ISSN: 2277-3754 ISO 9001:2008 Certified International Journal of Engineering and Innovative Technology (IJEIT) Volume 2, Issue 9, March 2013 76 Overview of the Effect of Post Welded Heat Treatment on Friction Stir Welding Of Aluminum Alloys M. Muthu Krishnan Abstract:-The objective of this investigation is to learn how and why certain alloys (primarily Aluminum alloys) can be strengthened by Precipitation Hardening heat treatment processes and their effects on the friction stir weldments of Aluminium alloys. The strength and hardness of some metal alloys may be enhanced by the formation of extremely small uniformly dispersed particles of a second phase within the original phase matrix; this must be accomplished by appropriate heat treatment. The process is called Precipitation Hardening or Age Hardening which involves three distinct steps: Solution Treatment to minimize segregation in the alloy, Quenching to create a supersaturated solid solution and Aging to facilitate the formation of coherent precipitates which strengthen the alloy by interfering with dislocation movement. I. INTRODUCTION Generally, Al alloys have many specific properties and can be applied to many structural parts that need both light weight and high mechanical properties. In these days, Al alloys have been used in the transportation industry such as high speed rail way [1], shipping and external fuel tanks of rockets .Structural parts and frames composed of Al alloys must be welded using the sound welding technique commonly employed in this industry. If Al alloys were welded using conventional fusion welding, many welding defects, such as voids, hot cracking and distortion, related to the melting and solidification were formed in the weld zone. Moreover, problems of fusion welded Al alloys were the precipitates resolution, the loss of work hardening effect and hard to melt for welding [4–7]. Therefore, the solid state bonding technique is highly recommended to solve these many problems. Friction stir welding, invented by TWI in 1991, is a promising technique because it resulted in superior joint quality of the varieties of Al alloys .The resulting weld is composed of three primary zones: the heat- affected zone (HAZ), the thermo mechanically- affected zone (TMAZ), and the weld nugget. FSW produces a dynamically recrystallized grain structure in the nugget, which has reached near-solution zing temperatures, before being “quenched” by the surrounding material back to room temperature at the completion of the weld. Because of this, the local microstructure is no longer stable and will begin to naturally age at room temperature. For heat treatable Al alloys, FSW processing generates a softened region within the weld zone because of the dissolution of strengthening precipitates [2]. For non-heat treatable Al alloys, softening is not observed, when alloys are not sensitive to strain hardening [3]. However, non-heat treatable and sensitive to strain hardening aluminum alloys exhibit softening in weld zone owing to decrease in dislocation density [4]. Most of the earlier studies on friction stir welding of similar type of Al alloys are confined in evaluating stirring zone microstructure, micro-hardness and bond strength [5]. II. PRINCIPLES OF PRECIPITATION HARDENING Heat treatment processes for increasing the strength and hardness of either wrought or cast aluminum alloys utilize the mechanism of precipitation hardening. All heat treatable aluminum alloys are strengthened by precipitation hardening. Precipitation hardening involves raising the temperature of the alloy into the single phase region so that all of the precipitates dissolve. . In order for an alloy system to be able to be precipitation- strengthened, there must be a terminal solid solution that has a decreasing solid solubility as the temperature decreases. One essential attribute of a precipitation hardening alloy system is a temperature and time dependent equilibrium solid-solubility characterized by decreasing solubility with decreasing temperature and then followed by solid-state precipitation of second phase atoms on cooling in the solidus region. These conditions are usually met by most aluminum equilibrium systems of heat treatable types, such as the Al-Si and Al-Cu alloys etc [6]. The alloy is then rapidly quenched to form a supersaturated solid solution and to trap excess vacancies and dislocation loops which can later act as nucleation sites for precipitation. The precipitates can form slowly at room temperature (Natural ageing) [7] and more quickly at slightly elevated temperatures, typically 100C to 200C (artificial ageing) .The degree of hardening obtained depends on the size, number and relative strength of the precipitates. These factors are determined by the composition of the alloy and by the tempering temperature and tempering line. During welding, liquid films form at grain boundaries adjacent to the fusion boundary. These liquid films lead to the formation of microscopic intergranular cracks after welding which may provide paths for subsequent brittle intergranular fracture. The pre-existing precipitation state can be modified in the following different ways: i) Dissolution of precipitates. ii) Growth of coarsening of pre-existing precipitates. iii) Transformation of metastable phases to