The effect of controlled shot peening on the fatigue behaviour of 2024-T3 aluminium friction stir welds

Abstract

The work examines the microstructural and fatigue properties of friction stir welds made of 2024-T3 aluminium alloy and provides extensive information towards their cyclic stress–strain behaviour, residual stress distribution and crack initiation sites. To eliminate the cost associated with the removal of the flow arm by milling and other costs associated with the quality control of the welding process (residual stress distribution, micro-hardness profile, welding scar, etc.), controlled shot peening is introduced. Tensile residual stresses introduced in the thermomechanical affected zone during welding are found to become compressive after peening. The effect can be held responsible for increasing the fatigue resistance of the weld beyond the values of the bare (parent) material.

Introduction

Commercial transport airplanes generally consist of a built-up structure where the skin-to-stringer, skin-to-clip and clip-frame joints are riveted, bolted or bonded. Such joints for many years have been the subject of extensive research, especially in terms of multiple site damage, widespread damage, fretting fatigue, etc. The proceedings of the International Conference of Aeronautical Fatigue provide an excellent source for referencing.

Friction stir welding (FSW) is a relatively new process patented by The Welding Institute (Cambridge, UK) in 1992 [1]. A friction stir butt weld is produced by plunging a rotating tool into the facing surfaces of the two plates. The tool consists of a shoulder and a profiled pin emerging from it. As the rotating pin moves along the weld line, the material is heated up by the friction generated by the shoulder and stirred by the rotating pin in a process similar to an extrusion. Since the temperatures are well below the melting point, problems associated with the liquid/solid phase transformation are avoided.

Besides the attractive mechanical properties, especially in fatigue and load bearing capacity strength, FSW integral structures are claimed to offer cost and weight savings [2], [3]. Therefore, FSW was recently identified by leading aircraft manufacturers as “key technology” for fuselage and wing manufacturing [4], [5]. Yet, problems associated with the fatigue behaviour of FSW are numerous and not well established.

Generally FSW produces five distinct microstructural zones [6], namely the weld nugget (N), the shoulder contact zone or flow arm region, the thermomechanical affected zone (TMAZ), the heat affected zone (HAZ) and unaffected zone or parent plate (PP). Consequently, the fatigue strength of FSW joints varies for each zone of the weld. The FSW weld zone is V-shaped and widens near the top surface due to the close contact between the shoulder of the tool and the upper surface [7]. The above indicates potential discontinuities in the strength and fatigue volume properties. Sato et al. [8] pointed out that the shape of the weld zone depends on the welding parameters and the material used. Dalle Donne and Biallas [9] show that with proper FSW tooling and welding parameter control, a reduction of only 20% compared to the base material values for the joint ultimate strength and fatigue endurance can be achieved. In addition, the zones have also been considered responsible for variations in the fatigue failure initiation sites. Booth and Sinclair [6] identified two forms of failure in the 2024-T351 FSW: (a) failure occurred from within the actual weld material (Nugget) and (b) failure occurred outside of the actual weld, either in the TMAZ or HAZ. Failure within the nugget region was associated with discontinuities in the material flow pattern at the surface. With no obvious defects being seen, the exact origins of crack initiation within this region were not clearly identifiable, whilst the failure in TMAZ and HAZ initiated by decohesion of large S-phase particles or transgranular failure. They suggest that heterogeneous precipitation at particle interfaces may influence the decohesion strength of the intermetallics at a specific location.

Differences in the fatigue behaviour are also manifested by the hardness characterisation in relation to the five microstructural zones. Jata et al. [11] reported for the 7050 Al alloy that the hardness of the top side is lower than the bottom side of the weld. They suggested that this is due to the fact that the top side is in full contact with the tool shoulder, and thus, experiences direct heat. The bottom side, on the other hand, is in indirect contact with a back plate that acts as a heat sink. Comparing the hardness between the zones, the hardness within the nugget varies depending on the alloy and its initial heat treatment. For 2024-T351, 7050-7745 and 6061-T6 alloys hardness profiles in the weld nugget show a local maximum value at the plate joint line or centre of the nugget [9], [6], [11]. Hardness profiles taken from nugget zone of 6063 Al alloy showed a minimum value among other regions. These differences in hardness value within the nugget have been correlated with the size of the precipitates present in the region [10], [12], [13], [14]. Investigations in the 2xxx and 7xxx aluminium series showed hardness minima within the TMAZ zone [6], [8], [15], [16]. The effect has been attributed to overaging [10].

Residual stress fields are widely believed to significantly effect catastrophic crack nucleation and growth. In [8], [17], [18], [19], residual stress distribution was reported to vary along the zones of the weld. Webster et al. [20] measured the residual stresses using Synchrotron X-ray technique and reported tensile residual stress in the nugget zone of 7108-T79. Similar finding were also reported by Bussu and Irving [10], Oosterkam et al. [21] for 2024-T351 and AA7108-T79, respectively. Nevertheless, Jata et al. [11] and Dalle Donne et al. [22] found a small compressive residual stress located at the centre of the nugget zone for 7050-T7451, Al–Li–Cu and 6013-T6.

Defects associated with the FSW process are strongly associated with fatigue resistance. In [13], [15], [23] it was reported that voids, inclusions and surface cracks dominate the nugget and represent potential sites for crack initiation. The above makes clear that quality process control and quality fatigue damage tolerance control over FSW joints is a complex requirement demanding extensive and well organised international research. Yet, driven from today’s market and societal needs for prompt innovation, cost and pollutant emission reduction [24], the fatigue behaviour of FSW joints needs to be improved and safeguarded without the need for an “in-depth” research. Such solution can be sought in terms of controlled shot peening (CSP).

CSP is a well established surface engineering treatment in the area of aeronautical and automotive engineering [25]. Pellets made of steel, ceramic or even dry ice, accelerated by either pneumatic or mechanical means are directed through controllable flow conditions onto the surface of the target material. The above results into the development of compressive residual stresses, strain hardening of the near surface region and surface roughening [26], [27], [28], [29]. Residual stresses are likely to benefit the fatigue resistance of high strength materials. Softer materials on the other hand are likely to experience fatigue resistance improvement owning mostly to strain hardening, since partial or even complete relaxation of the residual stresses may occur depending on the type of loading, stress level and the residual stress distribution profile [30], [31], [32]. Strain hardening is likely to increase the flow resistance of the material to plastic deformation.

Roughening of the surface is the major detrimental effect of CSP. Surface roughness, owing to the local intensification of the far-field stress, can account for the premature initiation and propagation of short fatigue cracks [33]. Rodopoulos et al. [34] suggested that a portion of residual stresses is consumed in order to counteract the detrimental effects of surface roughening. In brief, the elastic stress concentration provided by the surface roughness will increase the surface stress and hence the near-surface crack growth rate.

In this work, the use of the CSP technology to provide an improvement of the fatigue resistance of FSW has been selected on the grounds that: (a) the technology has been previously applied to aluminium tungsten-inert-gas (TIG) and metal-inert-gas (MAG) welds with exceptional results [35]; (b) FSW does not create softening effects typical to TIG which would prevent the complete development of residual stresses [35]; (c) CSP has the potential of altering the state and magnitude of residual stresses (from tension to compression) [36]; and (d) the strain rate from CSP is low enough not to affect microstructural properties like the explosive hardening treatment [36] which would have complicated the case. This work presents a detailed analysis towards the potential of CSP to improve the fatigue resistance of 2024-T3 aluminium alloy friction stir welds.