Papers by Author: Athanasios G. Mamalis

Abstract: A frequent application of electromagnetic forming in the industry is joining by electromagnetic compression of tubular parts. The goodness of the joints is determined by the strength of the joint, and it is considered excellent if it reaches the strength of the weaker material pair. Strength of these joint are generally tested for tensile and torsion loadings. In order to increase strength, one or more horizontal and/or radial grooves can be fabricated onto the male joining pair to utilize form fitting besides interference fit. In the present paper, the strength of the joint is studied by finite element simulations.

Abstract: The high pressure (50 MPa - 2 GPa) – high temperature synthesized MgB2 bulk materials are characterized by nearly theoretical density (1-2% porosity), 80-98% connectivity, extremely high critical current densities (e.g. at 20 K, in 0–1 T jc=1.3–1.0 MA·cm-2 (with 10% SiC) and jc= 0.92 – 0.73 MA·cm-2 (without doping)), large irreversibility fields (Birr(18.4 K) =15 T and Birr (0 K) = 32.5 T) and high upper critical fields (Bc2 (22 K) =15 T and Bc2(0 K) ~ 42.1 T). The transformation of grain boundary pinning to point pinning in MgB2-based materials with increasing manufacturing temperature (from 800 to 1050 oC) under pressures from 0.1 to 2 GPa correlates well with an increase in critical current density in low external magnetic fields caused by the redistribution of boron and the oxygen impurities in the material structure. As the manufacturing temperature increases, the discontinuous oxygen enriched layers transform into distinct Mg-B-O inclusions and the size and amount of inclusions of higher magnesium borides MgBX (X>4) are reduced. The effect of oxygen and boron redistribution can be enhanced by Ti or SiC additions.

Abstract: The contact corrosion phenomena occuring at the electrical networks when two dissimilar metallic parts are connected to each other causes serious technical and economical problems. One possible solution to reduce the contact corrosion is the application of properly prepared bimetallics. The paper introduces the explosive cladding technique which can be successfully applied for manufacturing bimetallics matched to the given electrical circuit furthermore application possibilities of the explosively cladded bimetallics at electrical networks, railway systems and vehicles.

Abstract: In the present work the determination of intrinsic properties in ferromagnetic materials by using magnetic non-destructive techniques is discussed. Barkhausen noise and bulk permeability measurements were used in various steels. The monotonic dependence of the magnetic response on microstructural features was verified.

Abstract: Electromagnetic forming is a high speed forming process, wherein the forming pressure is created by high energy density electromagnetic pulse. Besides direct shaping there are other application areas as well, so electromagnetic plastic forming is a potential field of creating joints between tube and rod-like components. Connecting components of dissimilar materials is an increasing demand in the manufacturing process of structures in the automotive industry. The application of new technologies, such as electrodynamic, especially electromagnetic forming, is a possible method to satisfy these demands. The article summarizes the most important fundamentals of electromagnetic forming; in particular, tube-rod joints, the main types of such joints; interference-fit and form-fit joints are described. Experiments, which were carried out producing tube-rod joints with electromagnetic forming, are also introduced. A new type of form-fit joints for tube-rod connections has been developed, which can withstand not only tensile loads but also torsion. Experiments and mechanical tests have proved the applicability of this kind of joints.

Abstract: Some of the activities of the Project Center for Nanotechnology and Advanced Engineering (PC-NAE), a joint initiative of the Greek National Center for Scientific Research Demokritos and the Russian Research Center Kurchatov Institute, in advanced manufacturing engineering are briefly outlined, focusing onto some recent trends and developments in manufacturing from macro-, micro-, to nanoscale of advanced materials in the important engineering topics nowadays from industrial, research and academic point of view: nanotechnology/ultraprecision engineering and advanced materials under shock loading, with industrial applications to net-shape manufacturing, bioengineering, energy and transport.

Abstract: The application of the High Energy Rate Forming (HERF) represents a new paradigm in the field of production of knowledge-based more components materials: furthermore, joining by plastic deformation of the materials is carried out directly, by high speed, high energy shock waves, without using energy transforming equipment as hydraulic presses etc. The energy sources of the HERF processes are either the electrical energy stored in capacitors or chemical energy stored in the high explosives. High explosives can be utilized for many metalworking techniques; however the three main types of explosive metalworking are: Explosive welding and cladding Explosive tubeforming Explosive compaction of powders and granulates. The present work briefly introduces the principles and practices of the three main types of the explosive metalworking techniques mentioned above and discusses aspects of their numerical simulation.

Abstract: Two types of electrodynamic forming process have been developed: electromagnetic and electrohydraulic forming. In the case of electromagnetic forming, the energy stored in a capacitor bank is discharged through a coil, which means that the electrical interaction between the coil and the plate or a tubular part to be formed results in deformation of the workpiece. However, in the case of electrohydraulic forming, the capacitor bank is discharged through a spark gap or filament wire; the deformation of the workpiece is due to the shockwaves, generated by the discharge process in a transmitting medium. In both processes, a large amount of energy is released in extremely short time, therefore these processes are considered to be high energy rate forming processes. These high energy rates, result in increasing the formability of the materials in many cases, and obtain significant deformations also for some materials that normally do not behave plastically. The utilization of the energy stored in the capacitor bank is significantly better in the case of electrohydraulic forming, because the released energy is converted directly to pressure waves, results in forming of higher strength materials. Both metallic and non-metallic materials can be formed by the technologies of electromagnetic and electrohydraulic technologies. In the present paper some aspects and applications of these high energy rate methods are briefly outlined mainly focusing on the automotive industry, involving expansion or compression forming of tubular parts, joining and assembly operations.

Abstract: The implementation of magnetostrictive delay lines in distribution measurement of temperature is presented in this paper. The sensing method and measurements as well as the proper selection and treatment of the amorphous sensing core are discussed.

Abstract: Superconducting (SC) and mechanical properties of spark plasma (or SPS) produced MgB₂ –based materials allow their efficient applications in fault current limiters, superconducting electromotors, pumps, generators, magnetic bearings, etc. The synthesized from Mg and B at 50 MPa, 1050 °C for 30 min material has a density of 2.52 g/cm³, critical current density, j_c = 7.1•10⁵ A/cm² at 10 K , 5.4 •10⁵ A/cm² at 20 K, and 9•10⁴ A/cm² at 35 K in zero magnetic field; at 20 K its field of irreversibility B_irr(20)=7 T and upper critical field B_c2(20)=11 T; microhardness H_V=10.5 GPa and fracture toughness K_1C =1.7 MPa•m^1/2 at 4.9 N-load. SPS-manufactured in- situ MgB₂-based materials usually have somewhat higher jc than sintered ex-situ. The pressure variations from 16 to 96 MPa during the SPS-process did not affect material SC characteristics significantly; the j_c at 10-20 K was slightly higher and the material density was higher by 11%, when pressures of 50-96 MPa were used. The structure of SPS-produced MgB₂ material contains Mg-B-O inclusions and inclusions of higher borides (of compositions near MgB₄, MgB₇, MgB₁₂, MgB₁₇, MgB₂₀), which can be pinning centers. The presence of higher borides in the MgB₂ structure can be revealed by the SEM and Raman spectroscopy.