Isothermal Time-Temperature-Precipitation Diagram for an Aluminum Alloy 6005A by In Situ DSC Experiments

Abstract

Time-temperature-precipitation (TTP) diagrams deliver important material data, such as temperature and time ranges critical for precipitation during the quenching step of the age hardening procedure. Although the quenching step is continuous, isothermal TTP diagrams are often applied. Together with a so-called Quench Factor Analysis, they can be used to describe very different cooling paths. Typically, these diagrams are constructed based on mechanical properties or microstructures after an interrupted quenching, i.e., ex situ analyses. In recent years, an in situ calorimetric method to record continuous cooling precipitation diagrams of aluminum alloys has been developed to the application level by our group. This method has now been transferred to isothermal experiments, in which the whole heat treatment cycle was performed in a differential scanning calorimeter. The Al-Mg-Si-wrought alloy 6005A was investigated. Solution annealing at 540 °C and overcritical quenching to several temperatures between 450 °C and 250 °C were followed by isothermal soaking. Based on the heat flow curves during isothermal soaking, TTP diagrams were determined. An appropriate evaluation method has been developed. It was found that three different precipitation reactions in characteristic temperature intervals exist. Some of the low temperature reactions are not accessible in continuous cooling experiments and require isothermal studies.

High-precision isothermal dilatometry as tool for quantitative analysis of precipitation kinetics: case study of dilute Al alloy

“…The b 00 ! b 0 transformation in Al-Mg-Si alloys, which could be resolved in detail here, is hardly accessible by other thermal techniques as stated, e.g., for standard DSC [16]. Also with standard dilatometry upon linear heating, the formation of b 00 in Al-Si-Mg alloys is much weaker discernible [23].…”

“…For nonferrous metals and in particular for light-weight alloys such as aluminum alloys, available data only cover a few selected chemical compositions [15]. For instance, in situ differential scanning calorimetry (DSC) has been applied to study TTP diagrams in Al alloys [16] and in a more recent work dilatometry has been performed under cooling conditions providing significant data in the field of precipitation analysis in Al alloys [17]. However, dilatometric measurements on this kind of alloys have so far been restricted to relatively highly alloyed Al alloys as used in the study mentioned before or to steels [18,19] which both exhibit volume changes upon precipitation much larger compared to that ones of the Al alloy investigated in this work.…”

Quench sensitivity of Al–Mg–Si alloys: A model for linear cooling and strengthening

“…[14,3,15,8,16,17]). In recent years significant improvements were obtained in the in situ investigation of the precipitation processes during cooling of Al alloys from solution annealing through the development of high sensitivity in-situ DSC techniques [18][19][20][21]. These technically and metrological sound DSC methods allow to measure the enthalpy changes over the whole cooling rate range of technical and scientific interest: from slow cooling with phase transformation close to equilibrium conditions up to cooling rates near the upper critical cooling rate.…”

Quench-induced precipitates in Al–Si alloys: Calorimetric determination of solute content and characterisation of microstructure

“…Therefore, precipitation kinetics of low-Si binary Al-Si alloys during cooling from solution annealing are investigated in a wide cooling rate range with advanced DSC techniques (e.g. [18,[21][22][23][24][25][26][27][28]). Direct DSC cooling experiments are limited to a certain slow cooling rate due to the relation between sample mass, cooling rate and heat flow.…”