Filippo Gabrielli

Computer aided engineering of the sheet bending process

Abstract The effects of the punch nose radius and sheet thickness on the elastic springback and residual stresses in the 90° V-punch and U-die bending process of AA 5083 aluminium alloy have been extensively studied by numerical simulations based on the finite element method. The results show that, irrespective of the sheet thickness and punch stroke, the springback ratio is not affected by the punch nose radius. Such results are in excellent agreement with the experimental ones obtained in similar conditions. As far as punch nose radius effects on the residual stresses are considered, significant differences were observed between residual stresses predicted with different nose radii. In particular, the shifting of the neutral layer and increase in the magnitude of the residual stresses predicted by the FEM code were found with decreasing punch nose radius.

Formability of Friction Stir Welded AZ31 Magnesium Alloy Sheets

The formability of friction stir welded AZ31 magnesium alloy sheets was investigated by means of uniaxial tensile and hemispherical punch tests performed under different process conditions. The results of the tensile tests were analysed in terms of flow stress and ductility at different temperature and strain rate; the hemispherical punch tests, carried out at different temperatures, provided the limiting dome height. The formability of FSW-ed blanks was compared to the one exhibited by the base material in order to evaluate the quality of the welded joints.

Warm Formability of AZ31 Magnesium Alloy Sheets under Different Process Conditions

The effect of the process parameters on the sheet formability of AZ31 magnesium alloy has been investigated by means of uniaxial tensile and hemispherical punch tests at different temperatures and strain rates. The results of the uniaxial tensile tests were analysed in terms of flow curves, ductility and microstructural evolution; the constitutive parameters were evaluated and related to the forming limit curves obtained by the hemispherical punch tests carried out at different temperatures and punch speeds.

Artificial neural-network-based control system for springback compensation in press-brake forming

The effect of the training set size and the number of input parameters on the predictive capability of a neural-network-based control system for springback compensation in air bending was investigated. An aluminium alloy, in the form of 2, 3 and 4mm thick sheets, given by two suppliers, was bent to obtain the database for learning. The influence of the number of input parameters on the network performances was studied using input patterns consisting of six and seven parameters; the punch stroke was the output. A neural network for each sheet thickness was built and trained using two training levels; an approach building a unique network, trained using the entire database, was also followed. The results obtained in the operational mode have shown that, if the training patterns are well balanced, the increase in the training set size and in the number of input parameters produces an improvement in the network performances. The networks trained with all data available were characterised by a generalisation capability that is slightly lower than the one for the networks trained using data from sheets with a given thickness.

Thickness effect on the formability of AZ31 magnesium alloy sheets

The thickness effect on formability of AZ31 magnesium alloy sheet has been widely investigated by means of uniaxial tensile tests, performed in the temperature range from 250 to 350°C, with strain rates varying from 10-4 to 10-1 s-1, using samples with different thickness values (from 1.5 to 3.2 mm). A preliminary microstructural study has shown that grain size and morphology are not significantly affected by both sheet thickness and heating just before the deformation step. The experimental results of tensile tests have been analysed in terms of flow curve shape, flow stress and strain to failure levels. They show that, in general, flow stress increases and ductility decreases with increasing sheet thickness even if such influence is strongly related to the temperature and strain rate conditions Finally, the analysis of the Zener-Hollomon parameter vs. peak flow stress data showed that the same mechanisms are operative in the investigated sheets.

Flow Curve Modelling of a ZM21 Magnesium Alloy and Finite Element Simulation in Hot Deformation

The present investigation deals with the development of a methodology to predict the flow behaviour of the ZM21 magnesium alloy in given intervals of temperature and strain rate by FEM simulation of torsion testing. Equations based on the hyperbolic sine of flow stress and on the multiple linear regression were proposed and implemented into the finite element code. The flow curve shapes obtained by simulation were compared with experimental ones that were not used in the building phase of the equations. It was found that the simulation of torsion tests allows, under given conditions of temperature, strain rate and deformation levels, to obtain flow curve shapes very similar to those obtained by experiments under conditions not included in the building of the models.

On the Formability of Magnesium Alloy Sheets in Warm Conditions

In the stamping industry, the knowledge enhancing on formability is a continuous need to be satisfied, in order to develop the application of new materials and manufacturing technologies. In the present study, the formability of Magnesium-alloy AZ31B sheets in warm conditions was investigated by conducting two kinds of experiments: an out-of-plane test using a hemispherical punch and the Wedge test, in order to investigate both the material formability and wrinkle behaviour. As expected, the executed tests highlighted a larger process window for higher temperatures, in the investigated range. The obtained results were then introduced into a Finite Element solver and applied as design tool for a specific case study. Basic evidences and results are accurately discussed in the paper. Introduction The new goal of the modern manufacturing is represented by the sustainable manufacturing. In this context, the use of lightweight materials, such as Magnesium alloys, becomes a new priority since they allow, for instance, a relevant fuel cost saving and pollution reduction in automotive field. In particular, when thin walled structural components have to be obtained, the sheet metal forming is preferred to die casting, but, due to the poor formability at room temperature of such alloys, they have to be formed at higher ones [1]. In the study here addressed, funded by Italian Ministry of University and Research, the forming behaviour of the AZ31B Magnesium alloy, in the temperature range varying between 200°C and 300°C, has been investigated. As usual, two different phenomena, leading to the forming limits, may occur, namely necking and wrinkling. The latter phenomenon occurs when blank-holder pressure is not sufficiently high. In other words, it is possible to define a formability window, which includes the set of the possible conditions that allow the process success. In addition, it is very interesting to characterise these regions by proper rules in order to use them as boundary conditions in a Finite Element Analysis. By thus way, FEA may constitute a strategic tool for both the process design and the process verify steps. For this purpose, an experimental equipment has been developed and set-up to execute two different tests in order to define the limits of the material forming behaviour. More in detail, the wedge test and an out-of-plane test have been performed; the former has been used to define the wrinkling limit curves (WLC), whilst the latter, based on the use of a hemispherical punch, to define the forming limit curves (FLC). In both the cases, the testing equipment has been properly heated using a dedicated furnace and the temperature controlled by means of different thermocouples placed in air, in the tools and on the specimen. In general, when temperature increases a formability increasing may be observed, according to theory. All the tests have been carried out on AZ31 Magnesium alloy sheets, characterised by a thickness of 1mm. Finally, the experimental evidences have been utilised in order to define proper material behaviour rules for the FE simulation of a simple geometry, in order to highlight its potentiality as design tool. Key Engineering Materials Online: 2007-07-15 ISSN: 1662-9795, Vol. 344, pp 55-62 doi:10.4028/www.scientific.net/KEM.344.55

A statistically based methodology in the identification of friction welding parameters

An approach is developed to find the values of the friction welding parameters to be used in the joining of Mg alloy sheets with thickness of 1.5 mm. A set of laboratory trials, based on the design of experiment method, is performed in order to determine the conditions under which the welding operation is feasible. In particular, a 2k factorial design (k = 3) is used with two different configurations. In the first configuration the rotational speed, the welding speed and the presence/absence of the pin (in the tool) are considered, whilst in the second, performed under “no‐pin condition”, the rotational speed, the welding speed and the shoulder diameter are varied. The results show a strong beneficial effect on the tensile strength of the joint, under the “no‐pin condition”, when the shoulder diameter is increased.

Evaluation of Friction Coefficient in Tube Drawing Processes

A methodology, based on a combined numerical‐experimental approach, was developed to evaluate friction coefficient at the die‐workpiece surface in tube drawing. In the experimental stage the upsetting sliding method, performed under contact conditions, in terms of contact normal stress and equivalent plastic strain, similar to those encountered in the tube drawing process under investigation, was used. The values of friction coefficient calculated were dependent on tube geometry.

Flow Modelling of AA 6082 Aluminium Alloy

The flow behaviour of AA 6082 aluminium alloy has been studied by means of torsion testing carried out at temperatures ranging from 425 to 500°C, with strain rates varying from 1 to 20 s −1. For a given temperature and strain rate, flow curves exhibit a peak followed by flow softening up to fracture. Moreover, for a constant strain, flow stress increases with increasing strain rate and decreasing temperature. In general, the hot ductility shows an increase with increasing temperature up to a maximum, occurring between 450 and 475°C, followed by a decrease to the lowest value experienced at 500°C. An equation relating the hyperbolic sine of flow stress to the temperature modified strain rate was used to describe the flow behaviour versus the working parameters. The dependence of constitutive parameters on strain was also analysed. A very good agreement was found between predicted and experimental flow curves.