STUDIES ON CONTROLLING THE STRUCTURAL INSTABILITY AND THEIR EFFECTS ON THE MECHANICAL PERFORMANCE

Abstract

Friction stir welding (FSW) of precipitation hardening Al-Zn-Mg-Cu aluminium alloy AA7075 was performed in butt joint configuration using varying welding speed and a constant rotary speed. The developed weld joints were subjected to visual inspection to reveal the presence of surface and subsurface defects, if any. Tensile tests were performed to determine UTS, % elongation and toughness. Microstructure was investigated using optical microscopy. Microhardness measurements across the transverse cross section of FSW joints were carried out to identify variations in microhardness in different zones. Process parameters influence the metallurgical as well as the mechanical properties of FSW joints. in order to produce sound welds free from any type of defects, the process parameters were optimized (Welding Speed 96 mm/min and Rotary Speed 635 rpm) on the basis of maximum tensile strength. With the increase in the transverse speed upto welding sped of 96 mm/mm, mechanical properties of the FSW joints increase, thereafter starts decreasing. Moreover, increase in welding speed shifts fracture location from I-IAZ on retreating side to l-IAZ on the advancing side. The optimum combination of process parameters was used to develop FSW joints so as to study on the effect of PWHT on the microstructure and mechanical properties of the friction stir welded aluminium alloys. EDAX was used to determine the distribution (weight %) of Zn, Mg and Cu in the base metal. Fracture surfaces after tensile tests have been investigated using scanning electron microscope (S EM) to investigate the mode of fracture. Solution heat treatment was used as a PWI-IT which influenced the microstructure and anatomy of a Al grains in the different zones developed by FSW by varying the heat treatment parameter i.e. time at a constant solutionizing temperature. At 30 min of solution heat treatment, the highest mechanical strength of FSW joints of AA7075 alloy was achieved followed by the PWHT time of 15 min, while heat treatment above 30 min showed AGG. On the other hand, solution heat treatment upto 60 rnin deteriorated the mechanical properties of friction stir welded joints of AA7075 aluminium alloys. Further, the location of the fracture as well as the fracture mode was also affected by PWIIT.