The accurate range of internal high pressure forming technology should be hydraulic forming technology. Now the international popular to replace the original statement in hydroforming hydroforming, because hydroforming can more accurately describe the essence of the machining process, the pressure through the liquid transfer, effect on the machined surface of the workpiece to form the final torque, the workpiece to achieve the required size and shape.  Hydraulic forming (Hydro-forming) is divided into 3 types: shell hydroforming (Shell hydroforming); plate hydroforming (Sheet hydroforming), the general pressure in 600~800MPa; hydroforming (Hydroforming), generally refer to the United States tube hydroforming (Tube hydroforming), or high pressure in Germany, Europe (Internal High Pressure Forming forming). The forming process is simple by pressure device to put some pressure on the closed pipe in the cavity filled with liquid, the liquid has high internal pressure and flow, forcing the flow tube wall to the inner shape of the forming, the highest pressure can reach 4000MPa.  Internal high pressure forming technology   the use of internal high-pressure forming process T type pipe joints as an example to illustrate the basic principle of internal high pressure forming process. In the special hydraulic press used in the high pressure forming process, a double sided mold capable of processing multiple workpieces at the same time is provided. Each end of the tube is provided with an axial sealing head (an axial extrusion cylinder) and a counter pressure head. The tube blank is placed on the lower die, then the die is closed, the two ends of the tube billet are sealed by a sealing pressure head, and the inner cavity of the tube blank is filled with a pressure medium, such as oil, water, etc.. In the actual forming process, the axial head extrusion tube blank causes the pressure medium on the inner wall of the tube to produce very high internal pressure. When there is a gap between the blank shape and the cavity shape, the pressure medium flows naturally to the space and acts on the tube wall at the same time to cause the tube to bulge until the shape of the tube is in conformity with the inner contour of the mold. In addition, the longitudinal head can also control the flow of longitudinal material. With the help of plastic pressure, the shape of the workpiece conforms to the contour of the mold completely. This makes it possible to achieve high accuracy in size and shape of parts. Finally, open the mold and take out the workpiece.  The liquidity pressure medium so that it can change with the external shape, it is applicable to any irregular shape; in the filling process, the tube wall can be pushed to the forefront of the mold cavity pressure medium, the envelope shape of the wall deformation, and consistency in each direction due to pressure transfer the parts everywhere produce consistent uniform plastic deformation. The expansion of a straight tube is much greater than that of a bent tube, because the geometry of the tube prevents the flow of material. The limit of bulging neck height decreases as the geometry complexity increases.  4 kinds of failure forms often appear in the bulging process: Flex (buckling), wrinkle, cracking (wrinkling) (bursting) (tube pushed inwards) and folding.  Critical forming force  Among them, the die force FZ=, the workpiece projection area * the maximum internal pressure. When the hollow slender workpiece is formed, the length direction material will have plastic deformation, and the axial force acting on the end of the workpiece will be reduced because of the friction force acting on the whole part, so the axial force must be increased. When the axial force exceeds the critical forming force of FU, will appear irregular wall compression instability.  Internal high pressure forming technology is a new manufacturing technology. Only when combined with special applications under specific occasions can the technology be advanced and breakthrough. Combined with the example of tubular sub frame (object in Figure 5), the application of internal high pressure forming technology to automobile is explained.