This paper presents an investigation of welded joints strength properties in aluminum-magnesium alloys produced with a friction stir welding technology. Welded joints have been produced under various conditions with specific key parameters for each condition, such as velocity of rotary tool translational motion and clamping force applied to assembled elements. Mechanical tests are performed using a static tension approach, and curves of strain hardening and evaluations of strength under critical strain and fracture conditions are obtained. Additionally, fracture surface structures of samples with various welded joints are analyzed and compared with structures of samples without welded joints. It is revealed that there are significant differences in deformation behavior of aluminum-magnesium alloys produced under different conditions of a friction stir welding. These differences are caused by structure defects (pores, micro cracks, etc.) at the welding area. The structure defects are responsible for changes in fracture mechanisms and decrease of strength properties. The obtained results appear to be promising for welded joints diagnostic methods development.