Key Engineering Materials Vols. 340-341

Abstract: Deformation mechanism of room-temperature superplasticity in Zn-22wt%Al alloy was investigated by the direct observation during deformation. It was revealed that the dominant deformation mechanism of room-temperature superplasticity was grain boundary sliding. Also, superplastic properties and microstructure of friction stir welded Zn-22wt.%Al alloy were investigated, where Friction Stir Welding (FSW) has received a great deal of attention as a new solid-state welding technique. A sound jointing material was obtained successfully, and extremely fine and equiaxed grains were created in the stir zone. In addition, it was indicated that superplastic forming of the FSWed Zn-22wt.%Al alloy could be feasible. However, the tensile strength and elongation of the joint at room temperature were lower than that of the base material.

Abstract: The presence of residual stresses is a determining factor in the buckling behaviour of steel beams. It is therefore necessary to know the magnitude and the distribution of residual stresses in welded beams. Experimental measurements of these residual stresses are expensive, timeconsuming and they give no insight in the evolution of the stresses during the welding process. For these reasons it is interesting to have a numerical model which allows a prediction of the residual stresses due to welding and which allows a study of the parameters that have an influence on the residual stresses (e.g. yield stress, heat-input, thermal expansion coefficient, …). Such a numerical model can result in a proposal for a welding process that leads to less detrimental residual stress distributions in welded I-beams. However, the results of the numerical model must always be checked with reality. This contribution contains the experimental verification of the finite element model for both single plates and I-beams with rather small dimensions. First, an overview of generally accepted influencing factors on the residual stresses is given. After this introduction, a setup which allows control of the parameters for making reproducible weld seams is presented. This setup is usable for both plates and I-beams. Next it is described how the residual stresses in a plate and a beam were measured. Finally the results obtained from different configurations in the experiments are compared with the results from the FE-model. It is shown that the FE-model gives reliable predictions for the residual stresses.

Abstract: The flaws such as crack or slag inclusion, in the welded seam of header of a 300MW steam turbine generator unit, were detected by the ultrasonic non-destructive inspection. Based on the crack modeling, the finite element method (FEM) was used to calculate and analyze the displacement field, stress field and stress intensity factors of the cracks under the conditions of warm start, cold start and normal operation, and the residual service life was predicted. Analyzing results showed that these flaws would not bring about primary brittle fracture and propagation of crack was harmless to safe in operation.

Abstract: This study is dedicated to three-dimensional finite element analysis of seaming process, which consists
of bending, curling and caulking process, of a large tubular mechanical bonded structure. The seaming process is often used to improve a high bonding strength as avoiding any kind of defect. Finite element simulations of the seaming process were preformed for two different initial conditions with pre-analyzed results and without those from bending process. The mechanical bonding strength of the seamed area in the large tubular structure was estimated and compared through finite element analysis among several different analysis conditions of the bending and the caulking. Tensile test for the specimen extracted from the large tubular mechanical bonded structure was also executed and compared with the results of finite element analysis, in order to verify which initial condition in finite element analysis was suitable for this kind of multi stage seaming process. As a result, the effect on an accuracy of finite element analysis for the multi stage seaming process was evaluated in this study. Finally, it is noted that the pre-analyzed results from bending process should be considered in order to obtain the accurate results from finite element analysis.

Abstract: The effect of geometric clearances on the stress and deformation distributions of a wedged-ring joint structure is studied, five cases of dimensional tolerance of each contact surface, i.e., the maximum, minimum, mean of maximum and minimum, mean of maximum and mean, and mean of minimum and mean are investigated, and the probable states that the wedged-rings attach with the upper cylinder or attach with the lower cylinder radially in the assembly process are considered as well. The results of finite element numerical simulations indicate that in the design tolerance zone, the effects of geometric clearances between contact surfaces on the displacement and stress distributions of the structural components are weak. Only when the wedged-rings attach tightly with the upper cylinder or with the lower cylinder, the displacement and stress distributions in the joint of the structure occur a dramatic variation, and the assembly processing measure making the wedged-rings attaching tightly with the lower cylinder can be adopted to effectively decrease the stresses and deformations in the joint of the structure.

Abstract: Friction stir processes (FSP) are important for enhancing mechanical properties of metal sheets, such as the tensile strength, the elongation, etc. The stress distribution of the tool pin is affected by the thermo-mechanical characteristics of the workpiece in FSP. Recently, magnesium alloy AZ31 is widely used in machine industries due to the light-weight material property. In this paper, a thermo-mechanical model for FSP using three dimensional FEM analyses is proposed for exploring temperature distributions, strain distributions and stress distributions of the workpiece. The heat generated from the plastic deformation and the friction between the head tool and workpiece is considered as the heat source in the simulation of the FSP process. A commercial finite element code – DEFORM 3D is used to carry out the simulation of the plastic deformation of AZ31 sheets during the FSP. The analytical results of temperature, strain and stress distributions of the workpiece and head tool can provide useful knowledge for tool pin design in FSP

Abstract: Welding is widely used for construction of many structures. It is well known that residual stress is measured on welded joint. In this paper, a new method for reduction of residual stress using ultrasonic vibration during welding is proposed. Two thin plates are butt-welded. When ultrasonic vibration is used on one plate, tensile residual stress is reduced at center of the bead. Second, ultrasonic vibrations with different frequencies are applied on both plates. In this case, reduction rate of residual stress increases. Next, reduction of tensile residual stress is demonstrated by simulation method using an analytical model with dynamic characteristics of welded joint.

Abstract: The joinability of three aluminium alloy sheets using a self-piercing rivet was evaluated from a finite element simulation and experiment. The self-piercing riveting is hopeful as replacement of spot resistance welding generally used for steel sheets, because it is not easy to apply the resistance welding to joining of three aluminium alloy sheets due to the high thermal conductivity. Defects in the riveting are categorized into the penetration through the lower sheet, the necking of the lower sheet and the short overlap of sheets to obtain optimum joining conditions. The penetration and the necking are caused by small total thickness. The short overlap tends to occur as the ratio of lower sheet and total thickness is small. In addition, the cross-tension test was simulated by the finite element method to evaluate the joint strength.

Abstract: Considering the productivity and the maintenance, most of machine and product has many joints (fastening, welding and adhesive joint). Especially, the thread joint has been used the most mainly as a machine element. However, many troubles on strength and reliability of these machine and product are generating in these joints. In this paper, we have evaluated the sliding and the self loosening behavior of thread joints under transverse loading, where the entire response of the thread joint portion exhibits an inelastic one. The finite element analysis using a simple model of thread joints with respect to the relative sliding motion between the parts was performed and compared with the experimental results under quasi-static loading. The critical relative slippage (Scr), less than the value which thread joints can keep the fastening, was obtained by the result of cyclic loading tests. In addition, this critical relative slippage was estimated according to the theoretically obtained equation considering the bolt bending and the geometrical constraint condition. The inclination compliance (kw) of the bolt head used in this equation was evaluated by the comparison of experimental and analytical results from theoretical equation. In consideration of the nonlinearity of kw with respect to the bolt axial tension, the Scr is well estimated by this equation.

Abstract: The formability in friction stir and arc welded 5083 Al alloys was investigated. The elongation in friction stir welded material under uniaxial tensile test increased with the decrease in friction heat flow during friction stir welding (FSW) and the value of dome height under biaxial bulge test increased with friction heat flow. The ductility improved according to changes in FSW condition and FS-welded alloy sheet exhibited excellent formability.