Friction Stir Welding and Processing

Welding and joining of materials is a key step in manufacturing of integrated systems. The present book deals with metallic structures and the joining of these is achieved by either mechanical fastening or metallurgical bonding. Most of the conventional welding processes involve local melting along the joint line and subsequent solidification leads to formation of a joint. Figure 1.1 shows a very generic classification of manufacturing processes and welding/joining of metals falls under consolidation processes. The consolidation/assembly processes can involve joining or mechanical fastening. The selection of joining technique depends on the scale of structures and design considerations. While large aircrafts use mechanical fastening, the shipbuilding uses welding techniques. Smaller systems like automobiles use a large number of techniques including welding and adhesive bonding. The impact of joining techniques on the workpiece is an important consideration. Figure 1.2 shows an overview of metallic material joining and its association with microstructural changes and reliability. The inset in figure is a classic example of liberty ship failure because of improper joining procedure. Another example often cited in welding courses is the Alexander Kielland disaster in the North Sea where a weld failure led to collapse of oil platform. The lessons from these always weave an intricate relationship between design, materials, welding processes and procedures, and final inspection. Residual stresses and related impact on long term service are an integral and important part of the entire scheme. Of course, fundamental understanding of these is critical not only to avoid these occurrences but also to maximize the structural efficiency of the systems.

For any manufacturing process, understanding its fundamental process mechanisms is vital for its long-term growth. In this chapter, we will outline the essential characteristics of friction stir process. As pointed out in Chap. 1, unlike fusion-based joining processes, there is no perceptible melting during friction stir welding (FSW). From the operational viewpoint, a friction stir welding run can be divided into three sub-procedures or phases:

All metal processing activities involve a combination of deformation and heat transfer where depending on the processing path a variation in microstructure occurs. For a thermo-mechanical process like FSP, variations in process parameters are accompanied by changes in deformation and heat transfer characteristic with consequent variations in the microstructure. In Fig. 3.1a, b the microstructure of an Al alloy (Al-Mg-Si type) obtained through two different casting routes are shown while Fig. 3.1c, d presents the differences in welded macrostructure of a AA356 alloy using the same FSW tool at different operating parameters. As is evident, depending on the processing conditions the microstructure differs drastically with consequent changes on the macrostructure.

The basic goal of any joining technique is to perform welding in as many configurations as possible to enhance the design space for components and structures. This chapter is split into two main sections: joint configurations and tool selection guidelines. There may be some overlap in these areas, and those are dealt with some minor comments wherever needed. Also, some discussion on defect formation during the friction stir process is included.

Aluminum, the third largest element available on the earth’s crust is the second highest consumed metal by weight and stands a distant second to iron and its alloys. The lower consumption by weight is in a large part a consequence of its low density, and is one of the prime advantages of this metal. This low density combined with the attractive strength properties of modern Al alloys makes it ideal for structural applications. The other distinctive properties of Al and its alloys include high ductility, low electrical resistance and high corrosion resistance. Such combination of desirable properties along with low density makes Al and its alloys an ideal replacement for steel, though the relatively high cost and low weldability has limited its scope to more niche applications.

Magnesium is the eighth most abundant element (%molar) in earth crust. Although Mg came into existence in its elemental form 1808 when Sir Humphrey Davy isolated it from its compound, it was not until 1886 when its industrial production started. The invention of Hall-Heroult process same year enabled economical production of aluminum feasible. The production of Mg however remained limited, to a few tons per year until the start of twentieth century. Its production soared to ~228,000 ton per year worldwide by 1943–1944 due to its demand in various military operations during World War II declining to ~10,000 ton per year after the war (King 2007). Figure 6.1 shows a graphical overview of world production of Mg from 1983 till 2010. In 1983, the primary Mg production reached 200,000 ton which remained same till 2000. However, from year 2000 onward a surge in production can be noted (Fig. 6.1). This is due to the increase in magnesium production in China as well as a change in methodology of data collection by the International Magnesium Association from year 2000 onward. Overall, a recent surge in interest in Mg alloys is observed. The main driver for such renewed interest is the transportation industries where weight reduction to improve fuel efficiency has become very important (increased governmental regulation on fuel emissions to tackle global warming). In fact, as per one report (Magnesium Vision 2020 2006), North American automotive industries have projected to replace 630 lb of ferrous and aluminum components with 350 lb per vehicle of magnesium parts by 2020 from its current use of an average of ~12 lb per vehicle. The decision of South Korean government to spend close to 90 million US dollars on research and development activities related to future magnesium technologies is another example of the surge in interest worldwide in magnesium. Hence, the production of primary Mg is expected to grow in the coming years.

High temperature alloys pose an interesting challenge to friction stir welding. In the previous chapters on aluminum and magnesium alloys, we dealt with matrix elements that do not undergo allotropic transitions. This implies that the microstructural changes were limited to grain size and precipitates. Among high temperature alloys of industrial significance, the present chapter is limited to titanium alloys and steels. Both alloy systems involve complexity of phase transformation, and therefore peak temperature as well as cooling rate during friction stir welding makes a significant difference. Nickel base alloys and copper alloys are not covered in this book, but much of what is discussed applies to those. Students and readers can apply the treatment of alloys covered in this book to nickel and copper alloys by selecting the right type of physical metallurgy.

The varied requirements of different regions or parts in a structure often require the use of different materials to meet the design expectation. So, the use of different materials in a given engineering system is a norm not an exception. Thus, to improve the fuel efficiency, automobile industry is encouraging the use of a combination of light metals like Al and Mg alloys. The increased use of light metal parts will necessitate their joining with existing structural member and most probably with dissimilar metals.

The intrinsic nature of friction stir process has two basic components as highlighted in previous chapters, material flow and microstructural evolution. The development of friction stir processing as a generic metallurgical tool for microstructural modification and a broader manufacturing technology is connected to these. Even though the adaption of these friction stir process based technological variants is slow, the potential of these is limitless. The focus of this chapter is to illustrate the linkages of basic friction stir process attributes to some illustrative examples of new technology development. The chapter is by no means comprehensive because many ideas can be built on these basics and each one can have its own niche area of application.

The stresses existing in an elastic body in the absence of external forces or loads (thermal or mechanical) are termed as residual stresses. This can be appreciated further from the schematic illustrated in Fig. 10.1 which shows an irregular externally loaded two dimensional body. The external forces or loads are denoted by symbol F_i, where i = 1–5 and acts on a small region indicated by A. After removal of external forces or loads, two possible scenarios for the region denoted by letter A are: (a) there are no internal stresses, and (b) there are internal stresses present in the absence of external loads F_i. The stresses within the body in state (b) are termed as residual stresses.