MATERIALS Figure 2. Tool for form tapping [2]. In form tapping, the maximum depth of the thread and the maximum thread pitch are the most important parameters (Figure 2). A form tapping is characterized by: 1) the entrance zone, at the tool tip, which is a tapered geometry where the lobes progressively rise the nominal diameter (ISO 8830); 2) the cylindrical part, which acts as a suppor- ting system, guiding the tap during threading operation. The maximum depth of thread is signi cantly higher than in the case of conventional threading (cutting) tools, being limited by the quality of the coolant and the tool length. The maximum pitch of the thread depends on the material properties of the workpiece. Indeed, not all the materials are suitable for form tapping; due to the intrinsic deformation involved, the material should have a minimum ductility (minimum failure resistance at 5%) and should not exceed a maximum mechanical resistance of 1,400 N/mm2. Another important aspect is the in uence of the pre-machining dia- meter. Too small diameters will lead to excessive rolling phenomena and forces during the process. Inversely, if this diameter is too large, the core section is not enough rolled and the core diameter will be too small. Regarding to possible changes in the material, due to deformation mechanisms, work hardening may arise too in the affected zone. This phenomenon is accentuated the greater the wear. Additionally, since there is crushing and deformation, internal compressive stresses appear which are advantageous to enhance the mechanical resistance of the thread. As in the previous section, a test series was designed to select the most suitable process parameters for consistent thread through holes generated by drilling, followed by trials of rolling friction. To do this, the same clamping system used for form drilling was employed (Figure 3). Friction Stir Welding The program of experiments was carried out on the FSW specialized machine of ISIM Timisoara (Figure 9), having the following main tech- nical characteristics: aAdjustable welding speed in the range: 10 - 480 mm/min; rotational speed of the welding tool in the range: 300-1,450 rpm; maximum vertical thrust force 10 kN; power of the motor for the FSW tool rotation 4 kW. To perform the experiments, this equipment was completed with the devices required to measure the thrust force Fz of the tool on the base metals, as well as a thermographic camera for measuring the temperature in the welding zone at a distance of a few millimeters behind the FSW tool. These devices are shown in the gure 4. The components of the FSW equipment are the following: 1 - Vertical Positioning servomotor; 2 - Vertical force transducer; 3 – Servomotor for FSW tool rotation; 4 - Command panel; 5 - Electrical panel; 6 - Thermographic camera; 7 - Force and temperature recorder; 8 - FSW tool; 9 – Thermocouple; 10 – Fixture jig; 11 - Welding plates; 12 - Equipment frame; 13 - Translation Table; 14- Translation servomotor. Figure 5. FSW tools; a. Threaded cylindrical pin; b. Flat conical pin; c. Pin with 4 at bevels. The shape and dimensions of the welding tool were chosen based on the following requirements [4]: - Obtaining an optimal temperature for good plasticization of the mate- rial to be welded; - Establishing the adequate geometry and sizes of the welding tool pin to provide optimal helical motion of the material ow to form the nugget; - Thickness of welding materials. Welding tools are made of steels X38CrMoV5 (AISI H11, DIN1.2343) and X40CrMoV5 (AISI H13, DIN 1.2344) for tools with threaded cylin- drical or at conical pin, respectively P20S sintered tungsten carbide for tools with conical smooth pin or pin with four at bevels / pyramid trunk) [4]. The maximum working temperature of the tool depends on the type of steel used to make them: those quenched in oil or water can be used up to 500 – 600 °C. The P20S material has a hardness of 1,500 HV. It is a wear resistant material, maintaining its properties even at 1,200 – 1,300 °C. Figure 3. Form tapping of a 5754 aluminium alloy sheet on a 50 x 50 steel tube. 40<<