Room Temperature Tensile Tests Research Articles

An approach for identifying the deformation-induced strain from tensile tests and x-ray diffraction measurements

ABSTRACT Non-uniform plastic deformation and phase transformation introduce residual stress (RS) in a material. The present study focuses on the influence of tensile strain on RS development in 304L austenitic stainless steel, wherein strain-induced martensite (SIM) transformation influences deformation behaviour. Room temperature tensile tests were interrupted at various tensile strains between the material yield and ultimate tensile strength at 5.21 × 10−4 and 1.3 × 10−2 s−1 strain rates. Microstructural characterisation of as-received and deformed samples by electron backscatter diffraction provides evidence of SIM formation and shear bands. Analysis of tensile flow and work hardening behaviour is discussed in correlation to the microstructural and dislocation density variations. In addition, RS in the austenite and martensite phases was measured using the x-ray diffraction method, and the RS balance that occurs between the longitudinal and transverse components and the two phases after tensile deformation is presented. It is shown that a combined analysis of RS and x-ray diffraction peak width could be utilised to find the deformation level in tensile fractured, press brake bent, and roll bent samples.

Influence of post-welding heat treatment on microstructure and peculiarity of 10Cr9Mo1VNb boiler steel welded by hot wire TIG

Hot wire tungsten noble gas (HW-TIG) soldering tests are conducted on 10Cr9Mo1VNb boiler steel with multi-layer and multi-pass, and the soldered joints are subjected to post-weld heat treatment (PWHT) at different temperatures and times from 740°C to 790°C and 1 hour to 4 hours. The influence of PWHT on the microstructure and peculiarity of the joints are analyzed by using hardness, metallography, room temperature tensile testing, Charpy impact testing, SEM, and other experimental methods. The results show that the PWHT can transform the martensite formed in the welding process into tempered martensite, which can significantly improve the uniformity of the microstructure and peculiarity of each area of the joint. When subjected to heat treatment at 740°C, both strength and ductility improve with the increase of heat treatment time, while the plasticity gets a slight decrease. However, when subjected to heat treatment at 790°C, strength, plasticity, and ductility decrease with the increase of time, mainly due to the carbide coarsening such as M23C6 in the joint.

Avoiding microstructural instabilities in high tungsten containing superalloys by adjusting Al content

The effects of Al contents on the microstructures and mechanical properties of superalloy with high tungsten content were studied. During solidification, Al was found to be enriched in the residual liquid, which led to the transformation of eutectic morphology from reticular to bulk-like. When the Al content increased to 8 wt%, β-NiAl phase formed in the inter-dendritic region, Thermo-Calc calculations confirms such the experimental phenomenon. The absolute value of the lattice misfit between the γ/γ' phases and the elastic strain energy increases, then the size of the γ' phase increases when the Al concentration rises from 7 wt% to 8 wt%. When the Al content increased to 7 wt%, the tungsten-rich α-W phase is precipitated and the volume fraction is 1.20%. The volume fraction of the α-W phase increases to 7.84% when the Al content reached 8 wt%, which is consistent with the Thermo-Calc calculation results. After the room temperature tensile and stress rupture life test at 975 °C / 235 MPa, it revealed that the precipitation of hard and brittle α-W and β-NiAl phases with high Al content, which resulted in the mechanical properties decreased due to the crack initiation at multiple locations and then interconnected. The higher density of γ' phase with 6 wt% Al makes it difficult for dislocations to bypass. The site of crack initiation is decreased due to the absense of brittle phase. The stress rupture life can reach 49.21 h of the alloy.

Liquid metal compatibility of pre-oxidized FeCrAl in flowing Sn

A flowing Sn thermal convection loop (TCL) experiment was conducted in FeCrAlMo (Kanthal alloy APMT) tubing for 1000 h with a peak temperature of 400 °C. Oxide dispersion strengthened (ODS) Fe-(10–12)Cr-6Al and APMT specimens were exposed and all were pre-oxidized to form an external alumina scale. Unlike static exposures at 400° and 500 °C where pre-oxidation prevented dissolution, large mass losses were observed in the TCL hot leg after the specimens were cleaned in Li. The observed pitting suggests that localized failure of the pre-formed alumina surface oxide caused significant metal loss. Specimen mass gains were not observed on the cold leg implying that mass transfer did not occur and the small levels of Fe and Cr in the Sn after the experiment implies that Sn-rich phases may have formed. Post exposure room temperature tensile testing showed limited impact of the Sn exposure on the 12Cr ODS FeCrAl alloy, comparable to a 1000 h anneal in Ar at 400 °C. These results suggest that pre-oxidation is not sufficient to protect FeCrAl alloys in contact with flowing liquid Sn at 350°-400 °C.

Tensile behavior of the designed SiCf/Ti2AlNb/Ti17 composites

SiCf/Ti2AlNb-reinforced Ti17 (SiCf/Ti2AlNb/Ti17) composites were fabricated through hot isostatic pressing. The designed SiCf/Ti2AlNb/Ti17 composites exhibited excellent tensile properties, with tensile strengths of 1657 MPa at room temperature and 1830 MPa at 450 °C. The microstructure, fracture morphology, and longitudinal section of the composites were characterized by X-ray diffraction, scanning electron microscopy, and electron backscatter diffraction techniques. The fracture process of SiCf/Ti2AlNb/Ti17 composites during room-temperature tensile tests was as follows: the SiCf/Ti2AlNb reaction layer first fractured, the crack extended into the Ti2AlNb matrix, the Ti2AlNb matrix crack extended to the adjacent reaction layer and promoted fiber fracture, the crack extended to the Ti2AlNb/Ti17 interface, ultimately the Ti17 capsule fractured. The fracture process of SiCf/Ti2AlNb/Ti17 composites during 450 °C tensile tests was as follows: the SiCf/Ti2AlNb reaction layer first fractured, multiple fiber fractures occurred, and the crack in the reaction layer entered the Ti2AlNb matrix, the crack extended to the Ti2AlNb/Ti17 interface, eventually the Ti17 capsule fractured. The room-temperature tensile fracture was fitted to the LLS model, while the tensile fracture at 450 °C was fitted to the GLS model. The different fracture processes indicated that the SiCf/Ti2AlNb/Ti17 composites were suitable for exploiting fiber strength at 450 °C and therefore exhibited excellent tensile strength.

Role of Austenite Stability in Elevated Temperature Mechanical Properties of Gas Metal Arc-Directed Energy Deposition Austenitic Stainless Steels

A gas metal-directed energy deposition process was used to fabricate builds using two commercial weld fillers used in power generation applications, 16-8-2 and 316H. Microstructure stability and mechanical properties were investigated through room-temperature and elevated temperature tensile testing and creep testing at 650°C, 750°C, and 825°C. 16-8-2 exhibited reduced austenite stability which resulted in athermal martensite formation after aging at 650°C for 1000 h and strain-induced martensite formation during room-temperature tensile testing. 316H exhibited relatively higher austenite stability due to increased alloying content, resulting in no athermal martensite or strain-induced martensite. Due to lower austenite stability, ferrite formed during creep at 650°C in 16-8-2, which resulted in reduced creep life and lower creep ductility compared to 316H. At 750°C and 825°C, when ferrite is no longer thermodynamically stable, 16-8-2 exhibited longer creep life and similar creep ductility as 316H. The formation of ferrite in 16-8-2 appears to have a greater impact on creep performance than the formation of embrittling topologically close-packed phases like the σ phase, as 316H exhibited superior creep performance while predicted to form 14 vol.% σ phase at 650°C.

Evaluation of Microstructure and Mechanical Properties of TiAl/HI-TEMP 820/ SCM440H Materials Manufactured through Vacuum Brazing according to Process Temperature

The TiAl alloy is attracting attention as a lightweight and heat-resistant material, because of its high specific strength, excellent high-temperature formability, and fatigue strength. However, its applications are limited by its high unit price and low room temperature ductility. To overcome this issue, dissimilarly bonded materials have been extensively employed. This involves joining a brittle metal to a low-cost metal that possesses excellent plasticity, using various dissimilar bonding techniques. In this study, TiAl/HI-TEMP 820/SCM440H materials were fabricated using a vacuum brazing process under different temperature conditions. After the brazing process, the microstructure of the interfacial area revealed seven distinct layers resulting from chemical reactions between the base metals and the filler metal. These reaction layers consisted of a Ni solid solution, intermetallic compounds (Ti<sub>3</sub>Al, TiNi<sub>2</sub>Al, Ti<sub>2</sub>Ni, FeNi), and borides (CrB, TiB<sub>2</sub>, FeB). To analyze the effect of brazing temperature on the relationship between the microstructure and mechanical properties at the interface of TiAl/HI-TEMP 820/SCM440H materials, conventional uniaxial tests and nanoindentation tests were performed. The measured nanohardness exhibited a significantly large distribution for each reaction layer, with the highest hardness values observed in the intermetallic compounds and borides layers. Additionally, room temperature tensile tests confirmed that fractures initiated in the highhardness and brittle intermetallic compounds and borides layers.

Microstructural evolution and high-temperature strengthening mechanisms of the IN 738LC superalloy prepared by selective laser melting

Defect-free SLM-IN 738LC superalloy samples were fabricated using selective laser melting (SLM). The as-built samples exhibit a cellular structure with fine carbides, γ′ phases, and high-density dislocations at the cell boundary and nano-size γ′ phases in the interior, resulting in excellent deformation resistance and favorable mechanical properties at room temperature, with a yield strength (YS) of 970 MPa, an ultimate tensile strength (UTS) of 1263 MPa, and an elongation (EL) of 33%. This study investigates the evolution behavior of the γ′ phase at different solid solution treatment (SHT) temperatures and its impact on alloy strength and toughness. The 1020 + AHT sample exhibits a uniform distribution of coarse γ′ phases. The samples subjected to 1070 + AHT and 1120 + AHT display a bimodal distribution of γ′ phases. When the SHT temperature was further increased, fine spherical γ′ phases are evenly distributed in the 1160 + AHT and 1200 + AHT samples. Moreover, the γ/γ′ eutectics at the grain boundaries gradually dissolve with increasing SHT temperature, whereas the size and fraction of carbides increase. The room temperature tensile test of the 1070 + AHT sample demonstrated a favorable balance between strength and plasticity (YS = 1217 MPa, UTS = 1472 MPa, EL = 9.25%). This is attributed to the synergistic strengthening effect of the bimodal distribution of the γ′ phase. The temperature-dependent deformation behavior of the 1070 + AHT sample was analyzed at 23 °C, 800 °C, 900 °C, and 1000 °C. At 23 °C, numerous dislocations accumulated around the γ/γ′ interfaces. When the temperature increased to 800–900 °C, the dislocations shearing γ′ phases is activated and the alloy remained high strength. At 1000 °C, both the dislocation shearing and dislocation bypassing mechanisms coexist, resulting in a decrease in the strength of the alloy and an increase in its plasticity. This work provides scientific and theoretical support for SLM-IN 738LC parts with favorable properties.

Improving mechanical performance of heat-resistant eutectic Al-Fe-Ni alloy by in-situ TiB2 particles

The mechanical performance and strengthening mechanism of in-situ TiB2 ceramic particles reinforced eutectic Al-Fe-Ni alloy were researched. The smaller-sized TiB2 particles are uniformly distributed inside the grain and larger-sized particles tend to gather at the eutectic grain boundary. The hardness and room temperature tensile tests show that TiB2 particles improve the comprehensive mechanical properties of the eutectic alloy. Meanwhile, the high temperature tensile strength of eutectic alloys is also enhanced by particles. The pinning effect of TiB2 particles on dislocation and eutectic boundary are the main strengthening mechanisms. The study on the microstructures and properties of TiB2 particles reinforced eutectic alloys provides guidance for enhancing the mechanical performance of eutectic Al alloys.

Effect of Cooling Rate on the Microstructure and Mechanical Property of Nickel-Based Superalloy MAR-M247.

Heat treatment is an important process for optimizing the microstructures of superalloys, and the cooling rate after solid solution treatment is one of the most critical parameters. In this work, we treated solid solution MAR-M247 alloys with water quenching, air cooling, and furnace cooling. Microstructure characterization, hardness, and room temperature tensile tests were conducted to investigate the effect of cooling rate on the microstructure and mechanical properties of MAR-M247 alloys. The results showed that the cooling rate after solid solution treatment mainly affected the precipitation behavior of the secondary γ' phase, but it had few effects on other microstructure characterizations, including grain size, γ/γ' eutectic, and MC carbide. The water-quenched sample had the highest cooling rate (400 °C/s) and hardness (400 HV) but suffered from premature fracture because of quenching cracks. A further decrease in cooling rate from 1.5 °C/s to 0.1 °C/s deteriorated hardness (384 HV to 364 HV) and yield strength (960 MPa to 771 MPa) but increased elongation (8.5% to 13.5%). Moreover, the deformation mechanism was transformed from dislocation shearing to Orowan bypassing. The decreased yield strength was mainly due to the weakened precipitation strengthening resulting from γ'-phase coarsening. The improved elongation was attributed to not only the higher work-hardening index caused by interface dislocation networks but also the more uniform deformation, which delayed necking.

Effect of heat treatment on microstructure evolution, strengthening mechanisms and mechanical properties of Zn modified Al–Mg alloys with Sc and Zr additions

In this work, a new Al–Mg–Zn alloy was designed based on thermodynamic calculations combined with the concept of cooperative strengthening and toughening. The influence of different aging processes on microstructure and mechanical properties of the designed Al–Mg–Zn alloy was studied using experimental techniques including hardness testing, room-temperature tensile testing, EBSD, and HRTEM. The results indicate that after four different aging heat treatments, fine and uniformly distributed nano T-Mg32(Al, Zn)49 phases have precipitated within the grains. Compared to single-stage aging (140 °C/36 h, 180 °C/7 h), two-stage aging (90 °C/24 h + 140 °C/18 h, 90 °C/24 h + 180 °C/5 h) resulted in a smaller size and an increased number density of precipitated phases within the grains. After the aging process of 90 °C/24 h + 140 °C/18 h, the grains exhibited the smallest precipitated phases, being of 2.77 nm in size and a number density of 1.06 × 1023 #/m³. Additionally, TEM diffraction spot analysis identified the presence of Al3(Sc, Zr) precipitates within α-Al matrix. Under the aging process of 90 °C/24 h + 140 °C/18 h, the alloy exhibited superior mechanical properties with a tensile strength of 566 MPa, yield strength of 477 MPa, and elongation of 16.5%. The primary strengthening mechanisms of the presently investigated Al–Mg–Zn alloy include precipitation strengthening, solid solution strengthening, and grain boundary strengthening. A yield strength model considering various strengthening mechanisms is used to predict the yield strength based on thermodynamic calculations and experimental data. The research findings will offer theoretical insights guiding the design of compositions and heat treatment schedule for aluminum alloys.

Precipitation and tensile behaviors of Allvac 718Plus superalloy during long-term thermal exposure

Allvac 718Plus alloy is a promising Ni-based superalloy with excellent thermal stability and mechanical properties up to 704 °C. The γ′ coarsening, σ precipitation and the resulting degraded tensile properties at room and elevated (704 °C) temperatures during long-term thermal exposure at 704 and 760 °C are systematically investigated in this work. The results showed that a higher exposure temperature and a longer exposure time significantly accelerate the γ′ coarsening and σ precipitation. The σ particles were detrimental to the alloy by acting as the preferential nucleation sites for cracks, and the reduction in tensile strengths was mainly caused by the increasing size of γ′ precipitates during thermal exposure. Furthermore, there were significant differences between the deformation mechanisms of room-temperature and elevated-temperature (760 °C) tensile tests. Concerning the deformation mechanism of γ-matrix, planar slipping dominates during tensile testing at room temperature, while the formation of stacking faults and microtwinning are primary during tensile testing at elevated temperature (760 °C). The dominant precipitation strengthening mechanism depended on the γ′ characteristics of different thermal exposure conditions and the tensile temperature. The 704 °C/100 h exposure sample shows a dislocation shearing mechanism when tested at room or elevated (760 °C) temperature. Notably, the 760 °C/1000 h exposure sample exhibits a mixed precipitation strengthening mechanism of Orowan bypassing and superlattice stacking faults shearing when tensile tested at room temperature while showing a dislocation and superlattice stacking faults shearing mechanism when tensile tested at elevated (760 °C) temperature.

Effect of a Gradient Temperature Rolling Process on the Microstructure and Mechanical Properties of the Center of Ultra-Heavy Plates

As there is a small amount of deformation in the center during the rolling process of ultra-heavy plates, it is extremely easy to cause poor mechanical properties in the center. Increasing the deformation in the center is the most feasible method to eliminate the deformation effects in the cross-section of ultra-heavy plates. In this study, the gradient temperature rolling (GTR) process is compared with the traditional uniform temperature rolling (UTR) process. It is found that the GTR process can significantly increase the deformation in the center and thereby refine the grains.. The room temperature tensile test and instrumented Charpy impact test are used to test the strength at room temperature and impact energy at low temperature. Combined with the obtained impact load/energy displacement curve, the deformation and damage process under impact load are analyzed. The microstructure morphology and impact fracture obtained by different rolling processes in the center are analyzed by experimental methods such as OM, SEM, EBSD, etc. The prior austenite grain (PAG) boundary morphology is analyzed and the densities of grain boundaries are statistically quantified. The results showed that the strength, plasticity, and low-temperature toughness of the GTR process are improved compared to the UTR process, with increased dislocation density in the center microstructure, the density of PAG boundaries, and the density of packet boundaries. The size of the PAG in the center is refined by ~49%, the density of PAG boundaries increased by ~140%, the density of high-angle packet boundaries increased by ~39%, and the density of low-angle packet boundaries increased by ~49%. The crack propagation in the instrumented Charpy impact test of the GTR process showed stable expansion, indicating a ductile fracture compared to the semi-brittle fracture of the UTR process. The densities of PAG boundaries and high-angle packet boundaries are the most important factors affecting the strength and low-temperature toughness.

Study of self-heating and local strain rate in polyamide-6 and short fibre glass/polyamide-6 under tension through synchronised full-field strain and temperature measurements

This paper studies thermomechanical coupling during room-temperature tensile testing of polyamide-6 (PA6) and 50 wt% short glass fibre/PA6. The tests were performed for different fibre angles (0°, 45°, 90°), moisture contents (dry, 50%RH), and strain rates (10−4, 10−2, 10−1s−1). Digital image correlation (DIC) was coupled with infrared thermography. The contribution of the local strains, strain rates and temperatures to the global mechanical behaviour was investigated throughout deformation. The initial thermoelastic response was used to estimate the coefficient of thermal expansion. In PA6, neck development caused significant self-heating at high strain rates, differently between dry and 50%RH. In glass/PA6, however, temperature rises were always small (<+3 °C) despite local strain rate peaks in the fracture zone. This was ascribed to limited plastic deformation, as confirmed by post-mortem microscopy. The full-field data can be highly valuable for the development of advanced constitutive models for both pure polymer and short fibre composite.

Influence of Mg/Si ratio on the microstructure and formability of 6061 cast-rolled sheets

Three kinds of 6061 aluminum alloy cast-rolled plates with Mg/Si mass ratios of 1.23, 1.45, and 1.7 are prepared using NF6–300 vertical twin-roll casting and rolling mill. Metallurgical microscope (OM), x-ray diffraction (XRD), scanning electron microscope (SEM), energy dispersive spectrometer (EDS), room temperature tensile test, and other analysis methods studied the influence law and mechanism of Mg/Si ratio on the microstructure and texture of 6061 cast-rolled sheet. The best Mg/Si ratio cast-rolled sheet is then analyzed for formability. Research indicates that with increasing Mg/Si ratio, the grain size shows first decreasing and then increasing trend. Morphology of precipitated phases at the edge and the central of the sheet are different, however, the texture type has no obvious change, and a strong {112} 〈110〉 texture and weaker rotating cube {001} 〈110〉 texture is always observed. When the Mg/Si ratio is 1.45, the average grain size is the smallest, the edge size is 39.78 μm, the central size is 31.31 μm, and the precipitated phase is evenly distributed at the edge and central of the cast-rolled sheet. In addition, the elongation reaches 8.9%, the n-value is 0.072, and the r-value is 1.161 when the cast-rolled sheet reaches 90° along the cast-rolled direction under T6 state heat treatment.

Enhancing the strength of Sc-containing Al-Cu-Li alloys by modifying the precipitation behavior through plastic deformation and heat treatment

Al-Cu-Li alloys are used extensively in the aerospace manufacturing industry for their outstanding properties, however, the addition of trace Sc elements will form high-temperature insoluble (Cu, Sc)-containing phases during homogenization leads to a decrease in age-strengthening, and there is a limited investigation on how to overcome this unfavorable factor. In this work, the effects of plastic deformation coupled with heat treatment on the microstructure and mechanical properties of Sc-containing Al-Cu-Li alloy were systematically investigated by using age-hardening curves and room-temperature tensile tests combined with characterization by TEM, EBSD, and SEM/EDS. The results show that plastic deformation before high-temperature treatment fragments and promotes the dissolution of coarse phases, reduces the development of (Cu, Sc)-containing enriched phases, and simultaneously increases the content of Cu atoms within the Al matrix. Among all precipitates (GPB zones, S phases, T1 phases) after T6 aging, T1 phases act as the dominant strengthening phase of the alloy. At the same time, we found that the decrease in the Cu-containing residual phase after solid solution treatment will result in a remarkably increasing number density and volume fraction of T1 phases at the peak aging. In a word, we obtain a good strength-ductility trade-off (the yield strength is ∼500 MPa while maintaining ∼10% elongation) Al-Cu-Li alloy with Sc element by optimizing the distribution of Cu elements and modulating the aging precipitation behavior. Our proposed process is expected to provide an experimental and theoretical guide toward the development of Al-Cu-Li alloys with superior properties.

Microstructure and mechanical properties of a modified 316 austenitic stainless steel alloy manufactured by laser powder bed fusion

A 316 austenitic stainless-steel alloy, with modified alloy composition, manufactured by laser powder bed fusion (L-PBF) has been investigated. The modification of the alloy composition included addition of niobium (Nb), tungsten (W) and copper (Cu), together with a reduction in the amount of molybdenum (Mo) and an increased amount of carbon (C). To find suitable process parameters, a parameter study by varying laser power, hatch distance and scan speed was performed, centered on typical parameters used for normal 316 L. As-built material from a selected parameter configuration was then subjected to different stress relief annealing heat treatments and ageing heat treatments. The effectiveness of the stress annealing was ranked using a deformation-based method. Microstructural characterization, hardness and room temperature tensile testing were done to evaluate the effect of stress relief and aging heat treatments.It was found that a higher volumetric energy was needed to build dense material, about ∼50 % higher compared to the volumetric energy input for normal 316 L. A subsequent aging heat treatment at 725 °C for 3 h increased the strength and hardness of the material. A reinforcement of the cellular microstructure by precipitation of carbides in between the cells is believed to be the main reason for this. To completely alleviate the residual stresses it was necessary to carry out a stress relief annealing process at 950 °C, which resulted in a removal of the cellular structure and a lower strength material.

Effect of CaO Content and Annealing Treatment on the Room-Temperature Mechanical Properties of AZ61 and AZ61-CaO Alloys

In the present study, the effect of annealing treatment on the room-temperature mechanical performance of AZ61, AZ61-0.5CaO and AZ61-1CaO was thoroughly investigated. The as-rolled samples were annealed at 400 °C for 1 h followed by furnace cooling. Microstructural characterization was carried out using optical microscopy (OM), scanning electron microscopy (SEM), transmission electron microscopy (TEM) and electron back-scattered diffraction (EBSD). Moreover, room-temperature uniaxial tensile tests were carried out on the non-annealed and annealed samples along the rolling direction at the strain rate of 10−3 s−1. Microstructural analysis showed the presence of profuse {101¯2} twinning in non-annealed samples and the twinning fraction was increased by the addition of CaO content. SEM showed the formation of precipitates in the AZ61-CaO alloys and TEM confirmed the precipitates to be (Mg, Al)2Ca. The room-temperature tensile tests showed that the mechanical properties of AZ61 were slightly reduced by the addition of CaO, which was attributed to the higher local stress concentration due to the twin–twin interactions. Furthermore, the annealing treatment led to significant enhancement in the YS and UTS of AZ61-1CaO, which is related to the precipitation hardening induced by the intermetallic precipitates.

On the importance of nano-oxide control in laser powder bed fusion manufactured Ni-based alloys to enhance fracture properties

In this study, a series of Ni-Cr-Mo based alloys (IN625, C22 and NA282), sourced from various powder suppliers and having varying oxygen concentrations (200 ppm to 800 ppm), were printed under similar conditions using laser powder bed fusion (LPBF). Nano-oxides in the range of 20–50 nm were observed in each alloy in the as-printed state, albeit with different number densities. Room temperature tensile tests and liquid nitrogen (LN2, -196 °C) Charpy impact tests were performed to assess the mechanical response of each alloy. The IN625 and C22 samples showed considerably lower impact energy values than the NA282, as well as lower room temperature post-necking tensile elongations, despite showing similar yield strengths and strain hardening behaviours. Fracture surface dimple sizes were shown to correlate with nano-oxide particle spacings. Nano-oxides were found in the middle of most dimples on fracture surfaces suggesting that the nano-oxides are the sites of void nucleation during fracture, despite being only 20–50 nm in size. The precursor metal powders were identified as the primary origin of the nano-oxide particles. This study highlights the critical, detrimental role that nano-oxides, typically present in LPBF metals, have on the fracture properties of these materials. In the case of the Ni-Cr-Mo alloys examined here, limiting oxide formation is critical to achieving optimal fracture properties.

Study on surface quality of ultrasonic assisted underwater laser polishing

Surface polishing is a critical factor in optimizing the practical utility of laser additive manufacturing (LAM) parts. This study explores the use of nanosecond pulsed lasers for polishing the surface of laser-directed energy deposition (LDED) TA15 titanium alloy. Various polished surfaces are achieved by adjusting laser processing parameters. Analytical techniques, including scanning electron microscopy (SEM), X-ray diffraction (XRD), laser confocal microscopy, and ultra-depth of field microscopy, are employed to assess phase composition, oxidation levels, and surface roughness under different laser polishing conditions. The study also investigates changes in surface properties and the underlying mechanisms that occur during laser polishing. Microhardness tests and room temperature tensile tests are conducted to evaluate the compressive strength and tensile strength of the TA15 alloy's surface after laser polishing. The results demonstrate that different laser polishing methods yield varying surface quality and mechanical properties. Notably, ultrasonic assisted underwater laser polishing (UWP) exhibits significantly lower surface roughness Ra (3.035 μm) and Sa (3.172 μm) compared to air laser polishing (AP) and underwater laser polishing (WP), indicating improved surface quality and enhanced surface strength and plasticity.