Experimental and numerical study on the hydroforming process for producing U-sectioned metal bellows

Bellows-type metal pipes are widely used for various purposes, such as damping vibrations, increasing heat transfer, compensating for expansions caused by temperature changes, and detecting precise dimensional changes in measuring instruments and sensors. Bellows can be manufactured in a wide variety of types depending on their diameter, material, wall thickness, and geometric properties. This master's thesis investigates the feasibility of producing bellows pipes, which are frequently used in industry, using the hydroforming method, which is not widely used in our country. The critical process parameters in the tube hydroforming method, namely pre-forming pressure, folding fluid pressure, axial feed, and the loading curves expressing their interdependent changes, were examined. In the studies conducted, preliminary experiments were first carried out using ST 37 (S235JR) steel pipes with an outer diameter of 16 mm and a wall thickness of 0.4–0.5 mm. As a result, it was difficult to obtain successful results due to the high thickness/diameter ratio (0.025–0.03) and insufficient pressing forces. In addition, the length of the gaps between the mould segments forming the curved structure being shorter than the loading and pressurisation area at the top of the pipe caused additional buckling loads, resulting in swelling of nearly 2/1 of the pipe diameter in the preliminary tests. In the 321 stainless steel samples of the same dimensions, tears occurred directly in the upper region. This situation highlights the difference in formability between plain carbon steel and stainless steel with high corrosion resistance. In other tests conducted with plain carbon steel, the expansion of the throat region of the pipe was prevented, and the initial formation of the bellows structure was achieved. Finally, preliminary experiments revealed that unnecessary volumetric voids in the areas forming the figure cavity during mould design would cause material displacement and leakage during pressure forming. Subsequently, based on the results obtained from preliminary experiments, a new mould design and manufacturing process were implemented to optimise errors according to the expanded pipe dimensions. It was intended to achieve success by selecting AISI 316 stainless steel pipes with a diameter of 25 mm and a thickness of 0.28 mm, which have a thickness/diameter ratio of 0.011, reducing the thickness/diameter ratio by almost 1/3 compared to the previous dimensions. To determine the process variables for the pipe dimensions to be manufactured using the selected 316 stainless steel pipe, which is partially thinner-walled, the manufacturing stage was modelled and simulated using LSDYNA finite element software, and the loading curves were determined in the simulations using the trial-and-error method. The produced parts were cut along the symmetry axis using the wire erosion method, and the bellows profiles were measured using an optical comparator, CMM, 2D optical measuring device, and form scanning systems. The results obtained demonstrate that the changes in the wall thicknesses at the bottom and top bends are reproducible and that the profiles can be produced within acceptable tolerances. In bellows pipe production, a thickness reduction of less than 10% is ideal and acceptable, while the 10-15% reduction determined as acceptable yielded 11.7% at the peak points according to finite element software results and was measured between 7% and 14% in normal values in the products produced after the process. Furthermore, according to the measurements taken, a springback movement of 80% was observed in the total width of the bellows bends. The width, which was 14 mm under the closing load applied by the stamping force, expanded to approximately 18 mm after the load was removed.