T.O. 33B-1-14-784.7.1.26 ConductivityOfAluminumAlloys.Conductivity measurement is applied most often to aluminum alloys. This application results from the extensive use ofaluminum alloys in the aerospace industry and the wide variation in the electrical conductivity and mechanicalproperties between different alloys and heat treatment. For most aluminum alloys in common usage, specificconductivity ranges have been established for each alloy and temper. Table 4-7 lists the conductivity ranges for most ofthe aluminum alloys commonly used in aircraft structural applications. These values represent a composite of valuesobtained from various airframe manufacturers and Government agencies. The ranges include all values obtained forstandard heat treatments except for extreme values obtained from one or two sources which were clearly outside theranges of all other lists. If a measured conductivity value for an aluminum alloy and temper is outside of the applicablerange, its mechanical properties should be considered suspect. Measured conductivity values should be in accordancewith Mlt,-H-6088 or a suitable ASTM standard.4.7.1.27 HeatTreatmentEffectsOnAluminumconductivity.An aluminum alloy has the highest conductivity and lowest strength when it is in the fully annealed temper. Afterquenching from the solution heat treating temperature, the strength is increased and the conductivity decreased. Manyaluminum alloys are unstable for a considerable period of time after solution heat treatment even if held at roomtemperature during this time, a certain amount of atom migration takes place to initiate the formation ofsubmicroscopic particles. This process, sometimes called natural aging, increases the strength of the alloy but haseither no effect on conductivity or a slight decrease in the conductivity value. Some aluminum alloys remain unstablefor such long periods after quenching that they are never used in the solution heat treated condition (7075 is anexample). If a solution heat treated alloy is precipitation hardened by heating at relatively low temperature (generally200-450°F), alloying atoms form small particles. At a critical size and distribution of particles, the strength of thealuminum alloy reaches a maximum. Conductivity increases during the precipitation hardening or artificial agingprocess. If aging is carried on beyond the point where optimum strength is obtained, strength will decrease, butconductivity will continue to increase.4.7.1.28 DiscrepanciesInAluminumAlloyHeatTreatment.Variations from specified heat treating practice can result in aluminum alloys with strengths below required levels.Heat treat discrepancies include deviations or misapplication of the following processes:a. Solution heat treating temperatureb. Solution heat treating time.c. Quenching practice.d. Aging temperature.e. Aging time.f. Annealing temperature and time.g. Uncontrolled temperature application4.7.2 ApplicationsOfconductivityMeasurement.4.7.2.1 SeparationOfAlloysAndTempers.Conductivity measurement can be used to separate mixtures of two or more alloys and/or tempers. Separation ispossible when the electrical conductivity of each grouping is clearly different. The process of separation may beaccomplished with an instrument calibrated in %IACS.
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