Subsurface Microstructure Research Articles

Investigation of the effects of heat treatment and hot isostatic pressing in micro-drilling of Ti6Al4V alloy fabricated by laser powder bed fusion

In this study, micro-drilling performance of laser powder bed fusion (LPBF) Ti6Al4V alloy with heat treatment (HT) and hot isostatic pressing (HIP) post-processing conditions was investigated and compared to wrought material. The tests were performed at two spindle rotational speeds (5000 and 10,000 rev/min) and a feed per tooth (4 μm/tooth) under dry and minimum quantity lubrication (MQL) conditions using conventional drilling (CD) and peck drilling (PD) strategies. Micro-drilling performance was evaluated in terms of thrust force, surface roughness, subsurface microstructure and microhardness, burr formation, and tool wear. The test results showed that LPBF-HT and LPBF-HIP Ti6Al4V alloys exhibited different micro-drilling performances than the wrought material due to their different microstructural and mechanical properties. The LPBF-HIP Ti6Al4V characterized by higher strength and lamellar microstructure with higher β phase fraction (33.2 %) led to higher thrust force, lower surface quality and higher burr height. On the other hand, the wrought Ti6Al4V with higher ductility, lower hardness, and lower β phase fraction (16.3 %) showed lower thrust forces, surface roughness, and burr height. Notably, employing MQL conditions with the PD strategy provided good surface quality and reduced burr formation. A higher degree of tool wear was obtained in micro-drilling of LPBF-HIP Ti6Al4V. The major wear types were adhesion, BUE, and flank wear. Besides, chippings were observed in micro-drilling of LPBF-HT and LPBF-HIP Ti6Al4V.

Investigation on the material removal mechanism in ion implantation-assisted elliptical vibration cutting of hard and brittle material

Ductile-regime machining has been used to generate damage-free surface of hard and brittle materials by setting the cutting depth to be smaller than the ductile-brittle transition depth (DBTD). However, the ductile-regime cutting of sapphire remains challenging owing to its extreme hardness, small DBTD, serious surface fractures, and severe tool wear. To solve this problem, ion implantation-assisted elliptical vibration cutting (Ii-EVC) has been proposed in this study to enhance the machinability of hard and brittle materials. Taking sapphire as an example, high-energy phosphorus ions were implanted into the workpiece to modify its surface. Nanoindentation tests revealed that the modified materials undergo plastic and elastic deformation more easily due to the decrease in hardness and modulus. Compared with nanocutting without implantation assistance, the DBTD of implanted sapphire has been increased by more than five times. The advantageous effects of Ii-EVC achieve great enhancement in machinability, including surface fractures suppression, tool-wear reduction, chips morphology transformation from discontinuous to continuous, and cutting force decrease. Furthermore, even near the cracks in the brittle region after Ii-EVC, the subsurface microstructure showed a more complete lattice arrangement and a strain distribution close to zero, indicating that crack propagation was effectively suppressed. Due to the promoted localized plastic deformation, the stress distribution in the implanted material is much smaller than that in pristine workpiece. Implantation-induced defects not only serve as a core for absorbing external energy from the high-frequency vibration and improving the in-grain deformation but also facilitate the formation of shear bands. The interface with high distortion between the modified layer and substrate can effectively dissipate strain energy and hinder crack propagation to the free surface. The turning experiments verified that Ii-EVC can achieve better surface quality, less tool wear and higher optical transmittance. Overall, Ii-EVC addresses the challenges of tool breakage and surface fracture caused by high-frequency collision between tool and workpiece in traditional EVC, overcomes the problem of limited modification depth in ion implantation, and increases the ductile-regime removal depth of extremely hard and brittle materials to several microns. Such findings demonstrate that Ii-EVC is a promising method for the ultra-precision manufacturing of advanced materials.

Magnetic and ultrasonic vibration dual-field assisted ultra-precision diamond cutting of high-entropy alloys

Despite the remarkable achievements in single-energy field-assisted diamond cutting technology, its performance remains unsatisfactory for processing high-entropy alloys (HEAs), targeted for next-generation large-scale industrial applications due to their exceptional properties. The challenge lies in overcoming the limitations of current single-energy field-assisted processing to achieve ultra-precision manufacturing of these advanced materials. This study proposes a multi-energy field-assisted ultra-precision machining technology, the magnetic and ultrasonic vibration dual-field assisted diamond cutting (MUVFDC), to address the current challenges. The phenomenological aspects of the dual-field coupling effect on HEAs are explored and investigated through comprehensive characterization of the workpiece material, ranging from macroscopic surface morphology to microscopic structural features. These analyses are performed based on experimental results from four different processing technologies: non-energy field, magnetic field, ultrasonic vibration field, and dual-field assisted machining. Research results demonstrate that MUVFDC technology effectively combines the advantages of a vibration field, which enhances cutting stability, and a magnetic field, which improves the machinability of materials. Additionally, this coupling technology addresses the challenges associated with single-energy field machining: it mitigates the difficulty of controlling surface scratches caused by tiny hard particles in a vibration field and suppresses the rapid tool wear encountered in a magnetic field. Furthermore, the gradient evolution of the subsurface microstructure reveals that the vibration field suppresses the severe matrix deformation induced by magnetic excitation. Simultaneously, the magnetic field reduces the size inhomogeneity of recrystallized grains caused by intermittent cutting. Overall, MUVFDC technology enhances surface quality, suppresses tool wear, smooths chip morphology, and reduces subsurface damage compared to single-energy field or non-energy-assisted machining. This work breaks through the performance limitations of traditional single-energy field-assisted processing and advances the understanding of the dual-field coupling effects in HEAs machining. It also presents a promising processing technology for the future ultra-precision manufacturing of advanced materials.

Fabricating ultra wear-resistant surfaces on titanium alloy by combining laser-induced modification with abrasive belt grinding

A novel method combining laser-induced modification and abrasive belt grinding (LM&BG) was proposed to prepare wear-resistant surfaces of titanium alloys. Comprehensively investigated the evolution mechanism of material modification and wear resistance of LM&BG samples. It was found that an oxide-ceramic-modified layer with increasing hardness (10 times) was formed by laser modification on the titanium alloy, mainly composed of TiO2, with the two phases Rutile-TiO2 and Anatase-TiO2. Moreover, the surface roughness after abrasive belt grinding of the modified layer is less than Ra0.1, with a uniform subsurface microstructure. Interestingly, the wear modes of the modified layer shifted from adhesive and oxidative wear to brittle microdetachment, significantly reducing the coefficient of friction, material wear, and subsurface damage, thus improving the wear-resistant capability.

Wear behavior and microstructure evolution of lamellar γ-TiAl alloy at room and elevated temperatures

The wear behavior of lamellar Ti-46Al-2Nb-2Cr was investigated by reciprocating sliding at room and service temperatures. The wear scar and subsurface microstructure were studied via scanning electron microscopy (SEM) and transmission electron microscopy (TEM). The coefficient of friction (COF) increased with the temperature. However, the wear volume loss first decreased and then increased, due to the formation of a relatively complete tribo-layer at 750 ℃. Abrasive wear, oxidative wear and adhesive wear were activated at elevated temperatures. Reciprocating sliding introduced gradient nano-grained structures covered with tribo-layers. Compared to room temperature, the oxidative tribo-layer and nano-grained layer formed at 750 ℃ were thicker. And the nano-grains were finer. Nano-grains at room temperature and 750 ℃ were related to defects and enhanced dynamic recrystallization, respectively.

Microstructural analysis and tribological characteristics of thermally sprayed Co-Ni-O ternary oxide coatings

Due to their high thermal and chemical stability, ternary oxide coatings have been extensively investigated as potential solutions for high-temperature solid lubrication. In this study, the tribological behavior of the thermally sprayed Co0.75Ni0.25O and Co0.5Ni0.5O oxide coatings was investigated. The dry sliding reciprocating friction tests were performed against an Al2O3 counterface at room and elevated temperatures using a ball-on-flat tribometer. The results indicated that the friction and wear of Co0.75Ni0.25O oxide coatings did not change significantly at atmospheric and elevated temperature conditions, whereas the friction and wear of the Co0.5Ni0.5O increased at elevated temperatures. The correlations between the worn surface morphologies, the subsurface microstructure induced by wear, and the wear behavior of the coatings were discussed based on the characterizations, (i.e., microhardness, scanning electron microscopy (SEM), and electron channeling contrast imaging (ECCI)).

Investigations on diamond rotary rolling treatment induced gradient microstructure and its effects on mechanical properties

Surface and subsurface gradient microstructures play a significant role in mechanical properties. To obtain gradient microstructures, a surface strengthening method, namely diamond rotary rolling treatment (DRRT), was proposed in this study. Varied gradient microstructures in 316 L stainless steel were obtained through different DRRT processing parameters. The micro-hardness distribution along the gradient microstructure was tested. The resulting gradient micro/nanostructures in terms of grain size, dislocation density and phase transformation were characterized and evaluated with electron backscatter diffraction (EBSD), transmission electron microscopy (TEM) and X-ray diffraction (XRD) analyses. The corresponding mechanical properties were investigated by slow strain rate test. The results showed that the DRRT could introduce a gradient hardening layer on the surface, accompanied by a gradient distribution of nano-grains, ultrafine grains, fine grains and coarse grains extending from the surface to the material interior. Besides, the gradient microstructure was associated with martensitic phase transformation, slip and dislocation. Compared to milled and untreated specimens, the DRRT-treated specimens exhibited a significant increase in yield strength and reduced ductility. Particularly, a well-designed DRRT process can enhance the yield limit while preserving higher ductility. Additionally, a quantitative relationship between the overall yield strength and the hardness distribution of the gradient microstructure was established.

Crack initiation during environment-induced cracking of metals: current status

Abstract Environment-induced cracking (EIC) research spanning the last 80 years for ferrous and non-ferrous metals in aqueous environments at ambient and elevated temperatures has concentrated on crack propagation. Studies clearly reveal EIC involves two differentiable processes, one controlling initiation and the other propagation. Utilization of advanced high-resolution electron microscopy over the last 20 years has enabled more focused studies of crack initiation for stainless steel and nickel-based alloys at elevated temperatures exposed to environments associated with the nuclear industry. More recently, when coupled with advanced in-situ experimental techniques such as time-lapse X-ray computed 3D-tomography, progress has also been made for aluminum alloys suffering EIC at ambient temperatures. Conventional wisdom states that chemical processes are typically rate-controlling during EIC initiation. Additionally, experimental evidence based on primary creep exhaustion ahead of the introduction of an aggressive environment indicates that time-dependent mechanically-driven local microstructural strain accommodation processes (resembling creep-like behavior) often play an important role for many metals, even for temperatures as low as 40 % of their melting points (0.4 Tm). EIC studies reveal initial surface conditions and their associated immediate sub-surface alloy microstructures generated during creation (i.e. disturbed layers) can dictate whether or not EIC initiation occurs under mechanical loading conditions otherwise sufficient to enable initiation and growth. The plethora of quantitative experimental techniques now available to researchers should enable significant advances towards understanding EIC initiation.

Residual stress and crystallite size analysis of SMAT on AA7075-T6

The study examines how SMAT influences the microstructure, dislocation density, and residual stress of AA7075-T6 alloy. SMAT involves the repeated impact of shots on a material surface within a vibration chamber, inducing severe plastic deformation and grain refinement. The study explores the interplay between vibration-induced shot effects and their impact on surface properties, emphasizing the importance of optimizing process parameters for improved material performance. Experimental analyses, including surface and subsurface microstructure examinations, X-ray diffraction (XRD) characterization, and residual stress evaluations, are conducted to elucidate the underlying mechanisms of SMAT. Results indicate that SMAT leads to significant plastic deformation, grain size reduction, increased dislocation density, and microstrain. The treatment produces a nanocrystallite layer, enhancing material properties. Residual stress measurements reveal a surface stress of −245 MPa, increasing to −360 MPa at 50 µm depth. The proposed multiple impact model accurately predicts deformation behavior, highlighting the deep pit regions formed during SMAT. This comprehensive investigation provides insights into the dynamic nature of SMAT and its potential for enhancing material properties. By integrating experimental findings with numerical modeling approaches, the study advances our understanding of SMAT-induced plastic deformation mechanisms and residual stress profiles.

In Situ Microstructure Modification Using a Layerwise Surface-Preheating Laser Scan of Ti-6Al-4V during Laser Powder Bed Fusion.

An innovative in situ thermal approach in the domain of LPBF for Ti-6Al-4V fabrication has been carried out with results directing towards an improved fatigue life without the need for post-processing. The thermal process involves an additional laser scan with different process parameters to preheat the selected regions of each layer of the powder bed prior to their full melting. This preheating step influences the cooling rate, which in turn affects surface characteristics and subsurface microstructure, both of which are directly correlated with fatigue properties. A thorough analysis has been conducted by comparing the preheated samples with reference samples with no preheating. Without any additional thermal processing, the preheated samples showed a significant improvement over their reference counterparts. The optimized preheated sample showed an improved prior β-grain distribution with a circular morphology and thicker α laths within the even finer prior β-grain boundaries. Also, an overall increment of the c/a ratio of the HCP α has been observed, which yielded lattice strain relaxation in the localized grain structure. Furthermore, a less-profound surface roughness was observed in the preheated sample. The obtained microstructure with all these factors delivered a 10% improvement in its fatigue life with better mechanical strength overall.

Effect of Solid Solution Treatment on the Wear Behavior and Subsurface Microstructure of 2205 Duplex Stainless Steel in Different Environments

Effect of Solid Solution Treatment on the Wear Behavior and Subsurface Microstructure of 2205 Duplex Stainless Steel in Different Environments

Microstructure Evolution and Fretting Wear Mechanisms of Steels Undergoing Oscillatory Sliding Contact in Dry Atmosphere.

Variations in the microstructure and the dominant fretting wear mechanisms of carbon steel alloy in oscillatory sliding contact against stainless steel in a dry atmosphere were evaluated by various mechanical testing and microanalytical methods. These included scanning electron microscopy and energy dispersive spectrometry with corresponding elemental maps of the wear tracks, in conjunction with cross-sectional transmission electron microscopy of samples prepared by focused ion beam machining to assess subsurface and through-thickness changes in microstructure, all as a function of applied load and sliding time. Heavily dislocated layered microstructures were observed below the wear tracks to vary with both the load and sliding time. During the accumulation of fretting cycles, the subsurface microstructure evolved into stable dislocation cells with cell walls aligned parallel to the surface and the sliding direction. The thickness of the damaged subsurface region increased with the load, consistent with the depth distribution of the maximum shear stress. The primary surface oxide evolved as Fe2O3 and Fe3O4 with increasing sliding time, leading to the formation of a uniform oxide scale at the sliding surface. It is possible that the development of the dislocation cell structure in the subsurface also enhanced oxidation by pipe diffusion along dislocation cores. The results of this study reveal complex phase changes affecting the wear resistance of steels undergoing fretting wear, which involve a synergy between oxidative wear, crack initiation, and crack growth along dislocation cell walls due to the high strains accumulating under high loads and/or prolonged surface sliding.

Investigation on Surface Integrity in Milling of Inconel X750: A Comprehensive analysis of Cutting Edges and Machining Parameters

For over a century, machining has been a crucial manufacturing method, with milling standing out as a versatile and effective process for various engineering materials. Ni-based superalloys, renowned for their exceptional properties, present challenges in machining, causing excessive tool wear and surface quality issues. Techniques such as heat treatments, environmental adjustments, and tool geometry alterations are employed to mitigate tool wear and affect surface integrity, crucial for determining the quality of machined work materials, especially in high-strength superalloys where shear deformation induces grain misorientation. In this research article, an experimental study was carried out to find the impact of the different cutting edges on surface integrity while milling Inconel X750. Response Surface Methodology was employed using a central composite rotatable design of experiments to investigate the surface roughness of the machined sample under various cutting conditions. A thorough analysis was undertaken to explore the surface integrity of Inconel X750, involving variations in milling machining parameters (i.e., low, mid, and high) and the utilization of diverse cutting tool inserts. The investigation centred on analysing the sub-surface microstructure, texture, and residual stresses of the machined surface, aiming to achieve a comprehensive understanding of the material's surface characteristics. The double-coated cutting tool T2, with a diameter of 0.8 mm, exhibited superior performance in surface quality and grain distribution, particularly at low cutting parameters (feed rate: 0.15 mm/rev, cutting speed: 35 mm/min, depth of cut: 0.1 mm), showing fine equiaxed and lognormal grain distribution alongside low residual stresses (−36.8 MPa compressive, maximum shear residual stresses: 7.2 MPa) and a surface roughness of 0.17 μm.

Surface modification and enhanced wear performance through severe shot peening treatment in 316L stainless steel manufactured by metal injection moulding

This study investigates the micro-to nanostructural evolution, hardness, and linear reciprocating wear resistance, using Al2O3 ball as a counterpart of 316L stainless steel fabricated by the metal injection moulding (MIM) process, with and without severe shot peening treatment (SSP). SSP treatment led to a slight increase in Ra but a decrease in Rz and the formation of nanocrystalline surface, α′-martensite and mechanical twin. The subsurface hardness increased significantly. Surprisingly, the coefficient of friction remained constant (0.37–0.56) and depended solely on the testing load, despite alterations in surface roughness and subsurface microstructure due to SSP treatment. Furthermore, wear resistance notably improved under 10 N and 20 N loads, attributed to the surface modification by SSP treatment, but showed no significant difference at 30 N. The wear mechanisms, including abrasive, adhesive wear, and oxidative wear, remained similar, with adhesive and oxidative wear dominating at lower testing loads (10–20 N). However, with a testing load of 30 N, the strengthening effect of SSP treatment was surpassed, leading to the dominance of severe adhesive and oxidative wear. Consequently, the wear resistance exhibited insignificant differences between specimens before and after SSP treatment.

Segmentation of anatomical layers and imaging artifacts in intravascular polarization sensitive optical coherence tomography using attending physician and boundary cardinality losses.

Intravascular ultrasound and optical coherence tomography are widely available for assessing coronary stenoses and provide critical information to optimize percutaneous coronary intervention. Intravascular polarization-sensitive optical coherence tomography (PS-OCT) measures the polarization state of the light scattered by the vessel wall in addition to conventional cross-sectional images of subsurface microstructure. This affords reconstruction of tissue polarization properties and reveals improved contrast between the layers of the vessel wall along with insight into collagen and smooth muscle content. Here, we propose a convolutional neural network model, optimized using two new loss terms (Boundary Cardinality and Attending Physician), that takes advantage of the additional polarization contrast and classifies the lumen, intima, and media layers in addition to guidewire and plaque shadows. Our model segments the media boundaries through fibrotic plaques and continues to estimate the outer media boundary behind shadows of lipid-rich plaques. We demonstrate that our multi-class classification model outperforms existing methods that exclusively use conventional OCT data, predominantly segment the lumen, and consider subsurface layers at most in regions of minimal disease. Segmentation of all anatomical layers throughout diseased vessels may facilitate stent sizing and will enable automated characterization of plaque polarization properties for investigation of the natural history and significance of coronary atheromas.

Understanding the Subsurface Microstructure and Thermal Behavior of Model Oxide Dispersion Strengthened Alloys Through FIB_SEM and TEM

AbstractA potential but an underexplored application of FIB_SEM is its ability to image the subsurface microstructure and capability for an associated chemical analysis. In this article, agglomerated microstructures of two model oxide dispersion strengthened alloy systems, consisting of ZrO2 and Y2O3 dispersions, are evaluated to understand its elevated temperature behaviors. The systems under evaluation are relevant as candidate nuclear structural materials for next-generation fast breeder reactors, and FIB_SEM technique is effectively used along with TEM and EDS for a comprehensive understanding of the material microstructure, and the results are discussed. Distinct microstructures are observed for the two systems with a difference in crystallite size distribution and presence of micron-sized dispersoids in Y2O3. The varied behavior of dispersoids is discussed in terms of its pre-annealing microstructures, and the superiority of ZrO2 over Y2O3 as a dispersoid for ODS alloys is emphasized.

Evaluation of abrasive belt grinding performance in nickel-based superalloy robot grinding

ABSTRACT In order to evaluate the abrasive belt grinding performance, this paper proposes to conduct nickel-based superalloy robot abrasive belt grinding experiment based on different types of abrasive belts with multiple grit sizes. First, the single grinding performance evaluation is performed through five performance parameters: material removal rate, surface roughness, tangential force, specific grinding energy, and specific wear height. Then, based on the evaluation results of single grinding performance, a weighted evaluation function is established to comprehensively evaluate the grinding performance. It is found that the structured abrasive belt has relatively better grinding performance. The main factors affecting the abrasive belt grinding performance are identified through the evaluation results, with the effect of grit size being more significant. In addition, this paper analyzes the characteristics of material deformation based on the grinding subsurface microstructure, and discusses the effects of abrasive belts on material deformation.

Evolution of tribo-magnetization during sliding of ferromagnetic materials

Sliding-induced subsurface microstructure evolution is believed to be decisive for determining the friction and wear performance of metallic contacts as well as the development of tribo-magnetization. This expects to develop a new prediction method of wear state by elucidating the correlation between subsurface microstructure evolution and corresponding magnetic domain changes. Herein, subsurface microstructure evolution including crystal and magnetic domain under tribological action is investigated experimentally. Our results demonstrate that dislocation mediated plastic deformation decisively influences microstructural changes during tribological contact, further determining the magnetic domain structure. Specifically, sliding-induced plastic deformation causes an increase in the width of magnetic domains, but depth-dependent derived microstructure formed under severe plastic deformation such as the refined grains and sub-grains, in turn, promoted the refinement of magnetic domains and their discontinuity, forming depth-dependent magnetic domain structure. These results are helpful to clarify the evolution of tribo-magnetization and the pinning effect of dislocations on magnetic domains.

Microstructure and tribology of cold spray additively manufactured multimodal Ni-WC metal matrix composites

Cold spray produces thick and dense deposits that can be applied in additive manufacturing or as a repair to recover damaged components. One potential materials system is WC-reinforced Ni composites, which are known for a combination of high hardness and toughness. WC particle size and distribution are some of the most important variables that impact the mechanical properties and wear resistance. In this study, thick multimodal Ni-24 vol%WC composite coatings were additively manufactured using the cold spray technique. Spherical Ni powder and two types of WC particles, i.e., cast and agglomerated, were used as feedstock materials. The microstructure, porosity, WC distribution, and microhardness of coatings were investigated in comparison with cold-sprayed Ni and Ni-agglomerated WC coatings with similar WC fill ratios. Dry sliding wear of multimodal Ni-WC coatings was studied with a sliding speed of 3 mm/s under normal loads of 5 and 12 N and was then compared to Ni and composite coatings made with only agglomerated WC. Multimodal Ni-WC coatings with an altered distribution of WC particles offered an improvement in friction and wear as compared to Ni-agglomerated WC coatings. Characterization of worn surfaces revealed the formation of tribofilms following compaction of wear debris. A more stable tribofilm that developed to a higher coverage was identified for coatings with multimodal WC particles at both tested loads. However, the protective role of tribofilms was reduced at a higher load, caused by subsurface crack propagation. Subsurface microstructure of worn surfaces that are induced by sliding wear was studied, and a correlation between the mechanical properties and microstructure and wear mechanism of coatings was made. Overall, our findings highlight the potential of cold spray additive manufacturing in producing multimodal Ni-WC composite coatings with enhanced wear resistance.

The Influence of Vibro-Assisted Abrasive Processing on the Surface Roughness and Sub-Surface Microstructure of Inconel 939 Specimen Made by LPBF

This paper presents the research results on the influence of vibration abrasive machining parameters on the surface layer integrity of elements made by LPBF technology from Inconel 939. The research was carried out on samples of various sizes on vibrating smoothing machines. The influence of the size of the processed elements, the type of abrasive shapes, the processing time, and the supporting fluid on the surface roughness and microstructure of the processed elements was analyzed. Tests have shown that as a result of using vibration processing, it is possible to reduce the surface roughness five times to the value of Ra = 1.1 µm. A significant influence of the type of abrasive shapes was found. There was no significant effect of the machining fluid on the process.