The potential of various manufacturing methods was evaluated for producing nickel-titanium graded composition material. The selected test case examined attachment brackets that join nickel-based metallic thermal protection systems to titanium-based launch vehicle structure. The proposed application would replace nickel-based components with graded composition components in an effort to alleviate service induced thermal stresses. Demonstration samples were produced by laser direct metal deposition, flat wire welding, and ultrasonic consolidation. Microstructure, general bond quality, and chemistry were evaluated for the components. IntroductionThe ability to fabricate bulk materials with graded composition offers the potential for unique solutions to engineering problems and offers advantages over conventional materials and traditional composites. Research and development of functionally graded materials has largely focused on surface coatings or modifications of surface layers to enhance wear, optical, or electrical properties. Application of functional grading in bulk form offers the ability to create components with tailored chemistries or microstructures which enables properties to be selectively enhanced in critical regions. The current study examines the potential of various manufacturing methods for producing bulk material with composition graded from nickel-based Inconel 718 to titanium alloy Ti -6A1 -4V (Ti-6-4). The test case for application of such material is an attachment bracket that joins nickel-based metallic thermal protection systems to titanium-based launch vehicle structure. The goal of the current study was to evaluate the compatibility of each method with the materials selected, examine the resulting microstructures and chemistries, and characterize any reaction products.

Aluminum-lithium (Al-Li) alloys offer attractive combinations of high strength and low density for aerospace structural applications. However, a tendency for delamination fracture has limited their use. Identification of the metallurgical mechanisms controlling delamination may suggest processing modifications to minimize the occurrence of this mode of fracture. In the current study of Al-Li alloy 2090 plate, high quality electron backscattered diffraction (EBSD) information has been used to evaluate grain boundary types exhibiting delamination fracture and characterize microtexture variations between surrounding grains. Delamination was frequently observed to occur between variants of the brass texture component, along near-Σ3, incoherent twin boundaries. EBSD analyses indicated a tendency for intense deformation along one side of the fractured boundary. A through-thickness plot of grain-specific Taylor factors showed that delaminations occurred along boundaries with the greatest difference in Taylor factors. Together, these suggest a lack of slip accommodation across the boundary, which promotes significantly higher deformation in one grain, and stress concentrations that result in delamination fracture.

The electron beam freeform fabrication (EBF3) layer-additive manufacturing process has been developed to directly fabricate complex geometry components. EBF3 introduces metal wire into a molten pool created on the surface of a substrate by a focused electron beam. Part geometry is achieved by translating the substrate with respect to the beam to build the part one layer at a time. Tensile properties have been demonstrated for electron beam deposited aluminum and titanium alloys that are comparable to wrought products, although the microstructures of the deposits exhibit features more typical of cast material. Understanding the metallurgical mechanisms controlling mechanical properties is essential to maximizing application of the EBF3 process. In the current study, mechanical properties and resulting microstructures were examined for aluminum alloy 2219 fabricated over a range of EBF3 process variables. Material performance was evaluated based on tensile properties and results were compared with properties of Al 2219 wrought products. Unique microstructures were observed within the deposited layers and at interlayer boundaries, which varied within the deposit height due to microstructural evolution associated with the complex thermal history experienced during subsequent layer deposition. Microstructures exhibited irregularly shaped grains, typically with interior dendritic structures, which were described based on overall grain size, morphology, distribution, and dendrite spacing, and were correlated with deposition parameters. Fracture features were compared with microstructural elements to define fracture paths and aid in definition of basic processingmicrostructure- property correlations.

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