Influence of Cooling Intensity Difference Between Upper and Lower Surface to the Residual Stress Distribution of 7050 Aluminum Alloy Plates

Residual stress of aluminum alloy thick plates causes distortion in subsequent machining operations for aircraft components. During spray quenching the different cooling rates applied across the plate thickness to produce residual stresses. If differential cooling is applied to the upper and lower surfaces, this could lead to an asymmetric distribution of residual stress on both sides of plate. This in turn causes potentially uncontrollable part distortion and dimensional inaccuracy during the machining operation. In this study, a finite element (FE) model, taking into account of the different cooling intensity on the upper and lower surfaces, was established to predict the residual stress. Residual stress was measured using an ultrasonic stress measuring apparatus. The results showed that the residual stress on the upper surface was −174 MPa when the applied water flux was 350 m³/h, and −194 MPa on the opposite surface where the corresponding water flux was 300 m³/h. This was found to be in good agreement with the simulation results. It was found that the residual compressive stress on the surface with the higher cooling intensity was smaller than the side with lower intensity which is contrary to conventional wisdom. Therefore, based on the difference of cooling intensity on the two surfaces, the guidance for utilizing the roller-quenching-equipment can be derived finally.