Laser Solid Forming Process Research Articles

Study on the micro-structure and the nano-indentation responses of laser solid formed epitaxial Ni-based superalloy

Laser solid forming (LSF) is one of the most promising technologies which has been extensively applied for the reshape and fabrication of the thermal parts in gas turbine and aero-engine. In this paper, the Ni-based single crystal superalloy thin-walled parts were prepared by using the LSF technology. Then, the electron back-scatter diffraction (EBSD) and the scanning electron microscope (SEM) technologies were employed to characterize the micro-structure morphology and crystallographic orientation of grains. The dislocation density of Ni-based superalloy thin-walled parts was analyzed. The results showed that a remarkable micro-structure gradient with respect to different heights of repaired Ni-based superalloy thin-walled parts was observed after the LSF process while the micro-structure distribution in width direction is comparatively more uniform. In addition, the misorientation of crystallographic orientation can be attributed to the dislocation density. A higher dislocation density contributes to aggregation of massive low angle grain boundaries (LAGBs) and formation of high angle grain boundaries (HAGBs), which in return conversely reduces dislocation density within the material. Finally, the mechanical properties of the LSF epitaxial Ni-based superalloy were investigated by the cyclic creep and Nano-indentation tests. The results showed that with the increasing of repair height, the residual indentation depth increased, while the elastic modulus increased first and then decreased. Meanwhile, a lower hardness of the stray grain region was obtained comparing with that of the substrate. Within the stray grain region, the hardness decreases along the building direction.

Microstructure and Phase Transformation Temperature of NiTiNb Shape Memory Alloy Prepared by Laser Solid Forming Using Mixed Powder

NiTiNb is a wide-hysteresis shape memory alloy. The Laser Solid Forming (LSF) technology can overcome the shortcomings of the traditional long cycle processing to prepare NiTiNb. In this work, we studied the microstructure and phase transformation temperature of the NiTiNb prepared by LSF., in which the Ni + Ti + Nb mixed powder was melted under different laser power P, scanning speed v, layer thickness t, and energy density EV. The results show that the combination of LSF process parameters with P = 2000 W and v = 900 mm/min can obtain a good metallurgical bond. As the laser power increases, the grain size increases, and the proportion of equiaxed crystals increases, the martensite transformation temperature increases. The inhomogeneity of the LSF-NiTiNb microstructure results in different phase transformation temperatures even in the same sample. The subsequent heat treatment at 850 °C for 3 h increases the phase transformation temperature and hysteresis of LSF-NiTiNb. The tensile properties of the LSF-NiTiNb samples with different building heights are significantly different. The maximum elongation reaches 8% and the minimum elongation is only 0.8%. The LSF parameter combination in this work has reference value for the parameter selection of subsequent preparation of NiTiNb.

Grain refinement and improved tensile properties of Ti5Al2Sn2Zr4Mo4Cr titanium alloy fabricated by laser solid forming

Laser solid forming (LSF) Ti5Al2Sn2Zr4Mo4Cr titanium alloy usually exhibits brittle fracture with a poor elongation when the tensile direction is perpendicular to the growth direction of columnar β grains. In the present study, the effect of powder feed rate on the columnar to equiaxed transition (CET) and grain refinement of Ti5Al2Sn2Zr4Mo4Cr titanium alloy during the LSF process was investigated. The results showed that increasing in powder feed rate could effectively promote CET during LSF process. However, keeping increasing in the powder feed rate did not further refine the β grains effectively, especially in the regions near the fusion lines. The pauses of the LSF process aimed to obtain equiaxed β grains also lead to the precipitation of very fine α laths inside the β grains since the heat accumulation was avoided. The microstructure containing equiaxed β grains and homogeneous α laths was obtained after heat treatment. The tensile test was performed both in the vertical (parallel to the build direction) and horizontal (perpendicular to the build direction) directions under the as-deposited and heat treatment conditions. The tensile properties, especially the ductility, were significantly improved compared to that in the previous studies. It is partly because the equiaxed β grains effectively prevented the crack propagation.

Solidification Effect on the Microstructure and Mechanism of Laser‐Solid‐Forming‐Produced Flame‐Resistant Ti–35V–15Cr Alloy

Ti–35V–15Cr alloy has become an indispensable material in aerospace industry due to its excellent flame‐resistance properties. Herein, the block and thin‐wall Ti–35V–15Cr samples are built by laser solid forming (LSF) under the same processing parameters, and the evolution of solidification microstructure is investigated. This work focused on the effect of molten pool solidification parameters and complex thermal cycling conditions on the morphology of β grains and the substructure in the grains. The microstructure of LSF‐produced Ti–35V–15Cr block sample mainly consists of epitaxial columnar dendritic grains, whereas that of the deposited thin‐wall sample is mainly composed of near‐equiaxed grains together with subgrains formed in the near‐equiaxed grains. Such different microstructure results from extremely sensitive thermal behavior condition in LSF process. The simulated thermal behavior characteristics using finite element method illustrate that the thermal cycles during block deposition are very complex and the temperature gradient at the last solidification in block deposition is much higher than that in the deposition of thin‐wall samples, resulting in significantly different solidification microstructure between thin‐wall and block samples.

A Real-Time Method to Detect the Deformation Behavior during Laser Solid Forming of Thin-Wall Structure

Laser solid forming (LSF) is a promising additive manufacturing technology. In the LSF process, deformation behaviors dictate the accuracy of the produced parts. In this study, by using a laser displacement detector based on laser triangulation principle, an accurate and effective real-time detection method was established to monitor the real-time deformation behavior of the key position during the LSF of a thin-wall structure. The results confirmed that increasing thin-wall length results in increasing final deformation of the edge. The displacement fluctuation range and value in the middle of thin wall are both smaller than that of the positions near the end, while the entire displacement changing direction in the middle is opposite to that of the end positions. When the deposition process is paused, the deformation of the thin wall during the cooling stage will deviate the position of the deposited thin wall, resulting in the dislocation between the subsequent deposited part and that before the pause, which affect the dimensional accuracy of the thin wall structure. This non-contact real-time detection method also confirmed the ability to monitor the initiation of cracking during the LSF process, and a potential to be used for the on-line feedback control of deformation of detected key position of deposited structure.

Surface evenness control of laser solid formed thin-walled parts based on the mathematical model of the single cladding layer thickness

Laser solid forming (LSF) is a new type of advanced manufacturing technology, which can achieve the rapid fabrication of complicated metallic components with high performance. Multitudinous parameters that need to control in the LSF process make it difficult to accomplish the parameters optimization successfully. Once the unsuitable process parameters are set, it is very easy to produce corrugated surfaces during the LSF process. In order to realize the surface evenness control of thin-walled parts, the authors have established a mathematical analytical model to reflect the correlation relationship between the single cladding layer thickness and input process parameters. The novelty of this model is that a relationship between the single cladding layer thickness and input process parameters was first established. The results show that the single cladding layer thickness is mainly determined by the parameters, such as powder feed rate, defocusing amount, carrier gas velocity, laser beam spot, and scanning speed. Finally, the theoretical calculation results were verified via a series of experiments, and a practical application example of the model in the surface evenness control of thin-walled parts was given. The theoretical calculation results of the single cladding layer thickness correlate well with the experimental results.

High strength and ductility of 34CrNiMo6 steel produced by laser solid forming

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.

HAZ Liquation Cracking Mechanism of IN-738LC Superalloy Prepared by Laser Solid Forming

The heat-affected zone (HAZ) liquation cracking mechanism of nickel-based superalloys with high (Al+Ti) content during the laser solid forming (LSF) process was investigated via laser remelting of an as-deposited IN-738LC superalloy. Microstructural HAZ analysis revealed that cracks consistently propagated from the HAZ to the remelting zone along the grain boundary (GB). The formation of a liquid film during GB liquation was mainly owing to localized melting of the semicontinuous γ-γ′ eutectic distributed along the GB. The solute segregation behavior of the IN-738LC alloy during LSF was analyzed using the Giovanola–Kurz model and Scheil models, revealing that a significant enrichment of γ-γ′ eutectic-forming elements in the residual liquid at the final stage of solidification (solid fraction ~ 0.87) in the molten pool was the main cause of semicontinuous γ-γ′ eutectic formation along the GB. Further, a B enrichment at the GB was identified in LSF-fabricated IN-738LC, which promoted cracking by lowering the GB liquation temperature and promoting wetting of the GB by the liquid film. Unlike the phenomenon observed in the welding of cast IN-738LC, the coherence between the γ′ phases and the γ matrix in LSF-fabricated IN-738LC can suppress the occurrence of constitutional liquation of the γ′ phase. To understand the interaction between the thermal stress and the liquid film in the LSF process, thermal stress as a cracking driving force was also estimated based on the measurement of the residual stress from the substrate to the remelting zone of the IN-738LC deposit by the Vickers micro-indentation method.

A cellular automaton simulation of W–Ni alloy solidification in laser solid forming process

An alloy cellular automaton (CA) model is developed for the microstructure simulation in directional solidification and laser solid forming (LSF) process. The CA model's capture rule was modified by a limited neighbor solid fraction (LNSF) method. A multiscale two-dimensional model is presented for simulating laser remelting process, which is the same as LSF without the addition of metallic powders into melt pool. The metallurgy process in melt pool was simulated, including temperature distribution, pool shape and solidification of microstructure. The microstructure evolution of tungsten–nickel alloy (W–Ni) during LSF is also simulated by present CA model.

Microstructure and deformation behavior of Ti-6Al-4V alloy by high-power laser solid forming

This work investigated the microstructure and tensile deformation behavior of Ti-6Al-4V alloy fabricated using a high-power laser solid forming (LSF) additive manufacturing. The results show that the post-fabricated heat-treated microstructure consists of coarse columnar prior-β grains (630–1000 μm wide) and α-laths (5–9 μm) under different scanning velocities (900 and 1500 mm/min), which caused large elongation (∼18%) superior to the conventional laser additive manufacturing Ti-6Al-4V alloy. The deformation behavior of the LSF Ti-6Al-4V alloy was investigated using in situ tensile test scanning electron microscopy. The results show that shear-bands appeared along the α/β interface and slip-bands occurred within the α-laths, which lead to cracks decaying in a zigzag-pattern in the LSF Ti-6Al-4V alloy with basket-weave microstructure. These results demonstrate that the small columnar prior-β grains and fine basket-weave microstructure exhibiting more α/β interfaces and α-laths can disperse the load and resist the deformation in the LSF Ti-6Al-4V components. In addition, a modified microstructure selection map of the LSF Ti-6Al-4V alloy was established, which can reasonably predict the microstructure evolution and relative grain size in the LSF process.

Formation mechanism of adhering powder and improvement of the surface quality during laser solid forming

Laser solid forming (LSF) is a promising manufacturing technology. During the LSF process, the adhering powder has a significant impact on the surface quality of the laser solid forming part. In this study, the formation mechanism of the adhesion particle was discussed. It was shown that only the powder particles fed into the rear edge of the molten pool might form the surface adhering powder. Then, an interaction model of powder particles and molten pool edge was established based on the theory of elasticity and force analysis. The critical condition of spherical particle adhesion and rebound at the molten pool edge was obtained, which indicated that the particle speed and particle size had an important influence on the formation of the surface adhering powder. The smaller the powder particle is, the higher the critical speed of particle rebound at the molten pool rear edge is. Further, the verification experiments were conducted and the results agreed with the analysis. Finally, the forming process improvement was made based on the formation mechanism of the adhering particle, and the components with improved surface quality were obtained.

Development of powder flow model of laser solid forming by analysis method

Laser solid forming (LSF) is a promising additive manufacturing technology. Powder flow plays an important role during the LSF process. In this study, based on the flux balance and the introduction of specific parameters, a practical model was developed to simulate the powder flow of the discontinuous coaxial nozzle with four symmetrical tips. The observation experiments of powder feed process were carried out, and the specific powder flow parameters and the powder flow feed behavior were obtained. It can be found that the simulations of the powder flow agreed well with the experimental measurements due to the introduction of specific powder flow parameters. In addition, it can be also found that the peak value of the powder mass concentration is above the theoretical focus position of the powder flow. Further, the analysis model was used for the calculation of the deposition layer height. It was showed that the deposition layer size varied significantly with the distance between the deposited surface and the nozzle exit plane due to the variation of the powder mass concentration. It can be concluded that the analysis model of powder flow has potential use for building the deposition layer formation model to analyze the LSF process.

Influence of powdered state on crystallization during laser solid forming Zr55Cu30Al10Ni5 bulk metallic glasses

Laser solid forming (LSF) is a viable and promising manufacturing technique for preparing bulk metallic glasses (BMGs) without size limitation. Owing to the structural heredity of alloy melts, the crystallization characteristic of the powder has an important influence on that of the deposit during LSF process. In this work, the as-prepared Zr55Cu30Al10Ni5 (Zr55) alloy powder and the Zr55 alloy powder annealed at 1000 K are used for LSF of Zr55 BMGs. The influence of the crystallization characteristic of Zr55 alloy powder on the crystallization behavior of the remelted zone (RZ) and heat affected zone (HAZ) in the deposit are investigated. It is found that the as-prepared Zr55 powder prepared by plasma rotating electrode process (PREP) is composed of the amorphous phase and Al5Ni3Zr2 phase. When the heat input of laser is low, there exist some Al5Ni3Zr2 residual phases in the amorphous matrix in the RZ, and there appear some Cu10Zr7, CuZr2 and NiZr2 phases besides the Al5Ni3Zr2 phase in the HAZ for the deposit fabricated by as-prepared Zr55 powders. With the increase of the heat input of laser, the RZ remains the amorphous state since the Al5Ni3Zr2 phase is completely remelted, while there are a large quantity of Al5Ni3Zr2 phases and some other crystallization phases precipitated in the HAZ because the heating and cooling rate decrease in the HAZ during LSF. Fabricated by the fully crystallized annealed powder, the deposit is mainly of the amorphous phase, and almost no Al5Ni3Zr2 phase is found even if the incident laser power is low. It is shown that the crystallization of the deposit fabricated by the annealed powder at the low heat input does not change remarkably with the increase of the deposited layers. The Zr55 deposit with five deposited layers could still keep large volume fraction of amorphous phase. This is mainly because the powder experiences the structure relaxation entirely during the annealing treatment, and the volume fraction of the short/medium-range ordered structure associated with the Al5Ni3Zr2 phase in the powder is reduced. Therefore, the volume fraction of the Al5Ni3Zr2 clusters in re-solidified amorphous RZ in the deposited layer decreases during LSF, which is conducible to the increase of the thermal stability of the already-deposited layer. In result, the area of the HAZ in the subsequent deposition decreases and the precipitation of Al5Ni3Zr2 phase is suppressed. In conclusion, increasing the heat input of laser aggravates the crystallization of the deposited layers, and the Al5Ni3Zr2 cluster in the powder has an important influence on the crystallization behavior of the Zr55 deposited layers.

The Machinability Comparison of Laser Solid Forming (LSF) and Forging TC4 Alloys

The laser solid forming (LSF) TC4 (Ti6Al4V) alloy is made throughmelting the raw material powder layer by layer with impulse laser beam. The mostrecent advancement of the LSF TC4 alloy exhibits comparable mechanical propertieswith conventional forging TC4 alloy, which enables wider application in the fabrication of structural components in the aerospace industry. However, the accuracy of the LSF process is limited to only about tens of microns due to the laser system,machining is still necessary for the application of LSF TC4 in precision conditions. In this paper, the machinabilities of LSF TC4 and forging TC4 are studied through the turning experiments. In addition, the signature analysis of the cutting force signal is carried out to correlate the anisotropic property of the LSF TC4 with the cutting force dynamic characteristics.

Effect of overlap rate on recrystallization behaviors of Laser Solid Formed Inconel 718 superalloy

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.

Microstructural changes in a laser solid forming Inconel 718 superalloy thin wall in the deposition direction

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 38.0μm 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.

激光立体成形工艺对TC11钛合金组织和力学性能的影响

激光立体成形工艺对TC11钛合金组织和力学性能的影响

激光立体成形工艺对TC11钛合金组织和力学性能的影响

激光立体成形工艺对TC11钛合金组织和力学性能的影响

Laser solid forming Zr-based bulk metallic glass

The crystallization characteristics of Zr 55Al 10Ni 5Cu 30 bulk metallic glasses BMGs during pulsed laser surface melting (PLSM) were examined, and the crystallization behavior during Laser solid forming (LSF) of Zr 55Al 10Ni 5Cu 30 BMGs with the pre-laid powder method on the amorphous substrates was further investigated. It was found that the BMG could keep the amorphous state after PLSM with six pulses and crystallization began to occur in heat-affected zone (HAZ) after PLSM with twelve pulses. There was no crystallization occurred in the deposit with one and two layers during LSF, and the volume fraction of amorphous phase in the deposit with seven layers deposit was about 92.44%. The crystallization degree did not increase remarkably with the increasing of deposited layers. The crystallization mainly occurred in HAZ during PLSM and LSF. A physical model was proposed to describe laser solid forming of BMGs, which explained the formation mechanism of BMGs during laser solid forming. It is shown that the crystallization during the PLSM and LSF process was mainly caused by the accumulation of structural relaxation in the HAZ. The size of HAZ should be smaller than the thickness of single pulsed laser deposited layer during LSF of BMGs without crystallization. Based on the present model and experiment results, we can reckon that bulk metallic glasses could be achieved by LSF without size limitation.

MICROSTRUCTURES AND MECHANICAL PROPERTIES OF LASER SOLID FORMED NICKLE BASE SUPERALLOY INCONEL 718 PREPARED IN DIFFERENT ATMOSPHERES

Due to the rapid melting and solidification in laser molten pool,the laser solid forming (LSF) samples have generally much finer microstructure than that of the conventional cast,and are also free of macrosegregation.To date,more and more researchers have reported the application of LSF technology in the forming of superalloy,which contains some easily segregated elements such as Nb,Ti and Al.To prevent the molten pool from oxidation,the LSF process usually should be performed within a protective atmosphere.But in practice,the forming of oxidation-resistant metals are always performed in the air atmosphere and only the molten pool is protected by noble gas. Therefore,it is important to investigate the microstructures and properties of LSF materials prepared in the air and noble gas.In the present study,the microstructures,tensile properties and high cycle fatigue properties of LSF Inconel 718 alloy samples prepared in the air and argon atmospheres were investigated.It is indicated that the microstructures in as-deposited and heat-treated LSF Inconel 718 alloy samples prepared in both atmospheres are similar.The as-deposited microstructure consists of columnar dendrites grew epitaxially along the deposition direction from the substrate,and changes to the equiaxed grains after solution heat treatment due to the recrystallization.The tensile properties of the two kinds of the samples after heat treatment reached the standard for wrought Inconel 718 alloy. The tensile strength of the sample prepared in air is slightly higher than that of the sample prepared in the argon atmosphere,and the high cycle fatigue lifetime of the latter is 30%higher than that of the former,which is related to the fewer defects such as micro pores and small size oxide inclusions existed in the former.The lower high cycle fatigue property of LSF Inconel 718 alloy than that of wrought Inconel 718 alloy is attributed to the metallurgical defects and the coarsening and uneven distribution of the recrystallized grains.