Metallographic Sections Research Articles

Optimization of the Anodic Dissolution of the Superalloy Inconel 718

Due to its particular mechanical and thermal properties, nickel-based superalloys such as Inconel 718 are widely used for aerospace industry. They are known to be the among the most difficult-to-cut- materials[1], so that conventional machining methods such as milling are inefficient and hard to proceed[2]. Because of its ability to generate high quality surfaces and to reproduce complex shapes. Pulse or Precise ElectroChemical Machining (PECM), an unconventional machining process (no cutting tools), can be useful to manufacture those alloys. This technic is based on the anodic oxidation of metals through a strong applied pulse current. The electrical signal is synchronized to the oscillation of the cathode corresponding to a rectilinear translational movement. Throughout the process, the electrolyte is continuously renewed that allows the removing of the solid residues of the dissolved metal and guarantees a good dissolution efficiency. The PECM is characterized by a very low inter-electrode distance (gap), from 0.01 to 0.2 mm, which ensures the dimensional accuracy. First step was to understand the physical-chemical phenomena related to the anodic dissolution of the Inconel 718 in the usual electrolyte NaNO3. Thus, polarization curves associated with current constant dissolution have been studied in 2,35 mol/L (20 % wt) NaNO3. Energy dispersive X-ray spectroscopy analysis of cross-sections were used to highlight the solid anodic products formed during anodization. They are mainly constituted of Ni and Nb oxides as well as γ '' (Ni3Nb) inclusions. From ICP analyses of the electrolyte, the dissolved elements in solution are mainly those of the austenitic matrix. Consequently, in order to optimize the dissolution process, the individual anodic behavior of both main components forming the superalloy, namely the matrix and the γ'' phase were studied in a mixture of NaNO3 and EDTA as a chelating agent. Linear Sweep Voltammetry (LSV) was performed to study the global behavior of each phase in the chelating medium. Chronopotentiometry was carried out to determine the dissolution yield. Finally, electrochemical impedance spectroscopy (EIS) measurements were performed to evaluate the electrochemical interface following the anodization process. It appears that, in presence of EDTA, the hydroxides formation in the electrolyte is limited, whereas the charge transfer resistance and the anodizing layer resistance decrease. As a final step, the electrolyte containing the chelating agent was subsequently tested for Inconel 718 in PECM configuration using a home – made experimental set up in order to determine the influence of the gap, the electrolyte flow and the pulse time on anodic dissolution. In addition, metallographic sections and dissolution products analysis were also made to identify the material integrity. [1] D. Dudzinski, A. Devillez, A. Moufki, D. Larrouquere, V. Zerrouki, et J. Vigneau, « A review of developments towards dry and high speed machining of Inconel 718 alloy », Int. J. Mach. Tools Manuf, vol. 44, no 4, p. 439-456, mars 2004. [2] M. Burger, L. Koll, E. A. Werner, et A. Platz, « Electrochemical machining characteristics and resulting surface quality of the nickel-base single-crystalline material LEK94 », J. Manuf. Process, vol. 14, no 1, p. 62-70, janv. 2012.

Reliability research on automatic ultrasonic testing (AUT) technology of land long distance pipeline with automatic welding joint

The complete automatic ultrasonic testing (AUT) reliability theory and evaluation system were established for automatic welding joint of long distance pipeline. The reliability of AUT system was verified by several indexes, including Probability of Detection (POD), Probability of Sizing (POS), Probability of False Alarm (PFA), Probability of Rejection (POR) and Receiver Operating Characteristic (ROC). In order to evaluate AUT process and analysis reliability of AUT system, the sample size was optimized to 46 and the quantitative data of defects in AUT, RT and macroscopic metallographic section were analyzed statistically, which could directly provide reliable quantitative bases for the automatic welding quality acceptance of AUT Technology. The results show that POD and reliability of AUT are not less than RT, which can provide support for AUT application to automatic welding of the domestic long distance pipeline.

Reliability studies of InnoLot and SnBi joints soldered on DBC substrate

PurposeThis paper aims to investigate the quality and reliability of solder joints prepared from Pb-free alloys on direct bounded Cu (DBC) substrates. Two types of solder alloys were studied: Sn90.95Ag3.8Cu0.7Sb1.4Ni0.15Bi3.0, with a high melting point of 225°C, and Sn42Bi58, with low a melting point of 138°C.Design/methodology/approachCapacitor components of size 1806 were soldered on DBC substrates by using convection reflow soldering and vacuum vapor-phase soldering technologies. A part of the samples was subjected to the thermal shock test. The structure of the solder joints and the content of the voids were investigated using three-dimensional X-ray tomography. The mechanical strength of the joints was evaluated using the shear force test, and the microstructure of the joints was studied on metallographic cross sections by using scanning electron microscopy.FindingsIt was found that the number of voids is not related directly to the mechanical strength of the solder joints. The mechanical strength of the solder joints depends more on the amount of Ag3Sn precipitation, Au precipitation and the intermetallic layer in the solder joints. In some cases, the thermal shock test caused micro-cracks around the Au precipitation because of a mismatch of Au, AuSn4 and Sn in terms of coefficients of thermal expansion.Originality/valueDBC substrates are usually used for power electronics, where the quality of the solder joints is even more important than in the case of commercial electronics.

A physical modeling and predictive simulation of the laser cladding process

A 3D finite element simulation model of the laser cladding process has been developed taking into account heat transfer, fluid flow, surface tension and free surface movement. All input parameters and data, which are independent of the process parameters but depend only on the material and machine properties, have been obtained from measurements. Thereby the melt pool and the resulting surface contour can be simulated without compromising assumptions or calibration, because the machine parameters are the only variable input parameters of the model. Thus, the model can easily be transferred to other material combinations or other machines. For the surface contour calculation a modified height function method is applied. The model surface follows this contour as an arbitrary Lagrangian Eulerian (ALE) method is used allowing for mesh deformations. The model was implemented using the commercial finite element software COMSOL Multiphysics and validated by comparing the simulation results with caloric measurements of the effective heat input and metallographic cross sections from experiments, where the nickel-base alloy MetcoClad® 625 in powder form was deposited on structural steel S235JRC + C and the process parameters of laser power, feed speed, laser beam spot size and powder mass flow were varied within a range of at least 50% of their mean value each. The maximum deviation of the simulation results compared to the experimental data regarding track geometry is 14% for the parameter sets without weld defects so that these parameter sets could be industrially applied, whereas the average deviation of track width and height is below 5.1%. Moreover, the maximum difference of the simulated absorptivity compared to the measurement results is 6.9%, while the average difference is only 1.7%, meaning the model shows a high predictive capability especially regarding the heat input.

Irreversible degradation of Nb3Sn Rutherford cables due to transverse compressive stress at room temperature

In the framework of the Future Circular Collider design study for a 100 TeV circular collider, 16 T superconducting bending magnets based on Nb3Sn technology are being developed. A pre-stress on the conductor during magnet assembly at room temperature (RT) is needed to counteract the Lorentz forces during operation. The superconducting properties of the brittle Nb3Sn superconductor are strain sensitive and excessive pre-stress leads to an irreversible degradation of the superconductor. In order to determine the level of acceptable pre-stress during the magnet assembly process, reacted and impregnated Nb3Sn cables were exposed to increasing transverse compressive stress up to a maximum stress level of 200 MPa at RT. After each stress cycle, the critical current of the cable specimens were characterized at 4.3 K in the FRESCA cable test station. No significant critical current degradation was observed up to 150 MPa, followed by degradation less than 4% after a nominal stress of 175 MPa. A dramatic permanent critical current degradation occurred after applying a nominal stress of 200 MPa. A comprehensive post analysis consisting of non-destructive micro-tomography followed by microscopic characterization of metallographic cable cross sections was carried out after the critical current test to reveal cracks in the Nb3Sn sub-elements of the loaded specimen.

Tool wear in TBM hard rock drilling – backgrounds and special phenomena

AbstractIn hard rock TBM drilling, wear issues play a crucial role in the success of a tunnelling project. Thus, excavation tools must be selected carefully and be adapted to ground conditions. Wear‐related changes in the tools that occur over the course of the tools' operating lives must also be taken into account. This paper presents five basic types of macroscopic wear for cutter rings: (1) abrasive/normal wear, (2) tapering, (3) mushrooming, (4) brittle fracture of the cutter ring (special wear type 1) and (5) blockade of the roller bearing (special wear type 2). These basic wear types lead to distinct cutter ring shapes which allow deriving some fundamental characteristics of the interacting rock types. In addition, a microscopic analysis of the surfaces and associated metallographic sections from worn cutter tools has been performed. As a result, the tribological processes under the cutter, leading to tool wear, can be better understood. In summary, the investigations may contribute to a better understanding to make it easier to choose the proper TBM disc cutter for a distinct rock type in order to minimize the risk of tool‐related downtimes.

Heat Treatment and Microstructural Contrasting on Low-Alloy Copper-Based Alloys

Abstract Copper is a material used in a wide range of applications. The combination of electrical conductivity and mechanical strength is a feature which is especially important when it comes to conductive and contact materials. This combination of properties in low-alloy copper-based alloys can be achieved when the material is subjected to a thermomechanical treatment consisting of rolling and annealing. Samples of the alloy CuNi1.5Si0.3 were taken at distinctive steps of the industrial manufacturing process for semi-finished products in order to examine the influence of the manufacturing parameters on the microstructure during solution annealing, rolling, and aging. The sample material for the respective chosen conditions was aged in the laboratory at various temperatures. Quasi-static tensile tests were performed, the electrical conductivity was determined, the microhardness was measured, and the microstructure was examined on metallographic sections in order to characterize the material conditions.

Behaviour of S31400 and S32205 steels in HCl‐ and H2S‐containing gas atmospheres under a low oxygen partial pressure between 480 and 680 °C

The effects of gaseous hydrogen chloride and hydrogen sulphide on the high‐temperature corrosion of the austenitic SS. S31400 and the duplex steel S32205 were investigated in an atmosphere without molecular O2. The testing gas atmosphere consisted of 3.8 vol% HCl, 1.9 vol% CO2, 0.3 vol% CO, 2.8 vol% H2, 0.02 vol% H2S, bal. N2. It showed a similar composition, which is present in the thermal cracking of anthropogenic resources. Tests were performed between 480 and 680 °C for 1, 3 and 10 days. Mass loss measurements were done for all experiments. After the reaction times the samples showed non‐adherent and porous layers of corrosion products, which were investigated by SEM/EDX and XRD. Metallographic cross sections of the corroded samples were prepared by water‐free polishing to investigate the microstructure of the corrosion products. Both materials formed large chromium sulphide crystals on top of a chromium oxide layer and a chlorine‐containing layer beneath the oxide. The ferrite phase of the duplex steel is selectively attacked, while the austenite phase remained without severe corrosion. Steel S31400 showed a uniform corrosion.

Effect of Width of Kerf on Machining Accuracy and Subsurface Layer After WEDM

Wire electrical discharge machining is an unconventional machining technology that applies physical principles to material removal. The material is removed by a series of recurring current discharges between the workpiece and the tool electrode, and a ‘kerf’ is created between the wire and the material being machined. The width of the kerf is directly dependent not only on the diameter of the wire used, but also on the machine parameter settings and, in particular, on the set of mechanical and physical properties of the material being machined. To ensure precise machining, it is important to have the width of the kerf as small as possible. The present study deals with the evaluation of the width of the kerf for four different metallic materials (some of which were subsequently heat treated using several methods) with different machine parameter settings. The kerf is investigated on metallographic cross sections using light and electron microscopy.

Precipitation of string-shaped morphologies consisting of aligned \u03b1 phase in a metastable \u03b2 titanium alloy

String-shaped morphologies consisting of preferentially aligned lath-shaped α precipitates were observed in the metastable β Ti-6Cr-5Mo-5V-4Al alloy after deformations at high strain rates and elevated temperatures. The morphology and 3-dimentional arrangement of this feature have been elaborated based on the characterizations via a combination of transmission electron microscopy, transmission kikuchi diffraction and atom probe tomography. The 2D projected morphology of the coalescent α laths observed in the etched samples by SEM depends on the metallographic section. All the microstructural observations indicate that dislocation structures are most likely the nucleation sites for the aligned α laths. In addition, an appropriate testing temperature, which can ensure a relatively high diffusion rate of solutes without inducing strong recovery of dislocation structures, is necessary for the occurrence of the string-shaped morphologies.

Influence of Ar-ion implantation on the structural and mechanical properties of zirconia as studied by Raman spectroscopy and nanoindentation techniques

In this study, structural and nanomechanical properties of zirconia polymorphs induced by ion irradiation were investigated by means of Raman spectroscopy and nanoindentation techniques. The zirconia layer have been produced by high temperature oxidation of pure zirconium at 600 °C for 5 h at normal atmospheric pressure. In order to distinguish between the internal and external parts of zirconia, the spherical metallographic sections have been prepared. The samples were irradiated at room temperature with 150 keV Ar+ ions at fluences ranging from 1 × 1015 to 1 × 1017 ions/cm2. The main objective of this study was to distinguish and confirm different structural and mechanical properties between the interface layer and fully developed scale in the internal/external part of the oxide. Conducted studies suggest that increasing ion fluence impacts Raman bands positions (especially characteristic for tetragonal phase) and increases the nanohardness and Young's modulus of individual phases. This phenomenon has been examined from the point of view of stress-induced hardening effect and classical monoclinic → tetragonal (m → t) martensitic phase transformation.

Modeling of cementite coarsening during tempering of low-alloyed-medium carbon steel

The kinetics of cementite coarsening in tempered martensite AISI 4340 steel system has been investigated in order to understand over-tempering behavior. Microstructural response during tempering at 550 and 650 °C has been recorded, analyzed, and modeled. A statistical analysis of metallographic sections was performed to yield measurements such as average particle size, volume fraction, morphology, volume number density. The coarsening behavior of this multicomponent alloy system has been modeled with the classical coarsening theory for multicomponent alloys. These results highlight the remarkable coarsening resistance of 4340 steel even for prolonged tempering.

Very high cycle fatigue behaviour of 42CrMo4 steel with plate-like alumina inclusions

Recent progress in steel refining shows significant reduction of non-metallic inclusions (NMIs) of which alumina (Al2O3) is one of the most problematic. Among other refining methods, metal melt filtration by ceramic foam filters shows promising results in steel cleaning. In the present work the influence of alumina inclusions on the fatigue behavior is investigated after the reaction of steel melt with filters. Different batches are compared where carbon-bonded alumina foam filters with different coatings were introduced into the steel melt of 42CrMo4 for 10 s (so called “finger test”). Fatigue tests were performed using ultrasonic fatigue testing (USFT) up to 109 cycles. Specimens were nitrided in order to prevent crack initiation from the surface and to study internal failure on NMIs. Surface hardening of quenched steel increased fatigue limit significantly. Metallographic sections were analyzed using optical and scanning electron microscopy (SEM) for the estimation of NMIs distribution properties. NMI size distribution analysis based on maximum Feret diameter (instead of area) is found to be an effective method for detecting plate-shaped inclusions. Fracture surfaces after fatigue tests were investigated by methods of SEM and confocal laser scanning microscopy (CLSM), revealing that plate-like NMIs initiate crack with all their area even being inclined to the crack plane. Properties of crack initiating NMIs – alumina plates and MnS dendrites – are compared and analyzed. Formation of alumina NMI as plate lead to significant enlargement of its stress-concentrating area in comparison to the spherical shape of the same volume. Thus, total NMI content reduction in steel could give no fatigue limit improvement if NMI morphology changes from spherical to plate-like.

Porosity formation and fatigue properties of AlSiCu cast alloy

The need for aluminium alloys having a good toughness, high strength, adequate damage tolerance capability, good fatigue resistance and good corrosion resistance for use in the industries applications of aerospace, automotive and even commercial products led to study of the properties and structure of the AlSi9Cu3 cast alloy. The most important metallurgical parameters affecting the aluminium alloy’s resistance to fatigue load are the amount, types and size of casting defects. Therefore quantitative analysis was used for determination of casting defects size by optical microscopy on metallographic sections and an analysis of the size distribution by extreme value statistics. The software NIS Elements was used for quantitative analysis of casting defects and results show that porosity have a great influence on to the fatigue properties of AlSi9Cu3 cast alloy.

Wpływ zużycia narzędzia na jakość połączeń zakładkowych blach ze stopu Aluminium 7075 T6 wykonanych metodą FSW

The article concerns the issues of tool wear effect on the quality of a friction stir welding joint quality. The experiment used aluminum alloy 7075 T6 sheet metal, which is used primarily in the aerospace industry. 1.0mm and 0.8mm thick lap joints were tested. Tool wear was determined based on multiple readings on a multisensory machine. The tool wear evaluation was done on the basis of a static tensile strength test and metallographic sections of the joints. The pin of the tool works in more demanding conditions and is more exposed to friction. This results from tooling operations performed at full depth dive in the jointed material. When also considering the small dimensions of the pin such as the diameter and the great forces occurring in this process, it is easy to see why this element is most susceptible to tool wear. The welding process causes the tool to undergo friction wear, which is the cause of reduced tool dive depth in the jointed material. As a result, it is paramount to constantly control the tool extension to achieve the desired quality parameters of the joint. After creating 200m of joints, a decrease in the strength of joints was observed as well as the repeatability of the results connected to a change in the stirring conditions in the material. The change in joint strength and tool wear is also confirmed in the metallographic analysis, which states that the continued degradation of the tool makes it subject to a decrease in size of the characteristic sizes of the thermoplastic zone that is the main determining factor of the joint strength.

In Situ Acoustic Monitoring of Thermal Spray Process Using High-Frequency Impulse Measurements

In order to guarantee their protective function, thermal spray coatings must be free from cracks, which expose the substrate surface to, e.g., corrosive media. Cracks in thermal spray coatings are usually formed because of tensile residual stresses. Most commonly, the crack occurrence is determined after the thermal spraying process by examination of metallographic cross sections of the coating. Recent efforts focus on in situ monitoring of crack formation by means of acoustic emission analysis. However, the acoustic signals related to crack propagation can be absorbed by the noise of the thermal spraying process. In this work, a high-frequency impulse measurement technique was applied to separate different acoustic sources by visualizing the characteristic signal of crack formation via quasi-real-time Fourier analysis. The investigations were carried out on a twin wire arc spraying process, utilizing FeCrBSi as a coating material. The impact of the process parameters on the acoustic emission spectrum was studied. Acoustic emission analysis enables to obtain global and integral information on the formed cracks. The coating morphology and coating defects were inspected using light microscopy on metallographic cross sections. Additionally, the resulting crack patterns were imaged in 3D by means of x-ray microtomography.

Characterization of Microstructure of Hadfield Steel

In this contribution, the preparation of metallographic sections and characterization of the microstructure of manganese Hadfield steels are discussed. The purpose of this paper is to provide information relevant to microstructural characterization of these steels. This type of steel is characterized by high resistance to abrasive wear, which is provided by surface strengthening through strain-induced martensitic transformation. Strengthening complicates the preparation of metallographic sections because the final microstructure can be influenced by the process and it can be eventually misinterpreted. Great attention must be paid to the choice of the etchant and the etching procedure. This contribution describes the entire metallographic characterization procedure, including the evaluation of grain size, micro-cleanness and presence of carbides on grain boundaries. It provides information for manufacturers and those, whose process and examine Hadfield steels with respect to their processing routes, wear resistance, non-magnetic properties and other aspects.

Reason for the transition of fatigue crack initiation site from surface to subsurface inclusions in high‐strength steels

AbstractThe fatigue crack initiation in high‐strength steels at surface inclusions is linked to high stress amplitudes. Low stress amplitudes are associated with a crack initiation at subsurface inclusions. The aim of the paper is to gain insight regarding the factors that lead to the transition of the crack initiation site from the surface to the interior. On the basis of fatigue tests and fracture mechanics evaluation, it was determined that the transition of the crack initiation site is effected by a constant threshold value for crack initiation at the surface and different crack propagation rates at the surface as opposed to the interior. It is shown that with lower stress amplitudes no critical inclusions with stress intensity factors above this threshold value are positioned at the specimen's surface. Therefore, the cumulative number of inclusions on the surface is calculated based on the cumulative number of inclusions in a metallographic section.

Finite Element Calculation of Clinching with Rigid Die of Three Steel Sheets

The clinching joining of three steel sheets (DC06, DX53D, and H220PD type) was studied by utilization of the finite element method. Clinched joints were also prepared experimentally by joining the above-mentioned steels with the thickness of 0.8 mm (DC06 and DX53D) and 1.0 mm (H220PD). The experimental tool works as single stroke tool, while prepared joints have circular axisymmetric shape, and the rigid die has no flexible segments. Finite element calculations were carried out in ANSYS software under simplified-axisymmetric conditions. Metallographic sections were also prepared from experimentally developed joints to make possible to directly compare the results of simulated and experimental approach. The results of computational approach are discussed and compared to the experimental ones.

Round Robin Test on the “Analysis of Graphite Shapes by Means of Microscopic Images of Cast Iron Samples” in the DGM Working Group “Quantitative Structural Analysis” of the Expert Committee Materialography

Abstract One of the main topics the working group “Quantitative Structural Analysis“ deals with is cast iron. The examination of metallographic 2D sections in order to eventually thus deduce the material properties of the three dimensional microstructure is common practice. Based on the results of a round robin test already performed in 2014, one of the then used samples was three-dimensionally measured in the IABG [1]. Using the thus obtained image data, a comparison between the 2D and 3D measurement results was performed in the second round robin test presented here. For the parameters graphite fraction and graphite size, a very good agreement could be achieved. However, there were important variations for the shape classification and the determination of the particle density. The main cause for this is the lack of appropriate specifications in the standard. Thus, the working group should further intensify the existing contact with the respective standardization bodies.