Strengthening Mechanisms in Microtruss Metals

Microtrusses are hybrid materials composed of a three-dimensional array of struts capable of efficiently transmitting an externally applied load. The strut connectivity of microtrusses enables them to behave in a stretch-dominated fashion, allowing higher specific strength and stiffness values to be reached than conventional metal foams. While much attention has been given to the optimization of microtruss architectures, little attention has been given to the strengthening mechanisms inside the materials that make up this architecture. This thesis examines strengthening mechanisms in aluminum alloy and copper alloy microtruss systems with and without a reinforcing structural coating. C11000 microtrusses were stretch-bend fabricated for the first time; varying internal truss angles were selected in order to study the accumulating effects of plastic deformation and it was found that the mechanical performance was significantly enhanced in the presence of work hardening with the peak strength increasing by a factor of three. The C11000 microtrusses could also be significantly reinforced with sleeves of electrodeposited nanocrystalline Ni-53wt%Fe. It was found that the strength increase from work hardening and electrodeposition were additive over the range of structures considered. The AA2024 system allowed the contribution of work hardening, precipitation hardening, and hard anodizing to be considered as interacting strengthening mechanisms. Because of the lower formability of AA2024 compared to C11000, several different perforation geometries in the starting sheet were considered in order to more effectively distribute the plastic strain during stretch-bend fabrication. A T8 condition was selected over a T6 condition because it was shown that the plastic deformation induced during the final step was sufficient to enhance precipitation kinetics allowing higher strengths to be reached, while at the same time eliminating one annealing treatment. When hard anodizing treatments were conducted on O-temper and T8 temper AA2024 truss cores, the strength increase was different for different architectures, but was nearly the same for the two parent material tempers. Finally, the question of how much microtruss strengthening can be obtained for a given amount of parent metal strengthening was addressed by examining the interaction of material and geometric parameters in a model system.

Keywords: microtruss
strengthening
copper
C11000
aluminum
AA2024
cellular
metal
mechanical
properties
compressive strength
densification energy
modulus
work hardening
precipitation hardening
heat treatment
nanocrystalline coating
aluminum oxide coating
coating
anodizing
electrodeposition
strengthening mechanisms
geometry
pyramidal
compression
buckling
architecture
truss
uniaxial compression
perforation
Ramberg-Osgood
Holloman
nickel-iron
annealing
relative density
periodic cellular
hybrid
composite
microhardness
stretch bend
peak strength
scanning electron microscopy
stress
strain
bending
stretching
failure
yielding
column
critical buckling strength
critical buckling stress
yield strength
forming
Advisor: Hibbard, Glenn D.
McLean, Alexander