It has been a challenge to efficiently make titanium alloy thin-walled components by sheet metal forming. Incremental sheet forming (ISF) method provides a promising way to increase the formability ...of low-ductility metallic sheets, but making titanium alloy sheet panels still needs heating by electricity or laser. The present work investigates the deformation mechanism in fabricating truncated cones of titanium alloy with tooth features realized by flexible free incremental sheet forming (FFISF) at room temperature. Experimental investigations on auxiliary sheets and tool path selections, analytical modeling, finite element simulation, and microstructure characterization have been conducted to evaluate the deformation mechanism in terms of the geometric deviation, thickness distribution and microstructure evolution of TA2 and TC4. Results indicate that the I-O loading path coupled with an optimized auxiliary sheets selection can ensure the successful fabrication of designed panel without defects. An analytical model is proposed to predict the free edge dependent law of thickness distribution in FFISF, indicating the material thinning is positively correlated with the distance from the free edge, while the thinning rate is inversely proportional to the yield strength. A more uniform thickness of TC4 panel is obtained compared with TA2. Finally, the improved formability of titanium alloys by FFISF can be attributed to grain subdivision, decreasing of intragranular deformation, and optimization of dislocation movement.
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•Obtained truncated cone with tooth features of titanium alloy by FFISF at room temperature.•Determined optimal process set with feasible auxiliary sheet set and I-O loading path.•Found out free edge dependent thickness distribution against Cosine law.•Revealed homogenous deformation with less intragranular deformation for improved formability.
Limited geometric accuracy in incremental sheet forming is a major obstacle for its wide adoption in industry. This paper develops a generic methodology, suitable for arbitrary part geometries and ...various ISF processes, for addressing one of the main causes of geometric inaccuracy, i.e., in-process springback. The methodology consists of three main elements: determination of key control points to treat geometric complexity, simplified simulation models to predict springback offline, and in-situ toolpath modification during forming. It is shown experimentally that the method provides an efficient and robust solution for various geometries with negligible setup cost.
Incremental sheet forming (ISF) is a flexible process for rapid manufacturing of complex sheet metal parts. An advantage of ISF is the improved formability than traditional sheet forming processes ...such as stamping. A number of fundamental studies have been conducted to investigate the enhanced ISF formability considering the effects such as bending under tension and through thickness shear. To further understand the ISF deformation mechanism and formability enhancement, this work presents a new analytical model which is focused on investigating the deformation stability and its effect on the metal sheet fracture. Based on this new model, the critical strain of deformation instability is obtained. Furthermore, influences of the work-hardening effect and bending effect on the deformation stability are investigated. To validate the analytical model, the fracture occurrence of two aluminum grades, AA1100 and AA5052, are investigated by using ISF experiment. Based on the analytical and experimental investigation, this study has concluded that bending plays a major role on ISF deformation stability. In addition, the ISF fracture depends on both deformation stability and the sheet material's ductility.
Comparison of formability: (a) AA1100; (b) AA5052. Display omitted
•A new model to analyze the deformation stability in SPIF is proposed.•Plane strain and equibiaxial tension conditions are both considered in the model.•Failure in SPIF results from excessive tensile stress or limited formability.
Single point incremental forming (SPIF) is one of the procedures with the most potential in aerospace development and human implants. However, twist phenomena occur frequently and affect the ...processing defects, which strongly influence the geometric accuracy of the formed parts. This work reveals the mechanism of twist phenomena in detail and figures out the optimal solution of the principal challenge during industrial application and artificial bone processing by SPIF. A new analytical model is proposed to calculate the twist angle based on the law of conservation of energy. Meanwhile, a novel alternate spiral tool trajectory is established according to the linear interpolation of the two adjacent contour lines. Based on the case studies including pyramid frustum shape with constant wall angle and truncated cone shape with varying wall angle, SPIF formed parts by using the alternate spiral trajectory are compared with that by traditional unidirectional spiral trajectory, to effectively understand the advantages of the developed tool trajectory strategy. It shows that the proposed alternate spiral trajectory provides an effective way to effectually restrain the twist, and finally improves the geometric accuracy of target part in practical application of SPIF.
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•The twist mechanism was revealed by experimental and theoretical analysis.•A new analytical model to calculate the twist angle was derived and verified.•A novel alternate spiral tool trajectory was established.•The twist defect was effectually restrained.•The wrinkle defect was completely eliminated.
This study investigates the deformation and fracture mechanisms of two testing methods, tension under cyclic bending (TCB) and tension under cyclic bending plus compression (TCBC) and their ...relationship to single point (SPIF) and double-sided (DSIF) incremental sheet forming processes. Experimental tests were carried out by using a bespoke TCBC test rig and a DSIF machine with grade 1 pure Ti samples. The results show the elongation-to-fracture has a high relevance to the bending depth and compression, which leads to detailed investigation to the stress and strain evolutions in the local bending region using finite element (FE) method. A new Gurson-Tvergaard-Needleman (GTN) model is proposed with a modified shear damage mechanism utilising experimental fracture strain loci to calibrate the Lode angle effect under low stress triaxiality. It is found the bending and reverse-bending stages correspond to different stress states and significantly affect the fracture occurrence in TCB, TCBC and SPIF, DSIF processes. For the first time, the stress paths in the plane of stress triaxiality and Lode parameter are used to reveal the transition of deformation modes from equi-biaxial to plane strain tension in SPIF and DSIF, as compared to the plane stress tension in TCB and TCBC. Using the new GTN model, the simulation gives accurate predictions to the elongation-to-fracture in TCB and TCBC, and the fracture depth in SPIF and DSIF with an error of less than 8% in comparison to the experimental results. Although there is a distinction between the equi-biaxial and uniaxial tension deformations, the study concludes that the TCB and TCBC tests provide an insight into the formability improvement and represent intrinsic deformation mechanisms of SPIF and DSIF processes, an ongoing research question, which has drawn considerable attention in recent years.
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•Report a comprehensive study on the fundamental mechanisms of incremental sheet forming.•Present a new Gurson-type model to capture damage from macro-scale point of view.•Show deformation mode evolutions using stress paths of stress triaxiality and Lode parameter.•Reveal local bending and compression as key factors to fracture occurrence.•Establish correlation of cyclic bending and tension plus compression to ISF.
•Deformation mechanism and damage evolution in ISF process are investigated.•a coupled anisotropic plasticity and continuum ductile damage models is adopted.•A VUMAT is employed to implement the ...material behavior.•Capability of the FE modeling to predict the material deformation process is investigated.
The incremental sheet forming (ISF) is a flexible forming technology in the sheet metal engineering. This process reaches a higher formability of manufactured sheets than conventional sheet metal stamping process. A deep insight into parameters influencing this process is crucial for more understanding of the process. This paper presents a numerical investigation of damage mechanism during the Single Point Incremental sheet metal Forming (SPIF) of a part with conical shape. An elasto-plastic constitutive model with quadratic yield criteria of Hill’48 and mixed isotropic/kinematic hardening behavior is adopted to simulate ISF operation. A user material subroutine (VUMAT) is employed to implement the material behavior of the sheet metal. The efficiency of the finite element model to predict the material deformation process is investigated by comparing numerical and experimental results. A comparison between two hardening models in terms of deformed shape after springback and thickness evolution is presented. Damage evolution is investigated by comparing the results of two hardening models. The final part is devoted to study the influence of some process parameters on the damage evolution and forming effort during the SPIF of studied parts.
Although the multi-stage incremental sheet forming strategy is beneficial for manufacturing complex geometrical shapes via objective transitional stages, the forming limit and geometric accuracy are ...highly dependent on intermediate shapes, which need further improvement. Therefore, a novel forming strategy by combining the multi-stage sheet forming with ultrasonic-assisted vibration (UV) was proposed in the present work. This paper investigates the effect of ultrasonic vibrations on forming forces, thickness distribution, and stress–strain distribution through experimental tests and finite element (FE) simulations. Initially, a series of tests were conducted to determine the forming limit for three proposed strategies based on parameters including depth, initial diameter, and angle, both with and without the assistance of UV. Afterward, the FE model was established in ABAQUS/Explicit based on the optimal strategy by adopting the modified crystal plasticity constitutive model. The results indicated that, after superimposing the ultrasonic vibration, forming forces were effectively reduced and plastic deformation was greatly improved due to the increased equivalent plastic strain. Meanwhile, the application of the UV field efficiently reduces the high-stress zones and significantly decreases the stress value. Finally, it was discovered that by imposing UV at the appropriate forming stages, formability and thickness distribution can be successfully improved. The present investigation is beneficial to well understand the affecting mechanism of UV in completing the multi-stage sheet forming technique and obtaining the enhanced sheet formability.
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