电子束焊接热冲击对GH4133A的微裂纹损伤研究

电子束焊接是一个复杂的强瞬态的热冲击过程,包括发生在表层的热过程和发生在一定深度的应力波与材料的交互作用。本文基于电子束焊接热冲击效应分析了高温合金电子束焊接接头影响区的微裂纹形成原因,研究了微裂纹损伤对GH4133A电子束接头高温力学性能的影响。结果表明,热冲击是高温合金电子束焊接热影响区的微裂纹损伤的主要原因,热冲击损伤效应导致接头力学性能的劣化。     

GH4169电子束焊黑线组织研究

GH4169合金在电子束焊后,多次发现在热影响区腐蚀检查时,有沿基体晶界类似裂纹的"黑线",对其进行金相分析、扫描电镜观察和能谱分析结果表明,GH4169合金在电子束焊后腐蚀检查沿晶界的黑线不是裂纹,而是由铌、锆等元素形成的合金相.     

高参数汽轮机材料焊接裂纹的形成机理与控制

620℃等级汽轮机转子用13Cr9Mo1Co1NiVNbNB(FB2)材料是一种新型9Cr马氏体耐热钢,通过合理控制B、N含量,提高了高温蠕变强度。该材料焊接热影响区易出现随机分布的微裂纹,本文通过Gleeble热模拟试验获取该材料的塑性回复温度高达1 150℃,具有较大的液化裂纹倾向,采用扫描电镜(SEM)及能谱(EDS)对母材进行分析,结果发现母材随机分布少量Laves相,在高温区的液化共晶反应形成沿晶分布的液化膜,在焊接应力作用下形成液化裂纹。通过降低焊接热输入、控制预热温度和层间温度,以减少焊接热循环中的高温停留时间,有效控制了液化裂纹的产生,提高产品质量。     

含Zr的Ni_3Al合金中焊接热裂纹机理的研究

含Ｚｒ的Ｎｉ３Ａｌ合金电子束焊接时的主要缺陷是结晶裂纹。对焊缝金属进行显微分析后发现：室温下凝固组织由基体γ′相和晶间Ｎｉ５Ｚｒ相组成,它们是高温下γ相与Ｎｉ５Ｚｒ相离异共晶反应的产物。选择合适的焊接速度是克服热裂纹的关键     

电梯主驱动轴开裂原因

某电梯主驱动轴在焊接后进行磁粉检测时发现有裂纹。采用微观分析、金相检验、力学性能试验、化学成分分析等方法对电梯主驱动轴的开裂原因进行了分析。结果表明:裂纹位于电梯主驱动轴焊接热影响区熔合线附近,属于低熔共晶产物存在引起的近焊缝区液化裂纹,该液化裂纹是由于焊接工艺不当而产生的。     

电子束焊接TiAl基合金接头组织结构及其裂纹产生的敏感性

研究了电子束焊接TiAl基合金Ti-48Al-2Cr-2Nb (原子分数/%)的接头组织形态及其演化机制,分析了接头区域易产生固态裂纹的原因.研究发现焊缝主要形成柱状枝晶,呈肋骨状分布形态,其微结构为块状γm和少量层片状α2/γ组织.裂纹的形成原因主要归因于冷速过快导致α相分解受到抑制,从而使焊缝区域残余更多的α2相.此外热致应力、α2与γ相间的热膨胀系数的差异以及由此形成的相变应力也对裂纹的产生发挥着重要影响.     

Inconel 718 激光焊接接头组织与热影响区裂纹研究

采用CO2激光器对板厚为11mm的Inconel 718合金进行激光焊接,并利用金相分析和扫描电镜对Inconel 718激光焊接接头各区域组织以及热影响区显微裂纹产生的原因进行了分析。研究发现:利用激光对Inconel718进行焊接,可以获得成形良好的焊接接头;焊缝区域组织为铸造组织,从熔合线到焊缝中心由较长的树枝晶逐步变为等轴晶;热影响区因受循环热输入的影响,晶界较母材出现粗化现象,且在钉头缩颈处出现沿粗化的晶界扩展的液化裂纹,裂纹两侧存在低熔点共晶体,主要富集Nb及Mo元素,低熔点共晶体在热输入影响下发生液化是裂纹产生的主要原因。     

先进航空材料焊接过程热裂纹研究进展

高焊接热裂纹敏感性是制约新一代合金材料在航空航天领域推广应用的技术瓶颈。本文分别从焊接热裂纹的产生机理和各类合金裂纹敏感性实验的角度梳理该方向的研究进展。焊接热裂纹主要包括凝固裂纹（在焊缝内部产生）和液化裂纹（在焊缝与部分熔化区交界处产生）。影响焊接热裂纹产生的因素包括材料成分、焊接热循环以及接头热应力。在梳理焊接热裂纹机理研究的基础上,分别总结了铝合金、镁合金、先进高强钢以及镍基合金焊接热裂纹的实验研究进展。建立考虑复杂多组元以及结晶形态对裂纹敏感性影响的量化判据,是该领域未来的重要发展方向。针对母材和焊材进行成分优化、添加形核剂或实施辅助工艺措施,是工程应用领域抑制热裂纹缺陷的有效方法。开展焊接热裂纹产生机理及其抑制方法研究,有助于突破新一代合金材料加工技术瓶颈,推进其在航空航天领域的应用。     

高温合金氩弧焊时冶金及工艺因素对形成热裂纹的影响

前言 航空喷气发动机上大量采用高温合金制造各种零部件,焊接是制造中的主要工艺。合金零件进行钨极氩弧焊时,焊缝金属中常出现结晶裂纹,热影响区则出现液化裂纹,如图1所示。金相分析表明,这些是沿晶界和核晶间形成的热裂纹。这些裂纹的存在严重地影响合金的使用以及产品的质量和寿命。研究其形成的原因和影响因素,并提出预防措施,具有重要的意义。 一、形成焊接热裂纹的影响因素 1.合金元素及组织的影响     

K6C、K18铸造高温合金焊接热裂纹敏感性的研究

本文通过刚性平板直焊缝及环形焊缝两种焊接裂纹倾向试验方法,着重研究了焊接线能量和不同填充焊丝对K6C、K18铸造镍基高温合金焊接热裂纹的影响,并测试了热裂纹敏感的温度范围,对两种合金焊接裂纹形成原因进行了金相分析,提出了改善焊接热裂纹敏感性的措施。     

Simulations of microcracking in the process region of ceramics with a cell model

Microcracking around a macrocrack and the consequential toughening in polycrystalline ceramics are simulated. The objective is to check the hypothesis that the suppression mechanism for opening of off-side microcracks does not work in certain ceramic materials because cracks open up, assisted by residual stresses, at a much reduced load in some grains, while, still encountering a high resistance to crack growth at the grain boundaries. A two-dimensional cell model of a polycrystalline material is investigated. Each cell represents one grain. The load-deformation law for the cell is assumed to contain two load peaks. The first peak is associated with microcrack nucleation in the grain, while the second peak is related to the resistance that the microcrack meets at the grain boundary. The cells are included in a finite element model. Grain to grain variations, for instance due to residual stresses, are taken into account by a Weibull distribution of the first load peak. Results from the simulations show that variations of the propensity for microcrack nucleation between different grains constitute a major factor responsible for the generation of microcrack clouds. Such cloud formation would otherwise be impeded by unloading effects from central microcracks. In addition, and in accordance with observations, the simulations also show high fracture energies (compared to what would be expected for a typically brittle material), as well as a period of stable crack propagation.     

A study on the formation of liquation cracks in the weld heat-affected zone of HY-80 quenched and tempered steel

Liquation cracking of heat-affected zones (HAZs) is often encountered during the welding of HY-80 steels. To reduce the sensitivity of this defect, the content of impurities, such as P and S, must be kept as low as possible. However, in the development of HY-80 steel, HAZ grain boundary liquation is still found even at very low impurity contents. In order to clarify the cause of this defect, the Gleeble hot ductility test and electron probe X-ray microanalysis (EPMA) were carried out. From the results of EPMA, it was evident that the grain boundary liquation in the heat-affected zone of HY-80 steel was due to the low-melting-point eutectic reaction between Cr, Ni and Mn, which had been swept up by the migrating grain boundaries in the welding-heating thermal cycle, and hence enriched at the grain boundary. In addition, the Gleeble hot ductility test results revealed that the HAZ liquation cracking sensitivity of HY-80 steel could be decreased by reducing the C, Ni and Cr contents of base metal, and by decreasing the dwell time at high temperatures during the welding thermal cycle.     

On preventing HAZ cracking in laser welded DS Rene 80 superalloy

Abstract

The heat affected zone (HAZ) cracking behaviour in a laser beam directionally solidified (DS) Rene 80 nickel based superalloy subjected to preweld heat treatments was studied. The HAZ cracks in the alloy are grain boundary liquation cracks caused by liquation reaction of both non-equilibrium secondary solidification product, MC carbides and equilibrium solid state reaction product, γ′ precipitates. In contrast to theoretical prediction based a preweld heat treatment that reduced grain boundary liquid film thickness did not result in a lower HAZ cracking, which can be related to concomitant reduction in the ability of the base alloy to relax welding stress. In addition, formation of intergranular M₅B₃ boride particles in preweld alloy appeared to have aided cracking susceptibility by lowering grain boundary liquation temperature and widening the brittle temperature range in the HAZ during cooling. Based on the analysis of the results, application of a new preweld heat treatment that prevents the formation of the intergranular borides and induces moderate base alloy hardness resulted in a nearly crack free HAZ in laser welded DS Rene 80 superalloy.     

Simulation of microfracture process of brittle polycrystals: Microcracking and crack propagation

Microcracking and crack propagation behavior are simulated for 2-dimensional alumina polycrystals which have thermal anisotropy within a grain. Microcracks are generated by thermally induced residual stresses at the grain boundary. Stress redistribution due to microcracking and stress intensity factors at the microcrack tip are obtained numerically by the body force method. The location at which microfracture occurs is determined by a competition between microcracking and crack propagation under external stresses. The microfracture stress increases with the progress of fracture and decreases after the maximum indicating a fracture strength. In many cases, the extension of microcracks induces an unstable fracture. With both increasing grain size and decreasing grain boundary toughness, the number of microcracks prior to the unstable state increases and the stress concentration due to the microcracks plays a significant role in the stable crack extension, resulting in lower strengths than the fracture-mechanical predictions.     

全层状TiAl基合金断裂中晶界的双重作用

通过SEM原位拉抻技术和双晶体压缩实验研究了全片层TiAI基合金晶界断裂行为。研究表明,在全层状组织的断裂行为中,晶界具有双重作用。一方面,微裂纹首先萌发于晶界区,其扩展方式取决于晶界两侧片层的取向。另一方面,不同类型的晶界对裂纹扩展的阻力不同,因而对全层状TiAI基合金韧性的作用不同,纵向晶界有助于断裂韧性的提高,而横向晶界对合金韧性不利。     

Preparation and thermomechanical characterisation of aluminum titanate flexible ceramics

Flexible ceramics may be useful, for example to process refractory materials with an improved resistance to thermal shocks. A natural flexible sandstone, itacolumite, is mainly constituted of interlocked quartz grains and contains microcracks. Its microstructure allows some free motion between grains that induces its flexibility. The aim of this study is to prepare and characterize flexible aluminum titanate ceramics by mimicking the microstructure of itacolumite. Aluminum titanate (AT) has a high thermal expansion anisotropy that induces grain boundary microcracking leading to flexibility. Here, the flexibility is the capacity of the material to endorse large strain-to-rupture level. This concept is also closely related to a low value of the stiffness induced by damage mechanisms. In this study, the flexibility has been estimated by the measurement of the deflection at fracture on three-point bending test. By preparing AT samples sintered according to different heating cycles, the correlations between the sintering cycle, the microstructure and the flexibility have been studied. Grain size and microcrack width have been observed by scanning electron microscopy. A major parameter for flexibility is the microcrack volume fraction within the sample. Three types of AT materials have been processed: non flexible (NF), flexible (F), and very flexible (VF). Their thermal and mechanical behaviors have been investigated and showed that NF has a brittle behavior while F and VF have a nonlinear ductile one. This was found to be due to grain boundary microcracks network and to the interlocking of grains. VF is more flexible than F because its microcracks are wider. Flexibility improves the thermal shock resistance: F and VF have a higher thermal shock resistance than NF. Moreover, thermal expansion measurements during thermal cycles showed anomalous effects induced by crack closure when heating and crack opening when cooling.     

Analysis of laser beam weldability of Inconel 738 superalloy

The susceptibility of pre-weld heat treated laser beam welded IN 738 superalloy to heat affected zone (HAZ) cracking was studied. A pre-weld heat treatment that produced the minimal grain boundary liquation resulted in a higher level of cracking compared to those with more intergranular liquation. This deviation from the general expectation of influence of intergranular liquation extent on HAZ microfissuring is attributable to the reduction in the ability of the base alloy to accommodate welding tensile stress that accompanied a pre-weld heat treatment condition designed to minimize intergranular liquation. Furthermore, in contrast to what has been generally reported in other nickel-based superalloys, a decrease in laser welding speed resulted in increased HAZ cracking in the IN 738, which can be attributed to exacerbated process instability at lower welding speeds.     

Microstructural analysis of fusion and heat affected zones in electron beam welded ALLVAC 718PLUS™ superalloy

The fusion zone and heat affected zone (HAZ) microstructures of electron beam welded superalloy 718PLUS™ (718 Plus) that has been newly developed by ATI ALLVAC were examined. The microsegregation pattern during solidification of the fusion zone indicated that while Fe, Co, W, and Cr segregated to the core of the gamma dendrites, Nb, Ti, and Al were extensively rejected into the interdendritic liquid. Electron diffraction and X-ray microanalysis using transmission electron microscopy (TEM) of the fusion zone showed that the major secondary phases that formed from the interdendritic liquid were gamma/MC type carbide eutectic and gamma/Laves eutectic constituents. HAZ microstructure showed partially melted zone immediately adjacent to the fusion zone and intergranular microfissuring associated with resolidified products which suggested that HAZ cracking in this alloy occurred by liquation cracking. Microstructural examination of the HAZ using analytical scanning electron microscope showed resolidified gamma/Laves eutectic on the cracked and backfilled grain boundaries. Fine resolidified MC type carbide particles were also observed in the HAZ. Causes of grain boundary liquation were identified and the solidification of intergranular liquid in the HAZ was discussed.     

Growth of short cracks during low and high cycle fatigue in a duplex stainless steel

Damage evolution during low- and high-cycle fatigue in an embrittled duplex stainless steel is characterized in this paper. Moreover, scanning electron microscopy observations (SEM) in combination with electron backscattered diffraction (EBSD) measurements and transmission electron microscopy (TEM) were employed in order to analyze microcracks formation and propagation. During low-cycle fatigue, microcracks initiate the ferrite phase either along slip planes with the highest Schmid factor (SF) inside the grains or at the α/α grain boundary. Then, microcracks propagation take place in ferrite or austenite grains with the highest SF. An analysis of the dislocation structure in the near-surface and in ferritic grains in the bulk of the specimen has shown that dislocation microbands are associated with microcrack initiation.In the high-cycle fatigue regime, damage generally initiates in the austenite by slip band formation followed by crack initiation either at an α–α boundary or at an α–γ boundary in the intersection of slip bands in the austenite. The microstructure in the austenite consists of a low density of dislocation pile-ups while the ferrite is practically inactive or develops only micro-yielding at boundaries.Despite the differences in both fatigue regimes, phase boundaries are an effective barrier against crack propagation because they delay the advance of the crack tip.     

力、电、热作用下晶内微裂纹的演化

基于表面扩散的经典理论及其弱解描述,对曲率、力、电和热共同作用下金属材料内部晶内微裂纹的演化进行了有限元分析。详细讨论了微裂纹初始形态、电场大小、应力大小和电致生热对微裂纹演化的影响。结果表明:对于形态比为的微裂纹,存在一临界电场值和临界应力值。当且时,微裂纹逐渐圆柱化;当或时,微裂纹分节为上、下或左、右两个小裂纹。热应力可减小的值,即有利于微裂纹分节。同时热应力可加快微裂纹的漂移速度,缩短分节时间。