双层保护气体、氮氧联合过渡与焊缝成形和性能控制

An innovative activated welding process named GPCA welding is proposed to simply and conveniently realize continuous, deep penetration and high quality welding. In this process, an inner layer inert shilding gas is used to protect electrode and pool metal, and an outer layer gas is used to simultaneously introduce oxygen and nitrogen elements. By adjusting the coupeld area between the outer layer activating gas and pool metal, the weld appearance and properties can be controlled. To develop this process, except a series of technical matters, several scientific problems have to be soled: 1.Combined transfer bahaviors of nitrogen and oxygen elements; 2. coupling behaviors between the gas and pool metal; 3. Mechanism of driving the weld pool flow; 4. Mechanism of improving the weld properties. In this project, a comprehensive approach including arc spectroscopy diagnosis, quenching method and numerical simulation of pool flow is ultilized to analyze the combined transfer behaviors of nitrogen and oxygen elements. A two-temperature steady-state arc plasma model based on a combined diffusion coefficient driven by temperature gradient, component gradient and ambipolar diffusion between different charged particles is developed to numerically simulate the coupling behaviors between the gas and pool metal.Numerical simulation is carried out to research the mechanism of driving the pool flow, in which a 3D steady-state pool model is applied. The dislocation dynamics and grain orientation of the weld metal are analyzed to find out the mechanism of strengthenning and toughening the weld metal.This study can strengthen the understanding on the mechanism of oxygen and nitrogen elements affecting the welding arc-pool system and weld properties, promote the application and investigation of the GPCA welding, which is of great significance.

提出了一种利用内层惰性气体保护电极和熔池，外层气体同时引入O、N元素，通过调节外层气体与熔池金属耦合程度控制焊缝成形和焊缝性能的新型活性焊接方法（GPCA焊），可简单、方便地实现连续、深熔深、优质焊接，是一种国内外尚未开发的创新性方法。开发该方法除处理相关工艺问题外还需解决以下科学问题：1、氮氧元素联合过渡行为；2、气体熔池耦合行为；3、熔池流动驱动机制；4、焊缝性能改善机理。综合采用电弧光谱法、骤冷法和熔池流动数值模拟方法建立O和N元素从电弧到熔池内的过渡模型；建立基于温度梯队、成分梯度和不同带电粒子双极扩散的联合扩散系数的双温度稳态电弧模型，数值模拟研究气体熔池耦合行为；建立三维熔池模型，数值模拟研究熔池流动驱动机制；进行位错动力学和金相位向分析，研究焊缝金属强韧化的机理。本研究对于增强对O、N元素影响电弧-熔池系统和焊缝性能的机理的认识，推动GPCA焊方法的研究和应用，具有重要意

针对所提出的新型活性焊接方法——GPCA-TIG焊，综合采用电弧光谱法、骤冷法和数值模拟等方法建立了GPCA-TIG焊中氧和氮元素从电弧到熔池内的完整过渡模型。外层气体中的O2和N2进入电弧后主要存在于电弧外围区域，经历解离、电离和复合等一系列物理过程后产生一定的致冷作用收缩电弧，由弱到强的次序为O2＜O2+N2＜N2；当耦合度增加时，这种收缩电弧作用增强。氧和氮元素以原子和分子形式吸附到高温液态熔池金属表面后，将以原子形式进入熔池内部，并由于熔池内的强烈对流而均匀分布。当外层气体中存在氧元素时将在熔池表面发生直接和间接氧化反应生成氧化物，上浮后在熔池表面形成氧元素的偏聚层，当超过溶解度后形成表面氧化渣层；当氮氧元素共同引入时，由于氧元素偏聚层及氧化渣层的存在，将妨碍氮元素从低温区的析出，使得熔池内的氮元素浓度增加。建立了电弧-熔池耦合模型，进行数值模拟发现熔深增加的主要机理在于所引入的氧元素在熔池表面的偏聚使得熔池表面张力温度系数变成正值，且随着径向距离越大数值越大，产生很强的指向熔池中心的Marangoni剪切力，形成强烈的向内环流，促进电弧热量有效地向熔池底部传输，形成窄而深的熔池形状；另外，引入氮元素将增强熔池内的电磁力，也会进一步增加熔深。外层气体引入对GPCA-TIG焊焊缝的拉伸强度影响较小，但对低温冲击韧性影响较大，单独引入O2时将明显降低，单独引入N2时降低较少，而同时引入O2和N2将增加焊缝金属低温冲击韧性。氮氧同时引入增强焊缝金属的低温冲击韧性的机理主要在于：焊缝金属中的氮氧含量相对较少，所形成的夹杂物较少，铁素体数量下降较少，却增加了奥氏体晶粒平均取向差和大角度晶界数量以及铁素体晶粒与奥氏体晶粒之间位向关系的匹配性，从而使得裂纹不易扩展，韧性增强。本项目的研究对于增强对氧、氮元素影响电弧-熔池系统和焊缝性能的机理的认识具有重要意义。