Metal Grain Size Research Articles

The Microstructures and Mechanical Properties of a Welded Ni-Based Hastelloy X Superalloy

The Hastelloy X superalloy is a widely used solid-solution Ni-based sheet alloy for gas turbines, aero-engine combustion chambers, and other hot-end components. To investigate the effect of microstructure, especially grain size, on its weldability, Hastelloy X alloy bars are homogenized, cold-rolled to thin sheets, and recrystallized under different conditions to obtain equiaxed grain microstructures with average grain sizes of ~5 μm, ~12 μm, and ~90 μm. The laser welding process is used for joining the alloy sheets, and then the alloy’s weldability is investigated through microstructural and mechanical property characterizations. The microstructures in weld consist of coarse columnar grains with dendrite, and grain sizes of these columnar grains are almost the same when grain size of Hastelloy X base metal increases from ~5 μm to ~90 μm. Moreover, although all welds exhibit lower yield strengths (YS), ultimate tensile strengths (UTS), and elongations to fracture (EF) than the base metal, the degrees of reduction in them become slight when the grain size of base metal increases from ~5 μm to ~90 μm.

Innovation near-net-shape casting process of manganese metal based on grain refinement principle

The heat loss in the process of casting Mn metal and the generation of Mn powder in the process of breaking Mn ingots have always been difficult problems in the industry. In order to realize the energy saving and loss reduction in the process of casting manganese metal, we propose a concept of near-net-shape casting of Mn metal based on Grain Refinement Principle, in which the Mn metal melt is cast to small product size and heat recovery is realized through water heat transfer. The challenge of this method is to control solidification structure and improve mechanical properties using small-size casting mould. Present work focuses on the study of the solidification microstructure evolution mechanism of Mn metal and its structure-activity relationship with mechanical properties of small size abrasives casting process. The results showed that the average grain size of Mn metal first decreases and then increases as the decrease in casting temperature. The average grain size is the smallest when the casting temperature is 1533 K. The grain size of Mn metal is mainly affected by its critical nucleation radius. The relationship between the critical nucleation radius of Mn metal and its average grain size is established thatr=69.15∙exp−exp0.05r*−37.16;R2=0.9460. The smaller the critical nucleation size is, the more dendrite fragments there are to achieve grain refinement. The hall Petch relationship between the average grain size and the comprehensive strength of Mn metal under different casting conditions is established thatσ=1755.34d−1/2−137.27,R2=0.93457, the average size of the formula is 50–70 µm Mn metal has good applicability. The mechanical properties of the single β-Mn crystalline form are significantly better than those of the mixed α + β crystalline form with a casting temperature below 1573 K. The influence of solidification structure on the mechanical properties of Mn metal is much more significant than that of crystal form. This primary molding technology is also applicable to the casting process of metals with high intrinsic brittleness, such as silicon, germanium, and titanium, so it has a broad application prospect.

Effect of a pulsed magnetic field on the tribological properties of amorphous/nanocrystalline composite coatings by supersonic plasma spraying

Pulsed magnetic fields can enhance ferromagnetic and paramagnetic materials by enhancing their stress structure, domain variation, phase structure distribution and grain size of both solid and molten metals. Thus, Ni-based coatings were prepared by pulsed magnetic field-assisted supersonic plasma spraying under a strong magnetic field of 5 T, based on the characteristics of a pulsed magnetic field and combined with the advantages of supersonic plasma spraying technology. According to the results, the porosity of the coatings prepared using this approach decreased, the magnetic domains appeared as sheet domains and showed a sinusoidal distribution, numerous amorphous/nanocrystalline structures appeared, enhanced grain orientation and the coatings had good tribological properties. This study offers a new perspective on the design of high strength metal base coatings for various applications.

Low supported nickel-cobalt catalyst for hydrogenation of maleic anhydride

In order to prepare supported catalysts with low-metal loading, a series of NiCo/Clay catalysts were prepared and used in the hydrogenation of maleic anhydride. The hydrogenation reaction was carried out in a fixed-bed for 8 h with reaction pressure = 1 MPa, reaction temperature = 180 °C, molar ratio of H2: maleic anhydride = 50, WHSV = 2 h−1. Ni4Co1/Clay (loaded 3 wt%Ni and 0.75 wt%Co) can maintain no obvious catalyst deactivation within 8 h. The overall conversion of maleic anhydride is not less than 95 %, and the selectivity of succinic anhydride is up to 100 %. N2 adsorption–desorption characterization, X-ray diffraction, temperature-programmed reduction, temperature-programmed desorption, X-ray photoelectron spectroscopy and transmission electron microscopy were performed to verify the changes in catalyst morphology, active species, reduction performance and hydrogenation activity after adding Co as a promoter. With the addition of Co additive, the distribution of active sites on the carrier is more uniform, the grain size of active metal becomes smaller and the reduction temperature decreases. The addition of Co not only increased the active center of the catalyst but also effectively avoided the catalyst's deactivation.

Study of Microstructure and Properties of AZ91 Magnesium Alloy Welded Joint with Magnetic Field and TiO2 Activated Flux

In order to improve the weldability and bearing capacity of AZ91 magnesium alloy welded joints, magnetic field and active flux were added in the TIG welding process. In the welding process, the magnetic field and welding parameters were unchanged, and the coating amount of active flux was adjusted. The formability, mechanical properties, and microstructure of the welded joints under different coating amounts of activated flux were analyzed, and the crystallization nucleation characteristics of molten pool were discussed. The experimental results reveal that the combined effect of the magnetic field and activated flux has a significant effect on increasing the penetration and promoting mechanical properties. When the coating amount of activated flux is 3 mg/cm2, the highest penetration of the welded joint is obtained, which is 141% of that without activated flux. Meanwhile, the mechanical properties reach the maximum, which is a tensile strength of 292 MPa, elongation of 11.2%, and weld zone hardness of 75.6 HV (0.5 Kgf). The combined effect of TiO2 flux and magnetic field does not change the phase composition and the grain orientation of a weld metal but can affect the grain size. The average grain size of a weld metal under an activated flux coating amount of 3 mg/cm2 is 18.2% smaller than that under an activated flux coating amount of 1 mg/cm2.

Enhancement of tensile properties of gas tungsten arc welds using Cu-coated CoCrFeMnNi filler and post–weld heat treatment

This study investigated the gas tungsten arc (GTA) weldability of cold-rolled CoCrFeMnNi high-entropy alloys (HEAs) using Cu-coated HEA filler, specifically through the application of various post–weld heat treatment (PWHT) temperatures. The GTA weldability of cold-rolled HEA was evaluated by applying the optimum condition of full penetration, and the effect of PWHT was investigated in the temperature range of 973–1173 K. No macro-defects were detected in the weld metal (WM) to which the Cu coated HEA filler was applied. All the PWHT-applied specimens, including the as-welded specimens, were composed of the FCC phase. The Cu component was solid-solutionized over the entire area of the WM and did not form a precipitate. The tensile properties of the as-welded specimens deteriorated in the presence of CrMn oxides. As the PWHT temperature increased, the grain size in the base metal (BM) increased and inclusions in the WM were re-dissolved. Furthermore, by increasing the PWHT temperature, the hardness of the BM decreased significantly by grain growth, and the WM softened slightly owing to the re-dissolution of inclusions. Therefore, the WMs improved the tensile strength and elongation with increasing PWHT temperature. The application of PWHT significantly improved the weldability of the Cu coated CoCrFeMnNi welds.

Physical mechanism interpretation of polycrystalline metals’ yield strength via a data-driven method: A novel Hall–Petch relationship

The Hall–Petch relationship σy=σ0+kyd−0.5 is widely used to describe the relationship between yield strength and grain size of polycrystalline metals, and the material constants σ0 and ky fitted by experimental data are interpreted as lattice friction resistance and coefficient of grain boundary resistance, respectively. The frequent deviations from the Hall–Petch relationship between σy and d in coarser or finer grains, as well as the physical essence of σ0 and ky, have always been the focus of material scientists. In this study, intrinsic factors of the fitted constants σ0 and ky in the traditional Hall–Petch relationship are mined via a data-driven machine learning method, which reveals that the key physical quantities affecting σ0 are valence electron distance (S), cohesive energy (W), and coefficient of linear thermal expansion (lt); meanwhile, the key physical quantities affecting ky are grain boundary interface energy (γ), Young's modulus (E) and coefficient of linear thermal expansion (lt). Then, a novel Hall–Petch model σy=79W/(S3lt)+1.2γE/ltd−0.5 with satisfying prediction accuracy is constructed by symbolic regression methods. There is no experimental fitting constant term in the novel model, which can directly predict the yield strength of polycrystalline metals by key physical quantities. The novel Hall–Petch model has an excellent generalization ability and can be extended to the correlation calculation between the composition, grain structure and mechanical properties of single-phase alloys, which provides a theoretical method for the trans-scale calculation of metallic materials.

Effect of Grain Size on Change in Surface Roughness of Carbon Steels Through Polishing Processes

A mirror-like reflecting surface is an important characteristic in many industrial metallic parts. Polishing is done to form a mirror surface on metals. However, the effect of the grain size of metals on surface roughness through polishing processes is not clear. Specifically, mirror surface formation of ultrafine grained materials is still unknown. Ultrafine grained steels and coarse grained steels with 0.02, 0.10, and 0.60 wt% carbon contents were prepared by warm caliber rolling and annealing. Average grain sizes were 1–2 μm and 4–40 μm. The changes in surface roughness, Sa, were measured with an atomic force microscope (AFM) via eight polishing steps, using emery papers of type #600, #1000, #1500, #2000, #2500, #4000, and free abrasive grains of 3 μm and 1 μm diamond. As the polishing process progressed, the surface unevenness was removed and the surface roughness, Sa, decreased in all steels. The differences of Sa at each polishing step were analyzed from the point of carbon content, Vickers hardness, and grain size. Carbon contents and Vickers hardness have little effect on Sa. However, grain size has a considerable effect on Sa in all steels. Ultrafine grained steels have smaller Sa in all polishing steps in all steels. This is because ultrafine grained steels have very small work hardening rate. After final polishing, Sa is 2.5–3.6 nm in coarse grained steels and 2.0–2.6 nm in ultrafine grained steels. To obtain a mirror surface with smaller Sa, grain size control is important.

Structure and mechanical properties of thick-walled joints of Ti-6-4 titanium alloy made by electroslag welding

The results of studies of the quality of the electroslag welds (ESW) of 100 mm thick Ti-6AI-4V plates are presented. ESW was performed using a consumable nozzle made of Ti-6Al-4V alloy with two welding wires made of SPT2 alloy. The study of the properties of the welded joints included X-ray inspection, chemical and gas analysis, optical metallography, phase composition analysis, mechanical tensile and impact tests and fractographic analysis. As a result of the research, the following was established. X-ray inspection and optical metallography of the welded joint showed the absence of surface and internal defects in the weld metal and heat-affected zone (HAZ). The gas composition of the weld metal meets the requirements of the standard for the Ti-6Al-4V alloy. The tensile strength of the weld metal is on average 90% of the strength of the base metal and the impact toughness (KCU) of the weld metal is 1.6 times higher than that of the base metal. The weld and HAZ metal is characterized by mixed-type transcrystalline fracture with areas of both ductile and brittle fractures. The mechanical properties of the welded joint are primarily related to the grain size and microstructure of weld metal and HAZ, which are formed under conditions of a low cooling rate and high heat input used for the ESW process.

Dependence of spallstrength on temperature, grain size and strain rate in pure ductile metals

When a shockwave, which can be generated by high velocity impact or explosive detonation, reflects from the free surface of a metal, it usually creates tensile stress inside the metal. While the tensile stress is large enough, voids nucleation, growth and coalescence happen inside the metal, causing the metal to spall. As one of the main contents of the spallation damage research, the spallation strength, which is often characterized by features of the free surface velocity history measured in spallation experiments, represents the maximum tensile stress that the material can withstand, and is actually a complex interaction among several competing mechanisms. Optimizing the spallation strengths of metals is important for their applications in the aerospace, automotive, and defense industries, and can be achieved by using the advanced manufacturing strategies, if we can know better the meaning and present analytic model of the spallation strength of metal. A large number of experiments show that the spallation strength of ductile metal is strongly dependent on the tensile strain rate, grain size and temperature of material. Based on the analysis of early spallation evolution and influence of grain size and temperature on the material, a simple analytic model of spallation strength is presented in this paper, which takes into account the effects of strain rate, grain size and temperature in materials. The applicability of this model is verified by comparing the calculated results from the model with the experimental results of spall strength of typical ductile metals such as high purity aluminum, copper, and tantalum.

Influence of Post Weld Heat Treatment on the Grain Size, and Mechanical Properties of the Alloy-800H Rotary Friction Weld Joints.

This study evaluated the microstructure, grain size, and mechanical properties of the alloy 800H rotary friction welds in as-welded and post-weld heat-treated conditions. The standards for the alloy 800H not only specify the composition and mechanical properties but also the minimum grain sizes. This is because these alloys are mostly used in creep resisting applications. The dynamic recrystallization of the highly strained and plasticized material during friction welding resulted in the fine grain structure (20 ± 2 µm) in the weld zone. However, a small increase in grain size was observed in the heat-affected zone of the weldment with a slight decrease in hardness compared to the base metal. Post-weld solution heat treatment (PWHT) of the friction weld joints increased the grain size (42 ± 4 µm) in the weld zone. Both as-welded and post-weld solution heat-treated friction weld joints failed in the heat-affected zone during the room temperature tensile testing and showed a lower yield strength and ultimate tensile strength than the base metal. A fracture analysis of the failed tensile samples revealed ductile fracture features. However, in high-temperature tensile testing, post-weld solution heat-treated joints exhibited superior elongation and strength compared to the as-welded joints due to the increase in the grain size of the weld metal. It was demonstrated in this study that the minimum grain size requirement of the alloy 800H friction weld joints could be successfully met by PWHT with improved strength and elongation, especially at high temperatures.

Weld Joint of Tantalum Sheet by Tungsten Inert Gas Welding

The 0.6 mm tantalum sheet was welded under argon atmosphere by Tungsten Inert Gas Welding (TIG) in order to obtain a welded joint with high-quality and high-reliability. Metalloscope, scanning electron microscopy (SEM), and energy dispersive spectroscopy (EDS) were applied to analyze the joint. The results showed that the grain size of the base metal which affected by the welding heat remained the original size or enlarged slightly to 40-70 μm. What’s more, the weld zone was found to be composed of two components with different oxidation degree. And the distribution of these two components was related to the protection atmosphere of the location. The fusion line of the two tantalum sheets was clear which means high welding quality.

Influence of the Heat Input on the Dendritic Solidification Structure and the Mechanical Properties of 2.25Cr-1Mo-0.25V Submerged-Arc Weld Metal

The alloy 2.25Cr-1Mo-0.25V is commonly used for heavy wall pressure vessels in the petrochemical industry, such as hydrogen reactors. As these reactors are operated at elevated temperatures and high pressures, the 2.25Cr-1Mo-0.25V welding consumables require a beneficial combination of strength and toughness as well as enhanced creep properties. The mechanical properties are known to be influenced by several welding parameters. This study deals with the influence of the heat input during submerged-arc welding (SAW) on the solidification structure and mechanical properties of 2.25Cr-1Mo-0.25V multilayer metal. The heat input was found to increase the primary and secondary dendrite spacing as well as the bainitic and prior austenite grain size of the weld metal. Furthermore, it was determined that a higher heat input during SAW causes an increase in the stress rupture time and a decrease in Charpy impact energy. This is assumed to be linked to a lower number of weld layers, and therefore, a decreased amount of fine grained reheated zone if the multilayer weld metal is fabricated with higher heat input. In contrast to the stress rupture time and the toughness, the weld metal’s strength, ductility and macro-hardness remain nearly unaffected by changes of the heat input.

A hierarchical scheme from nano to macro scale for the strength and ductility of graphene/metal nanocomposites

Recent experiments have shown that grain size of metals in graphene/metal nanocomposites has a significant effect on their strength and ductility. As the grain size decreases, the strength tends to increase while the ductility tends to decrease. But quantitative assessment of these observations remains a challenge. In this paper, we establish a hierarchical scheme from nano to macro scale to evaluate the dependance of these properties on the grain size and graphene volume concentration. In the nano scale, the elastic properties of graphene nanofiller and metal matrix are evaluated via the density functional theory (DFT). In the micro scale, the plasticity of the ductile metal is described by a dislocation density-based constitutive equation, and its degradation process is accounted for by the generation of micro voids. In the macro scale we consider the system of randomly distributed graphene nanosheets in the degraded metal matrix. The grain-size dependent stress-strain relation of the overall graphene/metal nanocomposites are calculated by a two-scale homogenization method with the assistance of the field-fluctuation approach. In this process, the grain-size dependent thermodynamic driving force for the progressive generation of micro voids and the influence of an ultrathin imperfect interphase surrounding the graphene nanofiller are also considered. It is demonstrated that the predicted results from the developed hierarchical scheme agree well with the experimental data of graphene/aluminum nanocomposites. The developed multiscale theory can provide a useful guideline for the design and applications of graphene/metal nanocomposites through the grain-size controlled process.

Chemo speciation of metals in surficial sediments of Nile delta coastal zone ecosystem, Egypt

River Nile, Egyp’s most important watercourse, flows into the Mediterranean Sea by two estuaries forming a large Delta, within which lie three lagoons of varying morphological and hydrodynamic characteristics. Most estuaries receive a large quantity of wastewater input from different sources and also domestic due to the densely populated highly industrialized, leading to severe pollution of the coastal areas. Surface sediment samples were collected from six sectors and were examined for grain size and concentrations of heavy metals As, Cd, Mo, and Sn using a sequential extraction method and ecological risk assessment method. The results indicated: (1) the total concentrations of these heavy metals were influenced by runoff, industrial wastewater, and sewage, and were all higher in the coastal area (2) sequential extractions suggested that As and Mo were mainly composed with residual, Cd was bound to exchangeable and Sn was dominated by iron and manganese oxides fractions; The Contamination Factor (CF) for As, Cd, and Sn showed a very high degree of contamination while Mo showed low contamination in all study areas. The pollution load index (PLI) values were above one (> 1) indicates an advanced decline of the sediment quality. The average of the Igeo values for the studied metals indicated heavily polluted. The largest bioavailability factors were measured in all studied areas for Cd and Sn, and for As in El-Tina area, reflecting a potential risk to aquatic life. The ICF results showed the highest ability of Cd and Sn to be released from the sediments to the overlying water.

The effect of thermal frequency and current amplitude on weldability, microstructural evolution and mechanical properties of AA7075 alloy joint in DP-GMAW process

In this study, the effect of dual-pulsed gas metal arc welding (DP-GMAW) parameters such as thermal frequency and current amplitude on weldability, microstructural evolution, and mechanical properties of AA7075 alloy joint was investigated. ER53356 filler metal was employed to make AA7075 alloy joints. Thermal frequency in three levels of 1, 2, and 3 Hz was used during the welding process. And current amplitudes of 9, 27, and 45 A were used in three levels as well. After the DP-GMAW process, the welded samples were first checked visually, by a liquid penetrant method, to make sure of their welding soundness. No surface defect or crack was observed. An optical microscope (OM) and a scanning electron microscope (SEM) equipped with energy-dispersive spectroscopy (EDS) were used for microstructural analyses. The hardness of the base metal (BM) and different areas of the welded parts was measured by a microhardness testing method. Results showed that no visual defects were visible on the surface of the joints made under different PD-GMAW variables. As well, microstructural analyses showed that increasing the thermal frequency and current amplitude not only reduced the grain size of the weld metal (WM) and the heat-affected zone (HAZ) but also increased the hardness of these regions. The most refined structure was obtained by employing 2 Hz as frequency and 45 A as current amplitude. The average grain size under these conditions was 71 μm and 52 μm, respectively. It was also observed that increasing the current amplitude had the highest impact in reducing the grain size of the WM. The hardness of the WM and the HAZ, under the optimum frequency, was 91 HVN and 153 HVN, respectively. These values were 85 HVN and 38 HVN under the optimum current amplitude. Moreover, hardness was increased in the HAZ compared with the base metal in all welding conditions. Increasing the thermal frequency not only decreased the distance between the ripples but also increased the number of peak waves created in the fusion line. This was particularly the case in the longitudinal section of the weld and reduced the distance between these waves. On the other hand, increasing the frequency and current amplitude has led to the reduction of porosity in the WM. Finally, increasing the frequency from 1 to 3 Hz led to a 33% reduction in the porosity of the WM. This is while 66% reduction of porosity in the WM was obtained by increasing the current amplitude from 9 to 45 A.

Effect of Asymmetric Material Flow on the Microstructure and Mechanical Properties of 5A06 Al-Alloy Welded Joint by VPPA Welding

The microstructure, texture, and mechanical properties of the asymmetric welded joint in variable polarity plasma arc (VPPA) welding were studied and discussed in this paper. The asymmetric welded joint was obtained through horizontal welding, where the effect of gravity caused asymmetric material flow. The results showed that the grain size and low angle grain boundary (LAGB) at both sides of the obtained welded joint were asymmetric; the grain size differed by a factor of 1.3. The average grain size of the Base Metal (BM), Lower-weld zone (WZ) and Upper-WZ were 25.73 ± 1.25, 37.87 ± 1.89 and 49.92 ± 2.49 µm, respectively. There is discrepancy between the main textures in both sides of the welded joint. However, the effect of asymmetric metal flow on the weld texture was not significant. The micro-hardness distribution was inhomogeneous, the lowest hardness was observed in regions with larger grain sizes and smaller low angle grain boundary. During tensile strength tests, the specimens fractured at the position with the lowest hardness although it has reached 89.2% of the strength of the BM. Furthermore, the effect of asymmetric metal flow and underlying causes of asymmetric weld properties in VPPA horizontal welding have been discussed and analyzed.

Microstructure and mechanical properties of ultralow carbon high-strength steel weld metals with or without Cu-Nb addition

Two types of ultralow carbon steel weld metals (with and without added Cu-Nb) were prepared using gas metal arc welding (GMAW) to investigate the correlation between the microstructure and mechanical properties of weld metals. The results of microstructure characterization showed that the weld metal without Cu-Nb was mainly composed of acicular ferrite (AF), lath bainite (LB), and granular bainite (GB). In contrast, adding Cu-Nb to the weld metal caused an evident transformation of martensite and grain coarsening. Both weld metals had a high tensile strength (more than 950 MPa) and more than 17% elongation; however, their values of toughness deviated greatly, with a difference of approximately 40 J at −50°C. Analysis of the morphologies of the fracture surfaces and secondary cracks further revealed the correlation between the microstructure and mechanical properties. The effects of adding Cu and Nb on the microstructure and mechanical properties of the weld metal are discussed; the indication is that adding Cu-Nb increases the hardenability and grain size of the weld metal and thus deteriorates the toughness.

Spall strength dependence on temperature, grain size and strain rate in pure ductile metals

层裂强度表征了材料内部最大动态抗拉能力，并与材料本身的力学性质以及损伤早期演化相关。建立层裂强度计算的解析表达式，深入认识层裂强度所包含的微细观物理涵义，有利于更好地优化延性金属材料的层裂强度。目前大量的实验表明：延性金属材料的层裂强度对加载拉伸应变率、温度效应以及材料初始微细观结构具有很强的依赖关系。本文基于对孔洞成核与增长的损伤早期演化特性的分析以及对温度效应和晶粒尺寸与材料本身力学性质之间关系的分析，给出了简单、实用的层裂强度的解析物理模型，物理模型的计算结果与典型延性金属高纯铝、铜和钽的层裂强度实验结果基本符合，从而验证了我们给出的层裂强度模型具有较好的适用性和预测性。

Metal structure prediction in cross-wedge rolling processes

A method of computer prediction of the size of metal grains, their disorientation, grain boundaries and dislocation density, depending on the modes of cross-wedge rolling, is considered. The regularities of the formation of the parameters of the metal structure depending on the stress state are revealed by methods of computer simulation. The stress state is described by two parameters: the average stress and the parameter of the third invariant of the stress deviator. The effect of the stress state in the deformation zone on the metal structure parameters was determined for the first time. The new method allows improvement of the quality of products by computer optimization of rolling modes. The results of determining the metal structure and parameters of the stress-strain state in the deformation zone during hot rolling of the water pump shaft of steel 45 are presented. The verification and analysis of the data of virtual experiments on the formation of the structure of structural steels in the processes of cross-wedge rolling are carried out. To analyze the output data of the simulation, the parameters for predicting the calculation of grain boundaries and grain size were used. The created computer model for predicting the characteristics of metal structures, depending on the modes of plastic deformation, provides, at minimal cost and without carrying out field experiments, finding the optimal thermodynamic and stress-strain modes of plastic flow of metal, which guarantee the highest operational properties of the products obtained.