Tube HydroForming

Tube hydroforming

Introduction

A liquid under pressure is used to plastically deform the metallic material and achieve the desired shape. The mechanical action of the fluid partially substitutes tools and molds typical of conventional pressing. During the Nineties, this technology developed for hollow profiles, exhaust, and chassis parts in the automotive industry. One of its greatest advantages is the possibility to realize complex geometries at lower costs than conventional processes: in fact, a decrease in fabrication steps is achieved (no assemblies). Moreover, the consequent reduction in the number of welds increases mechanical resistance.

Process

The process is based upon the mechanical action of pressurized water or oil on a tube inside a mold reproducing the final form of the component. The mold is closed under the action of a closure force. The pressurized fluid is injected into the tube and an axial force is laterally applied through cylinders. The tube expands into the cavity until assuming the final form of the mold. According to the required part, additional force can be useful (for example, exerted by a transverse punch to set the end of the expansion).

Parameters

Considering a cylinder with thin walls in plain strain condition, the radial stress can be defined as: ^σ_r = ^σ_θ = -^σ_z. It is possible to define an equivalent stress as: √³/₂ (̅σ) = -^σ_z + 1 = ^σ_z/2, ̅σ = ^σ_z/2. Where in the case of plain strain, ̅σ = √³²/1 ^σ₁.

There are different processing zones: in the guided zone, the radial stress is opposed to the internal pressure of the fluid. The hoop stress (^σ_θ) is small and positive, and the axial stress (^σ_z) is negative due to large friction. In the expansion zone, ^σ₂ ≤ 0, and the stresses ^σ₁ and ^σ₂ are positive. Therefore, displacements occur.

Quality and application

The quality and application of hydroforming are significant due to its ability to create complex shapes with fewer fabrication steps and reduced welding, enhancing mechanical resistance. This technology is primarily used in the automotive industry for parts like hollow profiles, exhaust systems, and chassis components.