Session O4: MS: Process and Structure Effects on Dynamic Response

Laser welding and powder bed fusion of uranium-6 wt. pct. niobium

The metals laser powder bed fusion process is known for creating structures with architectural complexity through the use of thin layers (\textless 100 \textmu m), fine powder and small laser spot size. Builds are often custom and the different scanning strategies, part geometries, number of builds per plate, layer size, laser parameters and material properties all contribute to complex, unique thermal conditions and solidification behavior from part to part. In alloys, crystallographic structure is dependent upon composition, which is dependent upon cooling rate. Understanding the degree of segregation which occurs during processing is critical to the development of post-processing solutionizing heat treatments. We address this issue in uranium-6 wt. pct. niobium (U-6Nb), which forms various metastable phases as a function of Nb concentration, through (1) performing laser welding of as-cast U-6Nb, (2) employing conductive and convective heat transfer models to calculate cooling rate across these welds, (3) building the microstructure (cell spacing/secondary arm spacing)-cooling rate relationship for this alloy, and (4) identifying the degree of segregation in multiple welds. The laser powder bed fusion microstructures are examined and the cooling rates which resulted in such microstructural development and resulting segregation are determined, using relationships built through the as-cast welding study. We demonstrate various heat treatments and their effect on homogenization, with a view of discussing microstructure-property behavior. This work was performed under the auspices of the U.S. Department of Energy by Lawrence Livermore National Laboratory under Contract DE-AC52-07NA27344 and has been assigned the document release ID{\#}LLNL-ABS-767978.   

Anelastic Effects on Reverse Loading -- Connection to Evolving Dislocation Structure

Many experiments of interest, particularly those meant to probe flow strength response, involve reversals in the loading condition. Transients during the material response during these reversals can significantly influence experimental observables. In materials for which deformation is mediated by the motion of dislocations, aspects of the transients are thought to be associated with motion of dislocations during the transient; and the strains so produced are sometimes described as being anelastic. Appropriate formulations for anelastic response can be distinct from standard models used to capture plasticity response over larger monotonic strains. We present results from a new anelasticity formulation and its numerical implementation. In this formulation, the anelastic strain production is influenced by the characteristics of the dislocation network, with these characteristics evolving over the course of material deformation.   

Effects of manufacturing processes on the dynamic properties of gold.

As part of a study into the effects of manufacturing processes, a series of experiments is being conducted on the dynamic properties of gold. In the present experiments, samples were prepared by two different techniques---casting and rolling (wrought) and continuous electrodeposition (ED). The wrought samples have typical microstructure, with somewhat elongated grains that appear similar in both in-plane- and transverse-view micrographs. The ED samples, on the other hand, have highly elongated grains oriented perpendicular to the plane of the samples, extending nearly through the thickness of the samples, with very different morphologies apparent in different micrographic orientations. Experiments were designed to subject both types of sample to identical impact conditions, with standard time-resolved Doppler velocimetry techniques used to obtain shock transit times and free-surface velocity histories, yielding data on the shock Hugoniots and spall stresses of the materials. The results will be presented and implications discussed.   

Dynamic deformation of additively manufactured lattice structures

Recent advances in additive manufacturing have allowed for the production of new structures with hierarchical topologies. As in any new material development cycle what remains to be investigated is the macroscale dynamic deformation response, which in this instance is expected to be quite unlike that of bulk, homogeneous solids. Dynamic deformation processes can now be observed in-situ using recent advances in time-resolved phase contrast imaging in conjunction with gas-gun drivers at the Dynamic Compression Sector. Recent work by Branch et al. demonstrated the formation of jets from the target material in a simple cubic ``woodpile'' structure. Here, we present results from an experimental study of this phenomenon over a wide range of velocities from 0.2 to \textgreater 2 km/s. In parallel with the experiments, direct numerical modeling relate the origins of the jets back to the underlying stress state in the target and projectile.