Gracious Ngaile

Determination of flow stress for sheet metal forming using the viscous pressure bulge (VPB) test

Abstract In sheet metal forming operations the mechanical properties of the sheet material (i.e. flow stress or stress–strain curve) greatly influence metal flow and product quality. Therefore, accurate determination of the flow stress is of paramount importance in process simulation via finite element method (FEM). In this paper the use of the viscous pressure bulge (VPB) test for determination of flow stress under biaxial state of stress is discussed. With the VPB test, larger strains, which are relevant for stamping operations, can be achieved compared to the standard tensile test used to-date. In this study, FEM simulations and experiments have been performed in order to study the interrelationship of the geometric and material variables such as dome wall thinning, dome radius, dome height, strain hardening index, material strength coefficient, and anisotropy. From the study a robust method to determine the flow stress under biaxial deformation conditions using a viscous material as pressure medium has been developed.

Optimizing tube hydroforming using process simulation and experimental verification

Abstract The success of a tube hydroforming (THF) process is highly dependent on the loading paths (axial feed versus pressure) used. Finite element (FE)-based simulation was used to determine optimum loading paths for hydroforming of structural parts with different tubular materials. Experimental and simulation results have demonstrated that FE-based loading paths can significantly reduce trial and error, enhance productivity and expand the THF capability in forming complex parts. The test results also demonstrated that the reliability of the FE-based loading paths is highly dependent on the accuracy of the material properties of the blank, interface friction, and how close the properties of the welding zone are to the base material of the tubular blank.

Influence of ultrasonic vibration on micro-extrusion

Micro-forming is a miniaturization technology with great potential for high productivity. Some technical challenges, however, need to be addressed before micro-forming becomes a commercially viable manufacturing process. These challenges include severe tribological conditions, difficulty in achieving desired tolerances, and short tool-life due to inability of available die materials to withstand the forces exerted on miniature dies and punches. Some of these problems can be mitigated using ultrasonic technology. The principal objectives of this work were to investigate the possibility of applying ultrasonic vibrations in the micro-forming process, to design a set of tooling for ultrasonic micro-extrusion and to observe experimentally how ultrasonic oscillations influences the forming load and the surface finish. The test results showed a significant drop on the forming load when ultrasonic vibrations were imposed, and also a significant improvement in the surface of the micro-formed parts. Based on the preliminary test results, the study demonstrated high potential for using ultrasonic oscillations as a way to overcome the difficulties brought by the miniaturization.

Lubrication in tube hydroforming (THF) Part I. Lubrication mechanisms and development of model tests to evaluate lubricants and die coatings in the transition and expansion zones

Abstract The lubrication mechanisms that occur at the tool–workpiece interface for the transition and expansion zones are discussed. Suitable lubrication systems for the transition and expansion zones are reviewed based on the mechanics of deformation and material flow at the interface. Details of two model tests for evaluating the performance of tube hydroforming (THF) lubricants and die coatings are given. The optimization of die geometries for the model tests is based on sensitivity analysis through the finite element method together with experimental verification. The details of these tests are given and their development is discussed.

Lubrication in tube hydroforming (THF) Part II. Performance evaluation of lubricants using LDH test and pear-shaped tube expansion test

Abstract Two model tests to evaluate lubricant performance under realistic tribological conditions occurring in the transition and expansion zones of a tube hydroforming (THF) process are presented. The model test for the transition zone is based on the limiting dome height (LDH) test principle. For the expansion zone, a pear-shaped tube expansion test (PET) developed by the authors is employed. Four lubricants were tested and ranked based on (a) dome wall thinning behavior (for LDH), (b) tube wall thinning, tube protrusion height (PH), tube bursting pressure (for PET), and (c) surface topography. Friction coefficients for the lubricants were estimated by matching the experimental and FE results.

Optimization of blank dimensions to reduce springback in the flexforming process

Abstract In sheet metal forming operations, springback of the part during unloading largely determines whether the part conforms to the design dimensions and tolerances. Finite element simulations were performed in order to study the interrelationship of the blank dimensions and interface conditions on the springback for an axisymmetric conical part manufactured by flexforming. Sensitivity analysis done using the finite element method (FEM) demonstrated that the magnitude of springback and the overall dimensional quality are highly influenced by the initial dimensions of the blank. A conventional optimization method combined with FEM was used to obtain optimum blank dimensions that can reduce springback.

Analytical Model for the Characterization of the Guiding Zone Tribotest for Tube Hydroforming

Common part failures in tube hydroforming include wrinkling, premature fracture, and unacceptable part surface quality. Some of these failures are attributed to the inability to optimize tribological conditions. There has been an increasing demand for the development of effective lubricants for tube hydroforming due to widespread application of this process. This paper presents an analytical model of the guiding zone tribotest commonly used to evaluate lubricant performance for tube hydroforming. Through a mechanistic approach, a closed-form solution for the field variables contact pressure, effective stress/ strain, longitudinal stress/strain, and hoop stress can be computed. The analytical model was validated by the finite element method. In addition to determining friction coefficient, the expression for local state of stress and strain on the tube provides an opportunity for in-depth study of the behavior of lubricant and associated lubrication mechanisms. The model can aid as a quick tool for iterating geometric variables in the design of a guiding zone, which is an integral part of tube hydroforming tooling.

Preform design for forging and extrusion processes based on geometrical resemblance

Abstract Preform design in multi-stage forging processes is critical to ensure the production of defect-free parts. Moreover, owing to the geometry and material-flow complexities in forging processes, finding the optimal preform shapes could be difficult and time consuming. This paper proposes an efficient preform design methodology based on geometrical resemblance, which requires several finite element analysis simulation iterations to obtain a good preform shape. The initial and subsequent simulations are carried out by constructing a slightly larger part that geometrically resembles the desired part. Initial finite element analysis simulation of the larger part is performed with a reasonably guessed preform shape, whose forming defects or flash formation would be corrected in subsequent steps. Then a series of intermediate parts of similar shape and between the largest part and the desired part in size are constructed. The undeformed shape corresponding to an intermediate part can be obtained by backwards tracing of material flow from the simulation results of the larger part. This undeformed shape is then taken as the preform shape of the intermediate part. The procedure is repeated until the intermediate part is geometrically close to the desired part, which leads to the preform shape. In order to verify this preform-design methodology, several case studies on forging and extrusion processes have been carried out. The methodology has been shown to be computationally efficient, requiring as few as three finite element iterations to obtain a good preform shape.

Characterization of Adhesive Strength of Phosphate Coatings in Cold Metal Forming

A test method is proposed to characterize adhesive strength of phosphate coatings based on the various deformation patterns at the tool-workpiece interface. The deformation patterns were induced by tools of different surface geometrical profiles, i.e., flat surface, sinusoidal surface, saw-tooth surface and multi-surface profiles, in a localized rod drawing technique. With change in the tool geometry, three deformation regimes were observed, i.e., full film lubrication regime, mixed regime, and seizure regimes, which were categorized by the level of friction coefficient attained, and the degree of galling observed on the surface of the drawn specimens. The full film lubrication regimes were noticed when flat dies were used. In this case, the friction coefficient was maintained at nearly μ = 0.065, irrespective of the change in the surface roughness of the tools and reduction. With sinusoidal surface and other non-flat dies, mixed regime and seizure regimes were observed, and the friction coefficient varied from μ = 0.1 to 0.3. To complement the friction data, surface analysis of the tool-workpiece interface was also conducted. The frictional range of μ = 0.065 to 0.3 obtained in this study, therefore, provides for a manageable characterization of phosphate coatings for cold metal forming of objects with intricate shapes.

Performance of Graphite and Boron-Nitride-Silicone Based Lubricants and Associated Lubrication Mechanisms in Warm Forging of Aluminum

Although water/oil-graphite emulsions are widely used in warm forging processes, they carry environmental concerns. In an attempt to replace graphite-based lubricants in warm forging of aluminum alloys, two variants of boron-nitride-silicone lubricants were formulated. The two variants were made by dispersing boron nitride powder in polydimethyl siloxane oil at concentrations of 1% and 8%. The formulated lubricants were initially tested for their thermal degradation characteristics using a thermogravimetric analyzer and compared to the thermal degradation behavior of graphite and silicone oil lubricants. Ring compression tests were then carried out at 260°C and 370°C. Boronnitride-silicone lubricant variants did not show significant difference in performance as die temperature was increased from 260°C to 370°C. This is in contrast to graphite, which performed much better at 260°C than at 370°C, due to thermal oxidation. On the other hand, silicone oil exhibited the worst performance at 260°C and the best performance at 370°C. In both boron nitride lubricant variants, the polydimethyl siloxane facilitated hydrostatic/hydrodynamic lubrication at 260°C, with boron nitride acting as a barrier film that reduced friction. However, the lubrication mechanisms changed at 370°C, where the depolymerization of polydimethyl siloxane led to formation of silica due to thermal oxidation. Silica, together with boron nitride, acted as a film barrier with low shear strength. The dual lubrication mechanisms make boron-nitride-silicone lubricants suitable for a wide range of aluminum forging temperatures. DOI: 10.1115/1.2805432