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Performance Testing and Modeling of Ultra-High Strength Steel and Complex Stack-Up Resistance Spot Welds

Peer, Andrea J
http://orcid.org/0000-0002-3480-9698
http://rave.ohiolink.edu/etdc/view?acc_num=osu1493403670252986

Abstract Details

2017, Master of Science, Ohio State University, Materials Science and Engineering.
Hot stamped boron steels, such as Usibor® 1500, have been increasingly used in automotive structural components for light-weighting and impact resistance. Classified as an ultra-high strength steel, these alloys have superior strength with tensile strengths exceeding 1500 MPa. The rapid heating and cooling thermal cycle during resistance spot welding can significantly alter the martensitic base metal microstructure, resulting in formation of coarse-grained and subcritical heat-affected zones (CGHAZ and SCHAZ) with inferior mechanical properties. The martensitic CGHAZ is adjacent to the weld nugget and experiences the most time above the AC3, which allows for austenite grain growth. The SCHAZ is next to the unaffected base metal and does not reach the AC1¬ during welding, thus the base metal microstructure is over-tempered into cementite and ferrite. The present research aims at developing the fundamental knowledge of plastic deformation and fracture behaviors of ultra-high strength steel resistance spot welds. As a resistance spot weld comprises highly inhomogeneous microstructure, the overall research approach is based on studying the local (or microstructure-dependent) mechanical properties for individual regions in the weld as well as their interactions with weld geometry on the deformation behavior. Specifically, optimal welding parameters are determined to produce welds of appropriate nugget diameter for 2T Usibor 1500 with a gauge thickness of 1.5 mm. Micro-hardness mapping and metallographic analysis allow for characterization of the weld metal, CGHAZ, SCHAZ, and base metal of the spot weld. Quasi-static tensile testing with digital image correlation (DIC) is used to determine the local stress-strain behaviors of each region using bulk microstructural samples created in a Gleeble® thermal-mechanical simulator. Conventional and innovative resistance spot weld mechanical testing methods are used to generate more knowledge on the deformation of joints under various loading conditions. Sectioned tensile shear testing and single-sided wedge testing procedures have been established to use 2-D DIC for in situ observations of local deformation on the exposed weld cross-section during testing. A mechanical model, developed using Abaqus finite element analysis (FEA) code by incorporating the local constitutive behaviors of RSW joints, is used to better understand the effect of weld nugget profiles on the stress state present during loading. The FEA model is validated by comparing the simulated strain fields to the experimentally measured strain fields. The knowledge generated in this study can help improve the accuracy of predicting spot weld fracture of ultra-high strength steels in the automotive industry. Particularly, the fine-resolution, coupon-scale model developed in this research will be useful for implementation into coarse-resolution, full-scale models for crash simulation and optimization of vehicle components.
Wei Zhang, PhD (Advisor)
Menachem Kimchi (Advisor)
David Phillips, PhD (Committee Member)
175 p.
Automotive Engineering; Automotive Materials; Engineering; Materials Science; Metallurgy; Transportation
Welding; Resistance Spot Welding; Automotive; Mechanical Testing; Finite Element Analysis; Mechanical Properties; Metallurgy; Crash Simulation;

Recommended Citations

Citations

  • Peer, A. J. (2017). Performance Testing and Modeling of Ultra-High Strength Steel and Complex Stack-Up Resistance Spot Welds [Master's thesis, Ohio State University]. OhioLINK Electronic Theses and Dissertations Center. http://rave.ohiolink.edu/etdc/view?acc_num=osu1493403670252986

    APA Style (7th edition)

  • Peer, Andrea. Performance Testing and Modeling of Ultra-High Strength Steel and Complex Stack-Up Resistance Spot Welds. 2017. Ohio State University, Master's thesis. OhioLINK Electronic Theses and Dissertations Center, http://rave.ohiolink.edu/etdc/view?acc_num=osu1493403670252986.

    MLA Style (8th edition)

  • Peer, Andrea. "Performance Testing and Modeling of Ultra-High Strength Steel and Complex Stack-Up Resistance Spot Welds." Master's thesis, Ohio State University, 2017. http://rave.ohiolink.edu/etdc/view?acc_num=osu1493403670252986

    Chicago Manual of Style (17th edition)

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This open access ETD is published by The Ohio State University and OhioLINK.