Study Of Stamping Formability Of Laser Welded Blanks And The Optimization Of Weld-line Location

Tailor welded blank(sTWB) are defined as two or more sheets of blanks with equal or different thickness, mechanical properties, or surface coatings welded together by laser before stamping. TWB has been widely used in European and North American countries just in recent years. The focus attention has changed into solving the problems of reducing automobile weight, oil consumption and air pollution from the initial application of wide blanks with equal thickness by laser welding. TWB plays the important role in reducing processing procedure, cost of production, consumption of materials and also in improving working efficiency. By using TWB in the automobile industry, one step stamping is used instead of the original multi-steps stamping. TWB has become a hot issue in the automobile industry and steel industry for its high quality, high profit and low cost of production,and has been widely used in automobile industry.In this paper, the application of TWB in the automobile industry is discussed, and the development,characteristics, superiority and research of TWB are introduced. As the shape of automobile's covering parts is very complex and it is difficult to estimate the formality by means of experience,the finite element numerical simulation method is proposed to forecast the drawing formality of the TWB.At first, we use square box, a typical part in sheet metal processes, for deep drawing simulation of TWB, in order to study on the movement law of welding seams. The movement of welding seams of TWB combined by same material and different thickness sheet metal is studied by using finite element code KMAS. The weld-line moves towards the thick part on the bottom of the square box and towards the thin part on the wall of the square box. The non-uniform deformation of tailor-welded stamping parts, the control method of the non-uniform deformation and the computer simulation results of the processes are presented. The methods of control the movement of weld seams are summarized. Such as increasing the binder hold force or drawbead forces of the thin part, adjusting the initial weld-line location or blank shape.The tests for TWB mainly include cupping test, tensile test and forming limit test. Forming limit diagram (FLD) is the most simple and straight way to evaluate the formability of metal sheet.Before the stamping simulation, draw step surface, binder wrap surface, punch and die round are designed according to the characteristics of auto door inner panel, so that three dimensional die-face CAD model is obtained. The key of the investigation of the stamping formability is set off finite element mesh. The size of mesh should be suitable, normals of elements should be consistent, rounds should be adaptive,and elements should not be repetitive. We use Hypermesh for checking and modification. The parent material model is comprised of 4-node quadrilateral BT shell elements. Weld properties and geometry are excluded from the model. A set of rigid links is used to tie adjacent nodes on the thin and thick sheets together.TWB combined by three sheets are used for the simulation of the stamping process of auto door inner panel. The material is 08Al, the thickness of the middle part is 0.8mm and the other parts are 1.2mm. We should meliorate the technical parameters to avoid the crack or wrinkle tendency. Comparing the material flowing of different blank shapes, we decide which blank shape to be used. It is very difficult for the material to flow into the internal side of the part by the FE analysis,so several holes should be cut.The drawbead adds additional restraining forces to the blank, so that the movement of the weld line is reduced and the wrinkle tendency is removed. Several drawbeads are setting to control metal flowing. It is found out that this part could be drawn successfully by 700kN holding force.Rational layout of welding seam is the key factors affecting stamping forming quality of TWB. The location of weld seams should avoid from the complex situation of forming for auto parts. As to investigating the movement of the weld seams during deep drawing, three different initial weld-line locations are set, then compared for their forming results, so that we can get a suitable weld-line location. The distance between the two seams should be suitable. The formability of door inner panel is greatly improved though the melioration of the technical parameter.The purpose of this study is to investigate the method of reduce the movement of the weld-line, the drawbead optimization are carried out in the drawing process.In order to confirm the reliability of numerical simulation of the KMAS, we compare the simulation result with the results of dynamic and implicit softwares. It is shown that the blank shape and the distribution of sheet metal thickness are homologous, the movements of weld seams are some a bit different, while the directions of movement are consistent. So this simulation result is reliable.