Recent advances in friction-stir welding – Process, weldment structure and properties

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Abstract

Friction-stir welding is a refreshing approach to the joining of metals. Although originally intended for aluminium alloys, the reach of FSW has now extended to a variety of materials including steels and polymers. This review deals with the fundamental understanding of the process and its metallurgical consequences. The focus is on heat generation, heat transfer and plastic flow during welding, elements of tool design, understanding defect formation and the structure and properties of the welded materials.

Introduction

Friction-stir welding (FSW) is a solid-state, hot-shear joining process [1], [2], [3] in which a rotating tool with a shoulder and terminating in a threaded pin, moves along the butting surfaces of two rigidly clamped plates placed on a backing plate as shown in Fig. 1. The shoulder makes firm contact with the top surface of the work-piece. Heat generated by friction at the shoulder and to a lesser extent at the pin surface, softens the material being welded. Severe plastic deformation and flow of this plasticised metal occurs as the tool is translated along the welding direction. Material is transported from the front of the tool to the trailing edge where it is forged into a joint. Although Fig. 1 shows a butt joint for illustration, other types of joints such as lap joints and fillet joints can also be fabricated by FSW.

The half-plate where the direction of rotation is the same as that of welding is called the advancing side, with the other side designated as being the retreating side. This difference can lead to asymmetry in heat transfer [4], material flow and the properties of the two sides of the weld. For example, the hardness of particular age-hardened aluminium alloys tends to be lower in the heat-affected zone on the retreating side, which then becomes the location of tensile fracture in cross-weld tests [5]; this is also the case for pure titanium [6].

Since its discovery in 1991 [1], FSW has evolved as a technique of choice in the routine joining of aluminium components; its applications for joining difficult metals and metals other than aluminium are growing, albeit at a slower pace. There have been widespread benefits resulting from the application of FSW in for example, aerospace, shipbuilding, automotive and railway industries [7].

FSW involves complex interactions between a variety of simultaneous thermomechanical processes. The interactions affect the heating and cooling rates, plastic deformation and flow, dynamic recrystallization [8] phenomena and the mechanical integrity of the joint. A typical cross-section of the FSW joint consists of a number of zones (Fig. 2) [9], [10], [11], [12]. The heat-affected zone (HAZ) is similar to that in conventional welds although the maximum peak temperature is significantly less than the solidus temperature, and the heat source is rather diffuse. This can lead to somewhat different microstructures when compared with fusion welding processes. The central nugget region containing the “onion-ring” appearance is the one which experiences the most severe deformation, and is a consequence of the way in which a threaded tool deposits material from the front to the back of the weld. The thermomechanically affected zone (TMAZ) lies between the HAZ and nugget; the grains of the original microstructure are retained in this region, but often in a deformed state.

A unique feature of the friction-stir welding process is that the transport of heat is aided by the plastic flow of the substrate close to the rotating tool. The heat and mass transfer depend on material properties as well as welding variables including the rotational and welding speeds of the tool and its geometry. In FSW, the joining takes place by extrusion and forging of the metal at high strain rates. Jata and Semiatin estimated a typical deformation strain rate of 10 s−1 by measuring grain-size and using a correlation between grain-size and Zener–Holloman parameter which is temperature compensated strain rate [13]. Kokawa et al. estimated an effective strain rates in the range 2–3 s−1 [14]. The plastic flow must clearly feature in any theory for the process, and the behaviour of the metal at high strain rates, its dynamic recrystallization behaviour and the effects of heating and cooling must also be considered.

The role of welding parameters, tool design, the initial process modelling, and the microstructure and properties of the FSW joints have been reviewed previously [15], [16]. It is nevertheless a relatively new process and one which is evolving rapidly. As a result, a periodic critical assessment of our understanding of the welding process as well as the structure and properties of the welded materials is likely to be helpful. This is the aim of the paper.

Section snippets

Heat generation

During FSW, heat is generated by friction between the tool and the work-piece and via plastic deformation. A fraction of the plastic deformation energy is stored within the thermomechanically processed region in the form of increased defect densities. In the weld, a mixture of recovery and recrystallization phenomena occur simultaneously [17]. Deformation not only increases the dislocation density but also the amount of grain surface and grain edge per unit volume [18] and by cutting

Aluminium alloys

Many aluminium alloys are strong by virtue of precipitation hardening through natural or artificial ageing from the solution-treated condition. The heat associated with welding changes the microstructure of the material. In Fig. 22, HVmin and HVmax represent the hardness in the solution-treated and precipitation hardened states. The effect of welding is to cause a drop in hardness from HVmax towards HVmin as the peak temperature experienced increases, curve (a), Fig. 22. This is because

Outlook and remarks

Friction-stir welding technology has been a major boon to industry advanced since its inception. In spite of its short history, it has found widespread applications in diverse industries. Hard materials such as steel and other important engineering alloys can now be welded efficiently using this process. Significant progress has also been made in the fundamental understanding of both the welding process and the structure and properties of the welded joints. The understanding has been useful in

Acknowledgements

We would like to thank Dr. G.G. Roy for many helpful discussions and his interest in this work. The research was supported by a grant from the Materials Division, Office of Naval Research, Dr. J. Christodoulu and Dr. J. DeLoach, contract monitors. One of the authors, Rituraj Nandan, is thankful to American Welding Society for a fellowship.

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