The factors which influence the conductivity of the metals and their alloys are temperature, chemical composition, microstructure, grainsize, and hardness. Hence, conductivity measurements are used as control for heat treatment process. Because of variations in chemical composition, resulting from the tolerance in alloy conditions, a conductivity range, than a specific conductivity value is obtained for each alloy.

The heat treatment conditions of aluminium alloys are featured by a combination of hardness and electrical conductivity measurements. Aerospace companies specify range of conductivity values for aluminium alloys, for acceptance.

The calibration blocks supplied with conductivity meters will be one high conductivity pure copper around 99% IACS (International Annealed Copper Standards) and another copper alloy standard of 13.5% IACS.

When calibrated to these standards, the equipment response to close range conductivity measurements of aluminium alloys will not be accurate. Hence, developing a set of aluminium alloy standards for conductivity measurements in the range of 25% to 55% IACS is a basic requirement for arriving at useful information in conductivity measurement of aluminium alloys, which are primary members on aircraft structures.

Four commonly used aerospace aluminium alloys AA 7075, AA 6061, AA 2014 and AA 2024 were selected, and were subjected to different heat treatment conditions (annealing, solutionising and precipitation). Eddy current conductivity measurements were conducted on these specimens, which were in the range of 25.5% IACS to 50.5% IACS. Brinell hardness test were also conducted. The results are tabulated and analysed.

A graph showing hardness (HB) vs. conductivity (% IACS) are drawn. It was observed that two different hardness conditions can provide same conductivity readings, in which case hardness is used to verify the condition.

Conductivity of these samples were measured by other methods (checking the resistivity in Micro-ohm. cm and conductivity in Metre/ohm. Mm²). The values obtained were converted to conductivity values in % IACS and recorded.

By subjecting aluminium alloys of different designations to different kinds of heat treatment processes, calibration standard blocks in the range of 25% to 55% IACS for eddy current conductivity measurements can be developed which are of useful practical significance.