Processing Procedures

Cleaning. Prior to processing of parts, all surfaces should be cleaned to be free of oil, grease, mill scale, crayon, weld slag or spatter, and other foreign materials. A typical cleaning procedure might involve acid pickling to remove mill scale, removal of oil and/or grease by vapor degreasing, as well as abrasive or grit blasting. Grit blasting should be done using aluminum oxide abrasives (60 to 320 mesh, depending on surface finish) at 400 to 550 kPa (60 to 80 psi) from a 75 to 150 mm (3 to 6 in.) standoff. Residual grit should be blown off with a clean source of compressed air. From this point on until packing, parts should be handled with clean cotton gloves to prevent contamination.

Powder Blending. Depending on the material being processed and the desired aluminum activity of the pack, the aluminum pack cementation powder mix contains the following typical range:

The two types of pack cycles are known as high activity and low activity (see the discussion on "Principles of Pack Diffusion Coating" in the previous Section of this article on "Diffusion Coatings for Gas Turbine Engine Hot Section Parts" ). A high-activity pack contains a relatively high fraction of pure aluminum (Table 2) as the source material, a halide activator, and an inert diluent, usually alumina; the process is performed at lower temperatures (650 to 815 °C, or 1200 to 1500 °F). This type of pack generally requires a subsequent high-temperature heat treatment under an inert atmosphere in order to complete diffusion and obtain the required surface aluminum content and diffusion depth. A low-activity pack contains a lower fraction of aluminum, usually in the form of an alloy with iron or chromium, and is performed at higher temperatures (815 to 1150 °C, or 1500 to 2100 °F) with no subsequent heat treatment.

Packing. Retorts can be constructed from a variety of alloys ranging from plain carbon steel to nickel-base superalloys, depending upon desired service life and acceptable costs. Retorts should be fabricated to best accommodate the job being processed. The parts to be coated in the retort should be completely covered with pack mix and should not be allowed to touch each other or the walls of the retort. Prior to sealing the retort, it is advised to make sure that the pack mix is well settled. Low-temperature (high-activity) packs can be run in air, but with high-temperature (low-activity) packs, a protective blanket of hydrogen or inert argon gas is required. For obvious reasons, the use of argon is preferable for safety considerations. High-temperature retorts must be constructed with provisions for gas input/removal kept in mind.

Coating Cycle. Depending on the base material to be pack aluminized, processing temperatures can range from 650 to 1150 °C (1200 to 2100 °F), as measured from a thermocouple inside the retort. At specific process temperatures, a maximum of ±14 °C (±25 °F) must be maintained. Process cycle times can range from 3 to > 40 h, depending on process temperature and base material to be processed. Transport of aluminum as aluminum chloride through the pack and solid state diffusion of aluminum through the base material is more influenced by process temperature than process time. Although higher process temperatures generally increase surface aluminum content and overall diffusion depth, in some cases the higher processing temperatures can have undesirable effects on the mechanical properties of the base material. In these cases, parts are processed at lower temperatures for longer periods of time.

Finishing. Retorts should be cooled to below 95 °C (200 °F) before opening. Parts should be removed and cleaned by brushing or by a light wet or dry abrasive blast. Certain alloys require controlled cooling cycles to maintain mechanical properties.

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