Michela Longo

Electrical Discharge Machining of Micro Holes on Titanium Sheets

Micro-Electrical Discharge Machining (μEDM) has become a widely accepted non-traditional material removal process for micro-manufacture of conductive materials considered difficult to be cut using traditional machining technologies. Moreover, EDM is an ideal process for obtaining burr-free micron-size apertures with high aspect ratios. Aim of this work was to investigate the feasibility of drilling micro holes on titanium using μ-EDM technology. Titanium plates having a thickness equal to 0.5 mm were taken into account and the holes were performed using a carbide electrode having a diameter equal to 0.3 mm. The Design Of Experiment (DOE) method was used for planning the experimental campaign and ANOVA techniques were applied to study the relationship between process parameters and final output. In particular, the most important process parameters such as peak current, pulse duration, frequency and electrode rotation speed were investigated as a function of material removal rate, wear rate and machining accuracy. Geometrical and dimensional analyses were carried out on micro-holes using both optical and scanning electron microscopes to evaluate both the over cut and the rate of taper.Copyright

Friction Stir Spot Welding (FSSW) of Aluminum Sheets: Experimental and Simulative Analysis

Friction Stir welding (FSW) is a solid state joining process developed by TWI (The Welding Institute) in 1991. This technology is suitable for joining different materials even considered difficult to be welded using more traditional techniques and it is appropriate to weld materials in different configurations (such as butt, lap, circumferential, T-joint etc). Recently, starting from the FSW approach, a new technology called Friction Stir Spot Welding (FSSW) was developed. In this case, instead of moving along the weld seam, the tool only indents two overlapped parts. In some applications, this technology can be considered as a valid alternative for single point joining processes like resistance spot welding (RSW) and riveting processes. This work deals with an experimental study of the FSSW process for the lap-joining of thin aluminum sheets. In particular, an experimental campaign was performed on AA6060 T6 aluminum sheets having a thickness equal to 2 mm. The FSSW process was applied on couples of overlapped sheets by varying the tool rotational speed, and by keeping fixed the other process parameters, such as axial feed rate, indentation depth, and dwell time. Welding forces distributions were recorded during the process. Preliminary tensile tests and metallurgical analyses were also performed to evaluate the quality of the joints as function of the chosen process parameters. A numerical model of the FSSW process was developed and implemented using the commercial FEM code Deform 3D. The model parameters were set according to the experimental evidence.

Control System Algorithm for the Prediction of Sprinback in Air Bending

The phenomenon of springback, which is ruled by strain recovery after removal of forming loads, is of remarkable interest in air bending of metal sheets. In this process, the final angle is affected by a number of parameters related to both process geometry (sheet thickness, die and punch radii) and material properties (elastoplastic stress-strain law); because of this, punch stroke has to be calculated in a nontrivial way and a number of input parameters should be taken into account. In this work the study of total load as a function of displacement is used to collect information about material stress-strain law; using this approach, load data may be exploited to fine tune the mathematical description of the material and, finally, to improve springback prediction. A customized press brake able to measure both displacement and force during bending was fabricated for this purpose. The press brake is equipped with a control system algorithm able to collect material information directly during the initial stage of the bending process. These collected data are used to feed a model based on a FEM simulation and the model output is the final punch displacement suitable to obtained a specific bending angle after unloading. The program utilized for the simulation is Deform 2D. Preliminary tests were executed on metal sheets having different thickness.

Coupled Simulative-Experimental Procedure for Studying the Solid State Bonding Phenomena

In extrusion operations, material solid state welding takes place thanks to the very high pressure and temperature at which the material undergoes between the several welding criteria developed, the attention was particularly focused on the Piwnik and Plata one. In this criterion a suitable parameter w, calculated as the interface pressure and the effective stress ratio integrated along the time, has been defined. According to this criterion the material should start to weld as this parameter exceed a limit value wlim. In the present paper a new procedure for the wlim identification as function of the temperature, based on coupled experimental-simulative strategy, is proposed. In particular, flat rolling experimental tests of sandwiches made of two rectangular specimens in AA6082 alloy were performed. The specimens were characterized by different heights in order to consider different compression ratio, that means different interface pressure and effective stress distributions. All the tests were repeated at different temperature. Once the experimental tests were performed, FEM simulations of the rolling process were run for the very same conditions. Thanks to a suitable user routine developed and implemented in the calculus code, it was possible to evaluate the steady state value of the w parameter for all the different conditions at the steady state conditions. By verifying if the actual experimental test demonstrated the presence or less of material bonding, it was possible to identify the wlim values as function of the temperature. Particular attention must be paid to the study of the macrostructure of the welded material in order to identify the influence of the process parameter on the weld quality. This means that it is possible to identify not only if the weld will take place, but also if it will be qualitatively adequate.

Experimental investigation of fatigue crack growth in the welding nugget of FSW joints of a 6060 Aluminum Alloy

The work investigates the fatigue behavior of friction stir welded butt joints by means of fracture mechanics techniques. FSW joints of artificially aged AA6060 T6 aluminum alloy were studied. The welding was performed on 8 mm thick butt joined sheets by means of a CNC machine tool. Welding speed in the range between 117 and 683 mm/min and tool rotational speed between 838 and 1262 rpm were considered. Fatigue crack growth tests were performed according to ASTM E647 standard on CT specimens, under constant load amplitude conditions, at 0.1 minimum to maximum load ratio, with the notch placed in the dynamic recrystallization zone of weld nugget, oriented along the welding direction. The comparison of results demonstrates the crack growth rate is always equal or lower than the base material at low values of stress intensity range. At ΔK values above 12 MPa√m, crack growth rate was found to be higher than base material for high feed, low speed and high feed/speed ratio.

Characterization of Friction Stir Welded Tubes by Means of Tube Bulge Test

Mechanical properties of friction stir welded joints are generally evaluated by means of conventional tensile test. This testing method might provide insufficient information because maximum strain obtained in tensile test before necking is small; moreover, the application of tensile test is limited when the joint path is not linear or even when the welds are executed on curved surfaces. Therefore, in some cases, it would be preferable to obtain the joints properties from other testing methods. Tube bulge test can be a valid solution for testing circumferential or longitudinal welds executed on tubular workpieces. The present work investigates the mechanical properties and the formability of friction stir welded tubes by means of tube bulge tests. The experimental campaign was performed on tubular specimens having a thickness of 3 mm and an external diameter of 40 mm, obtained starting from two semi‐tubes longitudinally friction stir welded. The first step, regarding the fabrication of tubes, was performe...

The Simulation of Metal Foams Forming Processes

Metal foams are two-phase compounds of a gas and a solid with several interesting physical and mechanical properties; in particular they have very low density, good rigidity, excellent energy absorption, high vibration damping. At now, the final shape of foamed devices is directly obtained through the foaming process itself and no further shaping steps are expected. Anyway, the plastic formability of metal foams, in order to both characterize the material itself and to produce more complex parts, seems to be useful for several industrial applications. Since metal foams are quite new products, the basic aspects ruling plastic deformation processes are still partially unknown and FEM methods may represent a valid tool for deepening these topics. This work deals with the formability of Aluminum Foam Sandwich (AFS) panels and it is focused on the FEM simulation of a compression processes. A numerical model was set up by using the FEM code Deform 2D v10.1. Foam behaviour was simulated by means of a compressible (porous) material model and the foam cracking was simulated using a damage model based on the foam density parameter. Some FEM routines were implemented into the FEM code to take into account both the non-homogeneous distribution and the strain hardening effect of the foam cells. An experimental campaign based on the compression of AFS panels made of close cells foam was carried out to fine tune and to validate the model. In particular, experimental data regarding load stroke curves and foam density were used to optimize the material description. An innovative solution, based on a non-linear relation between foam density and effective strain of the foam, was implemented into the FEM code.

Characterization Of An EBL System: The Influence Of Process Parameters On Thickness Resist And Engraving Shapes

In LIGA process, the low flexibility due to the use of masks might be overcome using the electron beam of a scansion electron microscope (SEM) instead of the synchrotron radiation. In this way, through the vector control of the beam, it is possible to irradiate a specific path on the resist without using any mask. Anyway, it is important to remark that the Electron Beam Lithography (EBL) can not include all the applications of the X‐ray LIGA technique but it could be a valid alternative only for some specific uses. In particular, some limits concerning the impression of high thickness resists are expected. An EBL system based on a SEM was recently implemented by the authors and some tests were carried out to characterize the device performances. Aim of this work is to asses the system performances in terms of maximum impressible thickness resist and shape of the engraved entities. Several tests were carried out by varying the resist thickness and the process parameters.

Electrical discharge machining of micro holes on titanium sheets: effect of process parameters and electrode shape

Micro-Electrical Discharge Machining (μEDM) has become a widely accepted non-traditional material removal process for micro-manufacture of conductive materials considered difficult to be cut using traditional machining technologies. Moreover, EDM is an ideal process for obtaining burr-free micron-size apertures with high aspect ratios. Aim of this work was to investigate the feasibility of drilling micro holes on titanium using μ-EDM technology. Titanium plates having a thickness equal to 0.5 mm were taken into account and the holes were performed using a carbide electrode having a diameter equal to 0.3 mm. The Design Of Experiment (DOE) method was used for planning the experimental campaign and ANOVA techniques were applied to study the relationship between process parameters and final output. In particular, the most important process parameters such as peak current, pulse duration, frequency and electrode rotation speed were investigated as a function of material removal rate, wear rate and machining accuracy. Geometrical and dimensional analyses were carried out on micro-holes using both optical and scanning electron microscopes to evaluate both the over cut and the rate of taper.Copyright