Closed-loop control of product properties in metal forming

Metal forming processes operate in conditions of uncertainty due to parameter variation and imperfect understanding. This uncertainty leads to a degradation of product properties from customer specifications, which can be reduced by the use of closed-loop control. A framework of analysis is presented for understanding closed-loop control in metal forming, allowing an assessment of current and future developments in actuators, sensors and models. This leads to a survey of current and emerging applications across a broad spectrum of metal forming processes, and a discussion of likely developments.Engineering and Physical Sciences Research Council (Grant ID: EP/K018108/1)This is the final version of the article. It first appeared from Elsevier via https://doi.org/10.1016/j.cirp.2016.06.00

Proportional loading of thick-walled cylinders

A generalized solution for small plastic deformation of thick-walled cylinders subjected to internal pressure and proportional axial loading is developed. The solution has been shown to reduce to the well-known Lamé\u27s elastic solution and Nadai\u27s general plane strain solution under appropriate assumptions. The influence of proportionality factor (ratio of axial strain to hoop strain) and hardening exponent on the induced strain, deformation fields and thickness reduction is systematically investigated. The formulation yields a singularity when the axial strain to hoop strain ratio is equal to \u27-2\u27 Based on the employed material parameters, power law constitutive model and proportionality factor, the maximum effective stress may occur at either the inside or outside of a tube shell. An equation to estimate ultimate internal pressure based on proportionality factor, material properties and tube geometry was derived. It is shown that maximum thickening occurs when the proportionality factor approaches \u27-2\u27. © 2004 Elsevier Ltd. All rights reserved

Optimization of material properties and process parameters for tube hydroforming of aluminum extrusions

Analysis of process optimization for hydroforming of central-bulge and T-branch from AA6063 tubes is conducted for W-temper and T4 heat-treated conditions. Systematic characterization of AA6063 mechanical properties as a function of aging time was also conducted. It was found that hydroforming in the W temper facilitates forming of a bigger T branch (due to available greater ductility), but limits the strength (hardness) of the final component compared to that formed in the T4 condition. By optimizing the material heat-treatment conditions and the process parameters during hydroforming, strains well in excess of the traditional forming limits can be achieved in the finished components. The relevant microstructural kinetics during hydroforming of the above two geometries in the two heat treated conditions and the associated strengthening mechanisms in aluminum alloys are discussed. Copyright © 2007 by ASME

Loading path optimization of tube hydroforming process

Optimization methods along with finite element simulations were utilized to determine the optimum loading paths for closed-die and T-joint tube hydroforming processes. The objective was to produce a part with minimum thickness variation while keeping the maximum effective stress below the material ultimate stress during the forming process. In the closed-die hydroforming, the intent was also to conform the tube to the die shape whereas in the T-joint design, maximum T-branch height was sought. It is shown that utilization of optimized loading paths yields a better conformance of the part to the die shape or leads to a higher bulge height. Finite element simulations also revealed that, in an optimized loading path, the majority of the axial feed needs to be provided after the tube material yields under the applied internal pressure. These results were validated by conducting experiments on aluminum tubes where a good correlation between the experimental results and simulations were obtained. © 2005 Elsevier Ltd. All rights reserved

Influence of end-conditions during tube hydroforming of aluminum extrusions

Tube hydroforming experiments were conducted to develop the forming limit diagram of AA6082-T4 by utilizing three types of end-conditions: (i) free-end , (ii) pinched-end or fixed-end and (iii) forced-end . It was found that free-end hydroforming gives the lowest forming limits followed by pinched-end and forced-end hydroforming. It was noticed that the tube failure occurs within 5° to the extrusion weld in the free-end experiments, within 7° in the pinched-end condition and extended up to 10° in the forced-end hydroforming experiments. Finite element simulations were carried out to capture the effects of the weld geometry, weld mechanical properties and the end-conditions of the extruded tube on the maximum induced stress and location of the maximum von Mises stress. It was found that the anisotropy of the weld material and the end-condition used during hydroforming experiments have the largest influence on the failure location with respect to the weld center. © 2004 Elsevier Ltd. All rights reserved

Mechanical properties and microstructural characterization of extrusion welds in AA6082-T4

Microstructural observations of extrusion welds in an A-pillar made of AA6082-T4 revealed that the observed extrusion weld is a composite of a seam weld (longitudinal weld) and a charge weld (transverse weld). To determine the mechanical properties of this weld region, tensile specimens were prepared with the weld located at 0°, 45°, and 90° to the tensile axis. For comparison purposes, specimens from no-weld regions were also prepared in the same orientations and tested. The specimens with 45°-weld exhibited the lowest tensile strength, followed by the specimens with 90°-weld, no-weld and 0°-weld specimens. Comparison of failure strains and fracture modes revealed that weld regions are less ductile than the no-weld regions. Microscopic observations of fractured surfaces and further analysis revealed that Mg 2Si precipitates that align along the charge weld cause premature failure at these locations. © 2004 Kluwer Academic Publishers

Experimental and numerical investigation of free-bulge formation during hydroforming of aluminum extrusions

Free-bulge hydroforming experiments were conducted to investigate the biaxial behavior of extruded aluminum tubes. A special fixture, capable of providing only internal pressure and free sliding of tube-ends, was successfully designed to conduct these free-bulge experiments. Forming limits obtained under these conditions were found to be significantly lower than those obtained with superposed axial feed. A finite element model was constructed to simulate the hydroforming process and assess the influence of friction between the die walls and the tube, tube material properties and tube anisotropy. A power law constitutive model based on stress-strain curves obtained for various test geometries was used in these simulations. It was found that the parameters obtained based on the tensile test data of the curved tube sections better correlated to the experimental bulge height measurements. The analysis also revealed that the material hardening coefficient had the most significant influence on the formability characteristics during hydroforming. © 2004 Elsevier B.V. All rights reserved