Fatigue Performance of Ti-6Al-4V Drilled Plates

Ti-6Al-4V drilled plates were subjected to cyclic loading tests in a previous study, which investigated the machining-induced effects on fatigue [1]. The majority of the specimens failed between low to moderate number of cycles, close to the surface of the hole, around the half thickness plane, with no clear crack initiation point. Differences in the performance of the plates were associated to the initiation of cracks from subsurface microstructure defects. However, a clear explanation of the mode of failure and the mechanisms behind it were not established due to the scattering of the fatigue data and the need for a more detailed examination of deformed microstructures and elastoplastic strain fields. This postulated the main research objective for the current study. A selected number of specimens after the fatigue tests were re-examined to identify the critical defects. The preliminary analysis included surface roughness, micro-hardness, LOM, and SEM of the machining-affected layer. Then, advanced characterization techniques, namely EBSD and FIB, were transformed into semi and fully quantitative methods to identify plastic strain gradients and residual stress profiles within the material. Despite the different drilling conditions, the specimens had similar roughness values. However, the plastic deformation and residual stress profiles within the material were directly related to each other and to the drilling conditions. All specimens displayed strain localization within smaller alpha grains of the sheared underlying microstructure, while tensile twins and slip bands were visible below the heavily deformed and strain hardened zone. Tensile, surface or near-surface stresses were measured for all drilling conditions at different locations around the hole. The results indicated that specimens with lower levels of deformation had superior fatigue performance. Since all specimens had identical defects, though to a different depth, it was deemed necessary to examine the fractured surface of the specimens. A closer inspection of the machined surfaces at the locations of the crack initiation revealed that chip fragments, embedded on the surface of the hole, were the critical defect dominating the fatigue life. Surface smearing and intense drilling marks were generating non-critical cracks. Chip cracking, voids within the chip, surface damage and cavities from the embedment of the chip, and shielding of surface twins by the chip were observed in the specimens. An exact description of the failure mechanism(s) was not possible because all of them appeared to be crack nucleation sites. The current study was concluded at this point providing the groundwork and the tools for future work

Residual stress measurement and structural integrity evaluation of SLM Ti-6Al-4V

Includes bibliographical references.The constant drive toward cleaner, more powerful and more efficient jet turbines in the aerospace industry has narrowed the gap between the aircraft performance envelope requirements and the material limits. The most advanced turbine engines are incredibly complex in design and the weight-saving requirements have placed significant pressure on material capabilities and the manufacturing systems. The next generation of manufacturing methodologies are being developed in the Additive Manufacturing (AM) arena from which Selective Laser Melting (SLM) has emerged as a promising candidate for producing highly complex components. Selective Laser Melting is a laser-based AM technique which builds 3-dimensionsal parts from CAD models in a layerwise fashion..

Control and qualification of titanium welds

This thesis was submitted for the degree of Doctor of Philosophy and awarded by Brunel University.The study was aimed at controlling the weld geometry of thin-plate titanium and one of its alloys (Ti-6Al-4V) by ultrasonic means and qualiFying the metals in the as-welded condition in terms of their grain sizes and mechanical properties. The alignment and symmetry of the weld pools were successfully tested by using ultrasonic shear waves. The grain sizes at the weld fusion zone were found to be related to their ultrasonic attenuation by a mathematical relationship. The temperature effect in locating weld pool radii in titanium was found at temperatures up to 600 °C. The ultrasonic velocity decreased as the temperature increased and the square of temperature affected the rate of change of the ultrasonic velocity. After compensation for the temperature effect, the maximum location error of the weld pool radius was 17 % which was comparable to previous measurement using different techniques.A positive relationship was seen between weld geometry (penetration depth and weld width) and heat input. A welding spectrum for titanium and its alloys of different thicknesses was obtained. Back shielding gas was beneficial in obtaining good welds. Both heat input rate and cooling rate were found to affect the grain size of the weld, with the cooling rate being the dominant factor. The grain size exhibited a Hall-Petch effect on mechanical properties, such as the tensile properties and fracture toughness of the weld. The phase transformation positively contributed to better mechanical properties in most cases, whilst the presence of interstitials worsened tensile properties. A system was developed in this study to utilise the above information and data for possible real-time and closed-loop control of the TIG welding process to give a desirable weld. Specifically, a process control data base was built up using software and a knowledge-based system for acceptable welding parameters, which were determined by acceptable penetration depth, grain size and mechanical properties. An algorithm was successfully written which relates the ultrasonic signal to the penetration depth of the weld. A hardware control circuit was built which took in the ultrasonic signal and converted it to a driving signal to change the welding speed and thereby change cooling rate

Approach to Qualification for Electron Beam Powder Bed Fusion in Ti-6Al-4V

Recent developments in additive manufacturing (AM) show promise for using AM manufactured components in a production setting. However, a crucial step for mass producing AM components is to certify these parts for use. One common method for certifying parts is to manufacture tensile coupons alongside any parts. These coupons are characterized and the results are related to the parts. This causes many researchers to focus on the process-material interactions while neglecting build setup. Another issue related to certification of AM parts is the lack of knowledge in the software calculations for a given process. Original equipment manufacturers (OEM), such as Arcam AB for electron beam powder bed fusion (E-PBF), need secrecy in their software to ensure their scan strategy is protected. Therefore, this practice provides researchers little information or confidence about changes made in process parameters. To provide insight into these areas of variation, the current work can be broken into two parts – (i) understanding how changes in selected process parameters can influence non-selected parameters and (ii) determining the effectiveness of current qualification methods for the E-PBF process.To better understand process parameters, changes in selected process parameters were simulated and compared with the Arcam provided parameter set. Results of these simulations show that speed function variable is only a function of melting time while modifications to the contour passes and surface temperature result in changes to the heat balance. Variations in the heat balance change the cooling rate of as-fabricated material, which causes microstructural evolution in titanium alloys. Preliminary results show that modifying the surface temperature for specific regions can be used to control microstructure.To better understand how build setup can influence parts in a build, build setup variables such as part melt order, build volume, and cross-sectional melt area were modified between two builds. Results of these changes show that performance in test coupons cannot be applied to performance in the other parts since changes in build setup influence each part differently. The current work provide challenges to applying traditional qualification methods to AM fabricated components in hopes that a process-based certification path can be achieved

Ambient temperature creep behavior of spark PLASMA Ti-6Al-4V/TiN composites

Abstract: Ti-6Al-4V alloys are well known for their high strength to weight ratio, corrosion resistance, fatigue and high temperature properties. In this Study, the effect of TiN addition on the mechanical properties of spark plasma sintered Ti-6Al-4V alloys was investigated. Ti-6Al-4V alloy powders with different proportions of TiN (2, 4 and 6%) were produced using Spark Plasma Sintering (SPS). Further, the spark plasma sintered samples were analysed using scanning electron microscopy, X-ray diffraction and optical microscope. The spark plasma sintered samples were then investigated for mechanical behaviours such as creep, erosion-wear and wear response. Creep, modulus of elasticity and hardness properties were investigated using the nanoindentation technique equipped with Berkovich indenter. On the other hand, high velocity solid particle erosion test was used to examine the erosion-wear behavior of Ti-6Al4V alloy with and without TiN additions. The aim and objectives of the research was to understand how different proportions of TiN addition affect the creep and erosion wear response of Ti-6Al-4V-TiN composite, investigate the mechanisms of creep on the using nanoindentation test, and evaluate the mechanism of wear occurring on the nano-composites and to characterize and analyse surface morphologies and fracture mode of SPS Ti6Al-4V-TiN composites. The results revealed that the use of SPS technique and TiN nanoparticles has led to transformation from alpha/beta lamellar to bimodal microstructure. It was also noticed that the presence of TiN improves the mechanical properties of Ti-6Al-4V to some extend and lead to deterioration as the volume fraction of TiN goes to 4% volume fraction. Nanoindentation results revealed that elastic modulus and hardness increased drastically with increase in TiN addition. Increasing the applied load reduced the hardness and elastic modulus. Ti-6Al-4V+ 6% TiN presented improved mechanical properties such as erosion wear, abrasive wear and creep resistance.M.Eng. (Engineering Metallurgy

Thermal Stability of Ultra-Fine Grained Microstructure in Mg and Ti Alloys

This chapter reviews the thermal stability of ultra-fine grained (UFG) microstructure in selected magnesium and titanium-based materials prepared by severe plastic deformation (SPD). The focus is on the wide palette of experimental methods applicable for investigation of microstructural stability. These methods include scanning electron microscopy (SEM), electron backscatter diffraction (EBSD), microhardness measurement, positron annihilation spectroscopy (PAS), and electrical resistance measurement. Microstructural stability of UFG commercially pure (CP) Ti and Ti-6Al-7Nb alloy produced by equal-channel angular pressing (ECAP) is studied ex situ after annealing by SEM, by microhardness measurements, and in situ during heating, by high precision electrical resistance measurements. Both materials show stable UFG structure up to 440°C. Further annealing causes recovery and recrystallization of the microstructure. At 650°C, the microstructure is completely recrystallized. Magnesium alloy AZ31 is prepared by hot extrusion followed by ECAP. UFG microstructure recovers and continuously recrystallizes during annealing. The microstructure of UFG AZ31 alloy is stable up to 170°C and subsequent grain growth is analyzed. Special attention is paid to interpret the activation energy of the grain growth. The superplastic properties of UFG AZ31 alloy are investigated in the temperature range of 170–250°C

The effects of titanium Ti-6Al-4V powders manufactured using electron beam melting (EBM) - additive manufacturing on metallurgical evaluation

Multiple methods of manufacturing Ti-6Al-4V powders for Additive Manufacturing (AM) are available. The effects of the powder quality, properties and post-processing conditions on microstructure and mechanical properties in Electron Beam Melting (EBM) process are investigated in this work. Two powders manufactured using Plasma (PA) and Gas (GA) Atomisation were fully characterised. Test specimens were built using default manufacturer’s (Arcam) parameters and mechanically tensile tested in different post-processing conditions: as built (near net-shape), heat treated using Hot Isostatic Pressing (HIP), and on surface machined. Each build specimen was cut and polished to analyse for porosity, defects, and microstructure. The microstructure of as-built samples was found to be of very fine and acicular morphology due to high solidification rate. HIP heat treatment has been observed to homogenise as-built anisotropic grain microstructure, with reduction and elimination of gas pores and defects for as-built EBM samples. However, this (HIP) also resulted in coarser grain microstructure. Both GA and PA specimens yield strength (YS) and ultimate tensile strength (UTS) measured, with PA found to have higher values in comparison to GA. The study found that lack of fusion/un-melted particles caused lower elongation for as-built PA samples due to un-optimised parameters and process instability. Spherical gas pores (argon trapped) in GA powders and parts were predominately found due to atomisation process thus inherited in as-built parts. Nonetheless, all samples had better and some above the minimum ASTM F294-14 titanium tensile requirement. The PA yield strength and tensile strength of the EBM as-built specimens were 850 and 925 MPa irrespectively, while GA yield strength and tensile strengths were 810 and 887 MPa irrespectively

Characterization of Porosity Defects in Selectively Laser Melted IN718 and Ti- 6A1-4V via Synchrotron X-Ray Computed Tomography

Additive manufacturing (AM) is a method of fabrication involving the joining of feedstock material together to form a structure. Additive manufacturing has been developed for use with polymers, ceramics, composites, biomaterials, and metals. Of the metal additive manufacturing techniques, one of the most commonly employed for commercial and government applications is selective laser melting (SLM). SLM operates by using a high-powered laser to melt feedstock metal powder, layer by layer, until the desired near-net shape is completed. Due to the inherent function of AM and particularly SLM, it holds much promise in the ability to design parts without geometrical constraint, cost-effectively manufacture them, and reduce material waste. Because of this, SLM has gained traction in the aerospace, automotive, and medical device industries, which often use uniquely shaped parts for specific functions. These industries also have a tendency to use high performance metallic alloys that can withstand the sometimes-extreme operating conditions that the parts experience. Two alloys that are often used in these parts are Inconel 718 (IN718) and Ti-6Al-4V (Ti64). Both of these materials have been routinely used in SLM processing but have been often marked by porosity defects in the as-built state. Since large amounts of porosity is known to limit material mechanical performance, especially in fatigue life, there is a general need to inspect and quantify this material characteristic before part use in these industries. One of the most advanced porosity inspection methods is via X-ray computed tomography (CT). CT uses a detector to capture scattered X-rays after passing through the part. The detector images are then reconstructed to create a tomograph that can be analyzed using image processing techniques to visualize and quantify porosity. In this research, CT was performed on both materials at a 30 μm “low resolution” (LR) for different build orientations and processing conditions. Furthermore, a synchrotron beamline was used to conduct CT on small samples of the SLM IN718 and Ti64 specimens at a 0.65 μm “high resolution” (HR), which to the author’s knowledge is the highest resolution (for SLM IN718) and matches the highest resolution (for SLM Ti64) reported for porosity CT investigations of these materials. Tomographs were reconstructed using TomoPy 1.0.0, processed using ImageJ and Avizo 9.0.2, and quantified in Avizo and Matlab. Results showed a relatively low amount of porosity in the materials overall, but a several order of magnitude increase in quantifiable porosity volume fraction from LR to HR observations. Furthermore, quantifications and visualizations showed a propensity for more and larger pores to be present near the free surfaces of the specimens. Additionally, a plurality of pores in the HR samples were found to be in close proximity (10 μm or less) to each other

Microstructural Evolution in Friction Stir Welding of Ti-5111

Titanium and titanium alloys have shown excellent mechanical, physical, and corrosion properties. To address the needs of future naval combatants, this research examines an alternative joining technology, friction stir welding (FSW). Friction stir welding uses a non-consumable tool to generate frictional heat to plastically deform and mix metal to form a consolidated joint. This work focuses on FSW of Ti-5111 (Ti-5Al-1Sn-1Zr-1V-0.8Mo), a near alpha alloy. This study aims to gain a fundamental understanding of the relationship between processing parameters, microstructure, and mechanical properties of experimental 12.7mm and 6.35mm Ti-5111 friction stir welds. The resulting weld microstructure shows significant grain refinement within the weld compared to the base metal. The weld microstructures show a fully lamellar colony structure with peak welding temperatures exceeding beta transformation temperature. The friction stir weld shows material texture strengthening of the BCC F fiber component before transformation to D2 shear texture in the stir zone. Transmission electron microscopy results of the base metal and the stir zone show a lath colony-type structure with low dislocation density and no lath grain substructure. In situ TEM heating experiments of Ti-5111 friction stir welded material show transformation to the high temperature beta phase at significantly lower temperatures compared to the base metal. Thermal and deformation mechanisms within Ti-5111 were examined through the use of thermomechanical simulation. Isothermal constant strain rate tests show evidence of dynamic recrystallization and deformation above beta transus when compared with the FSW thermal profile without deformation. Subtransus deformation shows kinking and bending of the existing colony structure without recrystallization. Applying the friction stir thermal profile to constant strain rate deformation successfully reproduced the friction stir microstructure at a peak temperature of 1000ºC and a strain rate of 10/s. These results provide unique insight into the strain, strain rates, and temperatures regime within the process. Finally, the experimental thermal and deformation fields were compared using ISAIAH, a Eulerian based three-dimensional model of friction stir welding. These results are preliminary but show promise for the ability of the model to compute thermal fields for material flow, model damage prediction, and decouple texture evolution for specific thermomechanical histories in the friction stir process

SMART ADDITIVE MANUFACTURING: IN-PROCESS SENSING AND DATA ANALYTICS FOR ONLINE DEFECT DETECTION IN METAL ADDITIVE MANUFACTURING PROCESSES

The goal of this dissertation is to detect the incipient flaws in metal parts made using additive manufacturing processes (3D printing). The key idea is to embed sensors inside a 3D printing machine and conclude whether there are defects in the part as it is being built by analyzing the sensor data using artificial intelligence (machine learning). This is an important area of research, because, despite their revolutionary potential, additive manufacturing processes are yet to find wider acceptance in safety-critical industries, such as aerospace and biomedical, given their propensity to form defects. The presence of defects, such as porosity, can afflict as much as 20% of additive manufactured parts. This poor process consistency necessitates an approach wherein flaws are not only detected but also promptly corrected inside the machine. This dissertation takes the critical step in addressing the first of the above, i.e., detection of flaws using in-process sensor signatures. Accordingly, the objective of this work is to develop and apply a new class of machine learning algorithms motivated from the domain of spectral graph theory to analyze the in-process sensor data, and subsequently, detect the formation of part defects. Defects in additive manufacturing originate due to four main reasons, namely, material, process parameters, part design, and machine kinematics. In this work, the efficacy of the graph theoretic approach is determined to detect defects that occur in all the above four contexts. As an example, in Chapter 4, flaws such as lack-of-fusion porosity due to poor choice of process parameters in additive manufacturing are identified with statistical accuracy exceeding 80%. As a comparison, the accuracy of existing conventional statistical methods is less than 65%. Advisor: Prahalada Ra