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Solidification Microstructure of Laser Additive Manufactured Ti——6Al——2Zr——2Sn——3Mo——1.5Cr——2Nb Titanium Alloy
Qiang Zhang, Jing Chen, Lilin Wang, Hua Tan, Xin Lin, Weidong Huang
J. Mater. Sci. Technol.    2016, 32 (4): 381-386.   DOI: 10.1016/j.jmst.2015.11.019
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Solidification microstructure of powder fed laser additive manufactured Ti——6Al——2Zr——2Sn——3Mo——1.5Cr——2Nb titanium alloy was investigated. The results showed that by deliberately increasing the powder feed rate, partially melted powders were retained at the top of the molten pool, which can promote heterogeneous nucleus. Thus, each cladding layer is composed of two regions: (i) randomly orientated cellular structure region caused by partially melted powders at the top of each cladding layer; and (ii) epitaxial cellular structure region adjacent to the fusion line. Usually, randomly orientated cellular structure region was totally remelted for a wide range of process conditions. The remelting effect ensures the continuity of epitaxial growth of cellular structure and leads to the formation of columnar β grains. In order to obtain equiaxed grains the scanning velocity and powder feed rate should be carefully selected to enlarge the randomly orientated cellular structure region.
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Microstructure Characteristics of Inconel 625 Superalloy Manufactured by Selective Laser Melting
Shuai Li, Qingsong Wei, Yusheng Shi, Zicheng Zhu, Danqing Zhang
J. Mater. Sci. Technol.    2015, 31 (9): 946-952.   DOI: 10.1016/j.jmst.2014.09.020
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Selective laser melting (SLM), an additive manufacturing process, is capable of manufacturing metallic parts with complex shapes directly from computer-aided design (CAD) models. SLM parts are created on a layer-by-layer manner, making it more flexible than traditional material processing techniques. In this paper, Inconel 625 alloy, a widely used material in the aerospace industry, were chosen as the build material. Scanning electron microscopy (SEM), electron back scattering diffraction (EBSD) and X-ray diffraction (XRD) analysis techniques were employed to analyze its microstructure. It was observed that the molten pool was composed of elongated columnar crystal. Due to the rapid cooling speed, the primary dendrite arm space was approximately 0.5 μm and the hardness of SLM state was very high (343 HV). The inverse pole figure (IPF) indicated that the growing orientation of the most grains was <001> due to the epitaxial growth and heat conduction. The XRD results revealed that the austenite structure with large lattice distortion was fully formed. No carbides or precipitated phases were found. After heat treatment the grains grew into two microstructures with distinct morphological characters, namely, rectangular grains and limited in the molten pool, and equiaxed grains along the molten boundaries. Upon experiencing the heat treatment, MC carbides with triangular shapes gradually precipitated. The results also identified that a large number of zigzag grain boundaries were formed. In this study, the grain formation and microstructure, and the laws of the molten pool evolution were also analyzed and discussed.
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Microstructure and Mechanical Properties of Al2O3/ZrO2 Directionally Solidified Eutectic Ceramic Prepared by Laser 3D Printing
Zhi Liu, Kan Song, Bo Gao, Tian Tian, Haiou Yang, Xin Lin, Weidong Huang
J. Mater. Sci. Technol.    2016, 32 (4): 320-325.   DOI: 10.1016/j.jmst.2015.11.017
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Directionally solidified eutectic ceramics such as Al2O3/ZrO2 are promising structural materials for applications in harsh environment with an ultrahigh temperature. In this work, through adopting assistant heating laser 3D printing, Al2O3/ZrO2 eutectic samples were manufactured with suppressing the formation of cracks. The dependence of the average rod spacing (λav) on the scanning rate (V) follows a relation with λavV0.5 = 1 µm1.5 s-0.5. Typical eutectic microstructures, so-called complex regular, were analyzed with respect to its evolution with modulating the growth conditions. Formation mechanism of the solidification defect, shrinkage porosity, was discussed and the defect is found to be significantly suppressed by optimizing the solidification parameters. The maximum hardness and fracture toughness are measured to be 16.7 GPa and 4.5 MPa m1/2, respectively. The interplay between the propagation of cracks and the Al2O3/ZrO2 interface is discussed.
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Microstructure and Fracture Behavior of 316L Austenitic Stainless Steel Produced by Selective Laser Melting
Casati R.,Lemke J.,Vedani M.
J. Mater. Sci. Technol.    DOI: 10.1016/j.jmst.2016.06.016
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Selective laser melting is an additive manufacturing method based on local melting of a metal powder bed by a high power laser beam. Fast laser scans are responsible for severe thermal gradients and high cooling rates which produce complex hydrodynamic fluid flow. These phenomena affect crystal growth and orientation and are believed to be the cause of material spattering and microstructural defects, e.g. pores and incompletely melted particles. In this work, the microstructure and texture of 316L bars built along two different orientations and the effect of different distribution of defects on their mechanical response and failure mechanisms were investigated. Partially molten powder particles are believed to be responsible for the scattering in elongation to failure, reduced strength, and premature failure of vertical samples.

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Friction stir based welding and processing technologies - processes, parameters, microstructures and applications: A review
G.K. Padhy, C.S. Wu, S. Gao
J. Mater. Sci. Technol.    2018, 34 (1): 1-38.   DOI: 10.1016/j.jmst.2017.11.029
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Friction stir welding (FSW) has achieved remarkable success in the joining and processing of aluminium alloys and other softer structural alloys. Conventional FSW, however, has not been entirely successful in the joining, processing and manufacturing of different desired materials essential to meet the sophisticated green globe requirements. Through the efforts of improving the process and transferring the existing friction stir knowledge base to other advanced applications, several friction stir based daughter technologies have emerged over the timeline. A few among these technologies are well developed while others are under the process of emergence. Beginning with a broad classification of the scattered frictions stir based technologies into two categories, welding and processing, it appears now time to know, compile and review these to enable their rapid access for reference and academia. In this review article, the friction stir based technologies classified under the category of welding are those applied for joining of materials while the remnant are labeled as friction stir processing (FSP) technologies. This review article presents an overview of four general aspects of both the developed and the developing friction stir based technologies, their associated process parameters, metallurgical features of their products and their feasibility and application to various materials. The lesser known and emerging technologies have been emphasized.

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Influence of Interfacial Bonding between Metal Droplets on Tensile Properties of 7075 Aluminum Billets by Additive Manufacturing Technique
Hansong Zuo, Hejun Li, Lehua Qi, Songyi Zhong
J. Mater. Sci. Technol.    2016, 32 (5): 485-488.   DOI: 10.1016/j.jmst.2016.03.004
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7075 aluminum billets were fabricated by micro droplet deposition manufacturing technique, and the influence of interfacial bonding between metal droplets on the tensile properties was studied. Three sets of samples were manufactured under different temperature conditions, and their mechanical properties were compared. The results show that the temperature of the metal droplets and substrate significantly affect the tensile strength of the sample. Moreover, with proper temperature setting, the 7075 aluminum billets manufactured by micro metal droplet deposition could achieve very good mechanical properties with a tensile strength of 373 MPa and an elongation of 9.95%, which are very similar to those of an extruded sample. Moreover, a metallurgical bonding diagram based on numerical calculations of interfacial temperature was established to predict the interfacial bonding state. In addition, the fracture morphologies of these specimens were observed. It is indicated that there was a significant transformation of failure mechanism with the improvement of metallurgical bonding, which agreed well with the numerical results.

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Metal Fabrication by Additive Manufacturing Using Laser and Electron Beam Melting Technologies
Lawrence E. Murr, Sara M. Gaytan, Diana A. Ramirez, Edwin Martinez, Jennifer Hernandez, Krista N. Amato, Patrick W. Shindo, Francisco R. Medina, Ryan B. Wicker
J Mater Sci Technol    2012, 28 (1): 1-14.  
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Selective laser melting (SLM) and electron beam melting (EBM) are relatively new rapid, additive manufacturing technologies which can allow for the fabrication of complex, multi-functional metal or alloy monoliths by CAD-directed, selective melting of precursor powder beds. By altering the beam parameters and scan strategies, new and unusual, even non-equilibrium microstructures can be produced; including controlled microstructural architectures which ideally extend the contemporary materials science and engineering paradigm relating structure-properties-processing-performance. In this study, comparative examples for SLM and EBM fabricated components from pre-alloyed, atomized precursor powders are presented. These include Cu, Ti-6Al-4V, alloy 625 (a Ni-base superalloy), a Co-base superalloy, and 17-4 PH stainless steel. These systems are characterized by optical metallography, scanning and transmission electron microscopy, and X-ray diffraction.
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Frontiers of 3D Printing/Additive Manufacturing: from Human Organs to Aircraft Fabrication
E. Murr Lawrence
J. Mater. Sci. Technol.    2016, 32 (10): 987-995.   DOI: 10.1016/j.jmst.2016.08.011
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It has been more than three decades since stereolithography began to emerge in various forms of additive manufacturing and 3D printing. Today these technologies are proliferating worldwide in various forms of advanced manufacturing. The largest segment of the 3D printing market today involves various polymer component fabrications, particularly complex structures not daihattainable by other manufacturing methods. Conventional printer head systems have also been adapted to selectively print various speciated human cells and special molecules in attempts to construct human organs, beginning with skin and various tissue patches. These efforts are discussed along with metal and alloy fabrication of a variety of implant and bone replacement components by creating powder layers, which are selectively melted into complex forms (such as foams and other open-cellular structures) using laser and electron beams directed by CAD software. Efforts to create a “living implant” by bone ingrowth and eventual vascularization within these implants will be discussed briefly. Novel printer heads for direct metal droplet deposition as in other 3D printing systems are briefly described since these concepts will allow for the eventual fabrication of very large and complex products, including automotive and aerospace structures and components.

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Solid-state additive manufacturing and repairing by cold spraying: A review
Wenya Li, Kang Yang, Shuo Yin, Xiawei Yang, Yaxin Xu, Rocco Lupoi
J. Mater. Sci. Technol.    2018, 34 (3): 440-457.   DOI: 10.1016/j.jmst.2017.09.015
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High-performance metal additive manufacturing (AM) has been extensively investigated in recent years because of its unique advantages over traditional manufacturing processes. AM has been applied to form complex components of Ti, Fe or Ni alloys. However, for other nonferrous alloys such as Al alloys, Mg alloys and Cu alloys, AM may not be appropriate because of its melting nature during processing by laser, electron beam, and/or arc. Cold spraying (CS) has been widely accepted as a promising solid-state coating technique in last decade for its mass production of high-quality metals and alloys, and/or metal matrix composites coatings. It is now recognized as a useful and powerful tool for AM, but the related research work has just started. This review summarized the literature on the state-of-the-art and problems for CS as an AM and repairing technique.

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