ORIGINAL ARTICLE Friction spot stir welding of polymers: control of plunging force F. Lambiase 1,2,3 & A. Paoletti 1,2 & A. Di Ilio 1,2 Received: 13 August 2016 /Accepted: 10 October 2016 # Springer-Verlag London 2016 Abstract This study investigates the influence of the plung- ing force in friction spot stir welding of polycarbonate sheets on the mechanical behavior of the welds. Experimental tests were carried out by varying the tool geometry and the applied plunging force. Mechanical tests based on single-lap shear tests were carried out for mechanical characterization of the welds. Thus, the morphology of the welds was analyzed to clarify the influence of the plunging force on geometry and defects of the welds. According to the achieved results, the control of the plunging force allows improving the mechanical behavior of the welds up to 37 % without requiring for addi- tional energy during the welding process or affecting the pro- cess production time. The increase of the weld strength is due to the reduction of porosities developing at the interface be- tween the stirred zone and the surrounding material. However, excessive plunging force results in weaker welds due to ex- cessive thinning of the punch-sided sheet. Under optimal con- ditions, the shear strength of the welds was 34.5 MPa that yields that of the base material. Keywords Friction stir welding . Friction spot stir welding . Welding . Polymers . Material testing . Thermoplastic . Joining . Force control . Plunging force 1 Introduction The wide employment of lightweight structures in transporta- tion industries is aimed at reducing vehicles weight, fuel con- sumption, and consequently improving the overall perfor- mances. To this end, materials other than metals such as poly- mers and reinforced polymers are being increasingly adopted due to their high strength-to-weight ratio. However, joining such materials requires special processes in order to prevent from stress concentration, to improve static and dynamic be- havior, and to avoid material damage and distortions. Different processes are available for joining thermoplastics including hot gas welding, speed tip welding, extrusion welding, hot plate welding, induction welding, injection welding, ultrasonic welding, friction welding, solvent welding, adhesive bonding, and mechanical fastening. However, these processes involve inefficient use of (welding) energy, specialized workers, significant environ- mental impact, surface pre-cleaning, etc. Thus, in the recent years, a number of newer processes have been developed based on combined thermo-mechanical joining including fric- tion spot joining, friction-based staking, and laser welding. Friction spot joining [25] allows to easily weld metal parts to polymers by heating the metal part by friction by means of a simple rotating tool. The process exploits the thermal conductivity of the metal part to let the frictional heat diffuse towards the metal-polymer interface allowing the achieve- ment of welding. However, this process involves more diffi- culties for welding polymer-to-polymer since low thermal conductivity of thermoplastics as compared to metals. Friction-based stacking requires extensive pre-working on the parts being joined: drilling a hole in the upper part and the presence of a stud, which will be thermoformed by means of a rotating tool [1], in the other part. Laser transmission welding (LTW) and laser-assisted direct joining (LADJ) * 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, Italy 2 CIRTIBS Research Centre, University of Naples Federico II, P.le Tecchio 80, 80125 Naples, Italy 3 Monteluco di Roio, 67040 L ’Aquila, Italy Int J Adv Manuf Technol DOI 10.1007/s00170-016-9586-0