Research On Die Wear Characteristics In Advanced High-strength Steels Sheets Forming

With the increase of automotive lightweight requirement, the use of Advanced High-Strength Steels(AHSS) sheets in automotive industry becomes wider and wider, which poses great challenges for researchers to understand their formability and fracture behavior, and develop predictive models for practical applications in stamping. Due to high strength and high hardness of AHSS sheets, there are more defects such as chipping, die wear and plastic deformation in the forming AHSS sheets processes, which results directly in die lifetime decreasing, fracture of workpieces, more severe springback and wrinkle. To analysis the mechanism of die wear/chipping and reduce their occurrence, numerical simulation is employed more and more popularly, which is very helpful to decreasing the occurrence of die wear/chipping, improving die lifetime and so on. The main research topics can be summarized as follows:1. Define the threshold values of fracture models by use of Forming Limited Diagram(FLD). In the numerical simulating process of blanking AHSS sheets, the occurrence of fracture depends on the threshold values of fracture models that are figured out using the combination of numerical simulations and actual experiments. In this study, a new method to define the threshold values of fracture models is presented using FLD. FLD with fracture data can be obtained by capturing Nakazima limited dome height experiment processes using digital inspection camera. The integral path of the threshold value of fracture models can be divided into two parts including linear loading path and plane strain state loading path, because necking occurs before fracture in the Nakazima experiment process. The threshold values of selected fracture models can be figured out after integration along the two paths and averaging.The reverse analyses were employed to evaluate the calculated threshold values of fracture models along two different paths. One started from necking point mapped by fracture point along plane strain state loading path, and the fracture points calculated by the selected fracture models can be located in the FLD figure. The other one was two-stage path starting from original point along linear loading path calculated by the threshold values of the selected fracture models, and the calculated fracture points can also be located in the FLD figure. After comparing the locations of fracture points between the two loading paths and experimental fracture points, there are great agreements between them. It can be seen that the threshold values of fracture models calculated by the two-stage integral method are very reliable and accurate.2. Evaluate wear volume loss based on the movement of nodes on the die surface. The Archard wear model was coded into ABAQUS/Standard user subroutine UMESHMOTION, in which the movement of nodes on the die surface was regarded as wear volume loss. The movement of nodes depended on the stresses distribution of nodes and their movement distances, and the wear volume loss can be figured out after the wear coefficient obtained from pin-on-disc tests was coupled with. Wear severe index(WSI) can also be presented and equal to the product of equivalent shear stress of nodes and their equivalent distances at die surface, to evaluate the extent of wear. The method quantifying wear volume loss was implemented in the numerical simulation of 2D flanging of DP780 sheet in ABAQUS/Standard, and it can be shown that the method is very reliable and accurate, after comparing with actual flanging experiments.3. Design and manufacture a pair of blanking die for X shape workpiece to compare and evaluate the wear characteristic among different die materials. One low-cost high carbon low chromium stainless steel 4Cr13 was chosen as one of testing die materials after special carburization to meet the requirement of blanking AHSS sheets. Both punch and die consisted of four quadrant inserts respectively, and die materials CALDIE, SKD11, A2, cemented carbide D60 and caburized 4Cr13 were tested. In detail, caburized 4Cr13,CALDIE,SKD11 and cemented carbide D60 are chosed as die inserts materials, and caburized 4Cr13,CALDIE,SKD11 and A2 are selected as punch inserts materials correspondingly. This was because the cemented carbide D60 sheet can not be welded on the punch insert smoothly, which resulted that A2 was selected as punch insert material in the corresponding quadrant of die insert cemented carbide D60. After 40,000 blanking strokes, the tested materials were compared and evaluated including worn topology, worn volume loss and workpiece edge quality, to find out which ones were suitable for blanking AHSS sheets.It can be seen that the wear resistance performance of cemented carbide D60 is the best among the tested die materials, however, the price of D60 is as high as the 15 times SKD11’s, which means that the priority for blanking AHSS sheets of cemented carbide D60 is beyond the top due to its pretty high price. The wear resistance performance of caburized 4Cr13 is lower than the ones of CALDIE/A2 and almost the same level as the popular die material SKD11, but the price of caburized 4Cr13 is only as low as the one third SKD11’s, which means that caburized 4Cr13 can be reguarded as a good die material alternative for blanking AHSS sheets.