Research On Orange Peel Generation Mechanism And Forming Limit Of Al-Li-S4Aluminum-lithium Alloy In Stretch Forming Process

Abstract:As a traditional method of sheet metal forming in aerospace manufacturing field, stretch forming technology is widely used in the production of the large skin parts of aircraft aerodynamic shape. In stretch forming process the generation of surface defects, which often become the main cause of parts obsolescence, seriously affect the life of the skin. Therefore, in recent years of the study about the skin surface quality becomes the one of research hotspots in the material components forming field.In this paper, the third-generation Al-Li-S4-T8aluminum-lithium alloys are used as the object of study. The influence of the strain rate, strain and other process parameters on the orange peel of the stretching sample was systematically studied. By exploring the criterion standard of the critical orange peel and using different strain path, forming limit diagram on the orange peel phenomenon is created. On this basis the effect of electrical pulses to improve the quality of the stretch forming on the stretching sample wad studied. And the forming limit diagram was used on the stretch forming simulation for Al-Li-S4aluminum typical skin components, which provides a theoretical reference for the development of the best stretching process. The main research and achievements are as follow:(1) The synchronization test system of stretch forming craft with surface defect detection was constructed based on a universal testing machine with non-contact optical strain gauge. The influence of the different stretching rate on the surface orange peel of Al-Li-S4aluminum-lithium alloy was comparative analyzed. And the best strain rate was obtained.(2) The influence of the strain on the surface orange peel of Al-Li-S4aluminum-lithium alloy was comparative analyzed at of the best strain rate. The surface morphology of the alloy was analyzed under the critical orange peel state.(3) Based on optical roughness tester, the variation of surface roughness was obtained with the variation of the strain, and the roughness of alloy surface was proposed as a criterion to describe the critical statues of orange peel. By this way, it was achieved the exact mathematical characterization of the critical state of orange peel.(4) By analyzing the microstructure of the alloy, it was identified the causes of Al-Li-S4-T8aluminum-lithium alloy easy to produce the orange peel phenomenon. And the generation mechanism of orange peel defects was analyzed.(5) Different notched specimens were designed to get different strain paths. Through the experiment, the forming limit curve equation based on orange peel defects of Al-Li-S4aluminum-lithium alloy was established. On this basis, in order to improve the stretch forming properties of materials and the surface quality of stretch forming sample, it was proposed to use electric pulse to assist stretch forming. It was found that the pulse current can delay the appearance of orange peel phenomenon. By the original method, the forming limit curve joining the factor of electrical pulse was established.(6) Based on MARC, A finite element analysis model of the stretch forming process for Al-Li-S4aluminum-lithium alloy typical skin was established. With Forming Limit Diagram introduced to stretch forming simulation analysis, which Provide a theoretical reference for improving the stretching process, the effect of different stretching process on formability was analyzed. Figures:54; Tables:13; References:96...