The clad material is a metallurgical joining composite of more than two materials at their interface surface. Thereby, it has two or more functional characteristics which one material does not hold in common. For instance, a tubular clad product, composed of good corrosion-resistant material and high heat conductible material can be effetctively used as a part of heat exchanger. Hydrostatic extrusion is particularly suitable for the extrusion of composite billets because it ensures that the surrounding pressure at the component interface is high enough to prevent the relative movement of the components of the composite billets and adhere the contact surfaces closely.
The bonding of bi-metal by hydrostatic extrusion at room temperature can be considered as a kind of cold pressure welding. Investigation of the bonding surface by cold pressure welding confirms this bonding mechanism to consist of the following progesses: (a) fracture of brittle surface layers, (b) expansion of the fractured surface to increase the area of virgin surface, (c) extrusion of virgin material through cracks of the original surface layer, and (d) establishment of real contact and bonding between virgin material. Successful manufacture of a clad bar by hydrostatic extrusion depends on whether the above four microstructural progresses, termed as local extrusion in this study, can happen at their interface surfaces. Design of hydrostatic extrusion process to produce a clad bar requires a criterion for the local extrusion as well as a tooling which can identify the process conditions to ensure that the local extrusion happens at their interface. In this study, the combined analysis of the global hydrostatic extrusion and the local extrusion at the contact surface has been attempted.
Of bonding mechanism ,the extrusion of virgin surface is considered as a plane strain backward extrusion through square dies under some assumption. Using a slip-line analysis, a critical extrusion pressure of virgin surface is introduced to be compared with the contact surface normal pressure for some conditions. Critical extrusion pressure is a function of flow stress instead of yield shear stress. The normal pressure as well as the surface expansion at the contact can be the most important patameters governing the bonding conditions.
In order to investigate the effects of process conditions such as extrusion ratio, die angle and clad thickness on the clad bonding, viscoplastic finite element simulations are preformed. A consistent penalty finite element formulation for the incompressible plastic flow problems has been effective to evaluate the mean stresses that is one of the most important factors in the analysis of local extrusion. The strain hardening, viscoplastic deformations of workpieces are characterized by an internal state variable model which consists of three parts: a yield condition, a flow law, and an evolution equation of the state variable. The specific model used in this study is a single scalar variable version of Hart's model.
Considering the steady state nature of a hydrostatic extrusion process, an Eulerian analysis is invoked. This necessitates an assumption about the shape and interface profile of the workpiece before the solution is obtained. Once a solution has been reached, the mesh must be checked to determine it does represent the correct interface profile. Usually the initial interface profile does not match the final shape and it must be corrected. In this work, the correct interface profile is predicted by tracking the particle path at the inlet contact of Eulerian domain. The variations of factors related to local extrusion are also investigated along that particle path.
The experiment has been carried out using 1.5MN hydrostatic extruder with variable speed ram, LVDT and load cell. It should be mentioned that the design of high pressure container, sealing device and dies are very crucial. Billet consists of aluminum 1050 and commercially pure copper and experimental conditions consists of the combinations of clad thickness, extrusion ratio and die angle. Subsequently, the inspection of bonding surfaces are carried out for some extruded products.
The predicted contact profiles and hydrostatic extrusion pressure are in good agreement with those obtained from experiments. The process map of local extrusion for the given extrusion conditions is suggested. The visual and microscopic inspection of some extruded products gives that the tendency of predicted process map matchs well qualitatively.
