3.1 Microstructure Characteristics of Tube with Gradient Grain
Fig. 2 shows the transverse microstructure grain size distribution of the tube. Figs. 2(a)–(c) show the microstructures of the spinning tube after annealing and holding at 450, 400, and 350 °C, respectively, for 1 h, in the annealed state. Fig. 2(d) shows the initial thickness of the 6063-T4 aluminum alloy tube microstructure. The grain size distribution of the initial tube microstructure was relatively uniform, and the microstructure of the spin-annealed tube exhibited a gradient distribution. Thus, the grains closer to the outer surface of the tube are thinner, whereas the sizes of those closer to the inner surface of the tube are comparable to the initial tube size.
Fig. 2(e) shows the grain size distribution of the initial tube and the gradient grain tube along the thickness direction. The grain size of the initial tube along the normal direction was between 50–110 μm, and the distribution was relatively uniform; the zone with a thickness grain size less than the initial thickness grain size minimum value of 50 μm is considered to be the refining zone. When annealed at 450 °C for 1 h, the grain size near the outer surface of the tube was approximately 40 μm. The fine grain region was 105 μm from the surface to the outer surface. The grain size beyond 500 μm near the outer surface exceeded the grain size of the initial tube. Abnormal grain growth may have occurred because of the high annealing temperature or long holding time. The grain size of the tube annealed at 400 °C for 1 h was approximately 25 μm from the outer surface. The fine grain region was from the surface to 470 μm from the outer surface, and the grain size was similar to that of the initial tube beyond 500 μm from the outer surface. The grain size of the tube annealed at 350 °C for 1 h was approximately 20 μm from the outer surface. The fine grain region was from the surface to 570 μm from the outer surface, and the grain size was similar to that of the initial tube beyond 760 μm from the outer surface. Therefore, the spinning tubes were annealed at 400 and 350 °C, and the grain size was refined to 50 μm with a fine thickness of 470 μm and 570 μm, respectively.
3.2 Mechanical Properties Under Biaxial Stress
Bulging experiments on three types of normal gradient grain tubes were carried out, and the true stress–strain curves and mechanical properties were obtained under the biaxial stress state. Fig. 3(a) shows an image after the bulging of the normal gradient grain tube. The maximum expansion rate of the bulging tube was measured—the gradient grain tube with a refined thickness of 105 μm had the largest expansion rate of 18%; the gradient grain tube with a refined thickness of 470 μm had an expansion rate of 17%; and the gradient grain tube with a refined thickness of 570 μm has the smallest maximum expansion rate of 10%. The larger the refinement thickness of the gradient grain tube, the poorer is the plasticity and the lower is the expansion rate.
Fig. 3(b) shows the true stress–strain curves, and Fig. 3(c) shows the effect of normal gradient grains on the mechanical properties of the tubes. The gradient grain tubes with refined thicknesses of 105, 470, and 570 μm exhibited yield strengths of 60, 68, and 86 MPa, tensile strengths of 79, 89, and 106 MPa, and work-hardening indexes of 0.19, 0.20, and 0.17, respectively. For a normal gradient grain tube with fine outside and coarse inside grains, the greater the thickness, the higher is the strength of the tube; the lower the work-hardening index, and the poorer is the ductility.
In summary, the strength of the tube with a refined thickness of 105 μm was too low, and the maximum expansion rate of the tube with a refined thickness of 570 μm was just 10%, which was too poor in terms of plasticity. Therefore, a tube with a refined thickness of 470 μm and which exhibited both strength and plasticity would be more suitable for the hydroforming of aluminum alloy tubular parts.
3.3 Microstructure Evolution
Fig. 4 shows the kernel average misorientation (KAM) and distribution of misorientation of the normal gradient grain tubes at an equivalent strain of 12%. Figs. 4(a–c) show the KAM and grain boundary distributions of the microstructure of the tubes with a refined thickness of 105, 470, and 570 μm, respectively. Different colors represent the different KAM values. The larger the KAM value, the greater is the degree of local deformation. The blue area represents the smallest degree of plastic deformation, where no plastic deformation occurs. The green area represents less plastic deformation, the yellow area represents a greater degree of plastic deformation, and the red area represents the greatest degree of plastic deformation. At the area bulged to an equivalent strain of 12%, the sum of the proportional scores of red, yellow, and orange in Figs. 4(a–c) were 0.172, 0.219, and 0.284, respectively. This shows that the degree of deformation was most uniform for the tube with a refined thickness of 105 μm during bulging. However, the deformation degree of the tube with a refined thickness of 470 μm was more uniform. The degree of deformation was concentrated for the tube with a refined thickness of 570 μm, and the deformation coordination was poor—that is, the greater the refined thickness, the worse is the plasticity.
Observing the change in the number of grain boundaries in the figure, the black line represents the large-angle grain boundary (15–180°)—that is, the grain boundary—and the white lines represent small-angle grain boundaries (2–15°)—that is, the intragranular dislocations. Fig. 4(d) is a statistical diagram of the orientation angle between the grains of the microstructure. In the same area which bulged at the equivalent strain of 12%, the proportions of small-angle grain boundaries were 0.41, 0.48, and 0.54 for the refined thicknesses of 105, 470, and 570 μm, respectively. The greater the proportion of small-angle grain boundaries, the more opportunities for pinning dislocations during intragranular deformation, the higher the strength of the tube. As a result, the higher the strength, the poorer is the plasticity, as in Fig. 4(a–c).