Donato Sorgente

Experimental Investigation on Fiber Laser Cutting of Ti6Al4V Thin Sheet

The high strength to weight ratio and good corrosion resistance of titanium alloys, have led to an increasing use of these materials, particularly in the aerospace industry. The laser cutting technique may be a promising tool in machining titanium alloy parts like those with subsequent welding requirement: in this case, surface quality of the kerf edges is of great importance. The low thermal conductivity and the high chemical activity of titanium alloys lead, in fact, to alterations of the surface properties of the machined zone. This paper presents the results of titanium alloy laser cutting using a 2 kW fiber laser. The cutting process was performed in continuous wave mode and using Argon as shear gas. Laser cuts were realized on titanium alloy Ti6Al4V sheets 1mm thick. Image analysis and microscopy, were carried out to examine the cutting edge quality features including thickness of the recast layer and heat-affected zone.

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.

Laser–arc hybrid welding of Ti6Al4V titanium alloy: mechanical characterization of joints and gap tolerance

This paper reports the results of a comparison between the mechanical characteristics of butt joints in 3.0 mm thick Ti6Al4V titanium alloy sheets, made by laser welding with no filler material and by laser–arc hybrid welding. Vickers hardness tests have been performed on cross-sections of the beads obtained. In particular, the influence of the gap between the sheets in test welds performed using CO2 laser–MIG hybrid welding has been analysed. Cross-sections of the weld beads obtained with several different gap sizes have been analysed morphologically. The welded joints have been subjected to draw testing, and joint deformation behaviour has been analysed using an optical deformation measurement system based on stereoscopic image capture (ARAMIS 3D analysis system).

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.

Analysis of Different Specimen Geometries for Tensile Tests in Superplastic Conditions for an Aluminium Alloy

Different geometries of tensile test specimens have been analyzed to get information about the variability of the measured physical quantities during tensile tests in superplastic conditions. A commercial aluminium alloy (AA5083) sheet has been used for the experimentation. The influences of main geometrical parameters on the elongation to failure, the measured stress and other test responses have been investigated using a universal testing machine and a split furnace. A numerical model has been used to better understand deformation phenomena involved in tensile tests.

Preliminary Study on the Formability of a Laser-Welded Superplastic Aluminum Alloy

In this work, the effect of the laser-material interaction on the formability of a superplastic aluminum alloy was investigated. In applications such as Tailor-Welded Blanks and in the manufacturing of very large components with a complex shape, laser welding combined with superplastic forming may be a very fitting industrial tool. Bead on plate tests were carried out in order to simulate the laser-welding process and then, free inflation tests were performed to evaluate the compatibility of these two processing techniques. The Al-Mg alloy used in this work has a very small grain size which ensures the superplastic behavior. With the aim of preserving this peculiarity, the following aspects on the formability were investigated: (i) the surface condition of the bead before the forming test (with and without the removal of the excess of metal); (ii) the effect of the travel speed of the laser source on the mean grain size; (iii) the introduction of a refiner, commonly used in aluminum casts, in the molten pool in order to further reduce the mean grain size.

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.

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.

Laser surface micro texturing to enhance the frictional behavior of lubricated steel

Surface micro-texturing has been widely theoretically and experimentally demonstrated to be beneficial to friction reduction in sliding contacts under lubricated regimes. Several microscopic mechanisms have been assessed to concur to this macroscopic effect. In particular, the micro-textures act as lubricant reservoirs, as well as traps for debris. Furthermore, they may produce a local reduction of the shear stress coupled with a stable hydrodynamic pressure between the lubricated sliding surfaces. All these mechanisms are strongly dependent both on the micro-texturing geometry and on the operating conditions. Among the various micro-machining techniques, laser ablation with ultrashort pulses is an emerging technology to fabricate surface textures, thanks to the intrinsic property of laser light to be tightly focused and the high flexibility and precision achievable. In addition, when using sub-ps pulses, the thermal damage on the workpiece is negligible and the laser surface textures (LST) are not affected by burrs, cracks or resolidified melted droplets, detrimental to the frictional properties. In this work several LST geometries have been fabricated by fs-laser ablation of steel surfaces, varying the diameter, depth and spacing of micro-dimples squared patterns. We compared their frictional performance with a reference nontextured sample, on a range of sliding velocities from the mixed lubrication to the hydrodynamic regime. The measured Stribeck curves data show that the depth and diameter of the microholes have a huge influence in determining the amount of friction reduction at the interface. Different theoretical interpretations to explain the experimental findings are also provided.