Fabio Rotundo

Advances in Plasma Arc Cutting Technology: The Experimental Part of an Integrated Approach

The experimental part of an integrated approach to design and optimization of plasma arc cutting devices will be presented; in particular results obtained through diagnostics based on high speed imaging and Schlieren photography and some evidences obtained through experimental procedures. High speed imaging enabled to investigate start-up transition phenomena in both pilot arc and transferred arc mode, anode attachment behaviour during piercing and cutting phases, cathode attachment behaviour during start-up transient in PAC torches with both retract and high frequency pulse pilot arc ignition. Schlieren photography has been used to better understand the interaction between the plasma discharge and the kerf front. The behaviour of hafnium cathodes at high current levels at the beginning of their service life was experimentally investigated, with the final aim of characterizing phenomena that take place during those initial piercing and cutting phases and optimizing the initial shape of the surface of the emissive insert.

Experimental analysis of the behaviour of high current electrodes in plasma arc cutting during first cycles

The behaviour of hafnium (Hf) cathodes at the beginning of their service life when operating at high current levels (250 A) in the plasma arc cutting (PAC) process has been experimentally investigated with the final aim of describing the phenomena that take place during those initial cutting cycles (CCs), and optimizing (with respect to expected service life) the initial shape of the electrode emissive surface. The experimental tests were carried out under realistic operative conditions for cutting mild steel plates with oxygen/air as the plasma/shield gas. Starting with an electrode with a new planar emissive surface, subsequent CCs were completed. With each cycle the dimensions of the growing concave recess naturally created on the insert, and the amount of Hf oxide (HfO2) collected on the nozzle surface and on the flat surface around the edge of the recess was recorded. Morphological and compositional analysis of the tested electrodes and nozzles led to a detailed description of Hf erosion mechanisms, modifications of the Hf insert morphological structure, and effects of HfO2 deposits on the nozzle inner surface. High-speed imaging during early stages of the transferred arc mode has been used to highlight the possible presence of non-destructive double-arcing phenomena taking place during the first few CCs as a consequence of deposition of HfO2 on the nozzle. Conclusions can be drawn concerning the optimization of the dimensions of the initial recess of the Hf insert fit to avoid the massive deposition of material on the nozzle inner surface that would cause a strong reduction of electrode and nozzle service life and a rapid degrading of cut quality.

A Study of Residual Stresses in Al/SiCp Linear Friction Weldment by Energy-Dispersive Neutron Diffraction

Linear friction welding (LFW) is a solid state joining process for bonding of two flatedged, complex geometry components through relative reciprocating motion under axial (compressive) forces. Although the proof of principle has been obtained some time ago, recently a number of studies have been published aimed at optimising the joining operations to obtain best joint strength and reduced distortion and residual stress. The present paper is devoted to the study of linear friction welds between components made from aluminium alloy 2124 matrix composite (AMC) reinforced with 25vol% particulate silicon carbide (SiCp). Neutron diffraction was used to measure interplanar lattice spacings in the matrix and reinforcement, and to deduce residual elastic strains and stresses as a function of distance from the bond line. Significant asymmetry is observed in the residual stress distribution within the two components being joined, that may be associated with the difference in the microstructure and texture.

Aluminum and Magnesium Metal Matrix Nanocomposites

The book looks into the recent advances in the ex-situ production routes and properties of aluminum and magnesium based metal matrix nanocomposites (MMNCs), produced either by liquid or semi-solid ...

Linear Friction Welding of a 2024 Al Alloy: Microstructural, Tensile and Fatigue Properties

The possibility of using linear friction welding (LFW) to produce high quality joints on an aerospace grade aluminium alloy (AA2024) was evaluated. In this solid state joining process the bonding of two flat edged components is achieved through frictional heating, induced by their relative reciprocating motion, under an axial compressive force. The Al joints were subjected to microstructural and mechanical characterization, including hardness and tensile tests. S-N probability curves were also computed after preliminary axial fatigue tests. No post-weld heat treatment was performed. The microstructural analyses showed substantially defect-free joints, with a relevant plastic flow in the thermo-mechanically altered zone. Maximum hardness decrease in the joint zone was approximately only 5% in respect to the base material. The joint efficiency was about 90% with respect to the ultimate tensile strength, with a slight reduction in the elongation to failure. Good fatigue performances were also detected.

Investigation of Thermal Nonequilibrium in a Plasma Arc Welding Process: Modeling and Diagnostics

In this paper, a plasma arc welding (PAW) process operating in pure Ar with constant current at 40 A has been characterized by means of a thermofluid-dynamic modeling and optical emission spectroscopy (OES). Excitation temperature of Ar atoms in the fringes of the arc measured using the OES has been found higher than temperature calculated from local thermodynamic equilibrium modeling, even if radiation reabsorption is taken into account using a discrete ordinate model; on the contrary, excitation temperature of Ar atoms is in a good agreement with electron temperature calculated using a two-temperature model for thermal nonequilibrium neglecting radiation reabsorption. Results reported suggest that for the investigated PAW process, thermal nonequilibrium due to steep radial temperature gradients induces a Boltzmann distribution of excited states at the electron temperature.

Comparing the effect of different atmospheric pressure non-equilibrium plasma sources on PLA oxygen permeability

Plasma technology is widely adopted for polymer surface modification. In this work polylactide (PLA) samples have been exposed to the plasma region generated by three different plasma sources operating at atmospheric pressure: a floating electrode dielectric barrier discharge (FE-DBD), a novel linear corona discharge and a DBD roller. The sources have been supplied with a high voltage generator capable of producing pulses with a rise rate in the order of several kV/ns in order to obtain diffuse plasma and avoid local damage to the membrane; air and argon have been used as working gases. Pure oxygen permeation tests in PLA films have been carried out by means of a closed-volume manometric apparatus working at 35°C with a pressure difference of pure O2 of about 1 bar applied across the membrane. Tests have been performed shortly after the plasma treatment and also replicated at different times in order to investigate the durability of surface modification. The effects of voltage, pulse repetition frequency (PRF) and exposure time on the membrane surface characteristics and barrier property have been studied.

Plasma arc cutting technology: simulation and experiments

Transferred arc plasma torches are widely used in industrial processes for cutting of metallic materials because of their ability to cut a wide range of metals with very high productivity. The process is characterized by a transferred electric arc established between an electrode inside the torch (the cathode) and another electrode, the metallic workpiece to be cut (the anode). In order to obtain a high quality cut and a high productivity, the plasma jet must be as collimated as possible and must have the higher achievable power density. Plasma modelling and numerical simulation can be very useful tools for the designing and optimizing these devices, but research is still in the making for finding a link between simulation of the plasma arc and a consistent prevision of cut quality. Numerical modelling of the behaviour of different types of transferred arc dual gas plasma torches can give an insight on the physical reasons for the industrial success of various design and process solutions that have appeared over the last years. Diagnostics based on high speed imaging and Schlieren photography can play an important role for investigating piercing, dross generation, pilot arcing and anode attachment location. Also, the behaviour of hafnium cathodes at high current levels at the beginning of their service life can been experimentally investigated, with the final aim of understanding the phenomena that take place during those initial piercing and cutting phases and optimizing the initial shape of the surface of the emissive insert exposed to plasma atmosphere.

A Study on Similar and Dissimilar Linear Friction Welds of 2024 Al Alloy and 2124Al/SiCP Composite

The aim of the present work is to evaluate the possibility of using the Linear Friction Welding (LFW) technique to produce similar and dissimilar joints between a 2024 Al alloy and a 2124Al/25vol.%SiCP composite. In this solid state joining process the bonding of two flat edged components is achieved through frictional heating induced by their relative reciprocating motion, under an axial compressive force. Microstructural characterization of the welds was carried out by optical and scanning electron microscopy, to investigate the effect of LFW both on the aluminium alloy matrix and the reinforcement particles. The mechanical behaviour of the welded specimens was studied by means of hardness and tensile tests. The mechanisms of failure were investigated by SEM analyses of the fracture surfaces. LFW joints in MMCs resulted substantially defect free, with a uniform particle distribution, while a partial lack of bonding at the corners was observed in the others welds. The hardness decreased by approximately 10% in the welded zone, with some data fluctuations due to the complex microstructural modifications introduced by the LFW process. The joint efficiency, evaluated in respect to the UTS, was 90% for the Al alloy joints and 80% for the MMC joints. A decrease in the elongation to failure was measured in all the LFW specimens, probably related to the orientation of the plastic flow in the TMAZ, where the fracture generally occurred.

Forming of Metal Matrix Composites

Metal matrix composites (MMCs), through a proper selection of matrix and reinforcement, offer unusual combinations of physical and mechanical properties such as high specific strength and stiffness, high thermal and electrical conductivity, high wear resistance, good corrosion resistance, and good impact and fatigue properties, together with superior thermal stability when compared to the unreinforced matrix alloys. Near net shape forming and improvements in microstructure, strength, and ductility can be achieved in discontinuously reinforced MMCs through the application of primary or secondary plastic deformation processes such as extrusion, rolling, forging, superplastic forming, or friction processing. Thermomechanical processing can be directly a part of the production process or follow a primary stage of fabrication or consolidation. Processing can be mainly divided in hot and cold working, the latter often followed by annealing. The set of processing conditions (e.g., temperature, total deformation, deformation rate) is of crucial importance with respect to the resultant microstructural and consequently mechanical properties, and must be optimized with respect to the system to be processed (matrix alloy, reinforcing hard phase, reinforcement size, and volume content).