A connecting piece for magnesium alloy SSRT experiment and a preparation method thereof

Abstract

The application relates to the technical field of magnesium alloy detection, and discloses a connecting piece for magnesium alloy SSRT (stress-strain-rupture-temperature) experiments and a preparation method thereof, the preparation method comprising the following steps: firstly, PA6 resin is pretreated by Soxhlet extraction to remove residual polymerization metal catalysts and small-molecule oligomers; secondly, the purified PA6 base material is mixed with a hydrolysis-resistant stabilizer, glass beads and other additives, special modified PA6 granules are obtained through double-stage vacuum melting, blending and granulation, then the connecting piece preform is injection molded by adopting a process of in-mold step pressure keeping and synchronous constant-temperature cooling, and after saturated steam gradient annealing in an inert atmosphere, a dense non-polar crosslinking passivation layer is formed on the surface of the blank body through low-temperature argon plasma activation and gas-phase in-situ deposition crosslinking, and finally the target connecting piece is obtained. The connecting piece prepared by the application has no risk of metal ion precipitation, and has excellent insulation, hydrolysis resistance and mechanical bearing performance.

Description

Technical Field

[0001] This invention relates to the field of magnesium alloy testing technology, and more specifically, to a connector for magnesium alloy SSRT testing and its preparation method. Background Technology

[0002] Magnesium alloys, as the lightweight and high-strength metallic structural materials with the best comprehensive performance in current industrial applications, have extremely broad application prospects in aerospace, new energy vehicles, and biomedical implants. However, stress corrosion cracking (SCC) under the synergistic effect of stress and corrosion is a core bottleneck restricting the large-scale engineering application of magnesium alloys. Slow strain rate tensile testing (SSRT) is currently the most mainstream standard test method for assessing the SCC susceptibility of magnesium alloys both domestically and internationally. This test requires applying a constant tensile load at an extremely low strain rate to the specimen in a highly corrosive medium containing chloride ions, simulating the actual service environment of magnesium alloys. Through complete load-displacement curves, fracture behavior, and other data, the SCC susceptibility of magnesium alloys can be accurately quantified. The test cycle usually lasts for tens to hundreds of hours, placing extremely high demands on the purity of the test system, the stability of load transfer, and the consistency of the test environment.

[0003] In the SSRT testing system, the connector is the core component that enables a rigid connection between the magnesium alloy specimen and the tensile testing machine fixture, ensuring stable coaxial transmission of tensile load. Its performance directly determines the accuracy and reliability of the test results. Currently, the commonly used connector solutions in the industry are not suitable for the specific and demanding working conditions of magnesium alloy SSRT testing. The core technical problem is that existing connectors cannot simultaneously meet the three core requirements of insulation against galvanic corrosion, corrosion resistance without ion precipitation, and stable mechanical properties under long-term corrosive environments. This directly leads to serious distortion of magnesium alloy SCC sensitivity test results, poor data reliability and repeatability, and an inability to provide accurate testing support for the performance evaluation and engineering applications of magnesium alloy materials.

[0004] Specifically, traditional metal connectors are prone to corrosion and dissolution in highly corrosive media. This not only releases metal ions into the test solution, contaminating the solution system and interfering with the corrosion process of magnesium alloys, but also forms strong electrical couples with magnesium alloys, which have extremely low electrode potentials. This causes unnatural accelerated corrosion at the sample clamping end, resulting in premature sample breakage and completely distorted test results. Existing general-purpose nylon connectors are not specifically designed for the specific needs of magnesium alloy SSRT testing. They have defects such as precipitation of residual metal catalysts from raw materials, poor hydrolysis resistance, rapid decay of mechanical properties under long-term loads, and large residual internal stress from molding. They cannot stably maintain insulation, dimensional accuracy, and mechanical properties in long-term corrosive environments, and still cannot fundamentally solve the core problems of distorted test results and insufficient data reliability. Summary of the Invention

[0005] In view of this, the present invention provides a connector for magnesium alloy SSRT experiments and a method for its preparation, aiming to solve the above problems.

[0006] On one hand, the present invention provides a method for preparing a connector for magnesium alloy SSRT experiments, comprising the following steps: S1. Resin purification pretreatment: PA6 resin is placed in a Soxhlet extractor and refluxed with anhydrous ethanol as the extractant to remove residual polymer metal catalysts and small molecule oligomers from the resin. After extraction, the PA6 resin is vacuum dried to control the resin moisture content to ≤0.03%, thus obtaining purified and dried PA6 base material. S2. Modified material compounding: Weigh 75-82 parts of purified and dried PA6 base material, 3-7 parts of hydrolysis-resistant stabilizer, 12-18 parts of glass microspheres, 0.3-0.8 parts of amide nucleating agent, 0.4-1.0 parts of composite antioxidant, and 0.6-1.5 parts of fluoropolymer lubricant according to the mass ratio. Place all raw materials in a high-speed mixer and mix evenly at room temperature to obtain a premix. S3, Two-stage vacuum melt blending and granulation: The premixed material is fed into a two-stage twin-screw extruder unit, where it undergoes first-stage melt dispersion and second-stage vacuum devouring blending in sequence. After extrusion through a die, cooling, and pelletizing, modified PA6 special granules are obtained. S4. Low internal stress precision injection molding: After the modified PA6 special granules are vacuum dried twice, they are sent into a closed-loop temperature-controlled injection molding machine and injection molded into a coaxial double-segment cylindrical solid connector preform with a central annular limiting boss. The injection molding process adopts in-mold stepped pressure holding and synchronous constant temperature cooling process. S5. Inert atmosphere saturated steam gradient annealing: The preformed connector is placed in a closed annealing furnace. After the air in the furnace is replaced by high-purity nitrogen, saturated water vapor is introduced to carry out a three-stage gradient heating-holding-cooling annealing process. After the process is completed, the furnace is cooled to room temperature to obtain a shaped billet. S6. Surface in-situ crosslinking passivation treatment: The shaped blank is placed in the plasma treatment chamber, and the surface is first activated by low temperature argon plasma. Then, fluorine-containing silane monomers are introduced to perform in-situ vapor deposition crosslinking, forming a dense non-polar crosslinking passivation layer on the surface of the blank. After removal, PA6 nylon connectors for magnesium alloy SSRT experiments are obtained.

[0007] Further, in step S1, the reflux extraction temperature is 75-85℃ and the extraction time is 12-24h; the vacuum drying temperature is 90-100℃, the vacuum degree is ≥-0.095MPa, and the drying time is 16-24h.

[0008] Further, in step S2, the hydrolysis-resistant stabilizer is polycarbodiimide, the glass microspheres are hollow glass microspheres modified with silane coupling agent, and the particle size is 5-20 μm; the composite antioxidant is a mixture of hindered phenolic primary antioxidant and phosphite auxiliary antioxidant in a mass ratio of 1:1-2; the mixing speed of the high-speed mixer is 800-1200 r / min, and the mixing time is 8-15 min.

[0009] Further, in step S3, the first-stage extruder of the two-stage twin-screw extruder unit is a co-rotating twin-screw extruder with a temperature of 225-255℃ in each section and a screw speed of 250-350 r / min; the second-stage extruder is a co-rotating twin-screw devouring extruder with a temperature of 230-250℃ in each section and a screw speed of 200-300 r / min, and 2-3 vacuum devouring ports are provided on the machine body, with a vacuum degree of ≥-0.098 MPa for each devouring port.

[0010] Further, in step S4, the temperature of the secondary vacuum drying is 85-95℃, the vacuum degree is ≥-0.095MPa, the drying time is 8-12h, and the moisture content of the dried granules is ≤0.03%; the barrel temperature of the injection molding is 230-250℃, the nozzle temperature is 240-245℃, and the mold temperature is 60-80℃.

[0011] Further, in step S4, the in-mold stepped pressure holding and synchronous constant temperature cooling process is as follows: first, the mold is held at a first pressure of 90-110MPa for 3-6s, and then held at a second pressure of 50-70MPa for 6-10s; during the pressure holding stage, the mold is kept at a constant temperature of 60-80℃ throughout the process, and after the pressure holding is completed, the mold is cooled at a rate of 1-2℃ / s to below 30℃ before opening.

[0012] Further, in step S5, the specific process of the three-stage gradient heating-holding-cooling annealing treatment is as follows: in the first stage, the temperature is raised to 85-95℃ at a heating rate of 5-10℃ / min, held for 2-4 hours, and the steam pressure inside the furnace is controlled at 0.1-0.15MPa; in the second stage, the temperature is raised to 115-125℃ at a heating rate of 3-5℃ / min, held for 3-6 hours, and the steam pressure inside the furnace is controlled at 0.15-0.2MPa; in the third stage, the temperature is lowered to below 50℃ at a cooling rate of 2-3℃ / min, and the heating system is turned off and the furnace is cooled to room temperature.

[0013] Further, in step S6, the process parameters for the low-temperature argon plasma surface activation are: vacuum degree 10-30 Pa, argon flow rate 20-40 sccm, power 80-120 W, and activation time 5-15 min; the process parameters for the vapor phase in-situ deposition crosslinking are: deposition vacuum degree 20-50 Pa, fluorinated silane monomer flow rate 10-25 sccm, deposition temperature 60-80℃, deposition time 15-30 min, and after deposition, heat preservation and curing in a vacuum environment for 10-20 min.

[0014] On the other hand, the present invention also provides a connector for SSRT experiments on magnesium alloys, which is prepared by the above-described method for preparing a connector for SSRT experiments on magnesium alloys. The connector is composed of two cylindrical segments of equal diameter, and an annular limiting groove is provided between the two cylindrical segments. The annular limiting groove is used to fix the magnesium alloy sample to be tested. The outer wall surface of the connector is provided with threads.

[0015] Furthermore, the present invention also provides a method for experimental evaluation of SSRT of magnesium alloys, comprising: Prepare a magnesium alloy sample to be tested, wherein the magnesium alloy sample is dog bone type; Place the clamping areas at both ends of the magnesium alloy specimen in the corresponding positions of the upper and lower clamps of the tensile testing machine; The above-mentioned connector for magnesium alloy SSRT test is used to pass through the connection hole of the magnesium alloy specimen and the connection hole of the upper and lower clamps of the tensile testing machine. Tighten and secure using the nuts that are compatible with the connector; The magnesium alloy sample was immersed in the corrosive solution, a slow strain rate was set, and a tensile test was started. The load-displacement curve was recorded until the sample broke. Remove the sample and bolt, and record the fracture location and corrosion morphology.

[0016] Compared with the prior art, the beneficial effects of the present invention are as follows: This invention utilizes a Soxhlet extraction reflux purification process to pre-remove residual polymeric metal catalysts and small molecule oligomers from PA6 resin. Combined with a metal-free modified formulation, it eliminates the risk of trace metal ions precipitating into the test solution and avoids interference from solution contamination and pH fluctuations on the magnesium alloy corrosion process. Simultaneously, relying on the excellent insulation properties of the PA6 substrate, it breaks the galvanic corrosion circuit between the connector and the magnesium alloy sample, eliminating the problem of unnatural accelerated corrosion at the sample clamping end.

[0017] This invention achieves uniform dispersion of functional additives in the substrate through the synergistic modification of polycarbodiimide hydrolysis-resistant stabilizer, surface-modified hollow glass microspheres, and PA6 substrate, combined with a two-stage vacuum melt blending granulation process. This significantly improves the material's hydrolysis resistance, tensile strength, stiffness, and creep resistance. The resulting connectors can maintain stable mechanical properties and structural integrity even after prolonged immersion in highly corrosive media containing chloride ions. They can reliably meet the low-rate tensile load transfer requirements of SSRT tests for tens to hundreds of hours, completely solving the problem of rapid mechanical property decay, creep deformation, and even fracture of ordinary nylon connectors under long-term corrosive environments. This effectively avoids test interruptions and ensures the continuity and stability of the test process.

[0018] This invention utilizes a low-stress injection molding process with in-mold stepped pressure holding and synchronous constant temperature cooling, combined with a three-stage gradient annealing treatment in an inert atmosphere saturated steam. This process effectively eliminates residual internal stress generated during the injection molding of connectors, significantly improving the dimensional accuracy and long-term dimensional stability of the connectors. It avoids stress cracking of connectors caused by internal stress, as well as additional bending stress introduced during assembly due to dimensional deviations and structural eccentricity, ensuring coaxial, uniform, and hysteresis-free transmission of tensile loads.

[0019] This invention utilizes a low-temperature argon plasma surface activation and in-situ vapor deposition crosslinking process to construct a dense, non-polar fluorinated silane crosslinked passivation layer on the surface of the connector. This significantly reduces the surface polarity and water absorption rate of the connector, further isolating the corrosive medium from contact with the substrate and enhancing the long-term corrosion resistance of the connector. At the same time, the passivation layer effectively prevents the adsorption and residue of corrosive media and corrosion products on the surface of the connector. After testing, it can be reused after simple cleaning, greatly extending the cycle life of the connector, reducing the cost of test consumables, and avoiding cross-contamination between different batches of tests. Attached Figure Description

[0020] To more clearly illustrate the technical solutions in the embodiments of the present invention or the prior art, the drawings used in the embodiments will be briefly introduced below. Obviously, the drawings described below are only some embodiments of the present invention. For those skilled in the art, other drawings can be obtained based on these drawings without creative effort.

[0021] Figure 1 A three-dimensional schematic diagram of a connector for magnesium alloy SSRT experiments provided in an embodiment of the present invention; Figure 2 A cross-sectional schematic diagram of a connector for magnesium alloy SSRT experiments provided in an embodiment of the present invention; Figure 3 A planar schematic diagram of a magnesium alloy sample provided in an embodiment of the present invention; Figure 4 This is a diagram of the magnesium alloy SSRT experimental scene provided in Embodiment 1 of the present invention; Figure 5 This is a diagram of the magnesium alloy SSRT experimental scene provided in Comparative Example 1 of the present invention. Detailed Implementation

[0022] 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.

[0023] To make the above-mentioned objects, features and advantages of the present invention more apparent and understandable, the present invention will be further described in detail below with reference to the accompanying drawings and specific embodiments.

[0024] See Figure 1 and Figure 2 As shown, the connector provided in this embodiment of the invention includes: a first cylindrical segment 11, a second cylindrical end 12, and an annular limiting groove 2 between the two cylindrical segments. The annular limiting groove 2 is used to fix the magnesium alloy sample to be tested. The magnesium alloy sample is dog-bone shaped, and its shape and size are as shown in the figure. Figure 3 As shown; the outer wall surface of the connector is provided with threads.

[0025] This invention also provides a method for experimental evaluation of SSRT in magnesium alloys, including: Prepare a magnesium alloy sample to be tested, wherein the magnesium alloy sample is dog bone type; Place the clamping areas at both ends of the magnesium alloy specimen in the corresponding positions of the upper and lower clamps of the tensile testing machine; The above-mentioned connectors are used to penetrate the connecting holes of the magnesium alloy specimen and the connecting holes of the upper and lower clamps of the tensile testing machine. Tighten the nuts that match the connector to secure it; The magnesium alloy sample was immersed in the corrosive solution, a slow strain rate was set, and a tensile test was started. The load-displacement curve was recorded until the sample broke. Remove the sample and bolt, and record the fracture location and corrosion morphology.

[0026] The application scenarios of the connector of the present invention will be described below with reference to specific embodiments.

[0027] Example 1: As-cast Mg-9Gd-3Y magnesium alloy (T6 aging)

[0028] Taking as-cast Mg-9Gd-3Y magnesium alloy as an example, a slow strain rate tensile test was conducted on a dog-bone shaped specimen in a 3.5% NaCl solution. PA6 nylon connectors were used as the connection device. The experimental procedure is as follows: Select as-cast Mg-9Gd-3Y magnesium alloy under T6 aging, based on Figure 3 The dimensions and shapes are processed slowly using wire electrical discharge machining to ensure smooth, burr-free edges.

[0029] Slow strain rate tensile tests were conducted in air, deionized water, and 3.5% NaCl solution, with a tensile rate of 1×10⁻⁶. -6 s - ¹, using PA6 nylon connectors for nut connection, immediately remove the sample after it breaks, rinse with deionized water, clean with alcohol, and dry.

[0030] The stress-strain curves of the tensile specimens were analyzed, and the mechanical properties are shown in Table 1.

[0031] Table 1. Mg-9Gd-3Y magnesium alloy C-ring (connecting device is PA6 nylon connector) in slow strain rate tensile test ( =10 -6 s -1 Experimental results in 3.5% NaCl solution

[0032] When using PA6 nylon connectors for connection, as follows: Figure 4 As shown, the solution remains clean overall, and its mechanical properties are reliable.

[0033] Example 2: As-cast AZ91D magnesium alloy (T6 aging)

[0034] Taking as-cast AZ91D magnesium alloy as an example, a slow strain rate tensile test was conducted using a dog-bone shaped specimen in a 3.5% NaCl solution with a strain rate of 10. -6 s -1 PA6 nylon connectors were selected for the connection device. The experimental procedure is as follows: Select as-cast AZ91D magnesium alloy under T6 aging, based on Figure 3 The dimensions and shapes are processed slowly using wire electrical discharge machining to ensure smooth, burr-free edges.

[0035] Slow strain rate tensile tests were conducted in air, deionized water, and 3.5% NaCl solution, with a tensile rate of 1×10⁻⁶. -6 s - ¹, using PA6 nylon connectors for nut connection, immediately remove the sample after it breaks, rinse with deionized water, clean with alcohol, and dry.

[0036] The stress-strain curves of the tensile specimens were analyzed, and the mechanical properties are shown in Table 2.

[0037] Table 2. As-cast AZ91D magnesium alloy C-ring (connecting device is PA6 nylon connector) under slow strain rate tensile test ( =10 -6 s -1 Experimental results in 3.5% NaCl solution

[0038] Example 3: As-cast WE43 magnesium alloy (T6 aging)

[0039] Taking as-cast WE43 magnesium alloy as an example, a dog-bone shaped specimen was subjected to a slow strain rate tensile test in a 3.5% NaCl solution with a strain rate of 10⁻⁶ s⁻¹. PA6 nylon connectors were used as the connection device. The experimental procedure is as follows: Select as-cast AZ91D magnesium alloy under T6 aging, based on Figure 3 The dimensions and shapes are processed slowly using wire electrical discharge machining to ensure smooth, burr-free edges.

[0040] Slow strain rate tensile tests were conducted in air, deionized water, and 3.5% NaCl solution, with a tensile rate of 1×10⁻⁶. -6 s - ¹, using PA6 nylon connectors for nut connection, immediately remove the sample after it breaks, rinse with deionized water, clean with alcohol, and dry.

[0041] The stress-strain curves of the tensile specimens were analyzed, and the mechanical properties are shown in Table 3.

[0042] Table 3. As-cast WE43 magnesium alloy (connecting device is PA6 nylon connector) under slow strain rate tensile test ( =10 -6 s -1 Experimental results in 3.5% NaCl solution

[0043] The following comparative examples use conventional tensile specimens subjected to slow strain rate tensile tests in air and NaCl solution, with a strain rate of 1 × 10⁻⁶. -6 s - ¹

[0044] Comparative Example 1: As-cast Mg-9Gd-3Y magnesium alloy

[0045] Taking as-cast Mg-9Gd-3Y magnesium alloy as an example, a slow strain rate tensile test was conducted using a dog-bone shaped specimen in a 3.5% NaCl solution. Iron bolts were used as the connecting device. The experimental procedure is as follows: Select as-cast AZ91D magnesium alloy under T6 aging, based on Figure 1 The dimensions and shapes are processed slowly using wire electrical discharge machining to ensure smooth, burr-free edges.

[0046] Slow strain rate tensile tests were conducted in air, deionized water, and 3.5% NaCl solution, with a tensile rate of 1×10⁻⁶. -6 s - ¹ Use iron bolts for connection. After breaking, immediately remove the sample, rinse it with deionized water, clean it with alcohol, and dry it.

[0047] The stress-strain curves of the tensile specimens were analyzed, and the mechanical properties are shown in Table 4.

[0048] Table 4. Mg-9Gd-3Y magnesium alloy C-rings (connected by iron bolts) under slow strain rate tensile tests ( =10 -6 s -1 Experimental results in 3.5% NaCl solution

[0049] When using iron bolts for connection, as Figure 5 As shown, with increasing corrosion time, Fe² + Cr³ + Ni² + If it dissolves into the corrosion solution, it will severely contaminate the solution composition and pH value, interfere with the corrosion process of the magnesium alloy itself, and cause the test results to be distorted, failing to truly reflect the SCC behavior of the magnesium alloy in a pure solution.

[0050] Comparative Example 2: As-cast AZ91D magnesium alloy

[0051] Taking as-cast AZ91D magnesium alloy as an example, a slow strain rate tensile test was conducted on a dog-bone shaped specimen in a 3.5% NaCl solution. Iron bolts were used as the connecting device. The experimental procedure is as follows: Select as-cast AZ91D magnesium alloy under T6 aging, based on Figure 1 The dimensions and shapes are processed slowly using wire electrical discharge machining to ensure smooth, burr-free edges.

[0052] Slow strain rate tensile tests were conducted in air, deionized water, and 3.5% NaCl solution, with a tensile rate of 1×10⁻⁶. -6 s -¹ Use iron bolts for connection. After breaking, immediately remove the sample, rinse it with deionized water, clean it with alcohol, and dry it.

[0053] The stress-strain curves of the tensile specimens were analyzed, and the mechanical properties are shown in Table 5.

[0054] Table 5. As-cast AZ91D magnesium alloy C-rings (connected by iron bolts) under slow strain rate tensile tests ( =10 -6 s -1 Experimental results in 3.5% NaCl solution

[0055] Comparative Example 3: As-cast WE43 magnesium alloy

[0056] Taking as-cast WE43 magnesium alloy as an example, a slow strain rate tensile test was conducted on a dog-bone shaped specimen in a 3.5% NaCl solution. Iron bolts were used as the connecting device. The experimental procedure is as follows: Select as-cast WE43 magnesium alloy under T6 aging, based on Figure 1 The dimensions and shapes are processed slowly using wire electrical discharge machining to ensure smooth, burr-free edges.

[0057] Slow strain rate tensile tests were conducted in air, deionized water, and 3.5% NaCl solution, with a tensile rate of 1×10⁻⁶. -6 s - ¹ Use iron bolts for connection. After breaking, immediately remove the sample, rinse it with deionized water, clean it with alcohol, and dry it.

[0058] The stress-strain curves of the tensile specimens were analyzed, and the mechanical properties are shown in Table 6.

[0059] Table 6. As-cast WE43 magnesium alloy (connecting devices are iron bolts) under slow strain rate tensile test ( =10 -6 s -1 Experimental results in 3.5% NaCl solution

[0060] This invention also provides a method for preparing a connector for magnesium alloy SSRT experiments, which will be described below in conjunction with embodiments.

[0061] Example 4

[0062] This embodiment provides a method for preparing a connector for magnesium alloy SSRT experiments, the specific steps of which are as follows: S1. Resin purification pretreatment: PA6 resin is placed in a Soxhlet extractor and anhydrous ethanol is used as the extractant. The resin is refluxed at 80℃ for 18 hours to completely remove residual polymer metal catalysts and small molecule oligomers. After extraction, PA6 resin is placed in a vacuum drying oven and vacuum dried at 95℃ and a vacuum degree of -0.096MPa for 20 hours. The moisture content of the resin is controlled to be ≤0.03% to obtain purified and dried PA6 base material.

[0063] S2. Modified Material Compounding: Weigh out 78 parts of purified and dried PA6 base material, 5 parts of polycarbodiimide hydrolysis-resistant stabilizer, 15 parts of hollow glass microspheres (particle size 5-20μm) modified with KH550 silane coupling agent, 0.5 parts of benzamide nucleating agent, 0.7 parts of composite antioxidant, and 1 part of polytetrafluoroethylene micro powder lubricant by mass ratio; wherein the composite antioxidant is a mixture of hindered phenolic antioxidant 1010 and phosphite antioxidant 168 in a mass ratio of 1:1.5; place all raw materials in a high-speed mixer and mix at room temperature at 1000r / min for 10min until uniformly mixed to obtain a premix.

[0064] S3. Two-stage vacuum melt blending and granulation: The premixed material is fed into a two-stage twin-screw extruder unit, where it undergoes a first-stage melt dispersion and a second-stage vacuum devolatilization blending. The temperatures of each section of the first-stage co-rotating twin-screw extruder along the direction from the feed inlet to the die head are 230℃, 240℃, 245℃, 250℃, and 245℃, respectively, with a screw speed of 300 r / min. The temperatures of each section of the second-stage co-rotating twin-screw devolatilization extruder are 235℃, 245℃, 245℃, 240℃, and 235℃, respectively, with a screw speed of 250 r / min. Two vacuum devolatilization ports are set on the die body, with a vacuum degree of ≥-0.098 MPa for each port. After blending, the material is extruded through a die, water-cooled, and pelletized to obtain modified PA6 special granules.

[0065] S4. Low-stress precision injection molding: Modified PA6 special granules are placed in a vacuum drying oven and subjected to secondary vacuum drying for 10 hours at 90℃ and a vacuum degree of -0.096MPa, controlling the moisture content of the dried granules to ≤0.03%. The dried granules are then fed into a closed-loop temperature-controlled injection molding machine for injection molding. The injection molding process parameters are: barrel temperature 240℃, nozzle temperature 242℃, and mold temperature 70℃. The injection molding process adopts an in-mold stepped pressure holding and synchronous constant temperature cooling process. Specifically, the first holding pressure is 100MPa for 4 seconds, and then the second holding pressure is 60MPa for 8 seconds. During the holding pressure stage, the mold is kept at a constant temperature of 70℃ throughout. After the holding pressure is completed, the temperature is uniformly reduced to 25℃ at a rate of 1.5℃ / s before the mold is opened, resulting in a preform of a coaxial double-segment cylindrical solid connector with a central annular limiting boss.

[0066] S5. Inert Atmosphere Saturated Steam Gradient Annealing: The preformed connector is placed in a closed annealing furnace. After replacing the air in the furnace with high-purity nitrogen, saturated steam is introduced for a three-stage gradient heating-holding-cooling annealing process. The specific process is as follows: In the first stage, the temperature is raised to 90℃ at a heating rate of 8℃ / min and held for 3 hours, with the steam pressure in the furnace controlled at 0.12MPa; in the second stage, the temperature is raised to 120℃ at a heating rate of 4℃ / min and held for 4 hours, with the steam pressure in the furnace controlled at 0.18MPa; in the third stage, the temperature is lowered to 45℃ at a cooling rate of 2.5℃ / min, and the heating system is turned off and the furnace is cooled to room temperature to obtain a shaped billet.

[0067] S6. Surface in-situ crosslinking and passivation treatment: The shaped preform is placed in a plasma treatment chamber and first undergoes low-temperature argon plasma surface activation. The process parameters are: vacuum degree 20 Pa, argon flow rate 30 sccm, power 100 W, and activation time 10 min. After activation, tridecafluorooctyltrimethoxysilane fluorinated silane monomer is introduced for in-situ vapor phase deposition crosslinking. The process parameters are: deposition vacuum degree 35 Pa, fluorinated silane monomer flow rate 18 sccm, deposition temperature 70℃, and deposition time 20 min. After deposition, the preform is kept in a vacuum environment for 15 min to cure. After removal, PA6 nylon connectors for magnesium alloy SSRT experiments are obtained.

[0068] Example 5

[0069] This embodiment provides a method for preparing a connector for magnesium alloy SSRT experiments, the specific steps of which are as follows: S1. Resin purification pretreatment: PA6 resin is placed in a Soxhlet extractor and anhydrous ethanol is used as the extractant. The resin is refluxed at 75°C for 12 hours to remove residual polymer metal catalysts and small molecule oligomers. After extraction, PA6 resin is placed in a vacuum drying oven and vacuum dried at 90°C and a vacuum degree of -0.095MPa for 16 hours. The moisture content of the resin is controlled to be ≤0.03% to obtain purified and dried PA6 base material.

[0070] S2. Modified material compounding: Weigh out 75 parts of purified and dried PA6 base material, 3 parts of polycarbodiimide hydrolysis-resistant stabilizer, 12 parts of hollow glass microspheres (particle size 5-20μm) modified with KH550 silane coupling agent, 0.3 parts of benzamide nucleating agent, 0.4 parts of composite antioxidant, and 0.6 parts of polytetrafluoroethylene micro powder lubricant by mass ratio; wherein the composite antioxidant is composed of antioxidant 1010 and antioxidant 168 compounded in a mass ratio of 1:1; place all raw materials in a high-speed mixer and mix at room temperature at 800r / min for 8min until uniformly mixed to obtain a premix.

[0071] S3. Two-stage vacuum melt blending and granulation: The premixed material is fed into a two-stage twin-screw extruder. The temperatures of each section of the first-stage co-rotating twin-screw extruder along the direction from the feed inlet to the die head are 225℃, 230℃, 240℃, 245℃, and 240℃, respectively, with a screw speed of 250 r / min. The temperatures of each section of the second-stage co-rotating twin-screw devouring extruder are 230℃, 235℃, 240℃, 235℃, and 230℃, respectively, with a screw speed of 200 r / min. Two vacuum devouring ports are set on the die body, with a vacuum degree of ≥-0.098 MPa for each devouring port. After blending, the material is extruded through a die head, water-cooled, and pelletized to obtain modified PA6 special granules.

[0072] S4. Low-stress precision injection molding: Modified PA6 special granules are placed in a vacuum drying oven and subjected to secondary vacuum drying at 85℃ and a vacuum degree of -0.095MPa for 8 hours, controlling the moisture content of the dried granules to be ≤0.03%. The dried granules are then fed into a closed-loop temperature-controlled injection molding machine for injection molding. The injection molding process parameters are: barrel temperature 230℃, nozzle temperature 240℃, and mold temperature 60℃. The in-mold stepped pressure holding and synchronous constant temperature cooling process is as follows: first, a first holding pressure of 90MPa is applied for 3 seconds, and then a second holding pressure of 50MPa is applied for 6 seconds. During the holding pressure stage, the mold is kept at a constant temperature of 60℃ throughout. After the holding pressure is completed, the temperature is uniformly reduced to 28℃ at a rate of 1℃ / s before the mold is opened to obtain the preform of the connector.

[0073] S5. Inert Atmosphere Saturated Steam Gradient Annealing: The preformed connector is placed in a closed annealing furnace. After high-purity nitrogen is introduced to replace the air in the furnace, saturated steam is introduced for three-stage gradient annealing. The specific process is as follows: In the first stage, the temperature is raised to 85°C at a heating rate of 5°C / min and held for 2 hours, with the steam pressure in the furnace controlled at 0.1 MPa; in the second stage, the temperature is raised to 115°C at a heating rate of 3°C / min and held for 3 hours, with the steam pressure in the furnace controlled at 0.15 MPa; in the third stage, the temperature is lowered to 45°C at a cooling rate of 2°C / min, and the heating system is turned off and the furnace is cooled to room temperature to obtain the shaped billet.

[0074] S6. Surface in-situ crosslinking and passivation treatment: The shaped preform is placed in a plasma treatment chamber and first undergoes low-temperature argon plasma surface activation. The process parameters are: vacuum degree 10 Pa, argon flow rate 20 sccm, power 80 W, and activation time 5 min. Then, tridecafluorooctyltrimethoxysilane monomer is introduced for in-situ vapor phase deposition crosslinking. The process parameters are: deposition vacuum degree 20 Pa, fluorinated silane monomer flow rate 10 sccm, deposition temperature 60℃, and deposition time 15 min. After deposition, it is kept in a vacuum environment for 10 min for curing. After removal, the target connector is obtained.

[0075] Example 6

[0076] This embodiment provides a method for preparing a connector for magnesium alloy SSRT experiments, the specific steps of which are as follows: S1. Resin purification pretreatment: PA6 resin is placed in a Soxhlet extractor and anhydrous ethanol is used as the extractant. The resin is refluxed at 85°C for 24 hours to completely remove residual polymer metal catalysts and small molecule oligomers. After extraction, PA6 resin is placed in a vacuum drying oven and vacuum dried at 100°C and a vacuum degree of -0.098MPa for 24 hours. The moisture content of the resin is controlled to be ≤0.03% to obtain purified and dried PA6 base material.

[0077] S2. Modified material compounding: Weigh out 82 parts of purified and dried PA6 base material, 7 parts of polycarbodiimide hydrolysis-resistant stabilizer, 18 parts of hollow glass microspheres (particle size 5-20μm) modified with KH550 silane coupling agent, 0.8 parts of benzamide nucleating agent, 1.0 part of composite antioxidant, and 1.5 parts of polytetrafluoroethylene micro powder lubricant by mass ratio; wherein the composite antioxidant is composed of antioxidant 1010 and antioxidant 168 compounded at a mass ratio of 1:2; place all raw materials in a high-speed mixer and mix at room temperature at 1200r / min for 15min until uniformly mixed to obtain a premix.

[0078] S3. Two-stage vacuum melt blending and granulation: The premixed material is fed into a two-stage twin-screw extruder. The temperatures of each section of the first-stage co-rotating twin-screw extruder along the direction from the feed inlet to the die head are 230℃, 240℃, 250℃, 255℃, and 250℃, respectively, with a screw speed of 350 r / min. The temperatures of each section of the second-stage co-rotating twin-screw devouring extruder are 235℃, 245℃, 250℃, 245℃, and 240℃, respectively, with a screw speed of 300 r / min. Three vacuum devouring ports are set on the die body, with a vacuum degree of ≥-0.098 MPa for each devouring port. After blending, the material is extruded through a die, water-cooled, and pelletized to obtain modified PA6 special granules.

[0079] S4. Low-stress precision injection molding: Modified PA6 special granules are placed in a vacuum drying oven and subjected to secondary vacuum drying at 95℃ and a vacuum degree of -0.098MPa for 12 hours, controlling the moisture content of the dried granules to ≤0.03%. The dried granules are then fed into a closed-loop temperature-controlled injection molding machine for injection molding. The injection molding process parameters are: barrel temperature 250℃, nozzle temperature 245℃, and mold temperature 80℃. The in-mold stepped pressure holding and synchronous constant temperature cooling process is as follows: first, a first holding pressure of 110MPa is applied for 6 seconds, and then a second holding pressure of 70MPa is applied for 10 seconds. During the holding pressure stage, the mold is kept at a constant temperature of 80℃ throughout. After the holding pressure is completed, the temperature is uniformly reduced to 30℃ at a rate of 2℃ / s before the mold is opened to obtain the preform of the connector.

[0080] S5. Inert Atmosphere Saturated Steam Gradient Annealing: The preformed connector is placed in a closed annealing furnace. After high-purity nitrogen is introduced to replace the air in the furnace, saturated steam is introduced for three-stage gradient annealing. The specific process is as follows: In the first stage, the temperature is raised to 95°C at a heating rate of 10°C / min and held for 4 hours, with the steam pressure in the furnace controlled at 0.15 MPa; in the second stage, the temperature is raised to 125°C at a heating rate of 5°C / min and held for 6 hours, with the steam pressure in the furnace controlled at 0.2 MPa; in the third stage, the temperature is lowered to 45°C at a cooling rate of 3°C / min, and the heating system is turned off and the furnace is cooled to room temperature to obtain the shaped billet.

[0081] S6. Surface in-situ crosslinking and passivation treatment: The shaped preform is placed in a plasma treatment chamber and first undergoes low-temperature argon plasma surface activation. The process parameters are: vacuum degree 30Pa, argon flow rate 40sccm, power 120W, and activation time 15min. Then, tridecafluorooctyltrimethoxysilane monomer is introduced for in-situ vapor phase deposition crosslinking. The process parameters are: deposition vacuum degree 50Pa, fluorinated silane monomer flow rate 25sccm, deposition temperature 80℃, and deposition time 30min. After deposition, the preform is kept in a vacuum environment for 20min for curing. The target connector is then obtained.

[0082] To verify the technical effects of the present invention, the following comparative examples are set up for comparison with the above embodiments: Comparative Example 1: Industry-standard 304 stainless steel bolts, with structural dimensions completely identical to those in Example 1, are standard connecting parts for existing magnesium alloy SSRT tests.

[0083] Performance testing and experimental verification

[0084] 1. Basic performance testing

[0085] Basic performance tests were conducted on the connectors of each embodiment and comparative example. The test environment was 25℃, and the test results are shown in Table 7.

[0086] Table 7 Basic Performance Test Results

[0087] Magnesium alloy SSRT test verification

[0088] Dog-bone shaped specimens of AZ91D magnesium alloy in the T6 aged as-cast state were subjected to slow strain rate tensile tests in a 3.5% NaCl solution at a strain rate of 1×10⁻⁶. -6 s - ¹ Assembly tests were conducted using the connectors from the above embodiments and comparative examples, and the test results are as follows: Examples 1-3: The test solution remained clear throughout the test, without discoloration or turbidity; there were no galvanic corrosion marks on the specimen clamping ends; all specimens broke within the gauge length, with no premature breakage at the clamping ends; the relative deviation of the fracture strength and elongation data of parallel specimens was ≤3%, demonstrating excellent data repeatability and reliability; after the test, the connectors were cleaned and showed no corrosion or deformation, and could be reused.

[0089] Comparative Example 1: After 24 hours of testing, the solution gradually turned yellow and turbid, and the bolts showed obvious corrosion; severe galvanic corrosion occurred near the sample connection hole, and all samples broke prematurely at the clamping end, making it impossible to obtain effective gauge length test data and reflect the SCC sensitivity of the magnesium alloy body.

[0090] The embodiments described above are merely preferred embodiments of the present invention and are not intended to limit the scope of the present invention. Various modifications and improvements made to the technical solutions of the present invention by those skilled in the art without departing from the spirit of the present invention should fall within the protection scope defined by the present invention.

Claims

1. A method for preparing a connector for magnesium alloy SSRT experiments, characterized in that, Includes the following steps: S1. Resin purification pretreatment: PA6 resin is placed in a Soxhlet extractor and refluxed with anhydrous ethanol as the extractant to remove residual polymer metal catalysts and small molecule oligomers from the resin. After extraction, the PA6 resin is vacuum dried to control the resin moisture content to ≤0.03%, thus obtaining purified and dried PA6 base material. S2. Modified material compounding: Weigh 75-82 parts of purified and dried PA6 base material, 3-7 parts of hydrolysis-resistant stabilizer, 12-18 parts of glass microspheres, 0.3-0.8 parts of amide nucleating agent, 0.4-1.0 parts of composite antioxidant, and 0.6-1.5 parts of fluoropolymer lubricant according to the mass ratio. Place all raw materials in a high-speed mixer and mix evenly at room temperature to obtain a premix. S3, Two-stage vacuum melt blending and granulation: The premixed material is fed into a two-stage twin-screw extruder unit, where it undergoes first-stage melt dispersion and second-stage vacuum devouring blending in sequence. After extrusion through a die, cooling, and pelletizing, modified PA6 special granules are obtained. S4. Low internal stress precision injection molding: After the modified PA6 special granules are vacuum dried twice, they are sent into a closed-loop temperature-controlled injection molding machine and injection molded into a coaxial double-segment cylindrical solid connector preform with a central annular limiting boss. The injection molding process adopts in-mold stepped pressure holding and synchronous constant temperature cooling process. S5. Inert atmosphere saturated steam gradient annealing: The preformed connector is placed in a closed annealing furnace. After the air in the furnace is replaced by high-purity nitrogen, saturated water vapor is introduced to carry out a three-stage gradient heating-holding-cooling annealing process. After the process is completed, the furnace is cooled to room temperature to obtain a shaped billet. S6. Surface in-situ crosslinking passivation treatment: The shaped blank is placed in the plasma treatment chamber, and the surface is first activated by low temperature argon plasma. Then, fluorine-containing silane monomers are introduced to perform in-situ vapor deposition crosslinking, forming a dense non-polar crosslinking passivation layer on the surface of the blank. After removal, PA6 nylon connectors for magnesium alloy SSRT experiments are obtained.

2. The method for preparing the connector for magnesium alloy SSRT experiments according to claim 1, characterized in that, In step S1, the reflux extraction temperature is 75-85℃ and the extraction time is 12-24h; the vacuum drying temperature is 90-100℃, the vacuum degree is ≥-0.095MPa, and the drying time is 16-24h.

3. The method for preparing the connector for magnesium alloy SSRT experiments according to claim 1, characterized in that, In step S2, the hydrolysis-resistant stabilizer is polycarbodiimide, the glass microspheres are hollow glass microspheres modified with silane coupling agent, and the particle size is 5-20 μm; the composite antioxidant is a mixture of hindered phenolic primary antioxidant and phosphite auxiliary antioxidant in a mass ratio of 1:1-2; the mixing speed of the high-speed mixer is 800-1200 r / min, and the mixing time is 8-15 min.

4. The method for preparing the connector for magnesium alloy SSRT experiments according to claim 1, characterized in that, In step S3, the first-stage extruder of the two-stage twin-screw extruder unit is a co-rotating twin-screw extruder with a temperature of 225-255℃ in each section and a screw speed of 250-350 r / min; the second-stage extruder is a co-rotating twin-screw devouring extruder with a temperature of 230-250℃ in each section and a screw speed of 200-300 r / min, and 2-3 vacuum devouring ports are provided on the machine body, with a vacuum degree of ≥-0.098 MPa for each devouring port.

5. The method for preparing the connector for magnesium alloy SSRT experiments according to claim 1, characterized in that, In step S4, the temperature of the secondary vacuum drying is 85-95℃, the vacuum degree is ≥-0.095MPa, the drying time is 8-12h, and the moisture content of the dried granules is ≤0.03%; the barrel temperature of the injection molding is 230-250℃, the nozzle temperature is 240-245℃, and the mold temperature is 60-80℃.

6. The method for preparing the connector for magnesium alloy SSRT experiments according to claim 5, characterized in that, In step S4, the in-mold stepped pressure holding and synchronous constant temperature cooling process is as follows: first, the mold is held at a first pressure of 90-110MPa for 3-6s, and then held at a second pressure of 50-70MPa for 6-10s; during the pressure holding stage, the mold is kept at a constant temperature of 60-80℃ throughout the process, and after the pressure holding is completed, the mold is cooled at a rate of 1-2℃ / s to below 30℃ before opening.

7. The method for preparing the connector for magnesium alloy SSRT experiments according to claim 1, characterized in that, In step S5, the specific process of the three-stage gradient heating-holding-cooling annealing treatment is as follows: In the first stage, the temperature is raised to 85-95℃ at a heating rate of 5-10℃ / min, held for 2-4 hours, and the steam pressure inside the furnace is controlled at 0.1-0.15MPa; in the second stage, the temperature is raised to 115-125℃ at a heating rate of 3-5℃ / min, held for 3-6 hours, and the steam pressure inside the furnace is controlled at 0.15-0.2MPa; in the third stage, the temperature is lowered to below 50℃ at a cooling rate of 2-3℃ / min, and the heating system is turned off and the furnace is cooled to room temperature.

8. The method for preparing the connector for magnesium alloy SSRT experiments according to claim 1, characterized in that, In step S6, the process parameters for the low-temperature argon plasma surface activation are: vacuum degree 10-30 Pa, argon flow rate 20-40 sccm, power 80-120 W, and activation time 5-15 min; the process parameters for the vapor phase in-situ deposition crosslinking are: deposition vacuum degree 20-50 Pa, fluorinated silane monomer flow rate 10-25 sccm, deposition temperature 60-80℃, deposition time 15-30 min, and after deposition, heat preservation and curing in a vacuum environment for 10-20 min.

9. A connector for magnesium alloy SSRT experiments, prepared using the method for preparing a connector for magnesium alloy SSRT experiments according to any one of claims 1-8, characterized in that, The connector is composed of two cylindrical segments of equal diameter, with an annular limiting groove between the two cylindrical segments. The annular limiting groove is used to fix the magnesium alloy sample to be tested. The outer wall surface of the connector is provided with threads.

10. A method for evaluating the SSRT (Self-Strain Tolerance) of magnesium alloys, characterized in that, include: Prepare a magnesium alloy sample to be tested, wherein the magnesium alloy sample is dog bone type; Place the clamping areas at both ends of the magnesium alloy specimen in the corresponding positions of the upper and lower clamps of the tensile testing machine; The connector for magnesium alloy SSRT test as described in claim 9 passes through the connecting hole of the magnesium alloy specimen and the connecting hole of the upper and lower clamps of the tensile testing machine. Tighten and secure using the nuts that are compatible with the connector; The magnesium alloy sample was immersed in the corrosive solution, a slow strain rate was set, and a tensile test was started. The load-displacement curve was recorded until the sample broke. Remove the sample and bolt, and record the fracture location and corrosion morphology.

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