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Ti-6Al-4V is an alloy increasingly used in aeronautics due to its high mechanical properties coupled with lightness. An effective technology used to manufacture titanium components with a reduced buy-to-fly ratio is laser beam welding. Previous studies showed that the key factor that rules the mechanical properties and the fatigue life of the joint is its morphology. The aims of this paper were to investigate the influence of the geometrical features of the joints (height of the top and root reinforcement, depth and radius of the underfill, and the valley-valley underfill distance) on their mechanical properties and also to conduct a finite element (FE) analysis on the real geometry of the welded joints. Ti-6Al-4V rolled sheets 3.2 mm thick were welded in butt joint configuration using a laser source and their performance was studied in terms of weld morphology, microstructure, Vickers microhardness and fatigue life. A full factorial plan, designed varying the welding speed and laser power, was carried out. The real geometry and then the joint morphology were studied through an innovative approach: for each specimen, both the total weld face and the total root surface were acquired using a confocal microscope. Finally, through these acquisitions, the clouds of points of the scanned surfaces were used in order to carry out a FE analysis capable of providing a stress concentration factor, K , value for each detected joint. The main results are the realization of a reliable FE model by an experimental agreement and the relationship found amongst the fatigue performances and some noticeable metallurgical and geometrical features, such as the underfill depth and the aspect ratio defined as the ratio between the maximum height of the joint and the valley-valley underfill distance. [ABSTRACT FROM AUTHOR]

The interest for natural fibers as reinforcement in composites is growing for a large number of products also for aeronautic applications, where the flame retardant requirement is fundamental. In this research, ammonium polyphosphate was incorporated as flame retardant into hemp/epoxy biocomposites produced by infusion technology. Cone calorimeter, vertical burning, dynamic mechanical analysis and bending tests were performed in order to study fire and mechanical behavior of the produced biocomposites with different percentage of ammonium polyphosphate. The results showed a decisive improvement of the flame properties; no-effect on their mechanical properties and on the technological feasibility of the process were observed. Therefore, the addition of ammonium polyphosphate enhanced flame retardancy of hemp/epoxy-resin biocomposites without sacrificing their mechanical properties. [ABSTRACT FROM AUTHOR]