Precipitate microstructures and resulting properties of Al-Zn-Mg metal inert gas-weld heat-affected zones
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1/4/04
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Precipitate Microstructures and Resulting Properties of Al-Zn-Mg Metal Inert Gas–Weld Heat-Affected Zones M. NICOLAS and A. DESCHAMPS 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.
I. INTRODUCTION
WELDING of aluminum alloys is increasingly a key joining process in the transportation industry. Medium strength 7xxx aluminum alloys (based on the Al-Zn-Mg system) have a good combination of properties making them suitable for many automotive applications where high specific mechanical properties are required. Besides solidification cracking susceptibility of the weld fusion zone,[1–5] the properties of the base material are also weakened outside the fusion zone due to the thermal cycle experienced in the so-called heat-affected zone (HAZ),[6] due to modifications in the precipitate microstructure, which is responsible for the major contribution to the yield strength.[7–10] When this is technologically feasible, one way to erase the welding history in the HAZ, i.e., to recover a structure with homogeneous properties after the welding process, is to proceed with post-welding heat treatments (PWHTs) either by natural aging[7] or by subsequent artificial aging.[10] The detailed study of precipitate microstructures in the HAZ of weld joints is a complicated task, since it is inherently spatially heterogeneous. Most of the existing studies have limited their investigation to microhardness,[8] at best some selected transmission electron microscopy (TEM) observations or differential scanning calorimetry (DSC) studies being made in critical points of the HAZ.[9,10] However, given the complexity of the microstructures that can be found, which include several types of precipitates of different strength and size, this methodology can lead to significant misunderstandings. Small-angle X-ray scattering (SAXS) provides an efficient technique to p
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