Effect of Post Weld Heat Treatment on Properties of ACTIG Welded Aa6063 Aluminium Alloy Joint

Abstract

AA6063 aluminium alloy joints were fabricated by alternating current tungsten inert gas (ACTIG) welding process and the effects of post weld heat treatment (PWHT) on the tensile properties, impact strength, microstructure, microhardness and fractography of the welded joints were investigated. The ACTIG welding process was adopted because it bears higher strength, more ductility and no apparent microstructure defect. The welded samples were divided into as-welded (AW) sample and PWHT sample. The PWHT method used on the samples was solution treatment (535 °C, 2h), water quenching and artificial aging (165 °C, 18h). The experimental results show that, compared with the AW samples, the microstructure characteristics and mechanical properties of the AA6063 joints after PWHT were significantly improved. The improvement of ultimate tensile strength, and impact strength are 3.72% and 22.22% respectively. Keywords: GTAW, PWHT, SEM, EDS. I.INTRODUCTION Aluminium alloys have been widely used in many fields such as the construction, transportation and aerospace owing to their excellent performance, including light weight, high strength and ductility, good corrosion resistance and abundant resources [1]. Among the various heat-treatable aluminium alloys, AA6063 aluminium alloys possess a good combination of strength and toughness with excellent weldability and extrudability, which makes this alloy an preferred choice for Automotive outer bodypanels, railcars, etc. buildings (doors, windows, ladder, etc), marine(offshore structure, etc), heating(brazing sheets, etc)]. This type of aluminium alloy contains Magnesium(Mg) and Silicon(Si) as its major alloying element. Welding is one of the most common joining methods for aluminium alloys. Tungsten inert gas (TIG) process and gas metal arc welding (GMAW) are the most-used welding processes [2]. GMAW possesses advantages of high deposition rate, high welding speed and deep penetration, but excessive heat input brings particularly in welding of thin aluminium sheets. Hence, TIG welding process is preferred over gas metal arc welding so as to obtain high quality weldments [3]. The TIG welding used in this work as AC tungsten inert gas (ACTIG). Moreover, the output of pulsed current can also strengthen the molten pool stirring to produce refined grains. In terms of weldability [6], AA6063 suffers from a substantial decline in strength after welding. The loss of strength is because the rapid melting and solidification process makes all the strengthening precipitates dissolve into the aluminium matrix and complete dissolution of the precipitates can not take place in weld metal and overaging appear in HAZ and meanwhile solute segregation and grain coarsening also come up [11]. Therefore, the post weld heat treatment (PWHT) can be implemented as an effective method of minimizing the softening and improving the properties of welded joints. PWHT has been demonstrated to be a practical option to regain the strength of the joints by modifying the size, shape and distribution of th...