G. Palumbo

Thermal stability of nanostructured electrodeposits

As a result of their unique, but well-behaved structure-property relationships, porosity free nanostructured electrodeposits in the form of thin and thick coatings, free-standing sheet, foil or wire, and complex shapes, are rapidly finding applications in many different areas. Because of the large driving force for grain growth in these materials, however, their thermal stability may be a critical issue for some applications. This paper reviews previous grain growth studies of nanostructured electrodeposits. Thermal stability has been evaluated using several different experimental approaches. Calorimetric studies of nanocrystalline nickel based electrodeposits have shown a general trend of increasing thermal stability by alloying with either P or Fe. There is, however, little agreement between the various studies in terms of suggested growth mechanisms. Indeed, because of the large range of annealed structures obtained from different annealing treatments, several distinct growth mechanisms have been suggested. Recent grain growth studies of nanostructured Ni electrodeposits, covering a much broader range of annealing conditions than used before, have shown that the multiple types of previously reported annealed structures are in fact the product of a multi-staged growth process.

Formability and Process Conditions of Magnesium Alloy Sheets

Comparing the formability with each other, extrusion and various rolling experiments were carried out to make fine-grained AZ31 Mg sheets, and uni-axial tensile tests were carried out at different strain rates and temperatures to investigate the effect of different variables. A warm deep drawing tool setup with heating elements, which were distributed under the die surface and inside the blank holder, was designed and manufactured, and deep drawing was performed. Extruded Mg alloy AZ31 sheets exhibit the best deep drawing ability when working in the temperature range 250-350°C. Extruded and rolled sheets of 0.8 mm thick were also deep drawn in the lower temperature range 105-170°C，showing good formability and reaching a Limit Drawing Ratio up to 2.6 at 170°C for rolled sheets. At last, a sheet cup 0.4 mm thick was deep drawn successfully at 170 °C.

Material Superplastic Parameters Evaluation by a Jump Pressure Blow Forming Test

In this work a method, based on bulge tests performed on a blow forming equipment, for evaluating the superplastic material characteristics is proposed. The pressure imposed on the sheet and the height of the dome of the specimen during the test are used as characterizing parameters. Different pressure levels are applied subsequently in the same test and the strain rate sensitivity index is calculated starting with analytical considerations and then with an inverse approach based on a simple finite element numerical model of the test. The change of the slope in the specimen dome height curve, due to the change of the pressure, is correlated to the strain rate in the sheet. The method has been verified applying other load profiles on the sheet and good agreement has been found between experiments and numerical results obtained by the inverse analysis.

Approach of using a ductile fracture criterion in deep drawing of magnesium alloy cylindrical cups under non-isothermal condition

Abstract There have been many ductile fracture criteria published for predicting fractures in the isothermal metal-forming process, especially at room temperature. However, many materials, such as magnesium alloy and aluminium alloy, show an improved formability at elevated temperatures under non-isothermal conditions. It is significant to predict the occurrence of fracture in metal forming under these conditions since a forming limit diagram is almost powerless here. In the present work, an approach is proposed for extending the application of existent ductile fracture criteria in a non-isothermal metal-forming process. Using the new approach, combined with the finite element method (FEM), the deep drawing of magnesium alloy AZ31 cylindrical cups under non-isothermal condition is analysed. The fractures located in the punch radius region and in the cup wall close to the radius region of the female die resulting from the low and high punch temperatures, respectively, are predicted.

Warm deep drawing of magnesium alloy sheets - formability and process conditions

The influence of deformation conditions on the formability of magnesium alloy was systematically investigated using the experimental method and finite element analysis. The research activity aimed to gain further insight into the formability of magnesium alloy AZ31 (aluminium 3 wt per cent, zinc 1 wt per cent) and to have first-hand knowledge of the correlations between the materials properties and the processing parameters. In this work, the mechanical properties of magnesium alloy AZ31 sheet were analysed according to the results of tensile tests. A set of experimental equipment with temperature control was employed to perform warm deep drawing of magnesium alloy AZ31 sheets under various forming conditions. The most important process parameters, including the punch speed, the forming temperature, and the geometrical shape of the blank, were taken into account and optimal values were proposed. Finite element analyses were also performed to evaluate the effects of the process parameters on the formability of rectangular cup drawing and to predict the process defects during the process.

Numerical-Experimental Characterization of a Superplastic AZ31 Magnesium Alloy

In this work the superplastic behaviour of a hot rolled AZ31 magnesium alloy sheet under a biaxial tension test with the blow forming technique is presented and reported. The specimen dome height and its thickness distribution, during and after the test, have been used as characterizing parameters. A numerical FE model of the test has been developed in order to easily characterize the material and to directly analyze experimental results. The influence of the rolling cycle on the microstructure and consequently on the material behaviour has been also analyzed. A synergic use of experimental results and of the numerical model has been done for finding material constants in different situations. The material flow parameters have been found and results are presented.

Warm Hydroforming of Magnesium Alloy AZ31 Sheets

In the present study, warm hydroforming of cell phone cases with magnesium alloy AZ31 sheets was investigated. Fine-grained magnesium alloy sheets were prepared by cross rolling. And the tensile tests were first conducted in order to determine the proper forming temperature. The results showed that the most suitable temperature range appears to be 150-200°C. At last, the magnesium alloy cell phone cases characterized with the small round radius of all edges were formed successfully at 170°C with the low punch velocity and the maximum pressure not less than 5MPa.

Warm Deep Drawing Of Rectangular Cups With Magnesium Alloy AZ31 Sheets

Recently, magnesium alloys have been widely applied in automotive and electronic industries as the lightest weight structural and functional materials. Warm forming of magnesium alloys has attracted much attention due to the very poor formability of Mg alloys at room temperature. The formability of magnesium alloy sheet at elevated temperature is significantly affected by the processing parameters. Among them the forming temperature, the punch speed, the geometrical shape of the blank, the blank holder force and the lubrication are probably the most relevant. In this research, the deep drawing of rectangular cups with AZ31 sheets was conducted at elevated temperatures with different process parameters. The finite element analyses were performed to investigate the effects of the process parameters on the formability of rectangular cup drawing and to predict the process defects during the process. The material yield condition was modeled using the isotropic Von Mises criterion. The flow stress data were obtained from tensile tests.

Experimental Procedure Definition for Evaluating the Formability at Warm Temperatures of AZ31 Magnesium Alloy

In the present work the definition of a test procedure for evaluating the formability of Mg alloy thin sheets was investigated taking into account both temperature and strain rate. A numericalexperimental approach was adopted by the authors: numerical simulations were run with the aim of: (i) defining the punch geometry of the formability test equipment in order to have a uniform, fast and constant temperature distribution on the specimen; (ii) setting the test operating conditions in order to force the specimen failure in a region where temperature and strain can be easily acquired. Some formability tests were performed and strain fields were measured using an optical measurement system.

Characterization of a superplastic titanium alloy with an experimental and numerical approach based on free-inflation tests

It’s well known that the microstructure dramatically affects the strain behaviour of superplastic materials. Virtually, each batch should be characterized ex novo: optimal ranges of temperature and strain rate as well as material constants have to be defined. An accurate and simple characterization methodology based on a strain condition close enough to the real forming process is of great industrial interest. In this work, a characterization methodology based on an experimental and numerical approach is proposed. Experimental free inflation tests with a pressure jump were carried out on a titanium alloy. Results were used as reference data for an inverse analysis based on the height evolution of the dome. Material constants were calculated by means of a genetic algorithm. The approach was verified with further experimental results and a good correlation was found.