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As the internal pressure increases, the interfacial contact area also increases, and adhesive friction may become dominant. Therefore, during the hydroforming process, different contact conditions can be controlled under various pressure conditions. This article, provided by Xingdiyuan Machinery, offers an overview of the friction coefficient in internal high-pressure forming as well as the types and selection of lubricants. I. Overview of the friction coefficient in internal high-pressure forming: As the internal pressure increases, the interfacial contact area also increases, and adhesive friction may become dominant. Therefore, during the hydroforming process, different contact conditions can be controlled under various pressure conditions. Conversely, various friction laws can be used to simulate friction conditions. For the low pressure levels in the initial stage of hydroforming, Coulomb friction can be used. According to Coulomb’s law of friction, the tangential (friction) stress (r) is proportional to the normal stress at the interface (on). The proportionality constant is called the friction coefficient (u). If the contact pressure approaches the flow stress of the tube material, the Coulomb friction model is no longer applicable, and a shear stress model must be used. According to the shear stress model, the interfacial tangential (friction) stress (r) is proportional to the low stress (a). In this case, the proportionality constant is called the friction coefficient. The morphology of tools and dies also plays an important role in the mechanism of tribology during the hydroforming process. In particular, in hydroforming, it is necessary to understand the effect of changes in surface roughness of parts subjected to recooling processing under varying stress conditions. In the early stages of this process, there are peak and valley surfaces at the contact points. Therefore, the friction conditions are severe, as the lubricant gets trapped in the rigid surface structure, which may not help separate the mold from the part surface. As the pressure increases, shape and roughness begin to disappear. Therefore, the friction conditions become less unfavorable on the surface. The friction coefficient at low pressure is higher than that at high pressure. Therefore, utilizing intentionally deformed tube surfaces can serve as another method for lubricating certain hydrofoil-shaped components. Hydraulic forming of aluminum can present additional challenges, as the surface of aluminum alloys is covered by a thin and hard oxide layer. During the deformation of the part’s surface, severe cold working causes the layer to break, exposing additional and unintended surfaces to the contact mechanisms. II. Types and selection of lubricants for internal high-pressure forming: Since these additional surfaces are not properly and sufficiently lubricated, they can lead to poor contact conditions, resulting in excessive thinning and premature failure: (a) dry lubricants (solid lubricants), (b) wet lubricants (solutions, emulsions, and compounds), (c) pastes, soaps, and waxes. Each group has its own advantages and disadvantages in terms of performance, applications, deletion, compatibility, and cost. Dry lubricants are generally considered to be more effective at reducing friction and extending tool life. Their application is simple and consistent with appropriate instruments. After proper drying, they have excellent compatibility with pressurized liquids. However, their removal requires special cleaning solutions. They prove to be more expensive than wet lubricants when dry time, application and removal processes, as well as initial costs increase. On the other hand, wet lubricant channels are cost-effective and easy to eliminate; they are compatible with pressurized fluids most of the time, but their performance is inferior to that of dry lubricants. Therefore, a trade-off is made based on the complexity of the part and its quality requirements. For a given die and tube material, as well as surface and loading conditions, selecting the appropriate lubricant is key to overcoming sliding friction, preventing adhesion and scuffing, reducing tool wear, axial forces, and excessive thinning that can cause noise, and producing acceptable hydrofoil-shaped parts. The choice to produce lubricants also requires reasons related to funding and a favorable environment. Before starting mass production, manufacturers need to verify and test these findings in order to compare them with the dimensional specifications required for the actual parts under production conditions. The wall thickness flatness and radius specifications must be verified against the specified values determined based on the vehicle’s NVH and crash requirements. Figure 1-19 illustrates the effect of lubricant selection on the critical thinning of structural orbital components ； Lubricant 2 shows the best effect on both initial tube thicknesses in the section and results regarding the critical expansion zone.