Conductivity Of Aluminum Alloys.
Conductivity measurement is applied most often to aluminum alloys. This application results from the extensive use of
aluminum alloys in the aerospace industry and the wide variation in the electrical conductivity and mechanical
properties between different alloys and heat treatment. For most aluminum alloys in common usage, specific
conductivity ranges have been established for each alloy and temper. Table 4-7 lists the conductivity ranges for most of
the aluminum alloys commonly used in aircraft structural applications. These values represent a composite of values
obtained from various airframe manufacturers and Government agencies. The ranges include all values obtained for
standard heat treatments except for extreme values obtained from one or two sources which were clearly outside the
ranges of all other lists. If a measured conductivity value for an aluminum alloy and temper is outside of the applicable
range, its mechanical properties should be considered suspect. Measured conductivity values should be in accordance
with Mlt,-H-6088 or a suitable ASTM standard.
Heat Treatment Effects On Aluminum conductivity.
An aluminum alloy has the highest conductivity and lowest strength when it is in the fully annealed temper. After
quenching from the solution heat treating temperature, the strength is increased and the conductivity decreased. Many
aluminum alloys are unstable for a considerable period of time after solution heat treatment even if held at room
temperature during this time, a certain amount of atom migration takes place to initiate the formation of
submicroscopic particles. This process, sometimes called natural aging, increases the strength of the alloy but has
either no effect on conductivity or a slight decrease in the conductivity value. Some aluminum alloys remain unstable
for such long periods after quenching that they are never used in the solution heat treated condition (7075 is an
example). If a solution heat treated alloy is precipitation hardened by heating at relatively low temperature (generally
200-450°F), alloying atoms form small particles. At a critical size and distribution of particles, the strength of the
aluminum alloy reaches a maximum. Conductivity increases during the precipitation hardening or artificial aging
process. If aging is carried on beyond the point where optimum strength is obtained, strength will decrease, but
conductivity will continue to increase.
Discrepancies In Aluminum Alloy Heat Treatment.
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 temperature
b. Solution heat treating time.
c. Quenching practice.
d. Aging temperature.
e. Aging time.
f. Annealing temperature and time.
g. Uncontrolled temperature application
Applications Of conductivity Measurement.
Separation Of Alloys And Tempers.
Conductivity measurement can be used to separate mixtures of two or more alloys and/or tempers. Separation is
possible when the electrical conductivity of each grouping is clearly different. The process of separation may be
accomplished with an instrument calibrated in %IACS.