Key Engineering Materials Vols. 340-341

Abstract: Micro-indentation has been widely used to evaluate the mechanical properties of materials. It has also been considered to be an important measure in the study of machinability of difficult-to-machine materials such as metal matrix composites (MMCs). Because of the complexity of deformation of an MMC and the interaction in the vicinity of contact zone between the indenter and work material, an analytical or experimental method is unable to predict the detailed deformation process. The present paper uses the finite element method to investigate the behavior of MMCs subjected to micro-indentation by a spherical indenter including the development of stress and strain fields in the MMCs during loading/unloading. Particle fracture, debonding and displacement, and inhomogeneous deformation of matrix material were explored and compared with the experimental results reported in the literature. The analysis also provides an insight for understanding the formation of residual stresses in machined MMC components.

Abstract: In this study, depth sensing nano-indentation test was carried out to investigate the plastic/viscoplastic behavior of beta titanium alloy. The indentation experiment results showed that both hardening and softening effect existed in indentation process and the residual penetration depth was deeper when the nominal indentation strain rate increased. That is opposite to the room temperature tension test results, which showed a strain rate hardening behavior. FEM simulation combined with viscoplastic model was carried out to simulate the indentation procedure. FEM results showed that the pile-up pattern changed with the consideration of the nominal indentation strain rate effect. Atom force microscope (AFM) observation gave a same result of pile-up patterns.

Abstract: In this paper, the forming limit of flange in radial extrusion process was analyzed by the rigid-plastic finite element method. The selected model material for simulation and experiments was AA 3105 aluminum alloy. The predictions from simulation were made in terms of axial and circumferential strains. Experiments also have been conducted to compare with the simulation results with regards to deformation pattern. Furthermore, the deformation pattern in forming of flange section was closely investigated and categorized in three cases such as sticking, separating and cracking. The analysis in this paper is focused on the transient extrusion process of material flow into the gap in radial direction for different gap heights and die corner radii. The results of present study were summarized in terms of evolution of surface strains in axial and circumferential directions measured from the finite element meshes located in the region where surface cracking occurred in experiments. The forming limit line was drawn in the relationship of circumferential and axial strain. It was concluded from this study that the forming limit line is influenced mainly by circumferential strain on free surface of flange. It was also predicted that ductile fracture on flange surface is likely to occur in the middle of flange gap under the condition of sticking deformation and near bottom of flange gap under the condition of separating deformation, respectively. The forming limit of flange in terms of flange diameter was expected about 2.5do, which is 2.5 times the diameter of original billet.

Abstract: Galvannealed steel sheets are being widely used in automotive application for better corrosion resistance. It is generally known that uncoated steel sheets have better mechanical properties than coated steel sheets due to presence of zinc coating. But frictional characteristics of coated steel sheets are very different from those of uncoated steel sheets. Therefore the study on mechanical and frictional characteristics of these steel sheets is needed. In this study, tensile test was performed to evaluate mechanical properties of coated and uncoated steel sheets. Cup drawing test was performed to measure friction-coefficient. And frictional characteristics were analyzed by using FE-analysis. The effect of mechanical and frictional properties on cup drawing was investigated. It was shown that the frictional properties more affected cup drawing.

Abstract: Extruded aluminum alloys, which are highly versatile, have relatively modest prototyping cost, good strength and corrosion resistance. Because there is no weld seam, the circumferential mechanical properties may be uniform and advantageous for hydroforming. However, surface defects such as die lines and pick-up can be generated during the extrusion especially due to imperfections on the die surface. In this study, the extent of the crack propagation caused by die lines is evaluated according to the deformed shape of the tube in hydroforming process. And when forming a extruded aluminum tube, the deformed surface of the tube frequently becomes rougher with increasing plastic strain. This is well known as orange peel phenomenon and it has a significantly effect not only on the surface quality of a final product but also on the forming limit. To evaluate the effects of the orange peel on the hydroformability, the inter-stage polishing has been performed. Through the several tests including hydroforming test, the effect of surface defects on the hydroformabilities are well defined.

Abstract: Preform design in tube hydroforming implies the design of an intermediate shape between initial tube and the final product enabling to be fabricated without defects and excessive loss of material. A carefully selected preform can contribute significantly to reduce production cost and improve formability, since thinned sections may not be able to endure internal pressure during expansion whereas excessive thickening may lead to wrinkles. Generally, preform design in hydroforming was mainly carried out through the trial-and-error approach. Even though a series of numerical simulations for several predetermined preformed shapes were conducted, optimum configuration could not be obtained and could not be suggested the general procedure for preform design as well. In this work, a simple numerical approach to the preform design for formability enhancement was introduced based on the deformation history during forward hydroforming simulation. The proposed approach was implemented to a hydroforming process of an automobile subframe component in order to be satisfied the required specification after hydroforming, and the conceptual application has been proved to be successful on its effectiveness and feasibility. Therefore, it is shown that preform design approach proposed in this study will provide one of feasible methods to satisfy the increasing practical demands for improvement of the formability in hydroforming processes.

Abstract: Formability of tube in elevated temperature is essential data to design the warm hydroforming process parameters, such as tube diameter, forming temperature and die geometries. Since the quantitative data of forming limit can be used to predict the failure on forming process, formability data available on the FE analysis is one of the very important information for the optimum design. In this study, the effect of heat treatment conditions and deformation temperature on the formability was investigated for the warm hydroforming of Al6061 tube. Full annealing and T6-treatment are applied for the heat treatment of Al6061 tubes. To evaluate the hydroformability, uni-axial tensile test and bulge test were performed at temperature ranges between room temperature and 300oC. The measured flow stresses were used as input parameters for the simulation of warm hydroforming process. The damage value and strain variation during hydroforming are analysed by FEM. A forming limit based on the ductile fracture criteria has been proposed by combining the results of experimental and FE analysis for the estimation of formability and optimization of warm hydroforming process.

Abstract: Fe was added to 6016 aluminum alloy as the impurity, and this alloy was used as the model of the recycled 6016 aluminum alloy. The content of the Fe was from 0.18 mass% up to 1.0 mass%. 6016 including impurity-Fe was cast into the strip using a high speed twin roll caster at speed of 60 m/min. The increase of the content of impurity-Fe makes castability better rather than worse. The formability was investigated by tension test and 180 degrees bending test. The increase of Fe did not have influence on the bending test. There was no crack at outer surface of T4-heat treated strip after 180 degrees bending. The formability of 6016 strip including impurity-Fe cast by the high speed twin roll caster was enough for hem forming. The ductility of Fe added 6016 was improved by the high speed twin roll caster. It is thought that Al-Si-Fe impurity became fine by the effect of the rapid solidification, and deterioration could be improved.

Abstract: Rheology forming is a novel processing method of semi-solid processing, which is different from traditional mold forging and conventional casting process. The rheological behavior of metallic alloys containing both solid and liquid phases was investigated with the low and high solid fraction ranges. Its obvious advantages are easier to produce complex work pieces because of excellent forming ability, more flexible to shape, and more compact in the inner quality for its high pressure. This research paper presents the theory of the rheology forming process and the results of the finite element simulation of rheology forming for aluminum alloys. In this proposed theoretical models for the rheology forming process involve simultaneous calculations performed with solid phase deformation and the liquid phase flow analysis. To analyze the rheology process, the new flow stress curves of rheology aluminum alloys and the viscosity for the simulation of two-phase flow phenomena have been proposed with as a function of temperature.

Abstract: Asymmetric rolling is a novel technique for giving rise to an intense plastic shear strain through the sheet thickness. The shear strain also develops shear deformation textures close to the {001}<110> and <111>//ND orientations, among which the latter is the most wanted component for the deep drawability, and give rise to the grain refinement. Previously we analyzed various rolling variables influencing the texture development and grain refinement in aluminum sheets obtained by asymmetric rolling with different roll-radius ratios at the same rotation rate and varied reduction per pass. In this study, AA1050 Al alloy sheets were asymmetrically rolled with a two-high mill of which two rolls had the same diameter, but rotated at different rotation rates, with emphasis on effects of combinations of shear directions in several passes. Textures and microstructures of the rolled sheets were investigated by x-ray diffraction and electron backscattered diffraction analyses.