Browsing by Author "Ramachandran, Dileep Chandran"

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A Study on Improving the Mechanical Performance by Controlling the Halo Ring in the Q&P 980 Steel Resistance Spot Welds
(Elsevier, 2022-01-17) Ramachandran, Dileep Chandran; Figueredo, Bruna; Sherepenko, Oleksii; Jin, Woosung; Park, Yeong-Do; Biro, Elliot
This study focuses on improving the mechanical performance of third-generation Q&P steel resistance spot welds using a double-pulse welding cycle. Single and double-pulse welding schedules were implemented to assess the mechanical performance of the welds. Single-pulse welds exhibited poor cross-tension strength (CTS) values, failed around the fusion zone, and were accompanied by poor energy absorption capability. However, the double-pulse schedule showed improved CTS values by 33%, with an associated 110% increase in absorbed energy. The failure path observed from interrupted cross-tension tests showed that, in welds made using both pulsing schedules, failure proceeded along the fusion boundary and CGHAZ. In the single-pulse welds failed in brittle fashion, whereas the welds made with a double-pulse schedule exhibited a mixed (ductile and brittle) fracture morphology. The high-density microhardness mapping confirmed the presence of a localized softened zone (halo ring) adjacent to the fusion boundary in single-pulse welds. Strong elemental partitioning of Mn, Si, and C in the vicinity of the fusion boundary during long welding time was the primary cause for the halo formation. However, the halo ring was eliminated by performing a double-pulse weld schedule with 30 ms cooling time in between pulses; resulting in improved mechanical properties.
Effect of Paint Baking on the Fusion Boundary Softening and Fracture Behavior of Q&P 980 Steel Resistance Spot Welds
(Elsevier, 2023-10-14) Ramachandran, Dileep Chandran; Betiku, Olakunle Timothy; Shojaee, Mohammad; Salandari-Rabori, Adib; Midawi, Abdelbaset R.H.; Kim, Ji-Ung; Bakhtiari, Reza; Macwan, Andrew; Biro, Elliot
During automotive assembly, vehicles undergo low-temperature heat treatment (paint baking) to harden the paint. Although paint baking occurs at a relatively low temperature, it can remarkably affect the weld's mechanical and fracture behavior. This work studies how paint baking improving the strength and fracture behavior of Q&P 980 spot welds exhibiting a halo ring; a low carbon enriched zone in the weld nugget. The mechanical behavior of the paint-baked welds reveals an increase in cross-tensile strength and absorbed energy when baked at 180 °C for 27 min. Microstructural observation showed that the martensite present in the as-welded conditions started to decompose into tempered martensite with ε-carbide in the martensitic matrix. Gleeble thermo-mechanical simulations of the upper-critical heat-affected zone (UCHAZ) were produced to understand the mechanical and fracture micro-and macro-mechanisms, before and after the paint baking process, by widening the regions of UCHAZ. The transmission electron microscopic analysis of the Gleeble simulated sample reveals the segregation of C, Mn, Al, and Cr along the prior austenitic grain boundary which will change the nature of bonding at these boundaries. Nevertheless, paint baking treatment helps to redistribute the segregated elements from the grain boundary to the grain interior and to eliminate the solidified liquation formed at the grain boundaries during welding. The transformation of martensite to decomposed martensite, elimination of solidified liquation due to the enhanced atomic mobility and growth of surrounding grains, and the redistribution of C, Mn, Al, and Cr from the grain boundary to interior regions of grains, are the main reasons for the improvement of mechanical properties and fracture behavior of the spot welds.
Effect of Paint Baking Treatment on Mechanical Properties of Resistance Spot Welded Q&P 980 Steel
(ISIJ International, 2024-05-15) Ramachandran, Dileep Chandran; Salandari-Rabori, Adib; Midawi, Adbelbaset R.H.; Macwan, Andrew; Biro, Elliot
This study investigates the impact of paint baking on the macro and micro-mechanical properties of resistance spot welds in quenched and partitioned 980 steels. It is observed that paint baking enhances both peak load and energy absorption during cross-tension tests, as indicated by load-displacement curves. Four different regions were identified from the load-displacement curves after paint baking. An intriguing observation was a quick increase in the loading rate following a prior decrease, attributed to change in crack propagation behavior rather than improved work hardening. The study further simulated the upper-critical heat-affected zone using a Gleeble thermo-mechanical simulator to evaluate flow strength and work hardening. The Kocks-Mecking strain-hardening model was employed to analyze work hardening behavior in the studied conditions.
Microstructural Evolution and Formation Mechanism of the Halo Ring in Resistance Spot Welding of a 3G Advanced High Strength Steel
(Springer, 2024-09-09) Ramachandran, Dileep Chandran; Salandari-Rabori, Adib; Macwan, Andrew; Biro, Elliot
The microstructure of the halo ring has been studied in quenched and partitioned (Q&P) steel resistance spot welds. The TEM and EBSD characterizations revealed the presence of an upper bainitic microstructure in the halo ring of the three-sheet stack-up welds. Stalking faults accompanied by nano-twins were identified surrounding the cementite. Diffusion of carbon towards the molten weld pool during solidification led to the formation of bainite at the fusion boundary, triggered the localized softening.