Structure and mechanical behavior of aluminum alloy 1570C processed by multidirectional forging with decreasing temperature

The effect of two-step isothermal multidirectional forging (MF) with decreasing temperature (325 → 250°C) on the structural and mechanical behavior of aluminum alloy 1570C (Al-5Mg-0.2Sc-0.08Zr) was studied. The first-step temperature (325°C) was chosen due to the optimal balance between dynamic recrystallization and grain growth, while the second one (250°C) was limited by the solvus temperature of the brittle β-phase (Al3Mg2) to avoid its undesirable precipitation and embrittlement. The first MF step produced an ultrafine-grained structure (≈2.0 μm, 78 % HABs). Subsequent deformation at 250°C refined the structure to ≈1.5 μm, increased strength (YS + 25 MPa, UTS +15 MPa, HV +10) with approximately 33 % elongation, and no β-phase was detected; however, thermal stability was reduced, with the onset of abnormal grain growth shifting from >450°C to 425°C. The two-step processed material exhibits dual superplastic behavior: at ≥450°C, superplastic performance deteriorates and elongation is limited to ≈1700 % (vs. 2800 % after 325°C forging); however, at around 400°C, high-strain-rate superplasticity (≈1000 % at up to 10−1 s−1) surpasses the 325°C-forged state (750 % at 5.6×10−3 s−1). This is attributed to enhanced dislocation density accelerating diffusion and dynamic recrystallization; however, at >450°C, the same feature causes abnormal grain growth, revealing the dual role of pre-stored defects in superplasticity. Comparison with subsequent rolling data showed that two-step MF provides a gain in sheet strength (up to 540 MPa) but significantly suppresses superplasticity. Thus, forging at 250°C may be considered a critical regime; further temperature decrease could risk structural instability and embrittlement. This route is recommended for bulk products or high-strength sheets when high-temperature processing is not required.

1. The work was carried out under the state assignment of IMSP RAS. Experimental studies were performed using the facilities of shared services center of IMSP RAS -