Thread Content
Compared to the stamping process, the high surface and contact stresses under internal high pressure, along with the distributed plastic deformation of the entire workpiece, result in more stringent tribological conditions in hydraulic forming. The contact stress level in hydraulic forming is significantly higher than that in stamping forming, but it is not as high as that in the forging process. Therefore, during the design process, great care should be taken to consider surface conditions, coatings, lubricants, and mold life. This article, brought to you by Xingdiyuan Machinery, covers the factors affecting the tribological conditions in hydroforming, as well as the friction state, sliding speed, and stress conditions. I. Factors affecting the tribological conditions in hydroforming: The tribological conditions in hydroforming, as in any typical metal forming process, are primarily influenced by the following factors: (a) the surface condition of the tube and the die, (b) the contact area and related stress states, (c) surface pressure, (d) sliding speed, (e) the materials of the tube and die and their mechanical properties, (f) contact temperature, (g) die coatings, (h) the positioning of the parting line. Structural frame components with particularly long lengths and different cross-sections require substantial axial feeding in order to form the mold cavities without them becoming too thick or expensive. II. Friction state, sliding speed, and stress state in hydroforming: The wide range of cross-sectional shapes, from circular to rectangular, requires minimal resistance to corner forming and material movement. The friction problem in this case becomes crucial. Choosing the appropriate lubricant and mold coating are important measures to overcome sliding friction, prevent sticking and scuffing, thereby reducing tool wear, axial force, and thinning. Generally, in a typical hydroforming process, depending on the metal flow, three main friction states, sliding speeds, and stress conditions are determined at the interface between the part and the die. These friction states and the resulting friction coefficients change continuously with position and time, as described below (Figure 1:18): (Prier and Schmoeckel, 1999 ； Koc and Altan, 2001): (a) Frictional contact pressure and axial compression in the guiding zone where the tube and die surfaces are located. The axial movement of the material is much faster compared to its circumferential movement; the material’s movement speed can range from 50 to 100 MMS-1. High surface pressure on the medium, high sliding speed, high axial pressure, limited surface expansion ； (b) The surfaces of bent parts and molds are under a triaxial stress state under contact pressure. Compared to the guided area, the material moves more slowly; its surface expands or contracts, and the sliding speed is lower than in the guided area. Nevertheless, there are still considerable stress concentrations and tensile circumferential stresses at certain points along the axis, with the tensile stress within the tube occurring in the circumferential direction ； (c) The friction in the axial feeding expansion and calibration regions is negligible, and a biaxial stress state exists ； Compared to axial motion, the movement of the material in the circumferential direction is dominant ； Tensile stress is present universally (axial and circumferential), the sliding speed is low, and the surface expands.