Formability limits and process window based on fracture analysis of 5A02-O aluminium alloy in splitting spinning

Abstract

Splitting spinning or rotary flow splitting is an advanced forming process for manufacturing axisymmetric integrated parts with bifurcated features and unique characteristics of highperformance and low-weight. During the process, under the kinematic effects of mandrel rotational movement and roller radial feed, plastic deformation occurs accompanied usually by undesirable fracture, which reduces the formability limit (FL). In this study, the kinematic effects on the FL of a 5A02-O aluminium alloy in the splitting spinning process were systematically investigated by finite element simulation based on a modified Lemaitre criterion and physical experiments. The results show that at a given roller feed speed or mandrel rotational speed (forming speed), the FL has a nonlinear relationship with forming speed, which increases firstly and then decreases. With the increase of forming speed, the maximum FL decreases, which appears at the larger forming speed. These variations of FL show that there exists a combined effect of the roller feed speed and mandrel rotational speed, thus a ratio between them, named as the roller feed ratio, is then used to investigate FL. It is found that there exists a critical roller feed ratio of approximately 2 mm/rev, independent of the speeds of roller and mandrel. Below this critical value, the FL increases with the roller feed ratio. While over the critical value, the FL decreases. In addition, the decrease of FL becomes more remarkable with the increase of mandrel rotational speed. Furthermore, the variations of stress triaxiality and tensile plastic strain were analyzed to see their effects on FL. The analyses show that the decrease of tensile plastic strain with the increasing roller feed ratio is dominant the increase of FL below the critical roller feed ratio value. The increase in the stress triaxiality is dominant in the decrease of FL when the roller feed ratio is over the critical value in combination with not too high forming speed, whereas both increases are dominant in the decrease of FL when it is over the critical roller feed ratio value in combination with high forming speed. Based on the kinematic effects of mandrel and roller, the process windows of the splitting spinning process were obtained to improve the FL. It is found that under the condition of the roller feed ratio within 1-2.5 mm/rev, the mandrel rotational speed within 8-100 rev/min and the roller feed speed within 0.5-4 mm/s are helpful to get high FL values. The experiments were carried out to verify the prediction on the FL and the process window. The research provides an in-depth understanding of FL and its affecting factors, and thus lays a basis for process optimization and process parameter configuration.

Fracture prediction in spin forming of anisotropic metal sheets

“…It can be observed that the equivalent plastic strain increases gradually along the path, and the value of equivalent plastic strain in the spin forming process is very large, far exceeding the fracture strain in uniaxial tensile test (about 0.52). Meanwhile, according to the study of Zhan et al 5 and Li et al, 45 the stress triaxiality fluctuates significantly and a material point is not only undergoes compressive stress state due to rolling of the rollers, but also the tensile stress state due to pulling of the rollers in the process, which is a typical plastic deformation process with a wide stress triaxiality range. To characterize the average stress state of material points during the spin forming process, an average stress triaxiality g 29 in equation ( 7) is adopted.…”

“…This is because not only this kind of aluminum alloy spun parts are widely used in aerospace engineering, and the high hardening rate makes the 2219 sheet easy to crack during the spin forming process, but also the material points experience complex deformation history during the process. 5 Therefore, this section mainly introduces the selected 2219 aluminum alloy sheet, the establishment of constitutive model, parameter determination in the constitutive model, and the FE model construction of the spin forming.…”

“…Parameters a, c, h and p can be calculated by the Lankford coefficients as Barlat and Lian. 13 In addition, the associated flow rule is used to describe the relationship between the normal direction of the yield surface caused by the stress change and the plastic strain increment de p , as equation (5).…”

“…The fracture strains in simple loadings could be easily presented in the above fracture loci space of g; e f ð Þ , while the material point undergoes a complex loading historiy 5,45 during the spin forming process and the fracture strains change drastically with the changable stress triaxiality, which made the damage accumulation in the process more complicated. To clarify the damage accumulation law in the spin forming process, the proportion of the accumulated damage w under different stress triaxiality ranges in the process was studied first, as shown in Figure 11.…”

“…Among the coupled DFCs, Zhan, et al 3 applied the Lemaitre damage model to predict the fracture of the LF2M aluminium alloy 1 State Key Laboratory of Solidification Processing, School of Materials Science and Engineering, Northwestern Polytechnical University, Xi'an, China 2 Shaanxi Key Laboratory of High-Performance Precision Forming Technology and Equipment, School of Materials Science and Engineering, Northwestern Polytechnical University, Xi'an, China 3 Laboratory of Aero-engine High-Performance Manufacturing, School of Mechanical Engineering, Northwestern Polytechnical University, Xi'an, China sheet during the spin forming process and found that the model can well predict fracture in the process. Furthermore, to distinguish the different damage accumulations under positive and negative stress triaxialities, Zhan, et al 5 applied a modified Lemaitre damage model in the split spin forming process of the 5A02-O aluminium alloy blank and found it can well predict the fracture position of the spun part. The results of Li et al 6 and Wang et al 7 showed that the initial GTN model can't well predict the fracture in the spin forming process due to neglecting the effect of stress triaxiality and shear deformation on damage accumulation.…”