A. Morão Dias

Residual stress analysis in orthogonal machining of standard and resulfurized AISI 316L steels

Abstract Residual stresses induced by orthogonal cutting in AISI 316L standard and resulfurized steels have been investigated, with attention given to the role played by the cutting parameters, such as cutting speed, feed rate, tool geometry and tool coating. Depth profiles of residual stress have been determined using the X-ray diffraction technique. The effect of cutting conditions and tool nature on residual stresses are analyzed in association with thermal and mechanical events, recorded during the cutting tests. The tool temperature distribution has been determined by a specific CCD infrared camera technique and the cutting forces by a Kistler table set up on the lathe.

Influence of EDM pulse energy on the surface integrity of martensitic steels

Abstract Electro-discharge machining (EDM) is essentially a thermal process with a complex metal-removal mechanism, involving the formation of a plasma channel between the tool and workpiece electrodes, resulting in metallurgical transformations, residual tensile stresses and cracking. These properties determine the operational behaviour of the material and can be included in one term: surface integrity. Results of different experimental analysis to characterise the surface integrity of steels used in the production of moulds after EDM are presented. The roughness of the surface, the metallurgical structure, the residual stress state and the surface crack network of the near-surface layers, in the electro-discharge steels studied, as well as their dependence on the main processing parameters, are examined and discussed quantitatively and qualitatively.

An experimental study on electro-discharge machining and polishing of high strength copper–beryllium alloys

Abstract An experimental study on the effect of electro-discharge machining (EDM) parameters on material removal rate (MRR) and surface quality, when machining high strength copper–beryllium alloys is presented. Processing parameters for rough, finishing and micro-finishing or polishing regimes were analysed, as well as the effect of the introduction of planetary motion and a relaxed oscillation waveform delivered by a relaxation circuit (RC) during finishing and micro-finishing cuts were studied. The surface integrity of electro-discharge machined (EDMed) is quantified by measuring the roughness values, crater diameter and white layer thickness (WLT). Quantification of the extended surface damage and its relation to the discharge power, as measured through the product of current intensity (I) and discharge time (td), controllable in a isopulse type generator, is carried out. “Thermal erosion models”, as justification for the optimal setting of different EDM parameters and achievement of distinct surface integrity using materials with different thermal properties, are discussed.

Deformation Asymmetry of AZ31 Wrought Magnesium Alloy

In this work, the mechanical behaviour of an AZ31 wrought magnesium alloy due to tensile, compressive and four-point bending tests was analysed. Several specimens of this material were taken in different directions from a rolled plate. Tensile and compression tests allowed to characterise the mechanical properties of the material in the rolling and cross rolling directions. To take the effect of the anisotropy induced by the rolling procedure itself into account, the bending tests were carried out using specimens with four different orientations. Six strain gages per specimen allowed to observe the evolution of the strain with the bending moment during the tests. All bending specimens were bent until a total compressive deformation of 2.5%. It was observed that the corresponding tensile strain was significant lower and slightly different for each orientation of the specimens. In addition, a characteristic non-uniform distribution of deformation twinning was observed. The induced residual stresses after bending were characterised by X-ray diffraction (XRD) and incremental hole-drilling (IHD). Due to the different mechanical behaviour in tension and compression, an asymmetric residual stress distribution after bending could be observed. The neutral axis was, in all cases, shifted towards the tensile side. This observation agrees with the strain measurements during bending tests. In addition, an example of the influence of the asymmetric deformation behaviour during fatigue was given.

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.

Surface Integrity of H13 ESR Mould Steel Milled by Carbide and CBN Tools

The quality of a mechanical component such as its geometrical accuracy stability and fatigue life are significantly affected by the surface integrity generated by machining process. Residual stresses are a major part of the mechanical state of a machined layer and they can be beneficial or detrimental depending of their nature and magnitude. This study concerns phase analysis and residual stress profile characterization by X-ray diffraction (XRD) technique and microhardness profile of AISI H13 ESR mould steel, milled using carbide and CBN tools. Analysis of the cross-section of the AISI H13 ESR samples, milled using both tools, reveal a martensitic microstructure, with a very thin layer heavily deformed due to the machining process. However, no phase transformation was detected by XRD. Concerning the residual stresses, the results show that they are predominantly compressive at the samples surface. However, depending of the cutting tools, the in-depth residual stresses profiles present different evolutions. This difference in the in-depth residual stresses profiles between the two kind of cutting tools is attributed to the different cutting tool parameters, including the tool geometry.

Residual Stresses around an Expanded Hole in an Aluminum Clad Sheet

Cold working introduces a compressive stress field around rivet holes, reducing the tendency for fatigue cracks to initiate and grow under cyclic mechanical loading. As it is well known, for the accurate assessment of fatigue lifetimes a detailed knowledge of the residual stress profile is required. Powerful experimental and numerical tools are nowadays available for that purpose. In the present work both types of tools, X-ray diffraction and 3D Finite Element Analysis (FEA), were used in order to evaluate the residual stress profile. A comparison of experimental and numerical data is presented and discussed.

Comparative Analysis of Shot-Peening Residual Stresses Using Hole-Drilling and X-Ray Diffraction Methods

An identical shot-peening treatment was applied to steel samples with different mechanical properties. The in-depth distributions of the residual stresses were analysed by the incremental hole-drilling (IHD) method and compared to X-ray diffraction (XRD) results. The strain hardening of the near surface layers was quantified by microhardness measurements, allowing to estimate a local yield strength. Due to the shot-peening treatment, the local yield strength increased thus reducing the influence of the plasticity effect on IHD results. In some cases, IHD can still give satisfactory results even though residual stresses exceed by far 60% of the bulk materials yield strength, normally limiting the application range of this method. Taking the local yield strength into account, a simple criterion for the assessment of IHD measurements is proposed to predict the influence of the plasticity effect in the case of shot-peening residual stress analysis.

Influence of Work Material Properties on Residual Stresses and Work Hardening Induced by Machining

Machining residual stresses are considered as part of surface integrity and a consequence of the machining process. Theses stresses are closely correlated with the corresponding process parameters, including the work material properties. As it is well known, not only the mechanical but also the physical properties of the work materials have great influence on machining residual stress. This was demonstrated in the present work through studying the residual stress and work hardening induced by the turning of AISI 316L and AISI 1045 steels. The residual stresses were determined at the workpiece surface and in-depth using the X-ray diffraction technique. To understand the influence of the work material properties on the residual stress and work hardening distributions, the mechanical and thermal phenomena occurring during the cutting process were studied, using a t developed experimental procedure. The experimental setup included a piezoelectric dynamometer to determine the cutting forces, and thermal imaging equipment developed to assess the temperature distribution in the deformation zone in turning. The results showed that the cutting forces and temperatures in the machining of 316L steel are much higher than those in the machining of 1045 steel. Thus, machining 316L steel, when compared to 1045 steel, results in higher superficial residual stresses and stronger in-depth residual stress gradients, higher superficial work-hardening and greater thickness of the work hardened layer.

Residual Stresses Induced by Machining of a Plain Carbon Steel Using Coated and Uncoated Commercial Tungsten Carbide Tools

It is well known that machining results in residual stresses in the workpiece. These stresses correlate very closely with the cutting parameters (cutting tool geometry and material, work material, and machining regime). This paper concentrates on the residual stress induced by turning of AISI 1045 steel. Particular attention is paid to the influence of the cutting speed and tool material. In experiments, the residual stresses have been measured using the X-ray diffraction technique (at the surface of the workpiece and in depth). Special attention was paid to the influence of the coating applied to the carbide inserts on the residual stresses due to machining. These residual stresses were analyzed accounting for the thermal and mechanical phenomena during machining. The temperature distribution on the tool and chip has been determined experimentally using specially designed infrared equipment installed on the lathe. Three orthogonal components (Fc, Ff and Fp) of the cutting force were measured using a piezoelectric dynamometer. For the range of cutting parameters used in the machining tests, the results show that the coated tool produce the tensile residual stresses, which are critical in the performance of the machined components.