Metallographic Characterization Research Articles

Development of a repair process for manmade structures in space by using aluminium brazing and friction stir spot welding


 
 
 Small-sized parts and micrometeoroids’ impacts represent an increasing threat to artificial components/structures. The damages caused by them can be fixed using e.g., mechanical fixture with epoxy adhesive. However, the repair process is costly and only intended for a temporary fixing the penetration. In order to increase the life time of the repair, an approach for a more reliable process using similar materials is needed. The objective of this work is to develop a process for repairing manmade structures in space by generating joint properties similar to the base material. In this work, two approaches for repairing complex structures were considered. The first approach aims to fill a hole using liquid aluminium phases by brazing, and the second approach aims to fill the holes by a solid-state process (friction stir spot welding). In this study, different binding mechanisms of these two approaches were analyzed preliminarily by means of mechanical and/or metallographic characterization under terrestrial, controlled atmosphere conditions. Both concepts are proven feasible under these conditions. It has been shown that it is possible to realize a good filling of the hole with the investigated materials and under selected process conditions with both presented concepts.
 
 

Microstructures and Metallographic Characterization of Superalloys

Superalloys are metallic alloys with specific microstructures and atoms in solid solution allowing good mechanical and chemical properties at high temperature, typically higher than 1000°C. They present many different types of {matrix – precipitates} combinations which can be characterized at each step of their elaboration, by specifying the chemical compositions and the crystalline network of the phases present. After test on a sample (laboratory scale), or after the deterioration in service of a component leading to its replacement by a new one (industrial scale), the superalloys constituting these pieces are often usefully examined. This aims to specify the changes occurred in their microstructures as well as in their surface or sub-surface, due to the endured thermal, mechanical and chemical solicitations. These characterizations can be simply done using basic low-magnification optical microscopic examinations. Other means such as electronic microscopy and other advanced apparatus may be usefully involved for further investigations. After a brief history about superalloys, an overview of the phenomenological aspects of their mechanical and chemical solicitations during their use at high temperature, the metallographic techniques which are usually employed to characterize them are presented and illustrated in the case of a chosen family of superalloys.

Influence of the Base Element on the Thermal Properties of Non- Ferrous Chromium-Rich TaC-Containing Alloys Elaborated by Conventional Casting: Part 3: Surface and Cross-Sectional Metallographic Characterization of the Oxidized Alloys

The six alloys the thermal properties of which and the tendency to oxide spallation of which were studied in the first two parts of this work, were here characterized after oxidation for 70 hours at 1250°C. The external chromia scale, and also the CrTaO4 subsurface oxide, formed for all the alloys, almost independently of the Co and Ni proportions in the base element content. But, because of the formation of more CrTaO4 for the nickel-richest alloys probably due to the higher availability of Ta in the matrix and its easier diffusion towards the neighbourhood of the oxidation front, the adherence of chromia was weakened and spallation, suggested by the thermogravimetric curves in the second part of this work, is here really observed and the denuded part of alloys clearly seen. The degradation of the subsurface, which can be in a first time summarized by the development of a carbide-free zone and a {Cr, Ta}-depleted zone, depends on the Co and Ni proportions. The microstructure of the bulk is differently affected by long exposure at elevated temperature. The changes in carbide population characteristics are stronger for the nickel-based alloys than for the cobalt-based ones. Finally, the isothermal oxidation behaviour is best for the nickel-richest alloys but the oxide spallation behaviour and the potential mechanical properties are the best for the cobalt-richest alloys.

Metallographic and morphological characterization of sub-surface friction stirred channels produced on AA5083-H111

The friction stir channelling process derives from friction stir welding, where the process itself is modified in order to produce sub-surface channels by removing material from the bulk of the workpiece and leaving its upper surface closed and at the same level it had before being processed. In the present study, sub-surface friction stirred channels were produced on rolled plates of AA5083-H111. Channels’ metallography and morphology were characterized using optical, scanning electron and confocal laser scanning microscopy. The microhardness profile at mid-thickness between the channel top and the upper processed surface, perpendicular to the channelling direction, was determined and the influence of the friction stir channelling process parameters on the microhardness distribution, near the channel, was assessed. It was observed that, the channel inner surfaces are topographically different: the retreating side is rougher than the advancing side and the channel bottom is smoother than its top inner surface. The average microhardness in the dynamically recrystallized zone (nugget), thermo-mechanically affected zone and heat affected zone is lower than that of the unprocessed base material. It was also found that the average microhardness of the nugget is not very sensitive to the tool rotation speed variation.

Investigation on Heat Exchanger Pipe Failure

Convection heat exchangers are commonly used for heat transfer in high-temperature furnaces and processes. Pipes from U-bundles of a convection section of a MIDREX direct reduction ironmaking unit heater which carries reducing gas have been investigated for failure analysis. Repeated failures had occurred at a specific location. Samples of the failed pipe were subjected to microscopic and metallographic characterization. Inner diameter erosion and metal dust, carbides, oxides, and carbon were found at the failure location. Optical microscope revealed severe carburization on the inner surface that was in contact with heat exchanger gas. Coarse carbides at the grain boundaries accelerated the wear and corrosion by intergranular cracking. The reducing gas composition (with CO), dust composition (with carbides), and exposed temperatures (> 400 °C) confirmed that metal dusting is the cause of the failure. The high-temperature corrosion of steel is caused by hydrocarbon gases which form thermodynamically metastable carbides of the base metal. Such failures can be avoided by maintaining the pipe surface temperatures below 400 °C or using pipes manufactured from high-temperature-resistant steels.

A Facile Technique to Produce Open Cell Titanium Foams

Nature prefers open cell order in most structures such as trees and skeletons. Engineers have been imitating the nature, to obtain high specific strength, to transfer heat, energy absorbance and filtering for various applications such as filters, heat exchangers, impact absorbers, bone implants. Although ceramic foams have been produced and used as heat shields and filters, recently a great effort has been put on metallic foams produced from aluminium, copper, iron, stainless steel, nickel and titanium. Studies on open cell structured titanium foams are steps forward among these materials due its good biocompatibility and high specific strength. In this study, open cell titanium foams are produced utilizing polymer impregnation process accompanied by a facile sintering method. In traditional foam making, long sintering durations reduce the efficiency as far as cost and delivery time for producers. In order to overcome these problems, an alternative solution is made in the production of open cell titanium foams. A facile method is designed to sinter the impregnated polymers by using induction heating. Titanium foams with open cell structures are successfully produced. SEM, XRD, metallographic characterizations are performed. Average hardness value is calculated as 815.093 ± 6.59 Hv0.1.

Effect of tool tilt angle on weld joint properties of friction stir welded AISI 316L stainless steel sheets

The effect of tool tilt angle on mechanical and metallographic characteristics of friction stir welded AISI 316L austenitic stainless steel joints was investigated. Three experimental trials were performed between 0° to 3° in steps of 1.5°, while keeping the other primary FSW parameters constant (tool rotational speed of 600 rpm, welding speed of 45 mm/min and axial load of 11 kN). The experimental findings prove that the tool tilt angle has significant effect on dynamic volume of the stir zone, peak temperature and the intensity of the shear band formation in the weld zone. The weld joint made at 1.5° was the best tool tilt axis that exhibited higher strength and superior microstructural features leading to onion ring formation at the weld stir zone. 0° tilt angle resulted in slight shoulder plunge whereas, 3° tilt angle paved the way to the formation of void in the weld zone.

Failure Analysis of Diode Components of Electronic Relay Packages Via Thorough Microstructural Characterization

Diode components used in the electronic relay packages of avionics of a weapon system had been found corroded. Two sets of the complete assembly of the damaged diode parts consisting of diode body, plain washer, spring washer and mounting nuts were analysed for finding out the root causes of failures. Detailed metallographic characterization of the failed materials was carried out using optical microscope, scanning electron microscope (SEM), electron dispersive spectroscopy (EDS) analysis within SEM and microhardness study. Inadequate electroless nickel (EN) plating along with significant amount of porosity and presence of MnS type of stringers affected the corrosion resistance of the diode body tremendously for both the sets. Inadequate electroplated Zn layer was observed on spring washer, plain washer and nut, wherein both uniform and pitting corrosion were detected. Important recommendations included: (i) thicker (more than 25 µm) EN coating with as-specified porosity level for diode body, (ii) replacement of electroplated zinc layers with galvanized zinc (of higher thickness) layer on all the fasteners, (iii) precaution in minimizing formation of galvanic couplings observed in the present system, like that between EN-coated diode body and zinc coated fasteners, between mild steel and stainless steel fasteners etc.

Metallographic, structural and mechanical characterization of low-density austenitic Fe-Mn-Al-C steels microalloyed with Ti/B and Ce/La in hot-rolling condition

ABSTRACTLow-density austenitic Fe-Mn-Al-C steels have high strength, high ductility and a significant weight reduction respect to other alloyed steels. However, this complex system exhibits second-phase precipitation, particularly κ-carbide. It is well-known that the microalloying elements addition to steel generates precipitation hardening, as well as grain refinement effect. It is worth noting that low-density steels can cause cracking during hot-rolling due to high Mn, Al and C contents and segregation in grain boundaries. Hot-rolling conditions play an important role in the dynamic recrystallization mechanisms, and therefore in the austenitic grain size. The main objective of this research work is the metallographic, structural and mechanical characterization of low-density steels microalloyed with Ti/B and Ce/La in hot-rolling condition. For this purpose Fe-(27-30)Mn-(7-8)Al-(1.2-1.8)C (wt.%) low density steels microalloyed with Ti/B and Ce/La were hot-rolled at 1200 °C in two stages. Metallographic, structural and mechanical characterization was carried out by optical (LOM) and scanning electron (SEM) microscopies, electron backscatter diffraction (EBSD) through quality images, inverse pole figures (IPF) and orientation distribution functions (ODF) maps, X-ray diffraction (XRD) and microhardness Vickers (HV) testing. In general, the first stage of hot-rolling exhibits a strongly bimodal microstructure of dynamically recrystallized austenitic grains, while the second stage shows more uniform recrystallized grain size. In the first stage of hot-rolling the austenite is the predominant phase, while in the second stage the α-ferrite phase is barely visible. Low-density steel microalloyed with Ti/B presented better grain size and microhardness values compared to steel microalloyed with Ce/La. Preferred crystallographic orientations were not found.

Characterizing the Mechanical Properties of Steel AISI-SAE 4140 to Apply a Plasma Nitriding Process

Surface treatments give physical and chemical properties to the materials, without them showing any change in their dimensions. Plasma nitriding is a thermochemical treatment, which increases some mechanical properties of metallic materials. In this article the characterization of the mechanical and micro-structural properties of an AISI-SAE 4140 steel is presented. To carry out the metallographic characterization, stress, hardness, abrasive wear and X-ray diffraction tests, the AISI-SAE 4140 steel was mechanized and prepared delivered in a hardened state (quenched and tempered) produced by Shah Alloys Ltd. making test pieces with their corresponding ASTM standards and nitrifying by plasma half of these at 500oC for 10 hours. A thickness of 6.78 μm was obtained from the nitride layer that the material acquired. A microstructural constitution was found formed by martensite and austenite retained in the base material, maintaining the same structure with the thermochemical process. The nitrided surface showed in its atomic chemical composition (At%) the presence of 8.04% N, 1.69% O and 1.38% Al (element of great importance to achieve great hardness), revealing an increase of 18.55% of surface hardness and 78.87% of wear resistance. Regarding the tensile properties, there is a 77.65% decrease in its elongation when subjected to medium and low tensile loads, presenting a deformation of 0.09 mm for the last stress effort of 600 MPa.

PROCESSABILITY OF WC-CO POWDER MIXTURES USING SLM ADDITIVE TECHNOLOGY

This contribution presents the findings from an experimental study which explored the possibility of processing WC-Co-based powders using selective laser melting (SLM) additive technology. Two commercially available WC-Co powder mixtures were studied. One is intended for thermally sprayed coatings and the other for isostatic pressing applications. Both were analysed using optical and scanning electron microscopy and then processed in an additive manufacturing (AM) machine. Process parameters were proposed on the basis of a literature search and metallographic characterization of the mixtures. Metallography was also employed for examining the prototype specimens. The mixtures were found to differ in morphology, technological properties and phase composition, which was also manifested during their processing. The powder mixture for thermally sprayed coatings proved completely inadequate for AM due to undesirable grain morphologies and phase composition. The powder mixture for isostatic pressing was found to be more suitable. Nevertheless, all the builds were highly porous which had an adverse impact on the product properties.

Corrosion Resistance of Stainless Steels Intended to Come into Direct or Prolonged Contact with the Skin.

The biocompatibility of materials in contact with a living tissue becomes a puzzle in the overall picture of assessing the toxic effects of chemicals that come into contact with us. Allergic reactions to substances are a significant and growing health problem affecting large parts of the population in Europe. Wristwatches are objects worn in prolonged contact with the skin, being subject to localized corrosion, especially pitting and crevice types, in sulfide-chloride medium, and high wear in the bracelets joints. Watches of medium quality are usually made of stainless steels. The X2 CrNiMo 17-12-2 316L grade as well as X1 CrNiMo 20-25-5 Cu 1 or 904L are commonly used, having good resistance to generalized corrosion. The passive layer is nevertheless insufficient to ensure complete immunity in all cases of localized corrosion encountered during wear. For this reason, a high-corrosion-resistant steel: X1 CrNiMo 18-15-4 N 0.15 or 317LMN, from three different suppliers was evaluated. Metallographic characterization was carried out. The corrosion behavior evaluation was performed for the generalized corrosion, pitting and crevice corrosion and galvanic corrosion. Galvanic couples steel 317LMN-gold 18K alloy 3N and gold 18K 5M were used. The results of the generalized and pitting corrosion test indicated three basic groups. All of the 317LMNs were similar. The 316L variants tested noticeably worse. The 904Ls were difficult to discern, but certainly easier than the 316Ls and, possibly, at least comparable to the 317LMNs.

Handling of Blackplate in Metallographic Preparation and Heat Treatment Optimization via Quenching Dilatometry

Abstract A prerequisite for the successful optimization of a heat treatment for high-alloyed steels is the knowledge about the present initial microstructure. The metallographic preparation and characterization of the initial microstructure poses a challenge, especially for blackplate, due to its low thickness. In this article, a method is presented which facilitates the handling of blackplate during metallographic preparation. With a precise knowledge of the initial microstructure, tests for the optimization of a heat treatment for such blackplate can also be performed. An optimization of the heat treatment process is particularly important for energy and resource efficient manufacturing processes. In this work, a preparation method was developed which allows winding blackplate to hollow cylinders to determine the optimized heat treatment parameters with the help of a quenching and deformation dilatometer. It will be shown that the two methods presented are an efficient but yet reliable way to optimize heat treatment processes of blackplate via laboratory tests.

Influence of the Base Element on the Thermal Properties of Non- Ferrous Chromium-Rich TaC-Containing Alloys Elaborated by Conventional Casting: Part 3: Surface and Cross-Sectional Metallographic Characterization of the Oxidized Alloys

Influence of the Base Element on the Thermal Properties of Non- Ferrous Chromium-Rich TaC-Containing Alloys Elaborated by Conventional Casting: Part 3: Surface and Cross-Sectional Metallographic Characterization of the Oxidized Alloys

Metallographic, Structural and Mechanical Characterization of a Low Density Fe-Mn-Al-C Steel Microalloyed with Ti/B in As-Cast and Homogenized Conditions

ABSTRACTLow density (LD) steels have shown particular characteristics in terms of mechanical properties and microstructure, since they have high strength, high ductility and density reduction up to 18%. On the other hand, the addition of microalloying elements such as Ti and B generate hardening by solid solution and precipitation, as well as grain refinement effect. LD steels generate nano-sized kappa phase precipitated from the austenite matrix, and these advanced steels can reach strength and elongation up to 780 MPa and 60%, respectively. The main objective of this research work is the metallographic, structural and mechanical characterization of a LD steel microalloyed with Ti/B in as-cast and -homogenized conditions. For this purpose a Fe-27Mn-7Al-1.2C (%wt) LD steel microalloyed with Ti/B was melted in a vacuum-induction furnace and cast in metallic mold. LD-Ti/B steel samples were homogenized at 1100 °C during 20, 50, 100, 150 and 200 minutes followed by water quenching. Metallographic, structural and mechanical characterization was carried out by optical (LOM) and scanning electron (SEM) microscopy, X-ray diffraction (XRD) and microhardness Vickers testing (HV10), respectively. In general, results showed a typical dendritic microstructure with average grain size of 1256 μm in the as-cast condition. On the other hand, the as-homogenized condition showed an austenitic equiaxial microstructure with average grain size from 164 to 940 μm. Austenite, ferrite and kappa phases were detected by X-ray diffraction (XRD). Also, second-phase particles such as AlN, TiC and MnS were detected by LOM and SEM-EDS analysis. LD steel microalloyed with Ti/B exhibited the highest microhardness Vickers value (235 HV10) in the as-cast condition, whilst in the as-homogenized condition microhardness gradually decreases from 223 to 198 HV10 as holding time increases.

Metallographic, Structural and Mechanical Characterization of REM-Containing Fe-30Mn-8Al-1.8C Low Density Steel in As-Cast Condition

ABSTRACTRecently, low-density steels have received increased attention as promising alternatives for automotive applications of the next generation of Advanced High-Strength Steels (AHSS), considering that vehicle´s weight decrease has been the subject of intense interest. It is well-known that the addition of rare earth metals (REM) has a remarkable effect on shape control and the modification of inclusions. Also, REM additions affect the grain size refinement as well as the tendency to form oxides and sulfides. The aim of this research work was to determine the effect of REM (Ce, La) addition on the microstructure and mechanical properties of the Fe-30Mn-8Al-1.8C low-density steel in as-cast condition. In order to clarify the REM effect on the Fe-Mn-Al-C system, non-microalloyed (LD-NM) and REM microalloyed (LD-REM) specimens were examined in detail by means of light optical and scanning electron microscopy for microstructural characterization. In the same way, the primary and secondary phases founded in the studied steels were identified by X-ray diffraction (XRD). Meanwhile, in order to evaluate the mechanical properties, ten microhardness measurements were carried out on the overall bulk by the Vickers hardness testing. In general, the results showed a dendritic refinement effect due to the addition of REM to low-density steel. REM acted as effective inoculants agents which reduced the primary and secondary arm spacing. Also, the strong segregation tendency at the grain boundaries in the liquid phase was limited. XRD profiles revealed the presence of austenite, ferrite, κ and DO3 phases. Low density steel microalloyed with REM showed a moderate increase in hardness compared to the non-microalloyed steel in the as-cast condition.

Development of 410S-Y2O3 oxide dispersion strengthened steel prepared by a powder metallurgy technique

In the present study, the Oxide Dispersion Strengthened (ODS) steels were synthesized by adding various concentration of Y2O3 (0, 0.25, 0.35 and 1 wt. %) into 410S steel. The sample preparation was carried out using a powder metallurgy technique. The steel-Y2O3 powders were wet mechanically alloyed for 3 h, dried and then compacted into green body. The samples were sintered in a vacuum furnace at elevated temperatures of 1000 °C and 1100 °C for 3 h. The phase analysis and structural characterization were carried out by XRD and SEM-EDX, respectively. The bulk density and hardness of the samples were measured using Archimedes principle and Vickers hardness tester, respectively. According to the results of metallographic characterization, ODS steels are composed mainly by (Fe,Cr) phase. The SEM observations indicate the presence of precipitates in the sintered steels. The density and hardness of the sample sintered at 1000 and 1100 °C increase with the increase of Y2O3 concentration. At 1100 °C, however, the density and hardness of ODS steel containing 1 wt. % Y2O3 have a tendency to decrease due to the formation of a compound consisting of Fe, Cr, Mn, Si and O.

Quantitative metallographic characterization of welding microstructures in Ti-containing TWIP steel by means of image processing analysis

Recrystallization and grain growth phenomena in Ti-containing Twining Induced Plasticity (TWIP-Ti) steel weld joints were studied. The joints obtained through the autogenous Gas Tungsten Arc Welding (GTAW) process at different heat inputs were characterized taking into account their grain size distribution and the particle pinning. The temperature ranges at which recrystallization and grain growth occurred in TWIP-Ti steel welds were determined. The optical micrographs were analyzed by means of MATLAB-based image analysis. In-house routines were added to perform grain size distribution/classification. The statistical analysis performed on micrographs by the image processing code indicated that TWIP-Ti steel showed a bimodal grain size distribution after the solution heat treatment. The static recrystallization (SRX) process detected in the low heat input weld joint, modified its bimodal grain size distribution to normal logarithmic (lognormal). The high heat input in TWIP-Ti steel weld kept the bimodal distribution. The presence of precipitated particles like Ti(C, N) and AlN generated microstructural stability in TWIP-Ti steel at temperatures <1050 °C. At higher temperatures, particle pinning by precipitates disappeared and abnormal grain growth was produced as a result of the pinning pressure decrease.

Processing and Characterization of a Copper Based Binary Alloy Achieved by Solid Phase Compaction and Sintering

In the present work, (Cu-Sn, Cu-Co) based alloys with different compositions have been obtained by using powder metallurgy (PM). These alloys were created with the purpose of increasing mechanical and structural properties of the industrial parts. The compacts are made according to the sintering manufacturing method, the uniaxial compressed cold samples. Metallographic characterizations, hardness and density measurements were carried out in order to study the influence of the addition of tin and cobalt, the variation of the compaction pressure and the sintering temperature on the finishing product. It has been proved that the addition of tin and cobalt to a copper powder mixture increase the properties of the sintered parts.

GAS NITRIDING WITH DEEP CRYOGENIC TREATMENT OF HIGH-SPEED STEEL

Nitriding with subsequent heat treatment in combination with deep cryogenic treatment (DCT) produces nitrided layers with specific properties. Layers with unique properties result from the dissolution of subsurface layers of iron nitrides and subsequent nitrogen diffusion into the substrate during austenitisation. Fine precipitates of carbonitrides eventually form during DCT and tempering. Intermediate deep cryogenic treatment was performed between the quenching and tempering steps. This work is based on comparing nitrided layers obtained using conventional treatment parameters with nitrided layers from novel processes. The experimental material was DIN 1.3343 (Czech Standard 41 9830) high-speed steel. Several treatment methods were compared in terms of the resulting hardness and metallographic characteristics.