Enhanced assessment of technological factors for Ti-6Al-4V and Al-Cu-Mg strength properties

OBRABOTKAMETALLOV MATERIAL SCIENCE Vol. 23 No. 4 2021 a b Fig. 4. Evolution of parameters as a function of the stress amplitude for VT6 ( Ti-6Al-4V ) samples with “1” and without a hole “2”; a) evolution of the temperature and the plastic mean axial strain; b) evolution of the temperature and the plastic axial strain amplitude average temperature ( ∆ T m ) appear at a stress amplitude above 245 MPa, for smooth samples – at 348 MPa, respectively (with p xm  = 0.015 %). This observation is consistent with the known softening effect of medium fatigue stress on metals, which is usually estimated from Haigh diagrams. At a stress amplitude of 348 MPa (Fig. 4, a ), the average component of irreversible longitudinal strain ( ) p xm  and temperature increment ( ∆ T m ) are 16 and 10 times greater, respectively, for samples with a concentrator than for samples without a concentrator. It can also be seen from (Fig. 4, b ) that the presence of a concentrator for a sample made of VT6 alloy leads to the fact that the amplitude of plastic axial strain has a higher value for a sample with a hole. When the stress amplitude ( σ a ) is close to 400 MPa, the amplitude of irreversible strain ( ) p xa  increases by 0.017 % for samples with a stress concentrator. The presence of a stress concentrator during periodic deformation of samples by a symmetric stress cycle with an average component leads to a decrease in the critical stress, at which irreversible processes are activated, by 30 %. This observation is consistent with the mathematical calculation presented in section « Comparison of experimental data and results of mathematical simulation ». To answer the question about the effect of the stress concentrator on dissipative heating and the average (amplitude) value of deformation in the cycle (stepping of the plastic hysteresis loop), a sample of D16 aluminum alloy with and without a weld was loaded according to a similar program (Fig. 3) with an average stress in cycle ( σ m ) and the maximum amplitude of the cycle stress ( σ a max ) equal to 167 MPa. Fig. 5 shows that in a sample of D16 material with a stress concentrator (weld), an increase in the average plastic strain (stepping of the plastic hysteresis loop) and heating caused by plasticity occur much earlier than in smooth samples (without a stress concentrator). We remind that these results correspond to a fi xed average stress σ m = 167 MPa. Thus, it can be seen from (Fig. 5, a ) that for D16 samples with a weld, the average values of irreversible plastic strain ( ) p xm  and the average temperature increment ( ∆ T m ) appear at a stress amplitude above 80 MPa, for smooth samples – at 130 MPa, respectively (with p xm  = 0,02 %). The increase in the average and amplitude components of temperature occurs with a minimum discrepancy up to a stress amplitude of 100 MPa. At a stress amplitude of 130 MPa (Fig. 4, a ), the average component of irreversible longitudinal deformation ( ) p xm  and temperature increments ( ∆ T m ) is 30 and 2 times higher, respectively, for samples with a weld than for samples without a weld.