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The residual stress distribution for two strategies of asymmetric quenching in Al-Zn-Mg-Cu aluminum alloy plates has been simulated using the finite element method. The results show that for asymmetric quenching between the upper and lower surfaces, the through-thickness asymmetric quenching residual stress distribution lies between the two distributions corre-sponding to the heat transfer coefficients on the upper and lower surfaces respectively. The surface and central stress magnitudes are equal to the average of the stress magnitudes corresponding to the two heat transfer coefficients. For asymmetric quenching of a single surface, the surface stress distribution is the same as the heat transfer coefficient distribution and the stress magnitude is equal to the stress magnitude corresponding to the average value of the heat transfer coefficients at each location. However, the center quench residual stress distribution is approximately uniform and the stress magnitude is equal to the average of the stress magnitudes corresponding to the maximum and minimum heat transfer coefficients.

Al-Zn-Mg-Cu aluminum alloys are widely used in the aircraft industry because of their high specific strength, specific stiffness and toughness, good machinability and excellent corrosion resistance [

The quench residual stresses of aluminum alloys Al-Zn-Mg-Cu have been studied both by experimental methods and numerical simulations [

In this paper, two strategies of asymmetric quenching in an Al-Zn-Mg-Cu aluminum alloy plate were simulated using finite element method. The distribution of surface and central residual stresses and the magnitude of the two asymmetric quenching strategies are discussed in detail. The relationship between residual stresses of asymmetric and uniform quenching is disclosed. In addition, a finite element model of residual stress quenching has been verified by neutron diffraction.

The material used in this study was a 27 mm thick Al-Zn-Mg-Cu hardened aluminium alloy plate with the composition (wt%) of Zn-7.99, Mg-1.90, Cu-2.20, Zr-0.11, Fe-0.056, Si-0.0037, rest-Al. The plate had dimensions of 270 mm (RD, rolling direction) × 81 mm (TD, transverse direction) × 27 mm (ND, normal direction). Neutron diffraction measurements were performed on a neutron reactor (CARR) and the Al {311} planes were chosen as the diffraction plane. The measurement points were distributed in the normal direction from the centre of the body to the centre point of the surface and four points with an equal interval of 2 mm were measured.

A sequential coupling of the temperature field and the stress field was applied in this simulation work, which was carried out by the ANSYS software. First, the temperature evolution and distribution were calculated through the boundary conditions and the heat transfer coefficient. Then, the results of the thermal simulation were used as initial conditions in the stress field simulation. Finally, the stress field result was obtained. An elastoplastic model was used in the study and bilinear kinematic quenching was used to describe the strain quenching.

The material used in the simulation work was an Al-Zn-Mg-Cu aluminum alloy plate. The starting temperature of the quenching process was 475˚C and it was assumed that there was no stress in the plate at the beginning. The water temperature was 20˚C. The material properties of the Al-Zn-Mg-Cu aluminum alloy used in the FEM analysis were obtained by experimental tests and are described in ref. [

Heat transfer coefficient distribution. A series of average heat transfer coefficients were used in this study to quantitatively simulate the asymmetric distribution of heat transfer coefficients on the surfaces of the hardened plate. The average heat transfer coefficients are 26,000 (20˚C), 20,500 (40˚C), 17,500 (60˚C) and 9500 (80˚C) W∙m^{−2}∙K^{−1}, which are taken from the literature [^{−2}∙K^{−1} at the centre line to 5000 W∙m^{−2}∙K^{−1} at the edge (by linear interpolation).

Verification of the finite element model by the neutron diffraction method. To illustrate the efficiency and accuracy of the finite element model, the residual stress distribution across the thickness of the plate hardened at 20˚C was measured by the neutron diffraction method, and the experimental results were compared with the simulation results. However, due to the limited testing time of the neutron diffraction method, only a few points in the center of the plate were measured, which was difficult to obtain by other methods. The comparison of the results is shown in

this study was applicable for the prediction of the asymmetric distribution of the quenching residual stress.

Asymmetric heat transfer coefficient distribution within two surfaces. Since the longitudinal and transverse components of the residual stresses are almost identical in large plates [

Asymmetric heat transfer coefficient distribution within a single surface.

Initial stress | Average stress | Stress (asymmetric) | |
---|---|---|---|

20˚C & 40˚C | −192.4 | −173.2 | −178.3/−169.3 |

−153.0 | |||

20˚C & 60˚C | −192.4 | −166.0 | −170.5/−165.0 |

−139.6 | |||

20˚C & 80˚C | −192.4 | −135.5 | −145.7/−141.0 |

−78.6 |

during uniform quenching. The quenching process can be assumed to be an approximately one-dimensional heat transfer process through the direction normal to the surface region. Due to the rapid cooling of the surface region, the temperature gradient difference between adjacent regions cannot be completely neutralized. Therefore, the corresponding residual stress magnitudes can be stabilized after cooling down. It can therefore be concluded that the surface hardening residual stresses for asymmetric quenching maintain the initial stress magnitude of the heat transfer coefficient at each location along the transverse direction.

^{−2}∙K^{−1} is 10 MPa. Based on the simulation results in

Initial stress | Average stress | Stress (asymmetric) | |
---|---|---|---|

20˚C & 40˚C | 120.3 | 108.8 | 108.9 |

97.3 | |||

20˚C & 60˚C | 120.3 | 101.2 | 101.4 |

82.0 | |||

20˚C & 80˚C | 120.3 | 75.2 | 76.2 |

30.0 |

When the top and bottom surfaces of the plate are in the asymmetric quenched state, the through-thickness asymmetric quench residual stress distribution lies between the two distributions corresponding to the heat transfer coefficients on the top and bottom surfaces respectively. The surface and center stress magnitudes are equal to the average of the stress magnitudes corresponding to the heat transfer coefficients of the two surfaces.

When the surface is quenched asymmetrically, the surface stress distribution is the same as the heat transfer coefficient distribution and the stress magnitude is equal to the stress magnitude corresponding to the average of the heat transfer coefficients at each location. However, the central quenching residual stress distribution is approximately uniform and the stress magnitude is equal to the average of the stress magnitudes corresponding to the maximum and minimum heat transfer coefficients.

This study was financially supported by National Key R & D Program of China (No. 2020YFF0218203, 2020YFF0218202) and Youth Fund Project of GRINM (G12620203129012).

The authors declare no conflicts of interest regarding the publication of this paper.

Li, Y.N. and Shi, G.H. (2021) Residual Stress Distribution of Asymmetric Quenching in Al-Zn-Mg-Cu Aluminum Alloy Plate. Journal of Materials Science and Chemical Engineering, 9, 11-18. https://doi.org/10.4236/msce.2021.94002