What matters should be paid attention to when using automatic sandblasting machine for surface treatment of parts and components in aerospace field?

A complete set of control precautions for surface treatment of automatic sandblasting machine for aerospace parts (adapting to AS9100, AMS2430 and aerospace special components specifications)

Core risks of aerospace parts: iron impurity pollution, hydrogen embrittlement, out-of-tolerance size, fatigue crack, chloride ion salt spray corrosion, FOD foreign bodies, and untraceable batches. The multi-spindle/turntable/conveying equipment of automatic sandblasting machine should be controlled from equipment, abrasives, tooling, technology, pre-and post-treatment, detection, cleaning and full dimension of documents.

I. Special equipment control (hardware modification and routine inspection of automatic sandblasting machine)

1. Whole machine isolation and cross-contamination prevention (aerospace red line)

Special purpose for special plane: titanium alloy, aluminum alloy, high-temperature alloy, high-strength steel and composite materials must be separated into independent automatic sandblasting cavities, and it is strictly forbidden to mix one machine; Before switching parts made of different materials, the whole machine shall empty, screen and wash the cavity, so as to prevent iron filings from being embedded in titanium and aluminum to form galvanic corrosion pitting.

The cavity is lined with stainless steel 304/316 to prevent iron powder from falling off the inner wall of ordinary carbon steel; All internal mesh belt conveyor, rotary table, spray gun bracket and separator are made of nonmetal/stainless steel, and carbon steel fittings are prohibited.

The automatic recovery system is compulsorily equipped with multi-stage magnetic separation+precision screening+cyclone dust removal: free iron impurities are removed by magnetic separation, and crushed fine powder is removed by screening, and the dust filtration efficiency is ≥ 99.9%; Clean up and screen the waste materials in each shift, and replace all the broken abrasives with a ratio of > 15%.

Air source system (space mandatory standard)

Three-stage treatment of compressed air: freeze dryer+precision oil removal filter+activated carbon sterilization filter element, oil-free and waterless dew point ≤-40℃; The pipeline is all stainless steel, and it is forbidden for ordinary rubber hoses to precipitate plasticizers to pollute the workpiece.

Online closed-loop monitoring of air pressure, pressure fluctuation control of ±0.02MPa, automatic overpressure shutdown alarm; The gasholder discharges pollutants, removes water and oil every week.

Automatic motion mechanism (multi-axis spray gun/rotary tooling)

The walking speed, spraying distance, angle and reciprocating layers of the spray gun are all solidified by the program, and the parameters cannot be modified at will; The repeatability of servo positioning is ±0.5mm, ensuring full coverage and no leakage.

Replace the nozzle immediately if it is worn over 20%, which will cause local overspray and uneven roughness; Check the nozzle inner diameter every 2 hours and record it.

Explosion prevention and dust control

Titanium and aluminum dust is flammable and explosive, automatic equipment is equipped with wet dust removal/inert gas dust suppression, and the dust concentration in the cavity is monitored in real time; The workshop has a constant temperature of 20 ~ 25℃ and a humidity of 40% ~ 55% to avoid electrostatic dust.

Second, the abrasive selection, control and acceptance (to prevent core pollution)

1. Match the special abrasive according to the aerospace substrate (mixing is strictly prohibited).

form

The core purpose of workpiece material is to allow abrasives to prohibit abrasives.

Ti-6Al-4V titanium alloy, high-temperature alloy high-purity white corundum (ALO ≥ 99.5%), ceramic sand, glass bead cast steel shot, brown corundum and iron-containing abrasive can prevent iron impurities from embedding and stress corrosion cracking.

2024/7075 Aluminum alloy ceramic beads, glass beads and stainless steel pellets Ordinary carbon steel sand can avoid iron pollution and scratch thin-walled structures.

Special stainless steel shot and reinforced ceramic shot for ultra-high strength landing gear steel (300 m), hard alumina (hydrogen permeable) are used to control the risk of hydrogen embrittlement, and shot peening is used for strengthening.

CFRP carbon fiber composite plastic microbeads, corn cob soft abrasive high hardness alumina and metal pellets prevent fiber delamination and matrix erosion.

2. Abrasive incoming and circulation control

Material report must be provided when entering the factory: iron oxide < 0.1%, chloride ion < 1 μ g/cm, no sulfide; The deviation of particle size distribution is less than 5%, and the hardness conforms to AMS2431 standard.

Before the new abrasive is put into use, it is fully screened, cleaned and dried to remove the production dust and salt; The circulating abrasive shall be sieved in each shift to eliminate broken powder and metal chips.

Abrasives shall be stored in separate zones, sealed and moisture-proof. Abrasives of different materials shall be marked in barrels, and mixing is prohibited. It is strictly forbidden to pile up moisture-absorbing salt in the open air.

Three, the workpiece protection and fixture clamping (automatic assembly line)

1. Pretreatment before sandblasting

Degreasing and degreasing the whole piece: alkaline cleaning+ultrasonic cleaning, and drying until there is no water; The oil stain on the surface will stick to the abrasive and embed into the matrix, which can not be cleaned later, causing the coating to fall off and corrode.

High-precision characteristic rigid shielding protection (automatic sandblasting machine must do in batch processing);

Threaded holes, precision mating surfaces, sealing grooves, sensor mounting surfaces and thin-walled edges are made of high-temperature resistant silicone plugs, metal cover plates and special shielding tools;

It is forbidden to use ordinary adhesive tape, and the residual adhesive layer is easy to fall off due to sandblasting impact, forming FOD foreign bodies.

Thin-walled parts, thin plates and slender shafts adopt multi-point support tooling, and the automatic turntable rotates evenly to prevent sand blasting impact deformation; Control the removal of single-sided material ≤0.02mm, without damaging the dimensional tolerance.

2. General requirements for automatic tooling

The material of the tooling is the same as that of the workpiece, and there is no contact between dissimilar metals; The tooling is regularly sandblasted to clean the surface sand to avoid secondary pollution.

Batch of small pieces are made of hollow special material frames to ensure that the whole surface is sprayed without shielding, and to prevent leakage of spraying caused by shielding of the frame body.

Four, automatic sand blasting process parameters accurate control (fatigue, roughness, residual stress core)

Spacecraft parts are divided into two categories: cleaning and sandblasting (removing scale) and * * strengthening shot blasting (improving fatigue life). Automatic equipment programs are solidified in different areas, and it is forbidden to adjust parameters at will on site:

injection pressure

Titanium/aluminum precision parts and composites: 0.1 ~ 0.3 MPa low-pressure micro-spray;

Scale removal of structural parts: 0.3 ~ 0.5 MPa;

Strengthening shot peening of high strength steel: 0.4 ~ 0.6 MPa;

Stop automatically when the pressure is out of tolerance to prevent overspray thinning and surface micro-cracks.

Spray gun attitude (multi-axis automatic program locking)

The injection angle is 40 ~ 60, and 90 vertical direct injection is prohibited (excessive impact causes pits and roughness exceeding the standard);

The spraying distance is 100 ~ 200 mm, the moving speed of the spray gun is 1 ~ 2 m/min, and the reciprocating 2 ~ 3 layers achieve 200% coverage.

Strengthening Shot Peening and Compulsory Use of almen Specimen

Standard test pieces are placed synchronously in each batch, and the arc height (strength) is measured to meet the requirements of the drawings; The coverage rate shall be over 100%, and the key load-bearing parts shall be fully covered by 200% to ensure uniform residual compressive stress layer (-200 ~-400 MPa).

Strictly control the hazards of overspray.

Overspray meeting: ① substrate thinning out of tolerance; (2) Micro-cracks are introduced due to excessive surface processing; ③ The roughness Ra is too large (> 6.3 μ m), which increases the corrosion active area; ④ Titanium alloy locally impacted at high temperature to form a dense oxide layer, which reduced the adhesion of the coating. Automatic program sets a fixed sandblasting time, and artificial extension is prohibited.

V. Special prevention and control of hydrogen embrittlement (fatal risk of high-strength steel and titanium alloy)

High-strength steel (tensile strength ≥1200MPa) is given priority to stainless steel shot wet spraying to reduce hydrogen ion infiltration; It is forbidden to dry sand blasting hard abrasive for a long time.

After sandblasting, the hydrogen removal baking is completed within 4 hours (keeping the temperature at 190 ~ 230℃ for 2 ~ 4 hours according to the process specification), and hydrogen atoms infiltrated into the surface layer are released.

Compressed air must be completely dehydrated, and water vapor will greatly increase the hydrogen absorption of the workpiece; Water is forbidden in the cavity, and the cavity is dried in each shift.

Vi. Cleaning after sandblasting (FOD, salt pollution prevention and control)

Off-line multi-stage cleaning process immediately after leaving the aircraft (space compulsory):

High-pressure clean compressed air purging → ultrasonic pure water cleaning → high-purity alcohol rinsing → vacuum drying; Deep holes, blind holes and inner cavities are washed by ultrasonic and high-voltage pulse to completely remove embedded abrasive dust.

Cleanliness detection: water membrane breaking test, ion chromatography for chloride ion (≤ 1 μ g/cm), and fluorescent magnetic particle for free iron impurities on the surface.

Immediately after drying, transfer to a class 100,000 clean area, wear dust-free and powder-free gloves for transshipment, and it is forbidden to touch the sandblasting surface with bare hands; Complete the subsequent processes such as anodizing, spraying and vacuum coating within 24 hours to avoid reoxidation.

Seven, environment, personnel and batch traceability (AS9100 system requirements)

The operation area is closed independently and physically separated from the machining and welding areas; The epoxy resin on the ground is antistatic, vacuuming and mopping the floor every day to prevent the ground iron powder from being brought into the cavity.

Operators should hold relevant certificates (aviation surface treatment qualification), wear anti-static and dust-free sandblasting clothes, air-supplied dust-proof masks and powder-free rubber gloves; Ordinary cotton yarn and brushes are prohibited from entering the sand blasting area.

Digital traceability of the whole process (data storage of automatic sandblasting machine)

Records of each batch: workpiece drawing number, batch number, abrasive model, air pressure, spray gun speed, number of spray layers, almen strength, roughness, cleaning records and equipment spot inspection records; The data shall be kept for ≥15 years, meeting the requirements of aerospace audit factory.

Check list of equipment in each shift: gas source oil content, nozzle wear, iron content in magnetic separation, screening crushing rate, dust removal pressure difference, emergency stop and calibration of air pressure sensor.

Eight, all kinds of typical aerospace parts special taboo

Engine blades (titanium/superalloy): only fine-grained ceramic sand is sprayed at low pressure, and large-grained abrasives are prohibited; Strictly control the removal amount of edges and leaf roots to avoid fatigue sources; Vertical spraying of airfoil surfaces is not allowed.

Aluminum alloy thin-walled parts of fuel tank: glass beads are sprayed at low pressure to prevent deformation; Thoroughly clean the inner cavity after sandblasting the inner wall to prevent abrasive residues from entering the fuel system and causing FOD.

Ultra-high strength steel landing gear: dry spraying of alumina is prohibited, and stainless steel shot peening is preferred; Strictly carry out hydrogen removal baking to prevent hydrogen brittle fracture.

Carbon fiber composite components: only plastic beads have low pressure, and hard abrasives are strictly prohibited to prevent fiber delamination and matrix damage and reduce bonding strength.

Precision valve body and micro-flow passage parts: micro-sandblasting+special tooling to block the flow passage, and high-pressure ultrasonic penetration cleaning in the later process to prevent sand particles from blocking the oil and gas path.

Nine, common failure risk summary (automatic sandblasting high frequency problem)

Iron impurities embedded in titanium and aluminum → cross contamination of ground, equipment and abrasive → galvanic corrosion and fatigue crack;

Chloride ion residue → pitting corrosion in salt spray environment and coating bulging off;

Overspray/parameter out of control → size out of tolerance, surface microcrack, fatigue life reduction;

Abrasive residual cavity → FOD foreign body stuck in valves and pipelines during service;

High-strength steel absorbs hydrogen but does not remove hydrogen → hydrogen brittle fracture;

Automatic program drift, nozzle wear → uneven roughness, local spray leakage;

The oil stain is not pretreated → the abrasive agglomerate is embedded in the matrix and cannot be cleaned.