Issue 47

K. Hachellaf et alii, Frattura ed Integrità Strutturale, 47 (2019) 459-467; DOI: 10.3221/IGF-ESIS.47.36 465 Tab. 4 shows the tensile strength values and their effectiveness on the weld bead for different parameters and for the two tools with cylindrical and conical pins. The cylindrical pin tool achieves maximum efficiency of the joint strength respectively 14.94% and 6.07% for advanced speed 24 mm/min and 40 mm/min and in parallel for the conical tool profile has a value of 13.12% for a speed of 24 mm/min. The experimental results shown in Fig. 7 (stress-strain curves) for tensile tests with a strain rate of 0.01s-1 at room temperature are compared to those of the no welded test specimen. According to these results of the FSW welding on HDPE, it can be seen that these results are very satisfactory, especially the tools T1 and T2 for the two advanced speed 24 and 40 mm/min, where a remarkable improvement is observed caused by the temperature welding effect on the material that is triggered at the beginning of the tensile stress on the elastic zone (Fig. 7). from Fig. 7, we remarked also that the maximum stress Rm = 24.3 MPa became important, an improvement of the modulus of elasticity with respect to that of the no welded specimen which was of value Rm = 20.41 MPa, at the same time or we have a reduced plastic part noticing on the curves where the plasticity domain of the material (plastic side on the tensile curve) was lost, therefore the material has become rigid, the latter linked to several phenomena and several parameters influenced by the FSW welding operation and even the material as there are other parameters that play an important role, all due to the influence of the machined weld joint that changes the mechanical behavior of the FSW welded material. These experimental results obtained allowed us to better understand certain phenomena resulting for example the flow of the material around the tool. M ICRO HARDNESS TEST he Vickers micro-hardness test is done on a very sophisticated automatic FT-ARS 9000 laboratory mechanical engineering tester with a load of 500 gf for 10 seconds at room temperature [15]. The determination of the hardness value of the weld seams for comparing the results to those of the non-welded material, the micro-hardness was measured on the mid-thickness of the welded joints perpendicular to the welding axis are made (Fig. 8) with a step of 1 mm between the points measured, where the measurements are carried out symmetrically with respect to the main axis of welding. Figure 8: Profile of welded joint on half thickness. The results of the Vickers micro-hardness (HV) shown in (Fig. 9), made on the average thickness of two welded plates (lateral side) of the welded bead (Fig. 8 ) with a spacing of 1 mm ,Within a limited range of (-15 to +15 mm) from the center of the weld, variable hardness applies over 4 main thermal zones, the core zone (or nugget zone NZ), thermo- mechanical affected zone (TMAZ), heat affected zone ( HAZ) and base material (BM) which is not affected by the FSW welding process. The mechanical properties vary between deferent component zones of the welding. The micro-hardness profile exhibits an increase in all cases which reaches a maximum value at the core zone (NZ) which is the equi-axed zone is harder than the other zones, the curve is fairly symmetrically to the axis, ie the core zone is very hard relative to the base material zone BM. the micro-hardness decreases towards the less hard zone, the latter represents the normal state of the non-welded material, this zone is located outside the thermal deformation during welding, so it retains its original granular structure without change. According to [16, 17] the hardness value in the NZ zone should be higher than the other zones because of its structure. The distribution of microhardness is almost symmetrical with respect to the median weld line because the field plastic flow on both sides of the weld center is uniform [16, 18]. T