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

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Abstract

The microstructural and mechanical property evolution of friction stir welded 7050-T7651 and 7075-T651 Al alloys were examined as a function of room temperature (natural) aging for up to 67,920 h. During the range of aging times studied, transverse tensile strengths continuously increased, and are still increasing, with improvements of 24% and 29% measured for the 7050-T7651 and 7075-T651 Al alloy friction stir welds, respectively. Microstructural evolution within the weld nugget and heat-affected zone was evaluated with both transmission electron microscopy (TEM) and differential scanning calorimetry (DSC). Formation of a high volume fraction of GP(II) zones produced a majority of the strength improvement within the weld nugget and HAZ regions. The rational for the microstructural changes are discussed in light of the mechanical properties.

Introduction

Since its invention in 1991 by TWI (Cambridge, United Kingdom) [1], friction stir welding (FSW) has emerged as a reliable method to join high strength 7XXX series Al alloys [2], [3], [4], [5], [6], [7], [8], [9], [10], [11], [12]. The high strength aerospace Al alloys, namely 7050 and 7075, are considered “unweldable” by fusion techniques due to solidification cracking or severely degraded mechanical properties within the joint [13], [14]. In contrast, solid-state FSW produces joints without recast material thereby eliminating issues with solidification cracking and weld defects.

Briefly, FSW is a solid-state welding process whereby a non-consumable rotating tool, with shoulder and probe (or pin), is plunged into the workpiece and translated along the contact line of the plates to be joined. Rotation of the tool creates heat by friction and metal deformation resulting in a narrow column of plasticized material surrounding the tool. The forward traverse of the tool disrupts any surface oxide layers along the joint line producing a recrystallized and equiaxed fine-grained microstructure behind the tool. In FSW, the probe length determines the weld penetration depth. Fig. 1 schematically depicts a moving FSW tool and associated nomenclature. For a more complete description of FSW, considerable details have been published elsewhere [1], [2], [3], [4], [5], [6], [7], [8], [9], [10], [11], [12].

Several detailed microstructural examinations have been published for friction stir welded 7075 (3, 4, and 12) and 7050 (6, 7, 9, and 11) Al alloys. Results from these published examinations are summarized in Section 2, and are used as a baseline for the present study. However, the effect of long-term natural aging after FSW has received little detailed examination. A majority of the literature notations regarding natural aging involves the justification for post-weld heat-treating, e.g., Merati et al. justified the implementation of a localized heat treatment to stabilize the long-term weld strength after noting a strength change in friction stir welded 7050-T7651 Al after one year of natural aging [11]. At present, the most detailed published study of natural aging following FSW is by Nelson et al. [12]. These investigators established the natural aging response of friction stir welded 7075-T7351 Al for up to 1000 h of natural aging. Nelson et al. [12] utilized tensile and microhardness data to demonstrate a direct relationship between increased mechanical properties of friction stir welds and increased workpiece cooling rate during and after FSW. The present study builds on the results found by Nelson et al. [12] and examines how both the microstructure and mechanical properties continue to evolve within friction stir welded 7050 and 7075 Al alloys for very long natural aging times. Microstructural evolution was studied using transmission electron microscopy (TEM) and differential scanning calorimetry (DSC) and the resultant mechanical properties were examined with microhardness measurements and tensile tests.

Section snippets

Precipitation in 7XXX Al alloys

In the Al–Zn–Mg 7XXX series Al alloys the supersaturated solid–solution decomposes in the following sequence [15], [16], [17], [18]:Supersaturated solid–solution  GP zone  η′(MgZn2)  η(MgZn2)

Decomposition of the supersaturated solid–solution simultaneously produces solute rich clusters, GP(I), and vacancy-rich clusters that transform or nucleate into GP(II) zones [19], [20], [21]. Both types of GP zones form at low-temperatures (20–125 °C) [22], [23], but the GP(I) zone dissolves at a lower

Friction stir welding

Bead-on-plate friction stir welds were produced on 6.35 mm thick plates of 7050-T7651 and 7075-T651 Al with an H13 tool steel FSW tool consisting of a concave (7° taper) 19.1 mm diameter shoulder and a 7.87 mm diameter cylindrical round bottom probe 6.2 mm long. Material for microhardness specimens was friction stir welded with tool parameters of 200 RPM and 109 mm min−1. The materials for tensile specimens were produced with tool parameters of 600 RPM and 101.6 mm min−1 (7050-T7651) or 76.2 mm min−1

Microhardness

Fig. 3, Fig. 4 show microhardness measurements of friction stir welded 7050-T7651 Al and 7075-T651 Al, respectively, as a function of natural aging time. Both Al alloys show hardness increases for the duration of natural aging. The commonly observed W shaped microhardness profile [8], [9], [10], [11], [12], [28], [29] was seen after only 48 h of natural aging. This hardness profile shape consists of a region of higher microhardness in the center (nugget), but significantly less than the hardness

Strength recovery

Natural aging strength recovery of friction stir welded 7XXX Al alloys is examined by comparing DSC and TEM results to the general microhardness trends as a function of the microstructural region. The analysis assumes that the 7050-T7651 Al microstructure is similar to the corresponding regions in 7075-T651 Al; in general terms this assumption seems reasonable as both alloys exhibit similar trends in microhardness as a function of microstructural region and natural aging time. One notable

Conclusions

This paper examined the long-term natural aging response of friction stir welded 7050-T7651 and 7075-T651 Al. Natural aging has a significant impact on the microhardness and tensile properties of 7XXX friction stir welds, especially after more than 50,000 h. For example, for the range of aging times studied, tensile strengths continuously increased, and are still increasing, with improvements of 24% and 29% measured for the 7050-T7651 and 7075-T651 Al alloy friction stir welds, respectively. TEM

Acknowledgements

The authors graciously thank The Boeing Company for support under their internal research and development funding. A special thanks to Kevin Colligan for supplying the legacy material for the 3.175 mm thick plate investigation.

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    Present address: Teledyne Scientific, 1049 Camino Dos Rios, Thousand Oaks, CA 93021, USA.

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