Preform design and microstructure-property analysis for isothermal extrusion of complex box-shaped components

The design of preform plays a key role in the precision forming of complex-shaped components with thin walls and large areas of webs. In the present work, two typical kinds of box-shaped forgings of 5A06 aluminum-alloy were selected as research objects, including the symmetric rectangular base forging and the asymmetric arc cover forging. The preform design method was proposed during isothermal back extrusion of boxlike forging, in which metal flow law of billets with equal thickness revealed by finite element (FE) simulation was quite useful for the billet design. The simulation results successfully predicted folding defects at the bottom of the base forging and inadequate filling at lateral walls of the cover forging, which were caused by inappropriate billet geometry and loading mode. According to the simulation and experimental results, the reverse loading of the rectangular base billet with variable thickness avoided the appearance of the folding at the bottom. The base forging without defects was formed successfully under reverse loading by using the billet with four 60 mm × 90 mm × 5 mm bosses and 15° transition surfaces, while the cover forging without defects was formed successfully by using the billet with 15° transition surfaces and two bosses whose heights were both 48 mm. Both the two boxlike forgings formed with modified billets exhibited perfect distribution of flow lines. Meanwhile, the lateral walls and webs possessed higher tensile strength, higher elongation, finer and more elongated grains than raw material. Therefore, the thermal backward extrusion was an effective process to precisely form complexly boxlike rib/web workpiece.