Mechanisms Of Boss Forming Process For Sheet Metal Based On Press-forging Technology

Press forging technology, a new important research issue in plastic formingindustry, has been widely used in manufacturing metal parts due to its advantagessuch as saving energy and material, high productivity, smooth surface finish andimprovement in mechanical properties. As a lightweight structure design in theforming of metallic cavity, box and shell, boss forming is widely used in automobileand electronic industries. In present paper, based on press forging technology, we aimto make an in-depth study on boss forming process combined with drawing andextrusion, using a synthesis method such as mechanical analysis, FEM numericalsimulation and sample experiments.Firstly, the process and principle of boss forming process combined with drawingand extrusion are introduced. Besides, based on Tresca yield criterion, a comparisonbetween back-extrusion method and press forging method on boss forming isdemonstrated. The results reveal that lower forming load in the press forging process.The mechanical characteristics of press forging process are analyzed using slabmethod, and the plastic flow law in the bottom blank sheet is focused. Fortunately, aseparation flow surface is discovered during the boss forming process and the natureof separation flow surface is actually referred to the zero value of Strain LodeParameter. Furthermore, the theoretical formula of separation flow surface is deducedand a new dimensionless parameter for characterizing the boss forming processnamed boss ability is brought up.The numerical simulations for boss forming process of both back-extrusion andpress forging are carried out by the software DEFORM-3D. Comparisons of theirpunch load, location of separation flow surface and boss ability are analyzed in details.The results are obtained as follows: the punch load of back-extrusion is three timeslarger than that of press forging; the location of separation flow surface inback-extrusion process is out-shift while in press forging it is ingression; the value ofboss ability in back-extrusion process is larger with the decrease of bottom sheetmetal while in press forging it is an approximate constant. Meanwhile, the evolutionof dimple defect occurred in boss forming process is investigated and a dimplemechanism diagram is developed.Finally, the corresponding forming die structures are designed for press forgingexperiments combined with deep drawing and extrusion. The experimental result ofpunch load shows a good agreement with the FEM simulation result and reveals alittle larger than the theoretical calculation load, which certifies the validity andfeasibility of those two methods.