The effect of hydrogen on aluminum alloys can manifest itself in a phenomenon known as high-temperature oxidation (HTO), also known as high-temperature deterioration (HTD). A case study involving 2024 and 7075 aluminum aerospace fasteners in which hydrogen-induced damage was found after solution heat-treatment and aging operations serves as an excellent example.
In simplest terms, HTO (Fig. 1) is a form of hydrogen diffusion that affects surface layers of a part during elevated-temperature treatment. This condition is often due to moisture contamination in the furnace atmosphere and is sometimes aggravated by sulfur or other furnace refractory contamination. The most common manifestation of HTO is surface blistering (Fig. 2). However, it may also appear in the form of surface voids or internal discontinuities. The symptoms of HTO are nearly identical to those of high gas content in ingots due to improper mill practices. The 7xxx-series alloys are the most susceptible followed by the 2xxx alloys.
Aerospace fasteners were being manufactured from 2024 and 7075 wrought aluminum bar stock. After heat treatment and during assembly, the hex end of multiple fasteners fractured and separated from the body close to the specified torque level. The quality department quarantined the parts in question. Visually, some surface pitting could be observed. Samples were analyzed, both in-house as well as by an independent laboratory to identify the root cause of failure.
The heat treatment of the two different fastener materials involved heating to 465°C (870°F) and 495°C (920°F), respectively, in an electrically heated air-circulation oven and holding for 75 minutes. The temperature uniformity of the oven was ±5.5°C (±10°F).
After soaking at temperature, a trap door opened beneath the single-basket load, which was then tilted so that the fasteners tumbled out of the basket into a chute that led to a water quench tank located below the oven. The loose parts were then collected in a perforated basket located beneath the chute. The water in the quench tank was kept at 32°C (90°F) via a heat exchanger before the introduction of parts. It was observed that the water was rising to 60°C (140°F) during the quenching operation.
The quench chute itself included an area that consisted of fine-mesh screening located around the circumference and extended for about 2 feet below the water level so as to allow water in the tank to move freely through the quench-chute area. A pipe with a series of holes designed to spray water across the top surface of the tank was located just at the water line in order to prevent steam from rising and entering the heating chamber as the parts were dropped from above.
Laboratory Investigative Work
Samples were gathered in the field and analyzed both as received using stereomicroscopy and after preparation by both optical and scanning electron microscopy. Samples were cut using a precision cutoff machine, and metallographic mounts were prepared in accordance with ASTM E3 using a conductive mounting material suitable for use in the SEM.
The scanning electron microscope was equipped with energy dispersive spectroscopy (EDS) capability. The EDS was capable of characterization of the near surface using both secondary electron imaging and backscatter electron imaging for evaluation of compositional variation (Fig. 3). Results from this analysis were documented through acquisition of digital photography, including EDS results of the corresponding spectra for each area analyzed. Multiple locations on each sample were evaluated to determine the degree of consistency in composition and morphology.
The result of the analysis was that the near-surface condition observed on a number of suspect parts was that of HTO. The presence of the subsurface porosity induced by HTO required the parts in quarantine to be scrapped and not used for production.
Field Investigative Work
Based on laboratory findings, water vapor present in the heating chamber was suspected to be the root cause. If water vapor rose from the quench-tank chute, it could enter the furnace through the trap door in the heating chamber. A high-humidity atmosphere was suspected as the source of hydrogen, which subsequently entered the part surface during the soak period.
Close inspection of the quench-tank area revealed two distinct problems. First, the exiting spray located at the water line of the tank was blocked, which limited spray and, in some cases, prevented spray from coming out of the holes. Disassembly found the pipe and holes to indeed be partially or fully clogged by mineral deposits and debris. Well water was being used to supply the system. Although not analyzed, it is a known source of such mineral deposits.
Second, the fine-mesh screen used to allow flow of water from the tank to the quench-chute area was completely blocked. This created a localized temperature rise in the chute area, creating steam that then rose into the oven proper. All components were cleaned, and a preventive-maintenance schedule was established. The problem did not reoccur.
Case studies are invaluable and offer us the opportunity to share practical lessons learned. Hydrogen-induced damage in fasteners is an industry concern most often negated by bake-out cycles. Phenomena such as the case of high-temperature oxidation, however, underscore the need for the heat treater to be ever diligent.
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