Aerospace Alloys Research Articles

Kinetics of Martensite Decomposition and Microstructure Stability of Ti-6246 during Rapid Heating to Service Temperatures

The aerospace alloy Ti-6246 was subjected to inductive heat treatments with high heating and quenching rates (up to 1500 K/s) while being applied to an in situ diffraction study at the HEMS beamline P07B at DESY. Thereby, the characterization of the emerging phases was possible at any point in the process. The heat treatment schedules include the preparation of Ti-6246 samples by means of a homogenization treatment and subsequent quenching to trigger α″-martensite formation. In order to simulate fast reheating within the scope of application, the samples were reheated to the upper range of possible service temperatures (550–650 °C) with a heating rate of 100 K/s. In a second heat treatment design, the homogenized and quenched sample state was exposed to high-temperature tempering at 840 °C, which aims for the elimination of α″. Again, fast reheating to the same service temperatures was executed. With the aim of this approach, the stability of the microstructure consisting of α-Ti, β-Ti and α″-martensite was characterized. Further, the martensite decomposition path was analyzed. It shows a two-tier nature, firstly approaching the bcc β-unit cell in the low-temperature range (<400 °C) but subsequently transforming into an hcp-like unit cell and later on into equilibrium α-Ti.

Prediction of heat generation effect on force torque and mechanical properties at varying tool rotational speed in friction stir welding using Artificial Neural Network

Friction stir welding (FSW) has played a significant role in joining aerospace alloys. During this process, the tool rotational (TRS) speed has been found to significantly affect heat generation compared to other parameters. Therefore, the study has investigated the effect of heat generation on force-torque and mechanical properties at different tool rotational speeds (TRS) in the FSW process through experimentation followed by Artificial Neural Network (ANN) technique. Further, the influence of different TRS ranging between 600 and 1800 rpm with an increment of 400 rpm on considered responses; namely thermal weld cycle, microstructure, and grain distribution in nugget zone (NZ) for 2050-T84 Al-Cu-Li alloy plates, welded using FSW were also investigated. It is observed that the vertically downward force (Z-force), longitudinal force (X-force), and spindle torque (Sp. T) decrease with increasing TRS. It is also observed an increasing (up to 1400 rpm) and then decreasing trend for tensile strength and hardness of welded samples. Moreover, the generation of frictional heat and grain size in NZ is increased with increasing TRS from 600 to 1800 rpm. However, the scanning electron microscope (SEM) micrographs of all-welded samples revealed a ductile mode of tensile fracture. Furthermore, the obtained experimental results were validated using the ANN technique. A quite better agreement has been established among the predicted outcomes from ANN with experimental results.

Finishing the surface micro-layer of additively manufactured TiAl alloy using electro-thermal discharge assisted post-processing

Selective laser melting (SLM) of titanium–aluminium (TiAl) alloy components has gained significant attention in the modern industrial world. The flexibility of the SLM process in producing complex shapes with minimum utilization of material and energy makes it dominant over other manufacturing techniques. As aerospace and biomedical industries demand complex-shaped TiAl alloy components, part fabrication using SLM becomes the ultimate solution. However, the unacceptable level of surface integrity and anisotropic behavior of SLM components demand post processing operations such as laser polishing, chemical polishing, and conventional polishing methods. In this study, a recently developed polishing method called wire electrical discharge polishing (WEDP) is performed on TiAl alloys for obtaining a smooth and defect-free surface. This study aims to investigate the micro-layer modification occurring to the WEDP-processed surface in detail. The experimental results establish the effectiveness of WEDP method in terms of improved surface integrity. The surface finish (Sa) got enhanced by ~88% after WEDP processing. In addition, the thickness of recast layer formed by WEDP was found to be minimum. Moreover, post-processing of TiAl alloy resulted in better surface morphology specifically at lower settings of peak current. It is noteworthy that the migration of wire material was minimum with zinc-coated brass electrode compared to the normal brass electrode. Hence, coated wire electrodes are recommended for WEDP process. In short, an excellent surface integrity can be achieved using WEDP process through favorable surface modification aided by lower peak current and coated wire electrodes. Furthermore, less electrode wear observed in WEDP process enables the deployment of lower feed rates leading to minimal electrode consumption.

Solid-state processing of surplus aluminium alloy powders through a combination of field-assisted sintering technology and hot rolling

ABSTRACT Metal additive manufacturing techniques typically operate using powders with limited particle size ranges, but atomisation processes produce significant amounts of particles outside these ranges, resulting in an accumulation of out-of-size specification metal powders without a clear use case. Field-assisted sintering technology (FAST) can provide an alternative, solid-state processing route to consolidate these powders into billets for subsequent processing, or directly into near-net shape components. In this study, surplus powders of A20X, an aerospace approved aluminium alloy developed by Aluminium Materials Technologies (ECKART GmbH), were processed using FAST and subsequently hot rolled to produce sheet material. Tensile properties were similar to hot rolled conventional cast material and comparable to additively manufactured product. This indicates that FAST is an effective option for converting surplus metal powders into useful products, while improving sustainability in the additive supply chain.

A novel hybrid model for prediction of distortions in milling

Distortion of machined monolithic thin-walled parts is a well-known issue and a perpetual concern in the aerospace industry. In this article, a novel analytical-FEM hybrid model for fast prediction of distortion of thin-walled parts is proposed. Moving cutting loads due to milling are calibrated and applied directly to the surface of the meshed model. The effects of measured initial stresses in the workpiece as well as cutting forces and heat flux are effectively modeled. Validation tests carried out on the aerospace alloy Al7050-T7451 showed the promise of the model in predicting the distortion of the part.

Resource-efficient performance testing of metalworking fluids utilizing single-point milling

ABSTRACT Metalworking fluids have the ability to extend cutting tool life and improve the machinability of materials. There is a need for the development of reliable machining tests which can be used to screen fluids with high confidence to allow for ranking in terms of performance. This study developed a novel methodology utilizing single-point milling to evaluate fluid performance in terms of tool wear and cutting forces across various aerospace alloys. The repeatability of the procedure was assessed and demonstrated by using standard deviation. The study showed alternative cutting fluid compositions could influence tool life performance across all the aerospace material variants. Inconel 718 was shown to be the hardest material to machine followed by Titanium Ti–5Al–5Mo–5V–3Cr and Titanium Ti–6Al–4V. However, with each material, there was a differentiation in fluid performance with up to 11% difference in average tool life between different fluids.

An in-depth evaluation of surface characteristics and key machining responses in WEDM of aerospace alloy under varying electric discharge environments

An in-depth evaluation of surface characteristics and key machining responses in WEDM of aerospace alloy under varying electric discharge environments

Mechanical property and microstructural evaluation of friction stir welded AA8090 T87 aluminium alloy

The aerospace alloy AA8090 T87 was friction stir welded with varying traverse speeds of 30 and 70 mm/min at rotation speeds of 900 and 1100 r/min to realize the effect of traverse speed and rotation speed on the joint strength. This variation in the rotation and traverse speeds produced variation in the thermal cycle in the weld joint that altered the microstructure of the stir zone and morphology of the grains. The increase in the rotational speed from 900 to 1100 r/min at 30 mm/min increased the grain size from 4.1 µm to 7.8 µm in the stir zone which was evident from the microscopy images and Schmid factor maps and contributed to the mechanical property changes. The increase in traverse speed from 30 to 70 mm/min has led to the hardness improvement and reduction in grain size in the stir zone. The nature of the fracture observed in the stir zone is ductile fracture having more dimples. An effective joint was produced at 900 r/min rotational speed and 30 mm/min traverse speed.

Additive manufacturing of TTFNZ (Ti-4.5Ta-4Fe-7.5Nb-6Zr), a novel metastable β-titanium alloy for advanced engineering applications

The alloy Ti‐6Al‐4V (Ti64; wt%) is extensively used for aerospace and medical applications. However, due to potential drawbacks and societal change, intensive research has been conducted to replace this material for medical use, as such various promising alloy systems have been proposed. By implementing additive manufacturing (AM) in the value chain of load-bearing implants, the question of the printability of these promising medical alloy systems arose as none were explicitly developed for AM. In a previous paper, an alloy development methodology for AM (NADFAM [Ackers et al., 2021]) was presented, which allows rapid screening of alloys for laser-based powder bed fusion of metals (PBF-LB/M; ISO/ASTM 52911–1). The method was applied to screen several new titanium alloys and rank the alloys according to target properties for PBF-LB/M of load-bearing implants. Following this screening, a novel metastable β-titanium alloy out of the quinary Ti-Ta-Fe-Nb-Zr system was selected for further investigations, and a refined alloy composition Ti-4.5Ta-4Fe-7.5Nb-6Zr (wt%, denoted as TTFNZ) will be presented and compared to Ti64 in this study. The motivation to investigate a system with a lower allotropic transformation temperature (β/α) than Ti64 derived from the idea that (I) it promises reduced cold cracking tendency in the course of rapid solidification (II) common post-print heat treatments could be performed at lower temperatures or could become redundant using printers with a preheating option and (III) it allows exploring a wide range of microstructures and mechanical properties through thermal processing. The present study reports on: (i) AM feedstock/powder production of TTFNZ, (ii) PBF-LB/M process parameter development, (iii) heat treatment optimisation, related microstructures and their effect on mechanical properties tested via indentation, compression and tensile testing. (iv) Corrosion properties and (v) the PVD coatability of TTFNZ were also studied to discuss its amenability for medical and advanced engineering applications. • First-time atomisation and additive manufacturing of Ti-4.5Ta-4Fe-7.5Nb-6Zr (TTFNZ). • TTFNZ surpasses well-known medical and aerospace alloys in a variety of disciplines. • Investigation of TTFNZ’s microstructure evolution using DSC, EDX/EBSD and XRD. • Compression, tensile and dilatometry results. • Evaluation of TTFNZ’s corrosion properties as well as initial PVD coating trials.

Synthesis, Microstructural Characterization, Mechanical, Fractographic and Wear Behavior of Micro B4C Particles Reinforced Al2618 Alloy Aerospace Composites

In the current studies an investigations were made to know the effect of 63 micron sized B4C particles addition on the mechanical and wear behavior of aerospace alloy Al2618 metal composites. Al2618 alloy with different weight percentages (2, 4, 6 and 8 wt. %) of 63 micron sized B4C particles reinforced composites were produced by stir cast process. These synthesized composites were tested for various mechanical properties like hardness, compression strength and tensile behavior along with density measurements. Further, microstructural characterization was carried by SEM/EDS and XRD analysis to know the micron sized particles distribution and phases. Wear behavior of Al2618 alloy with 2 to 8 wt. % of B4C composites were studied as per ASTM G99 standards with varying loads and sliding speeds. By adding 63 micron sized B4C particles hardness, compression and tensile strength of Al2618 alloy was enriched with slight decrease in elongation. Further, wear resistance of Al2618 alloy was enriched with the accumulation of B4C particles. As load and speed on the specimen increased, there was increase in wear of Al2618 alloy and its composites. Various tensile fracture surface morphology and worn surface behavior was observed by SEM analysis.

Microstructure Effects of the Electrical Discharge Machining Drill on Aerospace Super Alloys

This paper presents an analysis of the parent material damage and electrode wear of a handheld electrical discharge machining (EDM) drill. The motivation of this study is to investigate the material effects of EDM drilling on aerospace alloys at a microstructure level. The specific effects that are being investigated are heat affected zones (HAZ), grain structure changes, material composition changes, and fracturing. A secondary goal of this study is to determine if there is potential for electrode wear prediction models for supply chain management. The study consisted of making cuts into both Inconel and titanium test specimens while varying cut depths and electrode diameters. The results of the study found that the electrode wear while cutting Inconel showed some amount of predictable behavior. On the contrary, the titanium specimens did not indicate a predictable behavior and did not appear to follow an intuitive trend. Interestingly, when drilling into titanium, the active electrode increased in length, indicating that the titanium is being deposited onto the electrode. The EDM drill caused notable damage to the parent material, including a substantial recast layer of up to approximately 150 μm in thickness, incorporation of electrode material into the parent material through the recast layer, and formation of microcracks that can cross the recast layer/parent material interface. These noted characteristics can have an impact on material performance and longevity due to the nonuniform nature of the damage and the introduction of microcracks crossing from the recast layer into the parent material.

Corrosion Behavior of Friction Stir Welded AA8090-T87 Aluminum Alloy.

Aerospace alloys with reduced wall thickness but possessing higher hardness, good tensile strength and reasonable corrosion resistance are essential in manufacturing of structures such as fuselage. In this work, friction stir welding has been carried out on such an aerospace aluminum alloy AA8090 T87 which contains 2.3% lithium. Tool rotational speed of 900 rpm and traverse speeds of 90 mm/min., 110 mm/min. are the welding parameters. Hardness analysis, surface roughness analysis and corrosion analysis are conducted to analyze the suitability of the joint for the intended application. The samples were corrosion tested in acid alkali solution and they resulted in the formation of pits of varying levels which indicate the extent of surface degradation. Hardness of the samples was measured after corrosion analysis to observe the changes. The analysis suggests that the change in tool traverse speed transformed the corrosion behavior of the joint and affected both the hardness and surface roughness which mitigated the quality of the joint.

Comparative analyses and investigations of chamfered and honed-edge tool geometries on tool wear, chip morphology, residual stresses and end-burr formation

Cutting simulations for chamfered-edge and honed-edge tools for rough-to-semi finishing operations in machining an aerospace grade aluminum alloy AA2024 have been presented in this work. A finite element based approach has been developed to comprehend and investigate the relative effects of various cutting edge geometries on chip morphology, end-burr and residual stresses. Additionally, the role of chip-breaker geometry in segmentation and fragmentation has been thoroughly examined. Moreover, to estimate the expected wear in different sections of the tool, stress fields have been plotted. Furthermore, effects of chamfered-tool' macroscopic parameters: chamfer-length and chamfer-angle on various aspects of cutting process have been discussed. Presented coupled temperature-displacement numerical model has been validated for various results concerning chip morphology, cutting forces and segmentation frequency with previously performed experimental work. Finally, recommendations are made for proper selection of tool-geometry for better tool life, surface integrity and lesser burr formation in machining AA2024.

Large-scale serial sectioning of environmentally assisted cracks in 7xxx Al alloys using femtosecond laser-PFIB

In order to better understand the mechanisms of environmentally-assisted cracking (EAC) of 7xxx aluminium aerospace alloys, large scale serial sectioning using the newly developed femtosecond laser plasma focused ion-beam (laser PFIB) has been performed on both AA7050 and AA7085 alloys containing environmentally-assisted cracks introduced during accelerated laboratory testing. The samples were firstly scanned using X-ray computed tomography in order to reveal the long-range crack morphology and allow for targeted lift out of the crack tip regions for serial sectioning. This paper describes the optimisation of the methodology for both the liftout procedure and subsequent serial sectioning using laser-PFIB. Electron backscatter diffraction (EBSD) mapping at each slice allowed for 3D reconstruction of statistically valid volumes of material, up to 0.5 mm3 with a fine voxel size (1 μm). High resolution secondary electron imaging at each slice allowed for 3D reconstruction of the crack volume, and the datasets were combined in order to investigate the interaction of the crack with the microstructure. Optimisation of both the liftout geometry and the slicing procedure resulted in significant reduction in laser-induced curtaining artefacts and higher quality data acquisition. For these alloys, laser slicing alone was sufficient to provide a deformation free surface allowing for EBSD indexing rates up to ~95% acquired at high speeds of ~2000 Hz leading to reasonable acquisition times of ~4–5 days for each dataset. The datasets have shown that the grains in these alloys are much larger and more complex than previously realised, and the interaction of these cracks with the microstructure have shown that complex-shaped recrystallised grains can lead to significant crack deflection in these alloys and so may help to explain the observed differences in EAC behaviour.

Investigation of the cutting fluid’s flow and its thermomechanical effect on the cutting zone based on fluid–structure interaction (FSI) simulation

Cutting fluids are an important part of today’s metal cutting processes, especially when machining aerospace alloys. They offer the possibility to extend tool life and improve cutting performance. However, the equipment and handling of cutting fluids also raises manufacturing costs. To reduce the negative impact of the high cost of cutting fluids, cooling systems and strategies are constantly being optimized. In most existing works, the influences of different cooling strategies on the relevant process state variables, such as tool wear, cutting forces, chip breakage, etc., are empirically investigated. Due to the limitations of experimental methods, analysis and modeling of the working mechanism has so far only been carried out at a relatively abstract level. For a better understanding of the mechanism of cutting fluids, a thermal coupled two-dimensional simulation approach for the orthogonal cutting process was developed in this work. This approach is based on the Coupled Eulerian Lagrangian (CEL) method and provides a detailed investigation of the cutting fluid’s impact on chip formation and tool temperature. For model validation, cutting tests were conducted on a broaching machine. The simulation resolved the fluid behavior in the cutting area and showed the distribution of convective cooling on the tool surface. This work demonstrates the potential of CEL-based cutting fluid simulation, but also pointed out the shortcomings of this method.

Review of Cr-Free Coatings for the Corrosion Protection of Aluminum Aerospace Alloys

Aluminum alloys are known to have many advantages (e.g., light weight and low cost) but they are not immune to corrosion. So, it is important to assess their corrosion behavior, in particular under atmospheric conditions. To protect aluminum alloys against corrosion, paints are generally applied onto the materials. Corrosion protection in the aerospace industry consists of a conversion or anodized coating, an inhibited primer, and a top-coat. Chromate conversion coating (CCC) and primers containing chromate pigments have been widely used in the aerospace industry over the last decades. However, new environmental regulations have led to major changes for aluminum corrosion protection. By limiting or prohibiting some chemicals, for instance Cr(VI), the European regulation REACH (Regulation on Registration Evaluation, Authorization and Restriction of Chemicals) has induced major changes to some of the finishing processes of aluminum alloys (e.g., chromate conversion, chromic acid anodizing, and chromate sealing). Interesting results have been obtained while seeking replacements for Cr(VI), for example, with the incorporation of cerium, lithium salt, or nanocontainers loaded with corrosion inhibitors in organic coatings. For several years, hybrid sol–gel coatings able to replace the pre-treatment and primer steps have been under development, showing interesting results. New prospects for the future involve the use of photopolymerization to reduce the energy-intensive heat treatment needed in sol–gel technology. It will also be necessary to test these new technologies in service conditions or in accelerated corrosion tests before being able to conclude on the real effectiveness of these coatings. This review summarizes the recent developments in Cr-free coatings for aluminum alloys. Their advantages and drawbacks are also discussed.

Mechanical Strengthening of Lightweight Aluminium Alloys through Friction Stir Process

The aerospace industries are focused on lightweighting with alloys having good tensile strength, fracture toughness, fatigue resistance, and corrosion resistance. The friction stir welding technology is one of the productive techniques in the aerospace industry to join such alloys with little ease. This paper deals with the composition of alloying elements that makes the structure lightweight and the impact of the precipitates evolved out of the selected alloying elements on the mechanical properties such as tensile strength and hardness of the joint in the aerospace alloys such as AA2xxx conventional aluminium alloys, AA2xxx lithium-based aluminium alloys, and AA7xxx aluminium alloys.

A Novel Approach for Real-Time Quality Monitoring in Machining of Aerospace Alloy through Acoustic Emission Signal Transformation for DNN

Gamma titanium aluminide (γ-TiAl) is considered a high-performance, low-density replacement for nickel-based superalloys in the aerospace industry due to its high specific strength, which is retained at temperatures above 800 °C. However, low damage tolerance, i.e., brittle material behavior with a propensity to rapid crack propagation, has limited the application of γ-TiAl. Any cracks introduced during manufacturing would dramatically lower the useful (fatigue) life of γ-TiAl components, making the workpiece surface’s quality from finish machining a critical component to product quality and performance. To address this issue and enable more widespread use of γ-TiAl, this research aims to develop a real-time non-destructive evaluation (NDE) quality monitoring technique based on acoustic emission (AE) signals, wavelet transform, and deep neural networks (DNN). Previous efforts have opted for traditional approaches to AE signal analysis, using statistical feature extraction and classification, which face challenges such as the extraction of good/relevant features and low classification accuracy. Hence, this work proposes a novel AI-enabled method that uses a convolutional neural network (CNN) to extract rich and relevant features from a two-dimensional image representation of 1D time-domain AE signals (known as scalograms), subsequently classifying the AE signature based on pedigreed experimental data and finally predicting the process-induced surface quality. The results of the present work show good classification accuracy of 80.83% using scalogram images, in-situ experimental data, and a VGG-19 pre-trained neural network, establishing the significant potential for real-time quality monitoring in manufacturing processes.

Avoiding process vibrations by suppressing chip segmentation during machining of aerospace alloy Ti6Al4V

The characteristic segmented chip formation of titanium alloy Ti6Al4V leads to a periodic excitation and therefore to vibrations during the process. One way to avoid these chip formation induced vibrations is to position a counter element, the constraint, in front of the primary shear zone which suppresses the segmentation mechanism. In this paper, the simulative design of the geometry and position of the constraint, which are decisive for the successful suppression of the segmented chip formation, is shown. The most promising design is then manufactured and used in experimental investigations and the results are compared with those from the simulations.

On deformation characterisation of machined surfaces and machining-induced white layers in a milled titanium alloy

Machining-induced white layers and severely deformed layers are undesirable surface integrity features which can be formed when machining high-strength aerospace alloys. An orthogonal milling process has been designed and performed to assess the impact of cutting speeds, tool wear, cutting edge radius and climb vs conventional milling on white layer formation and plastic strain distribution. The plastic deformation in the machined surface associated with the formation of white layers in Ti-6Al-4V has been quantified using micro-grids of different length scales printed using the electron beam lithography technique. It was found that white layers formed via the severe plastic deformation mechanism, at equivalent plastic strain values in excess of 1.2 and in regions of the cutting arc with the instantaneous chip thickness of less than the cutting-edge radius and ploughing and rubbing being the dominant mechanisms. The results indicated that the magnitude of the measured strains and the depth of plastically deformed material was greater at lower cutting speeds, during climb milling and when machining with a larger cutting edge radius and tool flank wear land.