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SAVYASACHI NELLIKODE,이한주,허민,김영창,감동혁,김철희,장우석,박영도 대한용접·접합학회 2021 대한용접학회 특별강연 및 학술발표대회 개요집 Vol.2021 No.5
The effect of rivet length and die depth on joint quality of self-piercing riveting (SPR) of DP 590 and AA5052 aluminium alloy dissimilar sheets is investigated. Static cross tension strength and failure mode of DP 590 and AA5052 aluminium alloy dissimilar sheet Joints where estimated using a lower bound limit load based strength estimator. Considering deformed rivet diameter (Dt) and effective rivet leg length (teff) from cross sectional perspective as quality parameters, a combination of rivets and dies with different leg lengths and different die depths are compared respectively. Optimization of rivet leg length and die depth has been obtained for increased static cross tension strength. It is found that effective rivet leg length has more role than deformed rivet diameter in deciding the strength. These results provide an effective way to determine the effective rivet and die combination for self-piercing riveting joining to get increased joint strength
SAVYASACHI NELLIKODE,SIVA PRASAD MURUGAN,백영곤,정진성,김인찬,이찬훈,문준오,박영도 대한용접·접합학회 2021 대한용접학회 특별강연 및 학술발표대회 개요집 Vol.2021 No.5
The objective of the present study is to investigate the impact of bainitic microstructure and Martensite-Austenite constituents (M-A) formed at different inter-critical temperatures on toughness of 355 MPa HSLA steel for offshore structural applications. Inter-critically reheated coarse grained heat affected zone (ICCGHAZ) are produced using gleeble thermal simulation technique. Two samples where simulated corresponding to lower half of the inter-critical region and between Ac1 and Ac3. The fraction of M-A constituents was similar at each temperature. Notable difference in toughness has not been obtained in both samples. Bulk M-A constituents distributed at grain boundaries and film M-A constituents distributed in bainitic ferrite lath inside the grain are seen. Secondary cracks are initiated at MA constituents existed along prior austenitic grain boundaries and propagate linearly into granular bainite (GB)/ bainitic ferrite (BF) area in ICCGHAZ. BF and GB show straight cleavage cracks indicating brittleness. Major percentage of cracks are propagated through BF. Cracks through BF are across the lath created an angle~ 45 to 50 degree with the cracks in all samples. This crack might be a {100}-plane cleavage one as the bainitic ferrite area is known to be quite brittle.
SAVYASACHI NELLIKODE,SIVA PRASAD MURUGAN,백영곤,정승진,김인찬,이창훈,문준오,박영도 대한용접·접합학회 2021 대한용접학회 특별강연 및 학술발표대회 개요집 Vol.2021 No.11
The objective of the present study is to investigate and compare the impact of bainitic microstructure and M-A constituents formed at different inter-critical temperatures on the toughness of 355 MPa and 420 MPa HSLA steel for offshore structural applications. High strength low alloy steels of grade 355 MPa and 420 MPa steels for offshore applications are simulated using Gleeble thermo-mechanical simulator at different inter-critical temperatures between Ac1 and Ac3. In this study, the effects of microstructural features on ICCGHAZ on toughness and crack initiation and propagation were investigated. ICCGHAZ simulated at lower temperature region of Ac1 to Ac3 microstructure composed of bainitic ferrite (BF) and granular bainite (GB). MA constituents are distributed in the form of necklace at prior austenitic grain boundaries. ICCGHAZ simulated at higher temperature region of Ac1 to Ac3 microstructure composed of GB and BF with some acicular ferrite. MA constituents are distributed along the grain boundaries and lath boundaries. Secondary cracks are initiated at MA constituents that existed along prior austenitic grain boundaries and propagated linearly into GB/BF area in ICCGHAZ. BF & GB show straight cleavage cracks indicating brittleness. A significant percentage of cracks initiated through the necklace MA at the grain boundaries easily propagated through bainitic ferritic lath.
이한주,Savyasachi Nellikode,강태영,박영도 대한용접접합학회 2023 대한용접·접합학회지 Vol.41 No.6
The increasing demand for effective joining techniques in dissimilar materials has led to the widespread use of self-piercing rivet (SPR) mechanical joints, particularly in aluminum combinations. Studies in this field aim to enhance joint strength by optimizing process conditions, including the use of SPR-bonded processes with structural adhesives. Structural adhesives in SPR-bonded processes introduce different corrosion behavior owing to galvanic corrosion caused by potential differences between sheets over time. In this study, we utilize SORPAS® simulation to optimize SPR conditions and analyze stress and deformation. A tensile shear test was performed on a hot pressforming steel/AA5052-H32 combination for SPR and SPR-bonded processes. The experimental results indicated die sticking during SPR optimization, increased tensile strength through AA5052-H32 surface cleaning in the SPR-bonded process, varying corrosion conditions, and consistent SPR-bonded strength after salt spray exposure. The difference in failure modes between the processes concerning corrosion time can be attributed to the corrosion- mitigating effect of structural adhesive within the galvanic coupling region formed by potential differences. Additionally, adhesive and rivet failure modes were observed. Moreover, adhesive failure occurred in the SPR-bonded process after shear tensile tests, characterized by a fracture between the lower AA5052-H32 interface and adhesive. Furthermore, we found a linear relationship between the remaining adhesive area and shear tensile strength.