António Castanhola Batista

Residual Stress after EDM – FEM Study and Measurement Results

During electro discharge machining (EDM) a characteristic residual stress profile is formed. The process is essentially thermal and leads to tensile residual stresses at the surface. The profile form is independent from the discharge energy. However, the profile depth, the maximum value of the tensile residual stresses and the depth where the maximum is observed are found to be energy dependent. The residual stress profile is characterised by an increase from a certain tensile value at the surface to a maximum underneath the surface, followed by a decrease until slightly compressive residual stresses are reached. In the present paper, a finite element model (FEM) based on an individual discharge was built to calculate the residual stresses originating from EDM with different discharge energies. The model uses axisymmetric elements and takes the temperature dependencies of thermo-mechanical material properties of the workpiece into account, considering stainless steel AISI 304. For each temperature elastic-perfectly plastic material behaviour was assumed. The calculated residual stress profiles are in good agreement with experimental results.

Effect of activating flux and shielding gas on microstructure of TIG welds in austenitic stainless steel

Abstract The aim of this research is to study the effect of an activating flux, two shielding gases (100%Ar and 50%Ar z 50%He) and a range of weld currents on the microstructure of autogeneous A-TIG welds on an austenitic stainless steel. Metallographic, Mössbauer, X-ray diffraction and magnetic permeability methods were used in the study to evaluate ferrite content in the welds. The increase in welding current coarsened the microstructure and increased the retained ferrite content in welds made with and without flux. The activating flux increases the ferrite content and changes the distribution of ferrite in the welds. The influence of flux on ferrite content is less significant in Ar/He than in Ar shield welds. The process of filling steel samples, currently used in the Mössbauer method, drastically changes the microstructure of the parent and melted austenitic stainless steels.

Quantifying the Drilling Effect during the Application of Incremental Hole-Drilling Technique in Laminate Composites

In this work, a methodology to quantify the effect of the drilling operation, during the application of the incremental hole-drilling technique (IHD) for measuring residual stresses in laminate composites, in particular, the polymer matrix composites (PMC), is presented. This technique will allow the optimization of the drilling procedures and its parameters, enabling the quantification of the drilling effect. This quantification is obtained by using an experimental calibration procedure followed by a numerical simulation of the whole process. The direct comparison of the experimental and numerical results will allow quantifying the effect of the drilling operation. As example, the methodology was applied to the case of carbon/epoxy cross-ply laminate [0°/90°]5s. The holes have been made by using two different drilling procedures, but the same tool geometry. High speed milling powered by air compression, a process usually employed in the case of the application of hole-drilling technique to metal alloys and a conventional computer numerically controlled (CNC) milling machine, were used. The results seem to show that incremental hole-drilling could be a reliable technique to determine residual stresses in fibre-reinforced polymers.

Residual Stresses in Machining of AISI 52100 Steel under Dry and Cryogenic Conditions: A Brief Summary

Residual stress is one of the most important surface integrity parameter that can significantly affect the service performance of a mechanical component, such as: contact fatigue, corrosion resistance and part distortion. For this reason the mechanical state of both the machined surface and subsurface needs to be investigated. Residual stress induced by dry and cryogenic machining of hardened AISI 52100 steel was determined by using the X-ray diffraction technique. The objective was to evaluate the influence of the tool cutting edge geometry, workpiece hardness, cutting speed, microstructural changes and cooling conditions on the distribution of the residual stresses in the machined surface layers. The results are analysed in function of the thermal and mechanical phenomena generated during machining and their consequences on the white layer formation.

Residual stress evaluation on X 36 Cr Mo 17 HSM finished mould steel

The presented study was carried out in an industrial environment, using existing equipments, tools and materials. A set of tests was performed based on production demands and restrictions, aiming to achieve the lowest surface roughness and beneficial residual stress state. The experimental test surfaces were obtained using high-speed milling (HSM) and precision surface grinding on a DIN X 36 CrMo 17 steel work piece, widely used in injection moulding industry. The grinded surface was considered as a reference surface in order to evaluate the high-speed milling performance. For HSM tests, two types of tools were selected: tungsten carbide end mills with and without a TiAlN multilayer coating. The selected HSM main parameters ranged an upper, middle and lower limit value, considering the standard working values. The residual stress state at machined surfaces was evaluated by X-ray diffraction (XRD). Experimental results are discussed in order to achieve a procedure to select the optimal machining parameters that meet manufacturing specifications.

Phase Formation in Austenitic Stainless Steel A-TIG Welds

The aim of this research is to study the effect of the welding conditions and of an activating flux on the microstructure of the melted material of autogeneous A-TIG welds, made on austenitic stainless steel AISI 304. The increase of heat-input coarsened the microstructure, changed its morphology and decreased the δ-ferrite content retained in the microstructure. The use of a TiO2 activating flux does not significantly affect the microstructures. A thin layer tends to form in the surface of the welds, which microstructure and ferrite content are different from those observed in its core.

Evaluation of welding residual stresses using the incremental hole-drilling technique

In this work the reliability of the hole-drilling technique (HDT) for measuring welding residual stresses was analysed. HDT residual stress results were systematically compared with those determined by X-ray diffraction. A systematic overestimation of the residual stresses determined by HDT was observed, which was mainly attributed to the possibility of the so-called plasticity effect occurring. Experimental results were discussed taking the measurement principles of both techniques into consideration. In addition, preliminary results of a numerical study, using the finite element method, will be presented for a better understanding of the plasticity effect on HDT residual stress results.

Sensitivity of a Rotating Beam Sensor for Stress Evaluation in Aluminium Thin Films

The evaluation of stress in sub-micron tracks is critical for the microelectronics industry and there is a need for new methods of measurement. This paper advocates the use of a rotating beam sensor structure which can be fabricated on the wafer along side electronic devices and used to monitor stress generation and relaxation as a function of processing. The rotation can be observed with a reflected light microscope and correlated to the actual stress level. Several samples, assputtered and sintered, were prepared with the aim of having different residual stress states. X-ray diffraction with a low incident angle geometry, was used to evaluate the residual stresses on the aluminum layer. Computer simulations using ANSYS were also performed in order to correlate the sensor rotation with the experimental stress values. It was observed that the extrinsic stress from the mismatch in expansion coefficients between the aluminum layer and the silicon substrate dominates over the compressive stress from the sputter growth. Sintering the layers at temperatures above 150°C reduces this compressive stress due to the action of creep. The calibration of the rotation of the device with the direct measurements of the X-ray diffraction shows that the sensor has a resolution better than 2.8 MPa.

Spray-painting motion planning and quality analysis in powder coating systems

Abstract The paper reports results on the automation of the spray-painting process in industrial powder coating plants. The automation of the spray-painting process requires a painting model and the definition of appropriate metrics for painting task validation. Two main topics are addressed in the paper: motion planning of paint deposition and paint quality analysis. A paint quality function is formulated and results concerning experimental analysis are presented.

Industrial Powder Coating Systems: Sensor-Based Spray Trajectory Control

Abstract As the global economy becomes more competitive, industrial production requires more automation and process optimization. The paper reports the first phase of a project ( EP-Robot ) whose aim is the automation of the spray-painting process in industrial powder coating plants. The spray-painting machine should track the workpieces moved by an aerial conveyor, providing an adequate applicators trajectory and not be a bottleneck of the production system. For a high quality painting it is required that the spray-guns perform a suitable trajectory, i.e. the distance and orientation in respect to the workpiece surface, and the painting velocity should lie within specified tolerances. The global goal is to improve the quality of painting and reduce production and maintenance costs.