Control Of Welding Hot Crack By Welding With Trailing Electromagnetic Impact

The high susceptibility to hot crack is a shortcoming with welding thin plate-shell structures of high strength aluminum alloy. Basing on the fundamental principle of electromagnetic induction, a new technology of controlling welding hot crack by electromagnetic forces during welding is put forward. It could control magnitude of energy easily and prevent welding hot crack without mechanical contact, without changing components of weld bead and damaging surface layer and affecting fatigue performance.A pulse electromagnetic force device was first developed in this thesis. A electric capacitor was chose to accumulate energy, and the charging voltage could be changed between 0~1000V. The frequency of electromagnetic forces could be set between 9Hz~90Hz. Computer simulation proved the feasibility of design, and the parameters of main elements was selected appropriately. The experiment of the waveform indicated that the device can satisfy the design specification very much.Hall sensor AD22151 and HGT-2100 were employed in order to measure gap magnetic field between coil and workpiece. The measuring circuits were designed respectively. In virtue of them, the magnetic fields of a plane spiral coil, a plane spiral coil with field shaper and a coil with silicon-steel plate were obtained. The measurement results indicated that the magnetic field distribution of the plane spiral coil with field shaper became more concentrative, which contributed to controlling welding hot crack.The plane spiral coil with field shaper was used to study the influence of pulse electromagnetic force on welding hot crack. The results showed that the charging voltage and the frequency of electromagnetic force had great influence on welding hot crack. The technology could prevent welding hot cracking well only when the charging voltage and the frequency of electromagnetic force were in the appropriate range. Under the best condition, the hot crack rate decreased from 47.2% of the conventional weldment to 12.7% with the follow parameters: charging voltage of 200V, frequency of 18Hz and clearance distance of 2 mm between the field shaper and the weldment. Metallographic analysis indicated that pulse electromagnetic force improved the crystalline state of weld bead, refined the grain size and decreased the weld discontinuity . These all contribute to inhibit welding hot crack. Electromagnetic force generated the compression strain on the weld metal in the brittle temperature range, which counteracted the tensile strain leading to hot crack. It can be concluded that the two factors, mechanics and metallurgy, co-work to prevent hot crack.