Carlo Bruni

Friction Stir Welding of Magnesium Alloys under Different Process Parameters

Experimental and numerical investigations have been performed in order to study the effect of welding parameters on properties of FSW-ed AZ31 magnesium alloy sheets. The results, presented in terms of tensile strength and numerical field variables distributions, allow to understand the behaviour of such material when FSW-ed using different rotational and welding speeds for a given tool geometry.

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.

Tool Geometry in Friction Stir Welding of Magnesium Alloy Sheets

Friction Stir Welding (FSW) has been arousing a continuously increasing interest among joining processes since its invention in 1991. Although mainly used for aluminum alloys, it can also be applied to other light alloys. In the present work, experimental and numerical campaigns have been performed with the aim to study the effect of the tool geometry on the mechanical properties of FSW-ed AZ31 magnesium alloy sheets. The results, presented in terms of tensile strength, ductility, micro-hardness values and numerical field variables distributions, allow to reach a deeper knowledge on the behaviour of such relatively new material when FSW-ed, and can be used for a full optimization of the joints.

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

Friction Stir Lap Joining of Blanks under Different Conditions

The present investigation aims at studying the effect of the tool geometry and of rotational velocity of the tool, at different welding velocities, on the tensile shear strength of the friction stir welded joints realised with blanks of different thicknesses. The proposed trial and error methodology was based on experiments, numerical simulations and microstructure observations.It was observed that, at the lowest rotational velocity, the slender tool determines tensile shear strength values lower than those obtained with the thick tool in particular at the highest welding velocity investigated. The numerical simulation evidenced a wider stirred zone for the thick tool when compared with that realised with the slender tool at the lowest rotational velocity. Microstructure observations evidenced that the increase in the welding velocity determines reduced stirred zones and an homogenisation of material particularly relevant for the slender tool.

Friction Stir Lap Welding of Aluminium Alloys

The friction stir welding of lap sheets can be performed considering different variables in terms of process parameters, tool configuration, welding typology and so on. The proposed investigation deals with the friction stir welding of blanks, with the same thickness, performed under lap configuration with the sheets welded, in one-side and in both sides as well, with different tool geometries and tool rotation-wise. The double side allows to extend the weld through the whole thickness leading to better mechanical welding properties at the blank to blank interface. The weld morphology has been investigated through microstructure observations performed on the transverse area, with respect to the welding velocity, of each joint. The tensile shear strength of the joint in one-side weld is generally lower than that detected in two side weld.

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.