Laser Additive Manufacturing Technology Research Articles

Cracks Repairing by Using Laser Additive and Subtractive Hybrid Manufacturing Technology

Abstract A laser additive and subtractive hybrid manufacturing technology was proposed to repair Inconel 718 cracks in this paper. Microstructures and the mechanical properties such as the micro-hardness, the tensile strength, and the friction-wear of the repair zone were investigated in detail. The microstructure analysis showed that a metallurgical bonding could be achieved between the repair zone and the matrix. The typically columnar and dendrite crystal appeared in the repaired zone, and the crystal epitaxial grew along the deposition direction, and the heat-affected zone in the groove boundary was coarse equiaxial crystal. The mechanical test result showed that the micro-hardness and tensile strength of the repaired tissue was about 87% and 89% of the original Inconel 718 wrought substrate. And, the wear resistance of the repaired zone was similar to that of the substrate. It was found that the surface quality of the repair zone for laser polishing is better than that for mechanical milling.

Quantitative evaluation of hidden hierarchical pores in laser additive manufactured bulk metallic glasses via computed tomography

Laser additive manufacturing (LAM) technology presents a great chance to break through the size limitation and realize the wide applications of bulk metallic glasses (BMGs). However, one of the inherent problems associated with this technical route is the unavoidable hidden pore defects, which is detrimental to the mechanical properties of the BMGs. In the present study, a comprehensive investigation of hidden pore defects in a typical model BMG Zr51Ti5Cu25Ni10Al9 fabricated by LAM is performed by using the computed tomography (CT) technology. The overall porosity of the LAMed BMG samples is found to be increased with increasing laser power. However, the variation trends of the proportions for different-size pores are not the same. Specifically, the proportions of larger pores are increased with the consumption of small ones as the laser power increasing. This opposite evolution trends of pores with the increasing laser power is analyzed from the Marangoni effect in the molten pool flow.

Development of Laser Additive Manufacturing Technology for Metals

Laser additive manufacturing (LAM) technology for metals is one of the key fundamental technologies for realizing intelligent manufacturing in fields such as aeronautics, astronautics, and medicine. This study summarizes the current status and trends of the LAM technology for metals in China and abroad using questionnaires, on-site surveys, and literature review. It also analyzes the gap between China’s technology development and the international advanced level and proposes a tentative strategy for the development of the LAM technology in China, hoping to provide support for the top design of national technological and industrial strategies and for the formulation of development goals for the LAM technology by 2035. Currently, the research on LAM technology focuses on active control of structure properties while comparatively ignoring control of geometric shapes in LAM. To satisfy the requirements of high-quality manufacturing, process monitoring of LAM equipment is highly valued. To improve the manufacturing capability and efficiency of high-value components, hybrid additive/subtractive manufacturing equipment has become a new research and development hotspot. The sound development of the LAM industry requires integration among the whole industrial chain including material, process, equipment, test, standards, and personnel training. China should fully explore the functions of the material genome technology for strengthening its fundamental research and improve its independent development capacity of core components and research on equipment integration. Moreover, the engineering application of the LAM technology should be conducted gradually.

Broadband fluorescence emission in Bi-doped silica glass prepared by laser additive manufacturing technology

In this work, we proposed a feasible method to prepare the Bi/Al co-doped silica glass by using laser additive manufacturing technology. Bi was uniformly doped into the silica matrix. The hydroxyl content of the glass sample was measured to be 29.36 ppm. Using an 808 nm laser diode as the excitation source, a broadband near-infrared emission from 1000 to 1600 nm was obtained. The emission peak was centered at 1249 nm, and the corresponding FWHM was more than 400 nm. The results show that the laser additive manufacturing technology is promising to fabricate highly homogeneous Bi-doped core materials with broader emission band, which is beneficial to solving the communication capacity crunch and promotes the development of fiber communication in the upcoming fifth and sixth generation systems.

Laser Additive Manufacturing Technologies

Laser Additive Manufacturing Technologies

Modeling of laser power attenuation by powder particles for laser solid forming

Laser solid forming (LSF) is a typical laser additive manufacturing technology integrating rapid prototyping manufacturing and laser cladding. Laser power attenuation greatly influences the amount and distribution of laser energy. It is important to establish a theoretical model of laser power attenuation to effectively control the laser energy transmission and to ensure the stability of the molten pool. Considering the effects of overlap phenomenon among powder particles, a theoretical model of laser power attenuation is proposed by introducing the concept of the effective number of powder particles. The proposed model enables the accurate calculation of the laser power attenuation in arbitrary volume fraction of powder particles. The results indicate that the laser power attenuation is proportional to the powder feeding rate, and the increase rate of laser power attenuation is inverse proportional to the powder feeding rate. In addition, a simple method for measuring the laser power attenuation is also proposed to experimentally validate the proposed theoretical model. The experimental results validate the proposed method.

Laser additive manufacturing of bimetallic structure from Ti-6Al-4V to Ti-48Al-2Cr-2Nb via vanadium interlayer

As a potential candidate material, Ti6Al4V (TC4)/Ti48Al2Cr2Nb (TiAl) bimetallic structure (BS) material has a good development prospects in the field of aerospace engineering, and has a broad application prospects for the integrated manufacture of aero-engine turbine blades (TiAl) and turbine disks (TC4). However, via the direct bonding of TC4/TiAl BS, it is easy to produce brittle intermetallic compounds which increases the crack sensitivity. Therefore, to a certain extent, the application of TC4/TiAl BS is limited. In this study, we use laser additive manufacturing (LAM) technology to prepare the TC4/TiAl BS via a V interlayer, which further limits the formation of a brittle phase. The experimental results show that the V interlayer can effectively limit the formation of the brittle phase (Ti3Al), which reduces the crack sensitivity of the formed parts, and forms a good metallurgical joint at the TC4/TiAl BS interfaces. Results of room temperature tensile tests reveal that the tensile strength and elongation of the BS specimens are ~476 MPa, and ~2.8% respectively, which means that the strength of the joint exceeded that of the deposited TiAl alloy without the V interlayer.

High-throughput screening of laser additive manufactured metallic glass via ultrasonic wave

Laser additive manufacturing (LAM) technology provides an opportunity to fabricate bulk metallic glasses (BMGs) without any dimensional constraint and achieve the large-scale applications of BMGs. However, flaws, such as cracks, gas porosity, and crystalline phases, are always formed accompanied by the process of LAM, which seriously worsens the mechanical and physical properties of the resulting BMGs. Here, we present a novel method that involves ultrasonic wave technique to high-throughput screen the optimum process parameters for the LAM of BMG. A parameter library, constituted by a series of rectangular BMG samples, is rapidly fabricated by the LAM method under continuously changed combinations of laser power and travel speed. The ultrasonic attenuation factor, which is sensitive to the flaws, is used as the monitor to screen the parameters of the BMGs fabricated by the LAM. Using this approach, the laser power of 1300 W and travel speed of 600 mm/min are estimated as the optimum parameter combination for the LAM of a Zr51Ti5Ni10Cu25Al9 (Zr51) BMG with the slightest flaws. The amorphous-phase dominated microstructure and the sufficiently high tensile strength of the subsequent fabricated large-sized Zr51 BMG sample verify this optimum parameter combination.

Tm/Al Co-Doped Silica Glass Prepared by Laser Additive Manufacturing Technology for 2-μm Photonic Crystal Fiber Laser

In this article, we demonstrated the Tm/Al co-doped silica glass fabricated by the laser additive manufacturing technology for the first time. The measurements show that the glass was doped with Tm3+ concentration of 1.64 wt% without any crystallization. The Tm3+ and Al3+ were uniformly distributed, and no additional defects were introduced during the fabrication process. The glass exhibits low hydroxyl content level of 11.56 ppm. Using the prepared silica glass as the core rod, a Tm/Al co-doped photonic crystal fiber (PCF) was fabricated and performed 13.9% slope efficiency with nearly 600 mW output laser power. This novel technology provides an alternative method to fabricate Tm-doped fiber materials with high homogeneity and large core size, promoting the further spread and development of 2 μm fiber lasers.

Densified W[sbnd]Cu composite fabricated via laser additive manufacturing

Tungsten-Copper (WCu) composites are promising materials for electrical and thermal applications. However, their fabrication remains limited using conventional techniques, such as powder metallurgy, which are not suitable for manufacturing densified complex WCu components. In this work, laser additive manufacturing (LAM) technology was introduced for fabricating W-25 wt%Cu composites. A densified WCu composite, free of apparent cracks and pores, is successfully synthesized using optimum process parameters of laser power 2000 W and laser scan speed 360 mm/min in an Ar atmosphere with less than 10 ppm O. The densification of WCu composites is owing to the sufficient wetting of solid W particles by the molten liquid Cu and the surface smoothing of the W particles. The good wetting of the solid W particles by the liquid Cu is due to the low oxygen content and the combined effect of increased capillary force and reduced friction force. The W grains are rounded, indicating some solution-reprecipitation of sharp particle edges originating from the increase in solubility from curvature effects.

Laser additive manufacturing of TA15 - Inconel 718 bimetallic structure via Nb/Cu multi-interlayer

High quality fabricating of the bimetallic structure of TA15 and Inconel 718 (IN718) is widely used in the aerospace filed. In this study, we have used laser additive manufacturing (LAM) technology to fabricate the bimetallic structure and to prevent the formation of brittle phases via Nb/Cu multi-interlayer. The results of this study showed that Nb/Cu multi-interlayer was very effective in preventing the generation of brittle Ti–Ni and Ti–Cu phases. The good metallurgical bonds were presented at these interfaces between TA15 and IN718. Mechanical properties of the bimetallic structure revealed that the tensile strength reached that of the weakest metal, Cu.

Effect of Different Heat Treatments on the Microstructural Evolution and Mechanical Properties of Ni-Cr-Si Alloy Fabricated by Laser Additive Manufacturing

Ni-Cr-Si alloy samples were fabricated on mild steel via laser additive manufacturing technology. The microstructural evolution and mechanical properties of the samples were studied systematically in as-deposited conditions and under three different types of heat-treated conditions. The scanning electron microscopy, energy-dispersive x-ray spectroscopy and x-ray diffraction results showed that the structures of the as-deposited samples were mainly dendritic matrices of γ(Ni, Fe), continuous interdendritic eutectics of γ + Ni3B and discrete Cr-rich carbides and borides. After solid solution treatment, carbides (Cr7C3) and borides (CrB) precipitated on the boundary of the eutectic structure, distributing discretely in the matrix. In the solution plus aging heat-treated conditions, the interdendritic eutectics formed Ni3Fe intermetallic compounds with a layered distribution. After annealing, the grains were refined, and the hard phases (CrB) were uniformly dispersed along the grain boundary. Among the tested samples, the as-deposited specimens exhibited the highest microhardness and good wear ability at room temperature; however, the wear resistance of the as-deposited sample obviously decreased at 250 °C. The mechanical properties of the specimens were not greatly improved by solution treatment or solution plus aging treatment. The sample subjected to annealing treatment exhibited excellent wear resistance both at room temperature and at a service temperature of 250 °C.

Analysis of tool wear performance and surface quality in post milling of additive manufactured 316L stainless steel

Laser additive manufacturing (LAM) technology applied to the 316L stainless steel is attracting interest in the machining industry since it can shorten the production cycle and reduce numbers of machining steps. It still needs to be machined because of the poor surface quality, so the tool wear of post milling process after LAM needs to be studied. However, there are a few studies about the tool wear in milling of laser additive manufacturing stainless steel alloy. The aim of the paper is to evaluate the tool wear performance and surface quality when post milling the 316L stainless steel under different post milling (PM) time. The tool wear behavior was investigated using different analysis techniques. 3-D surface contour profilometer was used to measure the surface roughness and observe the morphology of small area; digital optical ultra-depth microscope analysis was carried out to evaluate tool wear and broken; the wear width of the blade was an indicator of the degree of the tool wear; different post milling methods were also compared. The obtained results demonstrate that the height of the LAM parts were connected with the heat dissipation effect; there are three stages of tool wear: initial wear stage, normal processing stage, and severe broken stage; usually the down-milling is better than up-milling at the bottom surface roughness, quality and edge morphology.

New Development of Metal Laser Additive Manufacturing Technology

New Development of Metal Laser Additive Manufacturing Technology

Microstructural Characteristics of Laser Metal Deposited Magnesium Alloy AZ31

Up to now, only a limited amount of metallic materials is investigated for laser additive manufacturing (LAM). However, the demand to widen the application possibilities by enlarging the range of materials for LAM is growing fast. By now, titanium and aluminium alloys are in the focus of research. In contrast, magnesium alloys are rarely used in the field of additive manufacturing, although they possess a low density in combination with a high specific strength. Currently, magnesium structures are mainly produced by casting but during the last years, the use of wrought alloys also increased. A reason for the rare use of magnesium alloys for LAM technologies might be the high flammability of magnesium powders. This difficulty can be avoided by using magnesium wire for laser metal deposition (LMD). In the present study, the microstructural characteristics of a LMD processed AZ31 magnesium alloy are investigated. For this purpose, optical microscopy and scanning electron microscopy were used. With the help of EDX and EBSD analysis, a change of the chemical composition and micro texture with structure height was identified. The relationship of microstructure and local mechanical properties was investigated with the help of Vickers micro hardness testing. Based on the obtained results it can be concluded that the microstructural characteristics of laser additive manufactured magnesium alloys differ from those of titanium and aluminium alloys. Thus, a wider application spectrum of LMD and magnesium alloys can be opened up.

The influence of Laves phases on the room temperature tensile properties of Inconel 718 fabricated by powder feeding laser additive manufacturing

In this paper, a powder feeding laser additive manufacturing technology has been used for fabricating the Inconel 718 super-alloy. Laves phases of different sizes and morphologies have been obtained by using three types of heat treatments. The influence of Laves phases on the room temperature tensile properties of laser additive manufactured Inconel 718 has been investigated. The results show that small and granular Laves phase can be gained after heat treatment at 1050 °C for 15 min (S-15 sample). When the holding time extends to 45 min, the morphologies of Laves phases basically remain unchanged while its volume fraction further decreases (S-45 sample). Nevertheless, irregular and long-striped Laves phases still exist in the samples only after direct aging heat treatment (DA sample). The room temperature tensile results reveal that the S-15 samples have better tensile strength and ductility than that of S-45 samples. Besides, the DA samples with irregular and long-striped Laves phases show the lowest tensile strength and ductility. Hence, a certain amount of small and granular Laves phases are presumably beneficial for the room temperature tensile properties of Inconel 718. Moreover, a model has been established to describe the fracture of the Laves phase. On the basis of the fracture model, the critical stress needed for the fracture of long-striped Laves phases is lower than that needed for the fracture of granular Laves phases. Therefore, the former generally suffered internal fracture while the latter often fail by interfacial decohesion. Through influencing the volume fraction, the size and the distribution of γ" phase, the effect of the Laves phases on the room temperature tensile property is achieved. Furthermore, a yield strength model has been developed to reveal this influence in terms of numbers. The yield strength increments caused by grains, solution elements and γ′ phase are almost the same for the three kinds of samples. The differences of the yield strength are mainly caused by γ" phase. In addition, in terms of ductility, granular Laves phases are more favorable than long-striped Laves phases.

Microstructure and mechanical property of 12CrNi2 high strength steel fabricated by laser additive manufacturing technology

12CrNi2 high strength steel thin wall with few defects was fabricated by laser additive manufacturing technology. The microstructure of as-built sample was studied using optical microscopy (OM), scanning electron microscopy (SEM) and energy dispersive spectroscopy (EDS). Additionally, the mechanical property was also investigated by microhardness tests. It was found that the microstructure of the as-built sample mainly consists of columnar dendrites aligned in building direction because of the fast directional cooling during LAM. Cellular grains, and equiaxial dendrites are also observed in different regions of the as-built sample due to heat accumulation effect. Furthermore, many types of microstructure, such as tempered martensite, M-A constituent, retained austenite, lower and upper bainitic ferrite, are appeared in the as-built sample resulting from complex heat history and wide temperature range during LAM process. The maximum and minimum microhardness (499 HV0.2 and 272 HV0.2) occur in the top and bottom region respectively, while microhardness of the middle region with value of 452 HV0.2 just slightly reduces compared to the top section of as-built sample.

Laser additive manufacturing of structural-graded bulk metallic glass

Extending the applications of metallic glasses for use as functionally graded materials could mitigate their intrinsic problems such as limited dimension and ambient brittleness. Laser additive manufacturing (LAM) technology, allowing us to tailor the microstructure of deposited materials, is an outstanding and reliable technique to fabricate structural-graded materials. In the present study, a Zr50Ti5Cu27Ni10Al8 (Zr50) alloy with excellent glass-forming ability was chosen as the model material to fabricate large-size metallic glass by the LAM technology. By optimizing the LAM process through the finite-element simulation, a structural-graded Zr50 alloy with the thickness of larger than 10 mm was successfully fabricated.

Effect of heat treatment on the microstructure of Co–Cr–W alloy fabricated by laser additive manufacturing

Stellite 6 cobalt-based alloy powder was used to produce Co–Cr–W alloy using laser additive manufacturing technology, and then different heat treatment strategies were carried out on the deposited sample. The characteristics of microstructure under different heat treatment conditions were investigated using scanning electron microscopy with energy dispersive spectroscopy, transmission electron microscope, and x-ray diffraction. The results show that the as-deposited sample has few cracks or pores, and the microstructure is typical dendritic structure, and lamellar eutectic carbides are rich in Cr in interdendritic. The matrix mainly consists of γ phases and a few ϵ phases. Some γ phases transform into ϵ phases after 900°C/6 h aging treatment and lamellar eutectic carbides transform into blocky carbides presenting as a network, most of the carbides are rich in Cr and a few are rich in W. When heat treated at 1200°C/1 h followed by water cooling and then treated at 900°C/6 h followed by furnace cooling, it can be found that some γ phases transform into ϵ phases. The carbides transform into elliptical M23C6 carbides that are rich in Cr with the size of 1 to 3 μm and a part of W-rich carbides.

Strengthening the ZrC-SiC ceramic and TC4 alloy brazed joint using laser additive manufactured functionally graded material layers

In order to improve the ZrC-SiC and TC4 brazed joint property, functionally graded material (FGM) layers (two SiC particles reinforced TC4-based composite layers) were designed to relieve the residual stress in the ZrC-SiC and TC4 brazed joint. The FGM layers were fabricated on the TC4 surface using laser additive manufacturing technology before the brazing. Then the TC4 coated with the FGM layers and ZrC-SiC ceramic were brazed using Ti-15Cu-15Ni (wt%) filler. According to the SEM and TEM results, the volume fractions of SiC particles in the FGM layers could reach 20% and 39% respectively. Ti from the braze filler and TC4 reacted with the ZrC-SiC ceramic to form TiC and Ti5Si3 adjacent to the ZrC-SiC ceramic. The shear test results indicate that the adoption of the FGM layers and the brazing temperature both affected the joint property significantly. The FGM layers could benefit the mitigation of coefficient of thermal expansion (CTE) mismatch between the ZrC-SiC and TC4, so that the residual stress caused by the CTE mismatch in the joint was relieved and the joint strength increased. The brazing temperature would affect the microstructure of the brazing seam and then control the joint strength. When the ZrC-SiC ceramic and TC4 coated with the FGM layers were brazed at 970 °C for 10 min, the maximum shear strength could reach 91 MPa, and cracks propagated in the ZrC-SiC ceramic substrate during the shear test.