Continuous cooling precipitation diagram of aluminium alloy AA7150 based on a new fast scanning calorimetry and interrupted quenching method

• The precipitation behaviour of high strength aluminium alloys was investigated by DFSC.
• The critical cooling rate for AA7150 alloy, was determined to be 300 K/s.
• The method enabled determination of precipitation start and end temperatures
• By this method the complete CCP diagram for AA7150 aluminium alloy was established.

Differential fast scanning calorimetry (DFSC) has been combined with a novel differential reheating method (DRM). Interrupted quenching at different temperatures followed by overcritical cooling enables both time- and temperature-dependent investigations of precipitation reactions in aluminium alloys during cooling from solution annealing at cooling rates between 1 K/s and 105 K/s. Together with conventional differential scanning calorimetry (DSC) results, for the very first time, a full continuous cooling precipitation (CCP) diagram for a highly concentrated wrought AlZnMgCu alloy has been developed. For alloy AA7150, three precipitation reactions with their respective start and end temperatures were observed by DFSC. The critical cooling rate for suppressing the high-temperature precipitation of S-Al2CuMg was found to be ~ 10 K/s, whereas for the medium-temperature precipitation of η-Mg(Zn,Al,Cu)2 it is ~ 100 K/s and for the low-temperature precipitation of Y-phase plates enriched in Zn and Cu it is ~ 300 K/s. The method can potentially be transferred to similar precipitate-forming metallic systems or related solid-state phase transformation systems also beyond the particular system analysed in the present study.