ORIGINAL ARTICLE Effect of tilt angle in FSW of polycarbonate sheets in butt configuration F. Lambiase 1 & V. Grossi 1 & A. Paoletti 1 Received: 10 November 2019 /Accepted: 10 February 2020 /Published online: 17 February 2020 # Springer-Verlag London Ltd., part of Springer Nature 2020 Abstract The influence of the tilt angle on the quality of Friction Stir welds made on polycarbonate sheets is investigated, experimentally. Welding tests were conducted by varying the tilt angle and the welding speed to determine possible interactions between these process parameters. Temperature distribution, material flow, and processing loads were analyzed. The quality of the welds was assessed by means of mechanical characterization and morphological analysis. The results indicated that the processing loads and temperature increased when higher tilt angles were adopted. Under low welding speed (20 mm/s), this enabled to improve the mechanical behavior of the welds. On the other hand, high tilting involved different issues. The tensile strength of the welds was severely compromised as high tilting produced severe thinning of the weld seam. In addition, tilt angles higher than 2° involved great amount of material ejection. This material formed a coherent side flash, which required subsequent machining, and worsened the surface appearance of the welds. Keywords FSW . Polymers . Mechanical characterization . Morphology . Material flow 1 Introduction High performing materials such as thermoplastic polymers are increasingly employed for structural applications in several fields including automotive, aerospace, and aviation. These materials may offer various advantages, including high strength to weight ratio, optical transparency, high flexibility of manufacturing, recyclability, and possibility to be formed in very complex geometries by means of different processes in- cluding injection molding and even additive manufacturing processes. However, several applications require different parts to be joined to produce very complex or large compo- nents. Nowadays, thermoplastics are widely joined by adhe- sive bonding [1], mechanical fastening, and even welding processes. However, since the limitation and disadvantages involved by these processes, new solutions are required. Laser transmission welding [2], ultrasonic welding, and fric- tion stir welding are achieving great consideration for joining thermoplastics materials. These processes are also capable to produce hybrid joints between polymers and metals [3–7]. Among the others, FSW enables to achieve high mechanical strength with low production costs, low energy absorption, poor thermal stress, and easiness of automation. Although FSW of metals has been extensively investigated and it is now a consolidated process for industrial applications, FSW of polymers is quite different as polymers exhibit different thermo-mechanical behavior as compared with metals. This is due to great sensitivity and phase-transitions with tempera- ture, e.g., the transition from glassy to rubber like to softened/ molten states. In addition, thermal behavior greatly changes with temperature. Polymers are affected by thermal degrada- tion that can influence the performance of the welds. These thermal transitions occur within a range of temperatures (rath- er than a specific temperature) as the different lengths if the polymer chains. Many polymers are highly hygroscopic; this may require pre-heating treatments to avoid development of bubbles originating due to moisture content [8]. Despite metals, polymers are characterized by low thermal diffusivity, and this modifies significantly the temperature distribution, the material flow, and consequently the influence of the pro- cess parameters on the weld quality. For example, during FSW of metals, the increase in the welding speed induces a reduction of the temperature within the weld seam [9]. On the * F. Lambiase francesco.lambiase@univaq.it 1 Department of Industrial and Information Engineering and Economics, University of L ’Aquila, via G. Gronchi 18, Zona Industriale di Pile, 67100 L ’Aquila, AQ, Italy The International Journal of Advanced Manufacturing Technology (2020) 107:489–501 https://doi.org/10.1007/s00170-020-05106-2