**1. Introduction**

Designed in the 1950s and initially used for compressor blades in gas turbine engines, Ti‐6Al‐ 4V titanium alloy has a high specific strength, stability at temperatures up to 400°C and good

© 2016 The Author(s). Licensee InTech. This chapter is distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited. © 2016 The Author(s). Licensee InTech. This chapter is distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

corrosion resistance. These properties have brought about an extensive use of Ti‐6Al‐4V in a variety of applications, such as aerospace, biomedical devices and chemical processing equipment. The aircraft industry accounts for more than 80% of this usage [1]. Airframes and aeroengine parts are the two most common applications for titanium alloys. The relatively high costs of both the raw material and the part fabrication continue to hinder the wider use of titanium in applications where weight and corrosion are not critical factors. In this context, more efficient technologies for producing complex titanium structures are required to broaden the application areas of titanium alloys and minimize the inherent cost problem. Laser beam welding (LBW) is a very promising joining technique that provides the possibility of high productivity, a single‐step process and the benefit of potential weight savings compared with riveting. Compared with most structural titanium alloys, Ti‐6Al‐4V is considered to be highly weldable. The high energy density of the laser beam enables relatively high welding speed, which results in low heat input, a narrow heat‐affected zone (HAZ) and low distortion. LBW has become increasingly competitive as a joining process over the last few decades owing to significant practical advantages over electron beam welding (EBW), requiring a high vacuum environment. However, additional measures must be taken to protect the weld zone from atmospheric contamination during LBW.

A number of researchers have investigated the influence of welding parameters on the quality and mechanical properties of the laser beam welded Ti‐6Al‐4V butt joints [2–5]. Generally, fusion zones exhibit higher strength and lower ductility than those of the base metal, and fracture of transverse‐oriented butt welds in tensile testing usually occurs in the unaffected parent material [2, 5]. Despite the generally higher tensile strength, inferior fatigue properties of laser beam welded titanium joints are commonly observed. Because Ti‐6Al‐4V alloy is primarily used in fatigue‐critical components, poor axial fatigue behaviour of laser beam welded Ti‐6Al‐4V joints is one of the main factors limiting their wide industrial application. In consideration of the above‐mentioned problems, more complete understanding of the reasons and mechanisms for fatigue failure of laser beam welded Ti‐6Al‐4V joints is of great scientific interest.

Because the control of microstructure is the primary key for obtaining required mechanical properties in titanium alloys, postweld heat treatment (PWHT), which results in the transfor‐ mation of the welding zone microstructure, is one of the options aiming to achieve improved fatigue performance of laser beam welded Ti‐6Al‐4V butt joints. Kabir et al. [6] investigated the effects of stress relief annealing and solution heat treatment followed by ageing on the microstructure, hardness and tensile properties of autogenously welded Ti‐6Al‐4V butt joints. Increased microhardness in the fusion zone after stress relief annealing was observed. The joint efficiency in terms of tensile strength was maximum for the as‐welded condition and was slightly decreased by nearly 5% for solution heat‐treated and aged conditions. Babu et al. [7] studied the influence of two types of heat treatment (700 and 900°C) on the fatigue properties of electron beam welded Ti‐6Al‐4V butt joints. The specimens annealed at lower temperatures exhibited longer fatigue lives and higher tensile strength. These results were attributed to the coarsening of the microstructure in the fusion zone after PWHT at high temperatures. Tsai et al. [8] showed that heat treatment at a temperature of 790°C for 1 h followed by air cooling can

noticeably improve mechanical properties and provide the electron beam welded joint with a superior combination of tensile strength and ductility. Friction stir welded Ti‐6Al‐4V joints subjected to various types of heat treatments were studied by Edwards et al. [9], who found that hardness and strength decreased with increasing heat treatment temperature, but ductility and fatigue performance improved. It was recommended that a high heat treatment temper‐ ature can be used to obtain the best combination of strength, ductility and fatigue performance. Thus, extensive research work has been carried out on welding of Ti‐6Al‐4V alloy and the subsequent PWHT. However, most of the published information is focusing on the static tensile properties. There is a lack of clarity about the influence of PWHT on the high cycle fatigue (HCF) behavior of the LBW Ti‐6Al‐4v butt joints. The present study was undertaken to quantitatively characterize the effect of heat treatment on the HCF performance of the laser beam welded Ti‐6Al‐4V butt joints and link this effect with microstructural transformations that took place during PWHT.
