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激光熔化沉积制备2A50锻造铝合金组织性能*

  • 赵宇辉1,2

    机 构：

    1. 中国科学院沈阳自动化研究所 沈阳 110016

    2. 中国科学院机器人与智能制造创新研究院 沈阳 110169

    ×
  • ， 孙力博1,2,3

    机 构：

    1. 中国科学院沈阳自动化研究所 沈阳 110016

    2. 中国科学院机器人与智能制造创新研究院 沈阳 110169

    3. 沈阳工业大学材料科学与工程学院 沈阳 110870

    ×
  • ， 赵吉宾1,2

    机 构：

    1. 中国科学院沈阳自动化研究所 沈阳 110016

    2. 中国科学院机器人与智能制造创新研究院 沈阳 110169

    ×
  • ， 王志国1,2

    机 构：

    1. 中国科学院沈阳自动化研究所 沈阳 110016

    2. 中国科学院机器人与智能制造创新研究院 沈阳 110169

    ×
  • ， 何振丰1,2

    机 构：

    1. 中国科学院沈阳自动化研究所 沈阳 110016

    2. 中国科学院机器人与智能制造创新研究院 沈阳 110169

    ×
  • ， 贺晨1,2

    机 构：

    1. 中国科学院沈阳自动化研究所 沈阳 110016

    2. 中国科学院机器人与智能制造创新研究院 沈阳 110169

    ×

1. 中国科学院沈阳自动化研究所 沈阳 110016；
2. 中国科学院机器人与智能制造创新研究院 沈阳 110169；
3. 沈阳工业大学材料科学与工程学院 沈阳 110870；

Microstructure and Mechanical Properties of 2A50 Wrought Aluminum Alloy Prepared by Laser Melting Deposition

  • ZHAO Yuhui1,2

    Affiliation：

    1. Shenyang Institute of Automation, Chinese Academy of Sciences, Shenyang 110016 , China

    2. Institutes for Robotics and Intelligent Manufacturing, Chinese Academy of Sciences, Shenyang 110169 , China

    ×
  • ， SUN Libo1,2,3

    Affiliation：

    1. Shenyang Institute of Automation, Chinese Academy of Sciences, Shenyang 110016 , China

    2. Institutes for Robotics and Intelligent Manufacturing, Chinese Academy of Sciences, Shenyang 110169 , China

    3. College of Material Science and Engineering, Shenyang University of Technology, Shenyang 110870 , China

    ×
  • ， ZHAO Jibin1,2

    Affiliation：

    1. Shenyang Institute of Automation, Chinese Academy of Sciences, Shenyang 110016 , China

    2. Institutes for Robotics and Intelligent Manufacturing, Chinese Academy of Sciences, Shenyang 110169 , China

    ×
  • ， WANG Zhiguo1,2

    Affiliation：

    1. Shenyang Institute of Automation, Chinese Academy of Sciences, Shenyang 110016 , China

    2. Institutes for Robotics and Intelligent Manufacturing, Chinese Academy of Sciences, Shenyang 110169 , China

    ×
  • ， HE Zhenfeng1,2

    Affiliation：

    1. Shenyang Institute of Automation, Chinese Academy of Sciences, Shenyang 110016 , China

    2. Institutes for Robotics and Intelligent Manufacturing, Chinese Academy of Sciences, Shenyang 110169 , China

    ×
  • ， HE Chen1,2

    Affiliation：

    1. Shenyang Institute of Automation, Chinese Academy of Sciences, Shenyang 110016 , China

    2. Institutes for Robotics and Intelligent Manufacturing, Chinese Academy of Sciences, Shenyang 110169 , China

    ×

1. Shenyang Institute of Automation, Chinese Academy of Sciences, Shenyang 110016 , China；
2. Institutes for Robotics and Intelligent Manufacturing, Chinese Academy of Sciences, Shenyang 110169 , China；
3. College of Material Science and Engineering, Shenyang University of Technology, Shenyang 110870 , China；

作者简介:

赵宇辉,男,1983年出生,博士,副研究员,硕士研究生导师。主要研究方向为激光增材制造与修复技术。E-mail:yhzhao@sia.cn;

贺晨(通信作者),男,1988年出生,博士,助理研究员。主要研究方向为高性能铝合金激光增材制造工艺与技术。E-mail:hechen@sia.cn

中图分类号:TG456;TB115

DOI:10.11933/j.issn.1007−9289.20211111001

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目录contents

    摘要

    工艺参数的协同调控决定了沉积工件的组织与性能，在锻造铝合金零件激光增材修复工程应用方面具有重要研究价值。 采用 OM、SEM、XRD 等试验方法，研究能量密度对激光沉积成形 2A50 铝合金构件组织与性能的影响规律。结果表明：当能量密度低于 200 J / mm2时，成形效果较差且产生粉末球化、未熔合等凝固缺陷；随着能量密度的提高，沉积试样底部和顶部一次枝晶间距均明显缩短、平均硬度由 85.7 HV 提高至 92.1 HV；过高的能量密度输入会导致熔池内部分低熔点合金元素蒸发形成气孔缺陷、同时削弱了合金元素的固溶强化效果。在优化的能量密度（333 J / mm2 ）条件下，激光沉积成形 2A50 锻造铝合金构件获得了较优的综合力学性能，其屈服强度、抗拉强度和延伸率分别为 85 MPa、207 MPa 和 14%。为航空重大装备关键零部件的激光增材修复探索出一条行之有效的技术途径。

    Abstract

    The cooperative regulation of process parameters determines the microstructure and properties of deposited workpieces, which has important research value in the engineering application of laser additive repair of forged aluminum alloy. With the application of OM, SEM, XRD and other methods, the effect of energy density on the microstructure and mechanical properties of 2A50 aluminum alloy are studied. The results show that when the laser energy density is less than 200 J / mm2 , the forming effect of the deposited sample is poor, resulting in solidification defects such as powder spheroidization and lack of fusion. With the increase of laser energy density, the primary dendrite spacing at the bottom and top of the deposited sample is significantly shortened, the average hardness increases from 85.7 HV to 92.1 HV. However, too high energy density input will lead to the evaporation of some low melting point alloy elements in the molten pool, forming porosity defects. Therefore, under the condition of optimized laser energy density (333 J / mm2 ), the 2A50 wrought aluminum alloy fabricated by laser melting deposition has better comprehensive mechanical properties, and its yield strength, tensile strength and elongation are 85 MPa, 207 MPa and 14% respectively. It explores a more effective way for laser additive repair of key parts of aviation major equipment.

  • 0 前言

  • 时效硬化型Al-Cu系合金具有较低的密度、较高的比强度、优异的可塑性、较好的耐腐蚀性、易于锻造和挤压等特点，广泛应用于国防军工、航空航天以及交通运输等领域[1-2]。2A50属于高强度锻造型铝合金，一般作为航空航天器的隔框、支架和摇臂等零部件的制造材料，传统的制造方法包括熔炼、铸造、锻造、挤压等步骤，工艺流程长、成材率低、无法小批量、定制化生产，显著增加了航空发动机关键零部件使用与维护成本[3-4]。激光熔化沉积（Laser melting deposition，LMD）工艺是一种典型的高效率、绿色化、智能化的先进增材制造技术，被誉为第三次工业革命的载体之一[5-6]。与选区激光熔化（Selective laser melting，SLM）技术相比，LMD技术具有热加工变形少、热影响区小、成形尺寸不受限制、成形效率高等优势[7-9]，特别适用于高成本、难加工航空发动机零部件的缺陷修复与表面熔覆[10-12]。

  • 铝合金粉末具有高温氧化性强、激光反射率高、熔融态流动性差等特点[13-14]，导致增材制造工件致密度较低、裂纹缺陷敏感性较高、综合力学性能明显低于相同成分的锻造工件[15-16]。目前，只有部分以中低强度Al-Si系铸造合金为核心的AlSi10Mg、 AlSi7Mg等牌号可以实现小批量、定制化零部件的生产[17-19]。然而，采用现有Al-Si系铸造铝合金对航空发动机零部件实施缺陷部位增材修复，仍无法满足航空器对使用材料强度、疲劳、摩擦磨损等性能的严格要求，因此也推动了对2000系[20-22]、5000系[23-24]、6000系[25-27]和7000系[28-30]等中高强度铝合金在激光增材制造领域的广泛研究。相比铸造铝合金，锻造型铝合金溶质元素含量较多、凝固区间较宽，在激光增材制造过程中容易产生裂纹等缺陷。 ZHANG等[23]提出，增材制造关键工艺参数及扫描策略的优化，可以显著改善沉积态工件的宏观形貌与组织形态，降低增材工件的孔隙率，从而实现材料的抗疲劳性能与耐腐蚀性能显著提高。 SPIERINGS等[24]和AGRAWAL等[31-32]开发出一种适合增材制造的特有的合金成分体系，研究了Sc、 Zr等合金元素对Al-Cu系，Al-Mg系合金孔隙率、晶粒度以及力学性能的影响规律，结果表明合金元素的添加析出了细小的Al3(Sc, Zr)初生相，增加了熔池中非均质形核质点的数量，在熔池边缘形成了细小的等轴晶带，降低了增材制造工件中的孔隙率，抑制了结晶裂纹与液化裂纹的形成。OTANI等[28] 重点研究了7075铝合金增材制造过程结晶裂纹的形成机理以及预防措施，结果表明Si元素的添加提高了熔池内铝合金熔体的流动性能，可以显著减少SLM工件中的孔洞缺陷、抑制结晶过程热裂纹的形成；而对于7075合金来说最佳Si含量为5%wt.，这是消除裂纹所需的最低Si含量，此时具有较高的抗拉强度和较好的延展性。

  • 综上所述，关于高强度锻造铝合金的增材制造，大部分基于小型实验室规模条件下，通过SLM铺粉式工艺制备的试验样品，而且没有实现基础研究成果向实际工程应用的转化。鉴于此，本文基于同轴送粉LMD技术制备出不同工艺条件下的2A50铝合金块体材料；探索了激光能量密度对沉积态铝合金试样微观组织与力学性能的影响规律；获得了激光熔化沉积制备2A50铝合金较优的工艺窗口，实现了该工艺技术在某航空发动机锻造铝合金关键零部件裂纹修复领域的成功应用，为航空重大装备关键零部件的缺陷修复探索出一条更为有效的技术途径，有望降低航空企业零部件的维护与制造成本。

  • 1 试验准备

  • 1.1 样品制备

  • 本文采用的2A50铝合金球形粉末是由北京有色金属研究总院气雾化法生产，采用等离子体原子发射光谱法（ICP-AES，Thermo Fisher Scientific， Inc.，Waltham，MA，USA）检测粉末的化学成分 （质量分数），如表1所示。图1为2A50铝合金粉末的微观形貌，由图可见粉末球形度较好且较为致密，尺寸范围为50～150 μm，其霍尔流动性数值为87s/50g，流动性较好且颗粒大小均匀；试验前采用真空干燥箱对粉末进行干燥处理，其工艺参数为120℃ × 4h；基板采用尺寸为φ108 × 20mm的圆柱形AlSi10Mg合金材料，试验前使用酒精擦掉表面油渍，并用砂纸打磨去除表面氧化层。

  • 表1 2A50铝合金粉末化学成分（质量分数）

  • Table1 Chemical composition of 2A50aluminum alloy powder

  • 图1 激光熔化沉积试验的2A50铝合金粉末形貌

  • Fig.1 Morphology of 2A50aluminum alloy powder for laser melting deposition experiment

  • 如图2所示，LMD试验是在中国科学院-沈阳自动化研究所自主开发的第四代激光增材制造装备上进行。试验平台包括：10kW光纤激光器 （ YLS-10000-KC）、高精度双路可调送粉器 （RC-PGF-D）、单温单控式水冷机组、瑞士Staubli五轴联动机器人、计算机控制系统、气体保护系统等。在试验过程中采用柔性密封的方式，确保沉积环境中的氧含量始终保持在0.05‰以下。铝合金属于高反射率材料，为了防止试验过程中产生的反射光对光学设备造成不可逆伤害，在扫描过程中通常采用一个偏转角度α，本文设定偏转角度α为8°。

  • 图2 2A50铝合金激光熔化沉积系统示意图

  • Fig.2 Schematic diagram of laser melting deposition system for 2A50aluminum alloy

  • 本文在不同能量密度下进行激光熔化沉积成形试验，能量密度如式（1）所示[29]：

  • ε=PVs×D
    (1)

  • 式中，ε为能量密度；P 为激光功率；V s为扫描速率； D 为光斑直径。

  • LMD试样的宏观形貌如图3所示，随着能量密度提高，熔池宽度逐渐增加，沉积试样高度上先升高后降低，而当能量密度小于200J/mm2 时，沉积试样成形效果较差，产生粉末球化、未熔合等凝固缺陷。因此本文工艺参数设定为：扫描速率3mm/s、送粉速率1.8g/min、光斑直径3mm，通过调整激光功率将能量密度控制在200～400J/mm2 范围内。采用不同能量密度制备单道6层试样和薄壁块体试样进行微观组织分析与力学性能检测，图3左边图片为单道6层试样的宏观形貌，能量密度分别为222、333、389J/mm2；图3右边图片为三道45层薄壁块体试样的宏观形貌，能量密度分别为222、333、389J/mm2。

  • 图3 激光熔化沉积制备2A50铝合金试样的宏观形貌

  • Fig.3 Morphology of 2A50aluminum alloy prepared by LMD

  • 1.2 结构表征及力学性能测试

  • 采用线切割、打磨、抛光制备金相试样，经Keller试剂（95ml H2O、2.5ml HNO3、1.5ml HCl and 1ml HF）腐蚀15s后在OM光学显微镜（Zeiss Vert.A1）和SEM扫描电子显微镜（Zeiss, EVO 10）以及配套EDS能谱仪对样品的表面进行形貌观测与表征，并利用Image-pro Plus软件测量枝晶尺寸； 采用X射线衍射分析仪（PANalytical, X'Pert PRO MPD）进行物相分析；采用显微硬度仪（Future-tech, FM-310）对进行试样截面区域进行硬度检测，载重200g，加载时间为15s；采用INSTRON 5982万能试验机进行拉伸试验，拉伸速率为1mm/min，并采用扫描电子显微镜观察断口形貌。如图4所示，采用本文自主开发的LMD技术对某航空发动机关键零部件使用过程中产生的疲劳裂纹进行了沉积修复试验，修复后缺陷部位熔合良好，无裂纹缺陷，实现了基础理论研究向工程实际应用的成功转化。

  • 图4 LMD技术成功修复某飞机上锻造铝关键零部件

  • Fig.4 LMD technology successfully repaired the key forging aluminum parts of an Aero engine

  • 2 结果与讨论

  • 2.1 微观组织

  • 如图5所示是激光熔化沉积2A50铝合金单道6层微观组织，分别是底部为柱状树枝晶区、顶部为等轴树枝晶区以及中间部分的过渡区域。为了更加清晰观察出不同能量密度对显微组织的影响，对各组试样的底部柱状树枝晶区以及顶部等轴树枝晶区进行高倍组织观察，单位面积内枝晶数量随能量密度提高而增加，微观组织明显细化。微观结构的生长形态与固液界面前沿组分过冷度有关，而组分过冷度为熔融金属平衡状态下相变温度和实际相变温度的差值。可以采用温度梯度（G）和凝固速率（R） 的比值衡量组分过冷度，如式（2）所示[33]：

  • 图5 不同能量密度2A50铝合金试样微观组织

  • Fig.5 Optical microstructure of 2A50aluminum alloy sample with different energy density

  • GR=2kT-T02εPVcos⁡θ
    (2)

  • 式中，k 为合金的导热系数；T 为金属液相实际温度； T 0 为初始温度；ε 为粉末对激光的吸收系数；P 为激光功率；V 为激光的扫描速度。

  • 靠近基板位置散热方式分为两种：一是通过基板的热传导，二是空气热传导，温度梯度 G 的数值较大，G/R 的数值较高，此时的晶粒容易沿着正温度梯度的方向生长成柱状树枝晶[34]；顶部位置的散热方式为通过前沉积层散热和空气散热两种，温度梯度 G 的数值较靠近基板处低，G/R 的数值较低，同时热流朝多个方向传导，此时容易促进等轴树枝晶的生长。

  • 如图6所示为Image-Pro Plus软件测量的一次枝晶间距的平均值。由图6可见，随着能量密度的提高，底部柱状树枝晶的一次枝晶间距由22.5 μm缩短到16.7 μm，而顶部等轴树枝晶的一次枝晶间距由10.6 μm缩短到8.2 μm。

  • 图6 2A50铝合金试样不同位置处的一次枝晶间距

  • Fig.6 Primary dendrite spacing at different positions of 2A50aluminum alloy

  • 在激光熔化沉积过程中，冷却速率与一次枝晶间距的关系如式（3）所示[35]：

  • λ=CV-m
    (3)

  • 式中，λ为一次枝晶间距；V 为冷却速率；C 和 m 为常数。冷却速率dT/dt 如式（4）表示[36]：

  • dTdt=-2KπVPΔT2
    (4)

  • 式中，k 为热导率；π为常数；V 为激光扫描速率； P 为激光功率；ΔT 为熔池冷却前后的温度差。

  • 由式（4）可知，当激光功率 P 增大，温度差ΔT 增大时，熔池冷却速率dT/dt 增大，一次枝晶间距逐渐减小。

  • 2.2 组成相分析

  • 如图7所示为沉积态2A50铝合金扫描电镜图像及相应的EDS能谱分析。激光熔化沉积是一个快速加热与快速冷却循环往复的过程，极快的冷却速率有助于合金中溶质元素的过饱和固溶，当能量密度较低为222J/mm2 时，熔池的升温速率与冷却速率均较慢，较多的溶质元素以非平衡共晶相的形式在晶界析出，如图7a在晶界上析出的少量亮白色第二相所示；当能量密度提高至389J/mm2 时，发现晶界处的结晶相密度明显降低，说明有较多的合金元素固溶如基体中，有助于后续热处理过程沉淀相的析出。通过图7d及面扫描的结果，可以看到晶界处聚集的第二相主要分为两种：一种为亮白色块状结晶相，另一种为长条状暗灰色结晶相。结合能谱分析结果，亮白色的第二相主要含有Cu元素，暗灰色第二相的主要含有Si、Cu及Mg元素。

  • 图7 2A50铝合金扫描电镜图像及相应的EDS图谱

  • Fig.7 SEM image and corresponding EDS maps of 2A50aluminum alloy

  • 通过XRD分析了能量密度对沉积试样组成相的影响规律，结果如图8所示。能量密度333J/mm2 与能量密度222J/mm2 试样的衍射峰与Al标准衍射峰对比均向大角度方向有一定的偏移，且能量密度333J/mm2 试样偏移的角度更大一些。当能量密度较低为222J/mm2 时，试样中检测到具有明显的CuAl2 的衍射峰；随着能量密度的提高，熔池内的升温速率与凝固速率均显著提高，Cu元素的过饱和固溶减少了晶界处非平衡共晶相的析出数量，因此检测到峰值强度有所降低。

  • 图8 不同能量密度2A50铝合金的XRD图谱

  • Fig.8 XRD pattern of 2A50aluminum alloy with different energy density

  • 2.3 力学性能

  • 图9 所示为不同能量密度沉积试样横截面处从底部到顶部的显微硬度分布。由图9可见，随着距离试样底部距离的增加，硬度值逐渐升高，并分别在不同微观组织区域的交界处出现了硬度的峰值。这是由于柱状树枝晶的晶粒较大而等轴树枝晶的晶粒较小，细晶强化作用导致显微硬度值较高，值得注意的是当能量密度从222J/mm2 提高至333J/mm2 时，硬度提高明显，这与图6中一次枝晶间距明显缩小的规律相一致。但是当能量密度从333J/mm2 提高至389J/mm2 后硬度值增加不明显。能量密度为222、333、389J/mm2 时的平均硬度值分别为85.7、90.7、92.1HV，这是因为能量密度的提高晶粒细化与固溶强化的综合作用导致硬度值增加。

  • 图9 激光熔化沉积2A50铝合金不同能量密度的显微硬度

  • Fig.9 Micro-hardness of LMD 2A50aluminum alloy with different energy density

  • 在室温条件下分别对不同能量密度沉积试样的力学性能进行测试，结果如图10所示。分析认为，能量密度为333J/mm2 试样的抗拉强度和延伸率最佳，分别为207MPa和14%。屈服强度随能量密度的增加而增加，当能量密度为389J/mm2 时达到最大值140MPa。能量密度从222J/mm2 提高至333J/mm2 时抗拉强度、延伸率、屈服强度都有较大程度的提高，这是由于能量密度提高后，试样的致密度提高，显微组织细化与溶质元素过饱和固溶使得沉积试样的综合力学性能有所提高。而当能量密度进一步提高至389J/mm2 时，反而出现抗拉强度降低，塑性下降的情况，分析认为过高的激光能量密度输入，导致熔池内部分低熔点合金元素蒸发与汽化，产生细小的孔洞缺陷成为后续拉伸变形过程裂纹源，因此导致综合力学性能有所降低。

  • 图10 LMD 2A50铝合金不同能量密度试样的力学性能

  • Fig.10 Tensile properties of 2A50aluminum alloy by LMD with different energy density

  • 试样的宏观断口形貌如图11所示。从宏观形貌图11a、11b、11c中可以观察到明显的孔洞缺陷，如图中黄色箭头所示。333J/mm2 能量密度下的样品的缺陷要明显少于222J/mm2 和389J/mm2。对其进行微观断口形貌进行观察可以发现，能量密度为222J/mm2 的沉积试样断裂形式为混合型断裂，其他两种试样为韧性断裂。相比222J/mm2 和389J/mm2 试样，333J/mm2 试样韧窝分布相对均匀，并且较深。能量密度为389J/mm2 试样在微观形貌下虽然有一定的韧窝，但是在宏观形貌图中可以观察到相对较多的气孔缺陷。在变形过程中，裂纹在孔洞附近萌生、扩展，导致塑性下降，如图10c中曲线显示的延伸率降低；能量密度为333J/mm2 和389J/mm2 的试样的韧窝尺寸与222J/mm2 相比较小。

  • 图11 不同能量密度LMD 2A50合金断口形貌

  • Fig.11 Fracture morphology of 2A50aluminum alloy prepared by LMD with different energy density

  • 3 结论

  • 采用同轴送粉激光熔化沉积法制备了锻造型2A50铝合金块体试样。对于不同化学成分合金熔化沉积过程，能量密度存在一个最优的阈值范围，当能量密度低于200J/mm2 时，沉积试样表面质量较差且产生了未熔合粉末缺陷。当能量密度高于389J/mm2 时，由于部分低熔点合金元素的蒸发与汽化，沉积构件的综合力学性能有所降低。

  • 在优化的工艺参数条件下，获得了性能较优的沉积试样，并实现了基础工艺研究向实际工程应用的成功转化。

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    • [32] ZHAO L Z,ZHAO M J,SONG L J,et al.Ultra-fine Al-Si hypereutectic alloy fabricated by direct metal deposition[J].Materials Design,2014,56:542-548.

    • [33] ROSENTHAL D.The theory of moving sources of heat and its application to metal treatments[J].Transactions of the ASME,1946,68:849-866.

    • [34] ZHANG H,ZHU H H,QI T,et al.Selective laser melting of high strength Al-Cu-Mg alloys:processing,microstructure and mechanical properties[J].Material Science and Engineering A,2016,656:47-54.

    • [35] TANG M,PISTORIUS P C,NARRA S,et al.Rapid solidification:Selective laser melting of AlSi10Mg[J].JOM,2016,68(3):960-966.

    • [36] TURCHI P E A,KAUFMAN L,LIU Z K,et al.Modeling of Ni-Cr-Mo based alloys:part I-phase stability[J].Calphad-computer Coupling of Phase Diagrams & Thermochemistry,2006,30:70-87.

  • 基本信息

    中图分类号: TG456;TB115
    DOI: 10.11933/j.issn.1007−9289.20211111001

    基金信息

    国家自然科学基金资助项目(51805526)；
    Supported by National Natural Science Foundation of China (51805526).；

    引用信息

    稿件历史

    收稿日期: 2021-11-11

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