Optimization of pulsed current in resistance spot welding of Zn-coated hot-stamped boron steels
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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.
KeywordsMetallic coatings Hot-stamped boron steel Weldability Resistance spot welding Pulsed current
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