Formation Mechanism Of Microstructure Research Articles

Formation mechanism of surface gradient microstructure and mechanical properties evolution of Mg–Y-Nd-Gd-Zr alloy by ultrasonic rolling

The evolution mechanism of the microstructure and mechanical properties of a Mg–Y-Nd-Gd-Zr alloy subjected to different numbers of passes of the ultrasonic surface rolling process (USRP) was investigated. The results demonstrated an initial decrease followed by an increase in the surface roughness of the alloy. The optimal surface roughness was achieved after four passes, with a contour mean deviation of 0.06 μm and micro-roughness 10-point height of 0.55 μm. After the USRP, the alloy exhibited a composite gradient structure characterized by increased grain size and decreased dislocation, lamination, and twin densities. The depth of the gradient layer increased with each additional USRP pass, ranging from 650 μm (1 pass) to 910 μm (8 passes). Furthermore, the USRP treatment effectively enhanced the alloy strength; compared with the untreated samples; the yield strength increased by 34.2% in the gradient sample after one pass. No significant change was observed in the YS with an increasing number of rolling passes for Mg alloys, whereas both the tensile strength and elongation showed improvements. High-density twinning played a crucial role in enhancing the YS of the gradient samples without significant variation across different numbers of rolling passes, thus following a consistent pattern. The formation of more nanocrystals on the surface of Mg alloys after multiple rolling cycles is key to preventing premature fracture failure and improving the work-hardening rate.

Phase-field simulation framework for modeling martensite and bainite formation in steel

In this study, we present a combination of phase evolution, chemical diffusion, temperature evolution, and finite strain elasto-plasticity to simulate the martensitic and bainitic transformation using the phase-field software library OpenPhase (OpenPhase, 2023). It is demonstrated how the carbon concentration significantly influences the martensite start temperature and the resulting microstructure. Furthermore, the kinetics of the transformation is strongly influenced by plasticity. For bainitic transformation, it is demonstrated how the holding temperature significantly influences carbon partitioning and the resulting microstructure: higher holding temperatures allow increased carbon diffusion and partitioning, stabilizing retained austenite, which is in good agreement with experimental observations. The present study offers new insights into the microstructure formation mechanisms during martensitic and bainitic transformations in low-carbon steel and offers a consistent modeling approach to model complex phase transformation scenarios in steel and other construction materials.

An improved preparation method of core-shell Al@Sn-Bi microspheres and its microstructure formation mechanism

This paper presents an improved method for achieving controllable preparation of the composition and size of the monotectic alloy core-shell structure microspheres. This advancement is expected to enhance the commercial application of monotectic alloy core-shell structure microspheres. By installing a heating device at the top of the drop tube in the pulsated orifice ejection method setup, we achieved in-situ heat treatment of falling droplets, resulting in the successful preparation of Al45(Sn42Bi58)55 core-shell structure microspheres. Furthermore, we investigated the formation mechanism of the Al@Sn-Bi core-shell structure. Nine sets of Al-Sn-Bi microspheres with varying in-situ heat treatment temperatures were prepared. Analysis via X-ray diffraction and transmission electron microscopy revealed that the microspheres consisted of Al-rich, Sn-rich, and Bi-rich phases, without any intermetallic compounds. Scanning electron microscopy results indicated that as the in-situ heat treatment temperature increased, the structure of the Al-Sn-Bi microspheres transitioned from irregular to regular core-shell configurations. This suggests that higher heat treatment temperatures prolong liquid phase separation time, leading to increased repetition of Marangoni motion within the droplet, Ostwald ripening of the minor phase droplets, and Soret effect of the matrix phase. However, at temperatures exceeding 700 °C, the solidification rate of alloy droplets decreased, causing the Al-rich phase core to deviate from the center and form a crescent structure due to Stokes motion.

Synergy effect of multi-strengthening mechanisms in CoNiCr-based high-entropy superalloy at cryogenic temperature

The evolution and formation mechanism of microstructures in a high-entropy CoNiCr-based superalloy induced by cold-drawing deformation at cryogenic temperature were investigated. The improvement of strength while maintaining the ductility during cryogenic-drawing deformation in the CoNiCr-based high-entropy superalloy originated from the activation of multiple deformation mechanisms including deformation twins, dislocations, 9R phase and hexagonal close-packed (HCP) phase transformation. Two dominant contributions have been suggested for the increase in strength, namely, nanotwins strengthening and the existence of 9R and HCP phase. In this paper, cryogenic-deformation-induced phase transformation from face-centered cubic (FCC) to HCP in the CoNiCr-based high-entropy superalloy was found, and the atomic-scale mechanism of FCC-9R-Twin/HCP transformation was revealed. A new crystallographic model was established to explain the FCC→9R→Twin/HCP transformation. The phase transformation is significant for the comprehensive understanding of the deformation behavior of this CoNiCr-based high-entropy superalloy at cryogenic temperature, and for guiding the design of high-entropy superalloy with advanced mechanical properties.

Grasping the behavior of magnetorheological fluids in gradient pinch mode via microscopic imaging

Magnetorheological (MR) fluids are suspensions of micrometer-sized ferromagnetic particles in a carrier fluid, which react to magnetic fields. The fluids can be operated in several fundamental modes. Contrary to the other modes, the rheology and microstructure formation of the MR fluid in the gradient pinch mode have been studied to a far lesser extent. The magnetic field distribution in the flow channel is intentionally made non-uniform. It is hypothesized that the Venturi-like contraction is achieved via fluid property changes, leading to a unique behavior and the presence of a pseudo-orifice. The main goal is to investigate the presence of the Venturi-like contraction effect in the fluid by means of optical imaging and hydraulic measurements. To accomplish the goal, a unique test rig has been developed including a fluorescence microscope and MR valve prototype. The Venturi-like contraction hypothesis was confirmed. The results indicate that the effective flow channel size decreases by 92% at the maximum magnetic flux applied. This has a direct impact on the flow characteristics of the MR valve. The variation of the pressure–flow rate curve slope with magnetic field was demonstrated. The results provide valuable information for understanding the rheology and microstructure formation mechanism in MR fluids in the pinch mode.

Effect of Al3+ on the surface microstructure and film formation mechanism of Zr-based conversion coatings with improved corrosion resistance performance on cold rolled steel

Most Zr conversion coatings are prepared on Al alloys, with Al3+ from the substrate contributing significantly to the film growth process. Thus, the effect of Al3+ on the electrochemical characteristics, surface microstructure and film-formation mechanism of Zr-based conversion coatings on old- rolled steel (CRS) substrates was investigated by growing the coatings with and without Al3+ as an additive. Electrochemical tests indicated that the anticorrosion performance of the Zr-Al coatings prepared in the presence of Al3+ was greatly superior to that of Zr conversion coatings obtained in the absence of Al3+. Field-emission scanning electron microscopy and X-ray photoelectron spectroscopy analysis indicated that adding of Al3+ in the conversion bath changed the surface microstructure of the Zr coating, greatly promoted the deposition of Zr and F on the CRS substrate and increased the film thickness and density. Both coatings were embedded into the etched CRS substrates. The presence of Al3+ additive induced the formation of different coating components. The Zr coating mainly consisted of ZrO2 and some FeFx, whereas the Zr-Al coatings comprised of ZrO2, ZrOxFy, Al2O3, AlFx and some FeFx. The present study results confirm the key role of Al3+ as an additive in the film formation and growth process of Zr coatings and its effect on enhancing the anticorrosion performance of CRS substrates in NaCl solutions.

Melting and solidification dynamics during laser melting of reaction-based metal matrix composites uncovered by in-situ synchrotron X-ray diffraction

Laser additive manufacturing (AM) of reaction-based metal matrix composites (MMCs) involves highly complex and non-equilibrium material transformation behavior, including melting, dissolution, precipitation, and solidification. Yet, the dynamics and interplay of these phase transformation processes remain poorly understood, posing substantial challenges in identifying the microstructure formation mechanism, and predicting and controlling the microstructure in the printed parts. Here we performed the in-situ X-ray diffraction experiment to characterize the phase evolution dynamics of the 316L + 10 vol.%TiC system during laser melting, which provides direct and quantitative insights of the complex phase reaction and evolution dynamics under rapid heating and cooling conditions relevant to additive manufacturing of reaction-based MMCs. Further in-depth thermodynamic and kinetic calculations revealed that most of the phase evolution behavior observed in the in-situ X-ray diffraction experiment cannot be solely explained by widely used equilibrium thermodynamic models, and diffusion-controlled nonequilibrium dissolution and precipitation kinetics must be considered to elucidate the complex phase evolution behavior, including incomplete TiC dissolution, and three-step TiC precipitation. The three distinct types of precipitates generate unique hierarchical TiC micro- and nanostructures, which enhances the yield strength from 513 MPa to 877 MPa by 71 %, tensile strength from 628 MPa to 1054 MPa by 68 %, and Young's modulus from 193 GPa to 221 GPa by 14 %. The findings of our research provide the knowledge foundation for the design of unique microstructures and advanced MMC materials through laser AM.

Laser ablation behavior and mechanism of Cf/C–SiC composites under different laser energy densities

Ceramic matrix composite (CMC) is typical difficult-to-machine material, laser assisted machining (LAM) is an effective method to solve the problem of poor machinability, and a deep understanding of the interaction between laser and material can lay a foundation for LAM. Aiming at the unclear mechanism of microstructure formation, crack propagation and material hardness evolution of laser-irradiated CMC, laser ablation behavior and mechanism of Cf/C–SiC composites under different laser energy densities were studied in this paper. The results show that Cf/C–SiC presents two different states of modification and ablation under irradiating. In modified state, fiber interface in central area is oxidized, and crystal transformation occurs. Amorphous Si–O–C, Si and spherical SiO2 exist in edge deposition area, and continuous phase disappears in heat affected zone. In ablative state, carbon nanosheets and clusters are formed in ablation groove. Surface of convex and deposition area are composed of sedimentary basement and clusters, and there are recast layer and heat affected zone in cross section. When thermal stress exceeds interfacial bond strength and critical matrix stress, the fiber interface debonding and matrix cracking occur, and both transgranular and intergranular fracture exist in the matrix. When laser scanning direction is perpendicular to fiber extension, crack propagation can be inhibited. The propagation and intersection of cracks and coarsening of SiC crystal phase in heat affected zone lead to the decrease of matrix microhardness. The results of this study can be used to provide references for parameters selection and theoretical support for LAM of CMC.

Quantification of melt pool dynamics and microstructure during simulated additive manufacturing

The dynamics of laser spot melting and metallic alloy solidification are investigated through synchrotron x-ray radiography, 3D electron backscatter diffraction data, and computational fluid dynamics (CFD) simulations on a model NiMoAl alloy. Solidification velocities are measured from in-situ images to validate a CFD model of the spot melt. TriBeam tomography is used to reconstruct the melt pool in 3D, characterize the microstructure, and validate the CFD model melt pool dimensions. 3D geometrically necessary dislocation (GND) density calculations reveal extensive deformation at the sample surface. GND calculations integrated with the CFD model reveal that the halo of small, equiaxed grains adjacent to the fusion line is a result of recrystallization that occurred in the heat affected zone. By combining in-situ and ex-situ data modalities across a variety of temporal and spatial length scales, the microstructure formation mechanisms and their relationship to melt pool solidification are quantified.

Effects of calcined phosphogypsum replacement on hydration and properties of calcium sulfoaluminate cement at different curing temperatures

The present research aims to understand the cement hydration and microstructure formation mechanism of calcined phosphogypsum (CPG) supplemented calcium sulfoaluminate (CSA) pastes cured under hot-weather temperatures and to ensure the appropriate application and effective service of CSA cement-based materials, as well as to address the problem of overstocking of CPG. The effects of CPG dosage on the heat of hydration, electrical resistivity, hydration products, and bound water content of CSA cement pastes were investigated at curing temperatures of 20 °C, 35 °C, and 50 °C. Hydration kinetics was studied to elucidate the hydration mechanism, and thermodynamic modelling was performed to predict the phase assemblages. The results show that the incorporation of CPG facilitated the formation of ettringite and significantly reduced the transformation of ettringite to monosulfate, and these phenomena were enhanced with an increase in the CPG. The addition of CPG at elevated curing temperature (35 °C and 50 °C) significantly improved the compressive strength of CSA cement pastes. The growth rate and nucleation rate predicted by the BNG model for the CSA cement pastes cured at 50 °C were substantially higher compared to those at 20 °C.

Microstructure evolution and formation mechanism of interfaces in parallel gap resistance welding of stranded Ag-plated Cu conductor to Ag interconnector

In space flexible solar arrays, stranded Ag-plated Cu conductors and Ag interconnectors are preferred materials for energy transmission. Parallel gap resistance welding is a solderless micro-joining process that favors the reliability of joints between stranded Ag-plated Cu conductors and Ag interconnectors. It was found the interfacial microstructure presented diversity and complexity while mechanical and electrical properties showed unusual nonlinear variations with welding voltage. This study aims to clarify the interfacial microstructure evolution and its effects on the joint properties while elucidating the microstructure formation mechanisms. A shift in the bonding mechanism at interfaces from solid-state diffusion to brazing was found as welding voltage reached a critical value (1.5 V). It eliminated micro gaps and built nano-scale interlocking structures, resulting in substantial improvement in mechanical properties. Increasing the welding voltage from 1.2 V to 1.4 V improved electrical conductivity due to the enlarged bonding area at interfaces. However, high welding voltage (1.6 V) led to degradation in the electrical conductivity of joints due to excessive Ag-Cu solid solution formed at interfaces. The key to fabricating high-strength and high-conductivity joints lies in achieving appropriate interfacial melting while reducing alloying by controlling peak temperature and shortening the duration above the Ag-Cu eutectic point.

Multiscale microstructure containing nanometer-scale precipitations and stacking faults yields a high-strength Al-5Cu alloy by electron beam freeform fabrication

Electron beam freeform fabrication (EBF3) additive manufacturing technique has been successfully employed to print the non-weldable Al-5Cu thin-walled parts through an optimized set of process parameters. Post-heat treatment is conducted to engineer the desirable microstructure containing multiple nanostructures, i.e., the nanoscale θ" and θ' precipitates, stacking faulted zones and Al-Cu-Cd clusters, leading to a high-strength Al-5Cu alloy with good ductility. The grain formation mechanisms are revealed via a combinatorial simulation and experimentation. It is shown that EBF3 process can produce three different types of grain microstructures: fully equiaxed, fully columnar, and the mixture. In the layerwise melt pool solidification process, wide partially melted zone is observed to favor the formation of equiaxed grains due to its inheritance characteristic. After solution and aging heat treatment, Al-Cu-Cd nanoclusters (∼ 2 nm in Dia.) are generated, which facilitates the formation of nanoscale stacking faulted zones (∼11 nm). Meanwhile, as heterogeneous nucleation sites, Al-Cu-Cd nanoclusters effectively promote θ'-Al2Cu phase (∼ 19 - 114 nm) precipitation to enable the coexistence with fine θ"-Al3Cu precipitates (∼ 7 - 37 nm). The multiscale microstructure, especially incorporating the newly identified nanoscale stacking faulted zones, activates multiple strengthening mechanisms. Consequently, its tensile properties at room temperature reach an ultimate tensile stress of ∼ 496.5 MPa, yield strength of ∼ 435.7 MPa, and elongation of ∼ 9.6%, which are superior to other as-cast and additively manufactured Al-Cu alloys. This work offers a promising processing route to directly fabricate high-strength near-net-shape Al-Cu alloy parts for the aerospace and automotive industries.

Microstructure Formation and Its Effect on Mechanical Properties for Duplex Stainless Steel 2205 Plasma Arc Welded Joint

The control of phase balance has always been a tough challenge for the welding of duplex stainless steel, which heavily restricts its optimal serving performance in engineering. The microstructure development and mechanical characteristics of SAF2205 plasma arc welded joints were thoroughly examined in this paper. It was proven that the phase balance can be well controlled by plasma arc welding, and the austenite content of the welded joints was about 60%. Despite successful phase control, there was still grain coarsening and distortion; i.e., at the center of the welded zone, the gain size was about eight times that of the base metal, and the austenite was mainly in the form of grain boundary austenite and intragranular austenite, while more Widmanstatten austenites were found in the heat-affected zone. In addition, a transition region between the heat affected zone and the center exhibited columnar ferritic grains. Furthermore, the plasticity and toughness of the welded joints were significantly decreased, especially the elongation in the longitudinal direction, which is about 10% lower than that of the base metal, and transversal tensile strength remained comparable to the base metal, with only a slight reduction in longitudinal tensile strength. Lastly, the formation mechanism of microstructure and its correlation with mechanical properties were revealed. This investigation offers valuable insights into the structural integrity of duplex stainless steel welded joints in engineering applications.

Insight on ultrasonic effect of Al[sbnd]Cu aluminum alloy joints fabricated by ultrasonic assisted non-thinning and penetrating friction stir welding

AlCu alloy joints of ultrasonic assisted non-thinning and penetrating friction stir welding (U-NTPFSW) and NTPFSW were prepared. The microstructure and mechanical properties of the joints were studied comparatively to elucidate the effect of ultrasonic. This study provided an in-depth understanding of the microstructure formation mechanism and strengthening mechanism of joints under the ultrasonic effect. Ultrasonics broadened the width of the stir zone (SZ) and stationary shoulder affected zone (SSAZ) by promoting the material plastic flow ability and changing the material flow at the weld root. The ultrasonic addition enhanced plastic heat production and promoted grain growth in SZ. Compared with NTPFSW, the SZ of both joints was dominated by continuous dynamic recrystallization (CDRX) and discontinuous dynamic recrystallization (DDRX) mechanisms, and geometric dynamic recrystallization (GDRX) grains were also present in U-NTPFSW. The mechanical properties of the joint were improved from 339 MPa to 348 MPa. Ultrasonics introduced numerous vacancies into the weld, which promoted the precipitation of the Al2Cu phase and increased the precipitated phase density of the thermal-mechanical affected zone (TMAZ). The interaction mechanism between the precipitated phase and dislocations was analyzed. The geometrically necessary dislocation (GND) density and residual stress of TMAZ were reduced due to ultrasonic vibration.

Multi-regional microstructure control using ultrasonic-assisted directed energy deposition for Al-Cu alloy

The uneven microstructure and element distribution between the inner-layer zone (INZ) and the inter-layer zone (ITZ) resulting from the thermal cycling effect during the directed energy deposition process with the electric arc energy source (DED-Arc) can lead to inferior mechanical properties. In order to address this problem, the ultrasonic treatment of the 2319 Al-Cu alloy during the DED-Arc process is investigated to control the multi-regional microstructure quantitatively. Therefore, the effect of ultrasonic vibration during the DED-Arc process on grain morphology, distribution of precipitates and mechanical properties in different regions are explored in detail. The formation mechanism of differential microstructure is elucidated by using an ultrasonic-heat-flow coupling simulation model for the DED-Arc process. The results indicate that ultrasonic vibration refines the average grain size, leading to 16% and 40% reductions at the molten pool center (MPC) and boundary (MPB), respectively. However, the grains in the semi-melting zone (SMZ) experience coarsening due to the reduced temperature gradient after ultrasonic treatment. The enhanced melt flow rate is considered responsible for both the homogenization of Cu and the fragmentation of the θ-Al2Cu phase. The higher subgrain boundary density in the ITZ leads to an average microhardness of approximately 60 HV, which is lower than that found in the INZ (72.3 HV). Meanwhile, ultrasonic treatment increases the average strength and elongation by 38.1% and 17.5%, respectively, mainly through precipitation strengthening, which results in a strength increment of 91.6 MPa. The slip directions tend to be consistent after ultrasonic vibration, which is conducive to crystal sliding and displays excellent ductility.

Effect of microstructure evolution on tensile properties of arc wire directed energy deposited Ti-6.5Al-3.5Mo-1.5Zr-0.3Si titanium alloy

In this study, the static globularization and static coarsening behavior of arc wire directed energy deposited (AW-DEDed) Ti-6.5Al-3.5Mo-1.5Zr-0.3Si (TC11) alloy and their affected on tensile properties were studied. The as-deposited sample and the 550-AC (550 °C/4h/air cooling) sample showed columnar prior-β grains, while the microstructure showed the continuous αGB grew up through the colony microstructure and the basket-weave microstructure. The driving force in the α/β interface was enough for static globularization and static coarsening, which formed crack-like primary α (crab-like αP), αG (globularized α), and the αs (secondary α) during 970-AA (970 °C/2h/air cooling+550 °C/4h/air cooling) or 990-AA (990 °C/2h/air cooling+550 °C/4h/air cooling). The microstructure evolution and formation mechanism of the crab-like αP, the αG, and the αs were discussed. The formation of the αG was mainly via grain boundary splitting, and the formation of the crab-like αP was mainly via terminal migration with different interchanged interfaces. The ultrafine lamellae α nucleated with the sympathetic mechanism grew until lost burgers orientation relationships with adjacent grain boundaries and formed αS. The 550-AC sample showed the highest yield strength (YS, 905 MPa) and the highest anisotropic ductility (ΔEL, 7.9) than those in the 970-AA sample and the 990-AA ample. It was because the crab-like αP, the αG, and the αs decreased the fracture resistance in the 970-AA sample and the 990-AA sample.

The microstructure and impact toughness of vacuum electron beam welded joints of a highly alloyed dual phase titanium alloy

Vacuum electron beam welding of 50 mm thick TC18 titanium alloy plates were carried out at different processing parameters, and full penetration welded joints were obtained. The welded joint presents three typical zones, namely, fusion zone, heat affected zone and base metal zone. It is found that processing parameters have no obvious influence on the width of heat affected zone. By using EBSD technology, the changes of phase composition, grain size and grain boundary density from the fusion zone to heat affected zone and then to base metal zone were clearly observed and quantitatively analyzed. The fusion zone shows coarse equiaxed/columnar β grains with an average diameter/width of more than 100 μm. The volume fraction of α phase in the heat affected zone decreases with the decrease of the distance to the fusion zone until it is completely dissolved into β phase. The formation mechanism of microstructure in different zones and its influence on impact toughness were discussed. The β grain size and undissolved/precipitated fine α grain on β grain boundary are two main factors affecting impact toughness.

Investigation on microstructure characteristics and mechanical properties of twin wire-directed energy deposition-arc fabricated TiAl alloy regulated by the line energy

The microstructure of TiAl alloy is extremely sensitive to the thermal history. During additive manufacturing, multiple thermal cycles and partial remelting of the TiAl deposition layers take place. Therefore, the heat input would exert significant influences on microstructural and mechanical characteristics of additively manufactured TiAl alloys. In this study, TiAl alloys were prepared successfully by twin-wire directed energy deposition-arc (TW-DED-arc). Systematic studies were conducted to regulate the microstructure characteristic and the mechanical property of the as-deposited TiAl alloys via the line energy. The results indicate strong effects of the line energy on the microstructure and performance of the TiAl alloys, which show alternative distribution of layer-band like microstructure with the dendritic grains and the fully lamellar colonies along the deposition direction. With the enhancement in the line energy, the layer-band like feature gradually weakens, and the fully lamellar colonies increase. In addition, the discontinuous coarsen phenomenon and the discontinuous dynamic recrystallization occurred, and the anneal twin was also formed. On the other hand, tensile fractures were observed in the dendritic grains region, and the maximum tensile strength was achieved with the medium line energy. However, the elongation and microhardness are proportional to the line energy. With the elevation in the line energy, the elongation and microhardness gradually increases and decreases, respectively. The microstructure formation mechanism of as-fabricated TiAl alloy is analyzed systematically, and the fracture mechanics was also discussed in detail. The findings of this research contribute not only to the methodology for fabrication of TiAl alloys with the novel TW-DED-arc process, but also to the deep understanding on the regulation of the microstructural and mechanical characteristics of the additively manufactured TiAl alloys.

On the role of cellular microstructure in austenite reversion in selective laser melted maraging steel

Cellular microstructure is a unique feature in alloys fabricated by selective laser melting (SLM). Abundant efforts have been made to reveal the formation mechanism of cellular microstructures and its influences on mechanical performances, while its potential role in microstructure architecting during post-heat treatment is rarely explored. In this work, we investigated the features of cellular microstructures in an SLM-fabricated 18Ni(300) steel and revealed how this microstructure influences austenite reversion upon aging. Segregation of Ti and Mo is experimentally detected at cell boundaries. It is interestingly found that a distinctive reverted austenite network forms rapidly along cell boundaries during aging, whereas much less austenite is found in conventionally treated 18Ni(300) steels. The rapid austenite reversion in SLM-fabricated material proceeds mainly via the growth of retained austenite on cell boundaries while the nucleation and growth of new austenite grains is negligible. Phase-field simulations suggest austenite grows in a fast, partitionless manner along cell boundaries where the chemical driving force for austenite reversion is substantially enhanced by Ti and Mo segregations, but in a sluggish, partitioning manner towards cell interiors. Contrary to conventional views that austenite fraction should be confined to avoid strength reduction, current SLM-fabricated 18Ni(300) steel containing ∼13% cellular austenite is found to have higher tensile strength compared to its counterparts with negligible austenite. The design of austenite also shows its potential to enhance fracture toughness. The current study demonstrates that cellular structures could substantially alter austenite reversion behavior, providing a new route for microstructure architecting in additively manufactured steels.

Study on microstructure of 42CrMo steel by ultrasonic surface rolling process

To explore the microstructure formation mechanism of 42CrMo steel under the strengthening of ultrasonic surface rolling process (USRP), the combination of theoretical analysis and experiment was used to conduct in-depth research on USRP. Firstly, according to contact mechanics and Hertz contact theory, the calculation model of contact stress distribution and elastoplastic strain between the rolling ball and the part during USRP is obtained. Secondly, the USRP processing test was carried out by single-factor experimental design method, and the microstructure of 42CrMo steel after USRP was analyzed by LEXT OLS5100 3D laser surface topography instrument and VEGA3 tungsten filament scanning electron microscopy, which found that with an increase in static pressure, residual stress and plastic strain gradually increase, the hardness firstly increases and then decreases, while surface roughness exhibits an initial decrease followed by an increase. The results show that USRP produces violent plastic deformation inside the material under the superposition of high-frequency impact and static pressure, at the same time, it refines the grains, so as to improve the surface performance of the part and improve its fatigue resistance.