Hydraulic St52 tube end orbital forming: improving forming limits and surface quality and dimensional precision through finite element analysis and experimental validation

Orbital end forming was applied to thick-walled St52 hydraulic tubes to evaluate the forming limits, process safety and dimensional accuracy under controlled operating conditions. Experiments were conducted at a constant feed rate of 10 mm/s with an expansion ratio of rp/r0 = 1.35 and a flaring angle of 37°, using various pressure and frequency combinations. Forming behaviour was assessed through experimental measurements and finite element analysis, focusing on forming force, equivalent stress, surface quality, roundness, and eccentricity. The average experimental forming force was measured at 3000 N, whereas numerical analysis predicted a maximum vertical force of 5000 N, and the maximum equivalent stress reached 481.25 MPa. This value remained below the ultimate tensile strength of the material (505 MPa), thereby confirming safe forming conditions without material failure. Optimal forming parameters of 4 bar pressure and a frequency of 13 Hz resulted in a 73% improvement in surface quality. Using a 60 HRC DIN X153CrMoV12 steel conical die minimised tool wear and ensured high geometric accuracy. The results demonstrate that the proposed orbital forming process provides a reliable, energy-efficient, and industrially applicable solution for precision end forming of thick-walled hydraulic tubes.