Mechanism and inhibition of welding cracks in microplasma arc of Inconel 713LC casting superalloy
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摘要:
针对Inconel 713LC铸造高温合金焊接增材时热影响区易产生裂纹这一问题，采用微束等离子弧焊接技术，运用扫描电镜和能谱仪对裂纹的微观形貌、合金元素分布进行分析，探究合金热影响区焊接裂纹的形成机理，研究裂纹抑制技术. 结果表明，热影响区焊接裂纹的出现主要是受焊接热输入作用晶间碳化物发生部分溶解和液化形成液相，同时晶粒受热长大推动原始晶界发生移动后与液相交汇形成更大面积的液相，晶粒及晶界中的Nb元素、Mo元素和Ti元素等合金元素向液相偏聚，液相冷却凝固后形成尺寸超过10 μm的碳化物，在焊接热应力作用下裂纹萌生于碳化物周围并沿晶界扩展，焊接裂纹数量和尺寸均随着热输入的增大而增大. 通过脉冲焊接降低焊接热输入，热影响区的高温停留时间变短，降低焊接过程中热影响区合金元素的偏聚，减少合金中有害碳化物的长大，抑制裂纹生成；对铸态母材进行固溶处理后，合金元素偏聚得到均匀化处理，母材硬度下降，塑性得以改善，减弱了合金元素的富集倾向，降低了焊接时的裂纹敏感性.
Abstract:Aiming at the problem that the heat-affected zone of Inconel 713LC casting superalloy is prone to cracks during additive welding, this paper adopts microplasma arc welding technology, scanning electron microscope and energy dispersive spectrometer to analyze the micro-morphology of cracks and the distribution of alloy elements. The formation mechanism of cracks in the heat-affected zone is explored, and the crack suppression technology is studied. The result shows that the appearance of welding cracks in the heat-affected zone is mainly due to the dissolution and liquefaction of intergranular carbide and the formation of liquid phase under the influence of welding heat input. At the same time, the grain is heated and grown to promote the original grain boundary to move and then intersect with the liquid phase to form a larger liquid phase. Nb, Mo, Ti and other alloy elements in the grains and grain boundaries are segregated into the liquid phase. Carbides with a size of more than 10 μm are formed after cooling and solidification of liquid phase. Under the action of welding stress, cracks initiate around carbides and propagate along grain boundaries. The number and size of welding cracks increase with the increase of heat input. Reducing the welding heat input through pulse welding, the high temperature residence time in the heat-affected zone becomes shorter, the degree of elements segregation in the alloy is reduced. The growth of harmful carbides in the alloy is slowed down, and the crack formation is inhibited. After the solution treatment of the as-cast base metal, the original element segregation of the base metal are homogenized. The hardness of the base metal decreases and the plasticity is improved, which weakens the enrichment tendency of the alloying elements and reduces the crack sensitivity during welding.
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Keywords:
- welding crack /
- microplasma arc /
- element segregation /
- pulse welding /
- solution treatment
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0. 序言
铸造高温合金具有良好的高温蠕变性能、热疲劳性能和抗氧化性能，广泛用于制造涡轮叶片、涡轮机匣等航空发动机热端部件，在复杂热场、流场作用下，容易产生高温氧化、外物击伤和摩擦磨损等损伤，采用焊接增材技术对损伤区域进行修复，恢复构件的外形和性能，是降低航空发动机全寿命费用的有效手段[1-2]. 铸造高温合金焊接性差，表现出高裂纹敏感性，限制了焊接增材修复技术在航空发动机部件的应用.
国内外学者采用电子束、激光等技术研究铸造高温合金元素、冷却速率等因素对焊接裂纹的影响. 研究发现B元素和Hf元素促进碳化物(MC + M23C6)形成，导致247LC高温合金的脆性凝固温度范围提高约200 ~ 230 K[3]；提高液固界面冷却速率抑制元素偏析、降低固态冷却速率减少开裂驱动力，有利于柱状晶沿增材方向生长，抑制Inconel 738铸造高温合金裂纹产生[4]；发现由晶界元素偏析所导致的热影响区液化现象[5]；除此之外，对于控制镍基高温合金中热裂纹的手段还有降低凝固过程中的收缩应力[6]等. 而在焊接增材制造领域中，对于Inconel 713LC铸造高温合金焊接开裂问题的研究较少，特别是对于裂纹的抑制研究更少，所以文中针对Ni-Cr-Mo系Inconel 713LC铸造高温合金易产生焊接裂纹的问题，采用微束等离子弧进行焊接增材，分析了裂纹生成机理，采用脉冲焊接控制温度场降低合金元素富集，采用焊前固溶处理减弱晶界合金元素偏聚，抑制热影响区裂纹生成.
1. 试验方法
焊接增材试验材料采用Inconel 713LC铸造高温合金，其化学成分如表1所示，通过线切割将铸锭制成厚度为1.5 mm的试片用于焊接试验. 试验方法是采用自主研发的BW 100型微束脉冲等离子弧叶片焊接修复机器人系统进行高温合金板材焊接增材，采用OLYMPUS 4100型激光共聚焦光学显微镜和ZEISS Sigma 300型扫描电子显微镜观察焊接接头微观组织和热影响区裂纹形貌；采用Quantax 400型能谱仪对裂纹区域进行元素成分分析；采用HVT-100A型显微硬度仪对铸态母材与固溶处理后母材的硬度进行测试.
表 1 Inconel 713LC高温合金化学成分(质量分数，%)Table 1. Chemical compositions of Inconel 713LC superalloyC Cr Mo Nb Ti Al B Zr Co Fe S Mn Ni 0.05 12.48 4.53 2.09 0.52 6.08 0.007 0.12 0.06 0.04 0.001 0.0001 余量 2. 试验结果及讨论
2.1 裂纹生成机理分析
焊接接头分为堆焊区、热影响区和母材区，堆焊区根据组织形貌细分为等轴晶区和树枝晶区，热影响区划分为近熔合线区与近母材区，如图1所示. 堆焊区的近表面区为等轴晶，随着凝固向熔合线方向靠近，晶粒逐渐长大，逐渐由较小的等轴晶过渡到树枝晶. 形成上述现象的原因主要是焊接过程中的单向热输入使堆焊区至母材区方向形成一定的温度梯度，近表面区冷却速度较快，晶粒生长时间较短，所以晶粒尺寸偏小，而在堆焊区的中间部分，高温停留时间长，晶粒有充足的能量和时间沿着温度梯度方向进行生长，尺寸较为粗大且呈现方向性. 热影响区主要为γ基体、弥散分布的γ′相及沿晶界析出的第二相颗粒如图2所示. 近母材区域晶粒尺寸在160 ~ 240 μm之间，约为母材晶粒尺寸的2倍，近熔合线部分晶粒尺寸在300 ~ 540 μm之间，为母材晶粒尺寸的4 ~ 5倍，沿晶界析出呈块状的第二相颗粒，其尺寸在5 ~ 20 μm之间，呈链状分布的第二相颗粒其尺寸在20 ~ 30 μm之间. 第二相颗粒出现团聚现象，部分第二相颗粒长大，发现沿晶开裂的裂纹，裂纹源为尺寸大于10 μm的第二相颗粒.
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图 2 热影响区裂纹微观形貌Figure 2. Microcosmic of crack in heat-affected zone. (a) crack in heat-affected zone; (b) enlarged of Fig.2 (a)对裂纹区域的A点和B点采用EDS点扫描进行合金元素成分分析，结果如图3所示. 结果显示，第二相颗粒(A点)的Nb元素、Mo元素和C元素含量比晶粒内部(B点)合金平均含量明显偏高，Ti元素含量也有所上升，A点和B点的Al元素含量低于合金平均含量. 同时面扫描结果如图4所示，裂纹附近Nb元素、Mo元素偏聚明显，母材原有质量分数为0.52%的Ti元素产生轻微偏聚，而母材原有质量分数为6.08%的Al元素在裂纹处出现贫瘠现象. 通过元素分析可知，裂纹周边的析出物为Nb-Mo-Ti-C元素偏聚，该类碳化物属于硬脆相，在后续热应力作用下极易成为焊接裂纹的起始点[7-8].
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图 3 热影响区裂纹周边析出相元素点扫描分布Figure 3. Point scanning distribution of precipitated phase elements around cracks in heat-affected zone. (a) A point; (b) B point![]()
图 4 热影响区裂纹元素面扫描分布Figure 4. Surface scanning distribution of elements of cracks in heat-affected zone结合微观组织及能谱分析，焊接裂纹的形成机理如图5所示. Inconel 713LC铸造高温合金所需的强度和延展性主要是由晶界处所形成的碳化物通过阻碍晶界滑动来提供的，且以NbC为主如图5(a)所示. 在焊接时的非平衡快速加热过程中，热影响区的碳化物无法快速溶解于基体，在高温下晶间与碳化物发生部分溶解和液化形成液相如图5(b)所示. 如图5(c)所示晶内及其附近晶界的Nb元素、Mo元素和Ti元素等元素向液相偏聚程度加剧，使得晶界处富集更多的合金元素[9-10]. 同时在焊接热输入作用下热影响区的晶粒长大会推动原始晶界发生移动后与液化相相遇，汇聚成更大面积的液相[11]. 在后续冷却过程中，晶界处形成更大尺寸的碳化物，当焊接应力大于晶粒与碳化物的结合强度时，裂纹在碳化物周围萌生并沿晶界扩展，形成较大尺寸的焊接裂纹如图5(d)所示.
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图 5 热影响区焊接裂纹形成机理Figure 5. Mechanism of welding cracks formation in heat-affected zone. (a) carbide distribution. (b) carbide dissolution and liquefaction; (c) element segregation; (d) crack formation2.2 裂纹抑制探究结果分析
2.2.1 焊接热输入
A1 ~ A5试样以铸态状态的铸造高温合金作为焊接对象，焊接电流模式采用直流输出，电弧电压U为21.5V，焊接速度v为1.3 mm/s，具体焊接工艺参数如表2所示. 直流电流的焊接热输入为
表 2 铸造高温合金直流焊接工艺参数Table 2. Direct current welding parameters of casting superalloy编号 焊接电流I/A 热输入E/(J·mm−1) A1 9 148.8 A2 11 181.9 A3 13 215.0 A4 15 248.1 A5 17 281.2 $$ {{E}} = \eta UI/v $$ (1) 式中：E为焊接热输入；I为焊接电流；$ \eta $为有效功率系数，值取1.
不同焊接电流下的焊接接头热影响区如图6所示. 当焊接电流为9 ~ 11 A时，由于焊接热输入小，热影响区高温停留时间短，晶界处析出的碳化物数量少，单个碳化物尺寸约为2 ~ 8 μm，热影响区未发现焊接裂纹，如图6(a)和图6(b)所示；电流增加到13 A，热影响区碳化物增多呈现链状，部分碳化物尺寸长大约为20 μm，发现1条焊接裂纹，尺寸约为250 μm，如图6(c)所示；电流进一步增大，碳化物和裂纹尺寸随之增大，数量随之增多，如图6(d)所示；当电流为17A时，晶界处的碳化物平均尺寸达到25 μm，裂纹数量增加至3条，最大尺寸超过270 μm，如图6(e)所示.B1 ~ B3试样以铸态状态的铸造高温合金作为焊接对象，焊接电流模式采用脉冲输出，电弧电压21.5V，焊接速度1.3 mm/s，具体焊接工艺参数如表3所示. 脉冲电流等效为直流电流的计算公式[12]为
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图 6 不同焊接电流下的焊接接头热影响区Figure 6. Heat-affected zone of the welded joints of different welding speeds. (a) A1; (b) A2; (c) A3; (d) A4; (e) A5$$ {I_{\text{E}}} = \sqrt {\delta I_{\text{P}}^2 + (1 - \delta )I_{\text{B}}^2} $$ (2) 式中：占空比为δ=tp/T；T为脉冲周期；IP为峰值电流；tP为峰值电流持续时间；IB为基值电流；tB为基值电流持续时间；δ为占空比；IE为脉冲等效电流.
表 3 铸造高温合金脉冲焊接工艺参数Table 3. Direct current welding parameters of casting superalloy编号 峰值电流IP/A 基值电流IB/A 热输入E/(J·mm−1) B1 17.5 4.4 215.0 B2 20.5 5.1 248.1 B3 23.4 5.8 281.2 由脉冲电流等效为直流电流计算后可知，B1的焊接热输入与A3相同，B2的焊接热输入与A4相同，B3的焊接热输入与A5相同. 由如图7所示的B1、B2和B3热影响区微观组织可知，脉冲焊接热影响区析出碳化物数量和尺寸与相同焊接热输入条件下直流焊接相比均明显减少，且未发现大于10 μm的晶界碳化物，未发现焊接裂纹. 试验结果表明，焊接热输入增加，热影响区高温停留时间变长，元素偏聚程度加剧，碳化物析出数量增多、尺寸增大，焊接裂纹倾向增大. 采用脉冲焊接时，其电流是在峰值和基值间进行循环转化，峰值高电流送丝形成焊接接头，基值低电流抽丝使热影响区温度下降，结合文献[13-16]可知脉冲焊接的热影响区温度低于直流焊接，减缓了合金元素在晶界的偏聚，晶间碳化物析出、生长的驱动力下降，降低了焊接裂纹倾向.
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图 7 不同脉冲焊接电流的焊接接头热影响区Figure 7. Heat-affected zone of the welded joints of different pulse welding speeds. (a) B1; (b) B2; (c) B32.2.2 固溶处理
对铸造高温合金进行固溶处理后观察微观组织，与未进行固溶处理的母材组织进行对比，结果如图8所示. 未经过固溶处理的铸造高温合金存在大量的合金元素偏聚，形成富Nb、Mo的MC型碳化物，如图8(a)所示；固溶处理后，晶体组织分布均匀，晶界仅有少量的细小MC碳化物和弥散分布的γ′强化相存在，未发现尺寸较大的碳化物，如图8(b)所示. C1 ~ C3试样是以固溶处理状态的铸造高温合金作为焊接对象，焊接工艺与A3、A4和A5相同，具体焊接工艺参数如表4所示.
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图 8 铸态母材固溶处理前后的显微组织形貌Figure 8. Microstructure and morphology of before and after solution treatment of as-cast base metal. (a) before solution treatment; (b) after solution treatment表 4 固溶处理高温合金焊接参数Table 4. Welding parameters of solution treated superalloy编号 焊接电流 I/A 热输入 E/(J·mm−1) C1 13 215.0 C2 15 248.1 C3 17 281.2 对比分析两种母材状态下热影响区的生成裂纹情况，通过观察3种焊接工艺下热影响区微观形貌可知，经固溶处理后母材的焊接接头热影响区均未发现焊接裂纹，仅有少量的细小碳化物零散分布，碳化物数量和尺寸显著下降，如图9所示. 试验结果表明，固溶处理工艺改善了铸造高温合金元素偏聚现象，使粗大的γ′相、γ-γ′及碳化物回溶于基体，结合文献[17-18]可知，这样可以消除基体中的元素偏聚结构，合金元素分布更加均匀. 对Inconel 713LC铸态母材和固溶后母材各取10个点测量其显微硬度，结果显示铸态母材的平均硬度为389.8 HV1，固溶后母材的平均硬度为360.4 HV1. 结合文献[19]可知经固溶处理后沉淀相向基体的溶解降低了母材硬度，改善其塑性，焊接接头热影响区所受拘束力降低，降低了焊接裂纹倾向.
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图 9 母材固溶处理后不同焊接电流下的焊接接头热影响区Figure 9. Heat-affected zone of the welded joint of different welding speeds of after solution treatment of as-cast base metal. (a) C1; (b) C2; (c) C33. 结 论
(1) Inconel 713LC铸造高温合金焊接接头的裂纹主要出现在热影响区. 受焊接热输入影响，晶间碳化物溶解和液化形成液相，晶内及附近晶界上的Nb元素、Mo元素和Ti元素等合金元素偏聚至液化晶界上. 在后续冷却过程中，液相凝固形成更大尺寸的碳化物，受焊接应力场作用裂纹在碳化物周围萌生并沿晶界扩展，最终形成较大尺寸的焊接裂纹.
(2)采用脉冲焊接，能够有效减少焊接热输入，高温停留时间变短，Nb元素、Mo元素和Ti元素等元素偏聚程度减轻，冷却凝固形成的碳化物尺寸较小，裂纹敏感性降低.
(3)对焊接母材进行焊前固溶处理，沉淀相溶解于基体，有效改善合金元素偏聚，在焊接过程中减弱了合金元素的富集倾向，有害碳化物尺寸得以抑制，从而消除了焊接裂纹.
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图 2 热影响区裂纹微观形貌
Figure 2. Microcosmic of crack in heat-affected zone. (a) crack in heat-affected zone; (b) enlarged of Fig.2 (a)
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图 3 热影响区裂纹周边析出相元素点扫描分布
Figure 3. Point scanning distribution of precipitated phase elements around cracks in heat-affected zone. (a) A point; (b) B point
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图 4 热影响区裂纹元素面扫描分布
Figure 4. Surface scanning distribution of elements of cracks in heat-affected zone
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图 5 热影响区焊接裂纹形成机理
Figure 5. Mechanism of welding cracks formation in heat-affected zone. (a) carbide distribution. (b) carbide dissolution and liquefaction; (c) element segregation; (d) crack formation
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图 6 不同焊接电流下的焊接接头热影响区
Figure 6. Heat-affected zone of the welded joints of different welding speeds. (a) A1; (b) A2; (c) A3; (d) A4; (e) A5
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图 7 不同脉冲焊接电流的焊接接头热影响区
Figure 7. Heat-affected zone of the welded joints of different pulse welding speeds. (a) B1; (b) B2; (c) B3
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图 8 铸态母材固溶处理前后的显微组织形貌
Figure 8. Microstructure and morphology of before and after solution treatment of as-cast base metal. (a) before solution treatment; (b) after solution treatment
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图 9 母材固溶处理后不同焊接电流下的焊接接头热影响区
Figure 9. Heat-affected zone of the welded joint of different welding speeds of after solution treatment of as-cast base metal. (a) C1; (b) C2; (c) C3
表 1 Inconel 713LC高温合金化学成分(质量分数，%)
Table 1 Chemical compositions of Inconel 713LC superalloy
C Cr Mo Nb Ti Al B Zr Co Fe S Mn Ni 0.05 12.48 4.53 2.09 0.52 6.08 0.007 0.12 0.06 0.04 0.001 0.0001 余量 
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表 2 铸造高温合金直流焊接工艺参数
Table 2 Direct current welding parameters of casting superalloy
编号 焊接电流I/A 热输入E/(J·mm−1) A1 9 148.8 A2 11 181.9 A3 13 215.0 A4 15 248.1 A5 17 281.2
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表 3 铸造高温合金脉冲焊接工艺参数
Table 3 Direct current welding parameters of casting superalloy
编号 峰值电流IP/A 基值电流IB/A 热输入E/(J·mm−1) B1 17.5 4.4 215.0 B2 20.5 5.1 248.1 B3 23.4 5.8 281.2
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表 4 固溶处理高温合金焊接参数
Table 4 Welding parameters of solution treated superalloy
编号 焊接电流 I/A 热输入 E/(J·mm−1) C1 13 215.0 C2 15 248.1 C3 17 281.2
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