Co-reporter:Chenglong Ma;Donghua Dai;Guanqun Yu;Mujian Xia;Hongyu Chen
CrystEngComm (1999-Present) 2017 vol. 19(Issue 7) pp:1089-1099
Publication Date(Web):2017/02/13
DOI:10.1039/C6CE02284G
Selective laser melting (SLM) was applied to prepare TiC/Ti–Ni composites by using a mixed powder composed of titanium powder, nickel powder and titanium carbide powder. The result indicated that fine TiC particles were transformed into in situ Ti6C3.75 dendrites based on the complete melting mechanism and coarse TiC particles just partly experienced melting to form epitaxial dendrites along the margin of the remaining TiC particles. Besides, laser scan speed was found to have a significant influence on Ti6C3.75 dendrite growth. To give a better insight into the thermodynamic behaviour of TiC within the mesoscopic molten pool which was difficult to be monitored by experimental methods, a numerical simulation method was used. Due to the existence of differences in thermal conductivity between TiC and the matrix, reverse thermal hysteresis within TiC particles was predicted, influencing the temperature and its gradient on the TiC particles. Furthermore, the melting mechanism of TiC particles and growth processes of Ti6C3.75 dendrites were discussed. Moreover, nanoindentation load–penetration depth curves were also measured, reaching a value of 6.84 GPa at the applied v of 350 mm s−1.
Co-reporter:Hongyu Chen, Dongdong Gu, Donghua Dai, Chenglong Ma, Mujian Xia
Materials Science and Engineering: A 2017 Volume 682() pp:279-289
Publication Date(Web):13 January 2017
DOI:10.1016/j.msea.2016.11.047
This paper systematically investigated the crystallization thermodynamics and dynamic process within melt pool of 5CrNi4Mo steel fabricated by selective laser melting (SLM). The experimental results in conjunction with finite element analysis (FEA) demonstrated that the nucleation rate during SLM process was determined by the combined effects of supercooling degree and transfer capacity of atoms near solid/liquid interface; variant nucleation rate in different region of melt pool caused microstructure heterogeneity. Chemical compositions, including Cr, Ni and C, were observed to be homogeneously distributed due to the rapid solidification of the material. Specimens built along different orientation exhibited discrepant tensile properties due to the different deformation mode during loading. All the as-fabricated SLM-processed tensile specimens showed unfavorable ductility due to heterogeneous microstructures and residual stress concentration. After post-vacuum heat treatment, for horizontally built specimens, the elongation was significantly elevated from 5.6–9.7% and the toughness was enhanced form 63.68 J/m3 to 134.12 J/m3. The tensile strength increased marginally from 1576 MPa to 1682 MPa. These promotions were mainly caused by pronounced relief of intrinsic residual stress and recrystallization effect.
Co-reporter:Jiapeng Xiong, Dongdong Gu, Hongyu Chen, Donghua Dai, Qimin Shi
Materials & Design 2017 Volume 120(Volume 120) pp:
Publication Date(Web):15 April 2017
DOI:10.1016/j.matdes.2017.02.022
•A quantitatively optimized simulation on the inclination angle of a modified negative Poisson's ratio structure is presented.•Optimal structural parameters are obtained, the inclination angle is 9° and the corresponding re-entrant angle is 79°.•In this study, the optimal scanning speed is 2000 mm/s while the laser power is 300 W.•Existence of fillets remits the stress concentration phenomenon of re-entrant struts and slightly lowers the value of Poisson's ratio.In this paper, a quantitative optimization of a modified re-entrant negative Poisson's ratio (NPR) structure whose overhanging struts were replaced with inclined ones to avoid the support structures which were generally required in the selective laser melting (SLM) process was carried out using finite element analysis (FEA) method. Poisson's ratios of models with inclined links of different strut inclination angles were calculated. The optimal simulated inclination angle of struts was 9° and the corresponding optimum re-entrant angle was 79° while the thickness of the strut was 0.9 mm. The influence of fillet radii on Poisson's ratio and stress concentration was also discussed and the Poisson's ratio decreased and the stress concentration phenomenon remitted to some extent with increased fillet radii. In order to verify the improvement of the manufacturability of structures with inclined links, AlSi10Mg specimens were fabricated by SLM process using different scanning speeds and compression tests were also developed. Experimentally obtained results and simulation predicted ones were in good agreement. Therefore, the custom-tailor service could be accomplished in this way and the modified NPR structure with better surface quality could be obtained.Download high-res image (147KB)Download full-size image
Co-reporter:Jiubin Jue, Dongdong Gu, Kun Chang, Donghua Dai
Powder Technology 2017 Volume 310(Volume 310) pp:
Publication Date(Web):1 April 2017
DOI:10.1016/j.powtec.2016.12.079
•The Al-Al2O3 composites are successfully fabricated by SLM.•The microstructure of Al2O3 phase experiences successive morphological changes.•A continuous and compatible interface is developed at appropriate laser parameters.•SLM prepared composites exhibit excellent hardness and wear performances.•Relationship of process, microstructures and mechanical properties is established.The promising selective laser melting (SLM) technology was introduced to prepare Al based composites reinforced by Al2O3 particles. The influence of SLM processing parameters on the densification behavior, microstructure, microhardness and resultant wear performance was studied in detail. The results revealed that the near fully dense composite part (97.3% theoretical density) was achieved with the optimized v of 550 mm/s applied. A proper decrease in the applied v to 550 mm/s was proved feasible to favor the Al2O3 particle dispersion homogeneity due to the trapping effect of Al2O3 particle with the advancing interface in the molten pool. Besides, a continuous and compatible interface was developed in this case. At an even lower v of 450 mm/s, the homogeneously dispersed Al2O3 reinforcements exhibited a novel ring structures along the boundaries of molten pool, but showing a significant coarsening morphology. The optimally prepared fully dense Al-Al2O3 composite part exhibited excellent hardness with a mean value of 175 HV0.1 and superior wear performance with a considerably low coefficient of friction of 0.11 and a significantly reduced wear rate of 4.75 × 10− 5 mm3 N− 1 m− 1.Download high-res image (355KB)Download full-size image
Co-reporter:Donghua Dai, Dongdong Gu, Reinhart Poprawe, Mujian Xia
Science Bulletin 2017 Volume 62, Issue 11(Volume 62, Issue 11) pp:
Publication Date(Web):15 June 2017
DOI:10.1016/j.scib.2017.05.007
A transient three dimensional model for describing the temperature behavior, thermo-capillary convection, microstructure evolution and the resultant mechanical properties during selective laser melting of AlN/AlSi10Mg composite is proposed. The powder-solid transformation, temperature dependent physical properties and the preservation of the heat are taken into account. The effect of the additive manufacturing multilayer feature on the molten pool dynamics, cooling rate, crystal size, microstructure morphology, micro-hardness and types of the residual stress has been investigated. It shows that the operating temperature and the thermo-capillary convection obtained within the molten pool generally increases as the processing multilayers are successively added, while the thermal effect depth is negatively reduced. The preferential direction of the heat diffusion generally changes from a downward pattern, then to the slightly strengthened horizontal direction and finally to a typically horizontal one for various deposited layers being processed. Therefore, the microstructure of the solidified part along the building direction (Region I to Region V) undergoes an interesting transformation: directional columnar cellular microstructure, crosswise-extended cellular microstructure, refined cellular microstructure, fragmentation microstructure and the coarse cellular microstructure. The tensile stress and the compressive stress are comprehensively obtained within the finally solidified layers, significantly influencing the micro-hardness.Download high-res image (298KB)Download full-size image
Co-reporter:Dongdong Gu, Beibei He
Computational Materials Science 2016 Volume 117() pp:221-232
Publication Date(Web):May 2016
DOI:10.1016/j.commatsci.2016.01.044
•Detailed method for the establishment of the thermal and stress model was revealed.•In-depth understanding of the residual stresses distribution during SLM of Ti–Ni powders was proposed.•The variations of residual stresses in different positions were comprehensively studied.•The corresponding experiments were conducted to verify the results of the simulation.A three-dimensional transient finite element method (FEM) model was established to predict the stress distribution of parts shaped by selective laser melting (SLM) technology, using Ti–Ni two-component powders as the raw materials. The moving heat source with a Gaussian distribution was applied in the simulation process. By simulating the laser beam scanning process, the peak values of the thermal stresses were first recorded at the onset of the first track where the first heating–cooling cycle occurred. After the whole part was cooled down, the largest residual stresses were found at the end of the first track and the last track. To verify the simulation results, the experimental investigation with the same parameters was conducted. The initial area of the first track of the fabricated part was fully dense where the residual stresses were small. The larger residual stresses obtained at the following track resulted in the formation of the cracks at the end edge of the parts, which testified that the results were in good agreement with the simulation predictions.
Co-reporter:Chenglong Ma, Dongdong Gu, Chen Hong, Beibei He, Kun Chang, Qimin Shi
Surface and Coatings Technology 2016 Volume 291() pp:43-53
Publication Date(Web):15 April 2016
DOI:10.1016/j.surfcoat.2016.02.013
•The in-situ TiO2 reinforced Ti–Ni composite coating was successfully prepared by LMD;•The microstructure evolution along the depth of the deposited layer as well as with the variant of E was analyzed;•The formation mechanism of in-situ TiO2 submicronflower was present;•The LMD-processed layer obtained at 96 kJ/m showed high average microhardness and improved tribological property.The in-situ TiO2 reinforced Ti–Ni composite coating on carbon steel was successfully prepared by laser metal deposition (LMD) using Ti–Ni as-mixed powder with an atomic ratio of 60:40. With the aim of in-situ reaction design during LMD processing, a trace of oxygen mixed with the shielding gas was introduced. Different “laser energy input per unit length” (E) by changing the laser power was set to investigate the influence on the deposition quality and attendant microstructure and mechanical property of the LMD-processed layer. TiO2 particles with unique flower-like structure formed when the applied laser energy E ≤ 96 kJ/m, while the apparent oxidation of grain boundaries was observed as E increased to 120 kJ/m. The formation mechanism of in-situ TiO2 particles with a flower-like structure was present. At the optimized process parameter of 96 kJ/m, the LMD-processed layer showed the highest densification degree free of any pores and cracks. The corresponding mechanical properties were measured, showing the relatively high average microhardness of HV0.2 790 and significantly improved tribological property containing lower coefficient of friction of 0.4 and more smooth worn surface.
Co-reporter:Dongdong Gu
Science Bulletin 2016 Volume 61( Issue 22) pp:1718-1722
Publication Date(Web):2016 November
DOI:10.1007/s11434-016-1191-y
Co-reporter:Mujian Xia;Guanqun Yu;Donghua Dai;Hongyu Chen
Science Bulletin 2016 Volume 61( Issue 13) pp:1013-1022
Publication Date(Web):2016 July
DOI:10.1007/s11434-016-1098-7
A mesoscopic model has been established to investigate the thermodynamic mechanisms and densification behavior of nickel-based superalloy during additive manufacturing/three-dimensional (3D) printing (AM/3DP) by numerical simulation, using a finite volume method (FVM). The influence of the applied linear energy density (LED) on dimensions of the molten pool, thermodynamic mechanisms within the pool, bubbles migration and resultant densification behavior of AM/3DP-processed superalloy has been discussed. It reveals that the center of the molten pool slightly shifts with a lagging of 4 μm towards the center of the moving laser beam. The Marangoni convection, which has various flow patterns, plays a crucial role in intensifying the convective heat and mass transfer, which is responsible for the bubbles migration and densification behavior of AM/3DP-processed parts. At an optimized LED of 221.5 J/m, the outward convection favors the numerous bubbles to escape from the molten pool easily and the resultant considerably high relative density of 98.9 % is achieved. However, as the applied LED further increases over 249.5 J/m, the convection pattern is apparently intensified with the formation of vortexes and the bubbles tend to be entrapped by the rotating flow within the molten pool, resulting in a large amount of residual porosity and a sharp reduction in densification of the superalloy. The change rules of the relative density and the corresponding distribution of porosity obtained by experiments are in accordance with the simulation results.
Co-reporter:Ting Rong, Dongdong Gu, Qimin Shi, Sainan Cao, Mujian Xia
Surface and Coatings Technology 2016 Volume 307(Part A) pp:418-427
Publication Date(Web):15 December 2016
DOI:10.1016/j.surfcoat.2016.09.011
The WC/Inconel 718 composites were fabricated by selective laser melting (SLM). The laser processing parameters played an important role in determining the microstructure and performance of the WC/Inconel 718 composite parts. With the decrease in the laser scanning speed, the densification rate increased and achieved 97.8% at a scanning speed of 350 mm/s. As an optimal scan speed of 450 mm/s was applied, the WC/Inconel 718 composite part obtained a mean microhardness as high as 393.2 HV0.1. At the same time, a regular and orderly gradient interface with a mean thickness of 0.27 μm surrounded by a diffusion layer was obtained. What is more, the chemical composition of the gradient interface and the diffusion layer were X3C17 and XC4 (X = W, Ni, Cr, Fe), respectively. Meanwhile, the composite acquired a considerably low coefficient of friction (COF) of 0.35 with almost no fluctuation and attendant wear rate of 2.5 × 10− 4 mm3 N− 1 m− 1 and the wear mechanism changed continuously from severe abrasive wear to adhesive wear. Subsequently the effects of the tailored gradient interface on wear performance were discussed. It revealed that the existence of gradient interface showed a very important role in improving the wear performance of the SLM-processed WC/Inconel 718 composite parts.
Co-reporter:Donghua Dai, Dongdong Gu
Applied Surface Science 2015 Volume 355() pp:310-319
Publication Date(Web):15 November 2015
DOI:10.1016/j.apsusc.2015.07.044
Highlights
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Thermal behavior and top surface morphology during SLM of composites are simulated.
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Top surface activity with various protective atmospheres is disclosed.
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Mechanism of the influence of metal vaporization on surface quality is elucidated.
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Simulation results show good agreement with the obtained experimental results.
Co-reporter:Dongdong Gu, Hongqiao Wang, Donghua Dai, Pengpeng Yuan, Wilhelm Meiners, Reinhart Poprawe
Scripta Materialia 2015 Volume 96() pp:25-28
Publication Date(Web):February 2015
DOI:10.1016/j.scriptamat.2014.10.011
A novel ring-structured nanoscale TiC reinforcement with a regular distribution was tailored along the grain boundaries of the matrix by selective laser melting (SLM) to produce TiC/AlSi10 Mg nanocomposite parts. Relative to the SLM-processed unreinforced AlSi10 Mg part, the TiC/AlSi10 Mg nanocomposite part with the novel reinforcement architecture exhibited elevated microhardness (188.3 HV0.1) and tensile strength (486 MPa) without a reduction in elongation (10.9%), due to the combined effects of grain refinement and grain boundary strengthening caused by the ring-structured nanoscale TiC reinforcement.
Co-reporter:Chen Hong, Dongdong Gu, Donghua Dai, Moritz Alkhayat, Wolf Urban, Pengpeng Yuan, Sainan Cao, Andres Gasser, Andreas Weisheit, Ingomar Kelbassa, Minlin Zhong, Reinhart Poprawe
Materials Science and Engineering: A 2015 Volume 635() pp:118-128
Publication Date(Web):21 May 2015
DOI:10.1016/j.msea.2015.03.043
Laser metal deposition (LMD) additive manufacturing process was applied to produce ultrafine TiC particle reinforced Inconel 625 composite parts. The effects of laser energy input per unit length (LEIPUL) on microstructure development, densification response, and mechanical performance including wear performance and tensile properties were comprehensively studied. A relationship of processing conditions, microstructural characteristics, mechanical performance, and underlying strengthening mechanisms was proposed for a successful LMD of high-performance Inconel based composite parts. It revealed that using an insufficient LEIPUL of 33 kJ/m lowered the densification behavior of LMD-processed parts, due to the appearance of residual large-sized pores in inter-layer areas of the parts. An increase in LEIPUL above 100 kJ/m yielded the near fully dense composite parts after LMD. On increasing LEIPUL, the TiC reinforcing particles became significantly refined and smoothened via the elevated melting of particle surfaces and the dispersion state of ultra-fine reinforcing particles was homogenized due to the efficient action of Marangoni flow within the molten pool. The dendrites of Ni–Cr γ matrix underwent a successive change from an insufficiently developed, disordered microstructure to a refined, ordered microstructure with the increase of LEIPUL. However, the columnar dendrites of the matrix were coarsened apparently at an excessive LEIPUL of 160 kJ/m because of the elevated thermalization of the input laser energy. The formation of the refined columnar dendrites of Ni–Cr γ matrix combined with the homogeneously distributed ultra-fine reinforcing particles contributed to the enhancement of wear performance of LMD-processed composites with a considerably low coefficient of friction (COF) of 0.30 and reduced wear rate of 1.3×10−4 mm3/N m. The optimally prepared TiC/Inconel 625 composite parts demonstrated a ductile fracture mode with a sufficiently high tensile strength of 1077.3 MPa, yield strength of 659.3 MPa, and elongation of 20.7%. The superior tensile properties of LMD-processed parts were attributed to the significant grain refinement effect of the matrix during laser processing and the efficient prohibition of ultrafine reinforcing particles on the mobility of dislocations.
Co-reporter:Fei Chang, Dongdong Gu, Donghua Dai, Pengpeng Yuan
Surface and Coatings Technology 2015 Volume 272() pp:15-24
Publication Date(Web):25 June 2015
DOI:10.1016/j.surfcoat.2015.04.029
•Novel in-situ Al4SiC4 + SiC hybrid reinforced Al composites were prepared by SLM.•Starting SiC particle size affected microstructural and mechanical properties.•In-situ Al4SiC4 had multiple crystal growth morphologies during SLM process.•Enhancement mechanisms of microhardness and wear performance were disclosed.Selective laser melting (SLM) additive manufacturing of the SiC/AlSi10Mg composite powder systems with different starting SiC particle sizes was performed to produce in-situ Al4SiC4 + SiC hybrid reinforced Al matrix composites. The influence of starting SiC particle size on the constitutional phases, microstructural features, and mechanical properties of the SLM-processed composite parts was studied. As the SiC particle size decreased, the extent of in-situ reaction between aluminum melt and SiC particles was enhanced, leading to the elevated formation of Al4SiC4 reinforcing phase. With the fine SiC particles (D50 = 5 μm) used, the residual SiC particles with a reduced size of 3 μm were dispersed homogeneously throughout the matrix, thereby enhancing the microstructural homogeneity of the part. With the enhancement of in-situ reaction, the formation of plate-like and particle-structured Al4SiC4 reinforcement was significantly accelerated, favoring the formation of (Al4SiC4 + SiC)/Al hybrid reinforced composites after SLM. Using fine SiC particles, the reinforcement/matrix wettability was improved, leading to a nearly full densification level of 97.2% theoretical density of SLM-processed part. The microhardness of 218.5 HV0.1 showed at least 50% improvement upon SLM-processed unreinforced AlSi10Mg. The fine SiC particles also reduced the coefficient of friction (COF) by 19% and the wear rate by 66% compared to the SLM part processed with coarse SiC particles.
Co-reporter:Yali Li, Dongdong Gu
Additive Manufacturing 2014 Volumes 1–4() pp:99-109
Publication Date(Web):October 2014
DOI:10.1016/j.addma.2014.09.001
Abstract
A three-dimensional model was developed for studying thermal behavior during selective laser melting (SLM) of commercially pure titanium (CP Ti) powder. The effects of scan speed and laser power on SLM thermal behavior were investigated. The results showed that the average temperature of the powder bed gradually increased during the SLM process, caused by a heat accumulation effect. The maximum molten pool temperature (2248 °C) and liquid lifetime (1.47 ms) were obtained for a successful SLM process for a laser power of 150 W and a laser scan speed of 100 mm/s. The temperature gradient in the molten pool increased slightly (from 1.03 × 104 to 1.07 × 104 °C/mm in the direction perpendicular to the scanning path; from 1.21 × 104 to 1.28 × 104 °C/mm in the thickness direction) when the scan speed was increased from 50 to 200 mm/s, but increased significantly (from 1.29 × 104 to 8.24 × 104 °C/mm in the direction perpendicular to the scanning path; from 1.53 × 104 to 9.84 × 104 °C/mm in the thickness direction) when the laser power was increased from 100 to 200 W. The width and depth of the molten pool decreased (width from 137.1 to 93.8 μm, depth from 64.2 to 38.5 μm) when the scan speed was increased from 50 to 200 mm/s, but increased (width from 71.2 to 141.4 μm, depth from 32.7 to 67.3 μm) when the laser power was increased from 100 to 200 W. Experimental SLM of CP Ti powder was carried out under different laser processing conditions and the microstructure of SLM-produced parts was investigated to demonstrate the reliability of the physical model and simulation results.
Co-reporter:Qingbo Jia, Dongdong Gu
Optics & Laser Technology 2014 Volume 62() pp:161-171
Publication Date(Web):October 2014
DOI:10.1016/j.optlastec.2014.03.008
•Oxidation performance of SLM-processed Inconel 718 at elevated temperatures was comprehensively studied.•Underlying physical and chemical oxidation mechanisms of SLM-processed superalloys were disclosed.•Inherent relationship of oxidation property, SLM process, relative density, phases and microstructures was established.•Laser process and its control methods were optimized to improve high-temperature performance of superalloys.This work presented a comprehensive study of high-temperature oxidation behaviors and mechanisms of Selective laser melting (SLM) processed Inconel 718 superalloy parts using different methods including isothermal oxidation testing, X-ray diffraction, scanning electron microscopy and energy dispersive X-ray spectroscopy. The experimental results revealed that the oxidation process of the tested parts processed at a lower volumetric laser energy density experienced the severe spallation. On reasonably increasing the applied volumetric laser energy density, the oxidation kinetics of the as-produced parts obeyed a parabolic law, exhibiting the significantly improved oxidation resistance performance. The constitutional phases within the oxidation film were identified and the corresponding formation mechanisms were elucidated in detail according to the thermodynamic principles. The cross-sectional morphologies of oxidized Inconel 718 parts indicated that the oxidation microstructure mainly consisted of an external oxidation layer and an internal oxidation zone. The oxidation process was controlled by the outward diffusion of oxide forming elements and inward penetration of oxygen, by which the interaction mechanisms between the microstructures and internal oxidation zones were clarified. On the basis of the experimental results and theoretical analyses, the physical oxidation mechanisms were accordingly established to illustrate the oxidation behaviors of SLM-processed Inconel 718 parts at elevated operative temperatures.
Co-reporter:Donghua Dai, Dongdong Gu
Materials & Design 2014 55() pp: 482-491
Publication Date(Web):March 2014
DOI:10.1016/j.matdes.2013.10.006
•Thermal behavior and densification activity during SLM of composites are simulated.•Temperature distributions and melt pool dimensions during SLM are disclosed.•Motion behaviors of gaseous bubbles in laser induced melt pool are elucidated.•Simulation results show good agreement with the obtained experimental results.Simulation of temperature distribution and densification process of selective laser melting (SLM) WC/Cu composite powder system has been performed, using a finite volume method (FVM). The transition from powder to solid, the surface tension induced by temperature gradient, and the movement of laser beam power with a Gaussian energy distribution are taken into account in the physical model. The effect of the applied linear energy density (LED) on the temperature distribution, melt pool dimensions, behaviors of gaseous bubbles and resultant densification activity has been investigated. It shows that the temperature distribution is asymmetric with respect to the laser beam scanning area. The center of the melt pool does not locate at the center of the laser beam but slightly shifts towards the side of the decreasing X-axis. The dimensions of the melt pool are in sizes of hundreds of micrometers and increase with the applied LED. For an optimized LED of 17.5 kJ/m, an enhanced efficiency of gas removal from the melt pool is realized, and the maximum relative density of laser processed powder reaches 96%. As the applied LED surpasses 20 kJ/m, Marangoni flow tends to retain the entrapped gas bubbles. The flow pattern has a tendency to deposit the gas bubbles at the melt pool bottom or to agglomerate gas bubbles by the rotating flow in the melt pool, resulting in a higher porosity in laser processed powder. The relative density and corresponding pore size and morphology are experimentally acquired, which are in a good agreement with the results predicted by simulation.
Co-reporter:Dongdong Gu, Yves-Christian Hagedorn, Wilhelm Meiners, Guangbin Meng, Rui João Santos Batista, Konrad Wissenbach, Reinhart Poprawe
Acta Materialia 2012 Volume 60(Issue 9) pp:3849-3860
Publication Date(Web):May 2012
DOI:10.1016/j.actamat.2012.04.006
Abstract
This work presents a comprehensive study of the densification behavior, phase and microstructure development, hardness and wear performance of commercially pure Ti parts processed by selective laser melting (SLM). An in-depth relationship between SLM process, microstructures, properties, and metallurgical mechanisms has been established. A combination of a low scan speed and attendant high laser energy density resulted in the formation of microscopic balling phenomenon and interlayer thermal microcracks, caused by a low liquid viscosity, a long liquid lifetime, and resultant elevated thermal stress. In contrast, using a high scan speed produced the disorderly liquid solidification front and considerably large balling, due to an elevated instability of the liquid induced by Marangoni convection. A narrow, feasible process window was accordingly determined to eliminate process defects and result in full densification. The phase constitutions and microstructural characteristics of SLM-processed Ti parts experienced a successive change on increasing the applied scan speeds: relatively coarsened lath-shaped α → refined acicular-shaped martensitic α′ → further refined zigzag-structured martensitic α′, due to the elevated thermal and kinetic undercooling and attendant solidification rate. The optimally prepared fully dense Ti parts had a very high hardness of 3.89 GPa, a reduced coefficient of friction of 0.98 and wear rate of 8.43 × 10−4 mm3 N−1 m−1 in dry sliding wear tests. The formation of an adherent, plastically smeared tribolayer on the worn surface contributed to the enhancement of wear performance.
Co-reporter:Dongdong Gu, Yves-Christian Hagedorn, Wilhelm Meiners, Konrad Wissenbach, Reinhart Poprawe
Composites Science and Technology 2011 Volume 71(Issue 13) pp:1612-1620
Publication Date(Web):9 September 2011
DOI:10.1016/j.compscitech.2011.07.010
Selective Laser Melting (SLM) Additive Manufacturing (AM) process was used to produce nanocrystalline TiC reinforced Ti matrix bulk-form nanocomposites. The influence of “volumetric energy density” (ε) on densification activity, microstructural feature, nanohardness, and wear behavior of SLM-processed parts was studied. The densification levels of TiC/Ti parts remained above 97% as ε ⩾ 120 J/mm3. A further decrease in ε lowered the densification rate, due to the occurrence of balling effect. The TiC reinforcement experienced an interesting morphological change from the coarsened dendritic TiC (360 J/mm3) to the accumulated whisker-structured TiC (180 J/mm3) and to the uniformly dispersed nanoscale lamellar TiC (⩽120 J/mm3). As ε of 120 J/mm3 was properly settled, the dynamic nanohardness (90.9 GPa) and elastic modulus (256 GPa) of SLM-processed TiC/Ti nanocomposites showed respectively ∼22.7-fold and ∼2.4-fold increase upon that of the unreinforced Ti. A uniform distribution of friction coefficient with a low average value <0.2 was obtained, leading to a considerably reduced wear rate of 1.8 × 10−7 mm3/(Nm). A disappearance of nanostructured TiC reinforcement at an elevated ε of 360 J/mm3 lowered the mechanical properties of TiC/Ti part consisting of the coarsened dendritic TiC.Highlights► SLM proves to be a novel method to produce Ti-based bulk-form nanocomposites. ► Integrated control of SLM process is realized by using volumetric energy density. ► Unique metallurgical nature of SLM produces novel nanostructure of TiC reinforcement. ► Homogeneous incorporation of nanostructured TiC favors excellent wear resistance.
Co-reporter:Chuang Li;Yifu Shen;Guangbin Meng;Yufang Li
Advanced Engineering Materials 2011 Volume 13( Issue 5) pp:418-425
Publication Date(Web):
DOI:10.1002/adem.201000377
Abstract
In the present work, in situ TiN/Ti5Si3 nanocomposite powder was prepared by high-energy mechanical alloying of a Ti and Si3N4 powder mixture via the following route: 9Ti + Si3N4 = Ti5Si3 + 4TiN. Constitution phases and microstructural features of the milled powders at different milling times were studied by XRD, SEM, and TEM. The operative formation mechanisms behind the microstructural developments were disclosed. It showed that the original Si3N4 and Ti constituents demonstrated two different reaction mechanisms during milling, i.e., a progressive mechanism of Si3N4 (≤20 h) and a speedy mechanism of Ti (≤10 h). The morphologies of the milled composite powders experienced a successive change: pre-refining – coarsening – re-refining on increasing the applied milling time. The variation of the operative mechanisms was ascribed to the existence/exhaustion of the ductile Ti constituent in the milling system due to the nonoccurrence/initiation of the in situ reaction. The 20 h milled powder was the typical nanocomposites featured by the nanocrystalline Ti5Si3 matrix reinforced with in situ TiN nanoparticles. The grain sizes of the in situ formed Ti5Si3 and TiN phases were generally ≤15 nm, exhibiting coherent interfacial structure between reinforcement and matrix.
Co-reporter:Dongdong Gu, Yves-Christian Hagedorn, Wilhelm Meiners, Konrad Wissenbach, Reinhart Poprawe
Surface and Coatings Technology 2011 205(10) pp: 3285-3292
Publication Date(Web):
DOI:10.1016/j.surfcoat.2010.11.051
Co-reporter:Dongdong Gu, Wilhelm Meiners
Materials Science and Engineering: A 2010 527(29–30) pp: 7585-7592
Publication Date(Web):
DOI:10.1016/j.msea.2010.08.075
Co-reporter:Dongdong Gu, Wilhelm Meiners, Chuang Li, Yifu Shen
Materials Science and Engineering: A 2010 527(23) pp: 6340-6345
Publication Date(Web):
DOI:10.1016/j.msea.2010.06.052
Co-reporter:Dongdong Gu, Yifu Shen
Materials & Design (1980-2015) 2009 Volume 30(Issue 8) pp:2903-2910
Publication Date(Web):September 2009
DOI:10.1016/j.matdes.2009.01.013
Balling effect, as an unfavorable defect associated with direct metal laser sintering (DMLS), is a complex physical metallurgical process. In this work, two kinds of balling phenomena during DMLS of 316L stainless steel powder were investigated and the metallurgical mechanisms of balling were elucidated. It was found that using a low laser power gave rise to the first kind of balling characterized by highly coarsened balls possessing an interrupted dendritic structure in the surface layer of balls. A limited amount of liquid formation and a low undercooling degree of the melt due to a low laser input was responsible for its initiation. The second kind of balling featured by a large amount of micrometer-scaled (∼10 μm) balls on laser sintered surface occurred at a high scan speed. Its formation was ascribed to laser-induced melt splashes caused by a high capillary instability of the melt. Feasible control methods were proposed to alleviate balling phenomena. It showed that increasing the volumetric density of energy input, which was realized by increasing laser power, lowering scan speed, or decreasing powder layer thickness, decreased the tendency of balling. The addition of a trace amount of deoxidant (H3BO3 and KBF4) in the powder yielded a smooth laser sintered surface free of balling.
Co-reporter:Dongdong Gu, Yifu Shen, Zhijian Lu
Materials & Design (1980-2015) 2009 Volume 30(Issue 6) pp:2099-2107
Publication Date(Web):June 2009
DOI:10.1016/j.matdes.2008.08.036
Co-reporter:Dongdong Gu, Yifu Shen, Guangbin Meng
Materials Letters 2009 Volume 63(Issue 29) pp:2536-2538
Publication Date(Web):15 December 2009
DOI:10.1016/j.matlet.2009.08.043
Selective Laser Melting (SLM) was used to consolidate the high-energy ball milled Ti–Al–C composite powder. The morphologies of the in-situ formed TiC grains under different laser processing parameters were investigated. It showed that with increasing the applied laser powers, the TiC grain morphologies experienced successive changes from a laminated shape to an octahedron shape, and then to a truncated near-octahedron shape, and finally to a near-spherical shape. It was believed that the continuously elevated SLM operating temperatures induced by the increasing laser powers played an important role in determining the TiC grain morphologies. Reasonable metallurgical mechanisms for grain growth behind the microstructural developments were proposed.
Co-reporter:Dongdong Gu, Yifu Shen, Zhijian Lu
Materials Letters 2009 Volume 63(18–19) pp:1577-1579
Publication Date(Web):31 July 2009
DOI:10.1016/j.matlet.2009.04.010
The Selective Laser Melting (SLM) Rapid Manufacturing (RM) of the high-energy ball milled Ti–Si3N4 composite powder with the mol ratio of 9:1 was performed in the present work. The microstructural characterizations revealed the formation of TiN reinforced Ti5Si3 matrix composites after laser processing via the in-situ synthesis reaction 9Ti + Si3N4 = 4TiN + Ti5Si3. The in-situ presented TiN reinforcing phase possessed a refined granular morphology and a uniform distribution throughout the Ti5Si3 matrix, showing a clear and compatible interfacial structure with the matrix. The metallurgical mechanisms for the in-situ synthesis of TiN reinforced Ti5Si3 matrix composites by SLM were also proposed.
Co-reporter:Dongdong Gu;Yifu Shen
Advanced Engineering Materials 2009 Volume 11( Issue 7) pp:573-578
Publication Date(Web):
DOI:10.1002/adem.200800440
Co-reporter:Dongdong Gu, Yifu Shen, Xiaojuan Wu
Materials Letters 2008 Volume 62(12–13) pp:1765-1768
Publication Date(Web):30 April 2008
DOI:10.1016/j.matlet.2007.09.087
The direct laser sintering of a submicron/micron composite system consisting of 40 wt.% submicron W–20Cu powder and 60 wt.% micron Cu powder was performed via liquid phase sintering mechanism. The SEM and EDXS characterization shows the presence of a series of regularly-shaped W-rim/Cu-core structures after laser sintering. The metallurgical mechanism for the formation of such a novel structure was addressed.
Co-reporter:Dongdong Gu, Yifu Shen, Long Zhao, Jun Xiao, Peng Wu, Yongbing Zhu
Materials Science and Engineering: A 2007 Volumes 445–446() pp:316-322
Publication Date(Web):15 February 2007
DOI:10.1016/j.msea.2006.09.057
This paper presents a detailed investigation into the influence of the rare earth (RE) oxide (La2O3) addition upon the densification and the resultant microstructural characteristics of the submicron WC–Co particulate reinforced Cu matrix composites prepared by direct laser sintering. It is found that the relative density of the laser sintered sample with 1 wt.% La2O3 addition increased by 11.5% as compared with the sample without RE addition. The addition of RE element favored the microstructural refinement and improved the particulate dispersion homogeneity and the particulate/matrix interfacial coherence. The metallurgical functions of the RE element in improving the sinterability were also addressed. It shows that due to the unique properties of RE element such as high surface activity and large atomic radius, the addition of trace RE element can decrease the surface tension of the melt, resist the grain growth coarsening and increase the heterogeneous nucleation rate during laser sintering.
Co-reporter:Mujian Xia, Dongdong Gu, Guanqun Yu, Donghua Dai, Hongyu Chen, Qimin Shi
International Journal of Machine Tools and Manufacture (October 2016) Volume 109() pp:147-157
Publication Date(Web):1 October 2016
DOI:10.1016/j.ijmachtools.2016.07.010
•A novel powder-scale model was developed to investigate the evolution of surface quality during additive manufacturing of Inconel 718 alloy.•The thermodynamic behavior within the molten pool was analyzed.•The evolution of surface quality under variable hatch spacing was evaluated.•The corresponding experimental results were in a full accordance with those obtained from simulations.A transient three-dimensional powder-scale model has been established for investigating the thermodynamics, heat and mass transfer and surface quality within the molten pool during selective laser melting (SLM) Inconel 718 alloy by finite volume method (FVM), considering the powder-solid transition, variation of thermo-physical properties, and surface tension. The influences of hatch spacing (H) on the thermodynamics, heat and mass transfer, and resultant surface quality of molten pool have been discussed in detail. The results revealed that the H had a significant influence on determining the terminally solidified surface quality of the SLM-processed components. As a relatively lower H of 40 μm was used, a considerable amount of molten liquid migrated towards the previous as-fabricated tracks with a higher velocity, resulting in a stacking of molten liquid and the attendant formation of a poor surface quality with a large average surface roughness of 12.72 μm. As an appropriate H of 60 μm was settled, a reasonable temperature gradient and the resultant surface tension tended to spread the molten liquid with a steady velocity, favoring the formation of a flat surface of the component and an attendant low average surface roughness of 2.23 μm. Both the surface morphologies and average surface roughness were experimentally obtained, which were in a full accordance with the results calculated by simulation.
Co-reporter:Dongdong Gu
Science Bulletin (November 2016) Volume 61(Issue 22) pp:1718-1722
Publication Date(Web):1 November 2016
DOI:10.1007/s11434-016-1191-y
Co-reporter:Mujian Xia, Dongdong Gu, Guanqun Yu, Donghua Dai, Hongyu Chen, Qimin Shi
International Journal of Machine Tools and Manufacture (May 2017) Volume 116() pp:96-106
Publication Date(Web):1 May 2017
DOI:10.1016/j.ijmachtools.2017.01.005
•A randomly packed powder-bed model was established to investigate the porosity evolution during selective laser melting of Inconel 718 alloy.•The thermodynamic behavior within molten pool was studied.•The porosity distribution on top surface and cross-section was simulated and experimentally validated.•The thermodynamic mechanism of porosity evolution under variable scanning speed was concluded.To further investigate the porosity evolution during selective laser melting (SLM) Inconel 718 alloy, a transient mesoscale model with a randomly packed powder-bed has been proposed by finite volume method (FVM), taking consideration of the phase transition, variation of thermo-physical properties and interfacial force. The thermodynamics within molten pool and resulting porosity evolution behavior of a set of laser scanned tracks with various laser scanning speeds were studied using numerical approach. The results evidently revealed that the operating peak temperature was reduced obviously as increasing the scanning speeds. Accordingly, the high cooling rate, short lifespan and limiting depth of pool and small velocity of molten liquid flow were obtained under a high scanning speed. Scanning speed played a crucial role in determining the type of porosity in the terminally SLM-processed Inconel 718 components. At a high scanning speed of 500 mm/s, the top surface was primarily dominated by open porosity, accompanying with large-sized inter-layer porosity on the cross section, due to a limiting energy input penetrated into the powder-bed and incomplete melting of powder. By contrast, as a relatively low scanning speed of 200 mm/s was employed, the top surface appeared to be smooth free of less metallurgical porosity and no apparent inter-layer porosity on the cross section surface attributing to the escaping of porosity, indicating an well metallurgical bonding of the neighboring layer towards the building direction. Simultaneously, the physical mechanism was thoroughly discussed. The simulated distribution of porosity was found to be consistent with the experimental measurements.Download high-res image (290KB)Download full-size image
Co-reporter:Mujian Xia, Dongdong Gu, Guanqun Yu, Donghua Dai, ... Qimin Shi
Science Bulletin (July 2016) Volume 61(Issue 13) pp:1013-1022
Publication Date(Web):1 July 2016
DOI:10.1007/s11434-016-1098-7
A mesoscopic model has been established to investigate the thermodynamic mechanisms and densification behavior of nickel-based superalloy during additive manufacturing/three-dimensional (3D) printing (AM/3DP) by numerical simulation, using a finite volume method (FVM). The influence of the applied linear energy density (LED) on dimensions of the molten pool, thermodynamic mechanisms within the pool, bubbles migration and resultant densification behavior of AM/3DP-processed superalloy has been discussed. It reveals that the center of the molten pool slightly shifts with a lagging of 4 μm towards the center of the moving laser beam. The Marangoni convection, which has various flow patterns, plays a crucial role in intensifying the convective heat and mass transfer, which is responsible for the bubbles migration and densification behavior of AM/3DP-processed parts. At an optimized LED of 221.5 J/m, the outward convection favors the numerous bubbles to escape from the molten pool easily and the resultant considerably high relative density of 98.9% is achieved. However, as the applied LED further increases over 249.5 J/m, the convection pattern is apparently intensified with the formation of vortexes and the bubbles tend to be entrapped by the rotating flow within the molten pool, resulting in a large amount of residual porosity and a sharp reduction in densification of the superalloy. The change rules of the relative density and the corresponding distribution of porosity obtained by experiments are in accordance with the simulation results.
Co-reporter:Donghua Dai, Dongdong Gu
International Journal of Machine Tools and Manufacture (January 2016) Volume 100() pp:14-24
Publication Date(Web):1 January 2016
DOI:10.1016/j.ijmachtools.2015.10.004
•Thermo-capillary flow and migration during SLM of composites are simulated.•Reinforcement activity with various laser energy inputs is disclosed.•Mechanism of the interaction of melt flow and reinforcement is elucidated.•Simulation results show good agreement with the obtained experimental results.A transient three dimensional model for describing the thermo-capillary convection and migration behavior and the resultant distribution state of reinforcing particles during selective laser melting of AlN/AlSi10Mg is proposed. The powder–solid transformation, temperature dependent physical properties and interaction between the reinforcement and the melt are taken into account. The effect of the laser energy per unit length (LEPUL) on the molten pool dynamics, cooling rate and the resultant sizes and distribution state of AlN reinforcement has been investigated. It shows that the thermo-capillary convection pattern changes from inward flow pattern to outward one, due to the appearance of the oxidation in molten pool. Therefore, the morphology of the top surface undergoes a continuous variation from the balling phenomenon, to the discontinuous tracks and finally to the formation of a flat and dense one. Meanwhile, both the clockwise and counterclockwise convection patterns are produced in the molten pool, caused by the interaction of reinforcing particles and the melt. An increase in LEPUL will significantly intensify the thermo-capillary convection whereas result in a decrease in the cooling rate of the molten pool. As LEPUL decreases from 1800 J/m to 450 J/m, the distribution state of AlN particles changes from the severe aggregation, then to the formation of partial aggregation and finally to the homogeneous distribution in the solidified matrix. The particle sizes of AlN reinforcement are experimentally acquired, which are in a good agreement with the results predicted by simulation.
Co-reporter:Donghua Dai, Dongdong Gu
International Journal of Machine Tools and Manufacture (January 2015) Volume 88() pp:95-107
Publication Date(Web):1 January 2015
DOI:10.1016/j.ijmachtools.2014.09.010
•Thermal behavior and top surface morphology during SLM of composites are simulated.•Top surface activity with various laser energy inputs is disclosed.•Mechanism of the influence of surface tension on surface quality is elucidated.•Simulation results show good agreement with the obtained experimental results.A selective laser melting (SLM) physical model of coupled radiation transfer and thermal diffusion is proposed, which provides a local temperature field. A strong difference in thermal conductivity between the powder bed and dense material is taken into account. Both thermo-capillary force and recoil pressure induced by the material evaporation, which are the major driving forces for the melt flow, are incorporated in the formulation. The effect of the laser energy input per unit length (LEPUL) on the temperature distribution, melt pool dynamics, surface tension and resultant surface morphology has been investigated. It shows that the surface tension plays a crucial role in the formation of the terminally solidified surface morphology of the SLM-processed part. The higher surface tension of the lower temperature metal near the edge of the melt pool and the thermal-capillary force induced by the surface temperature gradient tend to pull the molten metal away from the center of the melt pool. For a relatively high LEPUL of 750 J/m, the molten material in the center of the melt pool has a tendency to flow towards the rear part, resulting in the stack of molten material and the attendant formation of a poor surface quality. For an optimized processing condition, LEPUL=500 J/m, a complete spreading of the molten material driven by the surface tension is obtained, leading to the formation of a fine and flat melt pool surface. The surface quality and morphology are experimentally acquired, which are in a good agreement with the results predicted by simulation.
Co-reporter:Kaijie Lin, Xiaoying Li, Hanshan Dong, Shangfeng Du, Yaxiang Lu, Xiaochao Ji, Dongdong Gu
International Journal of Hydrogen Energy (26 January 2017) Volume 42(Issue 4) pp:
Publication Date(Web):26 January 2017
DOI:10.1016/j.ijhydene.2016.09.220
•The surface of 316 stainless steel bipolar plates was modified with Pt.•A dense, columnar structured and single phase Pt3Fe deposition layer was produced.•The Pt3Fe deposition layers were characterised by SEM, XRD, TEM and XPS.•The corrosion behaviour and ICR value met the DOE 2020 target for bipolar plates.•The single cell performance of bipolar plates was largely improved after treatments.316 stainless steel has been regarded as one of the promising candidates to replace graphite for bipolar plate application. However, the relatively high electrical resistance caused by the formation of passive oxide film and the insufficient corrosion resistance in long-term operation are two main concerns of 316 stainless steel bipolar plates. Low temperature active screen plasma alloying technology shows the ability to reduce electrical resistance and enhance corrosion resistance of 316 stainless steel bipolar plates to some extent, but still can not satisfy the Department of Energy (DOE) requirements. In this paper, active screen plasma co-alloying treatments with nitrogen and platinum are conducted to modify the surface of 316 stainless steel. The surface morphology, phase constitute, chemical composition and layer structure of treated 316 stainless steel are fully studied. A dense, columnar structured and single phase Pt3Fe deposition layer is produced on the surface of 316 stainless steel after treatments. Thanks to the excellent electrical conductivity and corrosion resistance of Pt3Fe, the surface electrical conductivity and corrosion resistance are greatly enhanced and satisfies the DOE requirements, contributing to the significant improvement of single cell performances.
Co-reporter:Dongdong Gu, Hongqiao Wang, Fei Chang, Donghua Dai, ... Wilhelm Meiners
Physics Procedia (2014) Volume 56() pp:108-116
Publication Date(Web):1 January 2014
DOI:10.1016/j.phpro.2014.08.153
The nanoscale TiC particle reinforced AlSi10Mg nanocomposite parts were produced by selective laser melting (SLM) additive manufacturing process. The influence of laser energy density (LED) on densification behavior, microstructural evolution, microhardness and wear properties of SLM-processed TiC/AlSi10Mg nanocomposites was studied. It showed that the near fully dense nanocomposite parts (>98% theoretical density) were achieved with increasing the applied LED. The TiC reinforcement in SLM-processed parts experienced a microstructural change from the standard nanoscale particle morphology (the average size 77-93 nm) to the relatively coarsened submicron structure (the mean particle size 154 nm) as the LED increased.The sufficiently high densification rate combined with the homogeneousdistribution of nanoscale TiC reinforcement throughout the matrix led to a high microhardness of 181.2 HV0.2, a considerably low coefficient of friction (COF) of 0.36, and a reduced wear rate of 2.94×10-5 mm3N-1m-1 for SLM-processed TiC/AlSi10Mg nanocomposite parts.
