Bridging behavior of molten pool and its effect on defects formation in ultra-thin sheets edge welding by micro-plasma arc

Abstract

The molten pool behavior and weld formation in ultra-thin (thickness ≤ 0.3 mm) sheets edge welding is extremely sensitive to the variation of thermodynamic conditions, due to its unique heat transfer conditions and molten pool dynamics caused by special joint form and extremely small molten pool size. In this paper, micro-plasma arc source was applied to join the edge joint composed of two 0.12 mm thickness 304 stainless steel diaphragms. The typical molten pool bridging behavior was observed by a high-speed microphotography system. In addition, the formation mechanism of lack of fusion (LOF) defects was analyzed. The experimental results showed that common disturbances could affect the continuity and symmetry of melting process. Due to the instability raised by this melting process, the liquid bridge fails to form or to maintain, which is the major cause for undesirable weld and defects. Unlike sound weld formation process, the molten pool behaviors in LOF defects formation process could be classified into three states: temporarily discontinuous bridging (TDB), cyclically discontinuous bridging (CDB), and not only cyclically discontinuous but asymmetric bridging (CDAB). Comparing the TDB state, the backflow of molten pool under the CDB state tends to be more intense, leading to the occurrence of defects in succession. During the CDAB process, the molten pool is subject to lateral misalignment due to the gravitational component, resulting in asymmetric weld with defects. This study offers a comprehensive insight into molten pool behavior and weld formation process, which can enhance the understanding of ultra-thin sheets edge welding.