P.A.R. Rosa

Innovative Testing Machines and Methodologies for the Mechanical Characterization of Materials

This paper is focused on the development of innovative testing machines and methodologies for the characterization of materials in working conditions similar to those found in mechanical processing of materials. Special emphasis is given to the design, fabrication, and instrumentation of a flexible drop weight testing machine, an electromagnetic compressive Hopkinson bar, and an electromagnetic cam-driven compression testing machine that are capable of performing the mechanical characterization of materials under mediumand high rates of loading. The fundamentals of plastic flow in selected forming and cutting processes are utilized to assess the validity and reliability of the proposed testing machines and associated experimental methodologies. Experimental data with technical pure Lead allows understanding the combined influence of strain, strain rate, and strain rate versus strain loading paths in the overall stress response of the materials. Results also show that the new proposed electromagnetic cam-driven compression testing machine equipped with a root type or logistic cam profile can successfully replicate the operating conditions of the two other testing machines.

Experimental study of micro electrical discharge machining discharges

Micro electrical discharge machining (μEDM) is an atmospheric-pressure plasma-assisted technology that uses point-to-plane discharges in liquid dielectrics to remove microscopic quantities of electrically conductive materials. In this work, an innovative μEDM prototype machine was specifically designed and fabricated to produce and control single spark discharges, thus, resolving the typical limitations of (multi-discharge) commercial machines. The work analyses the type of discharge and the micro-plasma electron-density values obtained for 0.5–38 μm gap sizes, 3–10 000 μs pulse durations, 75–250 V low breakdown voltages, and 1–20 A discharge currents, using different combinations of metallic electrodes in oil and in water. Results allow fitting, for micro-scale and low voltages, an empirical law between the maximum gap-size for breakdown, the breakdown voltage, and the effective stress-time. The electron density ne is obtained by optical emission spectroscopy diagnostics of the Hα-line Stark broadening (...

Invert-Forming of Thin-Walled Tubes Using a Die

Abstract The production of sound thin-walled tubular parts by invert-forming is generally limited to components having geometrical features within a compact range. Recent published works in the field present new results relevant to process and die design. However, gaps of knowledge can still be found in understanding the influence of design parameters and interface friction on the overall formability of the process. The aim of this paper is to extend the actual knowledge of the process by means of a comprehensive theoretical and experimental investigation. The theoretical investigation is supported by numerical predictions based on the finite element flow formulation and the overall methodology is assessed by means of experimental tests on AA6060 aluminium-alloy tubes under laboratory-controlled conditions.

On the Analysis of the Expansion and Reduction of Thin-Walled Tubes Using a Die:

Abstract The production of sound thin-walled tubular parts by expansion and reduction using a die is generally limited to components having geometrical features within a compact range. Basic design rules, relevant to process and die design are mainly derived from the accumulated experience of both manufacturers of tubular parts and suppliers of machine tools. However, gaps of knowledge can still be found in understanding the influence of process parameters on material flow and in providing an adequate description of the modes of deformation that are associated with the formability limits induced by ductile fracture, wrinkling, and buckling. The aim of the present paper is to extend the actual knowledge on the expansion and reduction of thin-walled tubes using a die by means of a comprehensive theoretical and experimental investigation. The theoretical investigation is supported by axisymmetric and three-dimensional numerical simulations based on the finite element flow formulation. The experimental work, performed on AA6060 aluminium alloy tubes, consisted of specially designed tests that were carried out under laboratory-controlled conditions with the intention of supporting and validating the overall investigation.

Interference-Fit Joining of Aluminium Tubes by Electromagnetic Forming

Interference-fit joining of tubes by electromagnetic forming is an innovative and environmental friendly technology that can successfully replace conventional joining technologies based on fasteners, structural adhesives, welding and brazing. The technology works at room temperature, allows joining dissimilar materials and offers potential to foster new applications in the assembly of lightweight tubular frame structures. As with all new technologies, there is a need to understand interference-fit joining of tubes by electromagnetic forming in terms of its major parameters with the aim of identifying their influence on the overall strength of the joints and establishing the useful range of process operating conditions. This article investigates the interference-fit joining of aluminum-alloy tubes (AA6082-O) with mandrels made from different metallic and polymeric materials (AA6082-O, AISI1045 and Erlaton 6SA). Results show that the strength of the joint and the associated failure mechanisms are directly related to process parameters and materials.

On the utilization of pin-on-disc simulative tests for the calibration of friction in metal cutting

Abstract This article proposes a new design for pin-on-disc machines and introduces a research methodology that aims at providing a new level of understanding for the influence of surface texture and roughness on the average value of the friction coefficient. The new design for pin-on-disc machines increases the overall stiffness, eliminates the need for counter weights, and allows tests to be carried out under variable loading conditions and includes a unit for producing and regenerating the desired texture and roughness in the surface of the discs after completion of each test. The comparison between the friction coefficient obtained with pin-on-disc tests and that acquired in metal cutting laboratory conditions allows concluding that pin-on-disc tests, when performed with an adequate control of texture and surface roughness, are capable of providing a good estimate of the average value of the friction coefficient in metal cutting applications.

The Role of Interfaces in the Evaluation of Friction by Ring Compression Testing

The role of interfaces in friction is most frequently not taken into account when performing the evaluation of the coefficient of friction by means of process and simulative tribology tests. Despite being difficult and time consuming to control surface roughness as well as the formation of oxide films through exposure to surrounding medium, it is very important to perform the experiments under conditions similar to those commonly found in real metal forming processes. As accuracy and reliability of the experimentally determined coefficients of friction depend on the similarity to real metal forming conditions, this paper is aimed to provide a comprehensive analysis on the influence of oxide films and surface roughness in the evaluation of friction by means of the ring compression test. The paper presents an innovative experimental approach for ring compression testing under controlled conditions of exposure to atmosphere and surface roughness. Quantitative data obtained in presence of inert and active gas shields and across typical values of surface roughness allow understanding the role of interfaces in friction and to build a conscience on its prospective source of modelling errors due to experimental drift from real metal forming. Results show an increase of the coefficient of friction up to 30% when active gas shields are employed.

Tribology in Metal Cutting

This chapter revisits tribology tests in metal cutting in order to obtain new fundamental knowledge on friction and to understand which technical modifications and operating parameters need to be developed and implemented in order to obtain good estimates of the coefficient of friction. The methodology draws from the development of new equipment and testing procedures focused on the interaction between surrounding medium, surface roughness and freshly formed surfaces to the independent determination of the coefficient of friction.

Mechanical characterization of materials for bulk forming using a drop weight testing machine

Abstract The main limitation of mechanical testing equipments is nowadays centred in the characterization of materials at medium loading rates. This is particularly important in bulk forming because strain rate can easily reach values within the aforesaid range. The aim of this article is twofold: (a) to present the development of a low-cost, flexible drop weight testing equipment that can easily and effectively replicate the kinematic behaviour of presses and hammers and (b) to provide a new level of understanding about the mechanical characterization of materials for bulk forming at medium rates of loading. Special emphasis is placed on the adequacy of test operating conditions to the functional characteristics of the presses and hammers where bulk forming takes place and to its influence on the flow stress. This is needed because non-proportional loading paths during bulk forming are found to have significant influence on material response in terms of flow stress. The quality of the flow curves that were experimentally determined is evaluated through its implementation in a finite-element computer program and assessment is performed by means of axisymmetric upset compression with friction. Results show that mechanical characterization of materials under test operating conditions that are similar to real bulk forming conditions is capable of meeting the increasing demand of accurate and reliable flow stress data for the benefit of those who apply numerical modelling of process design in daily practice.

How Hydrogen Dielectric Strength Forces the Work Voltage in the Electric Discharge Machining

An electro-thermal model based on the Joule heating effect is proposed to simulate a single discharge in an electric discharge machining process. Normally, the dielectric strength of the hydrocarbons oil is approximately 20 MV/m, but it varies with both the thickness of the film and its decomposition. After the breakdown, the hydrocarbon oil has an average dielectric strength value of 2 MV/m. This value is close to the dielectric strength of the hydrogen, which is the main gas that results from the hydrocarbon oil decomposition, at temperatures between 6000 K and 9000 K. Therefore, the electric discharge occurs in a hydrogen atmosphere that imposes both the discharge gap and the work voltage. A 200 V voltage is associated to a 100 μm discharge gap, leading to a 20 V work voltage. Therefore, the 3 V work voltage control corresponds to approximately 15 μm. In other words, the increase of the discharge gap originates other discharge during the discharge pulse. The work voltage control, together with the multiple discharge method, is taken into account. The 100 μm discharge gap corresponds to the higher value of the transitory discharge gap that over evaluates the material removal and the tool wear rates. The results of the numerical simulations are validated with experimental data.