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Access Type
WSU Access
Date of Award
January 2022
Degree Type
Dissertation
Degree Name
Ph.D.
Department
Mechanical Engineering
First Advisor
Xin Wu
Abstract
ABSTRACT The surface topology role is studied on the manufacturing processes of dual phase steels. The sheet metal forming, trimming and the resistance spot welding of the dual phase steel are extensively used in the automotive industry. For a better understanding of the edge cracking in sheet metal forming, the edge preparation methods, edge topological characteristics, and its evolution during plastic tensile deformation are investigated. Tensile specimens were prepared by waterjet cutting, milling, and EDM methods. The as-machined edges were measured with a high-resolution Laser Scanning Confocal Microscope (LSCM (Keyence VK-9710), which not only provides statistical roughness parameters, but raw data of surface 3D geometry with the Z-measurement resolution at up to 1nm (on surface height), thus provides opportunities for advanced topological analysis for identification of critical surface parameters related to critical edge parameters. The edge evolution during tensile straining was achieved by interrupted tensile tests at different strains, and replication of the edge surfaces without unloading using a metallurgical replica technique, for post-measurement after the tests. The quality of the three edge preparation methods and the effect on edge cracking in tension are ranked. Tooling durability for advance high-strength steels has been investigated by Auto/Steel Partnership at an industrial site, in a progressive press line consisting of multiple operations of flanging, piercing, trimming, forming and cut-off, with the use of several die materials and under different conditions. This research is one part of this study involving the trimming operation using AISI D2 die material. The produced workpiece samples were collected from different cycles from the first hit up to 100,000 hits in certain intervals. The selected samples at different cycle numbers were used for edge characterization as a means of investigating the effect of trim die cutting edge damage on the trimmed edge quality of sheet steels. The topological characterization of the trimmed edge was measured by a laser confocal microscope that provided 3D surface geometries, and further analyzed for subtracting important edge feature parameters, represented as a function of number of trimming cycle. It was found that, despite of initial portion of edge roughening and edge quality deterioration from the new trim die, in the majority portion of the trimming operation up to 100,000 hits the edge quality of the trimmed sheet metal edges show combined roughening and smoothing cycles, or called a “self-reconditioning” effect, with the edge quality to be within a stable roughness window. This reason of this phenomenon is further discussed. This finding leads to the need of redefinition of trim die failure or trim die reconditioning criterion. Regarding the resistance spot welding, the advanced high strength steel strength continues to increase. The applied welding force has a proportional relationship with the material strength to break down the surface asperities and maximize the contact area at the faying surface. The current electrodes are made from copper base alloys which have their yield limitation at relatively high temperatures during the welding processes. In this study, the performance of a compound copper-molybdenum electrode is numerically investigated. The design of the electrode is suggested by a registered US patent. Using ABAQUS software, a 3D coupled thermal-electrical-constructional model is formed using the Q3D8R element type. The DP980 steel is considered as the sheet metal and its properties that are required to achieve the simulation were extracted from the literature. The focus of the simulation is the comparison between the commercial and the compound electrodes. The contact pressure, stress distribution, nodal temperature and the nugget formation were the elements that studied in the simulation. The surface roughness role will not be limited to the electrodes, the dynamic resistance is another point of interest where the surface callouts impact can be observed. To explore the surface topology impact on the dynamic resistance, three different surface levels were created using 240,150, and 80 GRITs sanding papers in addition to the original surface. The grinding or polishing process was performed on the sheet-to-sheet contact as it has the highest contact resistance contribution to the total dynamic resistance. The DP1180 bare steel is used in the experiments and all the welding parameters were fixed for all the specimens. The surface peak related parameter is measured and correlated to the value of the minimum dynamic resistance on the curve at the end of surface breakdown stage.
Recommended Citation
Al-Shawk, Ameer, "Surface Roughness In Manufacturing Processes Of Advanced High Strength Steels" (2022). Wayne State University Dissertations. 3749.
https://digitalcommons.wayne.edu/oa_dissertations/3749