Atomic-scale grain boundary engineering to overcome hot-cracking in additively-manufactured superalloys

There are still debates regarding the mechanisms that lead to hot cracking in parts build by additive manufacturing (AM) of non-weldable Ni-based superalloys. This lack of in-depth understanding of the root causes of hot cracking is an impediment to designing engineering parts for safety-critical applications. Here, we deploy a near-atomic-scale approach to investigate the details of the compositional decoration of grain boundaries in the coarse-grained, columnar microstructure in parts built from a non-weldable Ni-based superalloy by selective electron-beam melting. The progressive enrichment in Cr, Mo and B at grain boundaries over the course of the AM-typical successive solidification and remelting events, accompanied by solid-state diffusion, causes grain boundary segregation induced liquation. This observation is consistent with thermodynamic calculations. We demonstrate that by adjusting build parameters to obtain a fine-grained equiaxed or a columnar microstructure with grain width smaller than 100 $\mu$m enables to avoid cracking, despite strong grain boundary segregation. We find that the spread of critical solutes to a higher total interfacial area, combined with lower thermal stresses, helps to suppress interfacial liquation.Comment: Accepted version at Acta Materiali

Characterisation of the relationship between surface texture and surface integrity of superalloy components machined by grinding

The surface texture of a machined component is influenced largely by the processing parameters used during machining and hence, there is a relationship between both the formation of the surface texture and surface integrity of the machined component. In the study to be reported in this paper, GH4169, a hard-to-cut superalloy, widely used in aero-engines, was selected for a detailed investigation into the relationship between the surface texture and the component-performance (surface integrity) of the machined components for which a series of grinding experiments with different grinding-wheels and grinding parameter-values was carried out in order to quantitatively analyze variations of the surface roughness with processing parameters. Further, considering that the features of the ground-surfaces measured are of a random nature, statistic properties of the produced surfaces were revealed and characterised with power spectral density function (PSD) and auto-covariance function(ACV) method respectively

Comparison of Flank Super Abrasive Machining vs. Flank Milling on Inconel® 718 Surfaces

Thermoresistant superalloys present many challenges in terms of machinability, which leads to finding new alternatives to conventional manufacturing processes. In order to face this issue, super abrasive machining (SAM) is presented as a solution due to the fact that it combines the advantages of the use of grinding tools with milling feed rates. This technique is commonly used for finishing operations. Nevertheless, this work analyses the feasibility of this technique for roughing operations. In order to verify the adequacy of this new technique as an alternative to conventional process for roughing operations, five slots were performed in Inconel (R) 718 using flank SAM and flank milling. The results showed that flank SAM implies a suitable and controllable process to improve the manufacture of high added value components made by nickel-based superalloys in terms of roughness, microhardness, white layer, and residual stresses.The authors wish to acknowledge the financial support received from the Spanish Ministry of Economy and Competitiveness with the project TURBO (DPI2013-46164-C2-1-R), grant number [BES-2014-068874], to HAZITEK program from the Department of Economic Development and Infrastructures of the Basque Government and from FEDER funds, related to the HEMATEX project and Vice-chancellor of Innovation, Social Compromise and Cultural Action from UPV/EHU (the Bizialab program from the Basque Government). Finally, thanks are also addressed to Spanish Project MINECO DPI2016-74845-R and RTC-2014-1861-4

A statistical data-based approach to instability detection and wear prediction in radial turning processes

Radial turning forces for tool-life improvements are studied, with the emphasis on predictive rather than preventive maintenance. A tool for wear prediction in various experimental settings of instability is proposed through the application of two statistical approaches to process data on tool-wear during turning processes: three sigma edit rule analysis and Principal Component Analysis (PCA). A Linear Mixed Model (LMM) is applied for wear prediction. These statistical approaches to instability detection generate results of acceptable accuracy for delivering expert opinion. They may be used for on-line monitoring to improve the processing of different materials. The LMM predicted significant differences for tool wear when turning different alloys and with different lubrication systems. It also predicted the degree to which the turning process could be extended while conserving stability. Finally, it should be mentioned that tool force in contact with the material was not considered to be an important input variable for the model.The work was performed as a part of the HIMMOVAL (Grant Agreement Number: 620134) project within the CLEAN-SKY program, linked to the SAGE2 project for geared open-rotor development and the delivery of the demonstrator part. Funding through grant IT900-16 is also acknowledged from the Basque Government Department of Education, Universities and Research

Experimental studies using minimum quantity cooling (MQC) with molybdenum disulfide and graphite-based microfluids in grinding of Inconel® alloy 718

In the paper, the results of experimental studies related with determination of the influence of the supply of a grinding fluid (GF) doped with powdered graphite and MoS2 into the machining zone with the minimum quantity cooling (MQC) method on the course and results of the reciprocating internal cylindrical grinding of rings made from Inconel® alloy 718 have been presented. As a grinding fluid, water aerosols were used. The aerosols delivered the following into the grinding zone: water slurry MoS2 with a concentration of 30 g/dm3, water slurry of graphite with a concentration of 30 g/dm3, 5% water solution of Syntilo RHS oil and pure demineralized water. The obtained results of carried out experiments showed that the most favorable conditions of grinding wheel operation were obtained when MQC-based delivering an aerosol of water slurry made from demineralized water doped with MoS2 and graphite with a minimum flow rate and when delivering an aerosol of 5% water slurry of Syntilo RHS oil. It was proved that doping GF with powdered MoS2 and graphite, with delivery in the form of an aerosol with a minimum flow rate, has a substantial influence on the intensity of clogging grinding wheel active surface (GWAS). Additionally, it has been demonstrated that the solid grease MoS2 and graphite particles reached the area of contact of the GWAS and the machined surface effectively, actively influencing its tribological conditions of the grinding process.Web of Science1011-466163

Super Abrasive Machining of Integral Rotary Components Using Grinding Flank Tools

Manufacturing techniques that are applied to turbomachinery components represent a challenge in the aeronautic sector. These components require high resistant super-alloys in order to satisfy the extreme working conditions they have to support during their useful life. Besides, in the particular case of Integrally Bladed Rotors (IBR), usually present complex geometries that need to be roughed and finished by milling and grinding processes, respectively. In order to improve their manufacturing processes, Super Abrasive Machining (SAM) is presented as a solution because it combines the advantages of the use of grinding tools with milling feed rates. However, this innovative technique usually needed high tool rotary speed and pure cutting oils cooling. These issues implied that SAM technique was not feasible in conventional machining centers. In this work, these matters were tackled and the possibility of using SAM in these five-axis centers with emulsion coolants was achieved. To verify this approach, Inconel 718 single blades with non-ruled surfaces were manufactured with Flank-SAM technique and conventional milling process, analyzing cutting forces, surface roughness, and dimension accuracy in both cases. The results show that SAM implies a suitable, controllable, and predictable process to improve the manufacture of aeronautical critical components, such as IBR.This work is based on TURBO project (DPI2013-46164-C2-1-R) of the Spanish Ministry of Economy and Competitiveness. Also, the authors wish to acknowledge the financial support received from HAZITEK program, from the Department of Economic Development and Infrastructures of the Basque Government and from FEDER founds, related to the project with acronym HEMATEX. Besides, the authors would like to thanks as well to BCAM for its collaboration. Finally, thanks are also addressed to Spanish Project MINECO DPI2016-74845-R and RTC-2014-1861-4

Identification of influent factors on surface integrity in nickel-base superalloy drilling

For the critical rotating components in aeronautical industry, the metallurgical quality achieved after machining conditions could determine their mechanical behaviour in fatigue. To guarantee this quality, the tools, materials and cutting conditions are frozen during the validation process by a cutup part following by an acceptable surface integrity. Even with the fixed parameters, perturbations can occur during the process and may have a direct impact over the metallurgical quality through the apparition of anomalies, which could reduce the calculated fatigue life. The aim of this study is to define a Process Monitoring technique able to detect the thickness affected by the machining taking into account the flank wear effect

Machinability of cobalt-based and cobalt chromium molybdenum alloys - a review

Cobalt chrome molybdenum alloy is considered as one of the advanced materials which is widely gaining popularity in various engineering and medical applications. However, it is categorized as difficult to machine material due to its unique combination of properties which include high strength, toughness, wear resistance and low thermal conductivity. These properties tend to hinder the machinability of this alloy which results in rapid tool wear and shorter tool life. This paper presents a general review of the materials’ characteristics and properties together with their machinability assessment under various machining conditions. The trend of machining and future researches on cobalt-based and cobalt chromium molybdenum alloys are also discussed adequately

Pitting and Repair of the Space Shuttle's Inconel(Registered TradeMark) Honeycomb Conical Seal Panel

During return to flight servicing of the rudder speed brake (RSB) for each Space Shuttle Orbiter, inspectors discovered numerous small pits on the surface of the #4 right hand side honeycomb panel that covers the rudder speed brake actuators. Shortly after detection of the problem, concurrent investigations were initiated to determine the extent of damage, the root cause, and to develop a repair plan, since fabrication of a replacement panel is impractical for cost, schedule, and sourcing considerations. This paper describes the approach, findings, conclusions and recommendations associated with the investigation of the conical seal pitting. It documents the cause and contributing factors of the pitting, the means used to isolate each contributor, and the supporting evidence for the primary cause of the pitting. Finally, the selection, development and verification of the repair procedure used to restore the conical seal panel is described with supporting process and metallurgical rationale for selection

Surface integrity evaluation and the effect of machining-induced surface integrity characteristics on part's performance

Surface integrity (SI) is the integrated surface behavior and condition of a material after being modified by a manufacturing process; it describes the influence of surface properties and characteristics upon material functional performance. As the leading-edge field of manufacturing research, SI finishing/machining and the consequent machining-induced complex combination of surface roughness, residual stress, work-hardening, macro and microstructure transformation, strongly affect the fatigue and stress behavior of machined parts. This kind of influence is particularly sensitive and pronounced in the difficult-to-machine materials, which are typically chosen for the most critical applications in the automobile, aerospace and nuclear industry. Thus, well-designed SI processing requirement and accurate SI evaluation model are essential to control and ensure the surface quality and functional performance for these key parts. In this thesis, an SI descriptive model for quantitative characterization and evaluation of surface integrity is proposed based on five principal SI characteristics. Considering the nature of surface integrity, a conceptual framework of an SI model for machined parts is established, in which the SI model is constructed based on the correlations between SI manufacturing processes, SI characteristics and final functionality. This model offers a theoretical basis and guideline for controlling SI characteristics and improving fatigue properties for machined parts. An empirical model for estimating the SI-characteristics-caused effective stress concentration factor (SCF) is established with fatigue life as the evaluating indicator. For a typical difficult-to-machine material, GH4169 superalloy, usually used in internal combustion engines, its grindability and the influence of processing parameters on the five principal SI characteristics are investigated in detail. The correlations between the processing parameters and the SI characteristics, between the processing parameters and the fatigue properties, and between the SI characteristics and the fatigue properties, are analyzed based on an orthogonally-designed grinding experiment and corresponding rotary bending fatigue testing for GH4169 samples within the selective range of grinding processing parameters. The feasibility and effectiveness of the proposed model for estimating the SI effective SCF are also validated by the experimental results, and this has actually offered an equivalent and convenient means for evaluation of SI and fatigue properties. Finally, the conclusions and contribution of the research are discussed, and potential future work to build on this research is identified