The effect of VAR process parameters on white spot formation in INCONEL1 718

A 20 inch diameter VAR ingot of Inconel 718 was produced at Special Metals Wiggin Limited (Hereford) under conditions of abnormally long arc gap. It was sectioned and analyzed in the as-cast state. White spots were found, along with unusual macrostructural features at the ingot surface. The experimental results are presented and discussed in relation to possible formation mechanisms, and it is shown that the level of niobium found in these white spots was too low to be explained by conventional mechanisms of formation.     

Alloy design and development of INCONEL718 type alloy

The effect of alloying elements such as Al, Ti, Nb, W and Co in INCONEL 718 type alloys on phase precipitation behavior has been investigated by means of Thermo-Calc software. The results show that both the solvus temperature and fraction of phases in 718 type alloys has been significantly changed with variation these alloying elements. The experimental results also reveal that precipitation kinetics of δ, γ″ and γ′ phases in developed 718 type alloys have been changed. The alloy with higher content of Al shows microstructural stability superior to that of conventional 718 alloy. Based on thermodynamical and experimental results, the optimum content of Al, Ti and Nb of the developed 718 type alloy without W and Co additions has been determined to be 1%, 1% and 5.5%, respectively.     

Effect of different heat treatments on the microstructure and mechanical properties in selective laser melted INCONEL 718 alloy

The microstructure and tensile properties of selective laser melted (SLM) Inconel 718 alloy were studied in the as-printed and different heat treat conditions. The SLM as-print microstructures exhibited columnar grain structures with very fine dendritic structure with segregation of elements. Apart from the standard heat treatment, three other heat treat cycle variants were carried out in an attempt to remove the extensive segregation of elements and modify the textured grain structure of the SLM as-print microstructure. Increasing the homogenization temperature reduced the segregation and coarsened the grain structure. However, the grains still remained columnar, and the material became softer with reduction in strength. After the ageing treatment, the tensile strength improved significantly for all the heat treated samples, which is typical for precipitation hardening of IN718 alloy. The microstructures of the heat treated samples exhibited the needle shaped δ, carbides, and finely dispersed γ″, γ′ phases.     

Electrical and magnetic techniques for monitoring arc behaviour during VAR of INCONEL1 718: Results from different operating conditions

Novel instrumentation and computer modelling are described which have been developed to detect and interpret current flows and magnetic fields during VAR. Results from different operating conditions during VAR of 20″ diameter ingots of INCONEL 718 at Special Metals Wiggin Ltd., Hereford are presented and discussed with reference to macrostructural findings. It is shown that the results support the viability of the measurement techniques, and that the techniques are capable of detecting subtle changes in arc behaviour. Freckles were formed under one set of experimental conditions, and it is shown that the distribution of the radial location of the arc centre (obtained from the magnetic field analysis) may help to explain this.     

Microstructure and mechanical properties of an inertia friction welded INCOLOY alloy 909 – INCONEL alloy 718 joint for rotating applications

Abstract

Inertia welding is a common practice to join axially symmetrical parts for aero-engine applications. The shaft for a new advanced high-pressure compressor will be produced by joining the high strength superalloys INCOLOY® alloy 909 (Incoloy909) and INCONEL® alloy 718 (IN718). IN718 is the work-horse nickel-iron-chromium alloy for a variety of parts for aero-engine applications due to a good combination of relevant mechanical properties, good corrosion resistance and easy fabricability. Incoloy909 is a nickel-iron-cobalt alloy used in aero-engines due to an interesting combination of a nearly constant low coefficient of thermal expansion combined with a constant modulus of elasticity and high strength over a wide range of temperatures. Both alloys are strengthened by precipitationhardening through additions of niobium and titanium. Ring shaped specimens of fully heat-treated Incoloy909 and IN718 are joined by inertia welding. The microstructure in the welded zone is investigated by optical microscopy and transmission electron microscopy with the focus on the Incoloy909 side. Post-weld heat-treatments (PWHT) are evaluated by microstructural investigations and by hardness measurements. The chosen PWHT is characterized by its tensile, creep and load controlled fatigue properties. The results of the mechanical tests are described in terms of the microstructural changes observed in the welded zone.     

Effect of rhenium on solidification of Inconel 718 alloy

Abstract

The effect of rhenium (Re) on the solidification of standard Inconel 718 (St-In718) has been investigated by using experimental alloys containing 2·4, 3·5, and 6·0%Re. Rhenium is one of the most powerful refractory elements that improve the high temperature mechanical properties of Ni base superalloys. Results indicate that solidification starts with precipitation of primary γ phase, which is followed by (γ+ NbC) eutectic and (γ+ Ni₂Nb)eutectic. The solidification temperature of the St-In718 is increased by 30 K with the addition of 6%Re. Moreover, the volume fraction of the primary γ is increased, while the volume fraction of eutectic (γ+ NbC) and (γ+ Ni₂Nb) are decreased by the addition of Re. Also, increasing Re content enlarges the secondary dendrite arm spacing (SDAS). Finally, the effects of Re on the partition coefficients k of alloying elements to primary γ and to eutectic γ+ NbC were evaluated and Re was found to segregate preferentially to the primary γ.     

Effect of process parameters on deformation and temperature rise during forging of TC6 alloy disc

Abstract

Effect of the deformation temperature, hammer velocity, height reduction and shear factor on the equivalent stress, strain and temperature rise has been studied in forging process of a TC6 titanium alloy disc with deformation temperatures of 1153–1213 K, hammer velocities of 1·2–12 000 mm min^?1 and shear factors of 0·1–0·4. The calculated results show that the deformation temperature, hammer velocity and height reduction have a significant effect on the equivalent stress and temperature rise, but have a slight effect on the equivalent strain. The predicted load–displacement curves are found to be in a good agreement with the experimental results.     

Effect of cooling rate in overageing and other thermomechanical process parameters on grain refinement in an AA7475 aluminium alloy

Fine-grained AA7475 aluminium alloy sheets were produced in this study by a thermomechanical treatment involving solution anneal, overageing, rolling and recrystallization steps. It has been found that the cooling rate after the intermediate overageing treatment should be fast to obtain the finest grain size. The fast cooling rate ensured the presence of relatively large particles of MgZn₂ and some supersaturation prior to cold rolling. Generally, the final grain structure was heterogeneous, with bands of fine grains lying parallel to the rolling direction. In material rapidly cooled after overageing, bands of fine grains were also observed in the transverse direction and these bands were associated with shear bands formed during rolling. The fine-grained AA7475 alloy sheets with an average grain size of about 9 m showed large tensile elongations of about 800% when deformed at 516 °C and with an initial strain rate of 5×10^–4s^–1.     

INCO718高温合金钎焊工艺研究

对于高温合金,把合金元素及化合物充分固溶于基体内,才能获得良好的高温性能。本文主要介绍了INCO718高温合金的应用及工艺特点,并对该材料的焊接性能进行了深入的研究分析,总结出高温合金表面氧化问题、钎焊热循环的影响、钎料与钎焊金属成分的影响、焊缝间隙和钎料量的多少、钎焊温度的影响以及真空钎焊设备的性能,都将直接决定着INCO718高温合金钎焊接头的焊接质量。本文着重阐述了高温合金表面氧化问题的解决和不同牌号的钎料对于高温合金钎焊质量的影响。     

Effect of austenite grain size on ε martensite formation in an Fe-Mn-Mo alloy

Abstracts are not published in this journal This revised version was published online in November 2006 with corrections to the Cover Date.     

Effect of scanning strategy on grain structure and crystallographic texture of Inconel 718 processed by selective laser melting

Two types of scanning strategies were adopted to study the effect of scanning strategy on grain structure and crystallographic texture of selective laser melted (SLM) Inconel 718. The results show that bidirectional scanning without and with a 90°-rotation for every layer produced the bimodal grain structure and the directional columnar grain structure, respectively. Controlling the heat flux direction between the successive layers via scanning strategy enabled the formation of such different grain structures. Furthermore, when the 90°-rotation was applied, the competitive grain growth mechanism became more pronounced and the strong cube texture developed.     

Research and Improvement on structure stability and corrosion resistance of nickel-base superalloy INCONEL alloy 740

INCONEL alloy 740 is a newly developed Ni–Cr–Co–Mo–Nb–Ti–Al superalloy in the application to ultra-supercritical boilers with steam temperatures up to 700 °C. By means of scanning electron microscopy (SEM), transmission electron microscopy (TEM), X-ray diffraction (XRD), micro-chemical phase analyses, and corrosion-resisting test, this paper investigates the structure stability of the alloy at elevated temperature and concentrates on coal ash corrosion performance of the alloy under the simulated coal ash/flue gas condition. Experimental results show that the most important structure instabilities of the alloy during prolonged aging are γ′ coarsening, γ′ to η transformation and G phase formation at grain boundary. The performance of corrosion resistance of the alloy would meet the requirement of ultra-supercritical boiler tubes. The phase computation by means of Thermo-Calc has been adopted in chemical composition modification for structure stability improvement. Two suggested new modified alloys in adjustment of the Al and Ti contents and in control of Si level, and also in maintenance of Cr content of the alloy were designed and melted for experimental investigation. These two modified alloys exhibit more stable microstructure during 760 °C long time aging.     

Investigation of the abnormal effects of phosphorus on mechanical properties of INCONEL718 superalloy

The effects of phosphorus on mechanical properties of the world’s most widely used superalloy INCONEL718 was examined. The results indicate that P has little effect on the tensile properties of INCONEL718 at room temperature, 500 or 650°C, but it increases stress rupture life and ductility. Phosphorus also improves the creep property and crack propagation resistance of INCONEL718. Further microstructure analysis by means of optical, SEM and TEM does not reveal the obvious effect of P on grain size and precipitation of INCONEL718. Some theoretical discussions were held aiming at further understanding the role of P in INCONEL718. It was suggested that the interaction of P–GB–M (metal element) complex might be responsible for this.     

波浪型倾斜板振动过程中合金组织的形成机理

研究了波浪型倾斜板振动过程中Al-6Si-2Mg合金的组织演化规律和球形品组织的形成机理.结果表明:Al-6Si-2Mg合金的非枝品组织形成过程主要受到两种机制的作用.第一种机制是熔体爆发形核和球状长人机制:倾斜板为形核提供了足够大的过冷度和异质形核基础,使熔体爆发形核,熔体均匀的成分场和温度场使部分晶粒球状生长;第_二种机制是枝晶臂颈缩断裂和磨圆熟化机制:流动和振动使晶粒低熔点处颈缩断裂,断裂晶粒不断磨圆热化形成近球形组织.     

Effect of P and B on the creep behavior of alloy 718

The effects of P and B on the creep behavior of 718 alloys are discussed in this paper. The result implies that P and B have little effect on the γ″ and γ′ phases of the alloy, but they can enhance the grain boundary strength by segregating there, which can be of beneficial to the higher creep properties. P also may increase the grains strength through solid solution. The beneficial effect of P and B were presented in both standard IN718 alloy and DA718 alloy. Adding P and B together does a better work than adding P alone.     

A study of cold metal transfer clads in nickel-base INCONEL 718 superalloy

A study of microstructural and geometrical characteristics of cold metal transfer (CMT) clads in nickel-base INCONEL 718 superalloy was carried out. The CMT process was demonstrated to be suitable for low-dilution cladding of INCONEL 718 superalloy, with the ability to produce defect-free clads, and has a great potential to be used as a repair method for the alloy. Microstructural study revealed that the clads were free from porosity and cracking, and complete bonding of the clads with the substrate was achieved in all weldments. Results from statistical analysis by using 3-level full factorial design showed the relationship between the selected welding parameters and the geometrical characteristics of the clads. Regression models were developed for predicting weld characteristics. Analysis of variance (ANOVA) was used to test the adequacy of the regression models, which showed that the models are useful for optimizing the welding parameters. Also, the wire feed speed appeared to be linearly related with heat input based on the parameters used during the synergic CMT welding, which consequently influenced the shape and size of the weld beads and the extent of dilution of the weld metal with the substrate when other parameters were kept constant. Additionally, the ease of adding successive weld passes for material build-up depends significantly on the contact angle between the weld bead and the substrate. The result showed that the closer the contact angle is to 90°, the more difficult it is to add successive passes to existing clads. A contact angle greater than 115° appears to be more convenient for adding successive passes. The outcome of this work showed that the relatively new CMT process, with the choice of suitable welding parameters, is useful for repair build-up of affected areas of worn-out and service-damaged components of gas turbines and other high-temperature equipment that are manufactured from nickel-base superalloys.     

Effect of cast and extruded stock on grain structure of EN AW 6082 alloy forgings

Abstract

There has been a rapid increase in the use of aluminium forgings in the automotive industry, where weight savings is a critical requirement for reduced fuel consumption and exhaust emissions. Extruded bar is the typical forging stock particularly in the manufacture of suspension components. There has been a great interest to replace the extruded forging stock with cast billets of the same diameter. The casting and extrusion routes for the manufacture of EN AW 6082 alloy forging stock were compared in the present work. The control arm produced via forging of the cast stock offers a very uniform grain structure and much superior surface quality with respect to its counterpart produced by the forging of the extruded stock owing to a much smaller grain structure at the surface of the forged component after the T6 heat treatment.     

非平衡晶界偏聚理论在INCONEL 718合金研究中的应用

评述了国际上将溶质晶界偏聚领域的最新研究成果－非平衡晶界偏聚理论，用于INCONEL718合金研究中所取得的最新进展。     

Effect of the pre-existing carbides on the grain boundary network during grain boundary engineering in a nickel based alloy

Grain boundary engineering was carried out on an aging-treated nickel based Alloy 690, which has precipitated carbides at grain boundaries. Electron backscatter diffraction technique was used to investigate the grain boundary networks. Results show that, compared with the solution-annealed samples, the aging-treated samples with pre-existing carbides at grain boundaries need longer duration or higher temperature during annealing after low-strain tensile deformation for forming high proportion of low-Σ coincidence site lattice grain boundaries (more than 75%). The reason is that the primary recrystallization is inhibited or retarded owing to that the pre-existing carbides are barriers to grain boundaries migration.