Improvement of formability for the incremental sheet metal forming process

In order to obtain competitiveness in the field of industrial manufacture, a reduction in the development period for the small batch manufacture of products is required. In order to meet these requirements, an incremental sheet metal forming process has been developed. In this process, a small local region of a sheet blank deforms incrementally by moving a hemispherical head tool over an arbitrary surface. In this work, an incremental sheet metal forming process controlled three dimensionally by a computer has been accomplished. It has been shown by the experiments that a sheet blank is mainly subject to shear-dominant deformation. Therefore, the final thickness strain can be predicted. The uniformity of thickness throughout the deformed region is one of the key factors to improve the formability in the sheet metal forming processes. Using the predicted thickness strain distribution, the intermediate geometry is decided in the manner that a shear deformation is restrained in the highly shear-deformed region and vice versa. This double-pass forming method is found to be very effective so that the thickness strain distribution of a final shape can be made more uniform.     

An experimental and analytical investigation of in-plane deformation of 2036-T4 aluminum sheet

Sheet aluminum alloy (2036-T4) specimens of several geometries were photogrid-ded and pulled in a tensile testing machine while precision photographs were taken of the photogrid. This technique allowed determination of strain distributions and load-displacement points. These results are compared with corresponding results obtained by Finite Element Modeling based on Hill's anisotropic plasticity theory and experimental tensile stress-strain data. FEM predictions and experimental results are in excellent agreement; verifying Hill's model for the case of in-plane deformation of 2036-T4 aluminum alloy between the strain states of plane strain tension and uniaxial tension.     

Effects of sheet thickness and anisotropy on forming limit curves of AA2024-T4

In this study, the effects of sheet thickness and anisotropy of AA2024-T4 on forming limit curve (FLC) are experimentally investigated according to ISO 12004-2 standard. A new limit strain measurement method is proposed by using the grid analysis method so as to determine limit strains conveniently and reliably. In addition to the regular test specimens, various widths are added to enhance the FLC’s accuracy at the plane strain condition (PSC). The accuracy and reliability of the proposed method are verified for different materials. Results illustrate that an increase in the sheet thickness increases the FLC level. The additional experiments for additional widths improve the accuracy of the FLC at the PSC, and the position of the lowest major strain value differs from the literature. However, the effect of anisotropy on the FLC is found to be insignificant. Finally, experimental and numerical case studies are carried out for conventional deep drawing, stretch drawing, and hydraulic bulge processes. Results reveal that different FLCs are necessary for different thicknesses for accurate predictions.     

Investigation of the dislocation slip assumption on formability of BCC sheet metals

This paper explores the right-hand side of the forming limit diagram (FLD) for a BCC material in order to test the crystallographic slip assumptions. BCC crystals are considered with either 24 or 48 slip systems (BCC24 or BCC48). Identical uniaxial stress responses are assumed in order to compare the predicted FLDs. FLDs are performed using a rate-dependent polycrystal viscoplastic model together with the Marciniak–Kuczynski (M–K) approach. It is verified that the predictions of the limit strains carried out with the full-constraints (MK-FC) model are strongly affected by the selected deformation modes, showing unrealistically high limit strains in balanced-biaxial tension. Much more reliable values are found with the viscoplastic self-consistent (MK-VPSC) approach using either a BCC24 or BCC48 assumptions, enhancing the relevance of the selected transition scale model. Discrepancies between the numerical results, obtained using MK-FC and MK-VPSC, are interpreted in terms of the differences in the active slip systems selected by each model, and consequently, in the predicted lattice rotations and local curvature of the yield locus. Finally, it is found that the calculation of the FLD with MK-VPSC, using 48 slip systems, successfully predicts the right-hand side experimental tendency observed in a low carbon steel sheet metal obtained by bulge test.     

Formability evaluation of friction stir welded 6111-T4 sheet with respect to joining material direction

In order to evaluate the formability of friction stir welded (FSW) automotive TWB (tailor-welded blank) sheets with respect to base material direction, the aluminum alloy 6111-T4 sheet was joined with three different types of combination: RD||RD, TD||RD, TD||TD (Here, RD and TD mean the rolling direction and transverse direction, respectively). Formability performance was experimentally and numerically studied in three applications including the simple tension tests, hemisphere dome stretching and cylindrical cup drawing tests. For numerical simulations, the non-quadratic orthogonal anisotropic yield function, Yld2004-18p and the isotropic hardening law were implemented into the material constitutive model. As for the failure criterion, the forming limit diagram (FLD) was utilized to determine the failure strain.     

A study on natural aging behavior and mechanical properties of friction stir-welded AA6061-T6 plates

Mechanical properties, microstructural events, residual stresses, and aging behavior of friction stir-welded AA6061-T6 were investigated in this work. Microstructural and mechanical characterizations of the friction stir-welded joints in as-welded and post-welded conditions were made by means of optical metallography, transmission electron microscopy, X-ray diffraction for determination of residual stresses, tensile testing, and hardness measurements. It was found that weld strength and hardness variations after welding are mainly dependent on the imposed heat input per unit length. Besides, the kinetics of natural aging in the welded samples was found to be noticeable within the first 14 days, and its effect decreases considerably in longer aging durations. The residual stress measurements show that subsequent natural aging leads to considerable relaxation of residual stress of about 22 MPa, while this effect is particularly significant in the stir zone and the thermomechanically affected zone.     

Microstructural and mechanical properties of dissimilar friction stir welds between AA6082-T6 and AA7075-T651

In the present work, similar and dissimilar friction stir welds have been produced on 6-mm-thick plates of AA6082-T6 and AA7075-T651. The microstructural characteristics and the mechanical response of both similar and dissimilar welds were investigated aiming to determine the major differences between them. Material mixing of the dissimilar weld nugget, which was created after the welding process, was studied in order to determine the produced different areas and their dominant alloying elements in this zone. Microstructural investigation was made in the welding zones of similar and dissimilar friction stir welds and indications of partial dynamic recrystallization were observed in the thermomechanically affected zone of the similar welds. Transverse and longitudinal microhardness distributions determined the heat affected zone as the weaker area in the welded specimen. After tensile testing, the fracture of the similar and the dissimilar welds at heat affected zone demonstrated the good bonding and weld quality of the similar and dissimilar weld nuggets.     

Effect of weld parameters on mechanical properties and tensile behavior of tungsten inert gas welded AW6082-T6 aluminium alloy

The effect of different welding parameters on the mechanical properties and tensile behavior of tungsten inert gas (TIG) welded joints was analyzed. Four different groove angles were chosen, 60°, 70°, 80° and 90°, to ascertain the tendency of microstructure formation and quality of the weld. Mechanical properties were assessed in the terms of Vickers HV1 hardness. Microanalysis of test samples produced using different current 165 A, 180 A, 200 A with same groove angle of 90° was done in fusion, partially melted, and heat affected zone; all the images showed good penetration and clear transition from one to following zone. The transverse tensile tests were accomplished on the welded joints to evaluate influence of welding parameters and groove geometry to the joint tensile strength and its behavior during exploitation. It was verified that the tensile strength of the welds is closely related to the welding parameters. The chosen 180 A welding current ensured highest tensile strength of test samples; the same as proper selection of groove angle (90°) provides good fusion and high quality of major welds. The results revealed that the weld penetration depends on welding current.

     

On the FSW of AA2024-T4 and AA7075-T6 T-joints: an industrial case study

The paper presents an artificial neural network-optimization hybrid model to predict and optimize penetration depth of CO₂ LASER-MIG hybrid welding used for 5005 Al–Mg alloy. The input welding parameters are power, focal distance from the work piece surface, torch angle, and the distance between the laser and the welding torch. The model combines single hidden layer back propagation artificial neural networks (ANN) with Bayesian regularization for prediction and quasi-Newton search algorithm for optimization. In this method, training and prediction performance of different ANN architectures are initially tested, and the architecture with the best performance is further used for optimization. Finally, the best ANN architecture is found to show much better prediction capability compared to a regression model developed from the experimental data.     

Formability and forming force in incremental sheet forming of AA7075-T6 at different temperatures

Journal of Mechanical Science and Technology - The AA7075-T6 sheet recently received attention, owing to its low weight and strength for use fabricating automotive parts (e.g., body, motor case)....     

Development of a layered manufacturing system using sheet metal-polymer lamination for mechanical parts

Layered manufacturing enables us to make 3D objects rapidly and directly from 3D-CAD data and enables us to handle any design changes. But materials such as light cured polymers, paper, or wax don’t have enough strength to be used directly as mechanical parts. In order to solve such problems, layered manufacturing using both sheet metal and polymers is proposed in this paper. By using sheet metal and polymers, high strength parts with high dumping functions, suitable for use as mechanical parts can be manufactured. Also, a special unit for layered manufacturing is developed, which is designed to be coupled with a desktop NC milling machine, and can make 3D objects easily.     

Formation mechanism and modeling of surface waviness in incremental sheet forming

Improving and controlling surface quality has always been a challenge for incremental sheet forming (ISF), whereas the generation mechanism of waviness surface is still unknown, which impedes the widely application of ISF in the industrial field. In this paper, the formation mechanism and the prediction of waviness are both investigated through experiments, numerical simulation, and theoretical analysis. Based on a verified finite element model, the waviness topography is predicted numerically for the first time, and its generation is attributed to the residual bending deformation through deformation history analysis. For more efficient engineering application, a theoretical model for waviness height is proposed based on the generation mechanism, using a modified strain function considering deformation modes. This work is favorable for the perfection of formation mechanism and control of surface quality in ISF.     

Stochastic modeling of friction force and vibration analysis of a mechanical system using the model

The squeal noise generated from a disk brake or chatter occurred in a machine tool primarily results from friction-induced vibration. Since friction-induced vibration is usually accompanied by abrasion and lifespan reduction of mechanical parts, it is necessary to develop a reliable analysis model by which friction-induced vibration phenomena can be accurately analyzed. The original Coulomb’s friction model or the modified Coulomb friction model employed in most commercial programs employs deterministic friction coefficients. However, observing friction phenomena between two contact surfaces, one may observe that friction coefficients keep changing due to the unevenness of contact surface, temperature, lubrication and humidity. Therefore, in this study, friction coefficients are modeled as random parameters that keep changing during the motion of a mechanical system undergoing friction force. The integrity of the proposed stochastic friction model was validated by comparing the analysis results obtained by the proposed model with experimental results.     

Effects of sheet thickness and material on the mechanical properties of flat clinched joint

Frontiers of Mechanical Engineering - The flat clinching process is attracting a growing attention in the joining field of lightweight materials because it avoids the geometric protrusion that...     

Analytical modeling and numerical simulation for three-roll bending forming of sheet metal

The three-roll bending forming of sheet metal is an important and flexible manufacturing process due to simple configuration. It is suitable for forming large sheet parts with complex, curved faces. Most researches on roll bending forming of large workpiece are mainly based on experiments and explain the process through macroscopic metal deformation. An analytical model and ABAQUS finite element model (FEM) are proposed in this paper for investigating the three-roll bending forming process. A reasonably accurate relationship between the downward inner roller displacement and the desired springback radius (unloaded curvature radius) of the bent plate is yielded by both analytical and finite element approaches, which all agree well with experiments. Then, the three-roll bending forming process of a semi-circle-shaped workpiece with 3,105 mm (length)?×?714 mm (width)?×?545 mm (height) is simulated with FEM established by the optimum tool and process parameters. Manifested by the experiment for three-roll bending forming of this workpiece, the numerical simulation method proposed yields satisfactory performance in tool and process parameters optimization and workpiece forming. It can be taken as a valuable mathematical tool used for three-roll bending forming of large area sheet metal.     

A study on mechanical characteristics of the friction stir welded A6005-T5 extrusion

In this paper, A6005-T5 extruded aluminum alloy sheets which are used for floor, roof or wall panels of railroad vehicles were welded by the friction stir welding (FSW) and gas metal arc welding (GMAW) techniques. The mechanical characteristics including the tensile strength, micro-hardness and fatigue strength of the FSW joint were compared to those of the base metal and GMAW joints. In order to determine the relationship between the welding variables of FSW and the mechanical characteristics of the joint, the response function was derived using the least square method and the sensitivity analysis was performed. The rotational speed, welding speed and tilting angle of the welding tool were chosen as design variables. On the basis of the Plackett-Burman design table, eight different FSW experiments were done, and then the effects of design variables on the mechanical characteristics of the FSW joint were analyzed. The result showed that the welding speed has a most significant effect on the tensile and fatigue strength. In the case of the micro-hardness, the effect of the tilting angle was the biggest.     

Mechanical behavior of an asymmetrically rolled and annealed 1050-O sheet

In this work, 1050-O aluminum alloy sheets are asymmetrically rolled and annealed. The asymmetric rolling process imposes intense shear deformations across the sheet thickness, leading to the development of shear texture and also grain refinement. The shear texture obtained is found to be retained after annealing. The improvements of the mechanical response and the texture evolution after heat treatment processing are inferred based on experimental shear tests and numerical simulations. It is proven that it is difficult to spread shear texture through the entire sheet thickness from a general asymmetric rolling process. Based on the fact, future research is discussed at closure.     

Fracture and mechanical properties of friction stir spot welds in 6063-T6 aluminum alloy

The influence of the tool dimensions and of the welding parameters on the fracture and lap shear properties of friction stir spot welds is investigated. Interrupted lap shear tests allow to follow the mechanisms leading to weld fracture. A triangular cavity opens at the hook during lap shear testing. The distance between this triangular cavity and the hole left by the pin is the main parameter controlling the type of fracture. A too short distance favors a fracture through the weld nugget and hence should be avoided. In particular, this happens when the tool pin diameter is too small and when the plunge rate is too large. Fracture initiating at the triangular cavity and following the thermomechanically affected zone, i.e., by the pullout of the weld nugget, is preferred. This fracture type leads to significant plastic deformation and generally favors a large ultimate force during lap shear testing. Large ultimate forces are observed when the welds are cooler (large plunge rates and low rotation speeds), but the welding conditions should be chosen so as not to lead to fracture trough the weld nugget.