## 3 Results and Discussion
### Stress Reduction Mechanism  
The inhomogeneous deformation induced from the temperature gradient during the cooling process is the main reason for the formation of quenched residual stress. It is related to the geometry, elasticity modulus, heat conductivity coefficient, expansion coefficient and microstructure of the component. Compressive residual stress may be generated on  
Figure 11: Longitudinal residual stress distribution of the cutting surface after different processes: (**a**) quenching; (**b**) stepwise cold pressing; (**c**) artificial aging.  
the area with low yield strength and large plastic strain after unloading the force. The heat exchange of the T-type rib is quite different to that of the plate during the quenching process, thus resulting in different residual stress distribution.  
When the quenched T-type rib is subjected to cold pressing, plastic strain first initiates on the surfaces of the component due to its large residual stress after the quenching stage. Then, the strain is penetrated into the center zone of the component. As shown in Figure 7, the inhomogeneous strain between the surface and the center zone of the component is induced during the cold pressing. This will generate a new distribution of the residual stress, which is tensile stress on the surface and compressive stress on the center zone. It is interesting that the induced residual stress distribution in the cold pressing is contrary to that in the quenching stage. The superposition of the residual stress in the quenched and cold pressed stage will decrease the value of the residual stress, thus achieving the reduction of residual stress.  
As shown in Figure 8, the distribution of the residual stress in the integral cold pressing is more uniform than that in the stepwise cold pressing, especially for the smooth area of the T-type rib. The stress reduction at the stiffened area is similar for the integral and stepwise cold pressing as the strain variation at the stiffened area is quite heterogeneous, as shown in Figure 7. Nevertheless, there is an overlap zone at the smooth area with the adjacent pass of the stepwise cold pressing. The strain at the overlap zone is quite different from that at the non-overlap zone, thus resulting in the inhomogeneous distribution of the residual stress and heterogeneous stress reduction. Therefore, less passes of the cold pressing should be adopted to decrease the inhomogeneity of the residual stress within the capacity of the cold pressing machine.  
The stress reduction mechanism of the artificial aging mainly consists of stress relaxation and the effect of second phases. The yield strength of the material decreases with the increased temperature, and plastic strain is generated when the residual stress exceeds the decreased yield strength. This is the mechanism of stress relaxation, and it contributes to the stress reduction during the aging process. In addition, precipitation  
Figure 12: Comparisons between FEM and experimental results: (**a,b**) for quenched stress along path ab and cd, respectively; (**c,d**) for cold pressed stress along path ab and cd, respectively.  
decomposition of the supersaturated solid solution will be generated in the initial stage of the aging treatment. The second phases of the GP zone and the strengthening phase \(\eta^{\prime}(\text{MgZn}_{2})\) may be precipitated. The compressive residual stress induced from the previous process decreases the lattice constant of the matrix and the tensile residual stress will be generated around the \(\eta^{\prime}\) phase. There is a superimposed effect between the high elastic stress field around the second phases and the residual stress generated in the previous processes of the component, thus decreasing the residual stress during the aging process.