Fatigue analysis of multipass welded joints considering residual stresses

Gas metal arc welding (GMAW) is one of the most used joining method in the industry. However, one of the main problems of this process is the generation of residual stresses (RS). There are different approaches to predict the fatigue life of welded joints, but in general, these approaches do not consider the real value of RS. Therefore, the current approaches to estimate fatigue life of welded components are conservatives.This paper describes an alternative method to asses high cycle fatigue (HCF) life prediction based on numerically estimated RS values. Results have shown good correspondence for the HCF range, with a maximum average error of 15% in stress for the studied configurations. The proposed method can be used as a valid tool to optimise the geometry of the component and thus decrease the economic cost.     

Improving weld formability by a novel dual-rotation bobbin tool friction stir welding

A novel dual-rotation bobbin tool friction stir welding (DBT-FSW) was developed, in which the upper shoulder (US) and lower shoulder (LS) have different rotational speeds. This process was tried to weld 3.2 mm thick aluminum-lithium alloy sheets. The metallographic analysis and torque measurement were carried out to characterize the weld formability. Experimental results show that compared to conventional bobbin tool friction stir welding, the DBT-FSW has an excellent process stability, and can produce the defect-free joints in a wider range of welding parameters. These can be attributed to the significant improvement of material flow caused by the formation of a staggered layer structure and the unbalanced force between the US and LS during the DBT-FSW process.     

Study of residual stress variation with depth of friction stir welded aluminium plates with different thicknesses

ABSTRACT

The residual stress may greatly vary through thickness due to the large temperature gradients and severe plastic deformation in depth for thick section friction stir welded plates. AA 2024-T351 plates with 6.5, 12 and 20?mm thicknesses were joined by friction stir welding to investigate the differences of residual stress variations with depth between thin and thick plates. The surface residual stresses were compared between the X-ray diffraction and contour measurements. Significant variations of stress peaks, root central residual stresses and widths of ‘M’ profiles were observed along the thickness for thick plates. The origins of the aggravated variations with depth were investigated from the temperature gradients and material flow variations through thickness.     

辅助电流对Cu/Al大功率超声波焊接的影响

为获得更高质量的Cu/Al异质金属接头，开展了Cu/Al电流辅助大功率超声波焊接工艺试验，研究了辅助电流对Cu/Al超声波焊接的界面温度、材料塑性流动、界面中间相分布及接头力学性能的影响。结果表明，复合焊件成型良好，其接头抗拉剪力为3030 N，接头的断裂模式为韧性-脆性复合断裂。在同样的焊接时间0.2 s内，随着电流的增大，Cu/Al界面温度增加，金属塑性流动以及界面扩散也随之增强，这说明辅助电流能明显促进界面冶金；相比长时间0.4 s的超声波焊接，辅助电流能在保证界面温度、材料塑性变形的前提下，能明显减薄了界面IMC层的厚度，这是电流增强Cu/Al接头的主要物理机制。研究结果为优化Cu/Al复合焊接头强度提供了参考。     

Non-keyhole Friction Stir Welding for 6061-T6 Aluminum Alloy

A novel non-keyhole friction stir welding technique was proposed to weld the butt joint of 6061-T6 aluminum alloy with the thickness of 6 mm. A sound joint was obtained by this technique, simultaneously eliminating the flash, shoulder mark and keyhole defects. The sleeve directly affected zone (SDAZ) and the sleeve indirectly affected zone (SIAZ) were divided into the joint according to the plunging position of the hollow sleeve. The lack of root penetration defect was avoided when the plunging depth of the hollow sleeve was only 4.2 mm, because the hollow part inside the sleeve improved the material flow below the sleeve. An S-shaped line was left at the SIAZ, and the height of it had the minimum value of 1.47 mm at 20 mm/min. Whether the failure location of the joint was in SIAZ/SDAZ or the heat-affected zone (HAZ) depended on the height and bonding strength of the S-shaped line. The joint fracture location changed from the SIAZ/SDAZ at 35 mm/min to the HAZ at 20 and 30 mm/min. The maximum tensile strength of 224.3 MPa was obtained at 30 mm/min which was 73.7% of that of the base material. The fracture surface morphology exhibited the typical ductile fracture.     

不锈钢电弧铆焊焊点组织及力学性能

采用电弧铆焊的方法对3 mm+5 mm厚的0Cr18Ni9不锈钢钢板采用搭接形式实现连接. 通过优化焊接电流及其作用时间的参数匹配，以期得到最佳的焊点显微组织和力学性能. 采用X射线检测焊点缺陷的存在，使用金相显微镜研究焊点熔深和熔核尺寸的变化，使用电子万能试验机测试焊点的抗剪力. 结果表明，电弧铆焊焊点呈蘑菇状，与基体圆滑过渡，成形良好，无气孔、裂纹等宏观缺陷. 随着第一段和第二段焊接电流的增大，焊点的熔深和熔核尺寸，及焊点抗剪力随之增大. 优化的工艺参数下，熔深尺寸可达4.07 mm，熔核尺寸可达7.73 mm，抗剪力可达23.654 kN，满足生产需求.     

钛与低碳钢的电阻点焊

采用电阻点焊方法对纯钛与低碳钢Q235进行焊接试验，利用扫描电子显微镜观察分析了熔核区组织特性，探讨了焊接电流对熔核尺寸和抗剪载荷的影响. 结果表明，受焦耳热的影响熔核直径随焊接电流的增加而增加，抗剪载荷则随焊接电流的增大而呈先升后降的变化趋势，焊接电流为8 kA时所得接头的抗剪载荷最大，约2.85 kN. 在钢侧熔核区观察到了靠近钢侧厚度约为30～50 μm的TiFe2+α-Fe共晶组织层和粗大TiFe柱状晶；钛侧熔核区主要由靠近钛侧约12 μm厚的TiFe+α-Ti共晶组织层和TiFe柱状晶构成，且观察到了宏观分层现象.     

摩擦变形量对异种金属摩擦焊组织和性能影响研究

针对摩擦变形量对异种金属的焊接质量影响情况认识不充分这一问题,开展了摩擦变形量对焊缝组织及力学性能影响规律的研究。基于微观组织、显微硬度和力学性能的对比分析,对调质态42CrMo与正火态36Mn2V管材不同摩擦变形量的连续驱动摩擦焊进行了研究,为其在工程应用提供理论依据。     

Microstructural Evolution and Mechanical Behavior of Friction-Stir-Welded DP1180 Advanced Ultrahigh Strength Steel

Friction stir lap welding of a DP1180 advanced ultrahigh strength steel was successfully carried out by using three welding tools with different pin lengths. The effects of the welding heat input and material flow on the microstructure evolution of the joints were analyzed in detail. The relationship between pin length and mechanical properties of lap joints was studied. The results showed that the peak temperatures of all joints exceeded A c3, and martensite phases with similar morphologies were formed in the stir zones. These martensite retained good toughness due to the self-tempering effect. The formation of ferrite and tempered martensite was the main reason for the hardness reduction in heat-affected zone. The mechanical properties of the lap joints were determined by loading mode, features of lap interface and the joint defects. When the stir pin was inserted into the lower sheet with a depth of 0.4 mm, the lap joint exhibited the maximum tensile strength of 12.4 kN.