Effect of Aging Process on Strengthening Phase Particles of 6056 Aluminum Alloy for Bolts

The distribution and morphology of second phase particles in 6056 alloy wires were analyzed by phase diagram calculations, scanning electron microscopy (SEM), and energy dispersive spectroscopy (EDS). Hardness and tensile tests were conducted to study the variations in hardness and the mechanical properties of 6056 aluminum alloy under different aging processes. In addition, the types and sequences of precipitates in the alloy during optimal aging were observed using transmission electron microscopy (TEM), three-dimensional atom probe (3DAP), and differential scanning calorimetry (DSC). The results show that a lot of second phase particles, composed of spherical, short rod and irregular plate and strip, are distributed within the matrix of 6056 alloy wires. Under the optimal aging process (540 °C for 1 h solid solution followed by aging at 180 °C for 4 h), the 6056 aluminum alloy reaches a hardness of 136 HB and a tensile strength of 415.5 MPa. The main precipitates in the alloy matrix are identified as short rod-shaped β′′ phases, plate-shaped Q′ phases and spherical Guinier–Preston (GP) zones. The transformation of GP zones into β′′ phases and subsequently into Q′ phases occurs gradually by altering the ratio of Al, Mg, Si, and Cu during aging. The content of Cu atoms increases progressively during the growth of the precipitates, and the precipitation sequence is GP zones → β′′ phases → Q′ phases → Q phases.