Surface Integrity of Inconel 718 by Ball Burnishing

Inconel 718 has wide applications in manufacturing mechanical components such as turbine blades, turbocharger rotors, and nuclear reactors. Since these components are subject to harsh environments such as high temperature, pressure, and corrosion, it is critical to improve the functionality to prevent catastrophic failure due to fatigue or corrosion. Ball burnishing as a low plastic deformation process is a promising technique to enhance surface integrity for increasing component fatigue and corrosion resistance in service. This study focuses on the experimental study on surface integrity of burnished Inconel 718. The effects of burnishing ball size and pressure on surface integrity factors such as surface topography, roughness, and hardness are investigated. The burnished surfaces are smoother than the as-machined ones. Surface hardness after burnishing is higher than the as-machined surfaces, but become stable over a certain burnishing pressure. There exists an optimal process space of ball sized and burnishing pressure for surface finish. In addition, surface hardness after burnishing is higher than the as-machined surfaces, which is confirmed by statistical analysis.     

Oxidation of Inconel 718 in Air at High Temperatures

Experiments were conducted with Inconel 718 at high temperatures to evaluate the rate of oxidation of the material over as wide a temperature range as possible, as well as to determine the high-temperature failure limit of the material. Samples of Inconel 718 were inserted into preheated furnaces at temperatures ranging from 973 to 1620 K and oxidized in air for varying periods of time. After being oxidized in air at a constant temperature for the prescribed time and then being allowed to cool, the samples were reweighed to determine their mass gain due to the uptake of oxygen. From these mass-gain measurements, it was possible to identify three regimes of oxidation for Inconel 718: a low-temperature regime in which the samples behaved as if passivated after an initial period of transient oxidation, an intermediate-temperature regime in which the rate of oxidation was limited by diffusion and exhibited a constant parabolic rate dependence, and a high-temperature regime in which material deformation and damage accompanied an accelerated oxidation rate above the parabolic regime.     

Surface integrity analysis of machined Inconel 718 over multiple length scales

Surface integrity characteristics of machined Inconel 718 have been measured using experimental techniques, such as FEG-SEM, EBSD, XRD, TEM, nano-indentation and 3D optical microscopy. Nanosized grains typical of severe plastic deformation are characteristic of the machined surface while deformation in the form of plastic slip bands is typical of subsurface layers. Correlations are presented between deformation features on the machined surface, and cutting parameters and tool wear.     

Inconel718合金的超塑性

Ｉｎｃｏｎｅｌ７１８合金在超塑拉伸变形过程中,随着初始应变速率ε的降低或变形温度Ｔ的升高,其流动应力σ降低、延伸率δ增加。同时,初始应变速率和变形温度影响奥氏体晶粒尺寸和析出相的分布。合金在超塑变形中,位错滑移起重要协调作用,使晶界滑动易于进行。     

Hybrid machining of Inconel 718

A new approach for machining of Inconel 718 is presented in this paper. It combines traditional turning with cryogenically enhanced machining and plasma enhanced machining. Cryogenically enhanced machining is used to reduce the temperatures in the cutting tool, and thus reduces temperature-dependent tool wear to prolong tool life, whereas plasma enhanced machining is used to increase the temperatures in the workpiece to soften it. By joining these two non-traditional techniques with opposite effects on the cutting tool and the workpiece, it has been found that the surface roughness was reduced by 250%; the cutting forces was decreased by approximately 30–50%; and the tool life was extended up to 170% over conventional machining.     

Surface integrity in cryogenic machining of nickel based alloy—Inconel 718

In machining processes, a major quality related output is integrity of the machined part surface. In machining of difficult-to-cut materials, a drastic decrease in tool-life makes the machining process even more difficult. By considering the broader perspective of the machining system tailored towards sustainable operations, in this work an alternative—cryogenic machining is evaluated for machining performance. The surface integrity characteristics of machined surface as a function of depth have been analyzed for different combinations of cooling/lubrication machining conditions. The residual stresses on the machined surface and sub-surface, surface hardness, and surface roughness are among the significant characteristics studied in this work. The results show that cryogenic machining processes can be implemented to improve all major surface integrity characteristics, thus improving the final product quality level.     

Cold Spray Forming of Inconel 718

Inconel 718 was cold spray formed to a 6-mm thickness on an 8-cm diameter aluminum alloy tube using Sulzer Amdry 1718 powder and the Plasma Giken PCS-1000 cold spray system. The effects of spray particle velocity and post-spray heat treatment were studied. Post-spray annealing was performed from 950 to 1250 °C for 1-2 h. The resulting microstructures as well as the corresponding mechanical properties were characterized. As-sprayed coatings exhibited very low ductility. The tensile strength and ductility of the heat-treated coatings were improved to varying levels depending on the heat-treatment and spray conditions. For coatings sprayed at higher particle velocity and heat treated at 1250 °C for 1 h, an elongation of 24% was obtained. SEM micrographs showed a higher fraction of interparticle metallurgical bonds due to some sintering effect. Corresponding fracture surfaces also revealed a higher fraction of dimple features, typically associated with ductile fracture, in the annealed coatings. The results demonstrate that cold spray forming of Inconel 718 is feasible, and with appropriate heat treatment, metallurgical bonding can be increased. The ductility of the spray-formed samples was comparable to that of the bulk material.     

Surface integrity when machining age hardened Inconel 718 with coated carbide cutting tools

Considerable attention has been given to the use of ceramic cutting tools for improving productivity in the machining of heat resistant super alloys (HRSA). However, because of their negative influence on the surface integrity, ceramic tools are generally avoided particularly for finishing applications. As a result the main high end manufacturers are more or less dependent on carbide cutting tools for finishing operations. Still the improper use of carbide cutting tools can also result in poor surface integrity. The objective of this investigation is to develop a set of guidelines, which will assist the selection of the appropriate cutting tools and conditions for generating favorable compressive residual stresses. This paper specifically deals with residual stresses and surface finish components of surface integrity when machining (facing) age hardened Inconel 718 using two grades of coated carbide cutting tools specifically developed for machining HRSAs. The cutting conditions were obtained from investigations based on optimum tool performance. The effect of insert shape, cutting edge preparation, type and nose radius on both residual stresses and surface finish was studied at this optimum cutting condition. This investigation, suggested that coated carbide cutting tool inserts of round shape, chamfered cutting edge preparation, negative type and small nose radius (0.8 mm) and coolant will generate primarily compressive residual stresses.     

Fatigue Behavior of Inconel 718 TIG Welds

Mechanical behavior of reference and TIG-welded Inconel 718 specimens was examined in the present work. Tensile, constant amplitude fatigue, and fracture toughness tests were performed in ambient temperature for both, reference and welded specimens. Microstructure revealed the presence of coarse and fine-grained heat-affected zones. It has been shown that without any post-weld heat treatment, welded specimens maintained their tensile strength properties while their ductility decreased by more than 40%. It was found that the welded specimens had lower fatigue life and this decrease was a function of the applied fatigue maximum stress. A 30% fatigue life decrease was noticed in the high cycle fatigue regime for the welded specimens while this decrease exceeded 50% in the low cycle fatigue regime. Cyclic stress-strain curves showed that Inconel 718 experiences a short period of hardening followed by softening for all fatigue lives. Cyclic fatigue response of welded specimens’ exhibited cyclically stable behavior. Finally, a marginal decrease was noticed in the Mode I fracture toughness of the welded specimens.     

Broaching of Inconel 718 with cemented carbide

Broaching is the standard process for machining complex-shaped slots in turbine discs. More flexible processes such as milling, wire EDM machining and water-jet cutting are under investigation and show promising results. In order to further use existing resources and process knowledge, the broaching process has to be improved towards higher material removal rates. Taking into account that the state-of-the-art broaching process is working with high-speed-steel tools, the higher thermal resistant cemented carbide cutting materials offer the potential to significantly increase cutting speeds, which lead to increased process productivity. The following article presents a broad study on broaching with cemented carbide tools. Different cutting edge geometries are discussed on the basis of process forces, chip formation and tool wear mechanisms. Furthermore, a detailed comparison to the standard process is drawn.     

超声振动方向对激光定向能沉积Inconel 718性能影响研究

目的 面向激光定向能沉积表面改性及再制造发展需求,揭示超声振动方向对激光定向能沉积Inconel 718零件的影响机制,为超声辅助激光定向能沉积高质量成形提供参考。方法 开展多方向超声振动辅助激光定向能沉积Inconel 718零件试验,结合原位熔池监测技术,研究沉积方向、搭接方向和扫描方向超声振动对熔池流动行为、微观结构和力学性能的影响。结果 不同方向超声振动显著影响了熔池的动态流动行为,其中搭接方向超声振动使熔池润湿面积提高了86.4%,并且形状更为规则,拖尾现象得到改善。微观组织细化方面,扫描方向振动效果最佳,晶体直径细化了46.9%,水平方向的晶体等轴晶转变优于沉积方向,而在沉积方向振动下,Laves相点球化和减少最为明显。相较于对照组的显微硬度210.4HV0.2,添加沉积、搭接和扫描方向超声振动后分别提高到了232.5HV0.2、230.9HV0.2和233.9HV0.2,变化趋势与晶体直径变化趋势基本一致。水平方向超声振动影响下,特别是扫描方向的超声振动在提高材料强度的同时,还在一定程度上保持了材料的延展性;在搭接方向上,超声振动辅助显示出较好的各向异性消除能力,获得了优异的硬度、强度和塑性匹配。结论 不同方向超声振动,通过不同方向的惯性力和热流改善了熔池的动态流动性,其中搭接方向超声振动在扩大熔池润湿面积和改善熔池动态形状方面具有优异的综合能力。由于超声强度方向性传播衰减和引起不同的声压梯度,微观组织细化和Laves相的细小点球状消减分别在扫描方向和沉积方向上最为显著,这些变化引起了显微硬度和拉伸性能的相关性提升。超声振动还均衡了材料的力学性能,提高了抗拉强度、屈服强度和延展性,尤其在搭接方向上,显示出较好的各向异性消除能力。可根据具体需求目标,选用相应的超声振动方向或者复合使用,以获得优异的Inconel 718零件熔池流动、微观形貌和力学性能匹配。     

高温合金Inconel718、Inconel706和Inconel706M电渣重熔组织均匀性

对比研究了高温合金Inconel 718、Inconel 706和Inconel 706M电渣重熔铸锭的枝晶组织和析出相差异。结果表明,在3种高温合金中,718铸锭中心区域的枝晶间距与边缘区域的差值最小,并且主要偏析元素Nb和Ti在铸锭中心的偏析率与边缘的差异也最小,718铸锭的宏观组织均匀性最好;706M铸锭中心与边缘的枝晶间距差值最大,且铸锭中心的Nb元素偏析率与边缘的差异达到0.91%,铸锭宏观组织均匀性最差。718铸锭枝晶间易富集正偏析元素Nb和Mo,最高含量分别达到6.82%和3.01%;706、706M铸锭枝晶间最高Nb含量均低于4%,Ti元素含量最高均达到2%以上,同时706及706M铸锭组织中Nb的偏析率依然很大。706铸锭组织中Laves相最多,中心位置Laves相含量达到3.9%,边缘含量降低超过2%,分布不均匀;706M铸锭组织中Laves相最少,中心及边缘含量均低于2%,含量分布比较均匀。与718和706相比,706M铸锭中心区域的碳氮化物平均长度与边缘的差异最大,且组织中存在更少的针状相。     

选区激光熔化成形Inconel 718合金孔隙缺陷的研究

目的 研究选区激光熔化成形Inconel 718合金的孔隙缺陷,对缺陷进行科学分类并探究其形成机制,建立熔池溅射特征与缺陷形貌的对应关系,优化工艺参数,抑制缺陷产生。方法 采用扫描电子显微镜（SEM）、能量色散X射线光谱仪（EDX）分别对Inconel 718粉末的显微组织和化学成分进行观测,使用数字视频显微镜分析成形件内部缺陷,利用高速摄像机拍摄金属液滴的动态飞溅过程,并定量分析溅射特征参数。结果 随着激光功率的增大,能量密度升高,总的溅射数量增大,孔隙数量增多;当扫描速度增大时,能量密度降低,总的溅射面积减小,孔隙尺寸变小。当缺陷的圆度Circ≥0.731或纵横比AR≤1.368时,缺陷形貌由不规则向规则演变。当能量密度E=95.24 J/mm3时,相对致密度达到99.94%。经测量,所有样品的孔隙率和孔隙尺寸的平均值分别为2.249%和2.774 μm2。结论 孔隙缺陷可分为不规则的匙孔缺陷和规则的气孔缺陷两类,存在发生演变的圆度/纵横比门槛值。熔池震荡引起溅射特征变化,对应产生不同形貌特征的缺陷。减小激光功率和增大扫描速度可降低能量密度,使熔池震荡程度减弱,从而抑制缺陷产生,提高成形件的相对致密度。     

镍基718材料表面修饰类型的有限元仿真优化设计

目的研究不同凹坑形状组合分布对表面特性的影响规律,进一步提高微造型表面的摩擦学性能和承载能力。方法利用有限元仿真技术和单因素仿真设计,分析比较半球形、方形、三角形三种凹坑形状混合分布的微造型表面,对镍基718高温合金摩擦学性能的影响。结果分析获得了不同速度和载荷下,不同微造型表面对油膜承载力和摩擦系数的影响规律。半球形、方形、三角形三种凹坑形状共同存在的微造型表面的承载能力最好,而半球形凹坑和方形凹坑组合分布的微造型表面的摩擦系数最小。结论对镍基718高温合金材料进行表面凹坑修饰,可以显著提高油膜的承载能力,有利于完整润滑油膜的形成。不同凹坑形状的组合分布对材料摩擦学性能和承载能力的影响非常大,随着速度和载荷的增大,各微造型表面的摩擦系数差别减小,凹坑形状及组合分布对摩擦学性能的影响减弱。     

In718合金反挤压成形数值模拟

本文基于粘塑性材料模型,应用有限元模拟技术对In718合金高温下的反挤压成形过程进行了数值模拟。分析了不同挤压工艺的金属流变行为和应力应变分布,得出:在高温条件下,In718合金进行等温反挤压,成形质量较好;摩擦不仅降低反挤压成形范围,并且加剧金属表面裂纹的产生;坯料的等效应力分布较均匀,最大等效应力值出现在凸模工作带端点处;无摩擦、凸模球心夹角=60°、反挤压成形温度T=1000℃时得到的等效应变值比较均匀,产品成形质量相对较好。     

Inconel718合金的蠕变孪晶机制

通过TEM研究了固溶态及直接时效态Inconel718合金的蠕变变形组织。结果表明：固溶态Inconel718合金，在550℃／220MPa条件下达到1％变形量时，蠕变组织的特征为位错交滑移；而直接时效态Inconel718合金，在680℃／650MPa条件下达到1％变形量时，蠕变组织中既有位错滑移后形成的变形带，又存在位错攀移，并且存在少量的孪晶变形。讨论了Inconel718合金中发生蠕变孪晶的机制，着重从γ＂强化相的晶体学特征来解释该现象。研究结果认为：具有特殊结构的γ＂强化相是影响合金蠕变变形机制的根源。     

脉冲等离子增材再制造Inconel 718合金的组织演变

采用脉冲等离子工艺增材再制造了Inconel718镍基高温合金,并深入研究了再制造合金的组织形态及析出相分布规律,采用KGT模型对枝晶生长动力学进行定量计算。结果表明：沉积态合金组织以柱状枝晶为主,具有较强的外延生长特性,层与层之间有明显的界线。由于再制造过程中温度梯度和冷却速率的变化,自底部向顶部呈现胞状晶→柱状晶→粗大柱状枝晶→等轴晶的转变趋势,初生枝晶间距自底部向顶部分别为19.2、29.3和34.1 μm。大量不规则的Laves相分布在枝晶间,MC碳化物弥散分布于晶界。在中部及顶部,有（γ+Laves）共晶相生成。     

Inconel718合金金属注射成形制备过程及力学性能

以Inconel718气雾化预合金粉末为原料,采用金属注射成形(MIM)工艺制备Inconel718合金材料.研究Inconel718合金烧结、热等静压(HIP)、热处理对合金显微组织、密度和力学性能的影响.结果表明:经1275 ℃烧结后,烧结体的相对密度达到98%.烧结体经HIP处理后,达到全致密.经烧结+HIP+热处理后,组织弥散析出了大量的γ″相和γ′相,其室温抗拉强度为1250 MPa,延伸率为21.7%;650 ℃抗拉强度为1177 MPa,延伸率为16.6%,其达到或超过了同牌号锻造合金的性能.     

Inconel 718镍基合金与304不锈钢电子束焊接

对Inconel 718镍基合金与304不锈钢进行了电子束焊接试验,分析了接头显微组织及力学性能. 结果表明,焊缝区中部由枝晶及细小的等轴晶组成,在近镍侧及近钢的熔合线,都由向焊缝中心方向生长的树枝晶组成. 各特征区域显微硬度值各不相同,焊缝区高于镍基合金侧,高于不锈钢侧. 当焊接束流为8 mA,焊接速度为700 mm/min时,接头的抗拉强度最高为722 MPa. 拉伸试样断裂发生于焊缝区内部,呈典型的延性断裂,断口可观察到明显等轴状韧窝.     

Microstructures and mechanical behavior of Inconel 718 fabricated by selective laser melting

In this study Inconel 718 cylinders were fabricated by selective laser melting in either argon or nitrogen gas from a pre-alloyed powder. As-fabricated cylinders oriented in the build direction (z-axis) and perpendicular to the build direction (x-axis) exhibited columnar grains and arrays of γ″ (body-centered tetragonal) Ni₃Nb oblate ellipsoidal precipitates oriented in a strong [2 0 0] texture determined by combined optical metallography, transmission electron microscopy, and X-ray diffraction analysis. Fabricated and hot isostatic pressed (HIP) components exhibited a more pronounced [2 0 0] columnar γ″ phase precipitate architecture parallel to the laser beam or build direction (spaced at ∼0.8 μm), and a partially recrystallized fcc grain structure. Fabricated and annealed (1160 °C for 4 h) components were ∼50% recrystallized and the recrystallized regions contained spheroidal γ′ precipitates distributed in a dense field of fine γ″ precipitates. The γ″ precipitates were always observed to be coincident with {1 0 0} planes of the γ-fcc NiCr matrix. Some δ phase precipitates in the unrecrystallized/recrystallized interfaces and recrystallized grain boundaries were also observed in the annealed samples. The microindentation (Vickers) hardness was 3.9 GPa for the as-fabricated materials, 5.7 GPa for the HIP material, and 4.6 GPa for the annealed material. Corresponding tensile properties were comparable with wrought Inconel 718 alloy.