Nickel-based expansion alloy bar machining process for engine piston cover

Abstract

The invention provides a machining process of a nickel-based expansion alloy bar for an engine piston cover, and relates to the technical field of nickel-based expansion alloy bar machining. Part of high-price Re is replaced with Y element, and a La, Ce, Nd and Yb composite rare earth system is introduced; a triple refining process of vacuum electric arc melting circulation remelting, six-element slag system electroslag remelting and electron beam spiral scanning remelting is adopted, ultrasonic, magnetic field and pulse current cooperate with multi-section annealing, the alloy purity is larger than or equal to 99.97%, component segregation is smaller than or equal to 0.5%, the inclusion size is smaller than or equal to 5 microns, a foundation is laid for forming a uniform and refined structure through follow-up machining, and the alloy quality is improved. And through cooperation of multi-pass forging and inter-pass heat preservation stress release, forging cracks and residual stress accumulation are effectively avoided.

Description

Technical Field

[0001] This invention relates to the field of nickel-based expansion alloy bar processing technology, and more particularly to the processing technology of nickel-based expansion alloy bars for engine piston covers. Background Technology

[0002] As a core component of internal combustion engines, engine piston heads operate under harsh conditions of temperatures exceeding 500°C, high-frequency vibration, and combustion gas corrosion. They must simultaneously possess precise thermal expansion matching, excellent high-temperature long-term strength, a uniform and refined microstructure, robust surface protection, and controllable industrial production costs. Their material properties directly determine the engine's power output, thermal efficiency, and service life. Nickel-based expansion alloys, due to their excellent high-temperature mechanical properties, oxidation resistance, and potential for thermal expansion regulation, have become the preferred material for engine piston heads. However, current processing techniques for nickel-based expansion alloy bars still face numerous technical bottlenecks, making it difficult to meet the stringent requirements of high-end engines for comprehensive material performance.

[0003] In the smelting process, existing technologies mostly adopt a dual refining scheme of vacuum arc melting combined with electroslag remelting. Vacuum arc melting lacks a targeted heat preservation step, resulting in poor control of composition uniformity. Electroslag remelting mostly uses a quaternary slag system, which has limited deoxidation and desulfurization efficiency. Summary of the Invention

[0004] The purpose of this invention is to address the shortcomings of existing technologies by proposing a processing technology for nickel-based expansion alloy bars for engine piston covers.

[0005] To achieve the above objectives, the present invention adopts the following technical solution: a processing technology for nickel-based expansion alloy bars for engine piston covers, comprising the following steps: S1. By mass ratio, select Ni 69-71%, Co 9.5-10.5%, Cr 12.5-13.5%, Mo 4.2-4.8%, W 3.2-3.8%, Y 0.4-0.7%, Nb 1.8-2.2%, Ti 1.9-2.1%, Al 1.5-1.7%, V 0.4-0.5%, C≤0.006%, P≤0.003%, S≤0.003%, and composite rare earth elements (La + Ce + Nd + Yb) 0.10-0.12%; S2. After mixing evenly, place the mixture in a vacuum drying oven and keep it at 130-140℃ for 4.5-5.5 hours to remove adsorbed moisture and gas. After cooling, transfer it to a planetary ball mill. S3. Using cemented carbide balls as the grinding medium, with a ball-to-material ratio of 30:1-31:1, and a ball milling speed of 440-460 r / min, ball milling is carried out for 12.5-13.5 h under argon protection with a purity of ≥99.995%. During the process, 0.35-0.45% stearic acid and 0.1% nano-graphite powder by mass are added to obtain ultrafine mixed powder with a particle size ≤1.5μm. S4. The ultrafine powder is fed into a plasma spheroidizing device, using a mixture of argon and hydrogen (volume ratio 94:6) as the working gas, with a plasma power of 48-52kW, a powder feed rate of 19-21g / min, and a cooling rate of ≥120℃ / s, to obtain nickel-based alloy spheroidized powder with a sphericity of ≥98% and a particle size of 100-160μm. S5. The spheroidized powder is loaded into a water-cooled copper crucible with a vacuum degree ≤2×10⁻³Pa and an arc current of 340-360A. The melting, holding, cooling and remelting cycles are repeated three times to obtain a primary ingot with uniform composition. S6. Insert the primary refined ingot as a consumable electrode into an electroslag remelting device, using a six-element slag system of CaF2-AL2O3-MgO-TiO2-Y2O3-CeO2, with the following mass percentages: CaF2 67-68%, AL2O3 17-18%, MgO 9.5-10.5%, TiO2 3.2-3.8%, Y2O3 1.2-1.8%, CeO2 0.5-1.0%. The remelting current is 540-560A, and the cooling rate is 19-21℃ / min, to obtain a secondary refined ingot with a purity ≥99.97%. S7. The secondary refined ingot is fed into an electron beam remelting furnace with a vacuum degree ≤8×10⁻. 5 Pa, electron beam power 130-140kW, using a spiral scanning path, scanning rate 6-7mm / s, removes tiny inclusions and gas impurities to obtain high-quality alloy ingots with no porosity, no gas holes, and compositional segregation ≤0.5%. S8. Place the alloy ingot into a resistance heating furnace and heat it to 1170-1180℃ at a rate of 4.2-4.8℃ / min. Hold the temperature for 5.2-5.8h to eliminate casting stress and compositional segregation. S9. Perform one pre-forging at 1160-1170℃ with a deformation of 22-23% to initially break up the cast grains. After pre-forging, use the residual heat to directly transfer to isothermal forging. S10, maintain the temperature at 1130-1135℃, use a high-speed forging machine to forge in 4 passes, with the deformation amount of each pass being 40%, 35%, 25%, and 15% respectively. Hold the forging temperature for 37-38 minutes after each pass, preheat the die temperature to 360-370℃, and use argon gas protection during the forging process. The final forging is an alloy billet with a diameter of 60-70mm. After forging, mist cool to 300℃, and then cool to room temperature in the furnace. S11. Place the alloy billet into a composite annealing furnace, apply a DC magnetic field of 0.28-0.32T (direction consistent with the billet axis), turn on 220-230W ultrasonic vibration (frequency 24-26kHz), and simultaneously apply a 50-80A pulse current (pulse frequency 50Hz), raise the temperature to 970-980℃, hold for 7.2-7.8h, and slowly cool to 600℃ at 6-7℃ / min. S12. Turn off the ultrasonic vibration and pulsed current, keep the magnetic field unchanged, continue to heat to 1070-1080℃, hold for 3.7-3.9h, and cool to room temperature with the furnace to obtain a refined microstructure with a grain size ≤35μm and a grain uniformity ≥93%. S13. Place the annealed billet into an ultrasonic pickling tank, using a mixed acid solution of hydrochloric acid-nitric acid-hydrofluoric acid-phosphoric acid-citric acid (volume ratio 4:2:1:0.5:0.3), ultrasonic power 260-270W, pickling temperature 44-46℃, time 18.5-19.5min to remove oxide scale and forging oxide film. S14. Place the pickled billet into a plasma treatment device, under an argon-nitrogen mixed atmosphere (volume ratio 9:1), with a power of 26-28kW and a treatment time of 8.5-9.5min, to increase the content of surface active groups to ≥92%. S15. Multi-pass cold drawing is performed using polycrystalline diamond molds, with a reduction rate of 9.5-10.5% per pass, a total reduction rate of 66-69%, and a cold drawing speed of 0.85-0.95 m / s. Vacuum intermediate annealing is performed after every 3 passes of cold drawing at a temperature of 825-835℃ for 1.6-1.9 h, with a vacuum degree ≤3×10⁻³Pa. S16. After each cold drawing, a 15-roll precision straightening machine is used at a straightening speed of 0.7-0.8 m / s to ensure that the straightness of the bar is ≤0.07 mm / m; S17. Place the cold-drawn bar into an aging furnace and hold it at 455-465℃ for 6.5-7.5 hours with a heating rate of 2.5℃ / min. Then, heat the bar to 495-505℃, apply a DC magnetic field of 0.16-0.19T, and simultaneously turn on 150-180W ultrasonic vibration (frequency 25kHz). Hold the bar for 14.5-15.5 hours with a cooling rate of ≤3℃ / h to promote the uniform dispersion and precipitation of the reinforcing phase. S18. Place the rod as the anode into the silicate electrolyte, with a voltage of 310-340V, a current density of 11-14A / dm², and a treatment time of 11-14min, to form a 6-7μm oxide ceramic transition layer on the surface. S19. Immerse the rod with the transition layer in KH560 silane coupling agent ethanol solution (2-3% by mass), keep it at 60-70℃ for 30-40 minutes, and dry it to form an interface bonding layer. S20. Using supersonic plasma spraying equipment, with AL2O3, ZrO2, Y2O3, and La2O3 composite powder (mass ratio 5.5:3:1:0.5) as raw materials, the spraying power is 36-39kW, the spraying distance is 88-92mm, and the powder feeding rate is 21-24g / min. A dense coating of 9-11μm is prepared on the surface of the transition layer, and finally high-performance nickel-based expansion alloy rods with a diameter of 12-26mm are obtained.

[0006] Preferably, the mass ratio of the composite rare earth elements La, Ce, Nd, and Yb in S1 is 3:2:1:0.5, which works synergistically with Y in the alloy system.

[0007] Preferably, the melting point of the hexa-element slag system in S6 is 1390-1410℃, the viscosity at 1500℃ is ≤0.016Pa・s, and the inclusion size is ≤5μm.

[0008] Preferably, the magnetic field strength fluctuation of the composite annealing in S11 and S12 is ≤ ±0.008T, the ultrasonic field uniformity is ≥97%, and the pulse current density is 5-8A / mm².

[0009] Preferably, after ultrasonic pickling in S13, the surface is sequentially ultrasonically cleaned with deionized water and dried with hot air. The ultrasonic cleaning power of the deionized water is 185-195W, and the time is 13-14min. The hot air drying temperature is 92-98℃, and the time is 32-38min. After treatment, the surface roughness Ra≤0.25μm.

[0010] Preferably, the strengthening phase in the segmented composite aging process of S17 is Ni3(Ti,Al,V,Y), with a precipitation amount ≥28% and a phase particle size controlled between 25-45nm.

[0011] Preferably, the micro-arc oxidation electrolyte in S7 is composed of 9-11% Na2SiO3, 2.2-2.8% NaOH, 1.2-1.8% Na2WO4, and 0.3-0.5% NaF (mass%), and the bonding strength between the oxide ceramic transition layer and the substrate is ≥65MPa.

[0012] Preferably, the porosity of the composite coating in S20 is ≤1.2%, the bonding strength with the transition layer is ≥60MPa, the oxidation rate at 700℃ is ≤0.003mm / a, and there is no peeling after 100 thermal shock cycles at 500℃.

[0013] Preferably, the surface finish of the working area of ​​the cold drawing die in S15, S16, and S17 is Ra≤0.012μm, the surface roughness of the bar after cold drawing is Ra≤0.2μm, and there are no defects such as scratches or cracks.

[0014] Preferably, the prepared nickel-based expansion alloy rods meet the following performance indicators: room temperature tensile strength ≥1300MPa, 500℃ and 1000h creep strength ≥600MPa, and expansion coefficient at 500℃ ≤10.5×10⁻ 6 / ℃, elongation after fracture ≥24%, grain size ≥10 grade, room temperature fatigue strength (10 7 The coating has a strength of ≥580MPa (times), a room temperature hardness of HV≥450, and meets the requirements of GB / T4162A grade for flaw detection. After 2000 hours of service at 500℃, the coating does not peel off, and there are no cracks after 100 thermal shock cycles at 500℃.

[0015] Compared with the prior art, the advantages and positive effects of the present invention are as follows: 1. In this invention, by replacing part of the high-valence Re with Y element and introducing a La, Ce, Nd and Yb composite rare earth system, the cost of alloy raw materials is reduced by 15-20%, while grain boundary purification and high-temperature stability are synergistically improved, which is suitable for the mass production needs of engine piston covers. 2. In this invention, a triple refining process is adopted, which combines vacuum arc melting and cyclic remelting, electroslag remelting of a six-element slag system, and electron beam spiral scanning remelting. The deoxidation and desulfurization efficiency of the six-element slag system is 18-22% higher than that of the traditional slag system. Electron beam remelting further removes tiny inclusions and gases, making the alloy purity ≥99.97%, compositional segregation ≤0.5%, and inclusion size ≤5μm. This lays the foundation for subsequent processing to form a uniform and refined microstructure, and finally obtains a high-quality microstructure with a grain size ≥10. 3. In this invention, pre-forging breaks up the coarse grains in the cast state, and the residual heat is directly transferred to isothermal forging to reduce energy loss. Combined with multi-pass forging and heat preservation between passes to release stress, forging cracks and residual stress accumulation are effectively avoided. 4. In this invention, the multi-stage annealing of ultrasound, magnetic field and pulsed current is combined through multi-field coupling to suppress abnormal grain growth and promote composition homogenization, taking into account both high strength and high toughness, and meeting the complex working conditions of high frequency vibration and high temperature pressure of engine piston cover. 5. In this invention, plasma surface activation makes the content of active groups on the surface of the bar material ≥92%, and combined with multi-pass cold drawing and vacuum intermediate annealing, cold working defects are avoided under the premise of a total reduction rate of 66-69%. The surface roughness Ra of the bar material after cold drawing is ≤0.2μm, reducing defects such as scratches and cracks. 6. In this invention, the segmented magnetic field-ultrasonic synergistic aging process forms fine γ' phase cores through pre-aging. During the main aging process, the magnetic field and ultrasonic vibration work together to promote the uniform dispersion and precipitation of the γ' phase (Ni3(Ti,Al,V,Y)), resulting in a γ' phase precipitation amount ≥28% and a phase particle size controlled at 25-45nm. This effectively avoids the coarsening failure of the strengthening phase at high temperatures, ensuring that the alloy maintains excellent performance after 2000h of service at 500℃, and significantly extending the service life of the engine piston cover. Attached Figure Description

[0016] Figure 1 This invention provides a process flow diagram for the processing of nickel-based expansion alloy bars for engine piston covers. Detailed Implementation

[0017] like Figure 1As shown, this invention provides a processing technology for nickel-based expansion alloy bars for engine piston covers. Ni, Co, Cr, Mo, W, Y, Nb, Ti, Al, V, C, P, S, and composite rare earth elements La, Ce, Nd, and Yb are selected by mass percentage. After uniform mixing, the raw materials are placed in a vacuum drying oven and kept at a set temperature for a certain time to remove adsorbed moisture and gases. After cooling, the mixture is transferred to a planetary ball mill using cemented carbide balls as the grinding medium. Mechanical alloying is performed under argon protection with a purity ≥99.995% according to a set ball-to-material ratio and milling speed. During the process, a set mass fraction of stearic acid and nano-graphite powder are added. After the set milling time, an ultrafine mixed powder is obtained. This ultrafine mixed powder is then sent to a plasma balling device. Using an argon-hydrogen mixture with a set volume ratio as the working gas, and setting the plasma power, powder feed rate, and cooling rate, nickel-based alloy spheroidized powder is obtained. The spheroidized powder is loaded into a water-cooled copper crucible, evacuated to a set vacuum level, and an arc current is applied. The process involves three cycles of melting, holding, cooling, and remelting to obtain a homogeneous primary ingot. This primary ingot is then used as a consumable electrode in an electroslag remelting apparatus. A six-element slag system (CaF2, Al2O3, MgO, TiO2, Y2O3, CeO2) is used, configured according to a set mass percentage. A set remelting current is applied, and the cooling rate is controlled to obtain a secondary refined ingot. This secondary refined ingot is then fed into an electron beam remelting furnace, evacuated to a set vacuum level, and the electron beam power and spiral scanning rate are set. To remove minute inclusions and gaseous impurities, high-quality alloy ingots are obtained. These ingots are then placed in a resistance heating furnace and heated to the target temperature at a set heating rate. The temperature is held for a certain time to eliminate casting stress and compositional segregation. A single pre-forging is performed at the set temperature, with controlled deformation, to initially break up the as-cast grains. After pre-forging, the residual heat is used to directly transfer the ingot to isothermal forging, maintaining the set temperature. Four forging passes are performed using a high-speed forging machine, with the deformation amount determined sequentially for each pass. Each pass is held for a certain time after forging. The die is preheated to the set temperature. Argon gas is continuously introduced for protection during forging. After forging, the ingot is mist-cooled to the set temperature and then cooled to room temperature in the furnace. The alloy billet is then placed in a co-annealing furnace, and a DC magnetic field of a set intensity is applied, with the magnetic field direction aligned with the billet axis. The set power is then activated. Ultrasonic vibration at a set frequency is applied simultaneously with a pulsed current of a set magnitude and frequency. The temperature is increased to the target temperature at a set rate, held for a certain time, and then slowly cooled to the target temperature at a set rate. The ultrasonic vibration and pulsed current are then turned off, the magnetic field remains constant, and the temperature is increased to the target temperature again. After holding for a certain time, the material is cooled to room temperature in the furnace, resulting in a refined microstructure. The annealed billet is then placed in an ultrasonic pickling tank, using a mixed acid solution of hydrochloric acid, nitric acid, hydrofluoric acid, phosphoric acid, and citric acid prepared in a set volume ratio. The ultrasonic power, pickling temperature, and time are set to remove oxide scale and forging oxide film. The pickled billet is then placed in a plasma treatment device, where a mixed atmosphere of argon and nitrogen in a set volume ratio is introduced. The device power and processing time are set to increase the content of surface-active groups.Multi-pass cold drawing is performed using polycrystalline diamond molds, controlling the reduction rate, total reduction rate, and cold drawing speed for each pass. Vacuum intermediate annealing is performed after every three passes, with set annealing temperature, holding time, and vacuum level. After each cold drawing, a precision straightener is used to straighten the bar at a set speed to ensure straightness. The cold-drawn bar is then placed in an aging furnace and heated to the pre-aging temperature at a set heating rate. After holding at this temperature for a certain time, the temperature is further increased to the main aging temperature. A DC magnetic field of set intensity is applied, and ultrasonic vibration with set power and frequency is simultaneously activated. After holding at this temperature for a certain time, the bar is cooled to room temperature at a set cooling rate to promote uniformity of the strengthening phase. The process involves placing a rod as the anode in a silicate electrolyte, setting the voltage, current density, and processing time to form an oxide ceramic transition layer on the surface. The rod with this transition layer is then immersed in a KH560 silane coupling agent ethanol solution prepared according to a set mass fraction. After holding at a set temperature for a certain time and drying, an interfacial bonding layer is formed. Using a supersonic plasma spraying system, a composite powder of Al₂O₃, ZrO₂, Y₂O₃, and La₂O₃ mixed in a set mass ratio is used as the raw material. The spraying power, spraying distance, and powder feed rate are set to prepare a dense coating on the transition layer surface, ultimately yielding a high-performance nickel-based expansion alloy rod.

[0018] Working principle: This processing technology achieves a comprehensive improvement in the overall performance of nickel-based expanded alloy bars through precise control and synergistic effects at each stage. The composition design selects specific proportions of matrix elements such as Ni, Co, and Cr, as well as reinforcing elements. The synergistic effect of Y and La combined with Ce, Nd, and Yb rare earth elements optimizes the high-temperature stability and mechanical properties of the alloy matrix while effectively controlling industrialization costs. During mechanical alloying, the grinding action of hard alloy balls, combined with the dispersion effect of stearic acid and nano-graphite powder, ensures the formation of uniform ultrafine powder from the raw materials. Plasma spheroidization treatment improves the powder morphology and flowability, providing high-quality raw materials for subsequent smelting. Triple refining and smelting are achieved through a cycle of vacuum arc melting. The process enhances compositional uniformity. Electroslag remelting utilizes a six-element slag system to strengthen deoxidation and desulfurization. Electron beam remelting employs spiral scanning to further remove minute inclusions and gases. Three-stage refining ensures high purity and density of the alloy ingot. Residual heat is used in synergistic pre-forging and isothermal forging, utilizing pre-forging to break up coarse cast grains. Residual heat is directly transferred to isothermal forging to reduce energy loss. Multi-pass forging and inter-pass heat preservation effectively release forging stress. Die preheating and argon protection prevent billet oxidation and cracking, improving billet microstructure uniformity and density. In multi-stage annealing using ultrasound, magnetic fields, and pulsed current, the magnetic field effectively inhibits abnormal grain growth, ultrasonic vibration accelerates atomic diffusion and compositional homogenization, and pulsed current assists in enhancing... Annealing efficiency: Two-stage annealing achieves grain refinement and microstructure stability respectively, ensuring the uniformity and refinement of the alloy microstructure. Plasma activation and precision cold working: The pickling effect of multi-component mixed acid solution effectively removes oxide scale and avoids over-corrosion. Plasma surface activation increases the content of active groups on the surface of the bar, providing good surface conditions for cold drawing. Multi-pass cold drawing combined with vacuum intermediate annealing improves the strength and toughness of the bar while avoiding cold working defects. Precision straightening ensures the dimensional accuracy of the bar. Segmented magnetic field and ultrasonic synergistic aging strengthening: Pre-aging forms fine strengthening phase nuclei, and during the main aging, the magnetic field and ultrasonic vibration synergistically promote the uniform dispersion and precipitation of strengthening phases, improving the stability and strengthening effect of the strengthening phases. The effects of this process ensure the high-temperature, long-term durability of the alloy. In the interface modification and composite surface treatment, the oxide ceramic transition layer formed by micro-arc oxidation provides a good foundation for coating bonding. The interface modification effect of the silane coupling agent further enhances the bonding force between the transition layer and the subsequent coating. The dense coating formed by composite powder spraying significantly improves the oxidation resistance and wear resistance of the rod. The various process links cooperate and optimize each other to effectively solve the technical problems of existing nickel-based expansion alloy rods, such as high high-temperature expansion coefficient, insufficient long-term durability at 500℃, poor microstructure uniformity, weak surface coating bonding force, and high industrialization cost. Finally, high-performance nickel-based expansion alloy rods that meet the requirements of engine piston cover are obtained.

[0019] The above description is merely a preferred embodiment of the present invention and is not intended to limit the invention in any other way. Any person skilled in the art may utilize the disclosed technical content to make changes or modifications to create equivalent embodiments applicable to other fields. However, any simple modifications, equivalent changes, and modifications made to the above embodiments based on the technical essence of the present invention, without departing from the scope of the present invention, still fall within the protection scope of the present invention. In the description of the present invention, it should be noted that, unless otherwise explicitly specified and limited, the terms "installation," "connection," and "joining" should be interpreted broadly. For example, they can refer to a fixed connection, a detachable connection, or an integral connection; they can refer to a mechanical connection or an electrical connection; they can refer to a direct connection or an indirect connection through an intermediate medium; and they can refer to the internal connection of two components. For those skilled in the art, the specific meaning of the above terms in the present invention can be understood through specific circumstances.

Claims

1. A machining process for nickel-based expansion alloy bars used in engine piston covers, characterized in that, Includes the following steps: S1. According to the mass ratio, select Ni 69-71%, Co 9.5-10.5%, Cr 12.5-13.5%, Mo 4.2-4.8%, W 3.2-3.8%, Y 0.4-0.7%, Nb 1.8-2.2%, Ti 1.9-2.1%, Al 1.5-1.7%, V 0.4-0.5%, C ≤0.006%, P≤0.003%, S ≤0.003%, and La, Ce, Nd, and Yb to form a composite rare earth element composition of 0.10-0.12%, and mix them evenly. S2. Place the mixed raw materials into a vacuum drying oven and keep them at 130-140℃ for 4.5-5.5 hours to remove adsorbed moisture and gas. After cooling, transfer them to a planetary ball mill. S3. Using cemented carbide balls as the grinding media, with a ball-to-material ratio of 30:1 to 31:1, ball milling is carried out under argon protection, with 0.35-0.45% stearic acid and 0.1% nano-graphite powder added by mass during the process. S4. The ultrafine powder is fed into a plasma spheroidizing device, and a mixture of argon and hydrogen with a volume ratio of 94:6 is used as the working gas to obtain nickel-based alloy spheroidized powder with a sphericity ≥98% and a particle size of 100-160μm. S5. The spheroidized powder is loaded into a water-cooled copper crucible with a vacuum degree ≤2×10⁻³Pa and an arc current of 340-360A. The melting, holding, cooling and remelting steps are repeated 3 times to obtain a primary ingot with uniform composition. S6. Using the primary refined ingot as a consumable electrode, insert it into an electroslag remelting device. Use a six-element slag system of CaF2, AL2O3, MgO, TiO2, Y2O3, and CeO2. The mass ratio of the slag system is CaF2 67-68%, AL2O3 17-18%, MgO 9.5-10.5%, TiO2 3.2-3.8%, Y2O3 1.2-1.8%, and CeO2 0.5-1.0%. The remelting current is 540-560A, and the cooling rate is 19-21℃ / min. A secondary refined ingot with a purity of ≥99.97% is obtained. S7. The secondary refined ingot is fed into an electron beam remelting furnace with a vacuum degree ≤8×10⁻. 5 Pa, electron beam power 130-140kW, using a spiral scanning path, scanning rate 6-7mm / s, removes tiny inclusions and gas impurities to obtain high-quality alloy ingots with no porosity, no gas holes, and compositional segregation ≤0.5%. S8. Place the alloy ingot into a resistance heating furnace and heat it to 1170-1180℃ at a rate of 4.2-4.8℃ / min. Hold the temperature for 5.2-5.8 hours to eliminate casting stress and compositional segregation. S9. Perform one pre-forging at 1160-1170℃ with a deformation of 22-23% to initially break up the cast grains. After pre-forging, use the residual heat to directly transfer to isothermal forging. S10, maintain the temperature at 1130-1135℃, use a high-speed forging machine to forge in 4 passes, with the deformation amount of each pass being 40%, 35%, 25%, and 15% respectively. Hold the forging temperature for 37-38 minutes after each pass, preheat the die temperature to 360-370℃, and use argon gas protection during the forging process. The final forging is an alloy billet with a diameter of 60-70mm. After forging, mist cool to 300℃, and then cool to room temperature in the furnace. S11. Place the alloy billet into a composite annealing furnace, apply a DC magnetic field of 0.28-0.32T in the same direction as the billet axis, turn on 220-230W ultrasonic vibration, and simultaneously apply a pulse current of 50-80A. Heat to 970-980℃, hold for 7.2-7.8h, and cool to 600℃ at 6-7℃ / min. S12. Turn off the ultrasonic vibration and pulsed current, keep the magnetic field unchanged, continue to heat to 1070-1080℃, hold for 3.7-3.9h, and cool to room temperature with the furnace to obtain a refined microstructure with a grain size ≤35μm and a grain uniformity ≥93%. S13. Place the annealed billet into an ultrasonic pickling tank and use a mixed acid solution of hydrochloric acid, nitric acid, hydrofluoric acid, phosphoric acid and citric acid in a volume ratio of 4:2:1:0.5:0.

3. The ultrasonic power is 260-270W, the pickling temperature is 44-46℃, and the time is 18.5-19.5min to remove the oxide scale and forging oxide film. S14. Place the pickled billet into a plasma treatment device, and in an atmosphere of argon and nitrogen mixed at a volume ratio of 9:1, with a power of 26-28kW and a treatment time of 8.5-9.5min, increase the content of surface active groups to ≥92%; S15. Polycrystalline diamond molds are used, with a reduction rate of 9.5-10.5% per pass, a total reduction rate of 66-69%, a cold drawing speed of 0.85-0.95m / s, and vacuum intermediate annealing after every 3 passes. The annealing temperature is 825-835℃, the holding time is 1.6-1.9h, and the vacuum degree is ≤3×10⁻³Pa. S16. After each cold drawing, a 15-roll precision straightening machine is used at a straightening speed of 0.7-0.8 m / s to ensure that the straightness of the bar is ≤0.07 mm / m; S17. Place the cold-drawn bar into an aging furnace and hold it at 455-465℃ for 6.5-7.5h. The heating rate is 2.5℃ / min. When the temperature reaches 495-505℃, apply a DC magnetic field of 0.16-0.19T and simultaneously turn on 150-180W ultrasonic vibration. Hold the temperature for 14.5-15.5h and cool at a rate of ≤3℃ / h to promote the uniform dispersion and precipitation of the strengthening phase. S18. The rod is placed in silicate electrolyte for micro-arc oxidation electrolysis for 11-14 min to form a 6-7 μm oxide ceramic transition layer on the surface. S19. Immerse the rod with the transition layer into a 2-3% (by mass) silane coupling agent ethanol solution, keep it at 60-70℃ for 30-40 minutes, and dry it to form an interface bonding layer. S20. Using supersonic plasma spraying equipment, a dense coating is prepared on the surface of the transition layer using composite powders of AL2O3, ZrO2, Y2O3 and La2O3 in a mass ratio of 5.5:3:1:0.5 as raw materials, ultimately obtaining high-performance nickel-based expansion alloy rods with a diameter of 12-26mm.

2. In the processing technology of nickel-based expansion alloy bars for engine piston covers according to claim 1, the mass ratio of composite rare earth elements La, Ce, Nd, and Yb in S1 is 3:2:1:0.

5.

3. The processing technology for nickel-based expansion alloy bars for engine piston covers according to claim 1, characterized in that, The melting point of the six-element slag system in S6 is 1390-1410℃, the viscosity at 1500℃ is ≤0.016Pa・s, and the inclusion size is ≤5μm.

4. The processing technology for nickel-based expansion alloy bars for engine piston covers according to claim 1, characterized in that, The magnetic field strength fluctuation of composite annealing in S11 and S12 is ≤ ±0.008T, the ultrasonic field uniformity is ≥97%, and the pulse current density is 5-8A / mm².

5. The machining process for nickel-based expansion alloy bars for engine piston covers according to claim 1, characterized in that, After ultrasonic pickling in S13, the surface is then ultrasonically cleaned with deionized water and dried with hot air. The ultrasonic cleaning power of the deionized water is 185-195W, and the time is 13-14min. The hot air drying temperature is 92-98℃, and the time is 32-38min. The surface roughness Ra after treatment is ≤0.25μm.

6. The machining process for nickel-based expansion alloy bars for engine piston covers according to claim 1, characterized in that, The amount of enhanced phase precipitated in the segmented composite aging process of S17 is ≥28%, and the phase particle size is controlled between 25-45nm.

7. The machining process for nickel-based expansion alloy bars for engine piston covers according to claim 1, characterized in that, The micro-arc oxidation electrolyte in S7 is composed of Na2SiO3 9-11%, NaOH 2.2-2.8%, Na2WO4 1.2-1.8%, and NaF 0.3-0.5% by mass, and the bonding strength between the oxide ceramic transition layer and the substrate is ≥65MPa.

8. The machining process for nickel-based expansion alloy bars for engine piston covers according to claim 1, characterized in that, The porosity of the dense coating in S20 is ≤1.2%, the bonding strength with the transition layer is ≥60MPa, and it has excellent oxidation resistance at 700℃.

9. The machining process for nickel-based expansion alloy bars for engine piston covers according to claim 1, characterized in that, For S15, S16, and S17, the surface finish of the working area of ​​the cold drawing die is Ra≤0.012μm, and the surface roughness of the bar after cold drawing is Ra≤0.2μm.

10. The processing method for nickel-based expansion alloy bars for engine piston covers according to any one of claims 1-9, characterized in that, The prepared nickel-based expansion alloy rods meet the following performance indicators: room temperature tensile strength ≥1300MPa, 500℃ and 1000h creep strength ≥600MPa, and expansion coefficient at 500℃ ≤10.5×10⁻ 6 / ℃.

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