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.     

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.     

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.     

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.     

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。     

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。     

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.     

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。     

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.     

Microstructural characterization of Inconel 718 alloy after pulsed laser surface treatment at different powers

An annealed Inconel 718 alloy was surface-treated by pulsed laser at three different powers (100, 50 and 25 W). Microstructural changes induced by the laser treatments were characterized by use of electron backscatter diffraction and electron channeling contrast imaging techniques. Results show that both annealing twins and strengthening precipitates profusely existing in the as-received specimen are dissolved at elevated temperatures during the laser irradiation. Meanwhile, in the melting zone (MZ), densities of low angle boundaries (LABs) are greatly increased with a large number of Laves phases preferentially distributed along such LABs. For different specimens, widths and depths of their MZs are found to be gradually reduced with decreasing the laser powers. Orientation analyses reveal that the columnar grains in the MZ of the 100 W specimen could inherit orientations existing in the matrix while lower laser powers promote the formation of more nuclei with scattered orientations to grow to be granular grains in the MZ. Hardness tests reveal that the MZs of all laser-treated specimens are softer than the matrix probably due to both precipitate dissolution and grain coarsening.     

Study of Inconel 718 weldability using MIG CMT process

Abstract

This work investigates the weldability of Inconel 718 using the metal inert gas (MIG) welding process called ‘cold metal transfer’. This arc welding process is reported for working with a lower heat input compared to other arc processes. The consequences are less base metal impaired, low deformation and low residual stress level in assembly. In order to check these abilities, metallographic, texture, chemical and residual stress analyses have been carried out. Results show that MIG cold metal transfer has good properties for the welding of Inconel 718 inasmuch as no defect has been detected and a lower level of residual stresses has been introduced in metal compared to classic MIG.     

Inconel718合金件裂纹产生原因分析

对Inconel718合金件在使用过程中裂纹产生的原因进行分析。结果表明：合金件在服役过程中主要受冷热应力与O、S元素的影响,过渡区域的棱边为应力集中区域,在热应力作用下为最容易破坏的位置。合金在服役过程中O和S扩散进基体,产生了氧化及硫化作用,使基体产生了氧化物和硫化物产物的的U型凹坑,加剧了应力集中区域的缺口敏感性,从而在冷热应力的作用下产生裂纹。裂纹的产生更易于O、S等有害元素向基体扩散,在应力作用下促使裂纹进一步扩展。在腐蚀、动态应力开裂、再腐蚀、冷热应力的作用下裂纹进一步扩展,是导致合金件失效的原因。