Age hardening of heat treated aluminum alloy. An investigation of the fluctuation of hardness with time of ageing of heat treated aluminum alloy sheet

Abstract

The authors have found what they believe to be a tendency for a certain Aluminum Alloy, namely 24-SO, to fluctuate in strength, and hardness, during the process of age hardening, after heat treatment. This variation in hardness of the material, while the ageing process is going on, is accompanied by a change in ultimate strength, and in the stress-strain relationship of the material; and in general the condition of the alloy is indicated by the hardness number. The limited scope of this research does not indicate that this phenomenon of fluctuation of hardening is constant in occurrence; as the number of tests made were insufficient to determine the regularity, or irregularity, of this characteristic of the material. All of the metal tested became stable within the commonly accepted range of ultimate strength for this alloy. From these few experiments it appears that although the hardness varies with time, the alloy, if allowed to age sufficiently, will develop its rated strength. Various reports have been made to the effect that sheet aluminum alloy, after being heat treated, quenched, and allowed to begin normal age hardening, did not respond to ageing as might be expected, nor did it follow the theoretical hardening curve with reasonable agreement. It was found that sheet material, although apparently properly heat treated, had not hardened, or acquired its full strength, when inspected by hardness testing methods. From observations it was found that the same piece, which was observed to be soft, when allowed to age for a longer period and tested again for hardness, had in some cases come up to the required standard, and in others had exceeded the inspection limits. To the best of the authors knowledge no systematic investigation of this phenomenon has been undertaken, and as a preliminary quest in this field of research a periodic measurement of hardness, and tensile strength, was carried out to determine the actual manner in which the selected material would harden with time, and to correlate its indicated hardness with the strength of the piece at the time of measurement.