Search

The microstructure evolution and tensile properties of laser directed energy deposition (LDED) 300 M steel with isothermal heat treatment at 320 ℃ for different holding time were investigated in this paper. The results showed that the microstructure of the as-deposited LDED 300 M steel consisted of martensite and bainite. After heat treatment, the microstructure of the LDED 300 M steel is still the mixture of martensite, bainite and retained austenite, while the size of the lower bainite significantly decreased. As the increasing holding time, the content of martensite gradually decreased, while the content of bainite and austenite gradually increased. When the holding time of LDED 300 M steel increased from 20 min to 40 min, the number and size of austenite increased slightly. However, when the holding time was 60 min, the number and size significantly increased. As holding time continuously increase, it contributes little to the number and size of austenite. With regard to the hardness of the LDED 300 M steel, it was 464 HV when the holding time was 20 min. With the increment of holding time, the hardness gradually decreased until 428 HV when the holding time was 80 min. While holding time had less effect on the tensile strength and yield strength. To be specific, the tensile strength decreased from 1348 MPa to 1332 MPa, and yield strength decreased from 853 MPa to 759 MPa. The elongation gradually increased, and it increases to 33 % when the holding time was 80 min, which was higher than that of the forged 300 M steel standard (15 %). Effect of various factors, like microstructure, dislocation density and e-carbides precipitated in the bainite on the ductility of the LDED 300 M steel with isothermal heat treatment was also studied.

Laser remelting can be used not only to improve the metal surface structure, but also to repair metal surface defects. 34CrNiMo6 belongs to European standard high strength alloy structural steel. It has been widely used in the manufacture of large-size shaft parts which are prone to defects such as cracks and wear during service. This study proposed a repair method for these defects by laser remelting the filled block and then performed heat treatment. The microstructure, m9icrohardness and tensile strength of the repaired sample and heat-treated sample were analyzed using scanning electron microscope (SEM), Vickers hardness tester and universal tensile tester. The final results showed that the metallurgical bonding between the substrate and the filler was good. The microstructure of repaired sample could be divided into three regions: I, II, and III. The main microstructure in region I and II was tempered martensite, and in region III was bainite and martensite. Different regions of repaired sample presented uneven hardness distribution, and the fracture mode was quasi-cleavage fracture. However, after heat treatment, the microstructure of the sample was tempered martensite. Its hardness became uniform, and the fracture mode of the heat-treated sample was ductile fracture. The average hardness value of the heat-treated sample was 296.4HV0.2, which was slightly higher than the substrate. The tensile strength value of the heat-treated sample reached 972.91MPa, which was about 99.3 % of substrate, and its elongation δ was 14.25 %. Overall, the mechanical properties were close to those of the forging.

In order to achieve the microstructure homogeneity at a lower temperature and improve the mechanical properties, a novel post heat treatment was introduced to Inconel 718 fabricated by laser direct energy deposition (LDED), by means of δ aging treatment + solution treatment. The microstructure analysis shows that the fully precipitation of δ phase in LDED Inconel 718 superalloy can “cut” the long strip Laves phase into small pieces. According with the δ phase solution treated at 1020 °C for 30 min, only tiny particle Laves phase was remained. The equilibrium volume fraction of Laves phase after δ phase solution treatment is about 1%, which is much lower than that of the as-deposited sample or the sample only solution treated at 1020 °C. Due to the solution strengthening and precipitation of γ″ phase during aging treatment, the tensile strength of δ aging + δ solution + double aging treated samples was 10.27% higher than that of directed solution + double aging treated samples, as well as a 31.38% and 52.71% increases in elongation and the reduction of area during tensile tests.

Uniaxial tensile tests and stress-controlled low-cycle fatigue (LCF) and creep–fatigue interaction (CFI) tests of Inconel 625 alloy manufactured by selective laser melting (SLM) were performed at 815 °C in air environments. The microstructure was characterized by optical microscopy and scanning electron microscopy after testing. The results confirmed that significant embrittlement and large scatter in LCF life are resulted from manufacturing defects. The CFI life is decreased sharply to approximately dozens of cycles with the accumulated creep strain; however, the selected dwell time (i.e., 60 s and 300 s) exhibits low sensitivity to the fracture time and elongation to failure. The embrittlement of SLM Inconel 625 was proposed to be due to the low grain uniformity and precipitation of carbides at the grain boundaries. Due to the quality of the SLM process, the accelerated initiation and propagation of fatigue crack are caused by the present unmelted powder particles, which result in the large dispersion of LCF life. Meanwhile, due to the accumulation of creep damage, cracks in the CFI test are initiated along the grain boundaries and then linked together, contributing to a significant decline in fatigue life.

AZ91D magnesium alloy matrix composites reinforced with nano SiC particles (n-SiCp) was fabricated by Friction Stir Processing, and the effects of n-SiCp on the β-Mg17Al12 phase precipitation at 180 °C as well as the age hardening response of AZ91D were investigated by X-ray diffraction, optical microscopy, scanning electron microscopy, transmission electron microscopy, and micro Vickers hardness measurements. The results show that, with the addition of n-SiCp, the microstructure of AZ91D after FSP was greatly refined, the texture strength was decreased, and the age hardening efficiency became inferior. As n-SiCp can efficiently block the migration of grain boundaries and grain growth, the composite exhibited a higher microstructure stability at elevated temperatures, and the discontinuous precipitation of β-Mg17Al12 phase that dominated the age process of AZ91D at 180 °C was remarkably restricted in the composite. Besides, n-SiCp promoted the continuous precipitation of β-Mg17Al12 phase and the precipitation rate in the composite was faster at the initial aging stage, but on further aging the precipitation rate became gradually slower than that in the AZ91D. The underlying mechanisms for the different age precipitation behavior and age hardening response were also discussed.

In this paper, crystallization characteristics of amorphous Ge2Sb2Te5 (a-GST) films induced by a Gaussian picosecond laser with different laser fluence were carried out using transmission electron microscopy (TEM), Raman spectra and ab initio molecular dynamics (AIMD) simulations. TEM observations presented a solid-state phase transition with nanocrystalline microstructure at lower fluence, while a liquid-solid phase transition accompanied by an ingot-like crystalline microstructure was found at higher fluence, including a central coarse and an outer fine equiaxed regions as well as a columnar crystal region between them. In spite of the remarkable difference in microstructure, the composition of different regions kept constant from the Energy Dispersive Spectrometer (EDS) measurements. Different phase change behavior and laser fluence affected the shift of Raman peaks due to the phase change stress and thermal stress. A laser irradiation coupled AIMD simulation was then developed to study the preferential nucleation possibility. AIMD simulation results indicated that a thermal process in the central coarse equiaxed regions, with a higher cooling rate (10 k/ps) generated by the Gaussian picosecond laser irradiation, reduced the nucleation rate remarkably, causing crystal nucleus have enough space to grow up even to the micrometer scale. This is different from the morphology induced by nano- or femtosecond pulsed laser.

Because of the excellent mechanical properties of 34CrNiMo6 steel, it is widely used in high-value components. Many conventional approaches to strengthening-steels typically involve the loss of useful ductility. In this study, 34CrNiMo6 Steel having high strength and ductility is produced by laser solid forming (LSF) with a quenching-tempering (QT) treatment. Tempering of bainite is mainly by solid phase transformation in the previous LSF layers during the LSF process. The stable microstructure of LSF consists of ferrite and fine carbides. The microstructure transfers to tempered sorbite after heat-treatment. The tensile properties of the LSF steel meet those of the wrought standard. The UTS and elongation of LSF sample at 858 MPa, 19.2%, respectively, are greater than those of the wrought. The QT treatment enhanced the ultimate tensile strength and yield strength of the LSF sample. The ultimate tensile strength, yield strength, reduction in area, and elongation of the LSF+QT sample at 980 MPa, 916 MPa, 58.9%, and 13.9%, respectively, are greater than those of the wrought standard. The yield strength of the LSF+QT sample is approximately 1.27 times that of the wrought. The LSF samples failed in a ductile fracture mode, while the LSF+QT samples showed mixed-mode failure. The defects have only a small effect on the tensile properties owing to the excellent ductility of the LSF sample.

Long-striped Laves phases are considered to be crack sources in laser repaired Inconel 718 during stress rupture test. In order to obtain fine Laves phases, Nd:YAG pulse laser is used to fabricate Inconel 718 superalloy on a wrought Inconel 718 substrate in this study. The microstructures and stress rupture properties of pulse laser repaired Inconel 718 superalloy after DA treatment (720 °C × 8 h/furnace cooling at 50 °C/h to 620 °C × 8 h/air cooling) and 900STA treatment (900 °C × 20 min/air cooling+720 °C × 8 h/furnace cooling at 50 °C/h to 620 °C × 8 h/air cooling) are investigated by using SEM and TEM. The results shows that granular Laves phases can be obtained when pulse laser is used. The formation of granular Laves phase is related to high cooling rate (G·R) and low ratio of temperature gradient to growth rate (G/R). The critical values of G·R and G/R are 104 and 107, respectively. After 900STA treatment, δ phase precipitates around the granular Laves phases. But there is no δ phases existing in the DA samples. The surrounding δ phases can induce stress concentration, resulting in that the elongation of the 900STA samples is worse than that of the DA samples. However, the regions denuded in Nb element around the δ phases can delay the formation of the vacancies, which leads to that the stress rupture life of the 900STA samples is better than that of the DA samples.

This work investigated the microstructure and its influence on room-temperature fatigue crack growth (FCG) behavior of Inconel 718 (IN718) alloy fabricated with laser-based directed energy deposition (L-DED). As-deposited specimens were heat-treated by direct aging (DA), solution treatment + aging (SA) and homogenization + solution treatment + aging (HSA) respectively to investigate the effect of post-heat-treatment. On comparison with as-deposited microstructure, which mainly consists of γ dendrite and Laves phases in the interdendritic regions. Dendritic segregation still appears after DA heat-treatment, which is certified to alter the precipitation kinetics of γ′′/γ′ precipitations of IN718 and induce the formation of more abundant γ′′/γ′ precipitations in interdendritic Nb-rich regions versus the dendritic trunk regions. After SA heat-treatment, Nb released from the dissolution of Laves phase is prone to diffuse to the ambient interdendritic regions, which facilitates the formation of δ-phase and the homogeneous precipitations of γ′′/γ′ phases in dendritic trunk region. After HSA heat-treatment, the Laves phases are eliminated completely, the dispersive γ′′/γ′ precipitates (~30 nm) are distributed within the bimodal recrystallized grains. The results of FCG testing reveal that as-deposited specimen possesses the lowest threshold for fatigue-crack initiation (ΔKth) and the highest FCG rate (FCGR). The ΔKth and FCGR of DA specimen do not have the notable improvement versus those of as-deposited specimen, due to its inferior strength-plasticity mainly caused by heterogeneous γ′′/γ′ phases and Laves particles. The fatigue-crack propagates easily along the precipitate-free zone around short-acicular δ-phase, which causes the crack branching and deflection, enhances the resistance to FCG of SA specimen to some extent. The ΔKth of HSA specimen increases by ~2.6 times compared with as-deposited specimen and reaches 10.8 MPa∙m1/2, due to its decent strength-plasticity (~1130 MPa, ~26.5%). Especially, the ΔKth and FCGR of HSA specimen are comparable to that of the wrought IN718. The FCG behaviors of all specimens predominantly occur in the transgranular ductile fracture mode. Overall, the present study demonstrates that a judicious post-heat-treatment can enhance the damage tolerance of L-DED IN718 by increasing ΔKth and decreasing FCGR.

Second-phase particle pinning has been well known as a mechanism impeding grain boundary (GB) migration, and thus, is documented as an efficient approach for stabilizing nanocrystalline (NC) materials at elevated temperatures. The pinning force exerted by interaction between small dispersed particles and GBs strongly depends on size and volume fraction of the particles. Since metallic oxides, e.g. Al2O3, exhibit great structural stability and high resistance against coarsening at high temperatures, they are expected as effective stabilizers for NC materials. In this work, NC composites consisting of NC Fe and Al2O3 nanoparticles with different amounts and sizes were prepared by high energy ball milling and annealed at various temperatures (Tann) for different time periods (tann). Microstructures of the ball milled and annealed samples were examined by X-ray diffraction and transmission electron microscopy. The results show that the addition of Al2O3 nanoparticles not only enhances the thermal stability of NC Fe grains but also reduces their coarsening rate at elevated temperatures, and reducing the particle size and/or increasing its amount enhance the stabilizing effect of the Al2O3 particles on the NC Fe grains.

The coarse columnar grains and special interface in laser solid formed (LSFed) Inconel 718 superalloy workpieces seriously affect their mechanical properties. To improve the microstructure and mechanical properties of LSFed Inconel 718 superalloy, electromagnetic stirring (EMS) was introduced to alter the solidification process of the molten pool during LSF. The results show that EMS could not completely eliminate the epitaxially growing columnar grains, however, the strong convection of liquid metals can effectively influence the solid–liquid interface growing mode. The segregation of alloying elements on the front of solid–liquid interface is inhibited and the degree of constitutional supercooling decreases correspondingly. Comparing the microstructures of samples formed under different process parameters, the size and amount of the γ+Laves eutectic phases formed in interdendritic area decrease along with the increasing magnetic field intensity, resulting in more uniformly distributed alloying elements. The residual stress distribution is proved to be more uniform, which is beneficial to the grain refinement after recrystallilzaiton. Mechanical properties testing results show an improvement of 100 MPa in tensile strength and 22% in elongation was obtained after EMS was used. The high cycle fatigue properties at room temperature was also improved from 4.09 × 104 cycles to 8.21 × 104 cycles for the as-deposited samples, and from 5.45 × 104 cycles to 12.73 × 104 cycles for the heat treated samples respectively.

Ni-Sn anomalous eutectic is obtained by single track laser cladding with the scanning velocity from 1 mm/s to 10 mm/s using the Ni-32.5 wt.%Sn eutectic powders. The microstructure of the cladding layer and the grain orientations of anomalous eutectic were investigated. It is found that the microstructure is transformed from primary α-Ni dendrites and the interdendritic (α-Ni + Ni3Sn) eutectic at the bottom of the cladding layer to α-Ni and β-Ni3Sn anomalous eutectic at the top of the cladding layer, whether for single layer or multilayer laser cladding. The EBSD maps and pole figures indicate that the spatially structure of α-Ni phase is discontinuous and the Ni3Sn phase is continuous in anomalous eutectic. The transformation from epitaxial growth columnar at bottom of cladding layer to free nucleation equiaxed at the top occurs, i.e., the columnar to equiaxed transition (CET) at the top of cladding layer during laser cladding processing leads to the generation of anomalous eutectic.

In this study, 20 mm thick AA7075-T6 alloy plates were joined by friction stir welding. The microstructure and mechanical properties of the nugget zone along the thickness direction from the top to the bottom was investigated. The results showed that the microstructure including the grain size, the degree of dynamic recrystallization, the misorientation angle distribution and the precipitation phase containing its size, type and content exhibited a gradient distribution along the thickness direction. The testing results of mechanical properties of the slices showed that the nugget was gradually weakened along the depth from the top to the bottom. The maximum ultimate tensile strength, yield strength and elongation of the slice in the nugget top-middle are obtained, which are 415 MPa, 255 MPa and 8.1%, respectively.

Nanocrystalline (NC) materials are expected to exhibit excellent irradiation resistant property because of their high volume fraction of grain boundaries. However, the NC materials are usually suspicious to grain coarsening at elevated temperatures. This strongly restricts their applications as practical irradiation resistant materials which are expected to serve at relatively high temperatures. In this work, the irradiation resistant property of a thermally stable NC Fe-1 at.% Zr alloy was examined. The NC alloy was prepared by ball milling method. The irradiation experiments were carried out in a Helium Ion Microscope by 28.4 keV He+ ions to 1 × 1017, 2 × 1017 and 4 × 1017 ions cm−2 at room temperature. The microstructures of the specimens were examined in a Transmission electron microscope. No He bubbles were observed in the irradiated NC Fe-1 at.% Zr alloy, whereas high number density of He bubbles were found to appear in the coarse grained Fe-1 at.% Zr alloy. The results indicate that the NC Fe-1 at.% Zr alloy exhibits an excellent irradiation resistance.

In this paper, creep-fatigue behaviors of the thin-walled tubular specimen with film holes were experimentally studied to address the influence of cyclic stress interruptions on creep strength and creep-fatigue life of a directionally solidified superalloy. The specimens with and without multiple holes were designed and manufactured to consider the effect of stress concentration produced by film cooling holes. The creep life of the specimen with holes has reduced significantly with the introduction of fatigue. An interesting result was found that the creep strength and life was enhanced on the stress concentrated specimens even if the creep loading was interruption by fatigue loadings, while creep fatigue life will be commonly shortened for notched specimen. Failure mechanisms related with cooling holes were analyzed on the basis of microscope observation.

The aim of this study was to elucidate cyclic hardening behavior and deformation mechanisms of the strain-hardened zones of friction-stir-welded (FSWed) dissimilar AA5083-to-AA2024 aluminum alloy joints (with a heterogeneous microstructure) after cyclic deformation via quaasi-in-situ electron backscatter diffraction (EBSD) technique and transmission electron microscopy (TEM) examinations. The extent of cyclic hardening was observed to decrease with increasing number of cycles at a constant total strain amplitude. Strain localization occurred mainly in the stir zone (SZ) and heat-affected zone (HAZ) on the AA5083 side, accompanied by an increasing density of dislocations during cyclic deformation identified via both EBSD and TEM. The strain hardening capacity of AA5083 SZ was significantly higher than that of AA5083 HAZ based on the density of geometrically-necessary dislocations (GNDs). The {111}-oriented grains exhibited a higher strain hardening capacity than other grains. Cracks initiated and propagated in the AA5083 HAZ along with the presence of shear bands. The evolution of GNDs played a predominant role in the AA5083 SZ, while the evolution of statistically-stored dislocations (SSDs) was dominant in the AA5083 HAZ during cyclic deformation.

In this study, the effect of solution temperature on the microstructure and mechanical properties of Hastelloy X fabricated by laser directed energy deposition (LDED) was investigated. As a result, the microstructure of as-deposited (AD) sample was mainly coarse columnar grains directionally growing along the deposition direction. Due to the segregation of Mo and Cr, a lot of Laves phase precipitates in the inter-dendritic. With the increase of solution temperature, the volume fraction of Laves phase gradually decreases. When the solution temperature increased to 1200 °C, the grains of the LDED Hastelloy X superalloy exhibited fully recrystallization, and then became coarse with the solution temperature of 1225 °C. With the dissolution of the Laves phase, the microhardness of the samples drops from 231 ± 5 HV to 211 ± 5 HV. The room temperature tensile results reveal that the as-deposited sample with irregular and long-striped Laves phase presents the worst tensile strength (652 ± 10 MPa) and elongation (30 ± 2%). After heat treatment, the strength and elongation of the LDED Hastelloy X superalloy gradually increased with increasing the solution temperature. When the solution temperature was 1200 °C, the tensile strength of the sample reached the maximum value of 720 ± 1 MPa. With the increase of solution temperature, the elongation increased and then reached the maximum value of 47 ± 2% with heat-treated at 1225 °C. The tensile fracture of LDED Hastelloy X superalloy with solution treatment presented a ductile fracture pattern.

Dissimilar Al–Li alloys (2198 and 2060) were laser welded without the addition of filler material. The effects of welding parameters on the formation of the welded joints, microstructure evolution, solute segregation, porosity and their relationships with the mechanical properties of the joint were investigated. It was found that reducing the weld heat input can effectively prevent grain coarsening, while decreasing porosity and reducing the tendency for hot cracking, thereby enhancing the properties of the joint. The dissolution of precipitates such as the T1 phase and θ′ phase in the base metal, and the variations in Cu and Mg content between grain interiors and boundaries, resulted from solute segregation during welding, leading to reductions in the strength of the weld.

K465 nickel-based superalloy is widely used in the manufacturing of hot section components of aeroengines such as turbine blades and guide blades due to its excellent high temperature strength and superior corrosion resistance. In the present work, K465 superalloy deposits are fabricated on 316 L stainless steel substrate by laser solid forming technique. The microstructure and tensile properties of the as-deposited samples are investigated. The results show that the as-deposited K465 samples mainly consist of columnar dendrites which grow epitaxially from the substrate and the dendrite are fine with the average primary dendrite arm spacing of 18 µm, only a few fine equiaxed grains appear at the top of the deposit. The γ′ particles in the deposit seem uniform in spherical shape and are much finer than that in the as-cast K465 samples. The average hardness of the as-deposited samples is measured as 468 HV, which is higher than that of the as-cast K465 sample. The tensile strength and the yield strength of the as-deposited samples are also high and measured to be 1205.4 MPa and 917.4 MPa, while their elongation is up to 8.5%.

In this research, the effect of microstructure on the fatigue crack growth rate (FCGR) of laser solid formed (LSFed) 300M steel was investigated. The results showed that the as-deposited microstructure of LSFed 300M steel consisted of martensite and coarse bainite. After heat treatment, the microstructure became homogeneous and was primarily composed of tempered martensite, bainite and a small amount of retained austenite. The FCGR of LSFed 300M steel gradually increases with increasing stress ratio (for a given stress intensity factor range and stress ratios between 0.1 and 0.5). The refined austenite grain of heat-treated LSFed 300M steel reduced the fatigue crack growth threshold (FCGT) and the FCGR. The fractography of as-deposited LSFed 300M steel was different from that of heat-treated LSFed 300M steel.

Laser repairing technology, based on laser additive manufacturing technology, has been used in repairing damaged components for saving time and economic cost. Due to the existence of laser deposited zone, the microstructure and mechanical properties of laser repaired (LRed) components are different from original wrought parts. In this paper, the tensile deformation behavior of LRed Inconel 718 was compared with that of wrought Inconel 718 by using digital image correlation (DIC) method. The microstructure and the failure mechanism of LRed Inconel 718 were also investigated. The results showed that the LRed Inconel 718 sample could be divided into two parts: laser deposited zone (LDZ) and substrate zone (SZ). The laser deposited zone consisted of columnar dendrite growing epitaxially along the deposition direction, where the γ″ phases gathered around the interdendritic Laves phases. However, the microstructure of the substrate zone was equiaxed grain with the uniform distribution of δ and γ″ phases. It means that the microstructure of the LRed Inconel 718 was non-uniform. The deformation compatibility of the LRed Inconel 718 was inferior to that of the wrought Inconel 718 because of the existence of the Laves phases and the heterogeneous distribution of the γ″ phases. As a result, strain concentrated in the laser deposited zone resulting in that most plastic deformation was clustered in this region. The hard and brittle Laves phases slipped and broke up into smaller parts due to local stress concentration. Microscopic holes formed at the interface of the Laves phases and the γ matrix and combined together to cause the fracture of the LRed Inconel 718.

Fabricating an Inconel 718 superalloy using electromagnetic stirring assisted laser solid forming (EMS-LSF) is a novel method to modify its microstructure and mechanical properties by consuming the Nb element in the γ phase to alleviate interdendritic segregation. The precipitate of the δ phase at 950 °C after EMS-LSF can help to achieve the uniform diffusion of Nb, and can also improve its mechanical properties. The precipitation behavior of the δ phase in an EMS-LSF Inconel 718 superalloy with different heat treatment processes has been investigated. The results show that the morphology of the δ phase changes from rod-like to a long-needle shape and tends to grow from the inter dendrite to the core dendrite with electromagnetic field intensity increasing, which is accompanied by the “cutting” and “dissolution” of the Laves phase. Through precipitation kinetics analysis, the precipitation rate of the δ phase is seen to increase with the electromagnetic field intensity increasing. Under a combination of electromagnetic stirring and laser solid forming, the microhardness of the Inconel 718 samples increased slightly due to the fact that a higher content of Nb was distributed in the core dendrite resulting from the serious convection of liquid metal, which can strengthen the matrix.

In this study, Inconel 718 (IN718) specimens, manufactured by laser directed energy deposition (LDED), are subjected to three different heat-treatments to have different microstructural characteristics. Fatigue crack growth (FCG) testing is performed on both as-deposited and heat-treated specimens. The coarse Laves phases with uneven γ′′/γ′ precipitates severely decrease FCG resistance of LDED IN718. The soft precipitate-free zone around short-acicular δ-phase partly reduces FCG resistance of LDED IN718. The FCG threshold ( Δ K th ) of a judicious heat-treatment specimen increases by ~ 2.6 times vs. as-deposited specimen and reaches 10.8 MPa∙m1/2, and its Δ K th and FCG curve are comparable to those of wrought IN718.

The influence of isothermal treatment on the microstructure and mechanical properties of laser solid formed (LSFed) 300 M steel was investigated within the isothermal temperature range of 280–440 ℃. The result showed that the microstructure of LSFed 300 M steel isothermal with treatment of 440 ℃ consisted of upper bainite and small amount of martensite. As the decreasing isothermal temperature, the amount of upper bainite gradually decreased, the content of the martensite and lower bainite gradually increased. In addition, the superledges have been formed in the lower bainite. Microstructure of LSFed 300 M steel was almost martensite when the isothermal temperature was 280 °C. The hardness in LSFed 300 M steel presented a significant change with the decrement of isothermal temperature. The hardness decreased positively with isothermal temperature when it is more than 400 ℃; while, it increased inversely with the decreasing the isothermal temperature when the temperature is lower than 400 °C. The tensile strength and yield strength changed little when the isothermal temperature decreased from 440 °C to 320 °C, while sharply increased when the isothermal temperature was 280 °C. The elongation of LSFed 300 M steel was the highest (27 %) when the isothermal temperature was 320 °C.

The influence of post-heat-treatment on the resulting microstructure and room-temperature fracture toughness ( K I C ) of Inconel 718 fabricated by laser solid forming (LSF), a kind of laser based directed-energy-deposition additive manufacturing technologies, is investigated. Detailed microstructure characterization was performed on as-fabricated and heat-treated samples using direct aging (DA), solution treatment plus aging (STA), homogenization plus STA (HSTA), respectively. The results indicate that as-fabricated sample mainly consists of γ columnar dendrites and a small quantity of (γ+Laves) eutectics in the interdendritic areas. After DA post-heat-treatment, the nonuniform γ′′/γ′ precipitates exist around Laves phases. After STA post-heat-treatment, the short-acicular δ-phases precipitate around/in the Laves phases, the micro-segregation reduces and the distribution of γ′′/γ′ precipitates in the dendrite arm is almost homogeneous. After HSTA post-heat-treatment, Laves phase mostly disappears, micro-segregation completely removes and the γ′′/γ′ precipitates (~30 nm) are distributed in bimodal recrystallized grains. The results of K I C testing indicate that as-fabricated sample possesses the lowest K I C mainly due to its lowest elastic modulus and yield strength. However, the K I C of DA sample (~89.8 MPa m ) is close to that of as-fabricated sample, mainly due to its low ductility and strain-hardening exponent. But, the K I C of STA and HSTA samples increases by ~55.8% and 90.8% compared with as-fabricated sample respectively, mainly because of the better performance in strength–plasticity of HSTA sample (~1114 MPa, ~26.1%). Particularly, the K I C of HSTA sample with bimodal grains is ~60 MPa m higher than the lower limit of wrought IN718 (AMS 5662) and reaches ~164.1 MPa m . There exists significant difference in the K I C fractographs of as-fabricated and heat-treated samples. Overall, this research illustrates that an appropriate post-heat-treatment possesses obvious toughening effect on the LSF Inconel 718 alloy.

To improve the high temperature oxidation resistance of TA2 titanium alloy, a gradient Ni-Ti coating was laser cladded on the surface of the TA2 titanium alloy substrate, and the microstructure and oxidation behavior of the laser cladded coating were investigated experimentally. The gradient coating with a thickness of about 420–490 µm contains two different layers, e.g. a bright layer with coarse equiaxed grain and a dark layer with fine and columnar dendrites, and a transition layer with a thickness of about 10 µm exists between the substrate and the cladded coating. NiTi, NiTi 2 and Ni 3 Ti intermetallic compounds are the main constructive phases of the laser cladded coating. The appearance of these phases enhances the microhardness, and the dense structure of the coating improves its oxidation resistance. The solidification procedure of the gradient coating is analyzed and different kinds of solidification processes occur due to the heat dissipation during the laser cladding process.

Four tools with taper (TA), triangular (TB), three-grooves (TC), and square (TD) pin-tip profiles were designed to friction stir weld (FSW) thick AA7075-T6 alloy plates aiming at improving the local and overall mechanical properties of the joints. The microstructure evolution, hardness, and mechanical properties of the FSW joints were studied by scanning election microscope (SEM), X-ray diffractometer (XRD), and transmission electron microscope (TEM). The results show that equiaxed grains in the bottom nugget zone (BNZ) obtained by using TC are noticeably refined compared to in the case of other tools. Also, finer and more dispersive precipitates are distributed in this BNZ, and higher contents of η phase and dislocations are observed. A similar trend is found in the middle of weld. The bottom slices welded by employing TC show superior strength and ductility with a highest ultimate tensile strength of 388 MPa, yield strength of 315 MPa, and elongation of 7.9 %, which are in agreement with the hardness profiles. This is owing to ultrafine grains, more high-angle grain boundaries, more dispersive and higher contents of precipitates, and optimum hardness distribution. Accordingly, the global mechanical properties of the overall joints prepared by using TC significantly improve.

Presetting Stellite X-40 alloy powder on the surface of a K403 casting superalloy plate was laser re-melted to obtain a protective cladding layer. The microstructures, interface characteristics and influence of defocusing amount on the microstructure were investigated. The results shown that, a sound bonding between the cladding layer and the substrate was obtained, and no cracks or pores were formed in the cladding layer. The molten pool depth when a negative defocusing amount was used was larger than that when a positive defocusing amount was used. The structure of the cladding layer was not uniform but stratified into several layers. Equiaxed grain was observed in the central area of the cladding layer, and a transition layer existed between the cladding layer and the substrate. An abnormal distribution of Al, Ti elements was detected and the concentration of Al, Ti is much higher than that of the cladding layer and the substrate.

Wire and arc additive manufacturing (WAAM), using cold metal transfer (CMT) as heat source, exhibits a great potential for additive manufacturing of magnesium alloys due to low heat input. With the purpose of revealing the relationship between the microstructure and mechanical properties of WAAMed AZ31 material, the present study has been carried out. The manufactured AZ31 thin-walled deposit is mainly composed of columnar dendrite arrays, including dendritic α-Mg matrix, interdendritic eutectics (α-Mg and β-Mg17Al12) and some dispersive η-Al8Mn5 phases. The average primary dendrite arm spacing increases from 17 μm at the bottom to 39 μm at the top of the deposit, and the volume fraction of the interdendritic eutectic decreases from 52.1% to 39.3%. The microstructure of each layer except the top layer consists of vertical columnar dendrites and direction-changed columnar dendrites in sequence. The top layer appears equiaxed dendrites due to columnar to equiaxed transition (CET). The tensile properties present obvious anisotropic characteristics because of the epitaxial columnar dendritic growth along the building direction. The tensile properties also show obvious variation from the bottom to the top of the deposit because of the differing microstructures in different regions. The results are further analyzed in detail through the microstructure evolution resulted from the new manufacturing method.

Laser forming repair (LFR) technology has considerable potential in high strength steel structure repair. 34CrNiMo6 steel has been widely used in high-value components, and it is imperative to repair these damaged components. In this study, two different thicknesses of repaired layers are deposited on the 34CrNiMo6 wrought substrate with five layers and 20 layers via LFR technology. The microstructure, phases, microhardness, and tribological properties are analyzed using optical microscopy, scanning electron microscopy, X-ray diffraction, Vickers hardness testing, and dry sliding wear testing. These results show that the 34CrNiMo6 repaired layers were successfully deposited on the substrate. The microstructure of the laser-repaired layers in the five-layer sample included bainite and retained austenite. For the 20-layer sample, the microstructure in the top of the repaired layers was still bainite and retained austenite, whereas that in the bottom of the repaired layers was transformed into ferrite and cementite. The average coefficients of friction of repaired layers is not significantly different from the substrate. The wear rate of the five LFR layers, 20-layer LFR, and substrate samples were 12.89 &times, 10&minus, 6, 15 &times, 6, and 23.87 &times, 6 mm3/N&middot, m, respectively. The laser forming repaired samples had better wear resistance compared to the substrate. The wear mechanism of laser forming repaired samples is abrasive wear, whereas that of the substrate is abrasive wear and fatigue wear.

AerMet100 ultra-high strength steel plates with a thickness of 2 mm were welded using a CO2 laser welding system. The influences of the welding process parameters on the morphology and microstructure of the welding joints were investigated, and the mechanical property of the welding joints was analyzed. The experimental results showed that the fusion zone of welding joint mainly consisted of columnar grains and a fine dendrite substructure grew epitaxially from the matrix. With the other conditions remaining unchanged, a finer weld microstructure was along with the scanning speed increase. The solidification microstructure gradually transformed from cellular crystal into dendrite crystal and the spaces of dendrite secondary arms rose from the fusion line to the center of the fusion zone. In the fusion zone of the weld, the rapid cooling caused the formation of martensite, which led the microhardness of the fusion zone higher than that of the matrix and the heat affected zone. The tensile strength of the welding joints was tested as 1 700 MPa, which was about 87% of the matrix. However, the tensile strength of the welding joints without defects existed was tested as 1832 MPa, which was about 94% of the matrix.

Two-millimeter-thick 2060 Al-Li alloy plates were friction stir welded under a welding speed of 95–150 mm/min and rotation speed of 750–1500 rpm. The effects of welding speed and rotation speed on formation quality, microstructure, secondary phase particles’ transformation, and mechanical properties of the joints were investigated. The results show that defect-free joints are produced for varying friction stir welding (FSW) parameters, and nugget size increases firstly and then decreases with increasing rotation speed or decreasing welding speed. The weld nugget zones (WNZs) have fine dynamically recrystallized grains, and the size decreases to 7.9 μm with increasing rotation speed to 1180 rpm or decreasing welding speed to 118 mm/min, while the grains are coarsened at 1500 rpm or 95 mm/min. A similar trend occurs in the transformation of secondary phase particles, whose size is the smallest in WNZ at 1180 rpm–118 mm/min. All joints exhibit softened zones where the hardness is the lowest, and the joints fracture from WNZs or heat-affected zones. The joints welded at 1180 rpm–118 mm/min perform the highest ultimate tensile strength of 495 MPa, yield strength of 380 MPa, and elongation of 10.2 %. With increasing rotation speed or decreasing welding speed, the strengths and elongation of the joints increase firstly and then decrease.

Four different tools with the pin eccentricity of 0.1 mm, 0.2 mm, 0.3 mm and 0.4 mm were designed to friction stir weld 10 mm thick AA7075-O plate. The effect of pin eccentricity on microstructure, secondary phase particles transformation and mechanical properties of the joints was investigated. The results show that the nugget area ( A NZ ) increases firstly and then decreases with increasing the pin eccentricity. When the pin with 0.2 mm eccentricity is applied, the A NZ is the largest; meanwhile the grains size is the smallest which is about 3 μm and secondary phase particles are the most dispersive in nugget zone compared with other tools. While the grains are coarsened to 7–11 μm as the eccentricity is more than 0.4 mm, some coarse hardening particles get to cluster in the thermo-mechanically affected zone. The joints produced by the pin with 0.2 mm eccentricity perform the highest tensile strength and elongation, which is attributed to better interfaces, finer grains and more dispersive secondary phase particles.

A356 alloy was solidified with cooling rate enhanced by phase transition cooling (PTC) of Ga + In + Sn mixed alloy, and then solution treated at 540 °C, 500 °C and 450 °C for different time, followed by quenching and immediately natural aging. Evolution of the corresponding microstructure and hardness with the solution temperature and time were studied. The as-cast sample was hardened due to the refined microstructures, the large solubility of Si- and Mg-clusters in Al matrix and the nano-scale Si precipitates. The modification of eutectic Si was verified to depend on the initially solidified morphology of Si, the solution temperature and time, for example, the Si particles subjected to high cooling rate and high solution temperature become more readily spheroidized. The age hardening, from the hardness tests and differential scanning calorimetry (DSC) results, was ascribed to be associated with the Guinier–Preston (GP) zones. It can be concluded that an optimal combination of solidification with heat treatment may achieve the better mechanical property but with the less product cost.

Adopting a fluxing purification and cyclic superheating technique, Co-10 wt pct Si and Co-15 wt pct Si alloys had been undercooled to realize rapid solidification in this work. It was investigated that the solidification modes and microstructures of Co-Si alloys were deeply influenced by the undercooling of the melts. Both alloys solidified with a near-equilibrium mode in a low undercooling range; the peritectic reaction occurred between the primary phase and the remnant liquids, and it was followed by the eutectic reaction and eutectoid transformation. With the increase of undercooling, both alloys solidified with a nonequilibrium mode, and the peritectic reaction was restrained. As was analyzed, a metastable Co3Si phase was found in Co-10 wt pct Si alloy when a critical undercooling was achieved.

Rapid directional solidification of three peritectic Fe–Ni alloys with Ni-concentrations of 4.1 at.%, 4.33 at.%, and 4.6 at.% is achieved by laser surface remelting. It is found that phase selection of these alloys is controlled by the maximum growth temperature criterion. For the first two alloys, when the solidification velocity ( V s ) is above a critical value ( V s crit = 0.031 m / s for Fe–4.1 at.%Ni alloy and V s crit = 0.012 m / s for Fe–4.33 at.%Ni alloy), the growth temperature of γ phase ( T i γ ) is higher than that of δ phase ( T i δ ), leading to a transition from the solidification of δ phase to the solidification of γ phase. While for the third one, within the achieved range of V s , T i γ is always higher than T i δ ; only the solidification of γ phase is resultant. At V s crit , an abrupt increase in cellular spacings (λ) of the first two alloys are observed, which can be attributed to the transition from the solidification of δ phase to the solidification of γ phase. For Fe–4.6 at.%Ni alloy, within the achieved range of V s , only γ phase solidifies, which causes a monotonous decrease of λ with increasing V s .

Repairing damaged parts using proper repairing methods has become an important means to reduce manufacturing and operational costs and prolong the service life of 34CrNiMo6 steel structures. In the conventional fusion repairing method, welding wire and powder are often used as filling materials. Filling materials are often expensive or difficult to find. Some metallurgical issues (such as solidification crack, higher distortion) were also found with these methods. At the same time, most of the equipment that requires welding wire and powder is expensive. In this study, a new method based on friction stir processing (FSP) was successfully employed to repair 34CrNiMo6 steel, using a block as filling material. Filling blocks are much cheaper than conventional fusion repair consumables. As a result of solid-state repair, this method can also avoid the metallurgical issues of fusion repair. The microstructure and mechanical properties of the repaired samples were investigated using OM (Optical Microscope), SEM, EDS (Energy Dispersive Spectroscopy), XRD, and a Vickers hardness electronic universal tensile tester. The results showed that 34CrNiMo6 steel was successfully repaired by this method, with no defect. Tensile tests showed that the maximum ultimate strength (UTS) was 900 MPa and could reach 91.8% of that of the substrate. The fracture mode of the tensile samples was ductile/brittle mixed fracture. Hence, the repairing method based on FSP appears to be a promising method for repairing castings.

Hypercooling was achieved in both Co 80 Pd 20 alloys and Ni 80 Pd 20 alloys by applying the fluxing method. Microstructures and substructures of the as-solidified hypercooled Co 80 Pd 20 alloys and Ni 80 Pd 20 alloys were investigated. High densities of low-angle grain boundaries, dislocations networks were observed in the as-solidified hypercooled specimens. These substructures suggest a partial recrystallization occurring in the as-solidified specimens. After recrystallization annealing, the coarse grained microstructures of the quenched alloy refined. These results are believed as the evidences of recrystallization mechanism of grain refinement in these highly undercooled single phase alloys.

Aluminum matrix composites reinforced by different contents of multi-walled carbon nanotubes (MWCNTs) were fabricated by friction stir processing (FSP). The microstructure of nano-composites and the interface between aluminum matrix and MWCNTs were examined using optical microscopy (OM) and transmission electron microscopy (TEM). It was indicated that MWCNTs were well dispersed in the aluminum matrix throughout the FSP. Tensile tests and microhardness measurement showed that, with the increase of MWCNT content, the tensile strength and microhardness of MWCNTs/Al composites gradually increased, but on the contrary, the elongation decreased. The maximum ultimate tensile strength reached up to 190.2 MPa when 6 vol.% MWCNTs were added, and this value was two times more of that of aluminum matrix. Appearances and fracture surface micrographs of failed composite samples indicated that the composites become more and more brittle with the increase of the MWCNT content.

The effect of overlap rate on the residual stress, recrystallization nucleation location and the distribution of recrystallized grains of LSFed Inconel 718 superalloy were investigated. It is found that the as-deposited microstructures with different overlap rates have the similar characteristics including that columnar grains grow along the deposition direction. The overlap area possesses higher residual stress compared with the inner-pass area, and the position of peak residual stress is closer to previous pass. The increase in the overlap rate will broaden the variation range of the residual stress. During heat treatment, the recrystallization nucleation first occurs in the overlap area with the lower overlap rate, and then expands to both overlap area and inner-pass area paralleling with the increasing of the overlap rate. Although the overlap rate is only one of basic factors in the Laser Solid Forming process, it plays an important role in controlling residual stress distribution, material microstructure and mechanical properties. The higher overlap rate adopted, the finer and the more evenly distributed grains could be obtained through recrystallization for LSFed Inconel 718 superalloy.

The microstructure variation of as-deposited Inconel 718 superalloy sample in deposition direction fabricated by laser solid forming (LSF) was investigated. It was found that the as-deposited microstructure was mainly consisted of predominant γ columnar dendrites directionally growing along the deposition direction. Primary dendrite arm space measurement revealed spaces of 11.5, 17.5 and 38.0 μm at the bottom, the middle and the top section of the LSF sample respectively. The result was further studied by comparing with the Kurz–Giovanola–Trivedi (KGT) dendrite growth model, which was in a reasonable agreement with the experimental result. The various microstructures and micro-segregation situations at different sections of the LSF sample, which were induced by the thermal accumulation during LSF process, were also carefully discussed.

Undercooled solidification of hypoeutectic Ni–3·3 wt-%B alloy has been studied. The microstructure evolution of Ni–3·3 wt-%B alloy indicates that a transition from regular lamellar eutectic to anomalous eutectic exists with increasing undercooling for eutectic transformation. Here, a new model was proposed to describe the cooling curves of undercooled solidification with two-step transformation. Fits of the present model to the cooling curves after different undercoolings show that the predictions of transformed fractions are compatible with the microstructure observation. Furthermore, for hypoeutectic alloy, the size of the primary phase mainly depends on the undercooling for primary solidification, while the fraction of the primary phase mainly depends on the undercooling for eutectic solidification.