预成形是内高压成形的关键工序，预制坯的形状直接影响到后续内高压成形的缺陷与壁厚分布。针对管件无内压支撑压制时因失稳导致的预制坯截面凹陷与尺寸不可控等问题，提出管件充液压制成形方法。对椭圆截面管件充液压制成形过程进行应力分析与试验研究，将充液压制与传统压制进行对比，分析充液压力和下压量对管件截面应力、壁厚及尺寸的影响。结果表明：管内充液可有效改善压制管件等效应力分布情况，充液压力越大，等效应力分布越均匀|相比于传统压制，充液压制过程中椭圆截面的壁厚变化并不明显，最大减薄处位于直壁部分中间区域，当充液压力为15 MPa时，其最大减薄率为2%；随着充液压力的增大，管件直壁部分的凹痕缺陷逐渐平复消失。
Preforming is the key to the hydroforming process. The forming defects and thickness distribution are directly affected by the shape of the preform. A hydro-pressing forming method is proposed to solve the problems in conventional mechanical pressing process without internal pressure, such as dent defects and size uncontrollable. The stress analysis and experimental research are performed for the hydro-pressing process of mild steel tube with elliptical sections. The differences are compared between the hydro-pressing and the conventional mechanical pressing. The effects of supporting pressure and pressing displacement on section stress, wall thickness distribution and section shape are investigated. It is demonstrated that the equivalent stress distribution of pressing tube was improved by hydro-pressing process. The greater the supporting pressure, the more uniform the equivalent stress distribution. Compared with conventional mechanical pressing, the thickness variation of the elliptical section is unobvious during hydro-pressing process. The maximum thinning is located in the middle region of the straight wall. When the supporting pressure is 15 MPa, the maximum thinning is only 2%. Dent defects located in the straight wall of the tube is decreasing with the increasing of supporting pressure.
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