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Title: An efficient methodology for warm-forming process design of bimetallic components using reverse simulation approach
Authors: Kong, Ting-fai
Degree: Ph.D.
Issue Date: 2012
Abstract: The success of warm forming process is identified by a complete filling of die cavity without occurrence of defects. Outcomes are highly influenced by the shape of the initial billet or workpiece. The traditional design of billet shapes, which is based on either practical experience or a trial-and-error approach by forward simulation, is inefficient and costly. Therefore, this study aims to develop an efficient methodology for a warm-forming process design of bimetallic components using a reverse simulation approach. This approach is able to directly predict the shape of billet by starting from the final shape of formed bimetallic component. The significance of this study is to develop an integrated algorithm that can be used effectively for reverse simulation. The weld interface of the bimetallic component was assumed to be a sticking condition at the beginning of the reverse simulation. When such simulation was being performed, the two metals had to be separated gradually, and their mating surfaces then restored to a sliding condition. The reverse shape could be determined by the proposed algorithm, which was established with a significant modification based on the "backward tracing method", proposed by Part et al. in 1983 as well as reasonable assumptions about the constancy of volume and convex expansion of lateral deformation surfaces. An original idea of billet-height minimization routine was incorporated into the finite-element package for tracing the hollow cylindrical shape as the desired shape of original billet. The straight-line-repair (SLR), boundary-edge-mirror (BEM), nearest-die-profile-repair (NDPR), and profile-offset (PO) methods were developed to reconstruct the reverse shapes, while at the same time, avoiding the concave-shape problem and compensating for both excessive and insufficient volume. These procedures were adopted sequentially under specified conditions in order to maintain the desired volume of the suitable reverse shape.
Three axisymmetric components were taken as examples to evaluate the effectiveness of the developed approach. Two such shapes were made of aluminium alloy 6063 (AA6063), while the third was a bimetallic component composed of AA6063 and stainless steel AISI 316L (SS316L). The flow stress data of these two specimen materials were obtained by uniaxial compression tests, which were conducted at elevated temperatures ranging from 20 to 900 °C with intervals of 100 °C, except for the larger interval between 20 and 200 °C. These material data, as well as the friction factors acquired by the ring compression tests were identified as significant information for process modelling. The bimetallic joint was achieved successfully by solid-state welding. The strength of the joint was an average more than 10% greater than that of AA6063. The thickness of the diffusion zone was 4 μm, which was extremely small compared to the height of the base metals. The deformation behaviour of the bonding interface had practically no effect on the flow stress of the bimetallic component. In other words, the materials behaved in accordance with their own properties except at their joint. Therefore, at the start of modelling the reverse simulation for warm-forming bimetallic components, the key step was to identify a large shear-friction factor as a nearly sticking condition at the weld interface. During the procedures of reverse simulation, the value of this shear-friction factor could be reduced in stages, dependent upon the forming load obtained from the increment of forward simulation. The cylindrical billets predicted by the reverse simulation approach in the three case studies were verified by forward simulation and practical experiments under compatible process conditions. The results confirmed that the proposed methodology was capable of predicting the billet shapes for warm forming not only axisymmetric components, but also bimetallic components. It is believed that the developed approach can be applied for actual production with substantial savings of raw materials and a reduction of numerous uncertain and iterative trials during the process design.
Subjects: Die-casting
Hong Kong Polytechnic University -- Dissertations
Pages: xxvii, 281 pages : color illustrations
Appears in Collections:Thesis

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