L. Tricarico

Thermo-mechanical simulation of a rolling process with an FEM approach

Abstract The authors propose a model for the study of a hot rolling process, and the approach is based on thermo-mechanical analysis using the Finite-Element Method (FEM). The model can be used to speed up and improve the design and evaluation of the roughing and finishing phases in plate and sheet production. It is able to calculate the temperature distribution in the roll and the plate, the stress and strain fields, throughout a transient analysis done starting from the first phases of the process. The main hypotheses adopted in the formulation are: the elasto-plastic behaviour of the material; and rolling under plane-deformation conditions. The main variables that characterise the rolling process, such as the geometry of the plate and the roll, the loads and the boundary conditions (radius of the rolls, rolling speed, initial and final thickness, initial temperatures of the plate and the roll), have been expressed in a parametric form, this approach giving a good flexibility to the model. During the simulation, an iterative procedure enables the calculation and updating of the load conditions, such as the heat produced by friction on the plate–roll contact arc, and that caused by plastic deformation. The congruence of the results has been evaluated using experimental and theoretical data available in the literature.

Design of process parameters for dual phase steel production with strip rolling using the finite-element method

Abstract Dual-phase steel are low-carbon micro-alloyed steels, characterized by a ferritic multiphase structure (bainite and residual austenite) in which martensite is dispersed. The dual-phase structure depends on the chemical composition of the steel, and on thermo-mechanical treatment realized with lower rolling temperatures. The properties derived from this microstructure give high performance to dual-phase steels in cold-forming applications. In this work the authors propose an approach to simulate tandem rolling for understanding the influence of the process parameters on the thermo-mechanical treatment. The approach is based on the finite-element method (FEM); two models have been developed, the first being a coupled thermo-mechanical model, which describes the behavior of the strip during its travel in each stand of the rolling train; whilst the second is a thermal model, which analyzes the strip transfer between two consecutive stands. The approach has been used also to verify how an existing rolling plant can be adapted to obtain dual-phase steels: the characteristics parameters obtained from an experimental tandem rolling process for dual-phase steel production have been used to design a new rolling process on an existing rolling plant, obtaining the desired thermal cycle of the material during the process.

Plasma plume oscillations monitoring during laser welding of stainless steel by discrete wavelet transform application.

The plasma optical radiation emitted during CO2 laser welding of stainless steel samples has been detected with a Si-PIN photodiode and analyzed under different process conditions. The discrete wavelet transform (DWT) has been used to decompose the optical signal into various discrete series of sequences over different frequency bands. The results show that changes of the process settings may yield different signal features in the range of frequencies between 200 Hz and 30 kHz. Potential applications of this method to monitor in real time the laser welding processes are also discussed.

A comparative study of cut front profiles and absorptivity behavior for disk and CO2 laser beam inert gas fusion cutting

Results of experimental investigations on disk and CO2 laser beam fusion cutting cold-work tool steel 90MnCrV8 are presented. The study was performed with the aim to detect features of the cut front geometry and differences in the corresponding absorptivity behavior as a function of the laser wavelength. Longitudinal sections of the cut front were prepared for different sheet thicknesses, focal plane positions, and cutting speeds. The digitalization of the geometrical cut front data enabled the determination of local inclination angles and the calculation of corresponding Fresnel absorptivity values. The analysis revealed that particular areas of the cut front geometry are preferably inclined to values close to the Brewster angle which offer the theoretical maximum absorptivity for both laser types.

A computer-aided approach for the simulation of the directional-solidification process for gas turbine blades

Abstract A computer-aided approach to simulate the directional-solidification process is proposed, a finite-element method being used to model the thermal gradients and the temperature distribution in the part during solidification. The model allows simulation of the influence of the mould shape, the chilling plate, the filling metal temperature, the oven temperature and size, the ceramic mould thickness, and the withdrawal speed, considering the thermophysical characteristics of the metal and the ceramic. The results of the simulation are compared with the results of an experimental analysis done using thermocouples fixed directly onto a specially-shaped specimen.

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.

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.

A comparative study on fusion cutting with disk and CO2 lasers

In recent years, several studies on laser beam fusion cutting demonstrated significant differences in the characteristics of the well-established CO2 laser cutting and cutting with solid-state disk and fiber lasers. The reasons for the observed differences in cutting efficiency and cut edge quality are still the subject of the current research and not finally clarified. In order to further the understanding of the involved phenomena, a series of cutting experiments with CO2 and disk lasers was carried out using a Design of Experiment (DoE) approach. The particular feature of the applied experimental setup was the similar geometry of both the CO2 and the disk laser beam with comparable values of the focus diameter and the Rayleigh length. Cutting trials on cold work steel test specimens with different sheet thicknesses were performed. The extensions of the generated cut kerf and of the heat-affected zone as well as the recast layer were analyzed in order to reach a better understanding of the physical mechanisms which take part in the cutting process. The experimental evaluation of transmitted and reflected energy losses throughout the kerf was realized by means of PMMA (Polymethylmethacrylate) blocks placed under the sheet during the cutting process.In recent years, several studies on laser beam fusion cutting demonstrated significant differences in the characteristics of the well-established CO2 laser cutting and cutting with solid-state disk and fiber lasers. The reasons for the observed differences in cutting efficiency and cut edge quality are still the subject of the current research and not finally clarified. In order to further the understanding of the involved phenomena, a series of cutting experiments with CO2 and disk lasers was carried out using a Design of Experiment (DoE) approach. The particular feature of the applied experimental setup was the similar geometry of both the CO2 and the disk laser beam with comparable values of the focus diameter and the Rayleigh length. Cutting trials on cold work steel test specimens with different sheet thicknesses were performed. The extensions of the generated cut kerf and of the heat-affected zone as well as the recast layer were analyzed in order to reach a better understanding of the physical mech...

A Quality Evaluation Method for Laser Welding of Al Alloys Through Neural Networks

The authors propose an integrated methodology to evaluate the quality of Aluminium alloy butt joints welded by laser. The method, starting from the observation of the results of an experimental investigation, focuses on the definition of a omni-comprehensive quality index for welded joints. This index is obtained using the limits of imperfections for the quality level defined by the ISO 13919 standard. A neural network system has been developed to classify and evaluate the different welds. The experiments were performed on Al 6110 T61 alloy, welded using a 6 kW CO2 laser beam on plane sheets with a continuous butt joint. Computerised image processing has been used to recognise and to quantify the imperfections in the weld cross section. The defects have been divided into groups, as required by the EN 26520 standard. Due to the huge number of measurements required to imperfections, the artificial neural network very greatly simplifies the relationship between the quality index and the main process parameters. The neural network was trained with a set of data containing very different welding parameter choices. Application of the system aids process parameter selection that has proved to be in good agreement with quality levels measured on experimental welds made under the same conditions.

Fusion cutting of aluminum, magnesium, and titanium alloys using high-power fiber laser

Abstract. The effects of cutting speed and assist gas pressure on laser cutting of 1-mm thick Al 1050, AZ31, and Ti6Al4V lightweight alloys are experimentally investigated. Fiber laser cutting of these materials is not broadly investigated and the acquisition of a new level of knowledge is of fundamental importance for applications like sheet metal trimming in automotive industry. The main process outputs are in depth compared with results reported in literature and obtained by cutting with CO2 and Nd∶YAG lasers. The good cut quality, the high productivity, and the easy delivery of the beam obtained at the same time, corroborate the advantage of using fiber lasers for thin sheets lightweight alloys cutting.