Mohamed Bouzekri

Liquid zinc embrittlement of a high-manganese-content TWIP steel

The cracking resistance of a twinning induced plasticity (TWIP) steel with high-manganese content has been studied in relation to the liquid metal embrittlement phenomenon. This phenomenon was investigated by hot tensile tests carried out on electrogalvanised specimens using a Gleeble® simulator at temperatures ranging from 600°C to 1000°C. The results show that this steel can be embrittled by liquid zinc within a limited range of temperature depending on strain rate.

An Investigation of Failure Types in High-Strength Steel Resistance Spot Welds

On the shop floor, as in laboratories, destructive testing remains the main means of quality control of spot welds. After hand or mechanized weld destruction, the so-called “plug diameter” is measured and is usually considered a good indicator of the weld quality. However, it turns out that the “plug” failure of spot welds is far from being the rule. Moreover, fracture may occur in different zones of the weld, leading to very different meanings of the plug diameter. Therefore, the plug diameter is most of the time not well-correlated with the weld strength, which is the main value of the spot weld. Partial or full interfacial failures (through the weld nugget) exhibit equivalent mechanical strengths and therefore should not be rejected. Through this work, it is shown that the failure type depends on various parameters (nugget diameter, sheet thickness, loading mode, …), and consequently, it is not an intrinsic property of the steel grade. In this way, recommended quality criteria are based on weld strength, weld diameter (including the interfacial fracture area if any), or absorbed energy independently of failure occurrence. In addition, non-destructive techniques might be key investigation methods and have to be developed further.

Embrittlement of a High Manganese TWIP Steel in the Presence of Liquid Zinc

In the past decade, new steels have been developed for the automotive industry in the framework of environmental requirements. Among them, high manganese austenitic steels combining exceptional properties of strength and ductility are particularly promising. These exceptional properties stem from a fully austenitic structure at room temperature and a twinning deformation mode in addition to the classical mechanism of dislocation gliding, known as the TWinning Induced Plasticity (TWIP) effect. In this study, the cracking resistance of the Fe22Mn0.6C TWIP steel was investigated in relation to the liquid metal embrittlement (LME) phenomenon. Indeed, liquid zinc has been found to have an embrittling effect on such steels. Electro-galvanized specimens were subjected to hot tensile tests using Gleeble® thermo-mechanical simulator. The influence of different parameters such as temperature and strain rate on embrittlement was studied. The results show that this steel can be embrittled by liquid zinc within a limited range of temperature depending on strain rate. A critical stress for cracking has been defined for each embrittlement condition.