Investigation of Tube Hydroforming Process Envelope for Macro/Meso Scalability
The tube hydroforming (THF) process requires the internal pressurization of a tube stock while the ends are axially fed inside a die cavity. As a result of this internal pressurization, the tube deforms to fill the die cavity. The objective of this study was to investigate the scalability of tube hydroforming and determine the potential for hydroforming at the micro and meso scales. This was accomplished by establishing trends which were identified at the macro scale and using these results to predict the formability at the meso and micro scale. To establish these trends, first the effect of material properties, tube geometries, die geometries, forming pressures and friction coefficients on the formability of the tubes was studied. From these results a pentagon die geometry was selected as the forming die, and stainless steel was selected as the tubing material for the scalability study. For the scalability study, five tube diameters ranging from 0.250 in to 2.250 in were selected. Tube thicknesses for each tube were scaled as a percentage of the outer tube diameter ranging from 1.25% to 5.00%. The forming operation was considered complete once the tube walled experience 20% thinning. For each simulation, the wall thickness and R/t ratio were calculated. The results from these simulations indicate that that trends exist in the formability of tubes as the scale of the tube is reduced. The forming pressures required to reach 20% thinning increased linearly as the tube wall thickness ratio was increased. The tube diameter did not impact the formability of the tube. The wall thickness percentage was shown to be the critical factor when changing the scale of tubes for hydroforming.
To verify the simulation results, experimental testing was performed. Because of the high forming pressures required for stainless steel tubing, copper tubing was used for the experimental testing. Both materials have an identical strain hardening exponent of 0.46, but the strength coefficient for copper is much lower than that of stainless steel. The results for the experimental testing, indicate that FEM can be used to predict the THF process. The measured tube thickness profiles align very consistently with the simulation results. There is an offset in most cases but this can be attributed to variation in tube tolerances due to the manufacturing process. The measured R/t ratios followed the trends of the simulation data, though there was a significant difference in the values. This is most likely a result of ?spring back? in the tube at the completion of the forming process. The FEM simulations only considered plastic deformation. All elastic behavior was ignored.
The results from this study indicate that tube hydroforming is feasible on the micro and meso levels. However, there are still many issues which must be addressed in the manufacturing process, such as sealing techniques, alignment of the axial cylinders, and potentially high forming pressures as a result of tubes on the micro scale having higher wall thickness to diameter percentages.
Advisor:Dr. Kara Peters; Dr. Jeffrey Eischen; Dr. Gracious Ngaile
School:North Carolina State University
School Location:USA - North Carolina
Source Type:Master's Thesis
Date of Publication:06/11/2007