Finding Through NDE the Thermal History and Metallurgical Status of a Heat Treatable Aluminum Alloy

In heat-treatable aluminum alloys it has long been accepted that decreased values of strength were accompanied by increases in electrical conductivity (C). In quality or processing control and trouble-shooting situations this has been useful for finding anomalies in or among aluminum alloy maill products. But the regression was always found as a wide scatterband where conductivity could not give a narrow range of possible strengths.It was discovered for several alloys and quantified for 2219, that the scatterband formed by data from several lots and sources actually could be divided into groups with different histories. When specimens produced by created combinations of quenching-time and aging-time had their Hardness (H) vs Conductivity plotted on a H vs C format a fan-like dispersion of coordinated points was seen. Drawing locuses thru like times divided this fan into age-time and quench-time grids. Any particular C-H coordinate in this envelope then was seen as identifying the thermal history of the piece with that of C-H value. It was also found that progress in one direction on this format marked out the increase in the 2219 hardening precipitates θ″ and θ′. Progress in the other direction marked out the increase in the softening precipitate θ. So that even the particular metallurgical status could be found from the C-H coordinate of the specimen.This work taught that the large C-H variations seen in accumulations of data most often represented variations in the material itself, not in measurement systems. The work also taught that variation in production material was tracible mainly to variation in quench quench times. Should such variations be reduced the standard deviation of strength would be reduced and higher design strengths could be assigned to the alloy. In practical situations increases in design strengths (which conversely means reductions in assembly weight) are seen at 12%. The NDE measurements can serve this end by identifying and certifying grades of material before pieces are put into service. This strategy involves avoiding that apparently sound material which will fail early in its service life.