不同热处理对SLM TC4组织性能及各向异性的影响 
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Ti-6Al-4V(TC4) titanium alloy exhibits impressive characteristics, including excellent corrosion resistance, high specific strength, and yield ratio, and thus, widely used in several fields, such as aerospace, navigation, and biomedical industries. Selective laser melting (SLM), a common method for manufacturing metal alloys, has multiple advantages, such as short production time, low cost, high integration, and high precision. Therefore, SLM is preferred for the fabrication of TC4 alloys with high density and good mechanical properties. However, the molten pool of SLM undergoes rapid melting and solidification under high temperature gradient. The microstructure of SLM TC4 is mainly composed of β columnar crystals, imparting poor plasticity and obvious anisotropy to the samples. Therefore, the deposited samples fail to meet the standard of commercial engineering and the simulation requirements of the titanium alloy. In this study, the TC4 samples prepared by SLM were heat-treated under three different conditions of solution aging, cyclic annealing, and cyclic spheroidizing annealing with solution aging. The effects of the three heat treatments on the microstructures and mechanical properties of the deposited TC4 were investigated with an optical microscope, a scanning electron microscope, and a tensile strength testing machine. The mechanisms of change in the microstructure and anisotropy during the three heat treatments are analyzed, providing a research basis for the application of TC4 in the aerospace field.
In our study, TC4 alloy powder with the particle size of 15-65 μm was used along with the rolled TC4 substrate. Before the experiment, the powder was placed in an oven and dried at 120 °C for 24 h to remove the moisture from the powder. The formation process and heat treatment were performed in the argon gas atmosphere. The size of the horizontal sample was 45 mm×8 mm×8 mm, while that of the vertical specimen was Φ8 mm×45 mm. According to the national standard GB/T228.1-2010, the samples were processed into standard tensile specimens and subjected to the tensile strength test at room temperature. Later, the samples were ground and polished, and the polished surface was corroded with Kroll reagent [V(HF)∶V(HNO3)∶V(H2O)=1∶3∶50 ]for 5 s. KEYENCE VH-600 optical microscope and TESCAN MIRA 3 LMH field emission scanning electron microscope were used to observe the microstructure and fracture.
The microstructure study showed that the substructure of SLM TC4 mainly comprised of martensite α′ and α″. The aspect ratio of martensite was greater than 20, and the dislocations and stresses produced by rapid heating and cooling provided sites for the nucleation of martensite. Post HM1[950 ℃/1 h/ air cooling (AC)+ 550 ℃/4 h/ AC] heat treatment, a bimodal microstructure was observed for SLM TC4 with the aspect ratio of lath α phase being about seven. During the heat preservation at 950°C, martensite α′ transformed into the lath α phase by downhill diffusion, the grain boundary α truncated when lath α phase grew, β phase nucleated and grew preferentially at the grain boundary, and the grain boundary broke under the combined influence of the above factors. During the aging process, small strips of secondary α phase precipitated from the metastable β phase. After HM2[(920 ℃×5 min+ 700 ℃×10 min)×5 / AC] heat treatment, the β phase in the microstructure of TC4 almost completely transformed into lath and equiaxed α phase, with the aspect ratio of the lath α phase being about 4, and the transformation mainly occurred as α′+ α″→α+ β. The spheroidization is mainly caused by thermal grooving and boundary splitting mechanisms. After five cycles, the volume fraction of α phase and the degree of spheroidization increased. After HM3[(920 ℃×10 min+ 800 ℃×30 min-550 ℃)×4/ furnace cooling (FC)+ 920 ℃×1 h/ AC+ 550 ℃×4 h/ AC) ]heat treatment, the microstructure of SLM TC4 mainly composed of lath α phase, equiaxed α phase, and net basket-like secondary precipitated α phase. The Burgers orientation relationship was satisfied during the phase change process. Due to the long heating time of cyclic spheroidizing annealing, the grain strength decreased more severely, and the subsequent solution aging made up for the strength loss during the cycle annealing process (Figs.3-4). As per the tensile strength test results, the tensile strength and the yield strength of SLM deposited TC4 were obtained as 1223.0 and 1054.2 MPa, respectively. The grain boundary hindered the movement of dislocations, therefore, the deposited sample exhibited high strength. After HM1 heat treatment, the tensile and the yield strengths of the TC4 depositions were 957.5 and 865.4 MPa, respectively, and the elongation was approximately 17%. Among the three heat treatments, the highest elongation of the deposited TC4 was observed after HM3 heat treatment, reaching up to 18.4%. After the heat treatment, the grain boundary α phase transformed into a continuous equiaxed α phase, which weakened the crack resistance, so that the strength decreased and elongation increased (Table 3). Analyzing the anisotropy data of the different mechanical properties of the deposited TC4 in the four states(As-builted、HM1、HM2、HM3), the anisotropy of the elongation and the reduction of the TC4 deposition area was relatively high (12.21% and 5.63%, respectively). (Figs.5-6). The anisotropy of tensile strength and yield strength of TC4 after heat treatment of HM1, HM2 and HM3 is less than 2%, and the anisotropy of elongation and area shrinkage decreases compared with that in the deposited state.
The deposited microstructure of SLM TC4 constitutes coarse β columnar crystals with martensite α′ and α″ as the intragranular substructures. The microstructure of SLM TC4 after solution aging heat treatment is bimodal, and a secondary α phase is generated during the aging process. During the cyclic annealing process, parts of the grain boundary α phase and lath α phase are broken to form an equiaxed α phase. After the cyclic spheroidizing annealing and solution aging treatment, the microstructure of the sample is mainly composed of the lath α phase, equiaxed α phase, and secondary α phase; the volume fraction of the equiaxed α phase is relatively high. The standard deviation of the mechanical property data of SLM TC4 is generally large, which is significantly reduced after the heat treatment. The strengths of the samples treated either solely by solution aging or a combination of cyclic spheroidizing annealing and solution aging are roughly equivalent. The sample plasticity obtained by the combined cyclic spheroidizing annealing and solution aging is better than that obtained by solution aging alone. After the heat treatment methods, the mechanical properties of the samples exceed the national forging standard. The plastic anisotropy of SLM TC4 depositions is high. However, the three heat treatment processes significantly reduce the plastic anisotropy. Among these processes, the solution aging treatment provides the deposited TC4 with the lowest anisotropy of the mechanical properties. SLM TC4 depositions have mixed fracture morphology, while the deposited TC4 after heat treatment shows a typical ductile fracture.
1 引言
TC4具有比强度高、耐蚀性好和耐热性高等特点，被广泛应用于航空航天领域，是航空发动机风扇、压气机轮盘、叶片和起落架等重要结构件的首选材料之一[1-2]。但是传统钛合金加工难度大、周期长、材料利用率低，使用钛合金结构件的成本较高，阻碍了钛合金的发展。选区激光熔化(SLM)技术具有成形快、成本低、集成化高、近净成形等优点，通过系统自带的切片软件对模型切片，运用高能量激光束逐层熔化合金粉末，最终累加成三维零件，材料利用率高，可以直接制备出力学性能好、致密度高的零件[3-5]。但SLM的熔池具有快热快冷、温度梯度高的特点，使得SLM TC4微观组织产生粗大的β柱状晶，柱状晶组织使得成形件具有明显的各向异性。断裂韧性、电导率、弹性模量等很多性能都具有方向性，各向异性对具备方向性性能的工程使用和仿真带来了很大困难，限制了钛合金在航天领域的应用[6-8]。
调控TC4合金力学性能及各向异性的方法主要有三种：一是通过变质处理改善组织形貌，提供异质形核来细化晶粒，常见的加入合金元素有硼(B)和铁(Fe)，硼元素可以细化合金组织，在晶界或相界产生TiB，含量越高，细化效果越明显，但硼含量过高会使塑性下降，硼含量在0.05%时TC4综合力学性能最好[9-10]；Fe的加入使TC4中相的数量和大小略有变化，α相容易形成更窄的片状，体积分数降低，β相的宽度和相分数也相应增加。通过增强晶格畸变和振动，从而提高杨氏模量和断裂韧性[11]。二是通过机械冲击、微锻造细化晶粒并降低各向异性，在增材制造的同时对成形层表面施加超声冲锻或振动，通过形成机械变形层来限制β柱状晶生长，从而改善各向异性。但此方法成本高、生产效率低[12]。三是通过特定热处理来控制组织并降低各向异性。Sabban等[13]通过热处理将SLM TC4合金的组织转变为主要由球状α相组成的双态组织，微观组织为双态组织的TC4合金具有更高的综合力学性能，通过在β相变温度附近但低于β相变温度进行重复热循环来形成球状α相，可以在不施加塑性变形的情况下获得双态组织，同时提出一种解释形成球状α相的新球状化机制。北京航空航天大学袁经纬等[14]分析了激光增材制造TC4合金在不同热处理状态下试样的电化学及室温压缩蠕变性能，结果表明，双重退火处理会显著减小增材制造TC4钛合金中α板条的长宽比和尺寸，而固溶时效可使α板条长宽比增大，尺寸减小，从而提高材料耐蚀性、屈服极限以及抗压缩蠕变性能。Wang等[15-17]将SLM TC4合金分别在810～990 ℃下保温2 h空冷(AC)，发现随着正火温度的升高，初生α-Ti相的长宽比和含量逐渐降低，在930 ℃和990 ℃正火时，板条间和晶粒内都有β-Ti相生成，当正火温度为990 ℃时，获得最佳的综合力学性能。
国内外有很多学者对SLM TC4的力学性能进行研究，但对成形件的各向异性和调控柱状晶组织的研究不够系统和全面。热处理可以有效降低TC4合金的各向异性，提高其综合力学性能[18-20]。本文设计了三种不同热处理工艺，通过对热处理后成形件的力学性能、微观组织、各向异性进行分析，从而获得可以有效降低SLM TC4各向异性，同时其塑韧性匹配良好的热处理工艺，为SLM TC4钛合金在航天领域的应用提供研究基础。
2 实验材料及方法
SLM TC4实验成形件在西安交通大学机械制造系统国家重点实验室自主研发的SLM-500A型激光熔化沉积成形系统上进行。本实验所用粉末为TC4合金粉末，成分如
表 1. TC4粉末具体化学成分
Table 1. Chemical composition of TC4 powder
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表 2. SLM TC4合金热处理工艺
Table 2. Heat treatment processes of SLM TC4 alloy
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在成形过程中使用99.9%高纯氩气防止样件在打印过程中氧化。影响SLM TC4成形件性能的成形工艺参数主要有激光功率、扫描速度、扫描间距和单层提升量，对以上4个工艺参数设计四因素四水平的正交实验，通过阿基米德排水法确定成形件的致密度，当激光功率为280 W、填充扫描速度为1200 mm/s、扫描间距为0.14 mm、提升量为0.03 mm时，其成形件致密度最高，可以达到99.76%。外轮廓扫描速度为800 mm/s，扫描功率为280 W。光斑直径为0.043 mm，工作室氧体积分数≤1.3×10－3，10 h后氧体积分数≤3×10－4，填充角度为67°。
水平试样和竖直试样各成形20个，水平试样尺寸为45 mm×8 mm×8 mm，竖直试样尺寸为Φ8 mm×45 mm，成形方式如
图 1. SLM TC4样件成形方式及尺寸示意图。(a)水平成形示意图；(b)竖直成形示意图；(c)水平成形实物图；(d)竖直成形实物图
Fig. 1. Schematic diagram of SLM TC4 sample forming method and size. (a) Schematic of horizontal specimen；(b) schematic of vertical specimen; (c) physical drawing of horizontal forming; (d) physical drawing of vertical forming
SLM表示沉积态TC4样件。HM1表示成形件先在950 ℃下固溶1 h，之后在550 ℃下时效4 h后空冷。HM2表示成形件加热至920 ℃保温5 min，后冷却至700 ℃保温10 min，如此循环5次后空冷。HM3表示先在920 ℃下保温10 min，之后冷却至800 ℃，之后降温到550 ℃后立即升温到920 ℃，重复循环4次后炉冷(FC)，之后进行920 ℃固溶1 h，550 ℃时效4 h空冷。三种热处理中的升温速度为10 ℃/min，降温速度为5 ℃/min。
固溶时效是TC4锻件常用的热处理工艺。固溶时效可以减少TC4锻件中的连续晶界α相，从而降低其各向异性，固溶处理后生成马氏体α′相和α″相或一定量的亚稳定β相，时效处理过程中亚稳定相按一定方式分解，产生强化效果，因此HM1采用固溶时效热处理工艺。循环退火过程中伴随着α、β相的多次转变，可以降低α相的长宽比，进而降低其各向异性，因此HM2采用循环退火的热处理工艺。通过在较低温度下保温和缓慢炉冷，希望能够获得尺寸均匀的晶粒组织。但长时间的加热会使α相粗化，因此希望通过固溶时效在组织均匀的前提下细化晶粒，以达到在降低TC4样件各向异性的同时仍保留较高强度，因此HM3采用循环球化退火＋固溶时效热处理工艺。
按照国家标准GB/T228.1-2010将样件加工成标准拉伸试样，并进行室温拉伸测试，选取力学性能最接近平均值的拉伸试样进行线切割，完成之后进行制样抛光。用Kroll试剂[V(HF)∶V(HNO3)∶V(H2O)＝1∶3∶50]对抛光面进行腐蚀，腐蚀时间为5 s，分别用水和酒精清洗试样附着物。采用低倍的光学显微镜(型号为KEYENCEVH-600)观察沉积态试样β柱状晶及晶界，采用高倍扫描电子显微镜(型号为TESCAN MIRA3LMH)观察β晶内亚结构，利用Image J染色处理对晶粒进行统计分析。
3 实验数据分析与讨论
3.1 沉积态与不同热处理TC4微观组织分析
图 2. SLM TC4成形件宏观形貌。(a)SLM-V；(b)SLM-H
Fig. 2. Macro morphology of SLM TC4. (a) SLM-V; (b) SLM-H
图 3. SLM TC4沉积态及不同热处理工艺的微观组织。(a)沉积态；(b)HM1；(c)HM2；(d)HM3
Fig. 3. Images of low-magnification microstructure of SLM TC4 deposited state and different heat treatment processes. (a) As-deposited; (b) HM1; (c) HM2; (d) HM3
图 4. SLM TC4 沉积态及不同热处理工艺下微观组织SEM图。(a)沉积态；(b)HM1; (c)HM2; (d)HM3
Fig. 4. SEM images of low-magnification microstructure of SLM TC4 deposited state and different heat treatment processes. (a) As-deposited; (b) HM1; (c) HM2; (d) HM3
3.2 不同热处理TC4力学性能分析
表 3. 不同状态下SLM TC4合金拉伸试样力学性能实验数据
Table 3. Mechanical properties of SLM TC4 alloy tensile specimens under different treatment conditions
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三种热处理中，HM1热处理后的成形件的强度最高，抗拉强度和屈服强度分别为957.5 MPa和865.4 MPa，延伸率约为17%。由
3.3 不同热处理TC4力学性能各向异性分析
对沉积态、HM1、HM2和HM3四种状态的力学性能进行各向异性计算分析，然后将各向异性结果以柱状图表示。各向异性的计算公式为
图 5. 沉积态和不同热处理工艺下力学性能的各向异性柱状图
Fig. 5. Anisotropic histogram of microstructure in deposited state and different heat treatment processes
图 6. 不同方向晶界所受拉应力状态及裂纹扩展路径示意图。(a)SLM-H; (b)SLM-V; (c)沉积态和热处理后的裂纹扩展路径示意图
Fig. 6. Tensile stress state of grain boundaries in different directions and crack propagation path diagram. (a) SLM-H；(b) SLM-V; (c) schematic diagram of crack propagation pathin deposited state and after heat treatment
3.4 拉伸断口分析
图 7. SLM TC4沉积态下试样拉伸断口形貌。(a)SLM-H(×80); (b)SLM-H(×2000); (c)SLM-H(×5000); (d)SLM-V(×80); (e)SLM-V(×2000); (f)SLM-V(×5000)
Fig. 7. Tensile fracture morphology of SLM TC4 in deposited state. (a) SLM-H(×80); (b) SLM-H(×2000); (c) SLM-H(×5000); (d) SLM-V (×80); (e) SLM-V(×2000); (f) SLM-V(×5000)
图 8. 不同热处理工艺的拉伸断口形貌图。(a)HM1-H(×80); (b)HM1-H(×2000); (c)HM1-V(×80); (d)HM1-V(×2000); (e)HM2-H(×80); (f)HM2-H(×2000); (g)HM2-V(×80); (h)HM2-V(×2000); (i)HM3-H(×80); (j)HM3-H(×2000); (k)HM3-V(×80); (l)HM3-V(×2000)
Fig. 8. Topography of the tensile fracture of different heat treatment processes. (a) HM1-H(×80); (b) HM1-H(×2000); (c) HM1-V(×80); (d) HM1-V(×2000); (e) HM2-H(×80); (f) HM2-H(×2000); (g) HM2-V(×80); (h) HM2-V(×2000); (i) HM3-H(×80); (j) HM3-H(×2000); (k) HM3-V(×80); (l) HM3-V(×2000)
4 结论
SLM TC4微观组织为粗大β柱状晶，晶粒内亚结构为马氏体α′相和马氏体α″相；SLM TC4经过固溶时效热处理后的组织为双态组织，时效过程中有二次α相产生；循环退火过程中部分晶界α相和板条α相破裂溶解形成等轴α相；经过循环球化退火＋固溶时效处理后，试样微观组织主要由板条状α相、等轴α相以及二次析出α相组成，等轴α相占比32%左右，晶粒尺寸最为均匀。经过三种热处理后α相宽度和长宽比值都有大幅下降。
SLM TC4成形件力学性能数据标准差较大，经过热处理后明显降低；经过固溶时效和循环球化退火＋固溶时效处理的样件强度指标较为接近，但是循环球化退火＋固溶时效的塑性相对更好，其延伸率最高可达19%，两种热处理样件的力学性能指标均超过国家锻件标准。循环退火成形件加热时间长，晶粒粗化严重，且没有二次强化，循环退火成形件的强度指标下降最大。
连续的β相晶界对裂纹的扩展有阻碍作用，因此SLM TC4沉积态塑性指标的各向异性较高，其延伸率的各向异性达到13%；三种热处理工艺中，固溶时效处理后的成形件的力学性能的各向异性最低，总体各向异性小于1.2%。SLM TC4成形件的断口形貌为混合断口形貌，边缘韧窝深度较浅，热处理后的成形件的断口都为典型韧性断口，与SLM TC4相比其韧窝小而均匀，且深度较深，部分大韧窝之中有小韧窝出现。
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Article Outline
窦振, 王豫跃, 张安峰, 吴梦杰, 王普强. 不同热处理对SLM TC4组织性能及各向异性的影响[J]. 中国激光, 2022, 49(8): 0802009. Zhen Dou, Yuyue Wang, Anfeng Zhang, Mengjie Wu, Puqiang Wang. Effect of Different Heat Treatments on Microstructure, Properties, and Anisotropy of SLM TC4[J]. Chinese Journal of Lasers, 2022, 49(8): 0802009.
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