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ISSN 1005-0302
CN 21-1315/TG
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      05 February 2019, Volume 35 Issue 2 Previous Issue    Next Issue
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    Orginal Article
    Strategies for creating living, additively manufactured, open-cellular metal and alloy implants by promoting osseointegration, osteoinduction and vascularization: An overview
    Lawrence E.Murr
    J. Mater. Sci. Technol., 2019, 35 (2): 231-241.  DOI: 10.1016/j.jmst.2018.09.003
    Abstract   HTML   PDF

    Additive manufacturing of porous, open-cellular metal or alloy implants, fabricated by laser or electron beam melting of a powder bed, is briefly reviewed in relation to optimizing biomechanical compatibility by assuring elastic (Young’s) modulus matching of proximate bone, along with corresponding pore sizes assuring osseointegration and vasculature development and migration. In addition, associated, requisite compressive and fatigue strengths for such implants are described. Strategies for optimizing osteoblast (bone cell) development and osteoinduction as well as vascularization of tissue in 3D scaffolds and tissue engineering constructs for bone repair are reviewed in relation to the biology of osteogenesis and neovascularization in bone, and the role of associated growth factors, bone morphogenic proteins, signaling molecules and the like. Prospects for infusing hydrogel/collagen matrices containing these cellular and protein components or surgically extracted intramedullary (bone marrow) concentrate/aspirate containing these biological and cell components into porous implants are discussed, as strategies for creating living implants, which over the long term would act as metal or alloy scaffolds.

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    Progress in additive manufacturing on new materials: A review
    Neng Li, Shuai Huang, Guodong Zhang, Renyao Qin, Wei Liu, Huaping Xiong, Gongqi Shi, Jon Blackburn
    J. Mater. Sci. Technol., 2019, 35 (2): 242-269.  DOI: 10.1016/j.jmst.2018.09.002
    Abstract   HTML   PDF

    Recent efforts and advances in additive manufacturing (AM) on different types of new materials are presented and reviewed. Special attention is paid to the material design of cladding layers, the choice of feedstock materials, the metallurgical behavior and synthesis principle during the AM process, and the resulted microstructures and properties, as well as the relationship between these factors. Thereafter, the trend of development in the future is forecasted, including: Effects of the particles size and size distribution of powders; Approaches for producing fine microstructures; Opportunities for creating new materials by AM; Wide applications in reconditioning of damaged components; Challenges for deep understanding and applications of the AMed new materials. The idea of “Develop Materials” or “Create Materials” by AM is highlighted, but a series of scientific, technological and engineering problems remain to be solved in future.

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    A review of selective laser melting of aluminum alloys: Processing, microstructure, property and developing trends
    Jinliang Zhang, Bo Song, Qingsong Wei, Dave Bourell,
    J. Mater. Sci. Technol., 2019, 35 (2): 270-284.  DOI: 10.1016/j.jmst.2018.09.004
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    Selective laser melting (SLM) is an attractive rapid prototyping technology for the fabrication of metallic components with complex structure and high performance. Aluminum alloy, one of the most pervasive structural materials, is well known for high specific strength and good corrosion resistance. But the poor laser formability of aluminum alloy restricts its application. There are problems such as limited processable materials, immature process conditions and metallurgical defects on SLM processing aluminum alloys. Some efforts have been made to solve the above problems. This paper discusses the current research status both related to the scientific understanding and technology applications. The paper begins with a brief introduction of basic concepts of aluminum alloys and technology characterization of laser selective melting. In addition, solidification theory of SLM process and formation mechanism of metallurgical defects are discussed. Then, the current research status of microstructure, properties and heat treatment of SLM processing aluminum alloys is systematically reviewed respectively. Lastly, a future outlook is given at the end of this review paper.

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    Fatigue behavior of Ti-6Al-4V cellular structures fabricated by additive manufacturing technique
    Dechun Ren, Shujun Li, Hao Wang, Wentao Hou, Yulin Hao, Wei Jin, Rui Yang, R. Devesh K.Misra, Lawrence E.Murr
    J. Mater. Sci. Technol., 2019, 35 (2): 285-294.  DOI: 10.1016/j.jmst.2018.09.066
    Abstract   HTML   PDF

    Porous titanium and its alloys have been considered as promising replacement for dense implants, as they possess low elastic modulus comparable to that of compact human bones and are capable of providing space for in-growth of bony tissues to achieve a better fixation. Recently, the additive manufacturing (AM) method has been successfully applied to the fabrication of Ti-6Al-4V cellular meshes and foams. Comparing to traditional fabrication methods, the AM method offers advantages of accurate control of complex cell shapes and internal pore architectures, thus attracting extensive attention. Considering the long-term safety in the human body, the metallic cellular structures should possess high fatigue strength. In this paper, the recent progress on the fatigue properties of Ti-6Al-4V cellular structures fabricated by the AM technique is reviewed. The various design factors including cell shapes, surface properties, post treatments and graded porosity distribution affecting the fatigue properties of additive manufactured Ti-6Al-4V cellular structures were introduced and future development trends were also discussed.

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    Effect of ultrasonic vibration-assisted laser surface melting and texturing of Ti-6Al-4V ELI alloy on surface properties
    Sourabh Biswas, S. Habib Alavi, Bhishma Sedai, Frank D.Blum, Sandip P.Harimkar
    J. Mater. Sci. Technol., 2019, 35 (2): 295-302.  DOI: 10.1016/j.jmst.2018.09.057
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    Ultrasonic vibration-assisted laser surface processing that involves application of vertical ultrasonic vibrations to the Ti-6Al-4V alloy substrates while being irradiated with a CO2 laser was performed for the development of laser melted and textured surfaces with potential applications in biomedical implants. The laser processing resulted in very consistent repeating undulating grooved surfaces, and the undulations were significantly more pronounced in the samples processed with higher ultrasonic power outputs. The phase evolution, studied by x-ray diffraction, confirmed that the laser processing triggered transformation of globular α → acicular α and martensitic α' as well as increased amounts of retained β phases, which were also reflected in the microscopic analysis. The surface texture developed by laser processing resulted in increased surface wettability with increasing ultrasonic power output. The textured surfaces exhibited marked decrease in coefficients of friction during sliding wear testing performed under simulated body fluid due to lubricant entrainment within the textured grooves. The texturing also resulted in significant reduction in surface contact area during the wear process, which considerably reduced the overall wear rates due to abrasive wear.

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    Additive manufacturing of Ti-6Al-4V lattice structures with high structural integrity under large compressive deformation
    Kun Yang, Jian Wang, Liang Jia, Guangyu Yang, Huiping Tang, Yuanyuan Li
    J. Mater. Sci. Technol., 2019, 35 (2): 303-308.  DOI: 10.1016/j.jmst.2018.10.029
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    Additively manufactured Ti-6Al-4V lattice structures have found important niche applications. However, they often show insufficient compressive ductility or insufficient structural integrity. In this study, a batch of 45 octahedral Ti-6Al-4V lattice structures was manufactured in three different strut diameters (0.5, 1.0, 1.5 mm) by selective electron beam melting (SEBM). The influence of post-SEBM annealing on the compressive deformation characteristics of the lattice structure was investigated. The as-built Ti-6Al-4V lattices fragmented when the compressive strain reached 13%-23% depending on strut diameter. Annealing at 950 °C (β transus temperature: 995 °C) only slightly improved the compressive ductility of the lattice structures. However, annealing at 1050 °C (β-annealing) fundamentally changed the compressive deformation mode of the lattice structures. The resultant compressive stress-strain curve was featured by a long smooth plateau and no facture occurred even after significant densification of the lattice structure had taken place (>50% of compressive strain).

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    Microstructure and mechanical properties of Ti-6Al-4V-5% hydroxyapatite composite fabricated using electron beam powder bed fusion
    César A.Terrazas, Lawrence E.Murr, Diego Bermudez, Edel Arrieta, David A.Roberson, Ryan B.Wicker
    J. Mater. Sci. Technol., 2019, 35 (2): 309-321.  DOI: 10.1016/j.jmst.2018.10.025
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    A novel, Ti-6Al-4V (Ti64)/Hydroxyapatite (HA at 5% by weight concentration) metal/ceramic composite has been fabricated using electron beam powder bed fusion (EPBF) additive manufacturing (AM): specifically, the commercial electron beam melting (EBM®) process. In addition to solid Ti64 and Ti64/5% HA samples, four different unit cell (model) open-cellular mesh structures for the Ti64/5% HA composite were fabricated having densities ranging from 0.68 to 1.12 g/cm3, and corresponding Young’s moduli ranging from 2.9 to 8.0 GPa, and compressive strengths ranging from ~3 to 11 MPa. The solid Ti64/5%HA composite exhibited an optimal tensile strength of 123 MPa, and elongation of 5.5% in contrast to a maximum compressive strength of 875 MPa. Both the solid composite and mesh samples deformed primarily by brittle deformation, with the mesh samples exhibiting erratic, brittle crushing. Solid, EPBF-fabricated Ti64 samples had a Vickers microindentation hardness of 4.1 GPa while the Ti64/5%HA solid composite exhibited a Vickers microindentation hardness of 6.8 GPa. The lowest density Ti64/5%HA composite mesh strut sections had a Vickers microindentation hardness of 7.1 GPa. Optical metallography (OM) and scanning electron microscopy (SEM) analysis showed the HA dispersoids to be highly segregated along domain or grain boundaries, but homogeneously distributed along alpha (hcp) platelet boundaries within these domains in the Ti64 matrix for both the solid and mesh composites. The alpha platelet width varied from ~5 μm in the EPBF-fabricated Ti64 to ~1.1 μm for the Ti64/5%HA mesh strut. The precursor HA powder diameter averaged 5 μm, in contrast to the dispersed HA particle diameters in the Ti64/5%HA composite which averaged 0.5 μm. This work highlights the use of EPBF AM as a novel process for fabrication of a true composite structure, consisting of a Ti64 matrix and interspersed and exposed HA domains, which to the authors’ knowledge has not been reported before. The results also illustrate the prospects not only for fabricating specialized, novel composite bone replacement scaffolds and implants, through the combination of Ti64 and HA, but also prospects for producing a variety of related metal/ceramic composites using EPBF AM.

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    Fabrication of commercial pure Ti by selective laser melting using hydride-dehydride titanium powders treated by ball milling
    Wei Xu, Shiqi Xiao, , Gang Chen, Chengcheng Liu, Xuanhui Qu
    J. Mater. Sci. Technol., 2019, 35 (2): 322-327.  DOI: 10.1016/j.jmst.2018.09.058
    Abstract   HTML   PDF

    Micro-fine sphericalpowders are recommended for selective laser melting (SLM). However, they are mostly expensive due to the complex manufacturing technique and low yield. In this paper, using low-cost treated hydride-dehydride (HDH) Ti powders, commercial pure Ti (CP-Ti) was successfully fabricated by SLM. After 4-h milling, the resulting powders become near-spherical with no obvious angularity, and have optimal flowability with the apparent density of 1.64 ± 0.02 g/cm3, tap density of 2.10 ± 0.04 g/cm3, angle of repose 40.11°±0.09°, and Carr’s index of 77.74 ± 0.15. The microstructure was determined with full acicular martensitic α′ phase. The CP-Ti can achieve superior mechanical properties with the ultimate tensile strength of 876.1 ± 20.5 MPa and elongation of (14.7 ± 0.5)%, which exhibit distinctly competitive compared to the as-cast CP-Ti or Ti-6Al-4V. Excellent mechanical properties together with its low-cost make SLM-fabricated CP-Ti from modified HDH Ti powders show promising applications.

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    Microstructure and compressive/tensile characteristic of large size Zr-based bulk metallic glass prepared by laser solid forming
    Xin Lin, Yuanyuan Zhang, Gaolin Yang, Xuehao Gao, Qiao Hu, Jun Yu, Lei Wei, Weidong Huang
    J. Mater. Sci. Technol., 2019, 35 (2): 328-335.  DOI: 10.1016/j.jmst.2018.10.033
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    The large size, crack-free Zr55Cu30Al10Ni5 bulk metallic glass (BMGs) with the diameter of 54 mm and the height of 15 mm was built by laser solid forming additive manufacturing technology, whose size is larger than the critical diameter by casting. The microstructure, tensile and compressive deformation behaviors and fracture morphology of laser solid formed Zr55Cu30Al10Ni5 BMGs were investigated. It is found that the crystallization mainly occurs in the heat-affected zones of deposition layers, which consist of Al5Ni3Zr2, NiZr2, ZrCu, CuZr2 phases. The content of amorphous phase in the deposit is about 63%. Under the compressive loading, the deposit presents no plasticity before fracture occurs. The fracture process is mainly controlled by the shear stress and the compressive shear fracture angles of about 39°. The compressive strength reaches 1452 MPa, which is equivalent to that of as-Cast Zr55Cu30Al10Ni5 BMGs, and there exist vein-like patterns, river-like patterns and smooth regions at the compressive fractography. Under the tensile loading, the deposit presents the brittle fracture pattern without plastic deformation. The fracture process exhibits normal fracture model, and the tensile shear fracture angle of about 90°. The tensile strength is only about 609 MPa, and the tensile fractography mainly consists of micro-scaled cores and vein-like patterns, dimple-like patterns, chocolate-like patterns and smooth regions. The results further verified the feasibility and large potential of laser additive manufacturing on fabrication and industrial application of large-scale BMGs parts.

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    3D-printed surface promoting osteogenic differentiation and angiogenetic factor expression of BMSCs on Ti6Al4V implants and early osseointegration in vivo
    Jinkai Zhang, Wenhui Zhou, Hui Wang, Kaili Lin, Fengshan Chen
    J. Mater. Sci. Technol., 2019, 35 (2): 336-343.  DOI: 10.1016/j.jmst.2018.09.063
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    Three-dimensional-printed (3D-P) titanium implants display many advantages, such as design flexibility, higher efficiency, the capability to easily construct complex or customized structures, etc., and is believed to potentially replace traditional implants. However, the biological performance of the 3D-P titanium surface has not been investigated systematically. Herein, we analyzed the surface characteristics of 3D-P Ti6Al4V implants and evaluated the biological responses of bone marrow derived mesenchymal stromal cells (BMSCs) to the 3D-P surface in vitro. Moreover, after implantation into the rat femoral condyle for 3 and 6 weeks, the osseointegration performance was evaluated. The results showed the 3D-P Ti6Al4V implant presented distinct fluctuant macroscale rough surface and relatively better hydrophilicity which enhanced the adhesion, proliferation, osteogenic differentiation and angiogenetic factor expression of BMSCs. Moreover, the in vivo osseointegration performance was also better than that of the control group at the early stage. The present study suggested the 3D-P titanium alloy is a promising candidate to be used as implant material.

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    Epitaxial growth and oxidation behavior of an overlay coating on a Ni-base single-crystal superalloy by laser cladding
    Jingjing Liang, Yongsheng Liu, Jinguo Li, Yizhou Zhou, Xiaofeng Sun
    J. Mater. Sci. Technol., 2019, 35 (2): 344-350.  DOI: 10.1016/j.jmst.2018.10.011
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    An overlay coating material was deposited on a single crystal superalloy SRR99 by laser cladding. The microstructure and oxidation behavior of this coating was investigated through scanning electron microscopy (SEM) and X-ray diffraction (XRD). The results indicated that although the composition of the coating was chosen based on the γ' composition in René N5 superalloy, the primary solidification phase of this coating during laser cladding was γ-Ni. Furthermore, under the laser cladding condition, fine parallel dendrites grew epitaxially in the coating from the substrate, indicating the single crystal structure of the substrate was reproduced. When the single crystal MCrAlY coating was oxidized at 1000℃, both θ-Al2O3 and α-Al2O3 formed during initial oxidation process. As the oxidation time proceeded, the presence of θ-Al2O3 facilitated the formation of NiAl2O4 spinel oxide. Once the spinel was observed, it flourished and induced some porosity in the scale. When the scale thickness increased to 6-7 μm, large area spallation of the scale began.

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    Grain boundary and microstructure engineering of Inconel 690 cladding on stainless-steel 316L using electron-beam powder bed fusion additive manufacturing
    I.A. Segura, L.E. Murr, C.A. Terrazas, D. Bermudez, J. Mireles, V.S.V. Injeti, K. Li, B. Yu, R.D.K. Misra, R.B. Wicker
    J. Mater. Sci. Technol., 2019, 35 (2): 351-367.  DOI: 10.1016/j.jmst.2018.09.059
    Abstract   HTML   PDF

    This research explores the prospect of fabricating a face-centered cubic (fcc) Ni-base alloy cladding (Inconel 690) on an fcc Fe-base alloy (316 L stainless-steel) having improved mechanical properties and reduced sensitivity to corrosion through grain boundary and microstructure engineering concepts enabled by additive manufacturing (AM) utilizing electron-beam powder bed fusion (EPBF). The unique solidification and associated constitutional supercooling phenomena characteristic of EPBF promotes [100] textured and extended columnar grains having lower energy grain boundaries as opposed to random, high-angle grain boundaries, but no coherent {111} twin boundaries characteristic of conventional thermo-mechanically processed fcc metals and alloys, including Inconel 690 and 316 L stainless-steel. In addition to [100] textured grains, columnar grains were produced by EPBF fabrication of Inconel 690 claddings on 316 L stainless-steel substrates. Also, irregular 2-3 μm diameter, low energy subgrains were formed along with dislocation densities varying from 108 to 109 cm-2, and a homogeneous distribution of Cr23C6 precipitates. Precipitates were formed within the grains (with ~3 μm interparticle spacing), but not in the subgrain or columnar grain boundaries. These inclusive, hierarchical microstructures produced a tensile yield strength of 0.527 GPa, elongation of 21%, and Vickers microindentation hardness of 2.33 GPa for the Inconel 690 cladding in contrast to a tensile yield strength of 0.327 GPa, elongation of 53%, and Vickers microindentation hardness of 1.78 GPa, respectively for the wrought 316 L stainless-steel substrate. Aging of both the Inconel 690 cladding and the 316 L stainless-steel substrate at 685 °C for 50 h precipitated Cr23C6 carbides in the Inconel 690 columnar grain boundaries, but not in the low-angle (and low energy) subgrain boundaries. In contrast, Cr23C6 carbides precipitated in the 316 L stainless-steel grain boundaries, but not in the low energy coherent {111} twin boundaries. Consequently, the Inconel 690 subgrain boundaries essentially serve as surrogates for coherent twin boundaries with regard to avoiding carbide precipitation and corrosion sensitization.

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    Laser additive manufacturing of Zn porous scaffolds: Shielding gas flow, surface quality and densification
    Peng Wen, Yu Qin, Yanzhe Chen, Maximilian Voshage, Lucas Jauer, Reinhart Poprawe, Johannes Henrich Schleifenbaum
    J. Mater. Sci. Technol., 2019, 35 (2): 368-376.  DOI: 10.1016/j.jmst.2018.09.065
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    Zn based metals have exhibited promising prospects as a structural material for biodegradable applications. Pure Zn porous scaffolds were produced by laser powder bed fusion (LPBF) based on data files of designing and CT scanning. Massive Zn evaporation during laser melting largely influenced the formation quality during LPBF of Zn metal. The metal vapor in processing chamber was blown off and suctioned out efficiently by an optimized gas circulation system. Numerical analysis was used to design and testify the performance of gas flow. The surface of scaffolds was covered with numerous particles in different sizes. Processing pores occurred near the outline contour of struts. The average grain size in width was 8.5 μm, and the hardness was 43.8 HV. Chemical plus electrochemical polishing obtained uniform and smooth surface without processing pores, but the diameter of struts reduced to 250 μm from the design value 300 μm. The poor surface quality and processing pores were resulted by the splashing particles included spatters and powders due to the recoil force of evaporation, and the horizontal movement of liquid metal due to overheating and wetting. The insufficient melting at the outline contour combined with good wetting of Zn liquid metal further increased the surface roughness and processing pores.

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    High strength and ductility of 34CrNiMo6 steel produced by laser solid forming
    Chunping Huang, Xin Lin, Fencheng Liu, Haiou Yang, Weidong Huang
    J. Mater. Sci. Technol., 2019, 35 (2): 377-387.  DOI: 10.1016/j.jmst.2018.09.062
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    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.

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    High mechanical strengths and ductility of stainless steel 304L fabricated using selective laser melting
    Q.B. Nguyen, Z. Zhu, F.L. Ng, B.W. Chua, S.M.L. Nai, J. Wei
    J. Mater. Sci. Technol., 2019, 35 (2): 388-394.  DOI: 10.1016/j.jmst.2018.10.013
    Abstract   HTML   PDF

    Achieving not only high mechanical strengths but also high ductility is recently established using an additive manufacturing technique called selective laser melting. In the present study, stainless steel 304L fully dense samples were successfully printed using the 3D systems - ProX 300 printing machine. The ductility and tensile yield strength were almost two and three times higher compared to those of ASTM cast’s alloy. Honey comb like nano-cellular structure with different orientation was observed in the fine grains (~4 μm) due to fast cooling rate. In addition, the formation of martensite phase in random grains is also a contributor to the strengths. Furthermore, negative residual stresses in the build and horizontal directions were detected and assisted further increase in the tensile strength. Fractography revealed the ductile feature of plastic deformation and the crack openings at unmelted particles or pores.

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    Effect of laser incident energy on microstructures and mechanical properties of 12CrNi2Y alloy steel by direct laser deposition
    Tingting Guan, Suiyuan Chen, Xueting Chen, Jing Liang, Changsheng Liu, Mei Wang
    J. Mater. Sci. Technol., 2019, 35 (2): 395-402.  DOI: 10.1016/j.jmst.2018.10.024
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    This work aims to establish the effect of laser energy area density (EAD) as the laser incident energy on density, microstructures and mechanical properties of direct laser deposition (DLD) 12CrNi2Y alloy steel. The results show that the density of DLD 12CrNi2Y alloy steel increases at initial stage and then decreases with an increase of EAD, the highest density of alloy steel sample is 98.95%. The microstructures of DLD 12CrNi2Y alloy steel samples are composed of bainite, ferrite and carbide. With increase of EAD, the microstructures transform from polygonal ferrite (PF) to granular bainite (GB). The martensite-austenite constituent (M-A) in GB transforms from flake-like paralleling to the bainite ferrite laths to granular morphology. It is also found that the average width of laths in finer GB can be refined from 532 nm to 302 nm, which improves the comprehensive properties of DLD 12CrNi2Y alloy steel such as high hardness of 342 ± 9 HV0.2, yield strength of 702 ± 16 MPa, tensile strength of 901 ± 14 MPa and large elongation of 15.2%±0.6%. The DLD 12CrNi2Y material with good strength and toughness could meet the demand of alloy steel components manufacturing.

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    Effects of microstructure on fatigue crack propagation behavior in a bi-modal TC11 titanium alloy fabricated via laser additive manufacturing
    Yafei Wang, Rui Chen, Xu Cheng, Yanyan Zhu, Jikui Zhang, Huaming Wang
    J. Mater. Sci. Technol., 2019, 35 (2): 403-408.  DOI: 10.1016/j.jmst.2018.10.031
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    In this study, the crack propagation behaviors in the equiaxed and equiaxed-columnar grain regions of a heat-treated laser additive manufacturing (LAM) TC11 alloy with a special bi-modal microstructure are investigated. The results indicate that the alloy presents a special bi-modal microstructure that comprises a fork-like primary α (αp) phase surrounded by a secondary α colony (αs) in the β phase matrix after the heat treatment is completed. The samples demonstrate a fast crack growth rate with larger da/dN values through the equiaxed grain sample versus across the equiaxed-columnar grain sample at low ΔK values (<13.8). The differences that are observed between the crack propagation behaviors (in the crack initiation stage) of the samples can be mostly attributed to the different size and morphology of the αp lamellae and αs colony within the grains in the equiaxed and columnar grain regions rather than the grain boundaries. The cracks prefer to grow along the α/β boundary with a smooth propagation route and a fast propagation rate in the equiaxed grain region, where the αp and α clusters have a large size. However, in the columnar grain region, small and randomly distributed αp lamellae generate a zigzag-shaped propagation path with a reduction in the da/dN value. Additionally, the change in the size of the αp lamellae in the equiaxed grains (heat affected bands, HAB) is also observed to influence the propagation behavior of the crack during the crack initiation stage.

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