Cross-sectional Metallography Research Articles

Data-driven analysis of surface roughness influence on weld quality and defect formation in laser welding of Cu–Al

The laser welding of Cu–Al alloys for battery applications in the automotive industry presents significant challenges due to the high reflectivity of copper. Inadequate bonding and low mechanical strength may occur when the laser radiation is directed toward the copper side in an overlap configuration welding. To tackle these challenges, a laser surface treatment technique is implemented to enhance the absorption characteristics and overcome the reflective nature of the copper material. However, elevating the surface roughness and heat-energy input over threshold values leads to heightened temperature and extreme weld. This phenomenon escalates the formation of detrimental intermetallic compounds (IMC), creating defects like cracks and porosity. Metallurgical analysis, which is time-consuming and expensive, is usually used in studies to detect these phases and defects. However, to comprehensively evaluate the weld quality and discern the impact of surface structure, adopting a more innovative approach that replaces conventional cross-sectional metallography is essential. This article proposes a model based on the image feature extraction of the welds to study the effect of the laser-based structure and the other laser parameters. It can detect defects and identify the weld quality by weld classification. However, due to the complexity of the photo features, the system requires image processing and a convolutional neural network (CNN). Results show that the predictive model based on trained data can detect different weld categories and recognize unstable welds. The project aims to use a monitoring model to guarantee optimized and high-quality weld series production. To achieve this, a deeper study of the parameters and the microstructure of the weld is utilized, and the CNN model analyzes the features of 1310 pieces of weld photos with different weld parameters.

Hot corrosion behaviour of HVAF coatings deposited on Fe[sbnd]25Cr[sbnd]20Ni support hanger material

Fe25Cr20Ni cast alloy is used as the material of construction for support hangers in fired heaters of oil refineries. These hangers support heater tubes in the fired heaters and are susceptible to hot corrosion due to Na2SO4 and V2O5 containing corrosive ash, especially when low-grade fuels are burnt. API recommends 50Cr50Ni cast-alloy for constructing the support hangers in highly corrosive atmospheres which, is highly expensive as well as brittle. To overcome this problem, in this work, coatings of 50Cr50Ni and 20Cr80Ni were applied on Fe25Cr20Ni substrates by HVAF process. The coatings were characterized by: Optical Microscopy, SEM-EDS Analysis, XRD Analysis, and Raman Spectroscopy. To study the hot corrosion behaviour of these coated specimens, a cyclic hot-corrosion test was carried out at 700 °C using 20 % Na2SO4–80 % V2O5 salt slurry. Along with the coated specimens, cast specimens of Fe25Cr20Ni and 50Cr50Ni were also subjected to the hot-corrosion test. Thus, Type-II hot corrosion behaviour of all four specimens were analyzed comparatively by weight-change measurements, cross-sectional metallography, SEM-EDS, XRD, and Raman-Spectroscopy. The Fe25Cr20Ni cast specimen displayed the least corrosion resistance. The corrosion resistance of the 50Cr50Ni coated specimen and the 50Cr50Ni cast specimen was found to be intermediate and quite comparable. 20Cr80Ni coated specimen recorded the best corrosion resistance. The corrosion mechanism and beneficial effects of high nickel-containing HVAF coatings against hot corrosion are presented.

Controlled Hydride Formation on Titanium Porous Transport Layer for Application in PEM Water Electrolysis

Titanium belongs to a group of valve metals that are known to possess a surface layer of passive oxide that protects the material from corrosion. Thanks to this attribute, titanium can withstand even harsh conditions prevailing in the anode compartment of PEM water electrolyzer. This makes titanium, together with its high electrical conductivity, a suitable material for the anode porous transport layer (PTL) and the bipolar plates in PEM water electrolyzers. However, too extensive passivation leads to an increase of contact resistance between the PTL and catalyst layer and consequently decreases the energy efficiency. This is the reason why titanium PTL is usually covered with a thin coating of precious metal (such as iridium or platinum) [1]. This thin coating hinders further titanium oxidation during electrolyzer operation and mediates good electrical contact with the catalyst particles. Recently, a much cheaper alternative approach based on titanium surface etching in acid has been introduced [2]. Titanium hydride that forms during this treatment has been shown to inhibit the passive layer growth, thus decreasing the contact resistance and improving the energy efficiency of the electrolysis.This work aims to further develop the ideas presented in reference [2] and investigate the role of titanium hydride during PEM water electrolysis. The hydridation treatment was done on pieces of titanium foil and consists of two steps. The first step is etching in boiling 35% HCl. This step removes the native oxide layer, increases the surface roughness, and introduces some amount of hydrogen into the crystal lattice of the metal. The second step involves cathodic polarization of the etched samples allowing more pronounced hydridation.By changing the condition of the cathodic polarization process, it is possible to tune the hydrogen uptake. The effects of polarization time, current density, temperature and concentration of electrolyte were studied with respect to the amount of the hydride layer formed, its thickness and penetration into the bulk phase. Treated titanium foils were characterized by various physicochemical techniques. X-ray diffractometry was used for hydride quantification, further analysis was done by scanning electron microscopy and cross-sectional metallography. Electrochemical properties of samples were studied by impedance spectroscopy and linear sweep voltammetry.The hydridation procedure was then used for titanium PTLs treatment which were then tested at the anode side of the PEM water electrolyser. Significant increase of current density was observed for etched and polarized Ti PTLs in comparison with pristine Ti PTL (Figure A). This can be attributed to significantly decreased contact resistance of treated PTLs (Figure B). A further performance increase was achieved when PTLs were treated prior to iridium coating (Figure A, B). It is thus possible to conclude, that the hydridation represents a promising approach to reduce precious metals demands in PEM water electrolysis construction.Financial support by the Czech Science Foundation within the framework of the project No.: 20-06422J is gratefully acknowlegded. Liu, C., et al., Exploring the Interface of Skin-Layered Titanium Fibers for Electrochemical Water Splitting. Advanced Energy Materials, 2021. 11(8): p. 2002926. Bystron, T., et al., Enhancing PEM water electrolysis efficiency by reducing the extent of Ti gas diffusion layer passivation. Journal of Applied Electrochemistry, 2018. 48(6): p. 713-723. Figure 1

Quantitative percussion diagnostics for evaluating porosity and surface roughness of cold sprayed and laser deposited materials

Quantitative percussion diagnostics (QPD) is a non-destructive evaluation method that has been used successfully in a number of applications. This technique involves a rod that is actuated to impact a specimen with a given amount of kinetic energy and the resulting mechanical response of the specimen as a function of time is measured using a piezoelectric force sensor. The mechanical energy returned to the rod is then analyzed. Because contact with the specimen is on the order of a few hundred microseconds, QPD can be used in situations and for specimen conditions (e.g. high temperature) that are not feasible for other nondestructive testing methods. The objective of the present study was to evaluate the use of QPD for characterization of defects in specimens fabricated using two additive manufacturing methods, i.e. cold spray deposition and laser powder directed energy deposition (LPDED). Cold spray specimens were produced using commercially pure nickel with varied process conditions including gas type (i.e. helium and nitrogen) and deposition rate. In addition, a Ti–6Al–4V specimen was manufactured using LPDED with varying porosity content. All specimens were characterized using both QPD and destructive methods (i.e. cross-sectional metallography) to compare results for observed defect characteristics. Cold spray specimens exhibited a lower energy return when they contained more porosity and/or surface roughness. Microscopic plastic deformation at highly porous surfaces was indicated during percussion testing that reached a saturation level after repeated percussion. Overall, the results showed that QPD can effectively evaluate cold spray and LPDED specimens for porosity and surface roughness.

Optimization and Characterization of Plasma Sprayed Zirconium Diffusion Barriers on Uranium-Molybdenum Monolithic Nuclear Fuel Plates

In this report, the CONVERT MP-1 team at Los Alamos National Laboratory (LANL) presents the development work for an industrial plasma spraying method to affix zirconium (Zr) metal fuel diffusion barriers on uranium-molybdenum alloy nuclear fuel plates, in order to prevent a reaction between the U-Mo fuel and the eventual aluminum cladding. The United States government is continually committed to strengthening nuclear security and nonproliferation to reduce the threat of radioactive materials acquisition by terrorists. To meet this important mission, the US Department of Energy and National Nuclear Security Agency’s (NNSA) Office of Material Management and Minimization (M3) Reactor Conversion Program (CONVERT) seeks to minimize the use of highly enriched uranium (HEU) in civilian applications through work with governments and facilities around the world to convert research reactors to the use of non-weapons-usable low enriched uranium (LEU) fuels. Currently, the CONVERT Program is committed to development of a high-density U-Mo fuel that will be able to convert high performance research reactors, while establishing an efficient and economic fabrication capability to manufacture the fuel. A micron scale Zr metal diffusion barrier is deposited onto bare U-10Mo (uranium-10% molybdenum alloy) monolithic metal foils by plasma spraying. Zr metal powders are heated in a high energy DC plasma and accelerated toward the foil substrate. The molten metal particle impact and deform on the substrate, to form a uniform dense coating. (Figure 1A) Multiple passes of the plasma torch increase the coating thickness layer by layer. In initial experiments, the DU-10Mo foil was significantly preheated, which promoted oxidation of the foil surface. Microscopic analysis showed a thin (100’s of nanometer) interaction layer rich in oxygen (Figure 1B).[1] The integrity of the Zr diffusion barrier deposition is determined by tensile strength adhesion testing and destructive cross-sectional metallography and microscopy.[2, 3] Despite best efforts of ensuring a quality vacuum and oxygen free environment, trace amounts of oxygen were present in the Ar-He plasma. During the preheating passes (i.e. no metal powder), the uranium surface was slightly oxidized. During deposition, uranium and molybdenum, as well as zirconium, would diffuse into this oxide, forming the oxygen rich interaction layer. This became a point of weakness and led to a temperature dependent bond strength. Post coating annealing shows nano-scale phase segregation and reduced strength. Further experiments eliminated the preheating steps; the Zr powder was deposited onto the foil immediately, then the plasma torch was used to heat the coated foils following deposition. With this approach, there is almost no oxide formation and minimal interaction at the interface. Contrary to expectations, this minimal diffusion and interaction leads to improved tensile adhesion strength. This improved bonding strength seems to be less dependent on the deposition temperature. The effects of processing on bond adhesion strength are reported in Figure 1C. A successful diffusion barrier requires adequate Zr layer thickness, uniform coverage, sufficient bonding at the Zr/fuel and Zr/aluminum cladding interface, and maintenance of the original nuclear fuel properties i.e. physical dimensions, alloy composition and dispersion, crystallographic phase concentration, neutronics, etc. This work outlines development of the Zr plasma spray processing techniques, parameters affecting the diffusion barrier/nuclear fuel bond strength, and characterization of the Zr diffusion barrier. Works Cited. 1. Cummins, D.R., et al., Plasma Sprayed Zirconium Diffusion Barrier Development for Monolithic U-Mo Metallic Fuel, in RRFM European Research Reactor Conference 2016. 2016: Berlin, Germany. 2. Hollis, K.J., et al., Bond Characterization of Plasma Sprayed Zirconium on Uranium Alloy by Microcantilever Testing. Journal of Thermal Spray Technology, 2013. 22(2-3): p. 233-241. 3. Cummins, D.R., K.J. Hollis, and D.E. Dombrowski, Bond Strength of Plasma Sprayed Zr Diffusion Barrier on U-Mo Fuels. Trans. Amer. Nucl. Soc., 2016. Figure 1. A) Cross-sectioned backscattered SEM (BS-SEM) of typical U-10Mo foil with both sides coated with Zr metal via plasma spray. B) BS-SEM image detailing the interface between the Zr layer and the U-10Mo foil when the sample is preheated by plasma, prior to Zr spray. C) Relation between average substrate temperature during plasma spraying (°C) and the measured bond adhesion tensile strength (MPa). Black squares show the bond strength results of the conventional twelve preheating passes prior to Zr powder depositions. The red circle present the results of the post heating treatment at 750°C for 1 hour of these same foils. The blue square relates to the experiment with only four preheating passes before Zr powder is introduced into the plasma spray system. The green squares show the results of no preheating passes. Figure 1

Detecting rare, abnormally large grains by x-ray diffraction

Bimodal grain structures are common in many alloys, arising from a number of different causes including incomplete recrystallization and abnormal grain growth. These bimodal grain structures have important technological implications, such as the well-known Goss texture which is now a cornerstone for electrical steels. Yet our ability to detect bimodal grain distributions is largely confined to brute force cross-sectional metallography. The present study presents a new method for rapid detection of unusually large grains embedded in a sea of much finer grains. Traditional X-ray diffraction-based grain size measurement techniques such as Scherrer, Williamson–Hall, or Warren–Averbach rely on peak breadth and shape to extract information regarding the average crystallite size. However, these line broadening techniques are not well suited to identify a very small fraction of abnormally large grains. The present method utilizes statistically anomalous intensity spikes in the Bragg peak to identify regions where abnormally large grains are contributing to diffraction. This needle-in-a-haystack technique is demonstrated on a nanocrystalline Ni–Fe alloy which has undergone fatigue-induced abnormal grain growth. In this demonstration, the technique readily identifies a few large grains that occupy <0.00001 % of the interrogation volume. While the technique is demonstrated in the current study on nanocrystalline metal, it would likely apply to any bimodal polycrystal including ultrafine grained and fine microcrystalline materials with sufficiently distinct bimodal grain statistics.

Irradiation of Minor Actinide–Bearing Uranium-Plutonium-Zirconium Alloys up to ˜2.5 at. %, ˜7 at. %, and ˜10 at. % Burnups

An irradiation experiment on minor actinide (MA)-bearing uranium-plutonium-zirconium (U-Pu-Zr) alloys, in which contamination by rare earth (RE) elements was considered, was performed up to ˜2.5 at. %, ˜7 at. %, and ˜10 at. % burnups in the Phenix fast reactor. All the irradiated metal fuel pins were subjected to nondestructive tests such as cladding profilometry and gamma spectroscopy. Then, cross-sectional metallography of the low-burnup and medium-burnup fuel alloys was performed, and the redistribution of the fuel matrix constituents”U, Pu, and Zr”in the low-burnup fuels was analyzed by energy dispersive X-ray spectroscopy. As a result, the irradiation growth of MA-rich and RE-rich precipitates was observed by comparing the low-burnup and medium-burnup fuels. From the postirradiation examinations carried out so far, it was confirmed that the irradiation swelling, the cross-sectional structures, and the migration of matrix constituent in metal fuels containing 5 wt% or less MAs and REs are almost the same as those in conventional U-Pu-Zr fuels.

Structure, phases, and mechanical response of Ti-alloy bioactive glass composite coatings

Porous titanium alloy-bioactive glass composite coatings were manufactured via the flame spray deposition process. The porous coatings, targeted for orthodontic and bone-fixation applications, were made from bioactive glass (45S5) powder blended with either commercially pure titanium (Cp-Ti) or Ti-6Al-4V alloy powder. Two sets of spray conditions, two metallic particle size distributions, and two glass particle size distributions were used for this study. Negative control coatings consisting of pure Ti-6Al-4V alloy or Cp-Ti were sprayed under both conditions. The as-sprayed coatings were characterized through quantitative optical cross-sectional metallography, X-ray diffraction (XRD), and ASTM Standard C633 tensile adhesion testing. Determination of the porosity and glassy phase distribution was achieved by using image analysis in accordance with ASTM Standard E2109. Theoretical thermodynamic and heat transfer modeling was conducted to explain experimental observations. Thermodynamic modeling was performed to estimate the flame temperature and chemical environment for each spray condition and a lumped capacitance heat transfer model was developed to estimate the temperatures attained by each particle. These models were used to establish trends among the choice of alloy, spray condition, and particle size distribution. The deposition parameters, alloy composition, and alteration of the feedstock powder size distribution had a significant effect on the coating microstructure, porosity, phases present, mechanical response, and theoretical particle temperatures that were attained. The most promising coatings were the Ti-6Al-4V-based composite coatings, which had bond strength of 20±2MPa (n=5) and received reinforcement and strengthening from the inclusion of a glassy phase. It was shown that the use of the Ti-6Al-4V-bioactive glass composite coatings may be a superior choice due to the possible osteoproductivity from the bioactive glass, the potential ability to support tissue ingrowth and vascular tissue, and the comparable strength to similar coatings.

Experimental investigation of laser surface processing of flexure silicon nitride ceramic

A continuous wave carbon dioxide (CO 2) laser (λ=10.6 μm) was employed to treat the surface of Si 3N 4 MOR (modulus of rupture) bars. The effects of the CO 2 laser process on physical and mechanical properties of ground Si 3N 4 samples were investigated. Scanning electron microscopy (SEM) analysis shows that the area occupied by cavities and fracture is decreased by about 49.4% after laser treatments. Cross-sectional metallography results indicate that the secondary YSiAION phase in the Si 3N 4 ceramic is softened/melted and flowed into the defects. Four-point bending tests show that the flexural strength of the treated samples is improved to 10.9%. Fractographic analysis show that the fracture origins move from the surface to subsurface. It is concluded that laser surface processing have significant effects on fracture behavior of flexure Si 3N 4 ceramic.

Corrosion of CrN and CrN/TiN coated heat-resistant steels in molten A356 aluminum alloy

The components of the equipment for processing aluminum melts into molded parts can be severely corroded by molten Al. In this study, a 6-μm CrN coating or a 4-μm CrN/TiN multilayer coating for providing physical and chemical barriers between molten reactive Al and the steel substrate were deposited by cathodic arc evaporation onto 10-mm-thick heat-resistant steel plates. Dipping tests were conducted in a 700°C A356 melt for 1–21 h at intervals of 3 h. The damage of the coated steel was evaluated by plane-view and cross-sectional metallography. Experimental results indicate that after a certain incubation period, the coated steel was locally attacked; forming hemispherical pits on both CrN and CrN/TiN coated steel samples. The incubation time for obvious pitting on the CrN coated steel was shorter than that for the CrN/TiN coated steel. Once the obvious pits were formed, the pitted areas increased with dipping time, regardless of the type of coating. Macroparticles were observed on the surface of both the CrN and CrN/TiN coatings. Both the CrN and CrN/TiN coatings exhibited a columnar structure that hardly changed after 21 h of dipping. The different incubation times of the distinct coatings were discussed in terms of the grain structure, macroparticles and thermo-mechanical stress associated with the physical vapor deposition-coated steels.

中性子照射を受けたステンレス鋼およびニッケル基合金の溶接性について

Degradation of weldability caused by helium, which is generated by nuclear transmutation in neutron irradiated material, is an important issue to be addressed in planning of proactive maintenance of light water reactor core internal components. In this work, the weldability of neutron-irradiated stainless steel and nickel-base alloy, which are major constituting materials for components, was practically evaluated. The weldability was first examined by TIG welding in relation to the weld heat input and helium content using various specimens (made of SUS304 and SUS316L) sampled from reactor internal components. The specimens were neutron irradiated in a boiling water reactor to fluences from 4 × 1024 to 1.4 × 1026 n/m2 (E>1 MeV), and resulting helium generation ranged from 0.1 to 103 appm. The weld defects were characterized by dye penetrant test and cross-sectional metallography. The weldability of neutron-irradiated stainless steel was shown to be better at lower weld heat input and lower helium content. To evaluate mechanical properties of welded joints, thick plates (20 mm) specimens of SUS304 and Alloy 600 were prepared and irradiated in Japan Material Test Reactor (JMTR). The helium content of the specimens was controlled to range from 0.11 to 1.34 appm selected to determine threshold helium content to weld successfully. The welded joints had multiple passes by TIG welding process at 10 and 20 kJ/cm heat input. The welded joints of thick plate were characterized by dye penetrant test, cross-sectional metallography, tensile test, side bend test and root bend test. It was shown that irradiated stainless steel containing below 0.14 appm of helium could be welded with conventional TIG welding process (heat input below 20 kJ/cm). Nickel-base alloy, which contained as much helium as stainless steel could be welded successfully, could also be welded with conventional TIG welding process. These results served as basis to evaluate the applicability of repair welding to actual reactor components.

Weldability of neutron irradiated austenitic stainless steels

Degradation of weldability in neutron irradiated austenitic stainless steel is an important issue to be addressed in the planning of proactive maintenance of light water reactor core internals. In this work, samples selected from reactor internal components which had been irradiated to fluence from 8.5 × 10 22 to 1.4 × 10 26 n/m 2 ( E > 1 MeV) corresponding to helium content from 0.11 to 103 appm, respectively, were subjected to tungsten inert gas arc (TIG) welding with heat input ranged 0.6–16 kJ/cm. The weld defects were characterized by penetrant test and cross-sectional metallography. The integrity of the weld was better when there were less helium and at lower heat input. Tensile properties of weld joint containing 0.6 appm of helium fulfilled the requirement for unirradiated base metal. Repeated thermal cycles were found to be very hazardous. The results showed the combination of material helium content and weld heat input where materials can be welded with little concern to invite cracking. Also, the importance of using properly selected welding procedures to minimize thermal cycling was recognized.

Study of interfacial reactions between Tin and copper by differential scanning calorimetry

The interfacial reactions at the solder joints of interest in microelectronic structures have been extensively investigated in order to control the soldering process and thereby to provide strong and reliable solder joints. These reactions comprise the dissolution of a base metal into a molten solder, the concomitant growth of intermetallics in the liquid state, and the growth of intermetallics in the solid state. Various experimental techniques employed include conventional cross-sectional metallography, scanning electron microscopy, electron microprobe analysis, x-ray diffraction and others. In this study, the interfacial reaction between tin and copper has been investigated by using differential scanning calorimetry which provides a simple means to study such reactions and complements information obtained from other techniques. The experimental results have been applied to understand the interfacial reactions in two important examples: one for a soldering process with copper metallization, and another for a high conductivity, Pb-free, electrically conducting adhesive which contains tin-coated copper powder as conducting filler particles.