Precipitate microstructures and resulting properties of Al-Zn-Mg metal inert gas-weld heat-affected zones

Abstract

Using the combination of small-angle X-ray scattering (SAXS) and transmission electron microscopy (TEM), the precipitate microstructure is quantitatively investigated in the heat-affected zones (HAZs) of Al-Zn-Mg metal inert gas (MIG)-welds, and the resulting mechanical properties are determined by hardness measurements. Three initial states prior to welding (T4, T6, and T7) are investigated, and the subsequent microstructure evolution during natural aging and postwelding heat treatments (PWHTs) is assessed. The critical part of the HAZ is shown to be the transition region where partial dissolution of the initially present precipitates occurs. In this transition zone, precipitate coarsening is shown to occur for the T6 and T7 initial states, contrarily to the T4 material. After PWHT, the T6 and T7 materials experience a weak region related to this coarsening behavior, whereas the T4 material HAZ is able to recover a homogeneous microstructure after a suitably chosen PWHT. Simple model ramp heat treatments are shown to describe the main phenomena involved in the HAZ. Finally, a precipitation hardening model is successfully applied to the microstructural data to describe the hardness profiles in the various HAZs.

Evolution of microstructure and mechanical properties in naturally aged 7050 and 7075 Al friction stir welds

“…This is not surprising since the weld nugget has the highest concentration of vacancies and solute supersaturation, thereby promoting rapid nucleation and growth of GP zones. Similar behavior has been exhibited in the fusion zone regions of arc welded Al-Zn-Mg alloys [34,35].…”

Microstructure of butt laser joints of aluminium alloy 6056 sheets with an AS12 filler

“…These phenomena are due to the welding thermal cycle: the temperature increase in the HAZ during the welding cycle results in dissolution of the GP zones present. 18 During subsequent natural aging, some reprecipitation of GP zones occurs, although it is not sufficient to reach the initial fraction present in the base material, resulting in the observed hardness decrease. Further, one can link the measured hardness variation to the fraction of GP zones.…”

Microstructure mapping in friction stir welds of 7449 aluminium alloy using SAXS

“…SAXS has proved to be a powerful technique for studying precipitation, especially for Al-Zn-Mg alloys [16][17][18][19][20][21][22][23]. In particular, average features of the precipitate microstructure, such as the mean radius of the particles and their volume fraction, can be evaluated non-destructively.…”