摘要
高强铝合金薄板-壳结构焊接时,存在焊接热裂纹敏感性高的缺点。本文从力学角度出发,根据电磁感应原理提出了一种利用电磁力随焊控制焊接热裂纹的新方法。该方法不改变焊缝成分,能量易于控制,可实现无机械接触控制热裂纹,焊件表面无损伤,不影响接头疲劳性能。
为满足研究需要,本文研制了一套脉冲电磁冲击装置,设计功率为10kW。该装置以电容为储能元件,充电电压在0~1000V之间灵活选择,可以产生9~90Hz的脉冲电磁力。通过计算机仿真验证了设计方案的可行性,并结合实际选择主要元件参数。实测结果表明该装置性能满足设计要求。
为测量线圈与工件间隙之间的磁场分布,选择霍尔传感器AD22151和HGT-2100作为测量元件,分别设计检测电路,并测量了平板螺旋线圈、平板螺旋线圈加集磁器和铁芯线圈的磁场分布。结果表明,平板螺旋线圈加集磁器后,可将磁场集中分布在焊缝区域,对控制焊接热裂纹更为有利。
采用平板螺旋线圈配合集磁器研究了随焊电磁冲击对焊接热裂纹的影响。试验发现,脉冲电容充电电压和脉冲电磁力频率对试件热裂纹率影响很大,电压和频率过大或过小均不利于热裂纹的抑制。最佳效果时,裂纹率由常规焊时的47.2%降至12.7%,采用规范为:脉冲电容充电电压200V,电磁力频率18Hz,集磁器与焊件距离2mm。
金相分析表明,随焊电磁冲击改善了焊缝金属的结晶状态,细化了焊缝晶粒,减少了焊接缺欠,增强了抗热裂能力。同时,电磁力挤压脆性温度区间的焊缝金属,产生横向压缩应变抵消凝固过程中的致裂拉伸应变。随焊电磁冲击防止热裂纹的机理可以归结为冶金因素和力学因素共同作用的结果。
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.
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