On formability of tailor laser welded blanks of DP/TRIP steel sheets

Abstract

This paper aims to evaluate the formability of tailor welded blanks of dual phase (DP600)/transformation induced plasticity (TRIP700) steel sheets. In this work, bead on plate butt joints of 2·5 mm DP600 and 1·2 mm TRIP700 steel sheets were performed using CO₂ laser beam welding. Microhardness measurements and transverse tensile testing were carried out to characterise the welds. The formability of base metals and welds were investigated by standard Erichsen test. In a perpendicular tensile test to the weld line, all specimens were fractured at the TRIP base metal, and the strengths were somewhat higher than those of base metal. There was a significant reduction in formability caused by welding of the DP600/TRIP700 steel sheets, and the formability increased with increasing welding speed.     

Influence of clamping support distance on distortion of welded T joints

Abstract

A study is presented on the influence of the clamping support distance on welding distortion of 6 mm thick gas metal arc welded structural steel T joints. Experiments were performed for two clamping conditions and are compared to simulations based on a previously presented model. It was found that clamping closer to the weld seam reduces angular as well as bending distortion. In order to assess the accuracy of the results, sensitivity analyses were performed on the influence of predeformation and material parameters. It is shown that an initial predeformation has an influence on the final welding distortion. Additionally, it has been found that selection of material properties within the range of variations permitted by the material norms has a strong influence on the predicted distortion. Results were improved using material specific data rather than generic data for the relevant material class. Also the assumed phase volume change plays an important role.     

Microstructures and properties of laser/arc hybrid welds and autogenous laser welds in pipeline steels

Abstract

Lasers are capable of producing welds with deep penetration, low distortion and faster travel speeds, compared to arc welding. More recently, laser/arc hybrid welding processes have also been generating interest for industrial fabrication. In this paper, six carbon–manganese, mainly pipeline, steels were welded using both autogenous Nd:YAG laser welding, and Nd:YAG laser/MAG hybrid welding. The improvements in weld microstructures and weld metal toughness that are possible when using the hybrid process are described and illustrated. Laser/arc hybrid welding is shown to be a process that can generate good quality welds in commercially available pipeline steels. It also has the potential to complete girth welds in these steels with significantly fewer welding passes than are currently required for arc welded pipelines, reducing the joint completion time.     

Numerical simulation of heat transfer and fluid flow in GTA/Laser hybrid welding

Abstract

In order to understand the temperature fields, cooling rates and mixing in the weld pool, a comprehensive, three-dimensional heat transfer and fluid flow model is developed and tested by comparing model predictions with two sets of experimental data. The first set of data was taken from the literature. The experiments varied the separation distance between the heat sources for three arc current levels at a constant laser power. The second set of experiments analysed the effect of varying laser power for a constant heat source separation distance. The results demonstrate that the distance between the two heat sources significantly affects the cooling rates. The calculated results showed that the hybrid weld pool was very well mixed with strong convection currents resulting from the interaction between the electromagnetic and Marangoni forces. The calculated and experimental results showed that hybrid welding increases the weld pool width and gap bridgability when compared with laser welding. The weld pool depth in hybrid welding was affected mainly by the characteristics of the laser beam. Hybrid weld pool penetration depth is maximised at an optimal distance between the arc electrode and laser beam. The cooling rate increases significantly when the heat sources are separated beyond a critical distance. At close separation between arc and laser, calculations show that the arc radius must be decreased to achieve the observed weld depths.     

Influence of process parameters on microstructural evolution and mechanical properties in friction stirred Al-2024 (T3) alloy

Abstract

Commercial Al-2024 (T3) alloy was friction stir welded at various heat index (HI) values using bead on plate approach. Quantitative analysis using electron microscopy and differential scanning calorimetry (DSC) revealed a complex variation in the precipitation evolution of Guinier-Preston-Bagaryatskii (GPB) zone and Al₂CuMg (S phase) precipitates in nugget and heat affected zone (HAZ). Differential scanning calorimetry data also suggested formation energy of GPB zone equal to ?236 J g^?1. Tensile properties attained a maximum for HI values close to 3·94 in the nugget region, which was attributed to corresponding minimisation of the volume fraction of coarse S phase to the profit of GPB zone. The precipitation of fine S phase precipitates resulted in higher tensile properties in HAZ as compared to those in nugget for all HI values. Experimental data were used to determine major strengthening mechanisms by using constitutive relationships.     

Influence of resistance heating on self-piercing riveted dissimilar joints of AA6061-T6 and galvanized DP590

The hybrid use of aluminum alloy and advanced high strength steel (AHSS) has become an inevitable trend for fabricating a lightweight auto-body. Self-piercing riveting (SPR) as a preferred cold-forming fastening method is facing problem like weak interlocking when joining dissimilar combinations with considerably unequal thickness. In this study, a hybrid joining method, named rivet-welding (RW) was proposed to improve the robustness and strength of the SPR joint, by applying an electric current to it. For better evaluating the new process, the effects of heating time and electrode design on the microstructure, micro-hardness distribution, and mechanical performance of the RW joints were studied and compared systematically with the traditional SPR ones. The results showed that the electric current could improve the microstructure of the steel rivet and bottom DP590, and under long heating time, the inter-metallic compounds (IMC) could be formed at the interface of trapped AA6061-T6 and bottom DP590. Meanwhile, the electric current could increase the micro-hardness of the rivet and bottom DP590, and soften the AA6061-T6 around the rivet leg. In addition, the RW process using lower annular electrode A (LAE_A) could obtain 12.1% higher tensile-shear strength compared with the traditional SPR process.     

保温时间对20G/316L异种金属TLP焊接接头组织及性能的影响

以非晶态Ni箔为中间层材料,采用高频感应加热系统,对20G/316L进行了瞬间液相扩散(TLP)连接试验,研究了保温时间对接头界面组织、元素扩散迁移情况以及力学性能的影响。结果表明,在焊接温度为1 120℃,保温时间为30～60 min时,20G碳钢与316L不锈钢之间能实现较好的连接;在焊接过程中,不锈钢侧基体中有碳化物析出,且随着保温时间的延长,类奥氏体组织逐渐形成;在较短的保温时间里,中间层Ni元素优先向不锈钢侧扩散,形成一定宽度的扩散层;随着保温时间的增加,碳钢侧界面出现弯曲,界面元素互扩散通量增大;当保温时间增加至50 min时,界面元素扩散充分,细小的奥氏体组织重新生成,接头抗剪强度达到峰值(351 MPa)。     

高温回火对Q235/P91异种钢熔焊接头组织结构与性能影响

采用光学金相显微镜、维氏硬度计等测试方法,研究焊后不同保温时间和高温回火热处理对Q235/P91异种钢熔焊接头显微组织特征及硬度分布的影响。结果表明,经过不同保温时间的高温回火处理,Q235钢侧热影响区靠近熔合线附近,铁素体组织比较细小;过热区附近的奥氏体晶粒发生严重长大;相变重结晶区将得到更加细小的铁素体显微组织。P91钢侧热影响区分别是完全淬火区和不完全淬火区组织。且随着焊接热输入的增大,热影响区组织显著变大,其硬度也显著提高;而保温时间越短,焊缝区组织越细小,硬度也越高。其中保温1.5 h的焊缝区硬度比保温2.5 h的高约40 HV,保温2.5 h的焊缝区硬度比保温3.5 h的高约40 HV。     

Structure and mechanism of formation of dissimilar welded joints in nuclear power plant made of austenitic and pearlitic steels

The structure and mechanism of the formation of welded joints in pearlitic and austenitic steels in power engineering structures are analyzed. The results of investigations of the crystal structure of metals and metallurgical processes taking place in the welding zone are presented. Recommendations are given for improving the welding technology and welding materials, and the chemical composition of the materials used in these applications is determined more accurately.     

Influence of the shape and position of the preform in the precision forging of a compressor blade

The influence of the shape and the initial position of the preform in precision forging of a compressor blade has been studied by 3D rigid–viscoplastic finite element method (FEM) in this paper. The results show the following: (1) a short and thick preform results in necking at the arc transition zone between the tenon and the blade body, and at the zone near the tip of the blade during deformation; (2) a long and slender preform produces bending in the middle of the blade body during deformation; (3) if a long and slender preform is bent in advance, an advisable product without flash can be obtained; (4) a reasonable initial position of the preform deviates to the thinner side of the blade die cavity; (5) this research is beneficial for the practice of the precision forging process of a compressor blade and it has a general significance for other types of blade.