Issue 37

G. Cricrì et alii, Frattura ed Integrità Strutturale, 37 (2016) 333-341; DOI: 10.3221/IGF-ESIS.37.44 339 Contact pressure applied by metal powder to the horizontal wall of die is almost equal to that exerted on punches. This is a confirmation of correct ratio of punches’ displacements. Pressure on vertical walls of die varies from 290 MPa to 390 MPa (corresponding to about 50-70% of punches pressure), showing a trend almost independent of friction coefficient value. Instead, friction stress is not negligible and is strongly dependent of friction factor between powder and walls, reaching the maximum values of 57 MPa and 28 MPa when   = 0.1 and  = 0.05 respectively. 1 MN MX X Y Z 285 326.111 367.222 408.333 449.444 490.556 531.667 572.778 613.889 655 JUL 29 2011 16:50:43 NODAL SOLUTION STEP=6 SUB =105 TIME=6 CONTPRES (AVG) DMX =6 SMN =295.49 SMX =651.511 1 MN MX X Y Z -42 -31 -20 -9 2 13 24 35 46 57 JUL 29 2011 16:53:23 NODAL SOLUTION STEP=6 SUB =105 TIME=6 CONTSFRI (AVG) DMX =6 SMN =-42.066 SMX =56.115 Figure 6 : Contact pressure and friction stress, respectively, at final stage of compaction process, just after the loading phase (  = 0.1). 1 MN MX X Y Z 285 326.111 367.222 408.333 449.444 490.556 531.667 572.778 613.889 655 JUL 20 2011 11:38:57 NODAL SOLUTION STEP=6 SUB =105 TIME=6 CONTPRES (AVG) DMX =6 SMN =286.16 SMX =614.57 1 MN MX X Y Z -42 -31 -20 -9 2 13 24 35 46 57 JUL 29 2011 16:56:12 NODAL SOLUTION STEP=6 SUB =105 TIME=6 CONTSFRI (AVG) DMX =6 SMN =-19.619 SMX =28.728 Figure 7 : Contact pressure and friction stress, respectively, at final stage of compaction process, just after the loading phase (  = 0.05). The mechanical behaviour of a pressed product is function of density obtained during compaction process. Indeed, defects and failures may occur most probably in components with lower density. Moreover, for several formed elements, such as filters, the compaction level is closely connected to their correct operation. Figure 8 shows the plot of relative density  r in the final compaction scenario. The value of relative average density obtained in numerical simulations is equal to 1.075, corresponding to a final density  s = 7.88 kg/cm 3 . The bulk and Young’s moduli after compaction are K = 156 GPa and E = 189 GPa respectively. Around the corners between punches and die and in the zone of object fillets numerical analyses provide with maximum and minimum density, which are about -5.8% and + 3.5% of average density (with  = 0.1) respectively. Figure 9 depicts the variation of the minimum, maximum, and average density during the loading and unloading phases, for both considered friction coefficients. Average density is not significantly influenced by variation of friction coefficient, whilst the deviation of maximum and minimum density from the average is lower in the case with  = 0.05 respect to the simulation with  = 0.1. Specifically, the previously mentioned deviation of maximum and minimum density is equal to - 4.2% and +1.9% respectively.