Key Engineering Materials Vols. 504-506

Abstract: Friction Stir Welding (FSW) is a relatively new solid state joining method that can be used to achieve very good weld quality. This technique is energy efficient, environment friendly, and versatile. The FSW process utilizes a rotating tool in which includes a pin and shoulder to perform the welding process. FSW applications in high strength alloys, such as stainless steel remain limited due to large welding force and consequent tool wear. It has been shown that applying the ultrasonic vibration on some processes such as turning and drilling the resultant forces are decreased and process condition is improved. In this paper the influence of applying vibration on FSW is investigated in simulating tools. For FSW modeling a proper transfer function of axial force has been proposed. The resultant axial force of conventional FSW and Vibration Assisted FSW (VAFSW) are compared in frequency and time domain state spaces. A good correlation between FSW simulation and experiments is observed. For further investigation of VAFSW the response surface of design of experiment (DOE) method is utilized. The influence of changing VAFSW process parameters is investigated. The simulation results indicate that vibration helps to decrease the welding force. Using DOE method the effects of implemented frequency and vibration speed amplitude in FSW are found.

Abstract: The present investigation aims at studying the effect of different tool geometries and process parameters on FSW of thin sheets in AZ31 magnesium alloy. In particular two properly designed tools, with shoulder diameters equal to 8 and 19 mm, were used; each of them was manufactured both in pin and pinless configurations. The effect of the different tool configurations and sizes, and welding parameters on mechanical properties of FSWed joints were analyzed in detail. The results were compared with those obtained on the base material. It was shown that FSWed joints are characterized by strength and ductility values lower than those of base material. Furthermore, the pin tool configuration, with a shoulder diameter of 8 mm, leads to the obtaining of strength and ductility values higher than those provided by the pinless one. A strong beneficial effect is obtained by increasing the shoulder diameter from 8 to 19 mm using the pinless configuration, whilst the FSW with the pin tool is critically affected by the welding conditions. The experimental work was joined to a numerical investigation based on finite element method (FEM) in order to study the material flow occurring during the welding process as well as the distribution of temperature, with the aim to identify a input window of the process parameters within which sound joints can be obtained.

Abstract: Friction stir welding (FSW) is an innovative joining technique, in which a solid state weld is obtained by means of frictional heating and plastic deformation of the processing material. In recent years, an increasing application of the FSW process has been observed, however, for the effective implementation of the technique in safety-critical components and to predict the fatigue behavior of FSWed assemblies, an accurate knowledge of the process-induced residual stress is needed. In this paper results, provided by an experimental analysis on the influence of process parameters on the residual stress in 4mm AA2024-T3 butt joints are reported and discussed. The contour method has been applied to evaluate the residual stress. An asymmetric stress distribution has been found in all samples. A significant dependence of the tensile and compressive stress peaks on the feed rate has been found, while a non monotonic influence of the rotating speed has been evidenced.

Abstract: Friction Stir Welding (FSW) is a solid-state welding process introduced and developed in last decades. In this process a rotating tool is pressed on the two parts to be welded (mainly two plates), driven into the material and then translated along the parts interface. Academic and industrial interest is focused on the characteristics of the joined part in terms of mechanical resistance and fatigue resistance of the joints. These characteristics are heavily related to the process parameters chosen since the material stirring and the material temperature greatly depend on the pin rotating and translating speed. In fact, the stirring phenomena and the friction acting between the shoulder of the pin and the sheets, greatly increase the part temperature so that the material greatly changes its structural characteristics due to softening effect: grain dimensions, local hardness, grain orientation. Moreover, due to the physical material movement different types of defects (mainly voids) can be present in the welded zone (nugget). In particular three different areas can be identified: the heat affected zone (HAZ), the thermo-mechanical affect zone (TMAZ) and the nugget. The extension and the characteristics of these zone are very important in order to define the joint quality. These investigations are very important especially when FSW is applied in industrial fields such as aerospace, automotive and naval. To cut and to investigate an experimentally obtained joint is interesting for understanding the weld quality, but FEM simulation of the process can add very useful information in defining how the process parameter influence the joint behaviour and the three different zone extensions. As an example the heat flux, and consequently the temperature distribution inside the material, depend on the combination of rotation and welding speeds. For this reason, in the last years several efforts were oriented to the numerical simulation of the process, in order to investigate thermo-mechanical aspects, stress and strain distributions, thermal flow, residual stresses. The present paper deals with the set up of a FE model for the simulation of the FSW process whose results are correlated with the experimental observations carried out when joining AA6060-T6 aluminium alloy plates 5mm thick with a cylindrical tool with flat shoulder. The experimental campaign was performed under different welding conditions varying the tool rotational speed and the welding speed. A three-dimensional piezoelectric load cell was used to measure the welding forces in the main directions. The numerical model was developed and set up in DEFORM 3D environment. The information obtained from the model helped in the understanding of the welding phenomena.

Abstract: In the FEM calculation of sheet metal forming processes to determine the failure case, the forming limit diagram is basically used. To determine the failure case at bending condition, the forming limit diagram can not be used. This behaviour was shown by many authors. Bending tests with an aluminium material (AC170PX) have shown that a high deformation ratio can be achieved without failure. Based on the loading conditions and the previous strain path through the deep-drawing process, a resulting bendability at a certain point can be obtained. Depending on the pre-damage and the mentioned loading conditions of the material failure will be occurring during bending at different times. Current developments of failure criteria consider the failure as in ductile fracture or shear fracture, which must be considered separately in the simulation. To rule out a separate analysis of the mode of failure in the post-processing, an existing failure criterion is extended and will be presented in this work. For the applications flanging and hemming the following extension of a stress-based failure criterion is proposed. Based on the triaxiality and the equivalent plastic strain a monitoring of the stress ratio is implemented in the FEM simulation. During the forming simulation the monitoring system observe the stress ratio based on the principal stresses resulted from the integration (Gauss) point of the shell element. According to the evaluation of the stress ratio evolution, a relevant definition will take into account how the damage will be accumulated. If the critical value of damage in the integration point of the shell element is reached, failure will be occur based on the position of the sheet thickness.

Abstract: A basic approach for lightweight construction on the basis of material design is to join different materials in one component. In practice bonding, joining-by-forming or the combination of both operations ore often used in these cases. The combination of bonding and mechanical joining has great potential. For assemblies, in automobile production for example, special features can be carried out, such as high strength and stiffness, leak-tightness, corrosion resistance, accuracy and crashworthiness. One challenge is the uncertainty about the stability of hybrid joining processes, which relies on deficits in the understanding of the adhesive flow during the joining operation. This article outlines how process understanding can be expanded due to numerical simulation. As a result, the mutual influences of mechanical joining and adhesive bonding are worked out numerically and experimentally.

Abstract: In this paper a method for simulating hybrid joining processes will be described. The simulation of joining processes with adhesives is necessary because mechanical joining processes are mostly applied in combination with adhesives. However, the simulation of hybrid joining processes is not state of the art. The reason is the Fluid-Structure-Interaction between the adhesive and the plates, which occurs due to the highly uneven stiffness of the materials. This problem is minimized by the use of an elastic‑plastic material model and a Lagrange formulation for the adhesives. The parameters for the material formulation and the friction have been evaluated by conducting a design of experiment. The correlation between simulation and experiment is ensured by an evaluation of the geometrical distribution of the adhesives within the clinch‑bonding point and the force-displacement diagram. Considering the very low computing time of approximately thirty minutes, the obtained result is very satisfying.

Abstract: A novel clinching process design to join thin metal sheets and foils is proposed. In this process a small cup is first stretched out of the sheet plane. A shape interlock between the two sheets is then created by a movement of the die and blank holder in the sheet plane lateral to the cup wall. Such an approach makes the undesired upsetting of the cup bottom redundant, which promises less energy consumption and a longer tool life when compared to conventional clinching processes. The process principle and its finite element analysis for austenitic steel sheets as well as deep-drawing steel sheets (both 0.3 mm thick) are presented. The numerical results suggest feasibility of the proposed design to produce a good shape interlock between two thin sheets.

Abstract: This contribution investigates the springback behavior of several advanced high-strength sheet steels (TRIP, Dual-Phase, ferrite-bainite) with thicknesses up to 4 mm. Samples were tested by means of the bending-under-tension (BUT) test. This test proved very useful to discriminate constitutive models, while avoiding the interference of friction in the springback investigations [1,2]. However, the interpretation and numerical simulation of the test have to be carefully performed [3,4]. The applicability of several guidelines from the literature was investigated experimentally and numerically, in the context of thick AHS sheets. The monotonic decrease of springback as back force increased was confirmed for this category of sheet steels, and a general trend for the non-linear influence of the tool radius was observed. The influence of numerical factors on the predicted values of springback was investigated. Conclusions and simple guidelines are drawn from the analysis with industrial sheet forming applications in mind. References [1] T. Kuwabara, S. Takahashi, K. Akiyama, Y. Miyashita, SAE Technical paper 950691 (1995) 1-10. [2] I.N. Vladimirov, M.P. Pietryga, S. Reese, Prediction of springback in sheet forming by a new finite strain model with nonlinear kinematic and isotropic hardening, Journal of Materials Processing Technology 209 (2009) 4062-4075. [3] W.D. Carden, L.M. Geng, D.K. Matlock, R.H. Wagoner, Measurement of springback, International Journal of Mechanical Sciences 44 (2002) 79-101. [4] K.P. Li, W.P. Carden, R.H. Wagoner, Simulation of springback, International Journal of Mechanical Sciences 44 (2002) 103-122.

Abstract: Bending in a V-die has been used to indicate the outcome of bending in cold roll forming, although little direct correlation has been performed. In this work direct comparison of the springback in both processes was performed using six samples of automotive steels in a conventional roll forming line where the transverse springback is measured. A bend of similar radius was formed in a V-die and the springback determined. In general, the springback in V-die forming was greater than in roll forming, in some cases by a factor of 2. The theoretical springback angle was determined for all steels using a simple and approximate analytical equation and compared to the experimental roll forming and bending results. While for the roll forming process good agreement was achieved the theoretical values significantly underestimated springback in the V-bending process.