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

Abstract

In contrast to isothermal aging, few reports document the non-isothermal aging of deformed Al–Mg–Si alloys. The knowledge of non-isothermal aging of pre-deformed Al–Mg–Si alloys is of primary importance to understand the thermal stability as well as to control the microstructure of the final product during industrial processing. Therefore, the present work has been focused to understand the microstructure evolution during the continuous heating of a cold rolled Al–Mg–Si alloy. This has been followed using dilatometry, Differential Scanning Calorimetry, X-Ray Diffraction and microhardness measurement. Based on the results obtained, it is shown that dilatometry is a powerful tool to study phase transformations in deformed Al-Mg-Si alloys, moreover, the microstructural evolution, of the cold rolled sample, can be described as follows: at the earlier stages of the non-isothermal aging, formation and then the reversion of fully coherent GP zones take place. This is followed by the simultaneous occurrence of β” and β’ precipitation and recovery reaction. By continuing aging, the next reactions which will take place are β” and β’ dissolution and recrystallization. Finally, one can observe the formation and then the dissolution of the equilibrium phase β.

Effect of thermomechanical treatment on the hardening of Al6060 alloy

“…The DSC thermogram matches the established ageing sequence and shows the usual features detected in such alloy [13]; it shows six enthalpic effects On the other side, the derivative dilatometric heating curves are in good agreement with DSC results. It is worth noting that the different contributions affecting the dilatometric effects are caused by the increase of the specific volume of the new phase, which is different from that of matrix and/or the changing in the lattice parameter of the matrix due to the redistribution of the constituent atoms accompanying the phase transformation.…”

“…The DSC thermogram matches the established ageing sequence and shows the usual features detected in such alloy [13]; it shows six enthalpic effects, three are exothermic (A [343–443 K], C [503–603 K], E [723–758 K]), and three endothermic (B [443–503 K], D [673–723 K], F [758–803 K]) The peaks A and B, respectively, are identified as the precipitation and dissolution of GP zones. The broad exothermic peak C with a maximum of 573 K, is often interpreted as two overlapped peaks, associated with the formation of metastable precipitates β ″ and β′.…”

“…The thermal expansion (3) with a maximum around 543 K is due to be the precipitation of metastable β′ phases. It is important to note that the explanation of the appearance of an expansion instead of a contraction as mentioned in WQ sample detailed in previous work [13]. For low heating rate 3 K min −1 , the curve provides more details; a separated expansion (2) at a maximum near to 463 K, is associated to the precipitation of metastable β ″ phase.…”

“…For all DSC curves, we notice the formation of GP zones giving exothermic peaks A 1 and A 2 in the range [343–403 K]. 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].…”

“…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].…”