Investigation of Acoustic Softening and Its Application in Ultrasonic Assisted Incremental Sheet Forming
Cheng, Randy
2022
Abstract
Agile manufacturing processes are in demand across a variety of sectors and applications. The ability to go from design to prototype to full production in a shorter amount of time and at lower cost represents the goal manufacturing companies set every year. Incremental sheet forming (ISF) is a process that has the potential to satisfy those needs. The process directs a rounded tool along a tool path in the shape of the desired geometry. Through the motion of this tool against the sheet metal, the sheet is deformed in iterative steps until the full geometry is created. There are multiple configurations of ISF; single-point ISF (SPIF) is die-less configuration where the sheet is free-hanging and has no additional support besides the clamps at the periphery. Two-point ISF (TPIF) utilizes a half-die to support the sheet and prevent undesired bending away from the tool contact area. Challenges to ISF include low surface quality from contact mechanics, low machine stiffness resulting in geometric deviations, and stored residual stress created by the bending. This thesis investigates a variant of ISF that incorporates ultrasonic vibrations to the tool. This ultrasonic-assisted ISF (UA-ISF) process seeks to optimize on two known effects of the vibration. One is reduced friction at the tool sheet interface. The second is flow stress reduction, called acoustic softening, in vibration assisted processes. Truncated cone geometries were produced using UA-SPIF and UA-TPIF on CP aluminum, AA2024-O and AA7075-O. Due to the free hanging nature of the sheet in SPIF, minimal reductions in forming force, up to 5%, was observed and the fracture limit of the variable-angle, funnel geometry was unaffected. Sheet vibrations were minimized when incorporating the support die in UA-TPIF leading to a 27% reduction in axial forming force with an amplitude of 6µm. The thickness reduction was much greater than the amplitude and supports the theory of acoustic softening. Surface characteristics were improved given the right process parameters. Parametric analysis showed the softening effect was most sensitive to the tool diameter and step size. In comparison to UA-compression (UAC) tests in literature, UA-ISF has a much lower softening magnitude; the deformation state and contact mechanics are hypothesized to limit the softening magnitude. To gain a better understanding of softening effects, UA compression (UAC) tests were conducted on a variety of sample geometries. From an energy-based perspective, smaller sample volumes exhibited greater effects, up to 67% softening. The relative strain, called amplitude strain, was found to be a better parameter to represent the softening response across different sample dimensions and explains the wide variability reported in literature. In addition, the softening was found to diminish relative to the state of strain of the sample; therefore, it’s a function of amplitude strain and sample strain history. The microstructure shows greater subgrain formation and broadening in texture intensity. The reduction in flow stress is hypothesized to be a combination of dislocation consolidation through subgrain formation and dislocation annihilation. UA-indentation tests were selected to mimic the contact mechanics found in ISF and isolate the deformation mode. The average softening was found to be similar to UA-TPIF results. FE simulations modeled the indentation deformation. Provided with the plastic strain distribution, an effective softening value was calculated. This brought the simulated force closer to experimental observation but further ultrasonic testing on shear-based deformation is recommended.Deep Blue DOI
Subjects
acoustic softening incremental sheet forming
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Thesis
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