Numerical Simulation And Experimental Investigation On The Forming Of Ultra Thin-walled Elbow Of 6061 Aluminum Alloy With Internal Pressure

Considering the application of ultra thin-walled elbows in the aerospace industry, ultra thin-walled elbows of 6061 aluminum alloy were investigated to explore manufacturing process and forming laws of seamless ultra thin-walled pipes. The basic mechanical properties and formability parameters were obtained by means of uniaxial tensile test, free bulging test, finite element simulation software DYNAFORM to construct a better correlation constitutive equation of elbows. Moreover, the effect of process parameters on the forming quality in hydro bending process was also studied. Furthermore, we analyzed the causes of some defects in the hydro bending experiments and put forward corresponding solutions. Eventually, the Φ50.5 mm × 0.5 mm seamless ultra thin-walled tubes of 6061 aluminum alloy were manufactured according to the requirement of the aviation manufacturing handbook.Firstly, the mechanical property parameters and anisotropy coefficients of ultra thin-walled and thin-walled tubes of 6061-O-state, W-state, T6-state were obtained by uniaxial tensile test. Four types of constitutive equations were established to describe the stress-strain curves of uniform plastic deformation stage. It was found that the power-exponent function of constitutive equation possessed the highest correlation coefficient. And the material models of finite element simulation were based on the power-exponent function of constitutive equation.Secondly, combining free bulging test and adaptive solving software for pipe forming property, the forming property parameters of thin-walled tubes were obtained more accurately. Especially for the thin-walled pipe of practical forming 6061-W-state aluminum alloy, the forming property parameters of proximal real property(K = 327, n = 0.298) were used to make the material models more practical. Thus, the practical forming results were predicted more exactly in the finite element simulation and the specific forming experiments were guided effectively.Then, the process of ultra thin-walled tubes by hydro bending and the effects of some important parameters on forming quality were investigated by the material models and numerical simulation software. The results showed that the pipes of 6061-W-state aluminum alloy had the best forming quality in the hydro bending simulation. The cross-section distortion rate reduced with the inner pressure uprising. And the optimal inner pressure of the pipes was 1.4 MPa before free bulging of tubes. The medial folds of elbow were alleviated significantly with the increase of bending pitch. And the effects were not obvious when the pitch was over 500 mm. The forming limit of ultra thin-walled pipes was modified and the thinning rate was reduced with the sinusoidal loading. The forming quality and limit of the hydro bending tubes were improved remarkably with the increase of wall thickness.Finally, seamless ultra thin-walled tubes were manufactured by means of hydro press machine. The results indicated that the biggest press amount of the ultra thin-walled pipes on hydro bending was 35 mm and the resilience amount of the ultra thin-walled pipes was 5 mm. The forming quality of pipes was ameliorated obviously by the restraint system in hydro bending process. The marks of the tubes made by the restraint system were relieved remarkably by polyurethane pads. Excessive thinning or cracking on the ultra thin-walled tubes would appear if the inner pressure or press amount was too large. Furthermore, the cross-section distortion rate and wall thinning rate of ultra thin-walled tubes of 6061 aluminum alloy were both bigger than those of thin-walled tubes. The higher cross-section distortion rate of ultra thin-walled tubes was easier to appear in the experiment than in the simulation.