Characterisation of microstructure, mechanical properties and corrosion behaviour of an AA2219 friction stir weldment

https://doi.org/10.1016/j.jallcom.2009.11.198Get rights and content

Abstract

A friction stir weldment of AA2219-T87 aluminium alloy was characterised for its microstructure, mechanical properties, corrosion resistance and stress corrosion cracking behaviour. The dissolution and coarsening of the strengthening precipitates in the weld nugget and TMAZ/HAZ regions, respectively, had resulted in the softening of these regions; nevertheless, the weld joint was found to have an efficiency of around 72%. The general corrosion behaviour of the weld nugget was superior to that of the parent. The TMAZ/HAZ regions, and the weld nugget were found to have been galvanically protected in the immersion tests. Both the parent alloy and the friction stir weldment specimens exhibited a good resistance to stress corrosion cracking in 3.5% NaCl solution as was demonstrated by slow strain rate tensile tests performed at nominal strain rates of 10−6 s−1 and 10−7 s−1.

Introduction

Age-hardenable aluminium–copper alloys are widely used for aircraft and aerospace applications owing to their excellent mechanical properties and good strength-to-weight ratio. Friction stir welding (FSW) has been a preferred process for the joining of the so-called difficult-to-weld heat-treatable aluminium alloys right since its inception in the year 1991 [1], [2]. This solid state welding process overcomes the issues of liquation cracking, porosity and distortion in the weldments which are often associated with the other welding processes viz. gas tungsten arc welding (GTAW) and gas metal arc welding (GMAW) [3], [4], [5], [6]. High-strength aluminium alloy weldments obtained by FSW were reported to possess good mechanical properties and were found to exhibit better joint efficiency than GTA/GMA weldments [7], [8], [9]. The joining of AA2219 alloy in different temper conditions was carried out by Chen et al. using FSW, and their microstructure–mechanical property correlation was well documented [10], [11]. The evolution of fine grained structure in the friction stir welds partly compensates for the dissolution and/or coarsening of the strengthening precipitates in the weld nugget, and thus provides better mechanical properties than those realised in the GTA welds. The corrosion and stress corrosion cracking (SCC) behaviour of various high-strength aluminium alloys and also that of dissimilar weldments comprising two different aluminium alloys have been investigated by many researchers [12], [13], [14], [15], [16], [17]. Paglia and Buchheit in their recent viewpoint paper reviewed the corrosion of aluminium alloy friction welds and reported the sensitization of the microstructure during welding as the responsible factor for the decrease in the corrosion resistance of weld nugget and heat-affected zone in friction stir weldments [18]. Further, the environmental cracking behaviour and the regions of susceptibility in the friction stir weldments of different aluminium alloys were reported to be influenced by the microstructure and chemistry. As there is not much published information on the corrosion and SCC behaviour AA2219 aluminium alloy friction stir welds, except a few [19], [20], the present work is aimed at the understanding of the corrosion and SCC behaviour of an AA2219-T87 alloy friction stir weldment in the as-welded condition in chloride environments.

Section snippets

Experimental

A 5-mm thick wrought AA2219 aluminium alloy plate with a nominal chemical composition (wt.%) of Cu 5.95%, Mn 0.27%, Zr 0.1%, V 0.09%, Ti 0.06%, Fe 0.12%, Si 0.05% and balance Al was used in this investigation. The alloy was received in the T87 temper, after having been subjected to the following heat treatment: solutionized at 535 °C for 45 min followed by cold rolling and a subsequent aging treatment at 165 °C for 24 h to obtain a stabilised microstructure.

FSW was carried out at Hi-Tech Vocational

Microstructure and hardness

Optical macrograph of the cross-section of the friction stir weldment is shown in Fig. 1. The micrograph of the parent alloy shown in Fig. 2 reveals elongated grains characteristic of the rolled material, with some dark intermetallic particles. The densely distributed plate-like semi-coherent and coherent strengthening precipitates observed in the parent alloy are shown in the transmission electron micrograph shown in Fig. 3a, and the higher magnification of micrograph shown in Fig. 3b clearly

Conclusions

The joining of AA2219-T87 aluminium alloy by friction stir welding could be accomplished with a joint efficiency of around 72%. The weld nugget/TMAZ interface was the weakest region in this weldment in terms of mechanical strength. The general corrosion resistance of the weld nugget was better than that of the parent AA2219 alloy in 3.5% NaCl solution. The increased corrosion resistance of the FSW nugget region was attributed to the dissolution/coarsening of the strengthening precipitates in

Acknowledgements

The authors gratefully acknowledge Indian Space Research Organization (ISRO) for providing the material for research work and Mr. Rajneesh Kumar (NML, India) for the help in FSW. PBS expresses his sincere thanks to the AvH foundation, Germany for the award of post-doctoral fellowship. KSA gratefully acknowledges the Deutscher Akademischer Austausch Dienst (DAAD) for financial assistance and opportunity to carry out a part of the research work in Germany as a DAAD Fellow.

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