Research Papers

This paper uses response surface methodology to analyze the effects of current density, duty cycle and time on coating thickness and hardness in hard anodizing of 6061-T6. Thickness is determined by current density and charge passed, whereas hardness depends on pore structure, so the two do not move in the same direction. The paper concludes that pulse power outperforms direct current in microstructure, hardness and corrosion resistance.

This is a summary of the hard anodizing technical bulletin of the Aluminum Anodizers Council (AAC). Relative to the original surface, half of the coating penetrates and half grows outward, perpendicular to the surface, so sharp edges require a radius. Alloy and temper are the largest variables affecting the result, and sealing increases corrosion resistance and electrical insulation while somewhat reducing wear resistance.

This review summarizes the key requirements of MIL-PRF-8625F Type III, the reference specification for hard anodizing. The values apply directly to design and inspection: a nominal thickness of 2 mil (50 µm) with a ±20% tolerance, a coating weight of at least 4,320 mg/ft² per mil, Taber abrasion of 1.5 mg/1,000 cycles or less, 336 h of salt spray, and no sealing when wear resistance is the purpose.

This thesis uses cross-sectional observation to show that, when Al-Si cast alloys are anodized, Si particles remain unoxidized in the coating, clouding it and obstructing its growth. At 5 wt% Si or more, the coating appears dark gray to black, and high-pressure die casting (HPDC) surfaces produce thinner coatings and larger hardness variation than other manufacturing methods.

This paper analyzes yellow patches that form after anodizing of high-pressure die-cast ADC12 parts using SEM-EDAX and XRD, and shows that they result from Fe-based intermetallic compounds segregated at the surface reacting with the electrolyte. It also reports that the patches are suppressed at lower electrolyte temperatures.

This thesis explains the principles of sealing, which closes the pores of anodic coatings. In hot water sealing, aluminum oxide reacts with water and converts to boehmite (AlO(OH)), and the resulting volume expansion closes the pores. In nickel salt sealing, nickel hydroxide also forms and closes the pores. Cold sealing near room temperature consumes less energy and is used alongside these methods in industry.

This review draws on published patents to summarize two points. When aluminum die castings are alkaline-etched, only the aluminum dissolves and Si, Cu, Fe, Mn and Ni remain on the surface as smut, which requires a removal step. Die-casting alloys for anodizing are designed with lower Si, Fe and Cu, with strength compensated by Mn and other elements, because Si above 2 wt% causes segregation and precipitation that produce a color different from the base metal.

This review summarizes two methods for measuring coating thickness (nondestructive eddy current, ASTM B244; destructive microscopical cross-section, ASTM B487) and the measurement locations and acceptance methods defined by the standards. The cross-section method is the referee method in disputes, ISO 7599 excludes areas within 5 mm of contact points from measurement, and MIL-PRF-8625 bases acceptance on the average of 8 points.

This patent proposes a pretreatment process and apparatus that clean and activate metal surfaces with an atmospheric-pressure plasma jet instead of chemicals. The claims cover a seven-step process—material preparation, mounting on the conveyor, power supply to the plasma generator, supply of surface modification gas, atmospheric-pressure plasma jet discharge, conveyor operation and passage through the treatment zone—and a generator structure consisting of a gas chamber, gas distribution body, electrode mounting body, injection body and spacers. The objective is to reduce the chemical, rinse water and wastewater burden of chemical pretreatment and to connect batch immersion processes to continuous production lines.

This patent covers a conveyance method for uniformly treating the entire outer surface of parts in plasma pretreatment. A toothed planet is engaged between a fixed track and a circulating track that runs outside it, so that when the circulating track turns, the planet rotates on its own axis while revolving along the fixed track. Parts hang below the planets and rotate and revolve with them, so even surfaces that a jet from a single direction cannot reach are exposed to the plasma.

This patent covers how parts of various shapes are hung on a plasma pretreatment line. A pair of cross-connected hanging members opens and closes like scissors about a hanging shaft to grip the part, an insert member fixes the angle between them, and the hanging members themselves fit into and detach from the planet disc. Operators can load and unload parts without entering the treatment zone, making product changeover and maintenance easier.

This patent covers a jig structure for parts that require partial treatment. Jig bars are fixed in series to a current-carrying center bar, the part is fitted into a holder formed by bending the end of the jig bar, and a cylindrical marking member fitted on the fixing pin blocks the groove in the non-treated area. The marking member has a two-step guide of different diameters, so it covers non-treated areas of different sizes without changing the member, and its simple structure offers advantages in fabrication and work speed.

This study developed a soft anodizing process suited to high-Si die-cast aluminum alloys. In die-cast parts, Si particles obstruct coating growth, making uniform thickness and roughness difficult to obtain. Test pieces were processed with the rectification method and electrolytic conditions as variables, and coating thickness and roughness were compared. The semiconductor-switching rectifier proved advantageous in thickness variation and roughness, and a range of conditions that yields uniform coatings was defined. Mass production application and evaluation for large parts remain as follow-up tasks.

This study analyzed the causes of black and yellow stains in anodized die castings. SEM-EDS analysis of sectioned stain areas showed that the black stains resulted from segregation of Fe-Si-Cu intermetallic compounds in Si-rich regions, which lowered light reflectance, while the coating continued without interruption at the same thickness. Through a review of release agent composition and step-by-step tracking of die casting → shot blasting → machining → anodizing, the yellow stains were determined to be degraded release agent residue embedded in the surface layer. The variations were confirmed to be appearance-only, with no abnormality in thickness, adhesion, hardness, corrosion resistance, dimensions or insulation.

Parts machined after forging have both forged and machined surfaces on a single part, which makes it difficult to judge coating uniformity by nondestructive measurement alone. Samples were sectioned at a location where both surfaces can be observed at the same time and, after hot mounting, leveling and polishing, coating thickness was measured by the microscopical cross-section method. Coating thickness on the machined and forged surfaces was equivalent; the forged surface only looked different because of the lower smoothness of the base metal surface. This procedure is used as the standard method for checking thickness by area.

EF Technology is preparing a chemical conversion coating process and a dedicated line for aluminum parts. Because its purpose, chemicals and control items differ from anodizing, separate condition development and verification of mass production stability are required. Drawing specifications are being finalized against the Type I/II and Class 1A/3 requirements of MIL-DTL-5541, and a test has been designed to compare three pretreatment routes—minimal etching, light etching and surface conditioning—within the same lot. Control criteria will be finalized after the appearance, corrosion resistance and electrical contact properties of prototypes are evaluated.

EF Technology is studying conditions for achieving a uniform black appearance and the required coating performance on high-strength 7075. Because of Zn-Mg-Cu intermetallic compounds and the effect of temper, 7075 can show significant smut, coating nonuniformity and color variation. To identify the process step at which stains first appear, a test plan was established that photographs the same location after pretreatment, after anodizing, after dyeing, after sealing and after final drying, changes one variable at a time, and records coating thickness, contact locations and part orientation. Standard process conditions will be finalized once prototype measurements are obtained.