Metallographic Characterization Research Articles

Influence of inclusions on the pitting behavior of cast duplex stainless steels and patch weld layers

Abstract The relationship between the corrosion resistance of cast duplex stainless steel welded joints and inclusions was investigated by metallographic characterization, electrochemical tests, in-situ tests, scanning electron microscopy (SEM) combined with energy dispersive X-ray spectrometry (EDS) and other methods. The results showed that inclusions were the main sites for pitting corrosion initiation in cast 2205 welded joints, and the MnS portion of the composite inclusions preferentially dissolved to form micro-pits, which triggered pitting corrosion. In addition, the density of inclusions in the heat-affected zone (HAZ) was high and the average size of inclusions was larger, resulting in poorer pitting resistance in the HAZ. And pitting was more likely to occur and subsequently a large pit was developed which ultimately leads to failure.

Advancement of the Oxidized State of an As-Cast Low-Mn, High-Cr Cantor’s Alloy in Natural Air at 1000 °C

In order to discover how the multiple oxides observed in the final external scales after long exposure of a low-Mn, high-Cr Cantor’s alloy to hot air were formed, oxidation tests in a furnace were performed for seven different durations. Metallographic characterization was carried out concerning the oxidation products obtained after each test duration. The different oxides did not appear one after the other, but simultaneously, early on in the exposure to hot air and after. They all thickened progressively and the chemical composition of each also evolved with time, more or less. Globally, the innermost oxide is almost entirely chromia, much richer in Cr than in Mn, while the outermost one contains principally Mn. The interrupted tests also allowed specifying the mass gain kinetic, which is parabolic and twice as fast as a chromia-forming alloy. Despite the lowered content in Mn, manganese still plays an important role in the oxidation phenomenon, starting very early.

Smart piezoelectric composite: impact of piezoelectric ceramic microparticles embedded in heat-treated 7075-T651 aluminium alloy

AbstractSignificant advances have been made in material synthesis in the last two decades, with a focus on polymers, ceramics, metals, and smart materials. Piezoelectric-based smart materials generate an electric voltage in response to loads, enabling distributed monitoring in critical structural parts. Friction stir processing (FSP) is a versatile approach that can enhance material performance in various engineering fields. The primary objective of the current research is to examine the sensorial properties of heat-treated AA7075-T651 aluminium plates that have been included with Lead Zirconate Titanate (PZT) and Barium Titanate (BT) particles via FSP. This study includes a comparative analysis of sensitivities with AA5083-H111 self-sensing material, metallographic and physicochemical characterization, and an assessment of the mechanical properties impacted by the incorporation of piezoelectric particles. The sensitivity of AA7075-PZT was found to be significantly higher than that of AA7075-BT. AA7075-PZT achieved a maximum sensitivity of 15.27 × 10−4 μV/MPa while AA7075-BT had a sensitivity of only 7.28 × 10−4 μV/MPa, which is 52% lower. Microhardness and uniaxial tensile tests demonstrated that the presence of particles has an influence on both mechanical strength and electrical conductivity of aluminium components, as opposed to those that do not have particles. The complete investigation intends to give significant insights into the performance and prospective uses of these innovative smart materials, therefore advancing materials science and engineering. Graphical abstract

Metallurgical characteristics and mechanical properties of dissimilar friction stir welded DH36 steel and UNS G10080 steel joints

The present study expanded the scientific comprehension of the friction stir welding process for dissimilar steels, namely high-strength shipbuilding grade DH36 steel and UNS G10080 steel. The effect of tool traverse speed and plunge depth on temperature history, microstructure characteristics, and mechanical properties is investigated experimentally. The metallographic characterizations were examined through an optical microscope and field emission scanning electron microscopy equipped with an energy-dispersive X-ray system. Microhardness, impact, and tensile tests were carried out on the friction-stir-welded specimens. Increasing the plunge depth and reducing the traversal speed resulted in an augmentation of the peak temperature, primarily attributable to higher heat generation. Within the range of process parameters used, the tool produced complex material movement, resulting in swirl-like and vortex-intercalated features, particularly adjacent to the stir zone/workpiece interface. These vortex-like features exhibited dynamically recrystallized fine-grained microstructures. The grain size in the stir zone and the thermo-mechanically affected zone is reduced by increasing the plunge depth and decreasing the traverse speed due to enhanced dynamic recrystallization, subsequently improving the hardness and toughness values. In the stir zone, the microstructure revealed the acicular-shaped bainite ferrite in the DH36 steel and the Widmanstatten ferrite in the UNS G10080 steel. The microhardness contours revealed the uneven hardness distribution across the weld cross-section due to the microstructural heterogeneity in the dissimilar steels. The maximum welding efficiency of 106 % and toughness of 46 J are obtained at 40 mm/min traverse speed with a plunge depth of 0.2 mm, which is attributed to sufficient heat generation and grain refinement.

Fracture mechanisms of Al-steel resistance spot welds: The role of intermetallic compound phases

This study explores the mechanical and metallographic characteristics of Al-Steel dissimilar resistance spot welds (RSW), with a particular focus on the intermetallic compound (IMC) phases and their impact on fracture mechanisms. Detailed metallographic analyses and novel miniature lap shear tests with in-situ Digital Image Correlation techniques were conducted to observe the crack propagation behavior. The findings revealed that the IMC phases significantly influence the crack path and fracture mechanisms, leading to variations in fracture energy. Specifically, three distinct IMC phases were identified at the weld interface, each exhibiting unique structural and mechanical properties, with corresponding fracture energies of approximately 0.03 kJ/m2, 1.1 kJ/m2, and 7.5 kJ/m2. These variations highlight the critical role of the IMC phase in determining the fracture behavior of the weld. The study further supported the development and validation of a finite element (FE) model, incorporating a Cohesive Zone Model to simulate debonding behavior and the Hosford-Mean fracture criterion to predict ductile fracture in the Al fusion zone, thereby successfully linking local material characteristics to mechanical properties.

Microstructure Characterization of Al-alloy AW 5454 Intended for Ecological Laser Hybrid Welding

The article deals with the presentation of the microstructure characterization of Al-alloy AW 5454, which is intended for laser hybrid welding. In the first part, the procedure of laser hybrid welding and comparison to other laser surface modification techniques is presented. In the following, the key methods of metallographic characterization and hardness measurements at selected locations of Al-alloy AW 5454 are presented, which made it possible to carry out a scientific evaluation of the obtained results. The goal of the research was to gain insight into the microstructure of Al-alloy AW 5454, which in the future will make it possible to determine those technological parameters of laser hybrid welding that have the greatest influence on the resulting final microstructure and thus on the properties of the weld.

Effect of the Mn and Cr contents on the oxidation and creep resistance at 1100°C of cast cantor–based HEAs

Cast High Entropy Alloys (HEAs) derived from the Cantor’s composition may represent alternative substitutional solutions to the superalloys which are constituted for more than half of the critical elements nickel and cobalt. Recently, MC–alloyed HEAs constituted by a Cantor’s type matrix associated to an efficient interdendritic network of MC carbides, have demonstrated promising creep resistance at elevated temperatures. Regrettably, they also show poor high temperature oxidation resistance. This bad oxidation behavior is possibly due to the deleterious effect of manganese and to a too low content in chromium. New alloys were elaborated and tested in oxidation at 1100°C with thermogravimetry follow–up and metallographic characterisation, and their creep behavior was controlled. With less Mn and more Cr than the original alloys, these new alloys demonstrated significant progress in oxidation resistance. These changes in chemical compositions did not modify their creep resistances which were globally maintained, for the carbide-free alloy as well as for the MC-containing ones. These Mn-decreased and Cr-increased MC-containing alloys can be considered as low-cost alternative to polycrystalline Ni or Co-based superalloys for working in moderate conditions of corrosive fluids and applied mechanical stresses.

Wear study of additively manufactured repair material for R260 grade rails

Additive Manufacturing (AM) has been considered as a promising method for repairing rails and extending their service life. In this work, the wear rates of twin-disc specimens produced through laser additive manufacturing against ER7 material are studied applying creepages from 0.4 % to 2.6 %. The approved E11018-G electrode material is atomised and fully cladded into twin-disc rail specimens by Laser Powder Directed Energy Deposition. After the weight loss tests at 1400 MPa, experimental results are fitted with the theoretical adhesion curve modifying FASTSIM coefficients. This adhesion curve is employed to numerically calculate the slip contact area of each test. A non-linear correlation has been found and defined between wear and the slip area including the non-linear zone. This relationship is identified for tribochemical wear for low creepages and delaminative wear for middle and high creepages by surface and metallographic characterisation. The obtained results allow to predict the wear of the additively manufactured E11018-G repair material under wheel-rail contact conditions when the slip area is known.

Mechanisms affecting the difference in fatigue resistance between welds and base materials were investigated based on in-situ fatigue method

Coiled tubing (CT) is manufactured using plastic processing and weld molding techniques, rendering the weld zone (WZ) susceptible to fatigue fractures under the continuous action of complex alternating loads during field applications. While this risk has garnered significant attention, the underlying mechanisms of its fatigue failure remain elusive. To address this knowledge gap, this study employs an in-situ SEM fatigue test method, coupled with metallographic analysis, hardness testing, SEM observation, and EBSD characterization, to deeply analyze the fatigue microdamage mechanism in the WZ by comparing the fatigue performance of the base material (BM) of the CT with that of WZ. It is found that the microstructure of the BM consists of different scales and shapes of ferrite and a small amount of pearlite nested and intertwined with each other, which has a strong coordinated deformation ability and can effectively relieve stress concentration, delay crack initiation and form a single fatigue source. In contrast, the WZ’s ferrite organization is large, uniform, and independent, making it prone to early-stage plastic deformation with weak inter-grain coordination, resulting in the formation of numerous slip bands and multiple crack sources. During the crack propagation stage, the abundant grain boundaries in the BM effectively deflect the crack expansion direction and slow down its progression; while multiple secondary cracks in the weld expand simultaneously and connect with each other, resulting in a higher crack expansion rate than the BM. The research findings provide crucial insights and guidance for enhancing the overall fatigue performance of CT.

The Role of Microstructure in the Gradient of Tensile Properties through Thickness in 7449 Aluminium Alloy Thick Plate

In this article, the yield strength, tensile strength, and the microstructure of the wrought aluminium alloy 7449 rolled thick plate have been studied through thickness under different temper conditions. For all heat treatments, it has been proven that the yield strength and tensile strength values increase from the surface to the centre. The largest difference between the centre and the surface, in both properties, occurs in the case of a sample aged at room temperature for 120 h (TTA temper). The sample artificially aged at 120 °C for 24 h (TTB temper) shows the best strength-gradient relationship of the tensile properties through the thickness. Metallographic characterisation carried out by optical and scanning electron microscopy shows much finer elongated grains in the region near the surface of the plate than in the centre, with incipient recrystallisation in the area near the surface. In addition, electron backscattered diffraction technique, used for micro-texture analysis, has proven the presence of a gradient of crystallography texture in the plate. This explains the yield strength gradient, since the rate of change of the Taylor factor through thickness correlates with the rate of the change of yield strength in the longitudinal direction for the samples studied.

Metallographic characterization of metallic heat shields, accidentally precipitation-hardened with tripled soaking time

Abstract Heat treatment (HT) problems, or the lack of necessary HT, are a regular topic of discussion in the papers published in this section Failure Analysis of Practical Metallography. In this present contribution, a case study is presented that shows how an erroneous furnace run almost caused the scrapping of an entire engine set of Metallic Heat Shields (MHS) of a heavy-duty gas turbine engine. The authors readily admit that the subject components did not fail. They were not even put in engine service yet. That means that this case study does not describe a failure in the original sense of the word. However, from a metallurgical point of view, it is still an interesting case that shows that without proper scrutiny during metallurgical investigation in the laboratory, considerable economic loss could have resulted from a simple fabrication problem, that involved a malfunctioning control system of a heat treatment furnace. It was concluded from the metallurgical investigation that is the subject of this contribution that an almost tripled soaking time during aging, i. e., precipitation hardening, of an entire engine set of MHS did not cause any microstructural alterations that could have detrimentally affected mechanical properties of the subject components.

Effects of Contaminations on Electric Arc Behavior and Occurrence of Defects in Wire Arc Additive Manufacturing of 316L-Si Stainless Steel

Additive Manufacturing is a manufacturing process that consists of obtaining a three-dimensional object from the deposition of material layer by layer, unlike conventional subtractive manufacturing methods. Wire Arc Additive Manufacturing stands out for its high productivity among the Additive Manufacturing technologies for manufacturing metal parts. On the other hand, the excessive heat input promotes increased residual stress levels and the occurrence of defects, such as pores, voids, a lack of fusion, and delamination. These defects result in abnormalities during the process, such as disturbances in electrical responses. Therefore, process monitoring and the detection of defects and failures in manufactured items are of fundamental importance to ensure product quality and certify the high productivity characteristic of this process. Thus, this work aimed to characterize the effects of different contaminations on the electric arc behavior of the Wire Arc Additive Manufacturing process and the occurrence of microscopic defects in thin walls manufactured by this process. To investigate the presence of defects in the metal preforms, experimental conditions were used to promote the appearance of defects, such as the insertion of contaminants. To accomplish the electric arc behavior analysis, voltage and current temporal data were represented through histograms and cyclograms, and the arc stability was assessed based on the Vilarinho index for a short circuit. Effectively, the introduction of contaminants caused electric arc disturbances that led to the appearance of manufacturing defects, such as inclusions and porosities, observed through metallographic characterization. The results confirm that the introduction of contaminations could be identified early in the Wire Arc Additive Manufacturing process through electric arc data analysis.

Effect of hydrogen on the pitting corrosion of 2205 duplex stainless steel welded joints

The effect of hydrogen on the pitting susceptibility of 2205 duplex stainless steel welded joints was investigated using metallographic characterisation, electrochemical techniques and immersion experiments. The results show that the diffusible hydrogen content in the welded joints is approximately twice that of base material (BM). Hydrogen significantly decreases the pitting potential (Epit) of the welded joints, while increasing the carrier density in the passive film, suggesting that hydrogen weakens the protective properties of the passive film and increases the pitting susceptibility of the welded joints. According to immersion experiments, the pitting susceptibility of the welded joint from high to low is heat-affected zone (HAZ), BM and weld metal (WM), the HAZ tends to be more prone to the formation of larger and more stable pits, and the number of inclusions at the HAZ is significantly greater than that at the BM and WM. For BM under hydrogen charging, austenite phase is usually corroded first, because hydrogen tends to be enriched in the austenitic phase of BM. However, in some regions of WM, ferrite phase corroded first, which is related to the enrichment of hydrogen in the ferrite in the weld area.

Performance analysis of conventional and high-feed turning tools in machining the thermally affected zone after plasma arc cutting of low carbon manganese-alloyed steel

The service life of cutting tools significantly affects machining costs. Establishing end-of-life criteria is crucial to mitigate wear effects on part quality, machine integrity, and maximize tool utilization. This is particularly important in machining operations with materials featuring numerous surface defects, as observed in those processed by prior thermal cutting operations, leading to accelerated tool wear rates. This study compares tool life and wear mechanisms in conventional and high-feed turning of low carbon manganese-alloyed steel bushes, previously profiled by plasma thermal cutting. Machinability tests utilized Class P carbide tools with triangular geometry for conventional turning and rhombic geometry for high-feed turning, both coated for enhanced performance. Tool's end-of-life criterion was established per ISO 3685 standard. The machining parameters adopted include a standard cutting depth of 1.5 mm, varying cutting speeds (140, 160, and 200 m/min) and feed rates (0.2; 0.3; 0.4 mm/rev for conventional turning, and 0.5; 0.75; 0.82 mm/rev for high-feed turning). Analysis involved assessing tool wear rates by observing flank wear and analyzing it using scanning electron microscopy and energy-dispersive spectroscopy techniques. Machined surface quality was quantified by roughness measurements. Metallographic characterization of bushes post-cutting indicated thermal cutting effects on the cutting edge's microstructure, microhardness, and residual stress. Machinability test results showed superior wear resistance in the high-feed turning tool (wear rate about 13 % lower). Surface finish evaluation demonstrated that, when isolating the effects of tool geometry and feed rate, the average surface roughness increased at a similar rate with tool wear for both conventional turning and high-feed turning. Identical wear mechanisms were observed: abrasive on the flank and adhesive on the rake face. Thus, the superiority of high-feed turning over conventional turning is established under the evaluated machining conditions.

Dislocation density-based fatigue approach for structural steel details

Global approaches have been the main procedure for designing structural details and components under fatigue loading. This procedure is easy to apply but it disregards not only the effect of the material type but also the influence of the geometry. On the other hand, local approaches rely on the specific local parameters that can be assessed for each type of material and detail geometry. The parameters derived from low cycle fatigue (LCF) tests are the most common damage parameters used to predict fatigue life and establish fatigue design approaches. Recently, Huffman proposed a fatigue damage model based on strain energy density and dislocation density of the material. In this regard, two structural steels were selected to perform a metallographic and mechanical characterization aiming to determine fatigue life parameters and dislocation density. The fatigue behavior of these materials was evaluated using strain-Morrow, SWT, and strain-Huffman parameters, and the dislocation density values obtained by XRD were analyzed. Additionally, experimental results from fatigue tests on structural details (plate with hole) were compared with fatigue life predictions based on each local model. It was found that the Huffman model based on dislocation density is a reliable approach to predict the fatigue life of structural steel details.

Exploration of TIG welding's impact on the structural integrity and metallographic characteristics of AISI 316/AISI 410 stainless steel joints

Exploration of TIG welding's impact on the structural integrity and metallographic characteristics of AISI 316/AISI 410 stainless steel joints

Electrochemical and metallographic characterization of Ni-NiO-NiBi3 equilibrium in molten lead-bismuth eutectic

This study aims to investigate the chemistry of the Ni-O-Pb-Bi system through a combined microscopic and electrochemical analysis of the Ni-NiO-NiBi3 equilibrium in molten lead-bismuth eutectic (LBE). Through SEM-EDX metallographic characterization, we examined to the precipitates formed on the solid nickel substrate in molten LBE, confirming the formation of the NiBi3 within the temperature range of 469–748 K. Subsequently, we induced the Ni(LBE)-NiO(s)-NiBi3(s) ternary equilibrium in LBE and measured the equilibrium oxygen concentrations at different temperatures. The equilibrium nickel activity was derived using the measured equilibrium oxygen concentration in the temperature range of 518–595 K. Finally, the Ni-Bi IMCs floating on LBE at the end of the measurements were collected and characterized by X-ray diffraction (XRD), confirming the presence of the NiBi3 phase.

Thermomechanical behavior and microstructural characteristics of 7055 aluminum alloy during friction stirring welding

This study employed a viscoplastic finite element model and re-meshing technique to investigate the thermomechanical response of 7055 aluminum alloy during friction stir welding (FSW). The stirring pin rotates at 800–1200 rpm and moves at 80–120 mm/min (i.e., welding speed) during the FSW process. The temperature field and viscoplastic flow were mathematically modeled based on a computational solid mechanics method and predicted by a three-dimensional coupled thermomechanical numerical simulation. To validate the simulation results, real-time temperature measurements at the welded joint were acquired via thermocouples embedded in the plate prior to the welding process. Additionally, optical microscopy and electron backscatter diffraction techniques were used to examine the metallographic structure and grain orientation of the welds, respectively. The results revealed a temperature difference of 5–10 ℃ between the advancing and retreating sides of the plate. The material on the advancing side was extruded upward in a circular motion and eventually reached the surface. Meanwhile, the material on the retreating side moved half a revolution around the pin and remained at the original depth of the plate. Complete dynamic recrystallization occurred in the weld nugget, resulting in the formation of fine equiaxed grains with random orientation. In the thermomechanically affected zone, the proximity to the weld nugget was associated with an increased proportion of high-angle grain boundaries and recrystallized structures. Simultaneously, the intensity of the deformation texture decreased, while the recrystallized texture showed an initial strengthening followed by a subsequent weakening. This comprehensive investigation contributes to a deeper understanding of the thermomechanical behavior and metallographic characterization of 7055 aluminum alloy during the FSW process.

Metallographic, structural and mechanical characterization of a low-density austenitic Fe–Mn–Al–C steel microalloyed with Nb in hot-rolling condition

Metallographic, structural and mechanical characterization of a low-density austenitic Fe–Mn–Al–C steel microalloyed with Nb in hot-rolling condition

Dynamic deformation behavior and constitutive model of a Zr–W alloy

In this paper, a Zr–5W alloy was fabricated via casting. In order to obtain the mechanical properties of the material, quasi-static compression tests at room temperature and split Hopkinson pressure bar tests at various temperatures were carried out. The x-ray diffraction result showed that the main components of the alloy were αZr and W2Zr, where αZr is the matrix and W2Zr is the reinforcement. The metallographic characterization results showed that there were two main forms of W2Zr in the material, namely, large particle boundary and small diffuse submicrometer precipitates. The reinforcements of both distributions have the effect of increasing the strength of the material, but the small submicrometer W2Zr precipitates would cause microcrack nucleation during the late plastic deformation stage, resulting in damage softening. In order to make theoretical calculations of the mechanical properties of materials, the Johnson–Cook (JC) constitutive model and Zerilli–Armstrong (ZAM) constitutive model of the material were obtained. It was found that the JC constitutive model had poor consistency in describing material properties. Although the consistency of the ZAM constitutive model was higher than that of the JC constitutive model, it still had obvious shortcomings. Combined with the deformation mechanism of the alloy, a modified constitutive relation was established by adding damage softening terms based on the hexagonal close-packed metal constitutive model inferred by the kinetics of heat-activated dislocations. The relative error results of all working conditions show that the correlation consistency of the improved constitutive model in this paper is significantly better than that of JC constitutive and ZAM constitutive.