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Factors affecting shrinkage voids in advanced high strength steel (AHSS) resistance spot welds
Vijeesh Vijayan,Siva Prasad Murugan,지창욱,손성국,박영도 대한기계학회 2021 JOURNAL OF MECHANICAL SCIENCE AND TECHNOLOGY Vol.35 No.11
Among various defects formed in resistance spot welds of advanced high strength steels (AHSS), shrinkage voids are seldomly studied. Apparently, the voids are thought to be formed towards the end of solidification due to the material volumetric shrinkage. Although, the voids are reported to be influenced by rich material chemistry of AHSS, the exact mechanism of its formation in RSW is yet to be understood. Hence, this paper attempts to study the influence of various RSW parameters on the void size and its formation mechanism. Computed tomography (CT) technique was utilized to visualize and quantify the void formed in a nugget and to study the influence of resistance welding parameters such as weld current, squeeze force and non-welding factors such as sheet thickness material strength on the void volume. A comprehensive analysis reveal that the parameters such as weld current (nugget size), sheet thickness and sheet strength had direct influence on the void volume, but the influence of electrode squeeze force was dependent on the nugget size. Besides, an attempt was also made to develop an empirical relation for the prediction of void volume formed in resistance spot welds of AHSS utilizing the results of present study.
Vijeesh Vijayan,Nazmul Huda,Siva Prasad Murugan,정찬영,노승만,강남현,박영도 대한기계학회 2017 JOURNAL OF MECHANICAL SCIENCE AND TECHNOLOGY Vol.31 No.9
The selection and application of the type coating to steel sheets in automotive industries depend on factors like corrosion protection, manufacturing compatibility and cost. The ‘first generation’ type primer coating improved the corrosion protection of chromate free pretreatments, however it deteriorated the resistance spot weldability of the sheets. The development of 2 nd generation primer coating with additives such as CNT showed dominance in corrosion protection properties along with zinc. The present paper investigates the resistance spot weldability of the CNT added primer coating. The weldability was evaluated by comparing the CNT added primer with 1 stgeneration primer for nugget size and electrode life. At lower welding currents, the nugget size of the CNT added primer was significantly lower than the primer coated steels. The increase in the dynamic resistance of the CNT coated steel after nugget formation led to higher nugget sizes at higher currents. The decrease in aluminum and increment in the Fe concentration at the electrode surface during welding of CNT based primer coated steels would improve the electrode life.
Optimization of pulsed current in resistance spot welding of Zn-coated hot-stamped boron steels
천주용,Vijeesh Vijayan,Sivaprasad Murgun,박영도,김재훈,유지영,지창욱 대한기계학회 2019 JOURNAL OF MECHANICAL SCIENCE AND TECHNOLOGY Vol.33 No.4
Metallic coatings have been widely used for hot-stamped boron steel in automotive applications. Surface coating in hot-stamped boron steels has improved corrosion resistance. However, a thick coating layer degrades resistance spot weldability. In the case of a Zn-coated hot-stamped boron steel, the high contact resistance and material stiffness with low vaporization temperature of the alloyed layers result in a narrow current passage at the faying surface. Expulsion occurs at low current due to rapid heat development, resulting in a narrow weld current range. The pulsed current can control the heat input by applying a fast cool time during welding to improve the mechanical properties of the joint and acceptable current range. In this study, Zn-coated hot-stamped steels were resistance spot welded using various pulse current procedures, and welding condition was optimized via three pulsed current steps. Step 1: Contact area was controlled without nugget formation. Step 2: Nugget growth was minimized and contact area was maximized. Step 3: The maximum acceptable current range and nugget diameter were achieved. These welding conditions improved the acceptable current range and mechanical properties of spot welds due to increased spot weld size.
The Resistance Spot Weldability of a Stainless Steel/Aluminium/Low Carbon Steel 3-Ply Clad Sheet
Siva Prasad Murugan,Muralimohan Cheepu,Vijeesh Vijayan,Changwook Ji,Yeong-Do Park 대한용접·접합학회 2018 대한용접·접합학회지 Vol.36 No.1
The present paper is aimed at investigating the resistance spot weldability of a 3-ply clad sheet comprised of stainless steel (SS), aluminium (Al) and low carbon steel (LCS). The weldability was assessed with the help of the weldable current range (WCR), the current range between the minimum current required to produce 4√t nugget diameter and a maximum current at which expulsion occurs. However, electrode-sheet sticking was found as the limiting factor instead of expulsion. It was observed that the introduction of aluminium to the SS/LCS clad increased the weld current range by shifting the expulsion to higher current, but resulted in melting of the electrode and subsequent electrode-sheet sticking.