What should I do after the surface treatment of the self-locking tuning screw is processed?
The surface treatment of self-locking tuning screws should meet both functional requirements (such as corrosion resistance, self-locking stability and tuning smoothness) and precision protection (to avoid dimensional out-of-tolerance and performance degradation), and the process plan should be formulated in combination with material characteristics and application scenarios (such as communication equipment, aerospace and medical electronics). The specific process and precautions are as follows:
1. Pretreatment before surface treatment: remove impurities to ensure coating adhesion.
Pretreatment is the basis of surface treatment, which directly affects the bonding strength and uniformity of coating. The following steps should be strictly implemented:
Precision cleaning: removing machining residues.
Core objective: to remove cutting fluid, metal chips and oil stains (such as emulsion and antirust oil) after turning/threading, so as to avoid residues affecting coating adhesion or causing local corrosion.
Process selection:
Ultrasonic cleaning (frequency is 28-40kHz, power is 500-800W) is preferred, and high-purity ethanol, isopropanol or special industrial cleaning agents (compatible with materials, such as avoiding strong alkaline cleaning agents for copper alloys) are used as solvents.
After cleaning, it needs to be rinsed with deionized water for 2-3 times (to remove the residue of cleaning agent), and finally dried in clean hot air (temperature 40-60℃, wind speed 0.5-1m/s) to avoid the formation of spots caused by water stains.
Quality requirements: the cleanliness should reach ISO 16232-10 level (particle size ≤5μm, the number of residual particles in a single piece ≤10), which can be detected by microscope or particle counter.
Surface activation: improve coating adhesion.
For different materials, it is necessary to remove the surface oxide layer and enhance the surface roughness (micro-level, without affecting the macro-size) by mild treatment:
Stainless steel (304/316): adopt "pickling activation" (5%-10% nitric acid solution, soaking at room temperature for 5-10 minutes) to remove the surface passivation film; Or "plasma activation" (argon atmosphere, power 100-200W, treatment time 30-60 seconds), to avoid chemical corrosion affecting the size.
Titanium alloy (TC4): After degreasing with "weak alkaline degreaser" (pH 8-9), it is slightly corroded with "hydrofluoric acid-nitric acid mixed solution" (volume ratio 1:3) (soaked at room temperature for 3-5 minutes) to form a microscopic rough surface.
Copper alloy (H62): adopt "citric acid activation" (5% citric acid solution, soaking at 40-50℃ for 8-12 minutes) to remove the surface oxide layer (patina) to avoid pinholes in subsequent nickel/chromium plating.
Key control: immediately enter the next process after activation (interval ≤30 minutes) to avoid surface secondary oxidation; At the same time, the treatment time and concentration should be strictly controlled to prevent excessive corrosion from causing thread profile deformation (such as crest wear ≤0.002mm).
Second, the core surface treatment process: select the scheme according to the functional requirements.
According to the three core requirements of "corrosion resistance, self-locking and tuning smoothness", mainstream technologies are divided into anti-corrosion, self-locking lubrication and appearance protection, which need to be selected in combination with use scenarios:
1. Anticorrosive surface treatment: prolong service life and adapt to harsh environment.
Suitable for humid, salt spray and chemical corrosive environments (such as outdoor communication base stations and marine equipment). The common processes are as follows:
Process name, applicable materials, process parameters, core advantages and precautions
Passivation of Stainless Steel Chromate Passivation of 304/316 Stainless Steel (concentration 8%-12%, temperature 50-60℃, soaking for 15-20 minutes) has low cost and no size influence. After passivation, it needs to be washed with water and dried to avoid hydrogen embrittlement caused by residual passivation solution.
Titanium alloy anodizing TC4 titanium alloy sulfuric acid electrolyte (15%-20%, temperature 20-25℃, voltage 15-20V, oxidation time 30-40 minutes) has strong corrosion resistance (salt spray test ≥100 hours) and high hardness (HV 300-500). The thickness of the oxide film is controlled at 5-10μm, and too thick will affect the thread fit.
Electroless nickel plating on copper alloy H62 copper alloy (nickel content 85%-90%, thickness 3-5μm, temperature 80-90℃) has better corrosion resistance than copper substrate. After uniform appearance plating, it needs "dehydrogenating treatment" (heat preservation at 180-220℃ for 2-4 hours) to prevent hydrogen embrittlement.
2. Self-locking lubrication surface treatment: to ensure tuning stability and self-locking performance.
In order to meet the requirements of "smooth tuning, anti-loosening and self-locking" (such as precision instruments and avionics), it is necessary to balance "low friction" and "self-locking". The common processes are as follows:
Solid lubricating coating (such as MoS₂ and PTFE)
Application scenario: high tuning frequency and long-term low friction (such as tuning screw of communication filter).
Process steps:
After the pretreatment of the substrate, spray solid lubricating paint (such as water-based paint with MoS₂ content ≥90%), and control the coating thickness to 2-5μm (the thread part should be covered mainly to avoid missing coating).
After low-temperature drying (60-80℃ for 30 minutes), high-temperature curing (200-250℃ for 1-2 hours) ensures that the coating is firmly combined with the substrate.
Quality requirements: the adhesion of the coating should reach Grade 4B or above (no large area shedding); The friction coefficient is controlled at 0.05-0.15 (detected by friction and wear tester) to avoid tuning jam or insufficient self-locking force.
Dry lubricant coating
Application scenario: small batch production and quick lubrication (such as tuning screws of medical equipment).
Process selection: PTFE dry spray is used, evenly sprayed at a distance of 15-20cm from the workpiece, with a thickness of 1-3μm, and dried at room temperature (10-15 minutes).
Precautions: To avoid coating accumulation caused by excessive spraying (such as the accumulation of thread root will affect the fit clearance), the excess spraying agent can be gently blown by compressed air.
3. Appearance protection surface treatment: both aesthetics and basic protection are taken into account.
Suitable for scenes that require appearance (such as consumer electronics and instrument panels). Common processes are:
Stainless steel wire drawing/polishing: for stainless steel screws, the surface of the head is treated by precision wire drawing machine (wire drawing mesh is 400-600 mesh) to form uniform texture; Or through diamond polishing (precision Ra≤0.2μm), the appearance texture is improved, and the surface smoothness is enhanced (dust adhesion is reduced).
Anodizing of aluminum alloy (color): If the screw is made of aluminum alloy (such as 6061), the appearance of black and silver can be achieved by anodic oxidation (the thickness of oxide film is 5-8μm), and the corrosion resistance and beauty should be taken into account, and the color uniformity (color difference δ e ≤ 1.5) should be controlled.
Third, quality inspection after surface treatment: to ensure that the performance and accuracy meet the standards.
After surface treatment, it is necessary to pass multi-dimensional inspection to prevent unqualified products from flowing downstream. The key inspection items are:
Re-inspection of dimensional accuracy
Core inspection: key dimensions such as thread pitch diameter, rod diameter and groove width (need to be inspected by CMM or thread micrometer), so as to ensure that the coating thickness does not lead to dimensional out-of-tolerance (for example, thread pitch diameter tolerance should be kept at IT5-IT7 level, and coating contribution error should be ≤0.003mm).
Focus on: thread compatibility-test with standard thread gauges (general gauge and stop gauge). The general gauge should be screwed in smoothly, and the screw-in depth of the stop gauge should be ≤2 threads, so as to avoid tight fit caused by too thick coating.
Functional performance test
Corrosion resistance: neutral salt spray test (GB/T 10125), stainless steel passivated parts have no corrosion for 48 hours or more, and titanium alloy anodized parts have no corrosion for 100 hours or more;
Self-locking performance: When the "loosening torque" is detected by a torque tester, it shall be more than or equal to 80% of the pre-tightening torque (for example, the pre-tightening torque of M3 screw shall be 0.5N・m, and the loosening torque shall be more than or equal to 0.4 nm);
Adhesion of coating: check method (GB/T 9286) or circle method (GB/T 1720) to ensure that no coating falls off or peels off.
Appearance and cleanliness inspection
Appearance: through visual inspection (or magnifying glass 10 times), there are no pinholes, bubbles, scratches and color differences, and there is no coating accumulation on the thread;
Cleanliness: after ultrasonic cleaning again, detect the residual particles (which must conform to ISO 16232-10 level) to avoid the debris formed by coating shedding from affecting the tuning.
IV. Matters needing attention in process core
Avoid the risk of hydrogen embrittlement: For electroplating (such as nickel plating and chromium plating) or pickling process, it is necessary to carry out "hydrogen removal treatment" (temperature 180-220℃, heat preservation for 2-4 hours) after treatment, especially for high-strength materials (such as stainless steel with tensile strength ≥800MPa), so as to prevent hydrogen embrittlement from causing the screw to break during stress adjustment.
Control the uniformity of coating thickness: complex parts such as threads and slots need to ensure complete coating coverage and uniform thickness (which can be detected by eddy current thickness gauge, and the thickness deviation of a single point is less than or equal to 10%), so as to avoid over-tolerance of size caused by local thickness or protection/lubrication failure caused by too thin.
Process compatibility: If multiple surface treatments (such as "passivation+solid lubricating coating") are required, it is necessary to ensure that the previous process will not affect the subsequent effect (such as thorough cleaning after passivation to avoid the reaction between passivation solution and lubricating coating).
summary
The surface treatment of self-locking tuning screws should focus on the principle of "function first, precision protection", and the adaptation process should be selected according to the materials and use scenarios. At the same time, through the whole process control of "pretreatment-process control-post-inspection", the final product can meet the requirements of corrosion resistance, lubrication and self-locking without sacrificing dimensional accuracy and tuning stability. For high-end fields (such as aerospace), it is necessary to combine with customer-specified standards (such as NASA-STD-8739.1 and MIL-SPEC) to further improve process reliability.
