Ultrasonic softening effects on materials ManufacturingX Lab
It is known that simultaneous application of ultrasonic (UA) energy during metal plastic deformation can effectively reduce the material flow stress. However, the fundamental physical mechanisms remain elusive. In this project, ultrasonically assisted micro-tensile tests with in situ high-speed micro-imaging and digital image correlation analysis are performed, along with post mortem multiscale microstructure characterizations. In addition, a dislocation density-based crystal plasticity constitutive material model will be developed to advance the fundamental understanding of material softening principles induced by UA vibration. In collaboration with University of Michigan, the project will enhance incremental sheet forming (ISF) process by effectively incorporating UA vibration to form complex free form geometries.
Results of UA-assisted micro-tensile tests on pure copper and TRIP-assisted steel were published in Materials Science and Engineering: A and Metallurgical and Materials Transactions A.
In Transformation-Induced Plasticity (TRIP) steel, drop in yield point as well as decrease in flow stress is observed. Significant decrease in work hardening rate is observed in stage 2 where TRIP effect occurs.
Stress-strain response under AU
Digital image correlation (DIC) analysis
Yield plateau is observed when UA is initiated around yield point. DIC analysis during stress plateau reveals that UA induces the formation of Lüders-like deformation bands(frame II). Without UA, higher strain builds up in a broad region in the middle of gauge section, while in UA condition, localized strain can be observed at both ends of gauge section, as marked in red dash line. In comparison, after yield plateau at frame iii, the difference in strain distribution can no longer be observed.
UA effect on microstructure
A considerable decrease in low-angle grain boundaries (LAGBs) length can be noticed when comparing UA samples with the regular ones at the same strain level. GNDs are stored during plastic deformation and directly related to the gradient in local misorientation. It is also noteworthy that the average density of GND drops significantly with UA. The reduction in GND density is most clearly seen in ferrite matrix. This can be explained by higher dislocation mobility under oscillating stress wave. Dislocations can travel longer distances and the chance for dipole annihilation is increased.
This project is sponsored by National Science Foundation.