Microstructure Distribution Characteristics of High-Strength Aluminum Alloy Thin-Walled Tubes during Multi-Passes Hot Power Backward Spinning Process ()
1. Introduction
High-strength aluminum alloy thin-walled tubes (HSATs) with the merits of high strength, excellent corrosion, and light weight, are widely used in aerospace, aviation, and weapons [1]. This type of tube can be manufactured by the multi-pass hot power backward spinning (MPHPBS) process because it can eliminate casting defects, refine microstructure and improve the plasticity of the tube [2]. However, MPHPBS is a complex and unsteady state-forming process coupled with multi-field, multi-dies, and multi-factor characters [3]. During the spinning process, the material of the tube experienced complex uneven deformation and microstructure evolution [4]. And this complicates microstructure evolution affects the tube forming quality as well as the performance [5]. Therefore, it is of great significance to study the microstructure evolution characteristics of the whole hot power backward process [6].
In this paper, microstructure distribution characteristics of high-strength aluminum alloy thin-walled tubes in the multi-passes hot power backward spinning process are analyzed based on the FE model. The achievements can provide a basis for the microstructure optimization, performance prediction, and control of the power spinning process of tubes [7].
2. Research Program
2.1. FE Model
A 3D coupled thermo-mechanical FE model (see Figure 1) coupled with a microstructure evolution model for the MPHPBS process of 7075 aluminum alloy tubes was established under the ABAQUS environment based on the solution of the key FE modeling technologies, such as geometric modeling, material modeling and loading boundary conditions [1].
2.2. Material Property
In this paper, the raw material used in the simulation is a semi-continuous cast
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Figure 1. The 3D elastoplastic finite element model of the multi-pass hot power backward of 7075 aluminum alloy tubes.
tube of 7075 aluminum alloy. The mechanical properties are seen in Table 1, and the Fields-Backofen model is used to represent the relationship (see Equation (1)) among the stress, strain and temperature during the MPHPBS process [8].
(1)
2.3. Characteristic Cross-Section
To analyze the microstructure distribution characteristics in circumferential direction of the spun tube, along the axial direction of the tubes from the root to the end, the seven characteristic sections of the equal proportion sections, such as S0, S1, S2, S3, S4, S5, and S6 (as shown in Figure 2) are selected. To analyze the microstructure distribution characteristics along the axial direction of the spun tube with different spinning passes, the axial symmetry section are selected (as shown in Figure 3) [9].
3. Results and Discussion
3.1. Dynamic Recrystallization Volume Percentage (RVF) Distribution Characteristics
From the Figure 4, it can be seen that under different spinning passes, dynamic
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Table 1. Mechanical properties of as-cast 7075 aluminum alloy.
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Figure 2. Characteristic sections along the axial direction of the tubes.
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Figure 3. Axisymmetric section of the tubes.
recrystallization only occurs in the spinning deformation zone, while there is almost no dynamic recrystallization phenomenon in the unspun area. With the increase of spinning passes, the dynamic recrystallization trend of deformation areas of the tubes gradually increased. Beside the first spinning pass, the dynamic recrystallization phenomenon is obvious in all other passes. In addition, with the increase of spinning process, dynamic recrystallization is occurred in the outer surface of the tube firstly, and then extended to the inner surface. But, the dynamic recrystallization in the inner surface has become a main zone.
3.2. Dynamic Recrystallization Grain Sizes (RGSs) Distribution Characteristics
Dynamic recrystallization grain sizes (RGSs) distribution characteristics are shown in Figure 5. From Figure 5, it can be seen that under different spinning passes, larger areas of dynamic RGS mainly distributes in the spinning deformation zone and near the root area of the tube. With spinning process progressing in the first spinning pass, large areas of dynamic RGS mainly distributes in the outer surface of the tube and in this area the dynamic RGS uniformly distributes; while in the inner surface, the areas of dynamic RGS is smaller and non-uniform distributed. With the increase of spinning process, dynamic recrystallization is occurred in the outer surface of the tube firstly, and then extended to the inner surface. The inner surface recrystallization area and grain size gradually increase. And the dynamic recrystallization in the inner surface has become a main zone.
3.3. Dynamic Average Grain Sizes (AGSs) Distribution Characteristics
Dynamic average grain sizes (AGSs) distribution characteristics are shown in Figure 6.
From Figure 6, it can be seen that under different spinning passes, the dynamic AGS at the end and the root of the tube are the large. The fine grain zone is mainly distributed in the spinning deformation large area and near the root zone of the tube. The fine grain zone is concentrated in the first, second and fourth passes and dispersed in the third pass. With the increase of spinning passes, the fine grain zone is distributed in the outer surface of the tube firstly, then extended
to the inner surface, and finally the inside surface is a main surface.
4. Conclusion
The dynamic AGS and the dynamic RGS are mainly distributed in the spinning
deformation zone and near the root area of the tube. The dynamic RVF occurs in the spinning deformation zone. And the dynamic RVF, the dynamic RGS, and the fine grain zone are extended from the outer surface to the inner surface and finally, the inner surface is the main surface. With the increase of passes, the dynamic RVF and the dynamic RGS gradually increase. The fine grain zone shows a trend from concentration to dispersion and then to concentration. And the fine grain zone gradually transfers to the root of the tube.
Acknowledgements
The authors would like to thank the National Natural Science Foundation of China (No. 52165051 and 51665041), the Primary Research and Development Program of Jiangxi Province (20202BBEL53010), the Major Science and Technology Specific Projects of Jiangxi Province (20194ABC28001) for the support given to this research.