PSI - Issue 47

Mario A. Sánchez Miranda et al. / Procedia Structural Integrity 47 (2023) 310–324 Author name / Structural Integrity Procedia 00 (2019) 000 – 000

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1. Introduction Friction Stir Welding (FSW) was introduced in 1991 by The Welding Institute (UK) [1]. In the first stages aluminum alloys were the predominate tested materials; nevertheless, in the recent years a diversity of materials have been tested, such as : polymers, different metals alloys, wood, composites and a large combination of these materials. FSW uses a no-consumable rotating pin which transfers heat and energy to the two overlapping or butt joining pieces [6-12]. This joining technique presents some advantages in regard to other joining modalities, within others: the heat dissipation is controlled by the process parameters, no toxic fumes release, no filler materials, low residual stresses and distortion, low comparative cost, good resulting mechanical properties, reduction on post processing task, among the most important. Recent investigations using FSW on dissimilar joining of polymers and aluminum alloys they found important findings related to thermomechanical effects and the material flow [13-16]: the friction stir process in metals results from the driven volumes, promoted by the shoulder and pin tool in the thermo-mechanical affected zone (TMAZ); whereas in thermoplastics only the pin driven flow volume is dragged into this zone; in thermoplastics the shoulder driven material is frequently melted and evacuated from the welding zone. Furthermore, in metals are presented discontinuities at the retreating side of welding (RS), and at the bottom of welded nuggets. Such effects are related to the evacuation of melting driven flow volumes, generated by shoulder and pin motion. Some researches related to FSW on thermoplastics have been carried out in the last years [17 - 20], main conclusions are the following: thermoplastics presenting high melting temperature and high viscosity need low advance speed and high rotational speed to produce better mechanical properties; pin profiles, such as: cylindrical, conical, hexagonal, threaded, etc., induce different material flow during welding. In addition, a set of traverse speed and rotational speed, together with approach speed and tilt angle, leads to best results on mechanical properties of joints; the fixed shoulder can reduce defects of peeling and fiber breakage on welded specimens. For polymers presenting low thermal conductivity, the process can be improved using preheating, leading to sound welding and improvement in mechanical properties. The square pin profile with high ratio: dynamic/static volume, leads to better mechanical properties, compared to cylindrical pin, when joining PP plates [18]. Other works have been oriented to investigate the effect of rotation speed, traverse speed and the axial force, in order to assess the joining performance on acrylonitrile-butadiene-styrene (ABS) plates [21]; finding best results for advance speed of 50 - 200 mm /min, axial force comprised between 0.75 and 4 kN, with a tensile strength attaining a maximum value higher than 60% the tensile strength of parent materials. The high tensile strength was obtained when high rotational and transverse speeds are used. The FSW joint of plates of ABS were investigated using same experimental parameters as the previous mentioned, but this time with a stationary shoulder and applying pre-heating; the strength and tension properties of welded joins were evaluated and associated with to the morphology of the welded beads [22]. Concerning mechanical tests on thermoplastics UHMWPE-PP, plates of HDPE were investigated under flexural loading; revealing high dependency on the process parameters [23]: tool rotation speed of 1400 rpm, tool transverse speed of 25 mm/min and shoulder temperature of 100 ° C, were revealed as the best parameters for the resulting mechanical properties. The last process parameters lead to flexural strength of approximately 96% the flexural strength of the base material. In these researches, high rotational speed and lower tool transverse speed have conducted to increase on the flexural strength and reduction of size of welding defects. High-density polyethylene (HMWPE) joins, have been studied using a fixed shoulder and preheating temperature of 50 and 80 ° C [24]; the tensile strength was as follows: 72% of strength compared to base material for specimens without preheating, 89% of base material with a preheating of 50 ° C, and poor results for preheating at 80 ° C, due to excessive heat dissipation of the two combined process: preheating and heat dissipation during the friction stir welding [25]. Preheating may enhance thermal distribution inside the testing material, leading to less time of welding and improving the mixing of welding parts and mechanical properties. UHMWPE is a popular polymer, which has increased his industrial applications: it can replace metals alloys in some medical applications [26]. On the other hand, PP is the most used thermoplastic, due to its flexibility, durability, high tensile strength and high corrosion resistance, light weight and low manufacturing and maintenance costs [27]. Thermoplastics can be joined by different modalities, the most popular are: process involving heat conduction, radiation (electromagnetic radiation) [28], laser welding, and by mechanical friction under vibration and rotation [29]. This work is devoted to advance on the knowledge of joining of these two dissimilar thermoplastics, using friction stir welding.