A DSC Investigation of the Effects of Cold Deformation and Low Temperatures Aging on the Microstructural Stability of a Peak Aged (PA) Al 6061 Alloy

Abstract

The effects of cold deformation and low temperature aging on the microstructural stability of a peak aged (PA) Al 6061 alloy were investigated by means of DSC and microhardness measurements. During aging at a relatively low temperature (100 °C) of the PA material, a small increasing of the mechanical properties was detected, which was explained by the formation of atomic clusters, GP zones and β phase. The response to the aging treatment of the cold deformed materials depends on both the level of the cold deformation and the aging temperature. During aging at relatively low temperature (100 °C), in contrast to the 75 % deformed material that shows a small variation in their mechanical properties, the mechanical properties, of the 30 % deformed material, are almost constant. This was attributed to the higher driving force of the recovery reaction in the heavily deformed material. In the other hand, aging at relatively higher temperature (140 °C) of the heavily deformed material, leads to a fast softening due to an increasing of the recovery kinetics.

A Dilatometric Study of the Non Isothermal Aging of a Cold Rolled Al-Mg-Si Alloy

“…The dissolution peak of both β" and β' occurred at the range of 340-400 °C (peak 4) and the formation and dissolution of equilibrium β phase occur at the temperature ranges [400-455 °C] (peak 5) and [455-505 °C] (peak 6) respectively. Interpretation of all reaction peaks has been made on the basis of The DSC thermogram of the ST and cold rolled sample, shows the occurrence of almost the same precipitation and dissolution reactions, however, one can state that cold rolling has introduced the following effects : -The displacement of all reactions towards low temperatures; this is due to the enhanced diffusion rate of solute atoms, because of the high density of dislocations, which promote the occurrence of all reactions at relatively low temperatures [11,16,17].…”

“…-The reduction of clusters/GP zones formed amount, which is mainly due to the fact that the clustering of solute atoms in solid solution at the early stage of phase decomposition is significantly influenced by the vacancy concentration [2 -4]. As dislocations are sinks for excess vacancies, the density of vacancies in the alloy decreases as dislocations are created during cold rolling [17]. It is also observed that the endothermic effect related to atomic clusters/GP zones dissolution is clearly composed of two small endothermic peaks which can be explained by the dissolution of two kinds of atomic clusters/GP zones; those formed during heating in the DSC apparatus (the first one) and those formed before DSC test (the last peak).…”

“…The decomposition of supersaturated deformed microstructures has been mainly monitored by the DSC technique [11, 14, 15 -17 and 20], and it is generally accepted that pre-deformation can change both the precipitation sequence and kinetics. However, due to the fact that the heats released during recovery and recrystallization reactions are very small, when compared to that corresponding to the formation of precipitates, the separation between recovery, recrystallization and precipitation by DSC is very difficult [11,17].…”

Effect of thermomechanical treatment on the hardening of Al6060 alloy

“…Their dissolution is recognised by the endothermic peak B 2 situated around 433 K. For CR sample, we note that the formation of GP zones (A 1 ) is happening during the experiments and not before CR, since the heavy plastic deformation suppresses their formation [12,13] while their dissolution is not detected on DSC. Moreover, the exothermic peak C 1 (around 503 K) is attributed to the precipitation of β′, since the plastic deformation promotes its heterogeneously distributed formation due to short-circuit provided by dislocation tangles [17]. On the derivative dilatometric curve in the same interval, a contraction (3) is observed.…”

“…The plastic deformation can be used to improve the ageing process since its effects are remarkable due to introduced defects. The plastic deformation before final age hardening can change the precipitation sequence, since the dislocations provide heterogeneous nucleation sites for the precipitates [12][13][14][15][16][17][18], by the suppression of GP zones [12,13] and β precipitates [13][14][15][16][17][18], as well as accelerating the precipitation kinetics [13][14][15][16][17][18]. The nucleation and growth of the intermediate phases are accelerated and the stable phase is precipitated at dislocations [12][13][14][15][16][17][18], as it was suggested that the increased density of defects in the crystal structure would enhance appreciably the diffusion distance of Si and hence promote the formation of more Si-rich particles [19].…”