Mitigating the influence of industrially relevant disturbances on LME severity of dissimilar resistance spot welded advanced high-strength steels

The third generation of advanced high-strength steels (3G-AHSS) has been developed to provide high strength and high ductility, which attract automakers. To protect these materials from corrosion during service, these materials are typically coated with zinc. During resistance spot welding (RSW), the zinc coating can melt, allowing it to penetrate into the grain boundaries (GBs), and lead to liquid metal embrittlement (LME) phenomena. Concerns regarding LME susceptibility have impacted the industrial application of 3G-AHSS; therefore, its mitigation has become a top focus for automakers. Several possible strategies for lowering LME severity by altering welding parameters have been proposed to mitigate LME in similar spot weld joints. However, these strategies were not tested on a dissimilar spot weld joint. Therefore, in this work, 1.4 mm gauge thickness galvanized (GI-coated) 3G-980 AHSS was joined with 0.6 mm thick Interstitial Free (IF) steel. In this work, current pulsation and ultra-short hold time were proposed to minimize LME severity. The robustness of the developed welding schedule was then tested on welds made with industrial disturbance factors such as pre-strained sheets (between 0 to 80% of 3G-980 material yield strength) and electrode misalignment (between 0° to 10° misalignment) compared to baseline parameters. In severe circumstances of disturbance factors, the resulting optimized welding schedule decreased LME cracking and showed improved resistance to LME, lowering LME severity by 41% for the extreme pre-strain condition and 27% for the extreme misalignment angle

Effect of strain-rate on the deformation response of D0 3 -ordered Fe 3 Al

The mechanical response of centrifugally cast Fe(3)A1 with the composition Fe-27A1 (at.%) containing microalloying additions of Nb, Zr, C, and B was investigated over a wide range of strain rates between 10(-4) and 10(3) s(-1) at room temperature. Tests were carried out in compression using a (i) screw-driven load frame, (ii) drop impact tester, and (iii) split-Hopkinson pressure bar at quasi-static, intermediate and dynamic strain rates respectively. Post deformation analysis was carried out by DSC, SEM/EBSD, TEM and micropillar deformation. In all instances, the stress-strain curves show initial hardening (-first 5% plastic strain) followed by a plateau in stress. A loss in work-hardening occurs at the highest strain rates examined (>10(3) s(-1)) and is likely associated with shear localization; in addition, (2 11) [11 1]-type twinning was observed at these high strain rates at room temperature. This observation is in line with previous theoretical calculations of the antiphase boundary (APB) energy. The consequence of a continuously increasing yield stress with strain rate and a loss in work hardening at the highest strain rates together yields a maximum in flow stress at the intermediate strain rate. (c) 2015 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved

Deformation response of ferrite and martensite in a dual-phase steel

Deformation response of ferrite and martensite in a commercially produced dual-phase sheet steel with a nominal composition of 0.15% C–1.45% Mn–0.30% Si (wt.%) was characterized by nanoindentation and uniaxial compression of focused ion beam-milled cylindrical micropillars (1–2 μm diameter). These experiments were conducted on as-received and pre-strained specimens. The average nanoindentation hardness of ferrite was found to increase from ∼2 GPa in the as-received condition to ∼3.5 GPa in the specimen that had been pre-strained to 7% plastic tensile strain. Hardness of ferrite in the as-received condition was inhomogeneous: ferrite adjacent to ferrite/martensite interface was ∼20% harder than that in the interior, a feature also captured by micropillar compression experiments. Hardness variation in ferrite was reversed in samples pre-strained to 7% strain. Martensite in the as-received condition and after 5% pre-strain exhibited large scatter in nanoindentation hardness; however, micropillar compression results on the as-received and previously deformed steel specimens demonstrated that the martensite phase in this steel was amenable to plastic deformation and rapid work hardening in the early stages of deformation. The observed microscopic deformation characteristics of the constituent phases are used to explain the macroscopic tensile deformation response of the dual-phase steel

Prediction of Mechanical Properties in the Sub-Critical Heat Affected Zone of AHSS Spot Welds Using Gleeble Thermal Simulator and Hollomon-Jaffe Model

Measuring the mechanical properties of weld Heat Affected Zone (HAZ) remains one of the main challenges in the failure analysis of spot-welded components. Due to the small size of the HAZ and variation in the temperature history, different peak temperatures and cooling rates impose a range of phase transformations across the resistance spot weld. Among the HAZ sub-regions, the sub-critical HAZ (SCHAZ), which experiences temperatures below AC1 (350–650 °C), usually shows a reduction in the hardness in most of the modern AHSS grades due to the martensite tempering phenomenon. SCHAZ softening may lead to strain localization during loading. Therefore, it is important to characterize the local properties of the SCHAZ region to accurately predict RSW failure. However, it is not feasible to extract standard mechanical test specimens out of the SCHAZ of the spot-welded structure due to its small size. In this work, the SCHAZ of the spot weld for two AHSS, 3G-980 and PHS-1500, was simulated using a Gleeble® (Dynamic Systems Inc., 323 NY-355, Poestenkill, NY 12140, USA) 3500 thermo-mechanical simulator. An in-situ high-speed IR thermal camera was used to measure the entire temperature field during the Gleeble heat-treatment process, which allowed for the visualization of the temperature distribution in the gauge area. The temperature and hardness data were fit to a Hollomon-Jaffe (HJ) model, which enables hardness prediction in the SCHAZ at any given temperature and time. Using the HJ model, a heat treatment schedule for each material was chosen to produce samples with hardness and microstructure matching the SCHAZ within actual spot weld coupons. Tensile specimens were machined from the coupons heat treated using simulated heat treatment schedules, and mechanical testing was performed. The results showed that the 3G-980 SCHAZ has a slight increase in yield strength and tensile strength, compared to the base metal, due to the formation of fine carbides within the microstructure. In contrast, the SCHAZ of PHS-1500 showed a significant reduction in the yield and tensile strength with yield point elongation behavior due to the reduction of the martensite phase and an increase in carbide formation due to the tempering process

Influence of selected alloying variations on liquid metal embrittlement susceptibility of quenched and partitioned steels

In this work, the influence of selected alloying variations on Zn-assisted liquid metal embrittlement (LME) susceptibility of Zn-coated advanced high strength steels (AHSS) is investigated. Cold-rolled AHSS alloys of different carbon (C), manganese (Mn), silicon (Si), and aluminum (Al) concentrations were continuous-annealed to generate a third generation AHSS microstructure (composed of martensite and retained austenite) via quenching and partitioning. High temperature tension tests using simulated spot-weld thermomechanical cycles revealed no significant influence of C and Mn variations on the Zn-LME susceptibility of AHSS. On the other hand, Zn-LME susceptibility was strongly correlated with the Si content of AHSS. A direct comparison of the (reacted) coating microstructures of the Si-alloyed and Low-Si AHSS variants revealed that Si in the AHSS substrate suppresses Fe–Zn alloying reactions and retards the nucleation and growth of Fe-Zn intermetallic phases at the coating-substrate interface in these spot weld simulations. The suppressed intermetallic formation at elevated Si concentrations is consistent with phase equilibria considerations in the Fe-Zn-Si ternary system. In the context of Zn-assisted LME, therefore, Si is hypothesized to aggravate LME behavior by increasing the liquid Zn availability for embrittlement and promoting direct contact between liquid Zn and the AHSS steel substrate