The overall goal of this work is to predict the mechanical response of an as-built Ti-6Al-4V directed energy deposition component by a dislocation mechanics-based internal state variable model based on the component's geometry and processing parameters. Previous research has been performed to connect additive manufacturing (AM) process parameters including laser power and scanning strategy to different aspects of part quality, such as porosity, mechanical properties, fatigue life, microstructure, residual stresses, and distortion. The lack of predictive capabilities to fully estimate residual stresses and distortion within parts produced via AM have hindered part qualification; however, modeling the AM process can aide in process and geometry optimization compared to traditional trial-and-error methods. The presence of unwanted thermally induced residual stresses and distortion can lead to tolerancing issues, reduced fatigue life, and decreased mechanical performance compared to similar components fabricated with traditional manufacturing methods such as casting and machining. A three-dimensional thermomechanical finite element model calibrated using dual-wave pyrometer thermal image datasets along with temperature- and strain rate-dependent mechanical data is utilized for this work. The purpose of this work is to understand the relationship between a component's temperature history and its resultant distortion and residual stresses.

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Ultrafast lasers are valuable research and manufacturing tools. The ultrashort pulse duration is comparable to electron-lattice relaxation times, yielding unique interactions with matter, particularly nonlinear absorption, melting, and ablation. The field of ultrafast laser manufacturing is rapidly evolving with advances in related laser technologies. The applications of ultrashort pulse lasers in additive manufacturing aim to fill gaps left by conventional techniques especially on the nano- and micro-scale. Concurrently, uptake of ultrafast fiber lasers for micromachining has increased, and may replace the Ti:Sapphire laser as the ultrafast laser of choice. Both additive and subtractive manufacturing are accomplished with ultrafast lasers which presents the possibility of hybrid, all-in-one devices using a single laser source. As one such combination of laser techniques, ultrashort pulse surface modification of additively manufactured metals is an area of limited investigation. This thesis aims to address the ever-changing landscape of ultrafast laser manufacturing by 1) reviewing ultrafast laser additive manufacturing techniques and recent advancements 2) comparing the design, operation, and micromachining potential of a commercial ultrafast Ti:Sapphire and ultrafast fiber laser, and 3) investigating femtosecond ablation of as-printed additively manufactured Ti-6Al-4V at a range of parameters to test the feasibility of surface feature control. Ultrafast laser additive manufacturing is still in its infancy with mostly niche applications. The ultrafast fiber laser architecture is found to deliver a platform that is easier to operate and maintain and has superior micromachining throughput relative to Ti:Sapphire lasers. In our experimental work, five main surface morphologies are obtained by femtosecond ablation of a rough Ti-6Al-4V surface: laser-induced periodic surface structures (LIPSS), undulating grooves, micro-ripples, grooves, and micro-cavities. Transitions between ablation regimes and evolutions of the surface under increasing pulse energy and number of pulses are observed. These patterns allow for control over the surface geometry without the need for post-printing polishing. / Thesis / Master of Applied Science (MASc) / Ultrafast pulsed lasers of <10 picoseconds pulse duration are commonly used to modify, melt, or ablate materials. As an important research and manufacturing tool, ultrafast lasers and techniques have seen great change in the past two decades. Additive manufacturing has emerged as an area in which ultrafast lasers are becoming increasingly prevalent. To make sense of this continuously evolving landscape, this thesis 1) reviews ultrafast laser additive manufacturing techniques, applications, and advances towards industrial use and commercialisation, 2) compares the setup, operability, and characteristics for two ultrafast laser designs, and 3) investigates the surfaces produced by ultrafast laser irradiation of an additively manufactured titanium alloy part. The surface morphologies that are produced are categorised into five main patterns: laser-induced periodic surface structures, undulating grooves, micro-ripples, grooves, and micro-cavities. Each is a distinct pattern that may allow for tuning of the surface properties with respect to the wettability and biocompatibility.

A detailed metallurgical analysis was conducted to correlate microstructure to axial strain-controlled high cycle fatigue of Ti-6Al-4V forgings made from cold isostatic pressed and sintered preforms of blended element powder metallurgy (BEPM) incorporating hydrogenated titanium. Analysis included fractographic examination by SEM, microstructure examination by optical microscopy, texture examination via EBSD, chemical analysis and fatigue strain mapping via digital image correlation (DIC). From a literature review and observations of findings, factors that were of primary concern were: maximum pore diameter, primary α volume fraction, primary α width, primary α particle count, oxygen equivalency (OE) and texture of the α phase. The primary α volume fraction was found to have the single most influential effect on fatigue, whereby decreasing volume fraction increased fatigue life. Using statistical analysis, multivariable regression analyses were performed to evaluate combinations of predictors on fatigue life. The resulting outcome of volume fraction and maximum pore diameter, having a 3.3 to 1 weighting, was the most significant at predicting the fatigue response. Improving fatigue life of forged Ti-6Al-4V made from BEPM should thus be primarily focused on microstructure refinement. It is suggested future experimentation also consider the effects of the number of primary α particles and OE when modeling fatigue strength. / Thesis / Master of Applied Science (MASc) / The mechanical properties of a metal are dictated primarily by the metal’s microstructure. The microstructure of a metal made from metal powder that has been pressed and heated to bind the powder together generally contains residual porosity. This generally leads to a reduction in metal fatigue resistance versus a metal that is pore-free. In studying metal fatigue of a titanium alloy made from metal powder, the resistance to metal fatigue varied considerably and did not achieve the same resistance of pore-free material, despite the titanium alloy in question being nearly pore-free. This titanium alloy was studied to determine what the cause of the poor metal fatigue resistance was. Through a methodical testing program, it was determined that volume fraction of a particular crystalline phase in the microstructure was more damaging than the pores themselves.

Layer by layer manufacturing (additive manufacturing, AM) of metals is emerging as an alternative to conventional subtractive manufacturing with the goal of enabling near net-shape production of complex part geometries with reduced material waste and shorter lead times. Recently this field has experienced rapid growth through industrial adaptation but has simultaneously encountered challenges. One such challenge is the ability of AM metal to withstand loading conditions ranging from static loads to complex multiaxial thermo-mechanical fatigue loads. This makes fatigue performance of AM materials a key consideration for the implementation of AM in production. This is especially true for AM in the aerospace industry where safety standards are strict. Defects in metal AM materials include rough surfaces, pores and lack-of-fusion (LOF) between build layers. These defects are detrimental to fatigue as they act as local stress concentrators that can give rise to cracks in the material.  Some defects can be avoided by careful build process optimization and/or post-processing but fully eliminating all defects is not possible. Because of this, a need arises for the capability to estimate the fatigue performance of AM produced critical components containing defects. The aim of the thesis is to increase understanding regarding the connection between defect characteristics and the fatigue behaviour in AM produced Ti-6Al-4V. Defect distributions are statistically analysed for use in a simple fracture mechanical model for fatigue life prediction. Other study areas include the impact of post-production treatments such as chemical surface treatments and hot isostatic pressing (HIP) on defects and fatigue behaviour. The thesis constitutes three scientific papers. The AM technique studied in these papers is Electron Beam Melting (EBM) in which an electron beam selectively melts pre-alloyed metal powder. In paper 1, defects were studied using X-ray computed tomography (XCT) and fatigue crack initiation was related to the observed defect distribution. In paper 2, XCT data was used to relate the surface morphology and roughness of post-production treated EBM material to the surface near defect distribution. The connection between this distribution and manufacturing parameter has also been explored. Paper 3 builds on and extends the work presented in paper 1 by including further fatigue testing as well as a method for predicting fatigue life using statistical analysis of the observed defect distribution. The impact of a defect on the fatigue behaviour of the material was found to largely depend on its characteristics and position relative to the surface. Production and post-processing of the material was found to play a role in the severity of this impact. Finally, it was found that a probabilistic statistical analysis can be used to accurately predict the life of the studied material at the tested conditions. / SUDDEN

Additive manufacturing has shown the ability to produce highly complex geometries that are not easily manufactured through traditional means. However, the implications of building these complex geometries regarding thermal history requires more attention. AM process simulations have proven to be computationally expensive and require large amounts of pre-processing to execute. This thesis will start with a review of additive manufacturing along with current modeling efforts. Then, the development of a pre-processing tool for finite element simulations of these processes is presented. It is shown that the pre-processing tool significantly decreases the total time-to-simulation by removing manual steps. Finally, a study using this tool is conducted to analyze the thermal histories of a cube and a cylinder with two different scan strategies and explore differences in resulting thermal history. It is shown that less temperature fluctuations and a lower final temperature result from an offset scan strategy and a cylindrical geometry.

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