediately after casting, turn on the circulating water and control the cooling rate to 10-20℃ / min. Continue cooling for 1.5-2.5h until the alloy is completely solidified, rapidly solidifying the low-melting-point lead-tin phase and refining the copper matrix grains.

[0088] Post-processing and assembly: The cast copper ring is machined (turning, grinding) and when embedded in the piston skirt, the copper ring can withstand 100-130MPa circumferential pressure without cracking.

[0089] The anti-fracture piston skirt copper ring processing method of this invention significantly reduces casting defects. By using bottom-pouring sequential solidification and circulating water quenching, it completely solves the problems of sweating, lead emission, and segregation in low-melting-point alloys, greatly improving the product qualification rate. The fracture resistance is greatly improved. Continuous quenching refines the grains, significantly improving the ductility of the copper matrix. Continuous quenching refines the copper matrix grains to 20-30μm, increasing the elongation from 7% to 15%-18%. There are no circumferential pressure cracks during embedding and assembly, resulting in high assembly reliability. The processing method of this application has strong process adaptability. The method is designed for lead-tin bronze alloys such as MB7 and can be directly applied to the mass production of piston skirt copper rings for marine diesel engines, with high market adaptability and high production efficiency.

[0090] It should be noted that the aforementioned electrical components are equipped with power supplies, and their control methods are existing technologies. To avoid redundancy, they will be described here uniformly. Furthermore, this application is primarily for the protection of mechanical equipment, so the control methods and circuit connections will not be explained in detail herein. In this document, relational terms such as "first" and "second" are used merely to distinguish one entity or operation from another entity or operation, and do not necessarily require or imply any such actual relationship or order between these entities or operations. Moreover, the terms "comprising," "including," or any other variations thereof are intended to cover non-exclusive inclusion, such that a process, method, article, or apparatus that comprises a list of elements includes not only those elements but also other elements not expressly listed, or elements inherent to such a process, method, article, or apparatus.

[0091] Although embodiments of the invention have been shown and described, it will be understood by those skilled in the art that various changes, modifications, substitutions and alterations can be made to these embodiments without departing from the principles and spirit of the invention, the scope of which is defined by the appended claims and their equivalents.

Claims

1. A method for machining a fracture-resistant piston skirt copper ring, characterized in that, Includes the following steps: Step 1: Mold preparation. A composite mold is made of a copper ring molten steel jacket and a resin sand core. A circulating water channel is opened on the inner wall of the molten steel jacket, and a bottom pouring gate is reserved in the core. Step 2: Alloy smelting. Melt the MB7 alloy raw material to 1150-1200℃, remove the gas, let it stand, and then keep it at that temperature. Step 3: Bottom-pouring sequential solidification casting, where molten metal is poured from the bottom of the mold, forming a unidirectional temperature gradient in the lower, upper, and central parts; Step 4: Continuous quenching with circulating water. During the solidification stage, quenching with circulating water is used, with the cooling rate controlled at 10-20℃ / min and the quenching time at 1.5-2.5h. Step 5: Post-processing and assembly. After machining, the copper ring is embedded into the piston skirt.

2. The processing method according to claim 1, characterized in that: The composition of MB7 alloy is similar to ZCuPb20Sn5, containing 70%-78% Cu, 18%-22% Pb, and 4%-6% Sn by mass percentage, with a total lead and tin content of 25%-35%.

3. The processing method according to claim 1, characterized in that: The molten steel jacket of the casting mold is made of low-carbon steel with a wall thickness of 15-25mm; the room temperature compressive strength of the resin sand core is ≥3MPa, and the gas emission is ≤10mL / g.

4. The processing method according to claim 1, characterized in that: The flow rate of molten metal in bottom-pouring casting is 0.4-0.6 m / s, and the temperature gradient difference between the lower and upper parts of the mold during the casting process is 50-80℃.

5. The processing method according to claim 1, characterized in that: The circulating water inlet temperature for quenching is 20-30℃, and the water flow velocity is 1.2-1.8m / s. During the quenching process, the temperature of the inner wall of the mold drops from 1000℃ to 300℃ in 40-60 minutes.

6. The processing method according to claim 1, characterized in that: In the alloy smelting step, argon gas is used for degassing, with an argon flow rate of 0.8-1.2 L / min and a degassing time of 10-15 min; the holding temperature is 1120-1160℃ and the holding time is 20-30 min.

7. The processing method according to claim 1, characterized in that: The post-processing steps include turning, grinding and surface polishing. The outer diameter of the copper ring is machined to IT6 grade, and the surface roughness Ra≤0.8μm. Before embedding and assembling, the inner surface of the copper ring is coated with a molybdenum-based anti-friction coating with a thickness of 5-10 μm.

8. The processing method according to any one of claims 1-7, characterized in that: The piston skirt copper ring prepared by the method has a copper matrix grain size of 20-30μm, a room temperature elongation of 15%-18%, and a Brinell hardness of HB80-HB95.

9. The processing method according to claim 1, characterized in that: The method is applicable to the mass production of piston skirt copper rings for marine low-speed diesel engines with cylinder diameters of 160-400mm.

10. The processing method according to claim 1, characterized in that: The circulating water channel of the molten steel jacket has a spiral structure with a rectangular cross-section, a width of 8-12mm, and a depth of 6-10mm. The circulating water channel is also designed to be detachable, which facilitates mold maintenance and cleaning.

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