6.1 Plastic Pressure
The effect of forming pressure on forming quality was studied, and the hydraulic pressure was linearly loaded to 140, 160, 180, and 200 MPa for analysis. The loading time was 0.18 s, and the friction coefficient between the tube fitting and the mold was 0.12. Figure 37 shows the distribution cloud of torsional beam wall thickness under different forming pressures. As can be seen from the figure, when the internal pressure reaches 140 MPa, the straight edge of the trapezoidal region of the torsion beam fits on the surface of the mold and is basically formed. However, the filling effect of the rounded corner area on the upper edge is not ideal. When the internal pressure is 160 MPa, the filling amount of transition zone and the rounded corner region of the torsional beam increases, and the thinning region gradually concentrates on the transition region between the rounded corner and the straight edge. When the internal pressure exceeds 180 MPa, the wall thickness of the torsion beam is not affected by the forming pressure, indicating that the mold has stayed in its original position under the action of pressure.
The wall thickness distribution of each measuring point under different forming pressure and the variation of wall thickness between different forming pressure were analyzed. The wall thickness of each section was measured at 64 points in the F-F section. The F-F section with the largest wall thickness difference is taken as the research object, and the distribution of points is shown in Figure 38. The wall thickness distribution of F-F segment is shown in Figure 39. Under different forming pressure, the thickness distribution of the straight side wall is unchanged, but the thickness of the rounded corner is different from that of the straight side transition zone. In the range of 140–180 MPa, the thickness of the transition zone near the rounded corner and the straight side increases. When the internal pressure is 180 and 200 MPa, the wall thickness is 2.76 and 2.74 mm, respectively. The wall thickness distribution of the two is basically the same, indicating that when the forming pressure of the torsional beam exceeds 180 MPa, the F-F section has been coated.
6.2 Loading Path
The minimum wall thickness of the torsion beam after hydroforming is located in the trapezoidal region, which is 2.76 mm, and the thinning rate is 21.14%. The area is in danger of breaking up. The effects of the feed rates of 50, 100, 150 and 200 mm/s on the quality of the forming parts were studied with the left and right axial feed rates as the research object. The loading path is shown in Figure 40. Figure 41 shows the wall thickness distribution of torsional beam at different feed speeds. When the feed speed increases from 50 to 150 mm/s, the minimum and maximum wall thicknesses of the torsional beam increase simultaneously. When the feed rates are 50 and 100 mm/s, the wall thinning is concentrated in the transition region between the small rounded corner and the straight edge of the trapezoidal zone. When the feed speed is 150 mm/s, the minimum wall thickness is located near the inside of the V-shaped zone and the transition zone, and the minimum wall thickness increases. When the feed speed is 200 mm/s, the end is wrinkled and cannot be flattened in the late pressure process. The results show that the forming quality is better when the feed speed is 150 mm/s and the minimum wall thickness δmin is 3.12 mm.
Figure 42 shows the curves of minimum wall thickness, maximum wall thickness, maximum thinning rate and maximum thickening rate under different coaxial feed speeds. It can be seen from the figure that with the increase of axial feed speed, the minimum wall thickness, maximum wall thickness and maximum thickness increase ratio of torsional beam all show an upward trend. The maximum thinning rate decreases with the increase of feed speed. Without axial feed, the minimum wall thickness δmin increases from 2.76 to 3.12 mm, and the maximum thinning rate η decreases from 21.14% to 7.71%. Figure 42 is the curve of minimum wall thickness, maximum wall thickness, maximum thinning rate and maximum thickening rate under different coaxial feed speeds. It can be seen from the figure that with the increase of axial feed speed, the minimum wall thickness, maximum wall thickness and maximum thickness increase ratio of torsional beam all show an upward trend. The maximum thinning rate decreases with the increase of feed speed. Without axial feed, the minimum wall thickness δmin increases from 2.76 mm to 3.12 mm, and the maximum thinning rate η decreases from 21.14% to 7.71%.
6.3 Friction Coefficient
In the process of hydraulic forming of torsional beam, the tube fitting is in direct contact with the mold, the relative motion between the tube fitting and the mold is in dry friction state, and the friction coefficient is significant. If lead, lubricating oil and graphite are coated on the contact surface between the tube and the mold, the friction will be transformed into a mixed friction state of fluid friction, boundary friction and dry friction, which effectively reduces the friction coefficient and enhances the metal fluidity. The contact surface between the mold and the tube fitting is coated with soft metal lead for solid lubrication, and the friction coefficient is 0.08–0.2. The contact surface is lubricated with synthetic oil and the friction coefficient is 0.04–0.1. Therefore, the five friction coefficients μ of this section are 0.04, 0.08, 0.12, 0.16 and 0.2, respectively. The effect of non-friction coefficient on forming quality was studied under the conditions of axial feed speed of 150 mm/s and forming pressure of 180 MPa. The forming results of torsion beam under different friction coefficients are shown in Figure 43. With the increase of friction coefficient, the maximum wall thickness of torsion beam increases gradually, while the minimum wall thick-ness decreases gradually. When the friction coefficient is μ = 0.2, the wall thickness of the transition zone between the straight side and the rounded corner of the V-shaped zone decreases sharply until the tube breaks.
Figure 44 shows the wall thickness distribution of F-F section under different friction coefficients. When the friction coefficients μ are 0.04 and 0.06, the wall thickness is low and uniform. When the friction coefficient continues to increase to 0.2, the wall thickness reaches the highest. The area affected by the friction coefficient is the transition area between round corner 1 and straight side 2, straight side 4 and round corner 4, because in the clamping stage, the round corner of the mold directly acts on the right side 2 and the right angle side. If friction increases, the flow of matter would be hindered. Therefore, the wall thickness is affected by the biaxial tensile stress and is severely thinned. Therefore, reducing the friction coefficient is beneficial to improve the friction between the tube and the die, so as to reduce the inhibition effect of the die on the tube material flow and increase the amount of material flowing into the rounded corner area.