Haiou Yang

The influences of processing parameters on forming characterizations during laser rapid forming

Abstract Laser rapid forming experiments were carried out with 316L stainless steel and nickel-base alloy to investigate the influences of the processing parameters on forming characterizations systematically. It is found that the height of a single cladding layer was very important for fabrication accuracy and forming stability of components of laser rapid forming because it was affected by almost all processing parameters and was quite difficult to precisely control. For the system with lateral powder feeding pattern, the powder injection point was the most important factor to the height control of single layer. The variation of the width of single clad, which was mainly affected by laser power, spot diameter and scanning velocity, was similar to that in laser surface melting. The surface quality was another important characterization for laser rapid forming and was remarkably affected by oxidation and the powder adhesion. In order to improve the surface quality, the flow flux of shielding gas should be ≮10 l min −1 and the powder stream cannot be injected to the tail part of the melt pool. Based on the investigation, some metal components were fabricated.

Laser direct forming of metal components: technical characterizations

Laser direct forming experiments were carried out systematically with 316L stainless steel and nickel-base alloy to investigate the technical characterizations deeply and some metal components were fabricated. It is found that, the height of single cladding layer, which was affected by almost all the processing parameters and was quite hard to be precisely controlled, was very important to laser direct forming for not only the accuracy of vertical direction but also the fabrication stability. The variation of the width of single clad, which was mainly affected by laser power, spot diameter and scanning velocity, was similar to that in laser surface melting. The surface quality was another important characterization for laser direct forming and was remarkably affected by oxidation and powder attachment. In order to improve the surface quality, the flow flux ofshielding gas should not less than 10l/mm.

Three-dimensional reconstruction of anomalous eutectic in laser remelted Ni-30 wt.% Sn alloy

Abstract Laser remelting has been performed on Ni-30 wt.% Sn hypoeutectic alloy. An anomalous eutectic formed at the bottom of the molten pool when the sample was remelted thoroughly. 3D morphologies of the α-Ni and Ni3Sn phases in the anomalous eutectic region were obtained and investigated using serial sectioning reconstruction technology. It is found that the Ni3Sn phase has a continuous interconnected network structure and the α-Ni phase is distributed as separate particles in the anomalous eutectic, which is consistent with the electron backscatter diffraction pattern examinations. The α-Ni particles in the anomalous eutectic are supersaturated with Sn element as compared with the equilibrium phase diagram. Meanwhile, small wavy lamella eutectics coexist with anomalous eutectics. The Trivedi–Magnin–Kurz model was used to estimate undercooling with lamellar spacing. The results suggest that the critical undercooling found in undercooling solidification is not a sufficient condition for anomalous eutectic formation. Besides, α-Ni particles in the anomalous eutectic do not exhibit a completely random misorientation and some neighboring α-Ni particles have the same orientation. It is shown that both the coupled and decoupled growth of the eutectic two phases can generate the α-Ni + Ni3Sn anomalous eutectic structure.

Laser Multi-layer Cladding Experiment on the DD3 Single Crystal Using FGH95 Powder: Investigation on the Microstructure of Single Crystal Cladding Layer

Lasermulti-layer cladding experiments were performed on the substrate of DD3 single crystal with FGH95 powder as cladding material. The solidification microstructure in the sample was investigated. It was found that the solidification microstructure was greatly influenced by the crystallography orientation of the substrate and the local solidification conditions. When the angle between the preferred orientation of the single crystal and the direction of heat flow in the cladding layer is less than 30°, single crystal cladding layers were acquired. Otherwise the crystallography orientation of the cladding layer will deviate from the orientation of the substrate and the microstructure with polycrystalline appears. Meanwhile, even when the experiments were performed on the same preferred crystal surface, the solidification microstructures will be different distinctly resulting from the variation of the local solidification conditions. The secondary arms were degenerated and the primary arm spacing was about 10–20 μm. Further investigation shows that the phases of the cladding layer are mainly made up of γ, γ•, the flower-like γ/γ• eutectic and carbide. The morphology of γ• was cubical and the size is less than 0. 1μm.

Morphology evolution of γ′ phase in K465 nickel-base superalloy during laser forming repairing

Laser forming repairing (LFR) was performed on the damaged K465 nickel-base cast superalloy parts. LFR presented the uneven microstructure characteristics at different positions of the LFRed K465 superalloy, such as substrate zone (SZ), heat affected zone (HAZ), and repaired zone (RZ), which experienced the different thermal history. Compared with the coarser γ′ precipitates in the cast SZ, the γ′ precipitates in RZ were refined obviously. As for HAZ, γ′ phase presented an incomplete solid solution and reprecipitation growth characteristics. This led to an increase in the microhardness from SZ, HAZ to RZ.

Microstructural evolution in laser rapid forming of a graded titanium-nickel alloy

The fabrication of near net shaping parts with functionally graded materials using laser rapid forming have been considered as a signification and promising technology. In this paper, a graded binary Titanium-Nickel alloy has been deposited using laser rapid forming from a blend of elemental Ti and Ni powders. A compositional gradient, from elemental Ti to Ti-23.2at%Ni, was achieved within a length of ∼25mm. The solidification behavior and the morphological evolution of Ti/Ni functionally graded material formed by a laser rapid forming process were investigated. a series of phase evolutions: α→α+β → α+β+Ti2Ni → β+Ti2Ni along the compositional gradient have occurred. Phase formation and microstructure evolution along the compositional gradient direction was analyzed.The fabrication of near net shaping parts with functionally graded materials using laser rapid forming have been considered as a signification and promising technology. In this paper, a graded binary Titanium-Nickel alloy has been deposited using laser rapid forming from a blend of elemental Ti and Ni powders. A compositional gradient, from elemental Ti to Ti-23.2at%Ni, was achieved within a length of ∼25mm. The solidification behavior and the morphological evolution of Ti/Ni functionally graded material formed by a laser rapid forming process were investigated. a series of phase evolutions: α→α+β → α+β+Ti2Ni → β+Ti2Ni along the compositional gradient have occurred. Phase formation and microstructure evolution along the compositional gradient direction was analyzed.

Influences of powder specifications and powder delivery on laser and powder particle interaction during the LRF process

With deep studying on the process of laser rapid forming (LRF) the researchers gradually meet the knowledge that it is very important to understand the mechanism of interaction between the laser and the powder particles since it is the key point to realize the effective control of the LRF process. The high-speed photography has been employed to realize in situ observation on the delivery process of powdered materials for the first time. A group of parameters -- delivery parameters of powder is put forward to characterize the delivery process in quantitative by dealing with the digital images obtained. On the basis of quantitative description of the powder delivery, an analytical model is presented to study the attenuation of the laser power caused by the cloud of the power particles. Another analytical model is also presented to study the temperature rise of the particle irradiated by the laser. It can be found that the attenuation ratio is determined together by the powder specifications, the powder feeding parameters and the powder delivery parameters. With the off axial powder nozzle being employed in the paper, the diameter of the powder steam was always bigger than the diameter of laser spot, thus the laser processing parameters have no effect on the laser attenuation. The temperature rise of the particle is determined by the powder specifications and the powder delivery parameters too. Meanwhile the laser processing parameters also affects the temperature rise of the particle. With the decreasing of the particle radius, the irradiation heating effect increases remarkably.

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.

Microstructure and Tribological Properties of Laser Forming Repaired 34CrNiMo6 Steel

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 × 10−6, 15 × 10−6, and 23.87 × 10−6 mm3/N·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.

Microstructure and hardness of laser forming repaired 300M steel

Microstructure and micro-hardness distribution in single-track laser forming repaired 300M steel were investigated. The microstructure in the repaired zone consisted of the lath martensite. There showed no obvious changes for the microstructure in the repaired zone with the increase of the deposited layer. A series of phase evolutions from the bottom to the top of heat-affected zone (HAZ) occurred: the ferrite and a small amount of carbides→ the ferrite and austenite→ lath martensite. With the increase of the deposited layer, the amount of the carbides reduced and the amount and size of the austenite increased gradually in the HAZ, and the morphology of the austenite changed from strips-like to mesh-like. There were the same variation trends of the hardness from the substrate to the repaired zone between the single-track single-layer repaired and single-track multi-layer repaired specimen. The hardness in the substrate zone was the lowest, it increased rapidly from the bottom to the top of the HAZ. The hardness of the repaired zone was the highest, and varied a little from the bottom to the top of the repaired zone.Microstructure and micro-hardness distribution in single-track laser forming repaired 300M steel were investigated. The microstructure in the repaired zone consisted of the lath martensite. There showed no obvious changes for the microstructure in the repaired zone with the increase of the deposited layer. A series of phase evolutions from the bottom to the top of heat-affected zone (HAZ) occurred: the ferrite and a small amount of carbides→ the ferrite and austenite→ lath martensite. With the increase of the deposited layer, the amount of the carbides reduced and the amount and size of the austenite increased gradually in the HAZ, and the morphology of the austenite changed from strips-like to mesh-like. There were the same variation trends of the hardness from the substrate to the repaired zone between the single-track single-layer repaired and single-track multi-layer repaired specimen. The hardness in the substrate zone was the lowest, it increased rapidly from the bottom to the top of the HAZ. The ha...