Surface Quality Requirements for Aluminum Alloy Structural Components Aluminum alloy materials are widely used in aerospace manufacturing due to their low density, high strength, excellent machinability, and favorable cost-performance ratio.
Surface quality control of aluminum alloy structural components directly impacts overall assembly, operational performance, and safety.
For instance, during machining, the metal substrate’s inherent low thermal conductivity and heat transfer rate can cause localized heat buildup and friction at elevated temperatures.
This instability leads to residual burrs, chips, oxide layers, and other contaminants on the metal surface, severely compromising subsequent processes and product aesthetics.
Therefore, establishing specific technical standards and management requirements for the surface quality of machined aluminum alloy structural components provides a basis for inspection and reference during actual production and delivery.
Machining production processes primarily include blank preparation, sawing, CNC milling, turning, sheet metal fabrication, fitting, heat treatment, surface treatment, and painting.
The fundamental principle for controlling structural component surface quality is that product appearance must meet inspection requirements or standards specified by the process and drawings.
Based on the delivery and inspection requirements for aerospace structural components, the fundamental technical standards for surface quality of machined aluminum alloy structural components are further specified:
Raw materials must comply with valid process specifications and exhibit no deformation, scratches, oxidation discoloration, or corrosion.
Component surfaces shall be smooth, flat, and free of burrs (no snagging sensation when touching any edge or corner along the part’s machined edges in any direction), deformation, rust, cracks, or dents.
Pre-embedded rivets shall be securely fastened without looseness. Threads shall be free of damage and corrosion. Tool marks and surface residues are not permitted.
Heat treatment methods shall comply with valid process specifications. Post-treatment, no defects such as overburning, deformation, warping, coarse grain structure, spotting, or rust shall be present.
Slight oxidation discoloration is permissible. Surface treatment methods shall comply with valid process specifications.
Prior to treatment, component surfaces must be free of oil, paint, metal shavings, marking colors, and other contaminants.
Post-treatment surfaces shall be smooth and flat, free of spots, scorching, blistering, coating peeling, incomplete plating, or acidic electroplating residues.
Minor, unavoidable fixture marks are permitted where not visible during assembly.
Slight roughness on edges and corners is acceptable. Coated areas (e.g., oxidized, plated, painted) must show no peeling or localized coating absence.
Cleaned component surfaces should exhibit the bright base material color. No oil, acid/alkali residues, water marks, or signs of over-etching are permitted.
Manufacturing marks refer to localized areas on the product’s outer surface exhibiting noticeable roughness, color variation, or tool/fixture imprints compared to surrounding areas.
This includes grinding marks, tool/fixture processing marks, and assembly/testing marks.
Defects are conditions that compromise the product’s ability to ensure proper form, assembly, and function under intended usage.
Manufacturing marks are permissible during production, but defects are not allowed.
From the perspective of surface defects, no structural component surface may exhibit deep scratches, cracks, corrosion, burrs, or similar flaws.
Deep scratches refer to surface coating abrasions or motion marks that penetrate to the substrate (exposing it) or create noticeable surface irregularities.
Cracks denote fissures within the material’s internal or external surfaces. Corrosion refers to rusting or oxidation of the surface metal caused by various factors (which cannot be cleaned or wiped away).
Burrs refer to metallic rough edges left on the part surface after machining or riveting.
Aluminum and aluminum alloys are highly chemically active and prone to surface corrosion.
To achieve good corrosion resistance, surface modification treatments such as chemical oxidation, anodizing, or electroplating must be applied.
Certain structural components require painting according to technical specifications after surface treatment.
Consequently, the quality of the treated surface directly impacts the success of the subsequent painting process.
The painted surfaces of components awaiting coating must be smooth, free of burrs, visible scratches, milling marks, weld spatter, and burrs.
Minor mechanical damage on part surfaces can be concealed through putty application and painting.
Only components meeting these requirements may proceed to subsequent processing.
All surfaces of aluminum alloy structural components must be free of exposed metal and contamination.
The oxide film must be continuous, uniform, and intact.
The oxide layer should exhibit no significant color variation (visual differences in gloss and color across different areas of the same surface with identical requirements, with a color difference value ΔE ≤ 5).
Due to variations in materials and surface conditions, color inconsistencies are permitted within the same component.
Color differences are allowed between aluminum-clad and non-aluminum-clad parts of the product.
Color variations may also exist between the aluminum-clad and non-aluminum-clad sections of a single part.
Watermarks (stains formed after electroplating or oxidation due to insufficient or incomplete drying of cleaning water) are not permitted on the outer surface.
Watermarks exceeding color difference control requirements are not permitted on the inner surface, but minor watermarks below the color difference control value are allowed, with no more than 3 locations per part.
Localized grinding is not permitted on the outer surface. The inner surface may exhibit uneven coating resulting from scratches or abrasions caused by metallographic sandpaper during pre-surface treatment.
Appearance requirements for electroplated aluminum alloy structural components: uniform and fine crystallization, continuous coating, and normal color.
Defects such as pinholes, pitting, peeling, flaking, scorching, or roughness are not permitted.
Minor watermarks and unavoidable fixture marks (localized coating absence where auxiliary hanging tools contact the part during surface treatment or plating) are permitted.
High-standard surface quality depends not only on the machining level and process control precision of each operation but also relies heavily on the clarity and implementation of quality management system standards.
Therefore, establishing clear, effective, and actionable surface quality management requirements is a critical means to ensure consistent product quality.
During product design and production preparation phases, design criteria, process requirements, and inspection standards must be clearly defined.
Specific implementation rules for product appearance inspection should be established to provide effective guidance and oversight throughout the production process.
Implementation rules for product appearance inspection must include:
They should also classify product surface grades and clarify inspection standards, requiring specific constraints for all typical product structures and surfaces to avoid disputes and ambiguous interpretations.
Simultaneously, surface quality inspection and control must be implemented at every stage.
First, during structural component manufacturing, risk points must be identified in advance.
From raw materials through each subsequent process, self-inspection and documentation must be conducted after each operation to systematically reduce potential sources of error.
Second, the inspection department must perform in-process inspections according to inspection management regulations and document results to ensure product conformity during production.
This guarantees that product functionality, performance, and appearance meet acceptance test procedure requirements.
When nonconforming products are identified during production, they must be immediately marked, isolated, and subjected to review, corrective action, and preventive control according to established procedures.
Additionally, technical training for operators, process engineers, inspectors, and managers should be intensified to enhance the production team’s professional competence and accountability.
Process engineers should be encouraged to explore new materials and techniques to diversify surface quality control methods, thereby strengthening the technical foundation for structural component surface quality management.
During production and processing, aluminum alloy structural components may develop surface defects such as burrs, scratches, vibration marks, and tool marks.
Inadequate protection during process transfers and product transportation can also cause surface scratches. These defects significantly impact surface quality and may disrupt subsequent processing steps, leading to issues like color inconsistencies after surface treatment.
Traditionally, manual grinding has been used to repair structural component surfaces.
However, this method offers low precision, limited improvement effects, and inefficient repair rates, adversely affecting product delivery quality and schedules.
The surface finishing process for aluminum alloy structural components directly influences product surface characteristics, performance, and reliability.
Therefore, exploring more efficient methods to address various surface defects in aluminum alloy structural components can effectively enhance surface quality.
This holds significant theoretical and practical importance for guiding the actual production of structural components.
Sandblasting is currently an economical and widely applied surface finishing process.
Its primary principle involves using compressed air as propulsion to propel abrasive particles at high velocity directly onto component surfaces.
The impact and grinding action effectively removes surface contaminants while enhancing mechanical properties and surface flatness. Sandblasting is primarily categorized into dry blasting and wet blasting.
Dry blasting utilizes compressed air to propel abrasive material into the blast gun.
The impact strengthening effect of the abrasive not only increases the hardness of the processed surface but also enhances its wear resistance.
Wet blasting involves using an abrasive slurry pump to pressurize a sand-water mixture of specific concentration—either through pump pressure alone or with additional compressed air—and eject it at high velocity through the gun nozzle.
The high-speed movement of sand particles or glass beads processes the part surface to achieve cleaning, strengthening, or finishing.
This method effectively improves surface finish without affecting dimensional accuracy.
The surface condition of structural components directly impacts the quality of surface treatment and coating, as well as the adhesion and protective efficacy of the film layer.
Sandblasting alters the surface roughness and condition of structural components, further enhancing the effectiveness of subsequent chemical surface treatment processes.
This yields coatings with high adhesion and a smooth surface finish.
This study employed both dry and wet sandblasting processes to treat aluminum alloy structural components prior to conductive oxidation.
By comparing different sandblasting parameters and the surface appearance of the structural components, the role of sandblasting in enhancing structural component surface quality was explored.
Dry sandblasting used brown fused alumina (particle size 0.15 mm) as abrasive.
The treated specimens exhibited brighter surfaces but darker colors after conductive oxidation.
Wet sandblasting employed glass beads (particle size 0.01 mm), which improved surface film color consistency while effectively enhancing post-treatment surface gloss, as shown in Figure 1.
As shown in Figure 1, wet blasting delivers more pronounced surface enhancement compared to dry blasting, resulting in superior surface gloss.
However, higher blasting pressures may cause deformation in thin-walled parts, while lower pressures may fail to remove surface scratches effectively.
Practical application requires adjusting process parameters based on the structural strength and geometry of the parts.
Wet sandblasting ensures consistent coating color while maintaining the glossiness of parts after conductive oxidation.
However, it requires corresponding industrial wastewater treatment equipment.
Post-blasting processes like drying and air-drying are necessary to remove surface liquid residues, making subsequent steps relatively complex.
Machining marks, surface hardened layers, and fusion layers left after high-speed milling on CNC machines are difficult to remove.
Currently, manual polishing is predominantly used to reduce surface roughness values, but this method is inefficient and cannot guarantee consistent surface quality.
Magnetic grinding represents a non-traditional surface finishing method.
Its core principle involves aligning magnetic abrasive particles along magnetic field lines using a magnet’s force, forming an abrasive brush that adheres to the workpiece surface.
When relative motion occurs between the workpiece surface and this abrasive brush, the brush functions like a cutting tool to grind the surface, thereby achieving surface finishing.
Magnetic grinding technology offers excellent flexibility, self-sharpening capability, contouring ability, and controllability.
The abrasive process involves micro-cutting, generating minimal heat during machining while maintaining dimensional accuracy.
It is widely applied for finishing flat surfaces, outer/inner cylindrical surfaces, complex curved surfaces, and micro-components.
For conventionally machined aluminum alloy structural components, post-machining defects such as scratches and uneven surface roughness may occur.
These imperfections often lead to inconsistent coloration during surface treatment, impacting product delivery schedules and cost control.
By employing magnetic grinding to address surface scratches, the process enhances product appearance and surface quality while maintaining dimensional accuracy.
It removes shallow surface scratches and burrs, improving first-pass yield rates and reducing rework costs and time.
This approach simultaneously elevates the aesthetic quality of delivered products and boosts customer satisfaction.
For structurally complex components, intricate features cannot be directly milled using CNC machines, necessitating electrical discharge machining (EDM).
Post-EDM processing, the part surface develops deposits like oxide films, directly compromising surface appearance quality.
Simple sanding with ordinary sandpaper not only fails to completely remove the oxide film but may also cause uneven surface roughness.
This compromises both the product’s visual integrity and functional performance while failing to meet appearance inspection standards.
In contrast, applying magnetic grinding finishing to EDM-processed surfaces effectively removes oxide films and machining marks, yielding a smooth, uniform surface.
This significantly enhances product appearance quality and surface consistency, fully meeting inspection requirements, as shown in Figure 2.
In summary, the surface quality of structural components directly impacts the final delivery quality and standards of the product.
This paper clarifies the technical standards and management requirements for the machined surfaces of aluminum alloy structural components.
It explores the fundamental principles and practical application effects of sandblasting and magnetic grinding processes.
It also provides theoretical references and practical foundations for further enhancing product appearance quality and quality management levels.
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