Philippe Bocher

Texture heterogeneities in αp/αs titanium forging analysed by EBSD‐Relation to fatigue crack propagation

The microstructure and the local texture of a large IMI 834 forging were characterized using the Electron Back Scattered Diffraction (EBSD) technique. Crystallographic domains called macrozones and formed by a majority of primary αp grains with their axes in nearly the same direction were found. They had a band‐like structure, parallel to the axial direction of the forging. The influence of these macrozones on the cold dwell‐fatigue properties was studied. Several samples were tested under cold dwell‐fatigue conditions. The crack initiation and the short‐distance propagation region optically matched a bright region that contained numerous quasi‐cleavage facets. The analysis of the EBSD measurements showed that this bright region was enclosed within a sharp textured region with axes at less than 30° from the loading axis. The crystallographic features of the crack nucleation site and the crack propagation path were also analysed.

Impact of startup scheme on Francis runner life expectancy

Francis runners are subject to complex dynamic forces which might lead to eventual blade cracking and the need for corrective measure. Damage due to cracks in runner blades are usually not a safety issues but might generate unexpected down time and high repair cost. Avoiding the main damaging operating conditions is often the only option left to plant operators to maximize the life expectancy of their runner. The startup scheme is one of the available parameter which is controlled by the end user and could be used to minimize the damage induced to the runner. In this study, two startup schemes have been used to investigate life expectancy of Francis runner using in situ measurements. The results obtained show that the induced damage during the startup event could be significantly reduced with change to the startup scheme. In our opinion, an optimization of the startup scheme with regard to fatigue damage could extend significantly the life expectancy and the reliability of Francis runner.

Multiphysics modeling of induction hardening of ring gears for the aerospace industry

Induction heating has been widely used for heat treating and especially surface hardening in a broad variety of applications, ranging from the automotive to the renewable energy market. However, the lack of precise knowledge about the interrelation between all the concurrent physical phenomena occurring within the part during the heating cycle has restricted its use to mass-production items (mostly gears). The benefits of this technology, which is clean, repeatable, and cost-effective, could boost its introduction into more conservative industry sectors, such as aerospace, where furnace-based treatments (e.g., carburizing) represent the golden standard. The major limitation is related to the optimization of the induction hardening process, which usually requires significant material know-how and can thus be very long and expensive. Computer simulation could provide a general tool for understanding and improving the critical aspects of each step of the process, thus speeding up the spreading of the induction technology into new markets.

Surface Finish and Residual Stresses Induced by Orthogonal Dry Machining of AA7075-T651

The surface finish was extensively studied in usual machining processes (turning, milling, and drilling). For these processes, the surface finish is strongly influenced by the cutting feed and the tool nose radius. However, a basic understanding of tool/surface finish interaction and residual stress generation has been lacking. This paper aims to investigate the surface finish and residual stresses under the orthogonal cutting since it can provide this information by avoiding the effect of the tool nose radius. The orthogonal machining of AA7075-T651 alloy through a series of cutting experiments was performed under dry conditions. Surface finish was studied using height and amplitude distribution roughness parameters. SEM and EDS were used to analyze surface damage and built-up edge (BUE) formation. An analysis of the surface topography showed that the surface roughness was sensitive to changes in cutting parameters. It was found that the formation of BUE and the interaction between the tool edge and the iron-rich intermetallic particles play a determinant role in controlling the surface finish during dry orthogonal machining of the AA7075-T651 alloy. Hoop stress was predominantly compressive on the surface and tended to be tensile with increased cutting speed. The reverse occurred for the surface axial stress. The smaller the cutting feed, the greater is the effect of cutting speed on both axial and hoop stresses. By controlling the cutting speed and feed, it is possible to generate a benchmark residual stress state and good surface finish using dry machining.

An Investigation of Machining-Induced Residual Stresses and Microstructure of Induction-Hardened AISI 4340 Steel

Excessive induction hardening treatment may result in deep-hardened layers, combined with tensile or low compressive residual stresses. This can be detrimental to the performance of mechanical parts. However, a judicious selection of the finishing process that possibly follows the surface treatment may overcome this inconvenience. In this paper, hard machining tests were performed to investigate the residual stresses and microstructure alteration induced by the machining of induction heat-treated AISI 4340 steel (58–60 HRC). The authors demonstrate the capacity of the machining process to enhance the surface integrity of induction heat-treated parts. It is shown how cutting conditions can affect the residual stress distribution and surface microstructure. On the one hand, when the cutting speed increases, the residual stresses tend to become tensile at the surface; and on the other hand, more compressive stresses are induced when the feed rate is increased. A microstructural analysis shows the formation of a thin white layer less than 2 µm and severe plastic deformations beneath the machined surface.

On the importance of crystallographic texture in the biocompatibility of titanium based substrate

The role of grain size and crystallographic orientation on the biocompatibility of commercially pure titanium was investigated. Samples, with significant differences in crystallographic texture and average grain size (from 0.4 to 40 µm) were produced by equal channel angular pressing (ECAP) and post deformation annealing. X-ray diffraction and electron back scattered diffraction (EBSD) were used to evaluate differences in texture and microstructural characteristics. The titanium oxide film present on the surface of the samples was analyzed to determine the oxidation state of titanium and the chemical bonds between oxygen and titanium using X-ray photoelectron spectroscopy (XPS). Biocompatibility experiments were conducted using MC3T3 preosteoblast cells. Cell attachment was found to be texture-sensitive, where the number of attached cells was higher on the samples with higher number of (0002) planes exposed to the surface, regardless of the grain size. A relationship was also found between the titanium oxide species formed on the surface and the crystallographic texture underneath. The surface texture consisting of more densely packed basal planes promote the formation of Ti-OH on the surface, which in turn, enhances the cell-substrate interactions. These surface characteristics are deemed responsible for the observed difference in cell attachment behaviour of surfaces with different textures. Finally, it is inferred that texture, rather than the grain size, plays the major role in controlling the surface biocompatibility of biomedical devices fabricated from pure metallic titanium.

Microscopic Strain and Crystal Rotation Measurement within Metallurgical Grains

This work describes an experimental procedure to measure the progressive strain localization and crystal lattice rotation within metallurgical grains. A digital image correlation software was implemented and associated with mechanical tests carried out inside a scanning electron microscope on specimens exhibiting nanometric grainy patterns. Cross-correlation analyzes between electron backscattering diffraction maps were also developed to quantify the corresponding local crystal rotation relative to the original structure. The microscale strain and rotation fields on the surface of a tensile-loaded specimen made of austenitic stainless steel 316L are presented as an illustration. Their direct spatial correlation between strain heterogeneities and the progressive activation of slip systems is put into evidence and discussed.

Modeling and Sensitivity Study of the Induction Hardening Process

Induction heating is a case hardening process used to improve performance of machine components by producing a hard martensitic microstructure and high compressive residual stresses at the surface layer. A reliable numerical model able to predict the hardness profile would shorten process development. However, the accuracy and the efficiency of the model are restricted by the coupling complexity between the electromagnetic and thermal fields, and the nonlinear behaviour of the material properties. The paper analyzes the sensitivity of the material properties values and of the finite element meshing onto the predictive modeling of the case hardening profiles. The material used is SAE-4340 low-alloy steel. The simulations are done using a computer-modeling software (Comsol) and the sensitivity analysis is conducted by using an experimental design method.

Study of induction heating process applied to internal gear using 3D model

The current paper is principally dedicated to the study of geometry and frequency effects for internal spur gears heated by induction. The overall work is realized by the simulation efforts performed on Comsol multi-physics software. The 3D model used during this study is built basing on coupling between Maxwell’s and heat transfer equations. This model is used to calculate the temperature profile in the gear in function of machine parameters. The module and the frequency are varied to determinate their effects. In fact, two gears having the same external diameter but different modules are exploited during this study and the frequency is varied from low to high level. The obtained results allow understanding the effect of module and frequency on the final temperature distribution. Finally, the optimal frequency value permitting to have the best temperature profile is found.

Comparison of Two X-Ray Residual Stress Measurement Methods: Sin2 ψ and Cos α, Through the Determination of a Martensitic Steel X-Ray Elastic Constant

a dorian.delbergue.1@ens.etsmtl.ca, b damien.texier@etsmtl.ca, c martin.levesque@polymtl.ca, d philippe.bocher@etsmtl.ca Abstract. X-ray diffraction technique for residual stresses measurement is usually associated to the sin 2 ψ method, a method based on the interception of the diffraction cone and line detectors. To overcome this loss of information, the cos α method is an alternative method which uses a single exposure to collect the entire diffraction cone via a 2D detector. The present paper compares both sin 2 ψ and cos α methods, through the X-ray elastic constant (XEC) determination of a quenched and tempered martensitic steel. The full-cone measurement method demonstrates a smaller scatter and a better repeatability of the measurements. This latter point is of considerable interest since larger scatter in XEC may result in large variation in residual stress values, especially at high stress levels.