FSW Weld Whitening and Anodizing Adaptation Technology

FSW Weld Whitening and Anodizing Adaptation Technology.Friction Stir Welding (FSW)  is a mature solid-state joining process. Featuring non-melting welding, zero porosity and cracking, and minimal workpiece deformation, it is widely adopted in mass production of aluminum alloy new energy structural parts and aluminum alloy liquid cooling plate components. However, continuous white strip patterns tend to form on FSW welds during actual processing. This surface defect becomes more severe after subsequent anodizing, resulting in inconsistent color difference on workpieces and directly lowering product yield. To resolve practical surface defects in mass production, this paper systematically analyzes the formation mechanism of whitening on aluminum alloy FSW welds from three perspectives: phenomenon, root causes and countermeasures, based on on-site production experience. It clarifies the correlation between anodizing processes and weld color difference defects, and establishes a complete process solution covering welding parameter optimization, post-weld pretreatment, anodizing parameter adjustment and precision CNC rework. This work provides practical guidance for quality control of anodizing on friction stir welded aluminum alloy workpieces.

1，Whitening Defect on Friction Stir Welding Welds

After aluminum alloy workpieces are processed via friction stir welding, continuous white or grayish-white strip marks often appear along the weld path traced by the stirring tool, creating a stark visual contrast against the base metal. On-site production observations prove this whitening is not merely surface dirt or machining residue. It mainly presents in two forms and acts as the primary trigger for uneven weld color after anodizing. Native White Bloom on Base Metal Surface Before any post-processing after welding, the weld surface is covered by a matte white film with patchy local distribution and no metallic luster. The surface feels smooth without protrusions yet shows prominent color deviation from the bright base metal zone.

This loosely structured, weakly adhered white bloom forms a shallow surface defect generated by high-temperature forging during FSW. It occurs on nearly all FSW aluminum alloy parts, with varying severity determined by welding parameters. This whitening stems from uneven internal microstructure in the weld zone rather than surface contaminants, representing a structural color defect and one of the most difficult cosmetic issues to eliminate fully in mass production. From a product performance perspective, FSW weld whitening is solely a cosmetic flaw that does not impair weld mechanical strength, workpiece tightness or overall service life. Nevertheless, when manufacturing precision heat sinks and exposed structural components with strict appearance standards, this defect directly renders products unqualified, standing as a key bottleneck lowering the mass production yield of anodized FSW aluminum alloy parts.

 2. Formation Mechanism of Weld Whitening

Unlike oxidation blackening and slag inclusion defects in conventional fusion welding, weld whitening is a unique issue of solid-state friction stir welding. Combined with metallographic characteristics and actual production conditions, this defect mainly originates from high-temperature oxidation on the weld surface layer, segregation of alloy precipitated phases and differentiated microstructure evolution across various weld zones. Meanwhile, the anodizing process will further amplify the original color difference defects. The detailed formation mechanism is elaborated as follows.

2.1 Formation of Porous Alumina Layer via High-Temperature Friction Oxidation

Friction stir welding relies on heat generated by rotational friction of the stirring tool to join aluminum alloys in a plastic state, with welding temperatures kept below the melting point of aluminum alloy. During welding, surface metal of the weld stays plastic at high temperatures for a long time and fully reacts with air to form a porous, loose alumina film. Unlike the dense native oxide film on base metal, this transient welding oxide film features uneven thickness, loose structure and poor bonding with the substrate, resulting in weak light reflection and macroscopically visible white foggy stripes. Heat input directly determines the severity of whitening: higher rotational speed, slower welding travel speed and longer high-temperature exposure will aggravate surface oxidation and thicken the white bloom. In addition, long-term abrasion of the stirring tool sheds tiny hard fragments of iron and tungsten, which mix into the weld surface and worsen grayish-white patchy surface defects.

2.2 Color Stains Caused by Agglomeration and Segregation of Alloy Strengthening Phases

Mass-produced 6061 and 6063 aluminum alloys belong to Al-Mg-Si series, with uniformly dispersed Mg₂Si strengthening precipitates inside the base metal to guarantee basic material properties. Local high heat from FSW completely disrupts the even distribution of alloy phases in the weld and heat-affected zone. Under high temperature, fine Mg₂Si precipitates inside the weld rapidly coarsen, agglomerate and accumulate heavily on the weld surface, while precipitates in the base metal remain fine and evenly distributed. The enriched precipitate zones show distinct light reflection performance compared with pure aluminum matrix, forming regular white strip marks on the workpiece surface. This is the key reason why whitening is most severe in the thermo-mechanically affected zone on the advancing side of FSW welds.

3. On-Site Process Optimization and Defect Treatment Solutions

Targeting two types of defects including surface white bloom on FSW welds and structural white spots after anodizing, this paper develops hierarchical treatment solutions from four dimensions of welding source control, post-weld pretreatment, anodizing process adjustment, and precision machining rectification based on the appearance quality requirements of different products, so as to balance mass production efficiency, cost and appearance quality.

3.1 Optimization of Welding Parameters to Reduce Defects at the Source

Reducing welding heat input by optimizing welding processes can effectively mitigate surface oxidation and precipitate segregation, and weaken weld whitening at the source, which serves as the optimal pre-control solution for cost reduction and efficiency improvement in mass production. Targeted parameter adjustments can be implemented in actual production: appropriately reduce the rotational speed of the stirring tool and increase the welding traveling speed to shorten the high-temperature residence time of workpieces, thereby inhibiting excessive surface oxidation and coarsening of strengthening phases; regularly inspect and replace stirring tools to ensure smooth, burr-free and wear-debris-free shoulder surfaces and avoid accumulation of metal impurities during friction forging; thoroughly remove oil stains, oxide scales and impurities in the weld area before welding to eliminate secondary oxidation defects during the welding process.

3.2 Fine Post-Weld Pretreatment for Complete Removal of Surface White Bloom

Native weld white bloom cannot be eliminated merely by anodizing. A combination of mechanical polishing and chemical treatment is mandatory to completely remove surface defects, which is an essential procedure prior to anodizing.

In the mechanical treatment stage, 800#~1200# abrasive paper or nylon drawing wheels are used for uniform grinding and wire drawing on the weld and surrounding base metal areas. This process thoroughly removes the loose white oxide layer and the deformed surface layer formed by welding forging, unifies the surface roughness of welds and base metal, and eliminates visual color difference on the surface. In the chemical treatment stage, the alkali etching time is appropriately shortened to half of the conventional process, so as to avoid aggravated structural differences caused by over-etching in the heat-affected zone and thermo-mechanically affected zone. Meanwhile, the acid pickling neutralization and pure water cleaning duration is prolonged to completely remove residual lye on the workpiece surface and prevent local corrosion and mottling.

3.3 Adjustment of Anodizing Process to Weaken Structural Color Difference

Structural whitening caused by internal structural differences cannot be completely eliminated by conventional processes, but fine adjustment of anodizing parameters can greatly reduce weld color difference, meeting the mass production appearance requirements of most standard products.During oxidation, the tank temperature is controlled within 18~20℃ with stable power supply at low current density, which ensures uniform growth of the oxide film on the workpiece surface and reduces the differences in film thickness and compactness between welds and base metal. For dyed products, appropriately extending the dyeing time can improve the color saturation of the film and reduce the white contrast of welds. A nickel-cobalt composite medium-temperature sealing agent is adopted in the sealing process to improve the overall uniformity of the film and reduce white spots and mottling. Compared with conventional natural anodizing and thin-layer dyed anodizing, hard anodizing features thicker films and stronger covering performance, which can effectively shield microstructural differences and achieve a more prominent color difference improvement effect. It is the preferred process for workpieces with moderate appearance requirements.

4，Conclusions & Mass Production Application Recommendations

Whitening of friction stir welding (FSW) seams is a typical surface defect in mass production of solid-state welded aluminum alloy parts, which falls into two main categories: surface oxidized white film and chromatic aberration caused by internal microstructure differences. For practical mass production quality control, tiered treatment strategies can be adopted based on product appearance requirements. For standard industrial components, core improvement measures include upstream process optimization, post-weld grinding pretreatment and fine-tuning of anodizing parameters, which can substantially cut the reject rate of whitening defects while keeping costs under control. For high-end precision decorative parts, an additional post-weld CNC milling process is required to completely eliminate microstructure-induced chromatic aberration. Overall production control shall follow the principle of “prior prevention first, post-process rework as supplement”. Strict control of welding heat input, standardized post-weld pretreatment and precise regulation of anodizing processes can effectively resolve the anodizing whitening issue on FSW seams and steadily raise the mass production yield of aluminum alloy welded parts. In short, adjusting preceding processes, implementing grinding and conducting anodizing treatment can eliminate this appearance defect entirely.