IN-SITU CHARACTERIZATION OF SURFACE QUALITY IN γ-TiAl AEROSPACE ALLOY MACHINING

The functional performance of critical aerospace components such as low-pressure turbine blades is highly dependent on both the material property and machining induced surface integrity. Many resources have been invested in developing novel metallic, ceramic, and composite materials, such as gamma-titanium aluminide (γ-TiAl), capable of improved product and process performance. However, while γ-TiAl is known for its excellent performance in high-temperature operating environments, it lacks the manufacturing science necessary to process them efficiently under manufacturing-specific thermomechanical regimes. Current finish machining efforts have resulted in poor surface integrity of the machined component with defects such as surface cracks, deformed lamellae, and strain hardening. This study adopted a novel in-situ high-speed characterization testbed to investigate the finish machining of titanium aluminide alloys under a dry cutting condition to address these challenges. The research findings provided insight into material response, good cutting parameter boundaries, process physics, crack initiation, and crack propagation mechanism. The workpiece sub-surface deformations were observed using a high-speed camera and optical microscope setup, providing insights into chip formation and surface morphology. Post-mortem analysis of the surface cracking modes and fracture depths estimation were recorded with the use of an upright microscope and scanning white light interferometry, In addition, a non-destructive evaluation (NDE) quality monitoring technique based on acoustic emission (AE) signals, wavelet transform, and deep neural networks (DNN) was developed to achieve a real-time total volume crack monitoring capability. This approach showed good classification accuracy of 80.83% using scalogram images, in-situ experimental data, and a VGG-19 pre-trained neural network, thereby establishing the significant potential for real-time quality monitoring in manufacturing processes. The findings from this present study set the tone for creating a digital process twin (DPT) framework capable of obtaining more aggressive yet reliable manufacturing parameters and monitoring techniques for processing turbine alloys and improving industry manufacturing performance and energy efficiency

Fast Determination of Soil Behavior in the Capillary Zone Using Simple Laboratory Tests

INE/AUTC 13.1

The development of an experimental technique to measure the influence of temperature on the mechanical properties of weldments

In large industries, such as in power stations, welds are widely employed to join different components together to meet various property requirements. The thermal gradient that develops during welding causes an inhomogeneous distribution of material properties, in areas adjacent to the weld, known as the Heat Affected Zones (HAZ). Welded joints subjected to elevated temperatures and loads during operations often experience a degradation of mechanical properties and performance of the joint. Studies have found that mechanical phenomena’s such as, fatigue and creep have compromised the structural integrity of weld zones. In essence a welded component acts as a composite material, for which it’s overall performance is dependent on its weakest material component. This study focuses on developing an experimental technique that is capable of measuring the influence of temperature on the mechanical and material properties across a weldment. The development of the experimental technique includes the design and optimisation of the hot zone of a welded tensile specimen, identification and characterisation of the different weld zones as well as, refining a strain recording strategy to detect the localised strains in each of the different weld zones. The application of the experimental technique is applied to welded components from turbine steam penetrations, which were extracted from a coal fired power station. The steam penetrations are a low Cr structural steel; (Cr 0.66, C 0.24 by wt. %) and have been in service for approximately 24 year (± 212 000 hrs). Two primary systems namely the Gleeble 3800 thermo-mechanical simulator and digital image correlation are used in this study. In order to accurately map the in-service evolution of material properties, each of the welds were mechanically loaded in tension and exposed to elevated operating temperatures. To induce mechanical loading at constant elevated temperatures, a Gleeble 3800 thermo-mechanical simulator with a tensile module was used to deform specimens at a strain rate of 50 µε.s1 . Experiments were conducted at various temperatures, ranging from room temperature (RT) to 535 o C. The evolution of material properties across the weldment was evaluated using Digital Image Correlation (DIC). DIC is a non-contact digital technique, capable of measuring localized strain during mechanical loading at elevated temperatures. In order to investigate the localized strain across the different weld zones, virtual strain gauges of one millimetre in length were simulated at intervals of one millimetre. It was found that there was a continuous accumulation of strain from the Fusion Line (FL) into the Parent Material (PM). This finding suggested that the HAZ nearest to the PM; which was the Fine Grained Heat Affected Zone (FGHAZ) was the weakest zone as it strained the most. The FL was found to be the least ductile region of the weld as most of the absorbed thermal energy provided during the welding process was used for strain hardening. At elevated temperatures, localised strain occurred at lower strain values than those at RT. This finding suggested that at elevated temperatures there was more thermal energy available for dislocation activation and mobilization. The influence of temperature on the local weld zones were evaluated by extending a specimen, containing just the parent material. A simulation of a virtual strain gauge across the monolithic specimen’s gauge length, revealed that necking occurred at the centre of the specimen which corresponded to the hot zone. In contrast, a simulation of virtual strain gauges across both welds revealed that necking occurred in the region between the HAZ and weld material. This finding inferred that the presence of a weld reduced the strength of the component, as the weld material was the weakest material. Furthermore, the in-service operating conditions was found to have significantly influenced the material behaviour of the welds. A weld that was exposed to a more elevated temperatures and loads, was found to have undergone a higher degree of material degradation, and strained to a larger extent when compared to a weld that was exposed to a more moderate operating environment

Micro, meso and macro materials processing using high-speed liquid projectiles

The objective of this research is developmentof aknowledge base of materials processing by the impact of high-speed liquid projectiles. The work involved experimental study of generation and applications of high-speed liquid projectiles. The projectiles were generated by the launchers, which used gunpowder as an energy source. The experiments were carried out at the low (O.35g of the powder), middle (1.2g) and high (10g to 70g) levels of energy consumptions and at several different launchers modification. Experimental investigation of effect of gun powder mass on energy of projectile was conducted for ductile and brittle targets. An array of experimental techniques for projectile\u27s external ballistics investigation was developed. Laser Particle Velocitimeter (PIN) and high speed filming were used for velocity measurements and visualization of images of water projectiles high speed filming revealed pulsing nature of projectile. A piezoelectric sensor and a pendulum were used to monitor the impact force and the projectile momentum. Range of materials was investigated in this study. Namely, investigation of deformation, forming, micro-forming, and welding of ductile materials was carried out. Demolition and boring of brittle materials was performed. Modes and mechanisms of deformation of ductile and brittle materials were studied and explained. High plasticity, high rate of deformation, temperature at the impact zone, hardness and micro-hardness distribution and degree of deformation work for ductile materials were determined and materials behavior knowledge base needed for materials processing was acquired. Modes and mechanisms of failure of ductile, brittle and composite materials were studied. Fractography study revealed three mechanisms: ductile overload fracture, brittle fracture and combination of the two. Six failure modes: brittle fracture, radial fracture, ductile hole growth, plugging, fragmentation and petaling were identified. An array of material processing operations using high speed projectiles impact was investigated. Full scale experimental investigation of terminal ballistics of high speed water projectiles was performed. Material processing operations included: piercing of metals, piercing of composite targets, explosive set ups neutralization, demolition of brittle materials, boring of granite and marble, punching of steel plates, complex shape punching in steel, forging of metals on macro and meso scale. Mechanisms of punching and forming of metals were identified and proposed. Welding of similar and dissimilar metals was conducted and high potential for novel stitch and spot welding formations was confirmed. Micro scale materials processing investigation involved range of studies. Submilimeter geometry scale forming of metals, fine stamping, micron scale forming and micron scale extrusion investigation were conducted and validation of novel technologies was achieved. Full scale topography and surface characterization of generated geometries was conducted and obtained quality proved to be at a competitive level with existing technologies. State of the art methods were used for investigation of generated samples. Scanning electron microscopy, infinite focus microscopy, 3 D digital microscopy, optical microscopy, 3-D digital profiler, Knoop and Vickers micro hardness testers, nano hardness indenter were used for characterization of generated samples. Full scale characterization on all levels of conducted materials processing was conducted and effect of high speed water projectile impact on mechanical properties of impacted materials was quantified and presented. Investigation of peculiarities of impact based micro-forming was conducted. The info acquired as result of investigation of geometry and topography of micro-forming processing. Accuracy of micro scale deformation was estimated, particularly it was shown that deviation of actual part from the die was at the acceptable level. Also was shown that size of generated parts was rather stable and roughness and waviness of the generated surfaces was in the acceptable range. The foundation of knowledge base for liquid based forming, welding and demolition processes was developed and the process technology will be developed on the base of the acquired knowledge. Theory of impact based high rate material deformation was enhanced. The emerging industrial scale demolition, forming and welding technologies will utilize the acquired knowledge

Crack paths under mixed mode loading

Long fatigue cracks that initially experience mixed mode displacements usually change direction in response to cyclic elastic stresses. Eventually the cracks tend to orient themselves into a pure mode I condition, but the path that they take can be complex and chaotic. In this paper, we report on recent developments in techniques for tracking the crack path as it grows and evaluating the strength of the mixed mode crack tip stress field

An Acoustic Emission Technique for Monitoring the Liquefied Natural Gas Cargo Tank

Increase in the market of supersized LNG (Liquefied Natural Gas) vessel, with doubled walled cargo tanks, has led to concerns regarding their safe operations. If both the primary and secondary wall of the cargo tank fails simultaneously, the hull of the vessel can be exposed to the LNG’s. This has the potential to cause brittle failure of the hull structure. This research presents a new Acoustic Emission (AE) technique that can be implemented to monitor the structural condition of the primary wall in the LNG cargo tank. The presented technique is able to provide information regarding critical damage so that appropriate maintenance can be carried out to avoid catastrophic failure. Acoustic Emission (AE) is a passive Non-Destructive Testing (NDT) technique, employed to identify critical damage in structures before failure can occur. Currently, AE monitoring is carried out by calculating the features of the waveform received by the AE sensor. User defined settings (i.e. timing and threshold) in the AE data acquisition system significantly affects many traditional AE features such as count, energy, centroid frequency, rise-time and duration. In AE monitoring, AE features are strongly related to the damage sources. Therefore, AE features, calculated due to inaccurate user defined acquisition settings can result in inaccurately classified damage sources. The new AE technique presented in this study is based on an AE feature of the waveform, which is independent of some user defined parameter (i.e. timing and threshold) used in the AE data acquisition system, unlike many traditional AE features. The presented AE feature is referred to as AE entropy in this research and is a measure of randomness in the waveform calculated using quadratic Renyi’s entropy. The effectiveness of AE entropy is evaluated by comparing it and traditional AE features under ideal conditions for a range of varying acquisition settings. Unlike the traditional feature, the AE entropy showed no variance with the acquisition settings and was effective in identifying different waveform shapes. The AE entropy was validated through fatigue and tensile tests on coupon specimens of austenitic stainless steel (material of the primary wall). The result suggested that AE entropy is effective in identifying the critical damages in austenitic stainless steel, irrespective of the data acquisition settings. Since AE entropy reduces the human involvement with the data acquisition system and can identify damages, it has the potential to be implemented in the commercial AE data acquisition system

Improvements in ultrasonically assisted turning of TI 15V3Al3Cr3Sn

Titanium alloys have outstanding mechanical properties such as high hardness, a good strength-to-weight ratio and high corrosion resistance. However, their low thermal conductivity and high chemical affinity to tool materials severely impairs their machinability with conventional techniques. Conventional machining of Ti-based alloys is typically characterized by low depth of cuts and relatively low feed rates, thus adversely affecting the material removal rates (MRR) during the machining process. Ultrasonically assisted turning (UAT) is an advanced machining technique, in which ultrasonic vibration is superimposed on a cutting tool. UAT was shown to improve machinability of difficult-to-machine materials, such as ceramics, glass or hard metals. UAT employment in the industry is, however, currently lacking due to imperfect comprehensive knowledge on materials‘ response and difficulties in obtaining consistent results. In this work, significant improvements in the design of a UAT system were performed to increase dynamic and static stiffness of the cutting head. Concurrent improvements on depth-of-cut controls allowed precise and accurate machining operations that were not possible before. Effects of depth of cut and cutting speed were investigated and their influence on the ultrasonic cutting process evaluated. Different cutting conditions -from low turning speeds to higher recommended levelwere analysed. Thermal evolution of cutting process was assessed, and the obtained results compared with FE simulations to gain knowledge on the temperatures reached in the cutting zone. The developed process appeared to improve dry turning of Ti-15-3-3-3 with significant reduction of average cutting forces. Improved surface quality of the finished work-piece was also observed. Comparative analyses with a conventional turning (CT) process at a cutting speed of 10 m/min showed that UAT reduced the average cutting forces by 60-65% for all levels of ap considered. Temperature profiles were obtained for CT and UAT of the studied alloy. A comparative study of surface and sub-surface layers was performed for CT- and UAT-processed work-pieces with notable improvements for the UAT-machined ones. Two- to three-fold reductions of surface roughness and improvements of other surface parameters were observed for the UAT- machined surfaces. Surface hardness for both the CT- and UAT-machined surfaces was investigated by microindentation. The intermittent cutting of the UAT-process resulted in reduction of hardening of the sub-surface layers. Optical and electronic metallographic analyses of cross-sectioned work-pieces investigated the effect of UAT on the grain structure in material‘s sub-surface layers. Backscatter electron microscopy was also used to evaluate the formation of α-Ti during the UAT cutting process. No grain changes or α-precipitation were observed in both the CT- and UAT-machined work-pieces