This research explores the use of two additive manufacturing processes for the fabrication of multi-material structures. Ultrasonic consolidation (UC) and laser- engineered net shaping (LENS) processes were used for parallel systematic investigations of the process parameters and methodologies for the development of multi-material structures. The UC process uses ultrasonic energy at low temperature to bond metallic foils. A wide range of metallic materials including nickel; titanium; copper; molybdenum; tantalum; MetPreg®; silver; stainless steel; and aluminum alloys 1100, 3003, and 6061 were bonded in different combinations. Material domains are inherently discrete in ultrasonically consolidated structures. The mechanical properties of some of the bonded structures were characterized to lay the groundwork for their real-life applications. LENS uses a laser beam to deposit metallic powder materials for the fabrication of fully dense structures. Mechanical testing was used to characterize the flexural and tensile properties of dual-material structures made of Ti6Al4V/10wt%TiC composite and Ti6Al4V materials. Experimental results show that the strength of transition joints in multi-material structures significantly depends on the joint design. Dual-material minimum weight structures, representing geometrically and materially complex structures, were fabricated using the results of the process parameters and fabrication methodologies developed in this work. The structures performed well under loading test conditions. It shows that function-specific multi-material structures ultrasonically consolidated and LENS fabricated can perform well in real-life applications.

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.

The goal of this dissertation is to develop a model relating LENSâ„¢ process parameters to deposited thickness, incorporating the effect of substrate heating. A design review was carried out, adapting the technique of functional decomposition borrowed from axiomatic design. The review revealed that coupling between the laser path and laser power caused substrate heating. The material delivery mechanism was modeled and verified using experimental data. It was used in the derivation of the average deposition model which predicted deposition based on build parameters, but did not incorporate substrate heating. The average deposition model appeared capable of predicting deposited thickness for single line, 1- layer and 2-layer builds, performing best for the 1- layer builds which were built under essentially isothermal conditions. This model was extended to incorporate the effect of substrate heating, estimated using an energy partition approach. The energy used for substrate heating was modeled as a series of timed heating events from an instantaneous point heat source along the path of the laser. The result was called the spatial deposition model, and was verified using the same set of experimental data. The model appeared capable of predicting deposited thickness for single line, 1- layer and 2- layer builds and was able to predict the characteristic temperature rise near the borders as the laser reversed direction.

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A compositionally graded Ti-xCr (10≤x≤30 wt%) alloy has been fabricated using Laser Engineered Net Shaping (LENSTM) to study the microstructural phase evolution along a compositional gradient in both as-deposited and heat treated conditions (1000°C followed by furnace cooling or air cooling). The alloys were characterized by SEM BSE imaging, XRD, EBSD, TEM and micro-hardness measurements to determine processing-structure-property relations. For the as-deposited alloy, α-Ti, β-Ti, and TiCr2 (C15 Laves) phases exist in varying phase fractions, which were influential in determining hardness values. With the furnace cooled alloy, there was more homogeneous nucleation of α phase throughout the sample with a larger phase fraction of TiCr2 resulting in increased hardness values. When compared to the air cooled alloy, there was absence of wide scale nucleation of α phase and formation of ω phase within the β phase due to the quicker cooling from elevated temperature. At lower concentrations of Cr, the kinetics resulted in a diffusionless phase transformation of ω phase with increased hardness and a lower phase fraction of TiCr2. In contrast at higher Cr concentrations, α phase separation reaction occurs where the β phase is spinodally decomposed to Cr solute-lean β1 and solute-rich β2 resulting in reduced hardness.

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