May 2021 Volume 3

MATERIALS

Thermal Processing Equipment Heat treating an aluminum alloy in compliance with industry and customer specifications requires the use of a qualified facility. A basic unit in a heat treating facility consists of a solution furnace, a quenching tank, and an aging furnace. Some heat treating facilities might include a station to give aluminum components a cold work step that is designed to reduce the level of residual stresses in the components after the quenching step. When such an intermediate step is specified for the component, the temper will indicate that. Al 6061 T651, for instance, indicates that the component was given a specified amount of cold stretching (1 to 5% of permanent deformation) after quenching and before aging. Al 6061-T652 means the same cold step was given but using a compressive operation instead. Solution furnace. This type of furnace is designed to operate at a medium temperature range. Most aluminum alloys or grades are solution-treated in the range 800-1000 °F for a time commensurate with the forging or part largest thickness. The operation is intended to bring the alloy to a point where the solute elements are all uniformly distributed right before the quenching operation. This chemical uniformity is essential to obtain uniform mechanical and physical properties after the subsequent aging step. The following features are key to the solution treating of aluminum alloys: Temperature control. Heat treating aluminum alloys requires a stringent control of both furnace and load temperatures. For instance, spec AMS 2772 requires that alloys 2219 and 7050 be solutioned within a very narrow temperature range of 20 °F. For solutioning of these alloys, a minimumClass 2 furnace with a +/- 10 °F tolerance is required. Heating the alloys outside of the specified range yields a non-compliant process per AMS 2772. These criticality is associated to the risk of underheating these alloys and undermining the degree of solute uniformity and aging precipitation that can be achieved, or the risk of an overheating that could cause to the alloy a microstructural damage such as incipient melting spots. For other grades the solution range might be less challenging. For alloy 6061 for instance, the solution range is about 145 °F which allows for an easier temperature control. Short quench delay capability. To comply with most frequently used specifications for heat treating of wrought aluminum, the quenching step must be conducted immediately after completing the solution cycle, and only a very short delay is allowed. Materials specifications such as AMS 2772/2770 and most customer´s own specs limit the quench delay to a maximum of 15 seconds. If this requirement cannot be complied with, the temperature of the entire load shall bemeasured and validated tobe above aminimumlimit that depends on the alloy type. This alternate method for complying with the quenching requirement can be easily monitored for small loads and short transfer distances, but it can prove to be extremely difficult to comply with for normal to large loads and large transfer distances. Well-established facilities would use a certain type of furnaces known as “drop-bottom furnaces” to provide a stable, productive, and reliable method for solution and quenching of wrought aluminum alloys. Figure 1 shows the arrangement between the solution furnace and the

quenching tank for a drop-bottom station. Automated doors and load lifters provide a mean for a quenching operation that can consistently meet the 15 seconds delay limit. Recommended quenching media are either water or water + polymer. In the case of watermedia, quenching at high temperatures is recommended to reduce residual stresses especially on parts that require high dimensional stability during finishing operations.

Aging furnace. This type of furnace is designed to operate at a low temperature range. Most aluminum alloys are age-treated in the range 200-400 °F (see also AMS 2772/2770). The aging step is intended to produce a uniform and effective precipitation within the alloys microstructure, in order to achieve the properties defined for a given temper. The following features are key to the artificial aging treating of aluminum alloys: Temperature control. Aging of aluminum alloys requires control of both furnace and load temperatures in the same way as with solution furnaces. A minimum Class 2 furnace is also required for aging aluminum components. Most specifications recommend a temperature set point and a time range depending on the alloy type. Some alloy grades, such as 7050, can be aged to tempers that require 2 or 3 distinct aging temperatures. Thus, the furnace must have capability to change temperatures with specified rates. The final cooling after aging is typically still air. Temperature Control Surveys and Tests Temperature Uniformity Survey (TUS). Most aluminum specs require that heat treatment be conducted in a furnace complying with pyrometry requirements per AMS 2750. This spec establishes strict requirements for the equipment and systems used to control the temperature inside the “qualified working volume” of the furnace. One of the most important requirements in AMS 2750 is a temperature uniformity survey (TUS) that must be conducted in a periodic basis. The purpose of this survey is to validate that (i) the furnace is capable of controlling the temperature inside the qualified working volume, with the required uniformity and within the +/- tolerance for the furnace´s Class, and (ii) that such uniformity and tolerance control are

FIA MAGAZINE | MAY 2021 27