TEM analysis of a friction stir-welded butt joint of Al–Si–Mg alloys

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Abstract

The microstructure evolution of a joint of Al–Si–Mg alloys A6056-T4 and A6056-T6 has been characterized by transmission electron microscopy (TEM). Metallurgical investigations, hardness and mechanical tests were also performed to correlate the TEM investigations to the mechanical properties of the produced friction stir-welded butt joint. After friction stir-welding thermal treatment has been carried out at 530 °C followed by ageing at 160 °C (T6). The base material (T4) and the heat-treated one (T6) were put in comparison showing a remarkable ductility reduction of the joint after T6 treatment.

Introduction

Light-weight aluminium alloys are widely used to obtain components for aerospace applications with high specific strength [1]. Indeed, recent studies focused on the possibility of using Al alloys in widespread range of activities [2]. In particular welded components reduce up to 30% the involved costs compared to mechanical fastening and numerical control machining, and a weight reduction of 10% respect to the weight components produced by mechanical fastening. On the other hand, traditional welding processes, when applied to several Al alloys, present a series of disadvantages that have sometimes discouraged the use of such a kind of welded materials.

Friction stir-welding (FSW) is a recent method of joining materials, patented by the welding institute (TWI) in 1991 [3]. This process constitutes a development of the classical friction welding methods. In the FSW process no melt of the joining parts occurs and the weld forms through solid-state plastic flow at elevated temperature. FSW assures the absence of porosity, distortion and residual stresses which are typical defects of the fusion processes. Moreover, it assures the possibility to operate in all positions with no protective gas. Alloys difficult to weld as 2000 and 7000 series have been successfully welded by FSW [4], [5], [6], [7], [8], [9], [10], [11], [12]. The process uses a shouldered rotating tool with a profiled pin, that penetrates the parts to join, firmly clamped, till the desired pressure; then the tool starts to move along the joint line. The resulting heat, produced during the friction, softens the alloy, and the pin stirs the material of the joint, until the sheets are joined.

The weld structure consists of a continuous consolidated nugget of forged material with a refined grain size. The FSW metal, solution heat-treated and artificially aged, can exhibit almost the same strength of the fully heat-treated parent material. The relationship between process parameters and final microstructure is very relevant to obtain a deep comprehension of the process. To reach this goal, a great deal of transmission electron microscopy (TEM) investigations has been recently performed on several friction stir-welded alloys, in particular on 2000 and 6000 series. The FSW joining process produces a thermally altered region where the grain morphology withstands deep structural changes.

The microstructure studies aimed at characterization of either the shape and dimension of the grains across the joint for different kinds of Al alloys, the distribution and size of precipitates, the dislocation density of the 6063 material and the types of orientation referred to the pin surfaces. Sato et al. [4], [6], [9] have found on joint a complex distribution of precipitates on the transverse section: needle-shaped ones diffused in unaffected material and rod-shaped ones closer to the join line. The variation of micro-hardness is also associated with the kind and shape of precipitates. Within the welded heat-affected zone, the density of strengthening precipitates changed considerably and influenced the mechanical properties of the joined materials.

Liu et al. [10] focused their studies on the complex precipitation phenomena, especially in the transition zone, and underlined the reduction of dislocation density in the centre of the weld. In the case of a 6061 T651 joint [12], it was found that the stir zone was basically constituted of small grains separated by high-angle grain boundaries. A great deal of dislocations characterize the cell–subgrain interior and, therefore, a strain gradient exists across the cell/subgrain boundaries. The subgrain structure transforms into a fine grain structure, promoted by lattice rotation associated to boundary sliding and subgrain growth. This kind of dynamic microstructure transformation is fully consistent to a continuous dynamic recrystallization process.

The aim of the present paper is to analyse the microstructure of friction stir-welded butt joint of the A6056 material by TEM and to compare the results of T4 and T6 treatment.

Section snippets

Experimental

The examined joined sheets had dimensions of 200mm×470mm×4mm and they were produced by Alenia Italia Aerospatial Section. The material, a 6056-T4 (i.e., as-welded) alloy, had the following chemical composition (wt.%): 0.65 Si, 0.15 Mn, 1.1 Mg, 0.23 Cr, Al bal.

Tensile tests have been made on specimens of different dimensions and kept in different positions, to measure the mechanical properties of base, as-welded and T6-treated material. The post-weld heat treatment (i.e., T6) consisted in

Results and discussion

A statistic evaluation of the grain size has been carried out either in the not-welded material and in the welded one. Fig. 1 shows the microstructure of the as-received material; the mean grain size has been calculated by means of statistic evaluations over different specimen areas. The mean grain size was 65 μm and clearly preferentially oriented along the rolling direction.

Fig. 2 shows the macrostructure of the transverse surface across the welding zone. The cross-section has the typical

Conclusions

FSW of A6056 material induced a dynamic recrystallization of the grains within the heat-affected zone. The elongated grain structure was also formed with a high dislocation concentration inside the deformed grains. A post-FSW T6 treatment produced a considerable increase in the yield and ultimate tensile strength of the joint. A remarkable decrease of the ductility response of the T6 material was basically due to a higher concentration of fine hardening Mg2Si particles formed during the ageing

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