PSI - Issue 39

M.A. Sánchez Miranda et al. / Procedia Structural Integrity 39 (2022) 161–172 Author name / Structural Integrity Procedia 00 (2019) 000–000

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years different materials such as: polymers, conventional and unconventional metallic alloys, wood and a combination of all these dissimilar materials have been tested [2-8]. Some important advantages of this joining technique, compared to the traditional ones, are: solid state process, low amount of generated heat, non-polluted atmosphere (due to absence of toxic fumes, shielding gases and filler materials), low distortion and resulting residual stresses, reasonable cost, good resulting mechanical properties, reduction of post-processing tasks. At the beginning of this joining technique, the aluminum alloys were the main tested materials [9-12]; nevertheless, in the last years a broad of materials have been incorporated, including dissimilar materials. Recent investigations for the friction stir welding of aluminum alloy and polymers have pointed out some important findings [ 13 ] : the main bonding mechanisms were located at the macro micro-nano scales, combined with physicochemical reactions, the effect of Van der Walls forces and the formation of a thin oxide layer at the interface polymer-metal; the maximum FSW efficiency reported in these investigations was 60 – 70% compared to polymer substrate. Since all these factors are related to energy production and dissipation under this joining technique, the process parameters, such as: rotational and transverse speeds, tool design and tool tilting angle, affect the resulting mechanical and physical properties. The dissimilar thermoplastic joining: Acrylonitrile Butadiene Styrene (ABS), and high-density polyethylene (HDPE), has been improved with the addition of multiwalled carbon nanotubes (MWCNTs) [ 7 ] . The content of MWCNTs improves the tensile strength, the elongation and microstructure of welded joint, which are associated with the increase on ductility; nevertheless, hardness is reduced at the same time. The multiwalled carbon nanotubes play the role of micro-bonding elements, increasing ductility and attaining 65% of the yield strength of the parent HDPE. Friction stir welding in metals and polymers present some main differences related to material flow and thermomechanical effects [14]: in metals the welding process results from the driven flow volumes induced by the pin and shoulder into the thermo mechanical affected zone (TMAZ); whereas in polymers most of the pin driven flow volume is dragged into the TMAZ, the shoulder driven material is largely melted and evacuated from the welding zone (mobile shoulder). Another main difference between metals and polymers joined by FSW, is the formation of important discontinuities at the retreating side in polymers and at the bottom of welded nuggets; these results are related again to the evacuation of melting driven flow volumes, propelled from the pin and shoulder. Investigations on thermoplastic polymer joining using FSW has been developed recently [15 - 18]; the main findings are as follows: polymers with high viscosity and high melting temperature require high rotational speed and low advanced speed to achieve good mechanical properties; differences in pin profile lead to different material flow; a set of rotational speed and tool traverse speed is observed to be the optimum for the resulting mechanical properties of welded parts. Fixed shoulder can reduce the fiber breakage of welded materials and defects such as material peeling. For polymers presenting low thermal conductivity, the pre-heating of welding parts can improve the resulting mechanical properties. Concerning the friction stir joint of ABS and PP, no references have been found by the authors; some attempts have been devoted to the joining of ABS sheets [19], in which the process parameters for the best resulting tensile strength were: high rotation speed combined with shoe temperature and low traverse or advance speed. Other investigations were oriented to the friction stirs joins of PP sheets [16], revealing that square pin with high ratio dynamic/static volume yielded better mechanical properties, compared to cylindrical pin. In this work is addressed the dissimilar joining of thermoplastics: ABS and PP, for which no previous investigations are available and present potential applications in car and aeronautical industries. 2. Materials and experimental procedures. 2.1 Polymeric materials used for FSW. The main mechanical and physical proprieties of commercial and tested polymers are listed in Table 1 and Table 2, respectively.