Metallographic Characterization Research Articles

Metallographic Characterization of Additive-Layer Manufactured Products by Electron Beam Melting of Ti-6Al-4V Powder

Abstract Optical metallography and electron metallography (SEM and TEM) techniques are applied to the characterization of additive-layer manufactured products and prototypes by electron beam melting (EBM) of Ti-6Al-4V atomized precursor powder. In addition, microstructure-property comparisons are made by hardness and tensile test measurements which demonstrate prospects for controlled property-performance variations by varying process parameters including temperature profiles and cooling rates in manufactured prototypes.

Composite zinc-fly ash coating on mild steel

Zinc-fly ash composite coatings were deposited on mild steel substrates with the help of electrodeposition technique. Metallographic and chemical characterization of the produced composite coatings was performed with the aid of Scanning Electron Microscopy (SEM) and Electron Dispersive X-ray Analysis (EDX). The corrosion behaviour of the composite coated mild steel substrate was studied in a 0.3 M NaCl solution (pH = 5.5) by means of anodic polarization curves. The wear of the zinc-fly ash coating was also investigated by using a pin-on-disk apparatus. The composite coating exhibited increased wear resistance, compared to the pure zinc coating and the mild steel substrate. The adhesion strength between the zinc-fly ash coating and the mild steel substrate was examined with a scratch testing apparatus. The adhesion strength between zinc-fly ash composite coating and the mild steel substrate was found to be higher than that of the pure zinc coating to mild steel.

Assessment of the effect of existing corrosion on the tensile behaviour of magnesium alloy AZ31 using neural networks

A concept has been devised to assess the effect of existing corrosion damage on the residual tensile properties of structural alloys and applied for the magnesium alloy AZ31. The concept based on the use of a radial basis function neural network. An extensive experimental investigation, including metallographic corrosion characterization and mechanical testing of pre-corroded AZ31 magnesium alloy specimens, was carried out to derive the necessary data for the training and the prediction module of the developed neural network model. The proposed concept was exploited to successfully predict: the gradual tensile property degradation of the alloy AZ31 to the results of gradually increasing corrosion damage with increasing corrosion exposure.

Examining the surfaces in used platinum catalysts

For the purpose of finding more advanced platinum catalyst manufacturing technologies and achieving a higher degree of ammonia oxidation, metallographic characterization has been done on the surface of catalyst gauzes and catalyst gripper gauzes made from platinum and palladium alloys. For the examined samples of gauzes as well as the cross section of the wires, a chemical analysis was provided. The purpose of this paper is the metallographic characterization of examined alloys carried out by way of electronic microscopic scanning, X-rays as well as chemical assays which contributed greatly to a better understanding of the surface deactivation, in other words a better consideration of structural changes occurring on the wire surface.

Characterization of 2024-T3: An aerospace aluminum alloy

The 2024-T3 aerospace aluminum alloy, reported in this investigation, was acquired from a local aerospace industry: Royal Malaysian Air Force (RMAF). The heat treatable 2024-T3 aluminum alloy has been characterized by use of modern metallographic and material characterization techniques (e.g. EPMA, SEM). The microstructural characterization of the metallographic specimen involved use of an optical microscope linked with a computerized imaging system using MSQ software. The use of EPMA and electron microprobe elemental maps enabled us to detect three types of inclusions: Al–Cu, Al–Cu–Fe–Mn, and Al–Cu–Fe–Si–Mn enriched regions. In particular, the presence of Al 2CuMg (S-phase) and the CuAl 2 (θ′) phases indicated precipitation strengthening in the aluminum alloy.

Spherical Fuel Element Concept for Small Reactor Design

The Pacific Northwest National Laboratory (PNNL) is currently developing a novel spherical fuel element concept that offers low fuel temperatures, low stored energy, and long core life. Fuel performance modeling has been conducted using the PNNL-developed Atoms for Peace Reactor (AFPR)-100 as a platform for demonstrating the potential of the fuel element concept. The AFPR-100 is a small [100-MW(electric), 300-MW(thermal)], water-cooled reactor concept that is designed to use established technology, be passively safe, and be proliferation resistant. The fuel performance modeling has demonstrated that this fuel element has a short thermal time constant, has low fuel temperature, provides a barrier for retention of fission products, and will have long-term dimensional stability.A technique for manufacturing these fuel elements was developed. A fabrication demonstration was conducted in cooperation with a commercial vendor to evaluate the feasibility of manufacturing the fuel elements. In order to demonstrate the proposed technique, the proposed spherical elements were produced using existing processes that could be scaled to large batch sizes. Surrogate ZrO2 kernels were substituted for the fuel in this demonstration. Thorough characterization of the fuel elements was performed at various stages in the fabrication process. The metallographic characterization included electron microscopic analysis of coating microstructure, and particular attention was paid to interface regions to search for deleterious reaction zones, debonding, and porosity. Although this demonstration is not complete, early results are promising and will be discussed in this paper.This paper will describe the fuel element, show the results of fuel performance calculations for this element, describe the proposed fabrication process, and discuss the results of a fabrication demonstration to date that has been performed for this concept.

New Approaches to Improve High Temperature Corrosion Resistance in Chlorine-Based Atmospheres

Chlorine gas is widely encountered in chemical industries, as well as in waste incinerators and plastic/polymer decomposition mills. The presence of chlorine may significantly reduce the life-time of the components. Under chlorine-based atmospheres, the process of scale formation may be considerably affected and the presence of chlorine usually impedes the formation of a long term protective dense oxide scale. Based on thermodynamic calculations and previous investigations, NiAl and NiAlMo APS-coatings were produced to be used as protection for conventional steels against chlorine corrosion. Indeed, thermodynamic diagrams showed that molybdenum should have a positive behaviour in “reducing”-chloridizing atmospheres, whereas aluminium has a positive behaviour in “oxidizing”-chloridizing atmospheres. Coatings of approximatively 300 μm thickness were thermally sprayed on Armco Iron and on a commercial ferritic 18 Cr steel. This work presents the corrosion behaviour of NiAl and NiAlMo APS-coatings under chlorine-based atmospheres at 800°C. In addition, metallographic characterisation as well as EPMA investigations of the coating cross sections were carried out before and after the corrosion tests.

Microstructures at high temperature of Fe-30 wt.% Cr-xC Alloys with x varying from 0 to 2 wt.%

AbstractDifferential thermal analysis experiments and treatments for 50 h at 1000, 1100 and 1200 °C were performed for a binary Fe30Cr alloy and for six Fe30Cr (0.2 to 2 wt.%) C alloys. A complete metallographic characterization was done for all samples. The solidus and liquidus temperatures, the natures and the surface fractions of the carbide and matrix phases were determined for the three temperatures. They were compared to thermodynamic calculation results. The values and the evolutions of the solidus temperature versus the carbon content are in good agreement between experiment and calculations. It is less true for the carbide volume fractions which were obtained either by image analysis, or from the matrix Cr content. Mismatches were also observed for the liquidus temperature. Comparisons were done with the Co30CrxC and Ni30CrxC systems and with Ta-containing Fe30CrxC alloys.

ASSIMILATION OF HIGH-SPEED OF CORD ROLLED WIRE ON MILL 150 OF RUP “BMZ”

It is shown that mechanical and metallographic characteristics of rod, rolled at speed 100 and 105 m/sec, correspond to ZTU 840-03-2006.

Electropolishing of Coronary Stents

Produce of coronary stents demands advanced precision. In the present study, electropolishing was performed on stainless steel slotted tube coronary stents made by laser cutting. The surface quality of stents has a significant influence on biocompatibility, therefore the optimal method for electropolishing were explored. Additionally, acid pickling as the pretreatment of electropolishing was also conducted. Pickling was necessary prior to electropolishing for decreasing roughness of the cutting zone and for removing the oxide films covering the stent surface. An optimal condition for electropolishing could also be established and it caused a smooth stent surface. Material removal (weight loss and strut width change) in the process of both pickling and electropolishing was investigated. Furthermore, material characterization of the stents was determined by means of composition analysis, metallographic characterization and microstructural analysis.

Stress-Rupture Characterization in Nickel-Based Superalloy Gas Turbine Engine Components

Today’s gas turbine engines utilize high volume fraction gamma prime (γ′) strengthened alloys for turbine airfoils, which typically operate at temperatures greater than ∼0.5Tm of the alloy. At these temperatures and at stresses below yield, time-dependent deformation (creep) of the airfoil can occur and, if left unabated, can result in complete separation of the airfoil. This process is commonly referred to as stress rupture. Insufficient cooling air, unintentional interruptions of cooling air as well as abnormal engine operating conditions are typical causes of stress-rupture failures in gas turbine blade components. Stress-rupture fractures are generally heavily oxidized, tend to be rough in texture, and are primarily intergranular and/or interdendritic in appearance compared to smoother, transgranular fatigue type fractures. Often, gross plastic yielding is visible on a macroscopic scale. Commonly observed microstructural characteristics include creep voiding along grain boundaries and/or interdendritic regions. Internal voids can also nucleate at carbides and other microconstituents, especially in single crystal castings that do not possess grain boundaries. Other signs of overtemperature include partial resolutioning of the γ′ strengthening precipitates, with the remaining volume fraction of γ′ commonly used to estimate blade metal temperatures. This article highlights the visual, fractographic, and metallographic characteristics typically encountered when analyzing stress rupture of turbine airfoils.

Mechanical testing of titanium/aluminium–silicon interfaces by push-out

Mechanical properties of Ti/Al–7Si assemblies produced by insert moulding were studied with a classical push-out test and a variant that is the circular-bending test. Special care has been taken for controlling both the reactivity at the Ti/Al–7Si interface and the metallurgical health of the Al–7Si matrix. Mechanical tests until complete debonding have been completed with interrupted tests, metallographic characterizations and FEM analysis of elastic stress state. A mean shear strength of the interface of about 120 MPa was obtained. When the Ti insert is solely fretted in the matrix, without chemical interaction between Ti and the Al–7Si alloy, the mean shear strength is significantly lower (48 MPa). This result clearly shows that chemical interaction at the interface (formation of a thin TiSi layer at the Ti side and a thick Al3Ti(Si) layer at the Al–7Si alloy side) improves the mechanical properties of the assembly. It is also shown that the failure sequence is characterized both by crack propagation from bottom to top and matrix yielding from top to bottom. Actually, interface damaging begins by crack initiation at the specimen bottom face (not at the top face and under the indenter) in a nearly pure mode I solicitation at a radial tensile stress of about 100 MPa.

Metallographic and numerical characterization of MgH2-Mg system

ABSTRACTThe remarkable ability of magnesium to store significant quantities of hydrogen has fostered intense research efforts in the last years in view of its future applications where light and safe hydrogen-storage media are needed. Magnesium material, characterized by light weight and low cost of production, can reversibly store about 7.7 wt% hydrogen. However, further research is needed since Mg has a high operation temperature and slow absorption kinetics that prevent the use in practical applications. For these reasons a detailed study of the interface between Mg and MgH2 is needed. Further insights are gained by characterizing the Mg-MgH2 system from both the experimental and the numerical point of view.The study of the MgH2-Mg phase transformation in powder samples has been performed to gain detailed metallographic information. A method for studying this phase transformation by cross sectional samples scanning electron microscopy observation of partially transformed material has been developed. This method exploits the peculiar features of this system where the MgH2 phase is insulating and the Mg is a metallic conducting phase. This difference can induce a contrast between the two phases owing to the different secondary emission yield. Further insights are gained by characterizing Mg-MgH2 interfaces by means of accurate first-principle molecular dynamics simulations based on the density-functional theory. Extensive electronic structure calculations are used to characterize the equilibrium properties and the behavior of the surfaces in terms of total energy considerations and atomic diffusion.

Casting Porosity and Long-Life Fatigue Strength of a Cast Al-Alloy

The sand casting process usually generates porosity that is highly detrimental especially to the fatigue behavior of Al-Si alloys. Since pores favor early fatigue crack initiation, the total fatigue life is crack-propagation-dominated and, for a given stress level, strongly depends on the initiating pore size. Here long life fatigue strengths were estimated using specimens extracted from different AlSi7Mg castings. Detailed metallographic characterization of specimen sections was carried out to determine pore size distributions and correlate it to test results. Murakami’s experimental procedure for applying the largest extreme value statistics to small defect characterization is applied and discussed.

Explosion Joining of Magnesium Alloy AZ31 and Aluminum

Explosion welding has produced a large number of dissimilar joints. But the explosion welding of materials of low impact toughness and brittle nature is considered to be difficult. Magnesium, with its HCP structure, has a low capacity for plastic deformation at room temperature and moreover it has a high chemical reactivity. Therefore, successful explosion welding of magnesium alloys, demands careful attention to be paid to prevent failure or formation of brittle reacted zones at the weld interface. Explosive welding of the wrought Mg-Al-Zn alloy and commercially pure aluminum is performed. The welds are analyzed through metallographic characterization and shear tests. High quality welds of the two materials possessing shear strength higher than the softer layer has been obtained.

Severe plastic deformation by linear flow splitting

AbstractDue to the new linear flow splitting forming process the production of bifurcated profiles from sheet metal without lamination of material becomes feasible. The continuous production of such structures takes place incrementally in a modified roll forming machine. By producing those complex parts process‐related changes in the material properties occur as a result of the cold forming. Thus, the assumption of homogeneous properties in the processed part is not valid and a reliable analysis of structural durability is only possible by considering changes in material properties. Investigations on linear flow split profiles show large gradients in the microstructure and properties over the profile cross section. In the areas of low plastification the degree of deformation and the flow stress can be determined by microstructure and hardness measurements. In the severely deformed areas, like the upper flange surface, this determination becomes doubtable. Therefore the electron backscattered diffraction method was used to investigate, if an ultrafine‐grained structure is occurring as it evolves in the processes of severe plastic deformation. Thus, the aim of this paper is the numerical analysis of the linear flow splitting process, the metallographic characterisation of bifurcated profiles and the numerical evaluation of the structural durability with consideration of the gained insights.

Characterization of laser treated steels using instrumented indentation by cylindrical flat punch

The aim of the present work is to assess the possibility of predicting the thickness of the hardened layers produced by High Power Diode Laser (HPDL) heat treatment. Instrumented indentation by cylindrical flat punch was carried out in order to evaluate the effectiveness of the surface-hardening heat treatments on milled AISI 1040 steel substrates by varying laser operational parameters as well as the morphology of steel substrates. In particular, the trend in load-penetration depth were obtained, supporting the identification of the depth at which the hardened layer starts transferring the deformation to the softer substrate, that is, the Critical Transition Depth (CTD) in addition to identifying the overall degree of hardening reached by the laser treated material. On the other hand, hardness profiles and the thickness of hardened layers were also monitored by using combination of Vickers micro-hardness tests and standard metallographic characterizations. Analytical examinations and simple mathematical simulations of the experimental results coming from indentation tests and conventional testing procedures showed good correlation between the extent of the heat affected zone (HAZ), hardness and CTD. In conclusion, this study demonstrates that indentation tests are an effective, sensitive, fast, and low-cost characterization technique for heat treated steels.

Thermal and metallographic characteristics of the Al–20% Si high-pressure die-casting alloy for monolithic cylinder blocks

Abstract Thermal analysis data were gathered to control the hypereutectic microstructure of the die-cast Al–20% Si cylinder block at a cooling rate of 1 °C/s. The liquidus temperature was approximately 691 ± 2.2 °C, the nucleation temperature of the Al–Si eutectic was approximately 567.1 ± 1.9 °C and the nucleation temperature of the Cu- and Mg-enriched eutectic was approximately 513.6 ± 1.4 °C. The fraction solid increases linearly from 0 to 21.6% when the temperature decreases from 691 to 567.1 °C. During the Al–Si eutectic growth the fraction solid rose to approximately 70% within a very narrow temperature range of 14 °C. The solidus temperature was recorded to be 479.9 ± 3.3 °C. The total solidification range for the investigated alloy was approximately 211 °C. The rapid quenching experiments during the alloy melting cycle revealed the dissolution of the primary Si particles with the increasing temperature followed by complete melting when the liquidus temperature were exceeded (i.e., 710.9 °C). It was found that a minimum superheat temperature of 80 °C was needed to achieve the adequate refinement of the primary Si particles as a combined effect of thermal and chemical modification. The results provided an extensive understanding of the thermal characteristics for the newly developed hypereutectic Al–20% Si alloy and might be used to optimize the liquid metal handling on the foundry floor with respect to obtaining the required alloy microstructural and service characteristics.

Compressive fatigue in titanium dental implants submitted to fluoride ions action

The aim of this study was to assess the influence of a fluoridated medium on the mechanical properties of an internal hexagon implant-abutment set, by means of compression, mechanical cycling and metallographic characterization by scanning electronic microscopy. Five years of regular use of oral hygiene with a sodium fluoride solution content of 1500 ppm were simulated, immersing the samples in this medium for 184 hours, with the solutions being changed every 12 hours. Data were analyzed at a 95% confidence level with Fisher's exact test. After the action of fluoride ions, a negative influence occurred in the mechanical cycling test performed in a servohydraulic machine (Material Test System-810) set to a frequency of 15 Hz with 100,000 cycles and programmed to 60% of the maximum resistance of static compression test. The sets tended to fracture by compression on the screw, characterized by mixed ruptures with predominance of fragile fracture, as observed by microscopy. An evidence of corrosion by pitting on sample surfaces was found after the fluoride ions action. It may be concluded that prolonged contact with fluoride ions is harmful to the mechanical properties of commercially pure titanium structures.

Metallographic Characterization of Gas Turbine Components

Metallographic Characterization of Gas Turbine Components