MECHANICAL ENGINEERING MS THESIS DEFENSE BY DENİZHAN AKBAL

Speaker: Denizhan Akbal

Time: January 10, 2023, 11:00

Thesis Committee Members:

Assoc. Prof. Arif Karabeyoğlu (Advisor, Koç University)

Prof. Demircan Canadinç (Koç University)

Prof. İbrahim Özkol (İstanbul Technical University)

Abstract:

In this study, Al6061-T6 plates are welded together with ‘Friction Stir Welding’ (FSW) method. Experimentations are performed within a design of experiments scheme. A full factorial design with three parameters such as; welding speed, rotational speed, and downward force at three levels (maximum, mean, minimum) with 27 design points is employed.

Half of the manufactured plates are then subjected to artificial ageing process resulting as a total of 54 different experiment points. Manufactured plates are cut into tensile specimens according to ASTM E8 standards. Tensile tests are performed in order to find out three mechanical properties: Yield Stress, Ultimate Tensile Stress, and Strain at UTS. Un-aged yield stress, un-aged UTS, aged yield stress, and aged UTS values were employed as responses to polynomial surrogate models taking process parameters (welding speed, rotational speed, downward force) as variables. Response surface methodology is then used to estimate the model constants associated with the singular or combinative effects of process parameters on measured responses. Model qualities for each response is measured by ANOVA analysis and are reported. With a very high level of confidence surrogate models with R2= 0.92 was obtained for aged UTS values.

Metallographic analysis is also performed to visualize selected experimentation points to determine the shape of formed weld zones namely, weld nugget zone (WNZ), thermos-mechanically affected zone (TMAZ), and heat affected zone (HAZ) as well as the amount of base material prior to the processing in corresponding zones.