Recrystallization Of Material Research Articles

Synthesis and characterisation of Al 7075 – Mg AZ31 alloy bimetal produced by friction stir additive manufacturing with Zn interlayer

ABSTRACT The bimetal formation of Al and Mg alloys has various metallurgical difficulties. To overcome these difficulties, Friction Stir Additive Manufacturing has been suggested as a promising technique for the Al and Mg alloy bimetal formation. In the present work, the defect-free bimetal of Al 7075 and Mg AZ31 alloys with Zn interlayer was successfully synthesised for the first time using Friction Stir Additive Manufacturing at a rotational speed of 1300 rpm and welding speed of 20 mm/min. The intermetallic compounds like MgZn and Mg32(Al, Zn)49 have been observed in the Friction Stir Additive Manufacturing through eutectic reaction at the temperatures 325°C and 330°C, respectively. The maximum strength up to 201 MPa and elongation of 5.4% have been achieved due to the proper mechanical mixing and dynamic recrystallization of material in the stir region. The maximum hardness has been observed in stir region. The hardness increases in top plate (Mg) and decreases in bottom plate (Al) as compared to their base material.

Adaptively remeshed multiphysical modeling of resistance forge welding with experimental validation of residual stress fields and measurement processes

Welding processes used in the production of pressure vessels impart residual stresses in the manufactured component. Computational modeling is critical to predicting these residual stress fields and understanding how they interact with notches and flaws to impact pressure vessel durability. In this work, we present a finite element model for a resistance forge weld and validate it using laboratory measurements. Extensive microstructural changes, near-melt temperatures, and large localized deformations along the weld interface pose significant challenges to Lagrangian finite element modeling. The proposed modeling approach overcomes these roadblocks in order to provide a high-fidelity simulation that can predict the residual stress state in the manufactured pressure vessel; a rich microstructural constitutive model accounts for material recrystallization dynamics, a frictional-to-tied contact model is coordinated with the constitutive model to represent interfacial bonding, and adaptive remeshing is employed to alleviate severe mesh distortion. An interrupted-weld approach is applied to the simulation to facilitate comparison to displacement measures. Several techniques are employed for residual stress measurement in order to validate the finite element model: neutron diffraction, the contour method, and the slitting method. Model-measurement comparisons are supplemented with detailed simulations that reflect the configurations of the residual-stress measurement processes themselves. The model results show general agreement with experimental measurements, and we observe some similarities in the features around the weld region. Factors that contribute to model-measurement differences are identified. Finally, we conclude with some discussion of the model development and residual stress measurement strategies, including how to best leverage the efforts put forth here for other weld problems.

Grain rotation mechanisms in nanocrystalline materials: Multiscale observations in Pt thin films.

Near-rigid-body grain rotation is commonly observed during grain growth, recrystallization, and plastic deformation in nanocrystalline materials. Despite decades of research, the dominant mechanisms underlying grain rotation remain enigmatic. We present direct evidence that grain rotation occurs through the motion of disconnections (line defects with step and dislocation character) along grain boundaries in platinum thin films. State-of-the-art in situ four-dimensional scanning transmission electron microscopy (4D-STEM) observations reveal the statistical correlation between grain rotation and grain growth or shrinkage. This correlation arises from shear-coupled grain boundary migration, which occurs through the motion of disconnections, as demonstrated by in situ high-angle annular dark-field STEM observations and the atomistic simulation-aided analysis. These findings provide quantitative insights into the structural dynamics of nanocrystalline materials.

Research insight into the change of aggregation structure of BOPP films and its influence on electrical properties under accelerated thermal aging

AbstractBiaxially oriented polypropylene (BOPP) film used in pulse film capacitors withstands high temperatures for a long time. This leads to frequent faults in pulse film capacitors, affecting the reliability of the pulse system. In this paper, the thermal aging tests of BOPP films at 100°C for 10–80 days are carried out in this paper, and the physicochemical and electrical properties of the aging materials are characterized. The results show that more deep and shallow traps to BOPP films are introduced during the thermal aging process. With the increase in aging time, the breakdown field strength gradually decreases, and the conductivity decreases first and then increases. Under the action of thermal aging for a long time, the recrystallization of materials and the destruction of molecular chains by thermal aging are the main reasons for the increase of traps and the decrease of breakdown field strength. The former is dominant in the early stage of aging, and the latter is dominant in the late stage of aging. Therefore, the conductivity shows the law of slow decrease first and then rapid increase. The relevant research provides a theoretical foundation for the process improvement and life prediction of BOPP films.

Selective growth of recrystallizing grains into well-characterized deformed aluminum at hardness indentation

The selective 3D growth of multiple recrystallization nuclei/grains around a hardness indentation into a deformed Al grain has been quantified through a new approach that combines two synchrotron techniques: 3D X-ray Laue micro-diffraction and diffraction contrast tomography. The former is used to characterize the deformed microstructure and nucleation, while the latter to track the nuclei's growth during interrupted in situ annealing. The combined 4D (x,y,z,t) datasets enable a detailed quantification of the local deformed microstructure consumed by the nuclei/grains and the boundary characteristics between them, including the stored energy and anisotropy of the orientation distribution of the deformation matrix, as well as the misorientation of the moving and non-moving boundary segments. It is found that only certain nuclei can grow into large sizes, and the growth is heterogeneous both among recrystallizing grains and along different directions. Neither the local stored energy nor the boundary misorientation, whether considered individually or in combination, can fully account for the observed growing and shrinking behavior. The growth difference along different directions is related to the deformation microstructure, specifically the geometrical alignment of dislocation boundaries. Furthermore, the effects of impingement of other recrystallizing grains, recovery of the matrix, and elastic strain/stress, on the growth heterogeneities are discussed in detail with a view towards developing advanced modeling with improved predictability of local recrystallization phenomena. The present study demonstrates the effectiveness of the new approach in enabling 3D analysis of recrystallization in complex materials, with potential applications in the development of advanced engineering materials.

Strain induced recrystallization in lanthanum zirconate ceramic

The focus of this work is to investigate the strain-induced recrystallization behavior on the surface of La2Zr2O7 ceramic through annealing. Unlike in metallic materials, the recrystallization phenomenon resulting from surface mechanical treatment in ceramics has not been extensively explored. In this study, we examine the strain-induced recrystallization in La2Zr2O7 that has undergone surface deformation and subsequent annealing at different temperatures. It reveals the presence of recrystallized grains in the region of the surface that was deformed, following the annealing process. To better understand the driving force for the recrystallization behavior, we analyze the variation of stored deformation energy in La2Zr2O7 using Raman spectroscopy. The study also highlights the microstructural evolution, revealing that shear bands, characterized by a high density of dislocations, serve as preferential sites for the nucleation of recrystallized grains. Finally, we discuss the mechanism of strain-induced recrystallization in ceramic materials.

LITHIUM IN THE SUBSOIL OF UKRAINE Part 6. Genesis of lithium-bearing objects: chamber pegmatites of the Korosten pluton, the Perzhansky ore node, hydrothermals of the Nagolnyi Ridge (Donetsk Basin)

The genesis of lithium-bearing objects was characterized in the following sequence: chamber pegmatites of the Korosten pluton, the Perzhansky ore node, hydrothermals of the Nagolnyi Ridge (Donetsk basin). Chamber pegmatites of Volyn were genetically considered in the following aspect: 1) connection of pegmatites with host rocks; 2) the main stages of pegmatite formation — magmatic, pneumatolite, mineral formation in sinkholes, hydrothermal-metasomatic; 3) methods of formation of minerals — growth of minerals in magmatic melt and formation of graphic zone; recrystallization of magmatic material and the formation of pegmatoid, block and monomineral zones; the formation of an overburden and the free growth of crystals, including giant crystals. Physico-chemical conditions for the formation of pegmatites are given based on the results of the study of the inclusions of the mineral-forming environment. The Perzhansky ore node is a complex multi-metallic structure in which genetically different types of mineralization coexist, which are not predicted by classical mineragenic concentrations. The authors develop a hypothesis according to which the formation of ore-bearing metasomatites was carried out under the action of solutions that separated from the magma that produced the Perzhansky granites and deep fluids. A new vision of the formation of the Perzhansky deposit was born according to which CO2 fluids — products of degassing, probably ultra-basic magma — took part in the crystallization of its mineralization. Gentgelvin crystals grew during metasomatization of granites (pneumatolithic-hydrothermal processing of rocks) at T = 500-250 °С. The Nagolnyi Ridge hydrothermals are a mysterious structure in the genetic sense, primarily because sulfide mineralization (galena, sphalerite, chalcopyrite, etc.) is associated with lithium mineralization (cookeite, donbasite). Based on the analysis of the results of a detailed study of the most common mineral of hydrothermal veins — quartz, and the general mineralogy of the veins, the following conclusion was made: vein formations of the Nagolnyi Ridge are not typical hydrothermal veins of either post-magmatic or metamorphogenic origin. According to the general history of their formation, they are closest to the so-called veins of the hydrothermal-alpine type. The share of impurity meteoric water in juvenile water could reach 50%.

Eco-friendly lubricants for improving performance and environmental impact in cold rolling

Cold rolling of strips is a well-known metal forming process, carried out below the material recrystallization temperature to improve the surface finish and the geometrical tolerance of the rolled strip. Friction plays a fundamental role in the rolling process and, even in case of a favorable roll bite condition, a non-optimal lubrication causes the rapid wear of the work rolls and high energy consumptions. Most of the currently used lubricants are based on synthetic or semi-synthetic formulations. However, the increase in the cost of petroleum-based products and the tightening of the environmental regulations is pushing the scientific and industrial community to search for more sustainable alternatives. Recently, the demand for green lubricants capable of matching non-toxicity, high biodegradability while keeping good lubricating performance is becoming of outmost importance. In the present work, the environmental impact and the lubricating performance of two innovative natural-based lubricants have been assessed using laboratory tests. An animal-based (AB) and a vegetable-based (VB) formulation were investigated and the results compared with a commercial mineral-based (MB) one. As concerns the environmental impact, emulsions at 2% oil concentration were chemically treated using a powder composed of bentonite and activated carbons and the Chemical Oxygen Demand (COD) and total Surfactants in the aqueous phase were evaluated. The lubricating performance of the emulsions at 2% oil concentration was assessed by conducting tribological tests using the Falex tribometer in accordance with the Standard. The experimental comparison revealed that it is not only feasible but also convenient in terms of both environmental impact and performance to replace the mineral-based lubricant.

Study on deformation behavior of Ti60 alloy based on multi-physics coupling

Titanium alloys are extensively employed in aerospace due to their superior performance, but their development is constrained by limited formability at room temperature, an electrically assisted forming (EAF) process emerges as a promising solution to reduce flow stress, improve formability, and accelerate material recrystallization. This study focuses on elucidating the interplay between pulse current, temperature, and flow stress in Ti60 alloy under combined electrical, thermal, and stress fields. To this end, electrically assisted tensile (EAT) tests were performed on Ti60 alloy using six different current densities. Currently, there is no unified theory for electro-plasticity in metal materials, and the absence of a suitable model hinders finite element simulation in EAF. The investigation encompasses four key areas: the impact of multi-physics coupling on temperature variations, the stress-strain curve characteristics, the microstructural analysis of tensile fractures, and the modification of the Johnson-Cook (MJC) model. Notably, temperature distribution proves more uniform across the width than the length. Tensile elongation initially rises and then falls with increasing current density, peaking at a critical current density (J=30 A/mm2). Higher current densities lead to a transition in fracture features, with deep, numerous dimples and tiny voids supplanting tearing ridges and facets. Despite this, brittle fracture remains the dominant mode, accompanied by granular oxide formation at J= 35, 40 A/mm2. The MJC model aligns closely with experimental outcomes, showing R and EAARE values of 92.7% and 7.15%, respectively. This model accurately predicts Ti60 alloy's deformation under multi-physics conditions, marking a significant advancement in integrating electro-plasticity into constitutive modeling.

Comparative Study on Primary Recrystallization of Metallic Materials Using Experimental and Numerical Simulations

Comparative Study on Primary Recrystallization of Metallic Materials Using Experimental and Numerical Simulations

Role of geometric dynamic recrystallization in nanocrystalline alloys

Nanocrystalline and near-nano-grained metals are plagued with an overall lack of ductility and formability. This is in part due to their inability to support the dislocation mechanisms or undergo the same types of transformations that conventional coarse-grained metals access inorder to increase their workability. Therefore, despite their advanced properties such as high strength, the manufacturing of such fine-grained metals, which exposes them to multi-axial states of stress in operations such as rolling, drawing, extrusion, forging, and bending, has not been possible. In this work, we address this limitation and highlight an important aspect of geometric dynamic recrystallization (GDRX) in nanocrystalline materials as a way to improve manufacturability. In particular, using two different alloy compositions and two different processing techniques, this study points to the fact that if nanocrystalline or near nano metals can be truly stabilized, they can repeatedly undergo the same types of processes that conventional coarse-grained metals endure, increasing both their formability and hence manufacturability. The results reveal that due to a large number of initial high-angle grain boundaries and large applied compressive strains, GDRX is activated in the absence of any discontinuous dynamic recrystallization (DDRX) or continuous dynamic recrystallization (CDRX). This is a unique aspect of stabilized nanocrystalline materials, as traditionally fine-grained metals are not microstructurally stable enough to accommodate such an event.

Forms of presence, interrelation and genetic significance of Al and Li impurities in quartz of gold deposits of the Darasunsky ore field (Eastern Transbaikalia, Russia)

Research subject. The distribution regularities of Al and Li impurities in gold-ore quartz. Materials and methods. The quartz of the Darasun, Teremkinskoye and Talatuy gold deposits of the Darasun ore field was studied. The gross contents of Al and Li impurities in quartz were determined by the LA-ICP-MS method; substitutional Al impurity concentrations were studied by the EPR method. The forms of Al impurity in quartz were determined based on the results of studying its behavior during material recrystallization. The genetic significance of Al and Li impurities in quartz was estimated taking into account the genetic information obtained during the study of the distribution of substitutional Al and Ti impurity concentrations. Results. It was found that Al is present in quartz in two main forms, i. e., as a substitutional Al impurity and Al complexes localized in the areas of high mineral defectiveness. Li+ ions are located in the structural channels of the mineral, serving as compensating ions for both Al impurity forms. The composition of Al complexes is assumed to include three Al3+ ions and one H+ or Li+ ion. Two stages of quartz recrystallization occurring at different temperatures of mineral formation were identified. The first, low-temperature stage leads to quartz enrichment with substitutional Al impurities. The second, high-temperature stage causes the decomposition of Al complexes. The recrystallization stages can be identified by the type of relationship between the gross concentrations of Al and Li. The increased content of Al impurity in ore quartz was found to be related to the presence of a large number of Al complexes. An assumption is made that these complexes formed during mineral crystallization from solutions with a high content of metal ions. Conclusions. The results obtained indicate that high Al impurity concentrations can serve as a genetic sign of ore quartz. At the same time, the decomposition of Al complexes during quartz recrystallization should be taken into account. A method for estimating the initial concentration of Al complexes is proposed, which is a more reliable genetic indicator.

Robust method for uniform coating of carbon nanotubes with V2O5 for next-generation transparent electrodes and Li-ion batteries.

Composites comprising vanadium-pentoxide (V2O5) and single-walled carbon nanotubes (SWCNTs) are promising components for emerging applications in optoelectronics, solar cells, chemical and electrochemical sensors, etc. We propose a novel, simple, and facile approach for SWCNT covering with V2O5 by spin coating under ambient conditions. With the hydrolysis-polycondensation of the precursor (vanadyl triisopropoxide) directly on the surface of SWCNTs, the nm-thick layer of oxide is amorphous with a work function of 4.8 eV. The material recrystallizes after thermal treatment at 600 °C, achieving the work function of 5.8 eV. The key advantages of the method are that the obtained coating is uniform with a tunable thickness and does not require vacuuming or heating during processing. We demonstrate the groundbreaking results for two V2O5/SWCNT applications: transparent electrode and cathode for Li-ion batteries. As a transparent electrode, the composite shows stable sheet resistance of 160 Ω sq-1 at a 90% transmittance (550 nm) - the best performance reported for SWCNTs doped by metal oxides. As a cathode material, the obtained specific capacity (330 mA h g-1) is the highest among all the other V2O5/SWCNT cathodes reported so far. This approach opens new horizons for the creation of the next generation of metal oxide composites for various applications, including optoelectronics and electrochemistry.

Initial grain orientation controls static recrystallization outcomes in cold-worked iron: Insight from coupled crystal plasticity/vertex dynamics modeling

Recrystallization of metallic materials is an important metallurgical treatment to bring cold-worked alloys to a usable state. Static recrystallization (SRX) occurs after deformation, once the material has built up sufficient plastic strain energy to trigger grain boundary motion. However, we have very limited knowledge about the combined effects on recrystallization kinetics of temperature, amount of cold working, triple junction mobility, and level of recovery in the deformed microstructure. In particular, little is still known about the microstructural factors that control the onset of SRX. Here we develop a comprehensive physical model based on coupling crystal plasticity and grain boundary kinetics utilizing curvature and differential plastic strain energy as driving forces for SRX in polycrystalline iron. The model links finite-element polycrystal plasticity simulations with a two-dimensional vertex dynamics approach, and is used to probe the large parameter space influencing static recrystallization. Our main finding is that the initial texture (defined by the polycrystal’s grain orientation and size distribution) is the dominant feature triggering and controlling SRX kinetics. We find a direct correlation between the degree of dislocation density buildup and the relative grain orientation across a grain boundary at the beginning of the deformation phase. Indeed, analysis of our results points to the maximum Schmid factor of a grain as the best predictor for activation of its grain boundaries during the recrystallization phase. We also find that the amount of post-deformation recovery only has a limited effect on the evolution of the recrystallized microstructure. Finally, using a definition based on the rate of transformation of the material, we find a recrystallization temperature of 600 ∘C, consistent with experimental observations in high-purity iron specimens.

Controlled precipitation and recrystallization to achieve superior mechanical properties of severely deformed Inconel 718 alloy

Severely deformed machined chips of Inconel 718 were subjected to an age-hardening treatment at 600 °C for 10 h in order to form a high fraction of γI and γII precipitates. These nano-sized precipitates help in pinning the grain boundaries and hence stabilizing the boundaries by impeding their motion at elevated temperatures. This strong pinning effect on grain boundaries is expected to result in increased strength and improved thermal stability. However, presence of a large fraction of nano-precipitates also induces precipitation hardening which results in limited ductility of the material. Therefore, in order to improve the ductility, age-hardened chips were given a subsequent heat-treatment at 700 °C, 800 °C and 900 °C for 15 min, hereafter called as post-aging heat treatment. It was anticipated that this heat-treatment at elevated temperatures would increase the dislocation mobility and result in dislocation annihilation, which can improve the ductility of aged samples. Results show that the recrystallization fraction increases with an increase in the temperature of post-aging heat treatment. Heat-treatment at 900 °C was found to result in a microstructure with highest fraction of recrystallized grains, a high fraction of coincidence site lattice (CSL) boundaries, comparatively smaller grain size and better ductility. Results also indicate that there are two counteracting effects undergoing at the same time during post-aging heat-treatment which determine the possibility of enhanced dislocation motion and associated improvement in ductility. One of these effects is related to the pinning force caused by the presence of precipitates at the boundaries. Higher pinning force by fine precipitates will lead to suppressed recrystallization by hindering the grain boundary motion and hence no improvement in ductility. Another effect is related to the driving force for recrystallization. Higher driving force, especially in the highly strained ultra-refined shear zones, will promote the reduction in overall dislocation density through recrystallization and hence leading to an improvement in ductility. Present study provides further insights into the competition between pinning and recrystallization of aged materials at elevated temperatures.

Modeling and characterization of dynamic recrystallization under variable deformation states

Flexible rolling with variable deformation states has brought new challenges to dynamic recrystallization (DRX) theory based on conventional constant deformation state. Modeling and characterization of DRX are investigated, inspired by the successful application of constitutive models and EBSD technique in the field of characterizing the recrystallization of metallic materials, combined with the characteristics of flow stress. Two constitutive models for constant and transient deformation states were considered. The dynamic recovery effect was integrated into the model for predicting the flow stress and DRX volume fraction. A hot processing map was established to predict the instability region. Various DRX synergistic regulation mechanisms and crystallographic orientation distribution under a transient deformation state were explored. The results suggest that DRX is the main softening characteristic under various deformation states. The predicted flow stress of the constant and transient models are in good agreement with the experimental values. The regulation mechanism of DRX transforms from continuous DRX to discontinuous DRX when the strain rate dynamic decreases. DRX grains display a random orientation distribution, and the crystallographic orientation is toward the 〈111〉 fiber parallel to the normal direction of the compression axis when the DRX volume fraction is high. These findings provide insight into the DRX constitutive description and microstructure evolution under variable deformation states.

Investigation of dust formation during changes in the structural and surface properties of plasma-irradiated materials

• The contribution of surface defects (cracks, craters, bubbles) and structural changes (recrystallization) exposed by the plasma beam candidate materials to dust formation is studied. • The scanning electron microscopy (SEM), and elemental and X-ray diffraction (XRD) analysis were used to study the surface morphology, chemical composition, and crystal structure of plasma beam irradiated materials. • On the surface of tungsten target the well-developed micro-cracks are formed, and defects such as melting and flaking of the surface layer, drop-shaped structures formation are also observed. The increasing of the pulse dose led to an increase in the width and depth of these microcracks. • Tungsten target erosion (mass loss) occurs due to the ejection of particles from the tungsten surfaces, both as droplets and as solid dust during crack development. • The measured peak shift in carbon was 0.03 degrees after x5 plasma load. Thereby, carbon is generally more resistant to plasma beam impact than tungsten. This paper presents the results of studying the impact of a plasma beam during the interaction of divertor candidate materials in a coaxial plasma accelerator, where graphite and tungsten plates were used as candidate materials. The authors consider the contribution of surface defects (cracks, craters, bubbles) and structural changes (recrystallization) in candidate materials to dust formation under the action of a plasma beam. The other aspect studied in the paper is the erosion of copper substrates (nearby components) under the action of a hot target dust cloud with account for the kinetic influence of hot dust particles on nearby components. It should be noted that these particles are formed by the erosion of the material by the plasma beam. Scanning electron microscopy (SEM), elemental and X-ray diffraction analysis (XRD) were used to study the material surface morphology, chemical composition, and crystal structure. The authors also consider some undesirable effects revealed after irradiation of divertor candidate materials. The obtained experimental results will be useful for evaluating the prospects for further application of these materials in existing and future fusion reactors (e.g., in the international fusion reactor ITER).

Study of material recrystallized regions on deformation and cracking behaviours in cast steel brake discs

Localized regions of recrystallized material have been observed on steel brake discs used in high-speed rail systems. Because the effects of recrystallized regions on steel brake disc cracking behaviour are not well known, further study is warranted. In the present study, the characteristics of localized regions of recrystallized material were characterized by transmission electron microscope, and the coupled effect of severe braking thermal cycles and frictional shear force was found to be the root cause of material recrystallization. In situ tensile testing was used to compare the deformation and cracking behaviours in samples taken from a in-service railway brake disc and a virgin brake disc. The results showed that the localized recrystallized material degrades the mechanical properties of the in-service brake disc due to strain localization. The localized recrystallization of the material also changed the sample fracture from a ductile mode in the virgin steel brake disc to a mixed mode. Electron back-scattered diffraction (EBSD) analysis showed that the nonuniform distribution of low angle grain boundaries (LAGBs) and high angle grain boundaries (HAGBs) in the localized recrystallized material reflected strain localization. HAGBs promoted local strengthening of the small-size grain region, resulting in grain-scale incompatibility deformation and enhanced strain localization. The difference in the degree of deformation between the large-size and small-size grain regions accordingly became irreconcilable, which led to stress concentrations and microcrack initiation. Based on these results, it was recommended that the microstructural condition of steel brake discs be monitored during maintenance inspections and that the sliding surfaces be turned to remove the recrystallized material if necessary.

Mechanical Behavior and Microstructure Evolution during Hot Torsion Deformation of Aluminum Alloy AA2219

This study comprehensively analyzes the mechanical behavior and microstructural evolution of aluminum alloy AA2219‐T87 undergoing high deformation and strain rates at elevated temperatures via hot torsion tests. An integrated experimental and modeling approach is employed, where a finite element model (FEM) is developed with ABAQUS to address local deformation and nonuniform temperature distribution along the specimen gauge section. A Johnson–Cook constitutive material model is calibrated based on the calculated and measured torque curves at different conditions. The grain structure distribution is characterized through optical microscopy and electron backscatter diffraction (EBSD) analysis. Combined with the ABAQUS calculated local thermomechanical conditions, microstructure modeling, using established literature‐based equations, is established to provide insights into grain size distribution and recrystallization mechanisms. The calibrated material constitutive model, recrystallization model, and grain size model can be utilized to model various manufacturing processes, which involve hot deformation of aluminum alloys in the intermediate strain rate range of 14–78 s−1. The constitutive model shows adequate matching of the experimental torque curves at 400, 450, and 500 °C. The recrystallization prediction model functions at over 94% accuracy and the grain size model provides an average deviation between predicted and measured grain size reduction of 11.5%.

Formation mechanism of white layer in the high-speed cutting of hardened steel under cryogenic liquid nitrogen cooling

The coupled thermal–mechanical effect in the high-speed machining of hardened steels induces the formation of a surface white layer structure, which adversely affects the mechanical properties and service performance of the machined component. However, knowledge about the white layer formation mechanisms is in severe dearth, particularly for high-speed machining under a cryogenic cooling condition. This paper presents an experimental study of the formation mechanisms for surface white layers in high-speed machining of a hardened steel. Both machining with cryogenic liquid nitrogen (LN2) cooling and dry cutting conditions are considered and the characteristics of the white layers formed under the two cooling conditions are compared to reveal the effect of process parameters. It is shown that the hardness of the white layers is increased while their grain size is decreased under the cryogenic LN2 cooling condition as compared to dry cutting. Under both the cooling conditions, no material phase transformation or recrystallization is noticed alongside the white layer formation, but severe plastic deformation is found to be the dominant reason for white layer formation. Tool wear is noticed to increase the white layer thickness. It is shown that during the cutting process, the work material undergoes two stages of intense plastic deformation. In the first stage, the material slips and approaches the first deformation zone, then gradually bulges and produces significant dislocations, and eventually forms a dense dislocation center around the shear plane. In the second stage, part of the material slides toward the tool flank face. The material dislocation slip induced by friction and material shearing further stretches the surface martensite lath bundles to form dislocation tangles and cellular substructures. The highly stretched martensite lath bundles are finally transformed into white layer structure under the interweaving multi-dislocation movement.