3.1 Effect of the end face angle of middle punch
Figure 10a shows the experimental result that the initial position of the middle punch with the inclination angle of 0 is far away from the tube blank. The left side of the branch tube is broken, and the contact area between the top of the branch tube and the middle punch is small. Analysis of the reason: Under the combined action of the left and right punch feeding and the booster pump, the tube blank is in a free bulging stage, the internal pressure of the tube blank is unstable, and the initial position of the middle punch is far away from the tube blank, the top of the branch tube can not get the acting force of the punch, so branch tube are prone to excessive thinning and consequent cracking.
Figure 10b shows the experimental result that the initial position of the middle punch with the inclination angle of 0 is closer to the tube blank. The contact area between the top of the branch tube and the punch is not complete, and a large cracking occurs on the left side of the branch tube. Analysis of the reason: In the free expansion stage of the tube, the branch tube has not yet been formed, and the middle punch cannot be pushed, causing the right side material to flow between the punch and the transition fillet of the die, and the left side of the branch tube cannot be replenished. The middle punch hinders the blank material flow. The left side of the punch is still far from the tube blank. As the internal pressure of the tube blank increases, the left side of the branch tube cannot get the acting force of the punch, which eventually leads to the cracking of the branch tube.
Figure 11 is the experimental result of the 13.5° middle punch. It can be seen from the figure that the forming quality of the branch tube is good, the contact surface between the punch and the branch tube is smooth, and the left guide zone is wrinkled.Reason for analysis: In the free expansion stage of the tube, the height of free bulging required for even contact between the end face of the 13.5° middle punch and the branch tube is lower than that of the 0° middle punch.so the branch is subjected to the acting force of the punch more quickly during the forming process, which prevents the branch from cracking. Finally, through the compound action of the left and right punches, the middle punch and the internal pressure, qualified parts are formed.
As shown in Fig. 12, It can be seen that the height difference between the left and right heights of the 0° middle punch in the mold cavity is 27 mm, which is higher than the 15° middle punch. The left angle between the 0° middle punch and the die cavity is 90°, which is less than the 15° middle punch. Because the forming smaller the angle in hydroforming, the higher the internal pressure required. Therefore, using the 0° middle punch to complete the initial expansion of the branch tube requires a higher internal pressure than the 15°middle punch, The higher internal pressure will result in a small hump on the right side of the brunch tube, which will restrict the flow of material. Therefore, 0° middle punch is more likely to cause cracking of the branch pipe with large transition fillet in Y-tube.
3.2 Effect of middle punch pressure
As shown in Fig. 13a, When the back pressure of the punch is 5MPa, the contact surface between the top of the branch tube and the punch is relatively flat, and the cracking occurs on the left side of the top of the branch tube. However, when the back pressure of the punch is 7MPa, the branch tube is formed well as shown in Fig. 11. Because the acting force of the 5MPa back pressure is too small, it is not enough to suppress the expansion velocity of the branch tube, so the material flow velocity cannot keep up with the expansion velocity of the branch tube, resulting in the cracking of the left side of the branch tube.
As shown in Fig. 13b, when the back pressure of the punch is 9MPa,the top of the branch tube is flat, but the left side of the branch tube is obviously sunken. The acting force of 9MPa back pressure is too large, resulting in the branch tube can not push the middle punch. Then, under the action of the left and right punch feeds, the material moves inward to form a dead wrinkle.
4.1 Effect of lubrication
As shown in Fig. 14ab, the top of the branch tube were cracking whenMoS2 and PTFE films were used in the half-ring area. The reason is that the coefficient of friction is large, which causes the material flow velocity decreased. Even though the middle header suppresses the expansion velocity of the branch tube, the material flow velocity still cannot keep up with the expansion velocity of the branch tube, resulting in the cracking of branch tube.The experimental results of scheme #1, #2, and #4 are shown in Fig. 14. The branch tube height of Scheme #1 is smaller than that of scheme #2, and the branch tube height of scheme #2 is smaller than that of scheme #3.It can be concluded that the smaller the friction coefficient in the expansion zone is, the more favorable the branch tube forming is.
As shown in Fig. 14d, when MoS2-PTFE joint lubrication is used for the half-ring area, and dry friction is used in other areas, the surface of the Y-shaped tube has no defects, and the height of the branch tube meets the requirements; As shown in Fig. 14cef, when MoS2 and PTFE film are used in other areas, the main tube area occurs wrinkling phenomenon. It can be concluded that when the friction coefficient of the non-bulging area is small, the material flows faster, and it is easy to cause annular wrinkles under the ladder loading path.
In order to more obviously explore the effect of differential lubrication method on the quality of hydraulic forming of Y-tubes, the wall thickness distribution is plotted as shown in Fig. 15. The minimum wall thickness is 1.35 mm and the maximum thinning rate is 10%. The maximum wall thickness is 2.82mm, and the maximum thickening rate is 88%. The wall thickness of the back zone of the Y-shaped tube is evenly distributed, and both ends are slightly thicker than the middle because of the initial length of the tube blank. The wall thickness distribution of the branch tube is smooth, without large local fluctuations In summary, the use of differential lubrication method can not only increase the material flow velocity in the branch tube area to prevent cracking, but also reduce the material flow velocity in other areas to prevent wrinkling in the main tube.