Effect of N2 on microstructure and mechanical properties of additive manufactured titanium matrix composites
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摘要: 鈦合金廣泛應用于航空航天、生物醫學等領域。但由于其力學性能不理想(如硬度低、耐磨性差)和加工性差,限制了其應用范圍。為了直接近凈成形出結構復雜且性能提升的鈦合金零部件。本文在選區激光熔化(SLM)Ti6Al4V鈦合金過程中通入氮氣(N2),通過Ti-N反應制備鈦基復合材料(TMCs)。該創新方法的成形原理為:激光誘導Ti6Al4V高溫熔池附近的N2分解生成N原子或者離子,并與熔融狀態的鈦原位反應生成TiN增強相,通過層層疊加,成形TiN增強鈦基復合材料。本文采用3種不同體積分數(3%、10%和30%)的N2氣氛SLM成形了鈦基復合材料,并對比了純氬氣(Ar)氣氛中SLM成形的Ti6Al4V鈦合金。采用掃描電子顯微鏡(SEM)觀察了材料的微觀組織。X射線衍射(XRD)圖譜表明部分N固溶進入Ti晶格中。能譜(EDS)證實了TiN的生成。高分辨透射電鏡(HR-TEM)圖進一步確認了基體相和第二相分別為Ti和TiN。這種原位合成的氮化物增強相分散均勻,尤其是在低體積分數N2氣氛(3%和10%)下制備的復合材料中均勻分布著大量納米級增強相。此外,在低體積分數N2氣氛(10%)下制備的復合材料強度和塑性同時提高。本文研究了不同N2濃度對鈦基復合材料微觀組織和力學性能的影響規律,并闡明了復合材料的強韌化機理。Abstract: Titanium alloys are extensively used in areas such as aerospace and biomedicine. However, inadequate mechanical qualities (e.g., low hardness and poor wear resistance) and poor machinability limit the scope of their application expansion. To directly manufacture near-net-shape titanium alloy components with complicated architectures and improved performances, titanium matrix composites (TMCs) were fabricated based on the Ti–N reaction by introducing nitrogen gas (N2) in the process of selective laser melting (SLM) of Ti6Al4V. The formation principle of this novel method is as follows: Laser-induced N2 decomposition near the melt pool of Ti6Al4V generates N atoms or ions, which react with Ti atoms in the melt pool to in-situ synthesize TiN-reinforcement particles. In turn, TiN-reinforced Ti6Al4V matrix composites are manufactured layer-by-layer. This approach has some important advantages, which are as follows: Above all, in-situ gas-liquid synthesized reinforcements are equally dispersed due to N2, good diffusivity and dispersibility. Furthermore, extremely small gas molecules have the potential to produce nanoscaled reinforcement. Moreover, the in-situ reaction mode produces a clean interface and strong interfacial bonding between the matrix and reinforcement. In this study, TMCs were prepared by SLM in three different N2 volume fractions of 3%, 10% and 30%, which were compared to the Ti6Al4V alloy fabricated in an argon atmosphere. The microstructures were observed by SEM. Interstitial solid solutions of N in the Ti lattices were confirmed by XRD patterns. The presence of TiN was verified by EDS. The high-resolution transmission electron microscope (HR-TEM) picture indicated that the matrix and reinforcement were TiN and Ti, respectively. Such in situ synthesized nitride reinforcements were uniformly distributed; in particular, numerous nanoscale reinforcements were uniformly dispersed in the composites manufactured in low volume fraction N2 atmospheres (3% and 10%). Additionally, the improved strength and plasticity were simultaneously achieved in a diluted N2 atmosphere (10%). The effect of varying N2 concentrations on the microstructure and mechanical characteristics of the TMCs was investigated. The content of the TiN particles increased with increasing N2 concentration due to the increased availability of N atoms and ions for nucleation and growth of the reinforcement. Nevertheless, the TMC produced in a high N2 atmosphere (30%) demonstrated degradation of the mechanical properties (particularly plasticity and ultimate strength) because of the presence of excessive N solid solutions and brittle TiN particles. The strengthening mechanisms were primarily grain refinement strengthening of the Ti matrix due to the "pinning" effect of TiN particles, the precipitation hardening and dispersion strengthening effects of uniformly distributed reinforcement particles, interstitial solid solution strengthening caused by the results from the portion of N in the Ti lattices, Orowan strengthening caused by the in-situ synthesis of nanoscaled reinforcements, and the load transfer effect from Ti matrix to TiN reinforcements because of the clean interface.
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Key words:
- selective laser melting /
- Ti6Al4V /
- N2 /
- composite materials /
- additive manufacturing
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圖 1 不同N2體積分數下SLM成形材料的微觀組織形貌. (a) 0% N2;(b) 3% N2;(c) 10% N2;(d) 30% N2;(e)~(f)分別是(a)~(d)的高倍放大圖;(f)~(h)白色亮點為氮化物
Figure 1. Micromorphologies of SLM-prepared samples under different N2 volume fractions: (a) 0% N2; (b) 3% N2; (c) 10% N2; (d) 30% N2; (e)–(f) are high magnifications of (a)–(d), respectively; the white bright spots in (f)–(h) are nitrides
圖 3 30%體積分數 N2氣氛下成形材料的EDS點掃描、面掃描和EDS線掃描圖譜. (a)SEM圖;(b)EDS譜圖1;(c)SEM圖,黑色箭頭為EDS線掃描方向;(d)~(g)分別為Ti、Al、V和N的元素分布圖; (h) Ti、Al和N沿線掃描方向的元素含量變化
Figure 3. Spot EDS measurement, elemental mappings, and EDS line-scan of the sample fabricated in a 30% N2 (volume fraction) atmosphere: (a) SEM image; (b) EDS of spectrum 1; (c) SEM image; the black arrow indicates the direction of the EDS line-scan; EDS elemental mapping for (d) Ti, (e) Al, (f) V, and (g) N; (h) element distributions from the EDS line-scan
圖 6 成形材料硬度(a)、極限強度(c)、屈服強度(d)和塑性(e)隨N2體積分數的變化關系圖;不同N2體積分數下SLM成形材料的壓縮應力–應變曲線(b)
Figure 6. Evolutions of the hardness (a), ultimate strength (c), yield strength (d) and plasticity (e) of the SLM-prepared samples dependence of N2 volume fraction, respectively; compression stress–strain curves (b) of the SLM-prepared samples in different N2 volume fractions
表 1 實驗中使用的球形Ti6Al4V粉末的化學成分(質量分數)
Table 1. Chemical composition of the spherical Ti6Al4V powder utilized in the experiment (mass fraction)
% Al V Fe C O Ti 5.8 4.1 0.3 0.1 0.13 Balance 表 2 SLM過程中的工藝參數及N2體積分數
Table 2. Processing parameters and N2 volume fraction of SLM
N2 volume fraction/% Ar volume fraction/% Power/
WScanning
speed/
(mm·s?1)Hatch spacing/
μmLayer thickness/
μm0 100 180 300 110 50 3 97 10 90 30 70 www.77susu.com -
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