Investigation of influencing factors in liquid metal embrittlement of advanced high strength steel

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This thesis explored the influence of temperature, steel type, galvanization method, and macro-strain level on the sensitivity of advanced high strength steels (AHSS) to zinc-based liquid metal embrittlement (LME). It is critical to understand the influencing factors of LME because zinc coatings are commonly used to protect steel parts from corrosion, and the use of advanced high strength steel in the automotive industry is increasing. Electro-galvanized and zinc free samples of a transformation induced plasticity steel, TBF1180, and a complex phase steel, CP1200, were studied to examine the sensitivity of each to LME. Hot-dip galvanized samples of CP1200 were examined alongside the electro-galvanized samples to investigate the effect of coating method on the LME effect. Hot tension tests were performed and ductility trough graphs were created for all samples to examine the effect of these factors on LME during fracture. Additionally, small-strain tensile tests were designed and performed on the steels to examine LME crack nucleation. From the results it was determined that LME response is temperature and steel dependent. It was shown that TBF 1180 nucleated LME cracks at 600 °C while CP1200 did not. It was also determined that hot-dip galvanized coatings more readily nucleate LME cracks than electro-galvanized coatings. Finally, these results suggest that macro-plastic deformation may not be required to initiate an LME response.

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Materials science