Effect of quenching cooling rate on residual stress and microstructure evolution of 6061 aluminum alloy

In this study, the cooling rate was manipulated by quenching with water of different temperatures (30, 60 and 100 °C). Surface and internal residual stresses in the quenched 6061 aluminum alloy samples were measured using hole-drilling and crack compliance methods, respectively. Then, the processability of the quenched samples was evaluated at cryogenic temperatures. The mechanical properties of the as-aged samples were assessed, and microstructure evolution was analyzed. The surface residual stresses of samples W30°C, W60°C and W100°C is −178.7, −161.7 and −117.2 MPa, respectively along x-direction, respectively; and −191.2, −172.1 and −126.2 MPa, respectively along y-direction. The sample quenched in boiling water displaying the lowest residual stress (∼34 % and ∼60% reduction in the surface and core). The generation and distribution of quenching residual stress could be attributed to the lattice distortion gradient. Desirable plasticity was also exhibited in the samples with relatively low quenching cooling rates at cryogenic temperatures. The strengthes of the as-aged samples are 291.2 to 270.1 MPa as the quenching water temperature increase from 30 °C to 100 °C. Fine and homogeneous β″ phases were observed in the as-aged sample quenched with boiling water due to the clusters and Guinier-Preston zones (GP zones) premature precipitated during quenching process.