Donald L. Erdman

Structure and mechanical behavior of Big Area Additive Manufacturing (BAAM) materials

Purpose This paper aims to investigate the deposited structure and mechanical performance of printed materials obtained during initial development of the Big Area Additive Manufacturing (BAAM) system at Oak Ridge National Laboratory. Issues unique to large-scale polymer deposition are identified and presented to reduce the learning curve for the development of similar systems. Design/methodology/approach Although the BAAM’s individual extruded bead is 10-20× larger (∼9 mm) than the typical small-scale systems, the overall characteristics of the deposited material are very similar. This study relates the structure of BAAM materials to the material composition, deposition parameters and resulting mechanical performance. Findings Materials investigated during initial trials are suitable for stiffness-limited applications. The strength of printed materials can be significantly reduced by voids and imperfect fusion between layers. Deposited material was found to have voids between adjacent beads and micro-porosity within a given bead. Failure generally occurs at interfaces between adjacent beads and successive layers, indicating imperfect contact area and polymer fusion. Practical implications The incorporation of second-phase reinforcement in printed materials can significantly improve stiffness but can result in notable anisotropy that needs to be accounted for in the design of BAAM-printed structures. Originality/value This initial evaluation of BAAM-deposited structures and mechanical performance will guide the current research effort for improving interlaminar strength and process control.

High-Temperature Galling Characteristics of TI-6AL-4V with and without Surface Treatments

Galling is a severe form of surface damage in metals and alloys that typically arises under relatively high normal force and low sliding speed and in the absence of effective lubrication. It can lead to macroscopic surface roughening and seizure. The occurrence of galling can be especially problematic in high-temperature applications like diesel engine exhaust gas recirculation system components and adjustable turbocharger vanes, because suitable lubricants may not be available, moisture desorption promotes increased adhesion, and the yield strength of metals decreases with temperature. Oxidation can counteract these effects to some extent by forming lubricative oxide films. Two methods to improve the galling resistance of titanium alloy Ti-6Al-4V were investigated: (a) applying an oxygen diffusion treatment and (b) creating a metal–matrix composite with TiB2 using a high-intensity infrared heating source. A new oscillating three-pin-on-flat, high-temperature test method was developed and used to characterize galling behavior relative to a cobalt-based alloy (Stellite 6B, HP Alloys, Windfall, IN). The magnitude of the oscillating torque, the surface roughness, and observations of surface damage were used as measures of galling resistance. Due to the formation of lubricative oxide films, the galling resistance of the Ti alloy at 485°C, even nontreated, was considerably better than it was at room temperature. The infrared (IR)-formed composite displayed reduced surface damage and lower torque than the substrate titanium alloy.

NRSF2 Load Frame: Design, Control, and Testing

A neutron diffraction load frame has been constructed for the Second Generation Neutron Residual Stress Mapping Facility (NRSF2) at the High Flux Isotope Reactor, Oak Ridge National Laboratory. The load frame is designed to study micromechanical behaviour of materials under applied tensile or compressive loads and to characterize specimens with stress risers (e.g., cracks) while under load, in combination with the existing mapping capabilities at NRSF2.

Constrained Thermal Fatigue Performance of Several Cast Ferrous Alloys

An experimental setup that was utilized to evaluate the constrained thermal fatigue (CTF) behavior of several cast ferrous alloys is described. The tests performed allowed the assessment of the relative performance of different materials in CTF loading. The stable hysteresis loop of the individual CTF tests further allowed the development of a unified parameter that determined the level of inelastic (plastic and creep) deformation and the CTF life under those conditions. A CTF life prediction methodology for cast ferrous alloys is outlined.

The Mechanical Response Evaluation of Advanced Claddings During Proposed Reactivity Initiated Accident Conditions

This study investigates the failure mechanisms of advanced oxidation resistant FeCrAl nuclear fuel cladding at high-strain rates, similar to conditions characteristic of design basis reactivity initiated accidents (RIAs). During a postulated RIA, the nuclear fuel cladding may be subjected to complex loading which can cause multiaxial strain states ranging from plane-strain to equibiaxial tension. To achieve those accident conditions, the samples were deformed by the expansion of high strength Inconel alloy tube under pre-specified pressure pulses, simulating strains rates occurring in a postulated RIA. The mechanical response of the advanced claddings, in the unirradiated state with ample ductility, was compared to that of hydrided zirconium-based nuclear fuel cladding. The hoop strain evolution pulses were collected in situ; the permanent diametral strains of both accident tolerant fuel (ATF) claddings and the current nuclear fuel alloys were determined after rupture. Both zirconium-based alloys and FeCrAl alloys exhibited ductile behavior. FeCrAl model alloys without microstructural control and strengthening mechanism were used in this demonstration study that showed reduced diametral strain (less than 0.15) compared to the diametral strain for the unirradiated zirconium-based alloy (approximately 0.2).