Microstructures and mechanical properties of Ti-6Al-3Nb-2Zr-1Mo alloy fabricated by CMT-wire arc additive manufacturing
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摘要: 采用基于冷金属过渡的电弧熔丝增材方法(CMT-WAAM)制备了Ti-6Al-3Nb-2Zr-1Mo合金试样，研究了CMT-WAAM Ti6321合金显微组织、力学性能及其各向异性. 结果表明，CMT-WAAM Ti6321合金显微组织由不规则的多边形原始β晶和晶界α相组成，CMT脉冲工艺(CMT+P)能够有效细化晶粒，组织中没有发现贯穿式的柱状晶，且未发现马氏体.CMT-WAAM Ti6321合金x向和z向的室温抗拉强度达到同级别锻件标准，断口形式均为典型的韧性断裂.成形组织中没有明显的织构存在，拉伸强度的各向异性也不明显，组织中的气孔导致z向的断后伸长率低与x向. x向和z向冲击韧性均不低于65 J，能够满足船用钛合金结构件的需求，冲击断口中存在大量的撕裂型韧窝，为典型的韧性断裂.Abstract: The microstructure, mechanical properties and anisotropy of Ti-6Al-3Nb-2Zr-1Mo alloy made by CMT-Wire Arc additive manufacturing(CMT-WAAM) were studied. The as-built microstructures exhibit irregular polygons prior β and grain boundary α. This technology can refine the grains, and no columnar prior β grain morphology is observed. No martensite phase was discovered. The tensile strength in both directions have reached the standard requirements of the same level forging. No obvious texture is observed, and the anisotropy in tensile behavior is not obvious. There is no obvious texture and anisotropy in the manufactured structure. The ductility in transverse specimens was limited by the presence of lack-of-fusion porosity. The impact toughness of x and z direction is not less than 65 J. The impact fracture is typical ductile fracture,which consists of a large number of dimples.
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Keywords:
- CMT /
- wire arc additive manufacturing /
- microstructure /
- mechanical property /
- anisotropy
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0. 序言
增材制造(additive manufacturing，AM)，即3D 打印技术，能提高钛合金材料的利用率，并且可实现复杂结构零件的快速制造，在航空航天、船舶、医疗、化工等领域有着广泛的应用前景[1-5]. 目前，钛合金增材制造主要以高能束、电弧作为热源，通过铺粉/送粉、送丝等形式进行构件的快速成形[6-7]. 针对钛合金的增材制造，国内外许多研究者开展了大量研究工作. 如Brandl等人[8]采用Ti-6Al-4V丝材进行了大尺寸构件的快速增材制造，获得原始β柱状晶组织，柱状晶内部为马氏体和网篮组织. Tan等人[9]发现电子束增材过程中快速冷却会导致成分的偏析和β柱状晶粗大，降低成形件的力学性能. Qiu等人[10]研究表明，贯穿沉积层的原始β柱状晶会导致显著的力学性能各向异性，成形后需通过热处理降低各向异性. Suo等人[11]研究表明，电子束熔丝成形Ti-6Al-4V合金室温拉伸性能有明显的方向性，水平方向的抗拉强度高于竖直方向，但水平方向塑性要明显低于竖直方向. 然而，增材制造钛合金在实际的推广应用中仍然面临着原始晶粒组织粗大以及力学性能各向异性等问题有待解决.
电弧熔丝增材制造(wire arc additive manufacture，WAAM)技术是以电弧为热源，利用金属丝材沉积直接制造金属零件[12]. 与其它增材制造技术相比，WAAM 技术具有设备投资少、运行成本低、材料利用率高以及沉积/生产效率高等优点，受到了国内外研究学者的广泛关注[13]. Horgar等人[14]利用传统熔化极气体保护焊技术，实现了铝合金构件的WAAM成形.Haden等人[15]对比了不锈钢WAAM构件与锻件的抗拉性能与耐磨性能，发现WAAM构件的性能接近于锻件. Wu等人[16]研究了钛合金WAAM过程中热积累对构件成形的影响，发现由于沿堆积方向散热路径发生改变以及冷却速率降低，构件的几何形状将发生改变. Davis等人[17]采用基于TIG的增材制造探索了双成分钛合金成形件的组织和力学性能. Xie等人[18]对基于TIG增材制造Ti-6Al-4V样件的疲劳裂纹扩展性能进行了深入的研究，发现竖直方向的疲劳裂纹生长速率高于水平方向的. 冷金属过渡(cold metal transfer，CMT)是一种热输入较低的焊接技术，在CMT焊接过程中通过焊接电源与焊丝回抽的配合能够周期性的熄弧、起弧，从而实现冷热交替焊接并降低热输入，有利于降低晶粒粗大的倾向[19-20]. 随着CMT技术的发展，基于CMT的WAAM技术在铝合金的快速制造中得到了应用研究[21-22]. Zhang等人[23]利用CMT工艺实现了Al-6Mg合金的增材制造，获得细小的晶粒组织和较高的强度.CMT-WAAM技术为获得组织与性能均匀钛合金提供了一种新的增材制造途径.
目前，船用钛合金增材制造的应用研究尚处于起步阶段[24-25]. CMT-WAAM具有无飞溅、搭桥能力好、成形均匀一致、热输入低、变形小等优点[26-27]，在船用大尺寸钛合金构件的增材制造方面有着良好的应用前景. 电弧熔丝增材在制备船用钛合金领域鲜有报道，影响其在船舶领域的推广应用. 因此，文中利用CMT技术对船用钛合金进行电弧熔丝增材，重点研究构件微观组织、力学性能及其各向异性，为船用钛合金电弧熔丝增材制造技术的应用提供基础.
1. 试验方法
采用ϕ1.2 mm的丝材作为成形原料，以24 mm厚轧制Ti6321钛合金板材为成形基板. 采用基于CMT的电弧增材制造系统进行钛合金熔丝增材，成形电流110 A，成形速度为10 mm/s，如图1所示.
在 Leica DMI5000M型金相显微镜(OM)上进行组织观察，并采用JEM-2100透射电镜(TEM)对精细组织进行观察. 采用EBSD技术对成形后组织进行观察. 金相试样采用75 mL H2O+10 mL HF+15 mL HNO3的腐蚀剂进行腐蚀，透射样品通过机械减薄和电解双喷获得. 试验沿着试样的水平方向(x向)和竖直方向(z向)进行取样，如图2所示.
按照GB/T 228—2010《金属材料室温拉伸试验方法》在SINTECH20/G型试验机上进行拉伸试验，按照GB/T229—2010《金属材料夏比摆锤冲击试验方法》进行冲击试验. 利用Quanta650扫描电子显微镜(SEM)进行拉伸、冲击断口和疲劳断口观察.
2. 试验结果及分析
2.1 显微组织
图3为CMT-WAAM Ti6321合金成形态的组织. 宏观上表现为平行于堆积层面的层带状纹理(图3a)，这与TIG和等离子束的熔丝成形组织相似[28-29].WAAM过程近似一个微区铸造过程，除了通过先沉积的组织向基板方向传热外，向周围环境的辐射传热效率低，单向传热且冷却速度慢，易生成沿沉积高度方向生长、呈贯穿式的粗大原始β柱状晶[30]. 而CMT-WAAM成形组织为不规则的多边形原始β晶(prior-β)和晶界α (αGB)组织，且原始β晶分布在2～3个层厚之间，如图3b所示.
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图 3 不同位置的CMT-WAAM Ti6321合金的金相组织形貌Figure 3. Optical microstructure of CMT-WAAM Ti6321 samples. (a) the macrostructure; (b) interlayer morphology熔融金属在凝固过程中，成分过冷对晶粒的形核、长大有很大影响[31]. 较大的过冷度(ΔTcs)会促进合金在固液界面以树枝晶的形式向熔池内延性生长，从而形成粗大的柱状晶，如图4a所示. 而控制增材制造组织中的贯穿式柱状晶和性能的各向异性是钛合金增材制造中“控形/控性”重要研究内容[3]. 为了改善凝固组织形貌，控制柱状晶的延性生长，研究人员做了大量的工作. 目前已经出现添加晶粒细化剂、采用超声振荡、磁场的凝固过程/组织干预方法[32-34]，但这些方法增加了金属增材制造工艺及设备的复杂程度或者具有引入新杂质元素的风险.
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图 4 CMT-WAAM形核结晶过程Figure 4. Nucleation process of CMT-WAAM. (a) columnar dendrites; (b) arc waveforms of P-CMT; (c) equiaxial dendrites文中采用CMT电弧中加入脉冲电流(CMT+P模式)，脉冲电流产生的电弧力能够对熔池产生强烈的搅拌作用，从而破碎柱状晶尖端的细小的枝晶阻止柱状晶继续长大[23,29]，如图4b，4c所示. 同时，CMT-WAAM的低热输入能够使得熔池中破碎的细小树枝晶能够作为新的形核质点长大形成新的晶粒而不至于熔化，这就增加晶粒数量达到细化晶粒目的.
如图5所示，CMT-WAAM Ti6321合金的原始β晶内以α片层为主，晶界附近存在少量的α集束.图5b为图5a中B区的微观形貌，在柱状晶内大量的α片层(α lamellas)，晶界处存在少量的晶界α相(αGB). 此外，在晶界处发现少量的α集束(α colony)，这是由于冷却速度较快，晶内与晶界处的α相同时形核长大，晶界处的α集束无法大量形核长大. 如图5c所示，成形组织可分为粗晶区(coarse grain region)和细晶区(fine grain region)，均由粗细不均的片层组成. 粗晶区热循环温度较低(低于Tβ)，初生α相(αp)未完全转变为β相，在受热长大成为粗α相；在快冷条件下初生α相周围已转变为β相的部分组织，快冷过程中生成细针状的次生α相(αs)，最终导致了粗晶区α相的片层厚度不同. 细晶区的组织经历的热循环温度较高(Tβ附近)，初始α相转变较为完全，在快冷下形成细针状的α相. 激光选区成形组织中存在大量的马氏体[35]，而CMT-WAAM金属热影响区中未发现马氏体组织，如图5d所示.
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图 5 CMT增材制造Ti6321组织形貌Figure 5. Microstructure of CMT-WAAM Ti6321. (a) overall fracture surface; (b) high magnification of B; (c) high magnification of C; (d) TEM images of D2.2 拉伸性能
Kok等人[36]研究表明，增材制造钛合金的力学性能存在明显各向异性，并指出导致力学性能的各向异性产生的主要原因是贯穿成形组织的柱状晶和组织内部的织构. 如上所述，CMT-WAAM能避免贯穿式的柱状晶出现，有效细化晶粒，从一方面降低了各向异性.
试验中采用以下公式对材料力学性能的各向异性进行描述，即
$$\varepsilon = \frac{{{{\rm{\sigma }}_{{x}}} - {{\rm{\sigma }}_{{{\textit{z}}}}}}}{{{{\rm{\sigma }}_{{x}}}}} \times 100{\text{%}} $$ (1) 式中：σx为x方向强度；σz为z方向强度.
分别沿x方向和z方向对CMT-WAAM Ti6321合金制备拉伸试样，所测拉伸性能如表1所示，达到同级别锻件标准，但z向的断后伸长率明显低于x向.
表 1 CMT-WAAM Ti6321合金拉伸性能Table 1. Tensile properties of CMT-WAAM Ti6321 samples抗拉强度Rm/MPa 屈服强度Rp0.2/MPa 断后伸长率A(%) 锻件标准 840 740 8 水平方向 849.3 752.3 14.2 竖直方向 844 760 8 各向异性 1.8% −1.1% 43.6% CMT-WAAM Ti6321合金试样的抗拉强度各向异性异性系数均不超2%，表明x方向、z方向的拉伸强度均匀，没有明显的各向异性.
如图6a所示，晶粒内部均由粗细不均的片层组成. 如图6b所示，从极图中可以看出，α相晶体取向表现出较大离散性，不存在明显的织构，这从另一方面解释了CMT-WAAM组织各向异性较低的现象. 如图6c所示，从β相向α相转变过程中，片层取向符合经典的Burgers取向关系：即{0001}α//{110}β，< 110 > α// < 111 > β，α片层夹角呈60°.
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图 6 CMT-WAAM Ti6321组织的EBSD图Figure 6. EBSD of CMT-WAAM Ti6321. (a) orientation map with inverse pole figure; (b) orientation map of pole figure; (c) distribution of misorientation angle图7为CMT-WAAM钛合金不同方向上拉伸试样的断口横断面、纵断面和微观形貌.图7a，7c，7e为x向拉伸断口，图7b，7d，7f为z向拉伸断口. 从图7c和图7d可以观察到不同方向的裂纹均以穿片层方式扩展. 如前所述不同方向的晶粒尺度差别不大，位错开动与裂纹萌生所需的激活能一致，因而拉伸强度无明显差异. 图7e，7f分别为x向、z向拉伸断口扫描像均呈现大小相间的韧窝，为典型的韧性断裂. 但图7f 中观察到的z向断口中的气孔，可能是导致x向和z向延伸率的差异的原因.Leuders[37]和Rafi等人[38]研究表明，钛合金增材制造组织中的气孔会导致竖直方向的断后伸长率低于水平方向.
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图 7 拉伸断口形貌Figure 7. Morphologies of tensile fracture. (a) horizontal macrostructure; (b) vertical macrostructure; (c) horizontal longitudinal section; (d) vertical longitudinal section; (e) horizontal transverse sectionand; (f) vertical transverse section2.3 冲击韧性
复杂的海洋服役工况对船用钛合金构件的冲击韧性提出很高的要求. 试验对Ti6321电弧熔丝成形组织的x向、z向的冲击韧性进行了研究. x向、z向冲击吸收能量分别为74.3和66 J，高于同级锻件的标准值(47 J) 能够满足船用钛合金结构件的需求. 邵晖等人研究表明[39]，冲击韧性与片层组织的厚度相关. CMT-WAAM Ti6321组织以粗细交织α片层为主，在片层之间还存在少量的β相. 在试样冲击过程中，在粗细片层和α/β界面裂纹扩展方向易发生偏转，导致扩展路径曲折，提高了冲击韧性. 如图8所示，图8a，8b为x向的冲击断口，图8c，8d为z向的冲击断口，裂纹在穿过片层组织时产生大量的韧窝，为典型的韧性断裂.
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3. 结论
(1) CMT-WAAM Ti6321合金显微组织由不规则的多边形原始β晶和晶界α相组成. CMT+P模式能够在低热输入下有效细化晶粒，没有发现贯穿式的柱状晶，成形组织中未发现马氏体.
(2) CMT-WAAM Ti6321合金没有明显织构存在，室温抗拉强度达到同级别锻件标准，各向异性系数为1.8%，各向异性不明显，均为典型的韧性断裂. 成形组织中的气孔导致z向方向的断后伸长率低于x向.
(3) CMT-WAAMTi6321合金在x向和z向冲击吸收能量均不低于65 J，能够满足船用钛合金结构件的需求. 冲击断裂过程中裂纹穿过片层组织并产生大量的撕裂型韧窝，为典型的韧性断裂.
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图 3 不同位置的CMT-WAAM Ti6321合金的金相组织形貌
Figure 3. Optical microstructure of CMT-WAAM Ti6321 samples. (a) the macrostructure; (b) interlayer morphology
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图 4 CMT-WAAM形核结晶过程
Figure 4. Nucleation process of CMT-WAAM. (a) columnar dendrites; (b) arc waveforms of P-CMT; (c) equiaxial dendrites
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图 5 CMT增材制造Ti6321组织形貌
Figure 5. Microstructure of CMT-WAAM Ti6321. (a) overall fracture surface; (b) high magnification of B; (c) high magnification of C; (d) TEM images of D
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图 6 CMT-WAAM Ti6321组织的EBSD图
Figure 6. EBSD of CMT-WAAM Ti6321. (a) orientation map with inverse pole figure; (b) orientation map of pole figure; (c) distribution of misorientation angle
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图 7 拉伸断口形貌
Figure 7. Morphologies of tensile fracture. (a) horizontal macrostructure; (b) vertical macrostructure; (c) horizontal longitudinal section; (d) vertical longitudinal section; (e) horizontal transverse sectionand; (f) vertical transverse section
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表 1 CMT-WAAM Ti6321合金拉伸性能
Table 1 Tensile properties of CMT-WAAM Ti6321 samples
抗拉强度Rm/MPa 屈服强度Rp0.2/MPa 断后伸长率A(%) 锻件标准 840 740 8 水平方向 849.3 752.3 14.2 竖直方向 844 760 8 各向异性 1.8% −1.1% 43.6% 
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