MECHANICAL PROPERTIES AND CORROSION BEHAVIOUR OF FSW JOINTS OF ALUMINIUM ALLOYS

Abstract

The wide spread application of heat-treatable aluminum (Al) alloys in all sectors of industry has grabbed great attention of manufacturers and technologists. However, the joining of these alloys is considered to be difficult using fusion welding techniques owing to the possibility of porosity, solidification cracking, heat affected zone softening etc. Therefore, the solid-state joining techniques are found to be more efficient and viable for the joining of these alloys. These techniques reduce the adverse effect on the microstructure, mechanical properties and corrosion behaviour of the heat-treatable Al alloys. Among the solid-state joining processes, the friction stir welding (FSW) is found to be the most suitable option. The FSW works on the principle of large scale plastic deformation of faying surfaces. The joints produced by FSW process, however comprises heterogeneities in the microstructure due to the formation of the various regions such as nugget zone (NZ), thermomechanical affected zone (TMAZ), heat affected zone (HAZ) and base metal (BM). Heterogeneity in the microstructure leads to the variation in the mechanical properties as well as corrosion behaviour of the weld joint. Furthermore, the HAZ of the precipitation hardenable aluminium alloys is found to be severely affected by the weld thermal cycle experienced during the welding. The HAZ experiences dissolution and coarsening of the precipitates as well as coarsening of the α-Al grains. In view of this, the focus of present work was on the improvement of the mechanical properties (such as tensile strength, microhardness, etc.) and corrosion resistance of the FSW joints using various approaches. Two different precipitation-hardenable aluminium alloys namely AA2014 and AA2024 were used during the present work. This work has been performed in the two major stages. In the first stage, the performance of the conventional FSW joint was studied and compared with the tungsten inert gas (TIG) weld joint; and then the effectiveness of various approaches such as parameter optimization, water cooling, reinforcement of the external particles on improvement of the mechanical properties and corrosion resistance of the conventional FSW joint of AA2014 was investigated. In the second stage, an indigenously designed stationary shoulder tool was developed for the FSW of the AA2014 and AA2024 aluminium alloys. Effectiveness of the stationary shoulder FSW on the improvement of the microstructure, mechanical properties and corrosion behaviour was investigated. The scientific evidences were collected using optical microscopy, FESEM, TEM vi and XRD analysis to support the results of mechanical properties and corrosion behaviour. Corrosion behaviour was studied using potentiodynamic polarization test (Tafel), electrochemical impedance spectroscopy (EIS), and immersion test. The weld thermal cycles were recorded using K-type thermocouples at 5 mm and 10 mm distance away from the weld centerline. The results obtained from the experimental investigations carried out in first stage revealed that the microhardness and corrosion resistance of the FSW joint were better than the TIG weld joint. Peak temperature measured during the FSW joint was significantly lower than the TIG weld joint. The heat-affected zone exhibited lower hardness and lower corrosion resistance than the nugget zone (NZ) in both types of weld joints. Area fraction of precipitates in FSW joint was found to be higher (11.67%) than the TIG weld joint (5.77%), which in turn improved the microhardness of the FSW joint. Thereafter, the FSW of AA2014 was performed using various rotational and traverse speed parameters. The optimum parameters to produce a sound weld joint with maximum mechanical properties and corrosion resistance were identified as 708 rpm and 931 rpm at the traverse speed of 41 mm/min. Subsequently, the feasibility of other approaches namely water cooling and external particle reinforcement during the FSW to improve the performance of the joint was investigated. Initially, water cooling was used during the FSW of AA2014 aluminium alloy. The water was supplied at the rate of 0.15 l/min behind the rotating tool. The performance of the water-cooled joint was compared with the naturally air cooled FSW joint and the parent metal (AA2014). The water cooled joint exhibited better mechanical properties and corrosion resistance as compared to the naturally air cooled FSW joint. The average size of the precipitates present in the NZ of the water cooled FSW joint and naturally air cooled FSW joint was 0.46±0.08 μm and 0.59±0.09 μm, respectively. In case of the water cooling, the fast cooling rate resulted in finer precipitates, finer α-al matrix grain size, and narrower HAZ than air cooled joint. The effect of the reinforcement of the boron carbide (B4C) powder particles in the NZ during the FSW of AA2014 was studied. A novel approach was used for the reinforcement of the powder particles. A slurry comprising the powder particles and acetone was pasted on the vii abutting surface of the plate to be joined by the FSW. The reinforced FSW joint exhibited better corrosion resistance and tensile strength as compared to the conventional FSW joint (without reinforcement). However, the reinforcement of B4C particles in the FSW joint reduced the ductility. Boron carbide particles are expected to offer the pinning effect at the grain boundaries during the tensile test, and increased the tensile strength of the FSW joint. Additionally, the powder particles also interrupted the continuity of the precipitates arranged at the grain boundaries, which is expected to have disturbed the flow of corrosion current at the grain boundaries, and thus improved the corrosion resistance of the FSW joint. Based on the aforementioned study, it was observed that the FSW using a conventional tool needs additional steps to reduce the width of the heat-affected zone of the joint. The rotating shoulder in the conventional FSW tool is a major source of the heat, which in turn produces a wide HAZ. Therefore, an indigenous tool with a stationary shoulder was designed for the FSW. In case of the stationary shoulder friction stir welding (SSFSW) tool, a drawback related to the accumulation of material in the body of the stationary shoulder was reported in the literature. The accumulated material has been reported to enter through the gap between the rotating subshoulder and stationary shoulder. The indigenously designed tool was able to remove the penetrated material continuously during the welding without any additional efforts. In this study, the metallurgical, mechanical, and corrosion behaviour of the SSFSW joint of AA2014 was investigated. The stationary shoulder FSW lowered the width of the HAZ (more than 50%) as compared to the conventional FSW joint. The heat input during the SSFSW was more localized in and around the nugget zone of the weld joint. However, in case of the conventional FSW, the heat is expected to spread over the large area owing to the large rotating shoulder. The size of the precipitates and α-Al grains observed in the SSFSW joint was finer than the conventional FSW joint. The SSFSW joint exhibited higher average microhardness, tensile strength, and corrosion resistance as compared to the conventional FSW joint. The corrosion potential of the HAZ of the SSFSW joint and conventional FSW joint was -643 mV and -664 mV, respectively, suggesting higher corrosion resistance of the HAZ of the SSFSW joint than the conventional FSW joint. The impact toughness of both types of weld joints was marginally different; however, both weld joints showed higher impact toughness than the parent metal. viii The post-weld heat treatment (T4) of both types of the FSW joints helped to recover the mechanical properties. The post-weld heat treatment homogenized the microstructure of the weld joints, however the abnormal grain growth (AGG) was also observed in the nugget zone. The size of the abnormally grown grains of the SSFSW joint was significantly finer than the conventional FSW joint. Further, the capability of the stationary shoulder tool to join other alloy systems was also examined by performing the FSW of AA2024, which is a high strength alloy with huge industrial applications. The efficacy of the stationary shoulder tool was studied by the execution of the FSW of AA2024 aluminium alloy at a wide range of speed parameters. It produced defect-free weld joints at all the parameters studied during present research work. The ability to develop sound weld joints at a wide range of parameters is an important characteristic of this tool. The SSFSW joint produced at 931 rpm and 13 mm/min exhibited maximum ultimate tensile strength (382±12 MPa). Thus, the SSFSW joints exhibited better mechanical properties and corrosion resistance as compared to the conventional FSW joints. The presence of the fine precipitates in the weld joint resulted in the higher strength and better electrochemical behaviour as compared to the coarse precipitates. Since the presence of the coarse particles and wide HAZ in the FSW joint of the precipitation-hardenable aluminium alloys are owing to the high heat input during the FSW; the stationary shoulder FSW tool can be effectively used to eliminate such problems.