Invariant stress state parameters for forging upsetting of magnesium in the shell

OBRABOTKAMETALLOV MATERIAL SCIENCE Vol. 23 No. 1 2021 Fig. 2. The distribution of stress intensity setting the relative coordinates z/h = 0; 0.5; 1; r/R = 0; 0.5; 1. Here, z and r are the current coordinates along the radius and vertical axis; h and R are half the height and radius of the workpiece. At these points, the values of the stress mean parameter, which corresponds to the concept of average stress (Fig. 1, b ), and the stress effective parameter, which corresponds to the concept of stress intensity σ and (Fig. 2), are estimated. Ta b l e 1 Value of σ /T at P1 - P9 control points z/h r/R 0 0.5 1 1 –1.99 –1.75 –3.02 0.5 –1.86 –2.01 –1.24 0 –2,15 –1,83 –1,21 Naturally, stretching medium stresses up to +102 MPa are preferably applied in the shell (Fig. 1, b ), their presence creates a danger of destruction, which is why such a plastic material as copper is used here. The distribution of average stresses in a magnesium billet resembles a picture of a “transverse crack”, known from the theory of forging. Here, the zone of increased (modulo) values is stretched along the diagonals of the billet longitudinal section with a voltage of 280 MPa. With conventional forging, it is along the lines of the “transverse crack” that a split of the billet is possible. In this case, the high (modulo) values of the average normal stress give hope for an increase in the level of plasticity, which should prevent destruction. A different picture is observed for the distribution of stress intensity (Fig. 2): they decrease from the center to the periphery, i.e. in the direction of the free surface. The stress gradient is especially large at the interface between the billet and the shell, which is explained by the difference in the mechanical properties of the materials. In the zones of dif fi cult deformation adjacent to the centers of the ends of the punches, the stress intensity decreases, which is due to the lack of metal hardening in this area. The calculated data on the stress state indicator at the control points P1-P9 are presented in Table 1. Since the plasticity of metals decreases with an increase in the index σ /T, it follows from the table that the hazardous zones are adjacent to the points with coordinates z/h = 0 and r/R = 1. At the same time, the values of σ /T have negative values everywhere, i.e. compression stresses prevail. The table shows that the indicator σ /T can vary at this stage of processing within –1.21...–3.02. The highest modulo values at the level of –3.02 are characteristic of the peripheral points adjacent to the contact surface. In this case, the unfavorable (the lowest modulo) values are located in the area of the lateral surface convexity. The results obtained can be compared to the results of a cylindrical billet upsetting by smooth strikers without friction. In this case, the stress state indicator for the entire volume of the billet is known to be the same and equal