i INVESTIGATION ON FRICTION STIR WELDING OF DISSIMILAR ALUMINIUM ALLOYS AA 7075 AND AA 1200 BUTT JOINTS BY ATTAH, Ileh Benjamin PhD/SEET/2016/910 THESIS SUBMITTED TO THE POSTGRADUATE SCHOOL, FEDERAL UNIVERSITY OF TECHNOLOGY, MINNA, NIGERIA IN PARTIAL FULFILMENT OF THE REQUIREMENT FOR THE AWARD OF THE DEGREE OF DOCTOR OF PHYLOSOPHY (PhD) IN MECHANICAL ENGINNERING (INDUSTRIAL& PRODUCTION) OCTOBER, 2021