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Chapter 1

CHAPTER 1

INTRODUCTION

1.1 Introduction

Friction stir welding (FSW) is an innovative welding process commonly known as a solid state welding process. This opens up whole new areas in welding technology. It is particularly appropriate for the welding of high strength aluminium alloys of the 2xxx and 7xxx series which are extensively used in the aircraft industry. Mechanical fastening has long been favoured to join aerospace structures because high strength aluminium alloys are difficult to join by conventional fusion welding techniques (Pouget. et al., 2007). Its main characteristic is to join material without reaching the fusion temperature. It enables to weld almost all types of aluminium alloys, even the one classified as non-weldable by fusion welding due to hot cracking and poor solidification microstructure in the fusion zone (Sandra et al., 2009). FSW is considered to be the most significant development in metal joining in a decade and is a ‘‘green’’ technology due to its energy efficiency, environment friendliness and versatility. The key benefits of FSW are summarized in Table 1.1 (Mishra et al., 2005).

Table 1.1 Key benefits of Friction Stir Welding

Metallurgical benefits

Environmental benefits

Energy benefits

  • Solid phase process
  • Low distortion of work piece
  • Good dimensional stability

and repeatability

  • No loss of alloying elements
  • Fine microstructure

  • No shielding gas required
  • No surface cleaning required
  • Eliminate grinding wastes
  • Consumable materials saving such as rugs, wire or any other gases
  • Improved materials use (e.g., joining different thickness) allows reduction in weight
  • Decreased fuel consumption in light weight aircraft automotive and ship applications

1.2 Friction stir welding process

The working principle of Friction Stir Welding process is shown in Fig. 1. A welding tool comprised of a shank, shoulder and pin is fixed in a milling machine chuck and is rotated about its longitudinal axis. The work piece, with square mating edges, is fixed to a rigid backing plate and a clamp or anvil prevents the work piece from spreading or lifting during welding. 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 (Nandan et al., 2008). The rotating welding tool is slowly plunged into the work piece until the shoulder of the welding tool forcibly contacts the upper surface of the material. By keeping the tool rotating and moving it along the seam to be joined, the softened material is literally stirred together forming a weld without melting (Rowe et al., 2005). The welding tool is then retracted, generally while the spindle continues to turn. After the tool is retracted, the pin of the welding tool leaves a hole in the work piece at the end of the weld. These welds require low energy input and are without the use of filler materials and distortion.

Fig.1.1 The principle of Friction Stir Welding (Thomas Wayne 1996)

1.3 Microstructural behavior of friction stir welding

The microstructure of a FSW weld depends on a few aspects like rotational and transverse speed, the pressure, the material and the tool design. This makes it difficult to describe the microstructure in general. However, following scheme was developed by TWI (The Welding Institute) and is accepted by the Friction Stir Welding Licensees Association. It divides the cross-section in four parts are shown in Fig1.2.

Fig. 1.2 Cross-section of weld nugget

A: Unaffected material: This is the region on a distance from the weld centre. It’s the region that is not affected by the generated heat. Although the material might have experienced a thermal cycle, it is not affected by this cycle. This means that the microstructure and the mechanical properties aren’t changed. It’s often referred to

as ‘parent material’.

B: Heat-affected zone (HAZ): This is a region a bit closer to the weld center. This has certainly experienced a thermal cycle and the mechanical properties and/or the microstructure is modified by it but it doesn’t show any plastic deformation. In the HAZ the changes in properties are comparable to those in the HAZ for other thermal processes. The shape of the HAZ is typically trapezoidal, as can be seen in above Figure

C: Thermo mechanically affected zone (TMAZ): This region has a change in microstructure and/or mechanical properties. But in contrast to the HAZ, the TMAZ has a plastic deformation. The grain size is similar to the grain size in the parent material.

D: Weld nugget: This is the part of the TMAZ that has been recrystallized. The grain sizes in this weld nugget are smaller than the grain sizes in the parent material.

1.4 Welding variables of friction stir welding

FSW involves complex material movement and plastic deformation. Welding parameters, tool geometry and joint design exert significant effect on the material flow pattern and temperature distribution, thereby influencing the microstructural evolution of material (Mishra et al., 2005). Therefore, welding speed, the tool rotational speed, the tilt angle of the tool, tool material and the tool design are the main independent variables that

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Verlag: BookRix GmbH & Co. KG

Tag der Veröffentlichung: 10.01.2018
ISBN: 978-3-7438-4965-5

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