The forming theory of elbow made by expanding diameter and pushing bend is introduced in this paper. The tube blank of elbow takes place combined deformation with compression deformation in the axial direction and in the circumferential direction. The compression deformation in the axial direction makes tube bending deformation under the restricted condition of ram-shape die, then the belly metal of elbow thicken after compression deformation, while the back metal of elbow don’t take part in deformation. The eccentric expanding diameter in the circumference direction ensures the thickened wall thickness flowing favorably from elbow belly to elbow back and supplying the reduced wall thickness of back elbow. Comparatively, the compression deformation and expanding diameter deformation of short radius elbow is larger than that of long radius elbow. According to metal flowing law, proper ram-shape die design and pushing rate, increasing heating temperature, improving temperature field distribution and lubrication condition can assure uniformity wall thickness.
TA24 titanium alloy pipe with 638mm diameter and 19mm wall thickness is carried out continuous load flatten test, and the stress of internal and external pipe wall during flatten process is studied in this paper. The results show that the TA24 titanium alloy tube has good flattening performance, and the flattening process has experienced original stage, flattened oblate stage, flattened straight wall stage, flattened depressed stage, flattened concave contact stage. During the flattening process, the outer layer of the upper and lower wall of the tube is subjected to compressive stress, and the inner layer material is subjected to tensile stress. The tensile and compressive forces cause the vertical part of the upper and lower walls to be concave. The outer layer of the left and right circular arc parts is subjected to tensile stress and the inner layer is subjected to tensile stress. The compressive stress also causes the radius of the arc to decrease due to the combined force of the tensile and compressive forces, that is, the flattening occurs. With the decrease of and pressing distance under continuous loading condition, the metal on the left and right sides of the pipe gathers toward the middle depression, which aggravates the deformation of the upper and lower walls until the upper and lower walls contact, and the arc radius of the left and right walls decreases until the outer surface cracks. The pipe microstructure changes significantly into elongated deformation structure during the flattening process. The most severe part of the deformation is the left and right end arc of the pipe, followed by the upper and lower end depression.
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