Cross-sectional Microstructure Research Articles

Influence of the Cooling Temperature on the Surface Quality in Integrated Additive and Subtractive Manufacturing of Aluminum Alloy

The surface quality of parts processed by laser additive manufacturing, especially laser-based directed energy deposition (LDED), makes it difficult to meet actual use requirements. In addition, defects generated during the long-term additive manufacturing process need to be removed in time. Therefore, laser additive and subtractive manufacturing is of great significance for additive manufacturing. The main difference between laser additive-subtractive manufacturing and pure subtraction is that a cooling temperature is required due to the laser process. Therefore, this work studies the temperature variation regularity during LDED and the milling processes, as well as the surface roughness, cross-sectional microstructure, and tool wear under different cooling temperatures for milling. The results show that there is a “turning point temperature” in LDED, and the value of the turning point temperature gradually increases with heat accumulation, which affects the initial temperature of the subtractive manufacturing. When subtracting, a high initial temperature improves surface quality and reduces tool wear, but an excessively high temperature will cause the aluminum alloy to adhere to the tool. Then, the smear metal is difficult to effectively remove, deteriorates the milling quality, and aggravates tool wear. It is found that the higher the cooling temperature generated, the wider the thermally insulated shear band. The insulated shear band may affect the quality of the additive and subtractive manufacturing. Finally, it is determined that the milling temperature of aluminum alloy in this work condition is about 100 °C.

Investigation of formation, microstructure and hardness of Ti-6Al-4V single tracks via Joule-fused filament additive manufacturing

Purpose This paper aims to investigate a novel additive manufacturing (AM) method for titanium alloy using Joule heat as the single heat source to melt TC4 wire, which intends to provide a new low-power, low-cost solution for the processing of titanium alloys. Design/methodology/approach When current flows through the wire and the substrate, Joule heat will be generated to melt the wire and join the wire with the substrate. By stacking the wire layer by layer, finally a part can be formed. The cross-sectional morphology, microstructure and hardness of TC4 single track deposits formed by Joule heat melting wire AM were investigated by various characterization methods. Findings The melting width and melting penetration decreased with the increase of printing speeds. There is no obvious change in single track morphology with the change of printing pressures. The melting width and melting penetration increased with the increase of printing currents. The observation of the internal microstructure of a single track reveals a decrease in grain size as printing speeds increase. The average hardness of the single track was about 363 HV, which is comparable to the hardness of the parts fabricated by selective laser melting process. The printing power is less than 300 W, which is lower than other AM processes. Originality/value This paper provides a novel solution for the processing of titanium alloy parts. Compared with other expensive energy sources, this work only uses an ordinary DC power supply as the energy source. The printing process is simple and the cost is low. The power is much lower than other AM processes.

Study on the mechanical properties of PES-HmA samples printed by selective laser sintering under various pre-heating conditions

The purpose of this study is to investigate the influence of pre-heating characteristics on the mechanical properties and forming process of selective laser sintering (SLS) printed PES-HmA samples. An experimental setup with four heating tubes was designed to study the pre-heating temperature distribution on the powder bed. The pre-heating temperature distribution on the powder bed was captured using a thermal imaging camera. A method for evaluating pre-heating temperature distribution based on the average and standard deviation of surface temperature was proposed. The heating tube installation position was optimized using a response surface experiment study based on the temperature distribution evaluation. By optimizing the installation position of the tubes, the temperature distribution on the powder bed tends to become uniform. The effect of pre-heating temperature value and distribution on the mechanical properties of the SLS printed PES-HmA samples was also experimentally investigated. The cross sectional microstructure of the printed samples were examined by scanning electron microscope to analyze the layer formation process at different pre-heating temperature. By increasing the pre-heating temperature from 70°C to 100°C, the material diffusion at the layers interface was improved, which made the tensile strength of sample increased by 376%, and the flexural strength increased by 224%.

A Comparative Study of Microwave Welding Using Multiwalled Carbon Nanotubes and Silicon Carbide Nanowhiskers as Microwave Susceptors

Recently, microwave welding has arisen as an advanced joining method due to its versatility and rapid heating capabilities. Among others, microwave susceptors play a crucial role in microwave welding, as different classes of microwave susceptors have distinct microwave heating mechanisms. In this work, polypropylene (PP) was utilized as a thermoplastic substrate and two types of microwaves susceptors, namely multiwalled carbon nanotubes (MWCNTs) and silicon carbide nanowhiskers (SiC NWs), were studied for microwave welding. The susceptor was first dispersed in acetone to form susceptor suspension. Next, the susceptor suspension was deposited onto the targeted area on substrate and paired with another bare PP substrate. The paired sample was then exposed to 800 W microwave radiation in a microwave oven. Afterward, the welded joint was evaluated using a tensile test and scanning electron microscopy to determine its joint strength and cross-section microstructure. The results showed that the joint strength increased as the heating duration increased. The welded joint formed using MWCNTs achieved a maximum strength of 2.26 MPa when 10 s was used, while the SiC NWs-formed welded joint achieved a maximum strength of 2.25 MPa at 15 s. This difference in duration in forming a complete welded joint can be attributed to the higher microwave heating rates and thermal conductivity of MWCNTs. However, increasing the heating duration to 20 s caused severe deformation at the welded joint and resulted in low joint strength. Overall, this study highlights the significance of understanding the microwave heating mechanism of different susceptors and provides essential insight into the selection of a microwave susceptor for microwave welding.

Evolution of fretting wear behavior of zirconium alloy cladding tube under gross slip regime in simulated primary water of pressurized water reactor

The evolution of fretting wear behavior of zirconium alloy cladding tubes mated with dimples under the gross slip regime (GSR) was investigated. The findings revealed that the primary wear mechanisms under GSR were delamination, surface fatigue wear and abrasive wear, and the fretting damage rate mainly depends on delamination. The cross-sectional microstructure of the worn area could be divided into the third-body layer, tribologically transformed structure layer, and general deformation layer, with their formation mechanisms analyzed. Furthermore, the mechanism of wear-induced grain refinement was identified as dynamic recrystallization (DRX), including both continuous DRX and discontinuous DRX. Additionally, the processes of fretting wear and DRX were discussed.

Influence of laser remelting on the microstructure and properties of Mo2FeB2 ternary boride coatings prepared by laser cladding

This study investigated the impact of laser remelting on the microstructure and properties of Mo₂FeB₂ coatings on AISI 1045 steel substrate. Different laser remelting powers were applied to pre-deposited Mo₂FeB₂ coatings. The cross-sectional morphology, phase composition, microstructure, microhardness, residual stress, fracture toughness, and wear resistance of the coatings were systematically analyzed. Compared to the untreated coatings, the remelted coatings exhibited reduced height and significantly refined microstructure. The primary phase observed in the coating was Mo₂FeB₂, accompanied with minor phases such as Cr₃C₂. Optimal microhardness (1209.4 HV0.5), fracture toughness (14.5 MPa·m1/2), and wear rate (5.4× 10−5 mm3/N·m) were achieved at a remelt power of 1200 W. Additionally, the residual stress in the coatings decreased with increasing power, reaching a minimum of 268 MPa at 1500 W. The wear mechanism transitioned from adhesive wear to abrasive wear with increasing remelting power. These findings highlight the beneficial effects of laser remelting in enhancing the performance of Mo₂FeB₂ coatings for industrial applications.

The influence of heat treatment on AlSi-polymer abradable coating

Abstract Abradable coatings, as one of the most important technologies for aero and industry gas turbine engines, have an extremely significant influence on engine performance. The macro-hardness of AlSi-Polyester for low-pressure compressor and as-sprayed coating degradation mechanism has been investigated by heat treatment process in imitation service condition. SEM (Scanning Electron Microscope) and OM (Optical Microscope) were used to observe the morphology of powder and coating microstructure in cross-section, respectively. Rockwell hardness tester was used to measure the macrohardness of coatings before and after heat treatment. It shows that AlSi-Polyester powder has an approximately spherical shape and uniform microstructure with alloy phase and polyester phase distribution in the coating prepared by air plasma spraying. Oxidation of the alloy phase and reduced polyester, as well as macrohardness in the as-sprayed coating, occur after the heat treatment process.

Investigation on Microstructures, Hardness, Friction, and Wear of Cold Sprayed Ti6Al4V Coatings With Coating Thickness of 100-3000 µm

The effect of coating thickness (CT) on the microstructures, hardness, and wear of cold sprayed Ti-6Al-4V (CS-Ti64) coatings was systematically investigated since the prolonged high pressure CS deposition could change their microstructures and porosity levels. In addition, the CT was relatively important for their durability, performance, and service life. Therefore, the CS-Ti64 coatings with different CT of 100-3000 µm were prepared on commercially available Ti64 (CA-Ti64) substrates via high pressure CS processes. The CS-Ti64 coatings had low porosity levels wherewith severely deformed Ti64 particles with a crescent-shape could be seen in their cross-sectional microstructures. The hardness of the CS-Ti64 coatings increased with increased CT probably due to their lowered bulk porosity levels associated with longer high pressure CS deposition. As a result, the increased CT from 100 to 3000 µm resulted in a 9.8% decrease in the wear of the CS-Ti64 coatings. The wear of the CS-Ti64 coating with 3000 µm was 16.8% lower than that of the CA-Ti64 as all the CS-Ti64 coatings had lower wear than the CA-Ti64. It could be concluded that the prolonged high pressure CS deposition for the thick CS-Ti64 coatings had an influence on their porosity, hardness, and wear.

Bonding Properties of Package-on-Package Stack Interconnection Using by 150 ㎛ Height Copper Posts

In this study, we used copper (Cu) posts, plated to a height of 150 μm, as interconnections to form electrodes between different package layers. The upper layer consisted of a chip-scale package (CSP), while the lower layer was made up of a flip chip-chip scale package (FCCSP). We plated a Cu post on the side of FCCSP substrate, and produced the FCCSP package by thermo-compression (TC) bonding with copper pillar solder bumps on the center of substrate. The surface of the bottom electrode for the upper CSP received a surface finish of electroless nickel, electroless palladium, and immersion gold (ENEPIG). The bonding surface of the lower FCCSP was the bare Cu surface of the epoxy-molded Cu post. The PoP joining process used a vacuum reflow process with solder paste and solder balls. To understand the package joint's characteristics, we measured voids and shear strength, and analyzed the cross-sectional microstructure. The temperature profile used in the PoP joining process demonstrated optimal joint characteristics with a joint void content of 2.3% and a joint strength of approximately 44 MPa when formic acid was utilized and the activation preheating time was extended. Cross-sectional warpage analysis of the PoP package revealed a minimal difference of approximately 31 μm between the center and the ends. This study successfully developed a stacked package with a PoP structure using Cu Post, and the bonding process was optimized to minimize warpage in the PoP package.

Structural, mechanical, and oxidation resistance properties of double glow plasma Ta[sbnd]W alloys

To enhance the performance of surface coatings on weaponry, TaW alloy layers with various W contents (7, 11.5, 13, 14, 14.5 and 16 at. %) were prepared on the surface of a PCrNi3MoVA gun steel substrate using double-glow plasma surface metallurgy technology. Mechanical properties, friction wear, and oxidation resistance were studied using scratch, hardness, and tribometer testers. The mechanical properties, interfacial properties, and effects on the oxidation performance of the gun steel substrate of the TaW alloy layer were calculated using first principles. The results indicated that the surface and cross-sectional microstructures of the TaW alloy layers were dense and free from obvious defects. With increasing W content, the thickness, hardness, and adhesive strength of the TaW alloy layer increased, and the coefficient of friction and wear rate decreased. The results of the calculations indicate that an increase in the W content enhances the elastic modulus of the TaW alloy and improves its brittleness, strength, and hardness. Doping with Ta and W atoms can increase the vacancy formation energy of Fe atoms in the Fe supercell, the adsorption energy of O atoms on the surface of Fe(110), and the diffusion barrier of O atoms in the Fe supercell and on the Fe(110) surface. Interdoping of Ta, W and Fe atoms at the Fe(110)/Ta-W(110) interface improved the bonding strength and stability of the interface, and the FeTa and FeW chemical bonds formed at the Fe(110)/Ta-W(110) interface ensured stable bonding at the interface. The incorporation of W into the TaW alloy layer enhanced both the mechanical and frictional properties of the alloy. It is well established that TaW alloys improve the antioxidant properties of gun steel substrates, thereby enhancing the performance of coatings on weaponry.

Improvement of AISI 316L laser cladding properties by high-frequency vibration process

In the present paper, a unique laser cladding-assisted high-frequency vibration device was applied to prepare the AISI 316L cladding layer by laser cladding on the surface of the AISI 316L stainless steel substrate. The macroscopic morphology, microstructure, microhardness, and wear resistance of the cladding layer were analyzed by optical microscope, scanning electron microscope, Vickers microhardness tester, friction and wear tester, and three-dimensional optical profiler. The laser cladding-assisted high-frequency vibration device uses a high-frequency oscillator with a fixed frequency (20 KHz) to transfer vibration energy through the probe in contact with the substrate and close to the molten pool. This paper explores the effect of different laser powers on the dilution rate of the cladding layer, compares and observes the cross-sectional morphology and microstructure of the cladding layer with and without high-frequency vibration, and analyzes the reasons for the different results.

Structural control in atomic layer deposited hafnium oxide thin films through vertical cavity surface-emitting laser (VCSEL)-Based ultra-rapid heating

HfO2 thin films were formed by plasma-enhanced atomic layer deposition (PEALD) using TDMAHf precursor as a Hf source and oxygen plasma as an oxygen source. Rapid annealing assisted by a 980 nm wavelength vertical cavity surface-emitting laser (VCSEL) was employed to assess the dielectric properties of the HfO2 thin films, compared to furnace annealing. Operational power and time of VCSEL was precisely controlled by computational simulation. In addition to decreased thickness of the interfacial layer, rapid annealing enabled reduced leakage current density as annealing temperature increased due to suppression of crystallization, resulting from the extremely short time duration of 0.98 s–8.14 s. Cross-sectional microstructures exhibited incomplete grain boundary formation of the rapidly annealed HfO2 thin film. VCSEL-based rapid annealing was verified to be an efficient process to enhance the dielectric characteristics of high-k dielectric oxide films.

Bending performance and failure mechanism of CFRP/Al hat-shaped thin-walled hybrid beams

Hat-shaped beams played an important role in the body load-bearing structure, such as B-pillar. Fiber metal laminates possessed great advantages of excellent stiffness/strength, high damage tolerance and lightweight effect. In order to improve the catastrophic damage mode of carbon fiber, the hat-shape thin-walled hybrid beams consisting of carbon fiber reinforced thermoplastic (CFRP) prepreg and aluminum (Al) sheet were proposed and fabricated by hot-pressing molding. The effects of carbon fiber layer numbers, CFRP/Al stacking sequence, and the CFRP layup orientation on the three-point bending performance and failure mode of the hat-shaped hybrid beams were investigated. The results showed that the more CFRP layer, the better the bending performance of the hybrid hat-shaped beams. The CA-2 hat-shaped beams generated overall lateral deformation. The ACA (Al/CFRP/Al) hat-shaped beams achieved a higher CFE (ratio of average load to peak force) value and better energy absorption effect. The deformation processes of AC (Al/CFRP) and ACA stacking structures was similar, the horizontal web concave and the side walls on both sides convex. In terms of fiber lay-up orientation, ACA-OR (orthogonal) possessed superior bending performance and energy absorption to the ACA-UD (unidirectional) hybrid beams. Furthermore, the failure mechanism of the ACA hybrid beams was analyzed and the cross-sectional microstructure at different sections were observed by SEM. Matrix cracking, interfacial delamination, fiber fracture and metal plastic deformation were detected, which played a supporting effect and energy absorption role.

A comparative study on microstructures of a high-Zn AlZnMgCuZr alloy fabricated by casting and melt spinning

A high-Zn Al–27Zn-1.5Mg-1.2Cu-0.08Zr alloy was prepared by traditional casting and melt spinning. The as-casted alloy consists of well-developed α-Al dendrites with many coarse η-phase and low solute concentration, coarse network precipitates at grain boundaries and the α-Al/η-phase eutectic structures at triple junctions. Due to these featured structures, the as-cast alloy exhibits a low tensile strength of 101 MPa with room temperature and high temperature (300 °C) damping capacity of 0.0058 and 0.027 respectively. Comparatively, the as-spun alloy reveals gradient cross-sectional microstructures after solidification: an ultrafine grained region near the wheel surface, a transition region and an equiaxed-grained region near the ribbon free surface, with changes of precipitate morphologies from granular shape to network shape via vermicular. High-density nano-sized η′-phase/GP-zones within α-Al grains result in a higher tensile strength (121 MPa) of as-spun alloy with room temperature and high temperature (300 °C) damping capacity of 0.0049 and 0.048 respectively. Fracture analysis shows that the as-casted alloy fails in a mixed-mode fracture, comprised predominantly of transgranular fracture, quasi-cleavage fracture, cleavage fracture and dimple fracture, whereas the as-spun alloy failed in cleavage fracture and dimple fracture.

High-temperature fretting wear behavior of IN738LC alloy formed by laser powder bed fusion

The fretting wear behavior of IN738LC alloy formed by laser powder bed fusion at 850 ℃ with various load (35 N and 100 N) were systematically studied. The results demonstrate that increases the load, leading the fretting wear mode gradually transitions from the gross slip region (GSR) to the mixed slip region (MSR). Judging with the radar map drawn by experimental data, the high-temperature comprehensive performances of fretting wear for supersolvus heat-treated sample improved significantly. The analysis of fretting wear cross-section microstructure indicates that the thermoplastic deformation of L-PBFed IN738LC alloy occurs under the interaction of different normal loading and force of tangential friction, which results in the accumulation of strong dislocation densities and diverse deformed tissue in damage zone.

Oxidation behavior of Ti55 alloy and TiBw/Ti55 composites and antioxidation mechanism of TiBw in composites at 960–1000 °C

The oxidation behavior of Ti55 alloy and TiBw/Ti55 composites at temperatures ranging from 960 to 1000 °C was investigated by characterizing the surface and cross-section microstructure of specimens. Results showed that TiBw reinforcement accelerated the occurrence of Ti6O/Ti3O by dissolving oxygen in titanium in the starting oxidation stage, and the Ti6O/Ti3O transformed into TiO2 with the progression of oxidation. Meanwhile, TiBw reinforcement promoted the formation of (101) crystal planes to be beneficial to the growth of TiO2 twins. The cross-sectional characterization showed that the oxide layer of Ti55 alloy and TiBw/Ti55 composites from outside to inside was TiO2+Al2O3, TiO2, Ti-Sn compounds, Ti6O/Ti3O in sequence, which was confirmed by calculating the standard Gibbs free energy of the oxide nucleation. The TiBw reinforcement accelerated the occurrence of suboxides Ti6O/Ti3O by dissolving oxygen in titanium, and promoted the formation of (101) crystal planes which were beneficial to the growth of TiO2 twins. The optimal addition of TiBw induced the TiO2 twins, promoted the random orientation of oxides and refined the oxide size of the TiBw/Ti55 composites with 3.5 % volume fractions of TiBw, resulting in the best resistance against oxidation.

Sn/Graphite/Cu reinforced aluminum: An experimental study on fabrication of hybrid surface composite as journal bearing material by friction stir processing

Friction stir processing (FSP) is an energy-efficient and environmentally friendly technique that provides tailored manufacturing possibilities for surface composites in solid-state conditions. This study focuses on manufacturing solid lubricant-reinforced aluminum matrix surface composite (AMSC) through FSP, which can be used as a journal-bearing material. Composite fabrications are performed utilizing Sn, Cu, and graphite particles (hybrid powder) with four passes of FSP. The cross-sectional macro and microstructures of obtained surface composites are investigated via optical microscopy (OM) and scanning electron microscopy (SEM). The stir zone (SZ) chemistry is analyzed with energy-dispersive X-ray spectroscopy (EDS) and X-ray photoelectron spectroscopy (XPS). The microhardness measurements show the effects of hybrid powder as reinforcement and FSP parameters on the mechanical features of the composite cross-sections. Wettability and tribological tests are also conducted to determine the surface properties of the fabricated composites. According to the findings, the wear rate is decreased by 8 % with the reduction of average CoF to 0.75, while a hardness increment is achieved over ∼35 % in manufactured surface composites compared to the base material. It is observed that the content and amount of the hybrid powder and FSP parameters employed can ensure the fabrication of desired characteristics compatible with the aim in surface composites.

Microstructure Evolution and Mechanical Properties of Extruded AlSiCuFeMnYb Alloy

This study investigates the impact of varying extrusion ratios on the microstructure and mechanical properties of AlSiCuFeMnYb alloy. Following hot extrusion, significant enhancements are observed in the microstructure of the cast rare earth aluminium alloy. Within the cross-sectional microstructure, the α-Al phase is reduced in size, and its dendritic morphology is eliminated. The morphology of the eutectic Si phase transitions from long strips to short rods, fine fibres, or granular forms. Similarly, the Fe-rich phase changes from a coarse skeletal and flat noodle shape to small strips and short skeletal forms resembling Chinese characters. The CuAl2 phase evolves from large blocks to smaller blocks and granular forms, while the Yb (Ytterbium)-rich rare earth phase shifts from large blocks to smaller, more uniformly distributed blocks. In the longitudinal section, the structure aligns into strips along the extrusion direction, with the spacing between these strips decreasing as the extrusion ratio increases. At an extrusion ratio of 22.56, the alloy demonstrates superior mechanical properties with a tensile strength of 325.50 MPa, a yield strength of 254.44 MPa, a hardness of 143.90 HV, and an elongation of 15.47%. These represent improvements of 27.8%, 36.5%, 38.9%, and 236.4%, respectively, compared with the as-cast rare earth alloy. In addition, the fracture surface of the extruded rare earth alloy exhibits obvious ductile fracture characteristics. Additionally, the alloy undergoes dynamic recrystallisation and dislocation entanglement during hot extrusion. The emergence of a twinned Si phase and a dynamically precipitated nanoscale CuAl2 phase are critical for enhancing deformation strengthening, modification strengthening, and dynamic precipitation strengthening of the extruded alloys.

Ultra-rapid development of ‘solid’ aerobic granular sludge by stable transition/filling of inoculated ‘hollow’ mycelial pellets in hypersaline wastewater

To overcome the long start-up period in cultivating aerobic granular sludge (AGS) under hypersaline environment, mycelial pellets (MPs) of halotolerant fungus Cladosporium tenuissimum NCSL-XY8 were inoculated to try to realize the ultra-rapid development of salt-tolerant AGS by stable transition of ‘hollow’ MPs into ‘solid’ AGS without apparent fragmentation. The granules directly met the standard of AGS after inoculating MPs (Day 0), and it basically satisfied relatively strict standards of AGS (SVI30 < 50 mL/g, D50 > 300 μm, D10 > 200 μm and SVI30/SVI5 > 0.9) under anaerobic/aerobic mode during whole cultivation processes. Microstructure of the granular cross section clarified that MPs with hollow/loose inner layer transitioned into solid/dense AGS under anaerobic/aerobic mode within 7 days, while formed skin-like floating pieces and unstable double-layer hollow granules under aerobic mode. Organics removal reached relatively stable within 13 days under anaerobic/aerobic mode, 6 days faster than aerobic mode. This study provided a strategy for ultra-rapid and stable development of AGS, which showed the shortest granulation period in various AGS-cultivation strategies.

Microwave Welding of Thermoplastic using Silicon Carbide Nanowhiskers as Susceptor: Effect of Heating Duration

Microwave welding is becoming more popular than conventional joining methods due to its advantages such as rapid and localised heating as well as applicable to components with complicated geometry. Previously reported susceptor, such as carbonaceous materials and conductive polymers, are toxic and the welding process involving these susceptors is time-consuming. Because of its exceptional microwave absorption and biocompatibility, silicon carbide nanowhiskers (SiCNWs) was employed as the microwave susceptor for microwave welding. Microwave welding in this study comprises of only three simple steps: SiCNWs suspension preparation, SiCNWs application and microwave heating. The weld strength of welded joint was then characterised using tensile test and energy dispersive x-ray spectroscopy equipped scanning electron microscopy (EDS-SEM) to study its mechanical properties and cross-section microstructure. The influence of microwave irradiation time was studied in this study, and it is found that the weld strength rose with the extension of microwave irradiation time, until a maximum weld strength of 1.61 MPa was achieved by 17 s welded joint. The development of SiCNWs reinforced PP nanocomposite welded joint layer is responsible for the enhanced weld strength. Prolonged heating duration may also result in flaws such as void formation at the welded joint, which subsequently lowered the weld strength to 0.60 MPa when the heating duration was extended to 20 s. In sum, a strengthen welded joint can be formed with rapid microwave heating under the proper control of heating duration.