Tailoring the microstructure and mechanical properties of wire arc additive manufactured Ti-6Al-4V alloy by trace TiC powder addition
-
摘要:
原位合金化是调控增材制造钛合金微观组织和力学性能的有效方法，文中研究了添加微量TiC粉末对电弧增材制造Ti-6Al-4V合金微观组织和力学性能的影响. 结果表明，添加微量TiC粉末后，Ti-6Al-4V合金中的柱状β晶粒尺寸减小，α-Ti相细化，微观组织为精细的网篮组织，抗拉强度和断后伸长率达到了
1029 MPa和14.8%，分别提高了12.8%和26.5%，实现了电弧增材制造Ti-6Al-4V合金强度和塑性的协同改善. 同时，沉积合金的显微硬度提高至362.9HV，相比Ti-6Al-4V合金提高了11.4%. 通过电子背散射衍射(electron backscatter diffraction，EBSD)分析得出， TiC粉末的添加降低了沉积合金的织构强度，增加α-Ti相的取向. 添加微量TiC粉末后，Ti-6Al-4V合金力学性能的提高主要归因于细晶强化和固溶强化效应，其中细晶强化是主要的强化机制. 对比分析两种沉积合金的拉伸试样断口得出，Ti-6Al-4V合金的断口形貌呈现出塑性-脆性混合断裂，添加质量分数0.5% TiC粉末后，断口形貌呈现出塑性断裂. 证实了TiC粉末作为电弧增材制造Ti-6Al-4V合金晶粒细化剂的潜力.Abstract:In-situ alloying has been demonstrated to be an effective method for modifying the microstructure of additively manufactured titanium alloys. In this paper, the influence of trace TiC powder addition on the microstructure and mechanical properties of wire arc additive manufactured Ti-6Al-4V alloy was investigated. The result showed that the trace TiC powder addition reduced the size of columnar β grains and refined the α-Ti phase in the Ti-6Al-4V alloy, and the microstructure was the fine basketweave structure, and the ultimate tensile strength and elongation reached
1029 MPa and 14.8%, which increased by 12.8% and 26.5%, respectively, synergistically improving the strength and ductility of wire arc additive manufactured Ti-6Al-4V alloy. Meanwhile, the microhardness of the deposited alloy was increased to 362.9HV, which was an increase of 11.4%. As analyzed by EBSD, the addition of TiC powder decreased the texture intensity and increased the orientation of the α-Ti phase. The improvement in mechanical properties of the Ti-6Al-4V alloy after adding trace TiC powder was mainly attributed to fine-grain strengthening and solid solution strengthening, where fine-grain strengthening was the main strengthening mechanism. Comparative analysis of the fracture of the tensile specimens of the two deposited alloys showed that the fracture morphology of Ti-6Al-4V alloy exhibited the mixed plastic-brittle fracture, while the fracture morphology exhibited a typical plastic fracture with a mass fraction of 0.5% TiC powder. The potential of TiC powder as a grain refiner for the wire arc additive manufactured Ti-6Al-4V alloys was demonstrated. -
0. 序言
搅拌摩擦焊(friction stir welding, FSW)作为一种固相焊接技术，具有焊缝质量高、变形小等优点[1-2]. 目前加工制造业对焊接智能化、高效化的要求日益上升，机器人搅拌摩擦焊得以更普遍的应用.在实际大型结构的FSW生产中，由于接头形式、板材加工精度以及工装夹具装配质量问题，焊接过程容易产生较大的间隙，对接头的成形和性能极为不利[3-4]，当工件之间的间隙超过工件厚度的10%时，很难获得无缺陷质量良好的接头[5]. 间隙的存在导致焊核区(weld nugget zone，WNZ)材料流动不充分，焊缝出现孔洞和隧道等缺陷[6]. 同时，工件被塑化的材料流入间隙，弥补材料缺失使得焊缝位置减薄严重，降低接头承载能力[7].
研究人员[8-9]采用粉末、焊丝或者补偿条作为填充材料对大间隙下的工件进行FSW，得到成形良好无缺陷的接头，接头与常规FSW接头力学性能吻合，然而，当焊接速度过快时，这些填充材料很容易飞出间隙，从而形成缺陷. 同时填充材料需要在焊前放置在间隙内，针对复杂结构间隙及焊接过程中产生的间隙，填充材料的尺寸以及填料的连续性受到限制.
基于传统搅拌摩擦焊方法，填充材料旁轴送料，将FSW与填料过程同时进行，实现大间隙机器人搅拌摩擦填丝焊，并对其接头进行盐雾腐蚀试验，分析搅拌摩擦填丝焊接头不同区域的腐蚀行为差异.搅拌摩擦填丝焊提高了FSW对工况条件的适应性，适用于高铁、船舶和飞机上大型及复杂结构焊缝，有望为工程实际应用提供理论依据和技术支撑.
1. 试验方法
试验材料为5A06铝合金轧制板材，尺寸为300 mm × 70 mm × 3 mm，填充材料为直径1.6 mm的5B06丝材. 机器人搅拌摩擦填丝焊焊接过程示意图及焊具尺寸如图1所示，对接板材焊接间隙为2 mm. 填充丝材经过高推力送丝系统从送丝孔连续输送到储料腔内部，高速旋转的螺杆将金属丝材剪切成粒状材料，粒状材料在自身重力及与螺杆侧壁的摩擦力的影响下，在储料腔内塑化从底部的缝隙流出. 轴向压力使储料腔与板材之间产生挤压效果，粒状材料发生变形堆积并被塑化. 在旋转的搅拌针的驱动作用下，塑化的填充材料发生流动并实现与基材的连接. 试验所采用的焊接工艺参数为转速3 000 r/min，焊接速度200 mm/min，送丝速度1.8 m/min，轴向压力5 000 N，倾角1.5°.
![]()
图 1 焊接过程示意图及焊具结构Figure 1. Welding process and the welding tool structure. (a) schematic illustration of wire-feeding friction stir welding; (b) dimensions of the welding tool2. 试验结果与分析
2.1 焊缝成形
图2为机器人搅拌摩擦填丝焊接头焊缝表面形貌. 焊缝表面光滑成形良好，无沟槽缺陷，在搅拌针的驱动作用下，塑化的填充材料发生流动后沉积弥补了间隙位置材料缺失，同时焊缝有一定程度的增厚，提高了接头的承载能力.
2.2 焊缝微观组织
图3为焊缝整体微观形貌及不同区域的微观组织. 焊接接头填充材料与基体母材结合良好，焊缝无孔洞及隧道缺陷，由于搅拌针的存在，搅拌针促进塑化的丝材和基材发生流动，提高了填充材料与基材的结合效果. 丝材经过螺杆的剪切及静轴肩的挤压作用，与焊核区受到搅拌针的搅拌作用一样，填充材料也经历了大塑性变形，发生动态再结晶，形成细小的等轴晶.
![]()
图 3 搅拌摩擦填丝焊接头微观组织Figure 3. Microstructures of wire-feeding friction stir welding. (a) microstructures of the cross-section; (b) top interface; (c) thermo-mechanically affected zone interface; (d) filler materials zone2.3 焊缝腐蚀性能
搅拌摩擦填丝焊接头经过7天盐雾腐蚀试验后接头各区域腐蚀形貌如图4所示. 接头表面均发生了点蚀坑的萌生, 表面出现腐蚀产物；焊核区及填充材料区域的点蚀坑尺寸较小，且分布较为均匀；母材点蚀坑分布不均匀，尺寸较大.热力影响区(thermo- mechanically affected zone，TMAZ)的点蚀坑随晶粒分布特征呈流线分布，热影响区(heat-affected zone，HAZ)的点蚀坑尺寸较大，且出现一定的聚集现象，点蚀坑发生扩展.焊核区和填充材料区表现出更好的耐腐蚀性能.
![]()
图 4 不同区域盐雾腐蚀形貌Figure 4. Salt spray corrosion morphologies in different zones. (a) WNZ; (b) filler materials zone; (c)TMAZ; (d) HAZ; (e)BM第二相分布及尺寸对点蚀坑的形成有巨大影响，第二相和基体之间形成微电偶会导致腐蚀现象发生.焊核区经过塑性变形后第二相颗粒被打碎，尺寸较小分布也更均匀，进而发生腐蚀现象后点蚀坑分布均匀细小；填充材料区域拥有更小且弥散分布的第二相颗粒，填充材料的加入增强了焊核区的耐蚀性.经过轧制后的母材中第二相颗粒尺寸较大且分布不均匀，耐蚀性较差易形成较大的点蚀坑；热力影响区点蚀坑呈流线分布，而热影响区第二相颗粒发生聚集长大，发生点蚀后有利于点蚀坑的扩展,导致热影响区的耐蚀性较差.
图5为热影响区点蚀坑SEM图及附近元素分布.发现在第二相Al6(FeMn)附近产生了明显的腐蚀现象, 点蚀坑发生扩展. 在盐雾环境中，铝合金表面虽然存在一层氧化膜，但是随着溶液中Cl−的侵入，Cl−破坏了表面氧化膜，促进点蚀现象发生. 同时热影响区第二相颗粒Al6(FeMn)与铝基体之间存在腐蚀电位差形成原电池，由于Al6(FeMn)电位高于铝基体[10]，第二相颗粒在腐蚀过程中充当阴极，促使周围基体发生腐蚀，因此在第二相附近形成环形腐蚀区域产生腐蚀坑并向四周扩展. 当第二相尺寸较大时，周围基体溶解的范围增大，点蚀坑的尺寸也会更大. 基于元素分布图可以看出，在腐蚀坑附近Al元素含量减少，点蚀坑内金属发生溶解，点蚀孔内阳离子浓度升高，Cl−就会不断侵入以维持平衡.随着Cl−浓度的升高发生水解，导致点蚀坑内部氢离子浓度升高，溶液酸化，促使基体进一步溶解，点蚀坑发生扩展.
2.4 接头力学性能
图6为经过7天盐雾腐蚀接头、未腐蚀接头及母材的拉伸测试结果.未腐蚀接头抗拉强度为388.9 MPa ± 1.4 MPa，断后伸长率为20.5% ± 0.4%，分别达到母材的99%及94%. 经过7天盐雾腐蚀后接头抗拉强度降低到356.6 MPa ± 1.2 MPa，断后伸长率为18.1% ± 0.9%，盐雾腐蚀后接头强度降低了8.3%，断后伸长率下降了11.7%，盐雾腐蚀试验后接头仍保持较优的力学性能. 盐雾腐蚀环境造成焊缝表面出现点蚀坑，而富Cl−环境使基体金属进一步溶解，点蚀坑发生扩展，减少了接头有效承载面积，在承受载荷时其易成为薄弱位置，裂纹在点蚀坑位置产生，降低了接头承载能力.
3. 结论
(1) 实现了大尺寸间隙下机器人搅拌摩擦填丝焊，焊接过程与填料过程同时进行，提高了搅拌摩擦焊对接头间隙的容忍性，消除了焊缝减薄问题.
(2) 填充材料与基材实现了良好的冶金连接，经过剧烈塑性变形后，焊核区和填充材料发生动态再结晶，表现为细小的等轴晶粒.
(3) 未腐蚀接头抗拉强度达到388.9 MPa ± 1.4 MPa，断后伸长率为20.5% ± 0.4%，分别达到母材的99%及94%. 在腐蚀过程中焊核区和填充材料区耐腐蚀性能优于热影响区与母材，点蚀坑细小且均匀分布，7天盐雾腐蚀后接头保持优异的耐蚀性能.
-
![]()
图 1 原位合金化过程示意图和取样位置示意图(mm)
Figure 1. Schematic diagram of in-situ alloying process and sampling location. (a) deposition process with TiC addition; (b) location of test samples
![]()
图 2 两种沉积合金的宏观组织
Figure 2. Macrostructure of the two alloys. (a) in longitudinal direction; (b)in horizontal direction
![]()
图 4 两种合金的微观组织
Figure 4. Microstructure of α-Ti in two alloys. (a) Ti-6Al-4V; (b) Ti-6Al-4V-0.5TiC
![]()
图 5 两种合金 α-Ti相的尺寸分布图
Figure 5. The size distribution of α-Ti in two alloys. (a) Ti-6Al-4V; (b)Ti-6Al-4V-0.5TiC
![]()
图 6 Ti-6Al-4V-0.5TiC合金的TEM明场像和SAED图案
Figure 6. TEM bright field image and SAED patterns of the Ti-6Al-4V-0.5TiC alloy. (a) TEM bright field image; (b) SAED pattern of the α-Ti phase; (c) SAED pattern of the β-Ti phase
![]()
图 7 两种沉积合金的EBSD结果
Figure 7. EBSD results of two deposited alloys. (a) IPF and IQ plots of Ti-6Al-4V alloy; (b) IPF and IQ plots of Ti-6Al-4V-0.5TiC alloy; (c) crystal orientation triangle; (d) PF of Ti-6Al-4V alloy; (e) PF of Ti-6Al-4V-0.5TiC alloy
![]()
图 8 两种合金的力学性能
Figure 8. Mechanical properties of the two alloys. (a) hardness; (b) tensile properties
![]()
图 9 两种合金的拉伸断口形貌
Figure 9. Tensile fracture morphology of the two alloys. (a) Ti-6Al-4V(60 × ); (b) Ti-6Al-4V(200 × ); (c) Ti-6Al-4V(
1000 × ); (d) Ti-6Al-4V-0.5%TiC(60 × ); (e) Ti-6Al-4V-0.5%TiC(200 × ); (f) Ti-6Al-4V-0.5%TiC(1000 × )表 1 Ti-6Al-4V丝材化学成分(质量分数，%)
Table 1 Chemical composition of Ti-6Al-4V wire
Al V C N H Fe Ti 6.1 4.05 0.11 0.012 0.02 0.067 余量 
下载: 导出CSV
表 2 电弧增材制造工艺参数
Table 2 Deposition parameters of WAAM
焊接电流
I/A送丝速度
vf /(mm·min−1)焊接速度
vt /(mm·min−1)电弧长度
La/ mm130 1.5 120 7
下载: 导出CSV
-
[1] Banerjee D, Williams J C. Perspectives on titanium science and technology[J]. Acta Materialia, 2013, 61(3): 844 − 879. doi: 10.1016/j.actamat.2012.10.043
[2] Barriobero-Vila P, Gussone J, Stark A, et al. Peritectic titanium alloys for 3D printing[J]. Nature Communications, 2018, 9: 3426. doi: 10.1038/s41467-018-05819-9
[3] Carroll B E, Palmer T A, Beese A M. Anisotropic tensile behavior of Ti-6Al-4V components fabricated with directed energy deposition additive manufacturing[J]. Acta Materialia, 2015, 87: 309 − 320. doi: 10.1016/j.actamat.2014.12.054
[4] Kim Y, Song Y B, Lee S H. Microstructure and intermediate-temperature mechanical properties of powder metallurgy Ti-6Al-4V alloy prepared by the prealloyed approach[J]. Journal of Alloys and Compounds, 2015, 637: 234 − 241. doi: 10.1016/j.jallcom.2015.03.019
[5] Wang F D, Williams S, Colegrove P, et al. Microstructure and mechanical properties of wire and arc additive manufactured Ti-6Al-4V[J]. Metallurgical and Materials Transactions A, 2013, 44: 968 − 977. doi: 10.1007/s11661-012-1444-6
[6] Zhuo Y M, Yang C L, Fan C L, et al. Grain refinement of wire arc additive manufactured titanium alloy by the combined method of boron addition and low frequency pulse arc[J]. Materials Science and Engineering A, 2021, 805: 140557. doi: 10.1016/j.msea.2020.140557
[7] Wang J, Lin X, Wang J T, et al. Grain morphology evolution and texture characterization of wire and arc additive manufactured Ti-6Al-4V[J]. Journal of Alloys and Compounds, 2018, 768: 97 − 113. doi: 10.1016/j.jallcom.2018.07.235
[8] Lin X, Yue T M, Yang H O, et al. Solidification behavior and the evolution of phase in laser rapid forming of graded Ti6Al4V-Rene88DT alloy[J]. Metallurgical and Materials Transactions A, 2007, 38: 127 − 137. doi: 10.1007/s11661-006-9021-5
[9] Zhuo Y M, Yang C L, Fan C L, et al. Effects of trace Sn and Cr addition on microstructure and mechanical properties of TC17 titanium alloy repaired by wire arc additive manufacturing[J]. Journal of Alloys and Compounds, 2021, 888: 161473. doi: 10.1016/j.jallcom.2021.161473
[10] Zhang F Y, Yang M, Clare A T, et al. Microstructure and mechanical properties of Ti-2Al alloyed with Mo formed in laser additive manufacture[J]. Journal of Alloys and Compounds, 2017, 727: 821 − 831. doi: 10.1016/j.jallcom.2017.07.324
[11] Bermingham M J, Kent D, Zhan H, et al. Controlling the microstructure and properties of wire arc additive manufactured Ti-06Al-4V with trace boron additions[J]. Acta Materialia, 2015, 91: 289 − 303. doi: 10.1016/j.actamat.2015.03.035
[12] Mereddy S, Bermingham M J, Kent D, et al. Trace carbon addition to refine microstructure and enhance properties of additive-manufactured Ti-6Al-4V[J]. Journal of the Minerals, Metals & Materials Society, 2018, 70: 1670 − 1676.
[13] Mereddy S, Bermingham M J, Stjohn D H, et al. Grain refinement of wire arc additively manufactured titanium by the addition of silicon[J]. Journal of Alloys and Compounds, 2017, 695: 2097 − 2103. doi: 10.1016/j.jallcom.2016.11.049
[14] Li S P, Wang X Y, Wei Z C, et al. Simultaneously improving the strength and ductility of the as-sintered (TiB + La2O3)/Ti composites by in-situ planting ultra-fine networks into the composite powder[J]. Scripta Materialia, 2022, 218: 114835. doi: 10.1016/j.scriptamat.2022.114835
[15] Zhang W J, Song X Y, Hui S X, et al. Tensile behavior at 700 ℃ in Ti-Al-Sn-Zr-Mo-Nb-W-Si alloy with a bi-modal microstructure[J]. Materials Science and Engineering A, 2014, 595: 159 − 164. doi: 10.1016/j.msea.2013.11.096
[16] Kok Y, Pan X P, Wang P, et al. Anisotropy and heterogeneity of microstructure and mechanical properties in metal additive manufacturing: a critical review[J]. Materials & Design, 2018, 139: 565 − 586.
计量
- 文章访问数: 185
- HTML全文浏览量: 35
- PDF下载量: 50
