脉冲GTAW焊过程辩识及PID控制

针对脉冲GTAW焊溶池动态过程，利用面积法、Laplace变换的极限理法及最小二乘法对焊接速度与溶池背面熔宽模型进行了辩识。结果表明，面积法更适用于熔池动态过程辩识；对辩识所得到的二阶熔池模型进行了PID控制器设计，并针对起焊时的时滞采用Smith预估器予以补偿，实验结果表明，Smith预估器时滞具有很好的补偿效果。     

Optimization procedures for GMAW of bimetal pipes

Autogenous gas tungsten arc welding (GTAW) and pulse rapid arc gas metal arc welding (GMAW) of butting bimetal (Bubi) pipelines were studied. GMAW was carried out from the outside of the pipe while GTAW was done from the inside to prevent lack of penetration and to promote a smooth internal weld bead surface. Current, welding speed, electrode diameter, shielding gas and orbital positions were defined as variables. The requirement for the GTA weld was to achieve 2 mm penetration depth without undercutting. The required penetration was difficult to achieve due to the outwards flow pattern in the molten pool driven by the Marangoni effect as a result of low sulphur content. It was shown that, under optimised conditions, it was possible to obtain sound welds with proper geometry and defect free. The conditions needed were a combination of current of 170 A, welding speed of 200 mm/min and an electrode angle of 30°, with shielding gas protection of He-25%Ar for narrow groove welding of a J-beveled pipe.     

镁合金焊接热裂纹机理的研究

采用交流氩弧焊对AM50镁合金进行焊接,研究结果表明,当电流达到一定值时,出现焊接热裂纹。热裂纹的产生是由于焊缝金属中出现粗大的晶间低熔点α+β-Mg17Al12共晶体所致。为防止焊接热裂纹的产生,应在工艺允许的情况下,尽量减少焊接线能量。     

Minimization of defects in nickel-based superalloy weld overlay deposited by the GTAW cold wire feed process

The objective of the present work was to evaluate the influence of the welding parameters in the formation of defects in weld overlays with nickel-based alloys, and its possible elimination through the correct adjustment of the welding parameters. Coatings were deposited with the nickel-based alloy types Inconel 625, Hastelloy C276 and Inconel 686 on C–Mn steel plates, using GTAW cold wire feed process. The planning of the experiments was accomplished by applying the Taguchi method. The control factors evaluated were the energy technique (ET), the welding heat input (E), the type of alloy (L), the shielding gas (G) and the type of arc oscillation (T). Other parameters were maintained constant, having previously been investigated. The results showed that the arc oscillation type is spiral; although it contributes significantly to reduce the dilution level, it causes a strong instability to the process, resulting in most of the cases in surface defects or defects among passes. The optimum condition to avoid defects among passes was identified by the Taguchi method, which was constituted by the following combination of control factors: 2-2-2-3-3, or by: I energy technique (TE-I); medium heat input level (E_medium); Hastelloy C276 alloy; shielding gas Ar + He; double-8 arc oscillation. The optimum condition for the welding without defects results in high dilution level not being indicated for the welding of resistant coverings to the corrosion.     

Corrosion behaviour of a dissimilar joint TIG weld between austenitic AISI 316L and ferritic AISI 444 stainless steels

The AISI 444 stainless steel (SS) has become an option to substitute the AISI 316L SS because of its low cost and satisfactory corrosion resistance. However, the use of AISI 444 alloy tubes in heat exchangers causes the welding of a dissimilar joint. The aim of this study was evaluate the corrosion resistance of the tube-to-tubesheet welded by a TIG process composed of AISI 316L and AISI 444. Preparation of samples was executed through replication of tube-to-tubesheet joints. In order to test the corrosion resistance of the welded joint, the following tests were applied: sensitisation, mass loss from room temperature up to 90 °C and electrochemical corrosion tests in 0.5 mol/L HCl and 0.5 mol/L H₂SO₄ electrolytes. The results have shown that the dissimilar joint suffers galvanic corrosion with increased degradation of the heat-affected zone of the AISI 444 tube. Nevertheless, the mechanisms of localised corrosion (pit and intergranular) were more active in the AISI 316L alloy. It is concluded that the dissimilar joint showed better corrosion resistance than the welded joint composed solely of AISI 316L at temperatures up to 70 °C, as the conditions observed in this work.     

7075铝合金电动自行车车架GTAW焊接过程温度场的三维有限元分析及工艺优化

采用焊接专用有限元分析软件对7075高强铝合金电动自行车车架GTAW焊接过程进行数值仿真,分析不同热输入条件下的温度场分布和熔池形态,探索其最优热源参数.结果表明:在最优热源参数下,达到准稳态时的熔池长度为10mm,熔宽为6mm,熔池最高温度不超过1100℃.此外,分析了最优热源参数下的焊接热循环特性.研究结果可为下一步预测焊接变形和残余应力的分布提供热学方面的理论借鉴.     

Two-dimensional arc stagnation pressure measurements for the double-electrode GTAW process

Double-electrode gas tungsten arc welding (GTAW) is a relatively new GTAW variant that has been studied and developed over recent years, with the aim being to achieve a robust high-speed and high-current process with subsequent productivity improvements. In this present study, two-dimensional arc stagnation pressure measurements were performed, allowing a broad visualisation of the arc pressure distribution at the area above the welding torch, with a 0.5?mm resolution. The influence of the distance between the electrodes was evaluated and the results were compared with the conventional GTAW process. It was found that this distance greatly modifies the resulting arc morphology and, consequently, the arc pressure distribution. The results showed that the double-electrode process has much lower pressure than the single-electrode GTAW, for example, using the minimal possible distance between the electrodes results in about half of the maximum pressure measured for the single electrode, which would enable the development of high productivity applications.     

Structural and operating aspects of rapid solidification of surface layer of spheroidal graphite iron castings

The relationship between the surface layer microstructure of iron castings after gas tungsten arc weld (GTAW) surfacing and their service properties (friction wear resistance and microhardness) has been investigated. A structural analysis of the fused (remelted) zone, in particular evaluation of the interphase spacing in the cementite eutectic λ_av, is presented. Relationships between the GTAW process parameters, microstructure and microhardness of the fused zone have been developed and are presented. Formulae relating hardness to λ_αv and relating friction wear rate to λ_av and hardness are presented. IJCMR/426     

氩弧原位合成TiC颗粒增强Fe基复合层

通过在Fe基自熔合金粉末中添加一定比率的强碳化物形成元素Ti及石墨,采用氩弧熔敷法在中碳钢基体上制备原位形成的TiC颗粒增强Fe基合金复合涂层.利用扫描电镜、电子探针、X射线衍射和图像分析系统,对熔敷层显微组织及其影响因素进行了观察与分析.研究结果表明,利用氩弧熔敷技术,可以原位形成细小、弥散分布的TiC颗粒;TiC颗粒沿熔敷层深度呈梯度分布.通过调整预置涂层的厚度、熔敷工艺参数以及石墨的加入量,可以调整控制熔敷层的成形及组织与性能.     

SAF2507超级双相不锈钢焊接工艺

采用GTAW和SMAW两种不同的焊接工艺对SAF2507双相不锈钢板进行对接试验,对比分析两种不同焊接工艺接头的金相组织、力学性能和耐点蚀性。结果表明,使用含N2保护的GTAW焊接接头的双相比例比SMAW更优;采用GTAW焊缝中的奥氏体相含量比SMAW中更高,且前者的延伸率、抗拉强度和冲击功均优于后者;在50℃,6%Fe Cl3溶液中,采用GTAW比SMAW焊缝的耐点蚀性更好。