VPPA welds of Al-2024 alloys: Analysis and modelling of local microstructure and strength

Abstract

The microstructural features of variable polarity plasma arc welded Al-Cu-Mg 2024-T351 with 2319 filler have been studied by TEM, SEM and DSC. Fusion zone, partial melting zone, resolutionising zone, overageing (for S phase), peak ageing (for S phase) and under ageing zones (for S phase) have been identified. The Ω phase has been observed between re-solutionising zone and peak ageing zone. The hardness profile contains two peaks. The microstructure development, and resulting hardness and yield strength profiles are modelled using a model which combines primary precipitation, resolution, partial/full melting and resolidification (Scheil type) and reprecipitation, in a two precipitate -two mechanism approach. Hardness profiles and microstructures are accurately predicted. The hardness peak in the re-solutionising zone is due to re-solutionising and subsequent Cu-Mg co-cluster formation; and the second hardness peak is caused by S phase strengthening.

Two types of S phase precipitates in Al–Cu–Mg alloys

“…1 shows the DSC curves of the AA2324 alloy in three different conditions. Five main effects may be identified in these thermograms [25][26][27]: an exothermic peak, A, between 60ºC and 110ºC, is due to the formation of co-clusters [17,20]; an endothermic effect, B, between 160ºC and 240ºC may be attributed to Cu-Mg co-cluster dissolution (with possibly some GPB2 dissolution); an exothermic effect C (containing two overlapping reactions, see below) between about 230ºC and 340ºC, is attributed to the formation of S phase precipitates; a broad endothermic effect, D, 340ºC-500°C, is identified as progressive dissolution of the S precipitates [25,28]; an endothermic sharp peak, E, between 505ºC and 515ºC is believed to be due to the (partial) melting of S+θ eutectics. In this study, exothermic effect C is analysed.…”

A model for co-clusters and their strengthening in Al–Cu–Mg based alloys: a comparison with experimental data

“…Thus the type of precipitation strengthening caused by co-clusters is very different from classical strengthening effects such as Orowan (precipitate by-passing) strengthening, strengthening by shearing of ordered precipitates and chemical strengthening, which all possess (in good approximation, for dilute alloys) a volume fraction to the power ½ dependency [19][20][21][22][23][24][25][26][27][28][29][30]. This distinct difference with other precipitation hardening mechanisms follows from the assumption that all enthalpy change is related to bonds of CuMg pairs and that interaction with moving dislocations is determined exclusively by the breaking of this bond.…”

The strength of friction stir welded and friction stir processed aluminium alloys

“…In terms of strength predictions for a given predicted micro/nano structure, existing models provide a reasonable estimate of yield strength [3,6,32,33]. A better understanding of superposition laws would still be welcome in situations where a large number of different obstacles to dislocations are present (forest dislocations, several types of precipitate phases, small grains).…”

“…These processes occur within a short time interval of typically a few seconds in zones as small as a few hundreds of microns to a few mm wide. A full understanding of the process requires models of processes occurring over a scale of millimetres (e.g., the thermomechanical deformation around the rotating pin) down to the subnanometer scale, where formation of small atom clusters (GP zones) can have a strong influence on strength in heat-treatable alloys [5,6]. Prediction of strength of FS welds requires 3 main models: i) a model for heat generation and heat diffusion, ii) a model for evolution of the nano/microstructure as a function of temperature and deformation, and iii) a model for strength as a function of the nano/microstructure.…”