Multi-stage Incremental Forming Research Articles

Improving the formability during ultrasonic-assisted multi-stage incremental sheet forming

Multi-stage sheet forming is an encouraging technique to obtain complex geometries by change of the design of intermediate stages. Due to the occurrence of stepped feature during multi-stage forming, the final geometry shape is affected and the formability is deteriorated. To eliminate fracture and enhance formability, ultrasonic assistance is integrated with the multi-stage sheet forming process. The present work aims to investigate the effect of ultrasonic-vibrations (UV) on formability and thickness evolution using two multi-stage strategies through experiments. A frequency of 29 kHz and an amplitude of 10 μm are taken for the ultrasonic-assisted incremental forming process. Firstly, two distinct strategies are compared by experiments. Secondly, thorough investigation of formability experiments is performed by applying the UV at only one or two stages instead of the complete multi-stage forming process. The results show that formability of the part is increased while applying UV at intermediate stages. Similarly, it is confirmed that UV is more advantageous during latter stage of experiment. This work provides an effective strategy to improve the formability of the incremental sheet forming process by using the ultrasonic vibrations in a smart way.

Influence of Different Zoning on Part Thickness in the Multi-Stage Incremental Forming Based on the Extrusion from the Forward and Reverse Side of the Sheet

In the multi-stage incremental forming of straight-wall parts based on the extrusion from the forward and reverse side of the sheet, different forming area zoning will have different effects on the part thickness. The larger the area of the sheet metal participating in forming is, the better the thickness distribution is. However, the forming area zoning can be adjusted by using the inclination angle α and the height H of the auxiliary zoning surface. Research results show that the forming area of the sheet metal participating the forming will increase and the thickness of the formed part will also increase with the inclination angle α decreases and with the height H increases.

Deformation characteristics and formability enhancement during ultrasonic-assisted multi-stage incremental sheet forming

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.

Research on the Negative Multistage Incremental Forming of Straight-Wall Parts Based on the Extrusion from the Forward and Reverse Side of the Sheet

Because the forming area involved in traditional reverse multistage incremental forming is only located inside the model, the sheet-metal thinning rate is relatively large. Particularly, the straight-wall parts with a narrow internal space cannot be formed using traditional multistage incremental forming. Therefore, a negative multistage incremental forming that extrudes the sheet from the forward and the reverse side of the straight-wall part is proposed in this paper. In this method, firstly, the auxiliary surface is generated to divide the straight-wall part model into three forming regions; secondly, the first- and second-stage forming are carried out from the forward side of the straight-wall part with support, respectively; Thirdly, the third-stage forming is carried out from the forward side of the straight-wall part without support. The software system for auxiliary-surface generation, the straight-wall parts partition, each intermediate-stage-forming model, and each stage-forming toolpath generation was developed by using C++, VC++, and OpenGL library. In order to verify the feasibility of the proposed method in this paper, the forming experiments of a 1060 aluminum sheet were conducted using traditional reverse multistage forming and the proposed method in this paper, and the forming effects were compared and analyzed. The results show that compared with traditional reverse multistage incremental forming with forward-side extrusion, the multistage incremental forming method with the forward and the reverse-sided extrusion proposed in this paper can increase the area of the sheet participating in the deformation and avoid the problem of excessive thinning of sheet thickness, especially suitable for the straight-wall part model with narrow internal space.

Toolpath Planning and Generation for Multi-Stage Incremental Forming Based on Stretching Angle.

To solve the problems that exist in the multi-stage forming of the straight wall parts, such as the sheet fracture, uneven thickness distribution, and the stepped feature sinking, a new forming toolpath planning and generation method for the multi-stage incremental forming was proposed based on the stretching angle. In this method, the parallel planes that were used for forming toolpath generation were constructed by using the stretching angle so that the distances between the parallel planes and the forming angles were gradually reduced. This makes the sheet material flow become changed and the thickness thinning is relieved. The software system for the toolpath generation was developed by using C++, VC++, and OpenGL library. In order to verify the feasibility of the proposed method, numerical simulation and forming experiments were carried out for the single stage forming, the traditional multi-stage forming, and multi-stage forming based on the proposed forming toolpath, using 1060 aluminum sheets. The comparative analysis of the thickness distribution, profile curve, strain curve, and sheet material flow shows that the proposed method is feasible, and the profile dimension accuracy is better, the thickness distribution is more uniform, and the sinking and bulging are significantly reduced. The formed sheet part with the stretching angle of 15° has higher dimensional accuracy, smaller bottom subsidence, and larger thickness than that of the stretching angle 5°.

Experimental investigation on AISI 304 steel sheets formed by multi stage incremental forming

Incremental sheet forming (ISF) is an sheet metal process which can be used widely in the area of automobile, medical and aeronautical application. The required shape of the final product is achieved by moving the rotating tool along the pre-determined path controlled by computer numerical controlled (CNC) milling machine. This paper explains the multi stage process to achieve the maximum wall angle in the cylindrical cup shaped component. The experiments were conducted using austenitic stainless steel AISI304 of 0.8mm thickness. In this work multi stage forming process was carried out in two stages (i). Starting from wall angle of 50° and (ii). Increasing the wall angle from 50° to 70° by keeping the tool traversing (1000 mm/min) and incremental depth (0.5mm) as constant for various tool rotational speeds of 600, 800 and 1000 rpm to maximum possible depth without the occurrence of fracture. In multi stage forming, with a wall angle of 70°, the maximum depth achieved was 21.3 mm before fracture at a tool parameters of 1000 rpm tool speed, 1000 mm/min tool traversing speed and 0.5mm incremental depth within the experimented process parameters.

Multi-Stage Two Point Incremental Sheet Forming

In incremental sheet forming, the thickness of parts decreases with the sine law as the forming slope increases, resulting in serious thinning or even fracture of parts. For industrial products, extreme thinning of parts can not meet the design requirements, seriously affecting the application of incremental sheet forming in the industrial field. Multi-stage incremental sheet forming is found to effectively improve the thickness distribution of parts. For multi-stage two point incremental sheet forming, there are a great variety of multi-stage strategies, such as parallel linear tool-path strategy, parallel arcs tool-path strategy, variable angle straight lines tool-path strategy, and stretch-bend assist multi-stage strategy. However, which multi-stage strategy is the best multi-stage strategy is not conclusive. Therefore, in this paper, a square cone is selected to analyse the influence of tool-path strategy on thickness distribution. Then, the best multi-stage strategy is obtained by comparing the thickness distribution with different strategies. Finally, by analysing the change of strain condition with an increase in stage, the causes of different strategies for increasing the thickness distribution are obtained.

Finite Element Simulations Of Multi Stage Incremental Forming Process

The limitation over the maximum formable wall angle in single stage incremental forming can be overcome by adopting multi stage forming strategy. This paper focuses on some preliminary finite element studies on deformation behavior of EDD steel in multi stage incremental forming in terms of thickness distribution, strains and geometric accuracy. The study has been carried by forming a conical cup with 850wall angle in four stages. Thickness distribution results revels that the sine law cannot predict the thickness with increase in number of stages. The major and minor principal strain history shows that the material is under plane strain condition in stage#1 and is towards bi-axial stretching with increase in number of stages. The simulation results also captured the geometric inaccuracies due to the formation of residual cone in Multi Stage Incremental Forming (MSIF). The finite element simulations can be used successfully to decide the number of stages and geometric accuracy parts in MSIF.

Analysis of formability in incremental forming processes

Incremental forming process is a relatively new sheet metal forming process with superior formability characteristics over other conventional forming processes. This paper reviews the reasons for improved formability in incremental sheet forming process. Further, the effect of process parameters on formality has been discussed. The formability in incremental forming is generally evaluated by maximum formable wall angle and fracture forming limit diagram. Different procedures adopted by the researchers to find the maximum wall angle and construction of fracture forming limit diagram have been discussed. The formability assessment in other variants of ISF processes such as multi stage incremental forming and incremental hole flanging are also presented

Tool path generation for the five-axis CNC multi-stage incremental forming

Because the intermediate-stage forming surface and the intermediate-stage forming tool path of the complex shape model are difficult to generate, the existing multi-stage CNC incremental forming takes the regular rotary body model or the square model as the research objects, and all adopt the three-axis CNC incremental forming mode. In this paper, a method for generating the intermediate-stage surface by using a longitude line that can reflect the personality of the surface and the five-axis CNC multi-stage incremental forming tool path was proposed. Firstly, the vertexes of the triangular facets of the STL model are used to generate the longitude lines which can reflect the characteristic of the surface, then the longitude lines are offset according to the multi-stage forming strategy and the characteristics of each surface associated with the longitude lines so that the intermediate-stage longitude lines could be generated, and then the intermediate-stage surfaces are built using the intermediate-stage longitude lines. Finally, the cutter location points of each intermediate stage are obtained by cutting the intermediate-stage surfaces, and the postures of the five-axis CNC pressing tool are determined according to the normal vector of the cutter location points. The case studies show that the proposed method can well generate the five-axis CNC multi-stage incremental forming path for the complex shape sheet metal part. The results of the numerical simulation analysis and forming experiments show that the proposed method is applicable.

Prediction of forming forces in single point incremental forming

Incremental sheet forming (ISF) is a cost effective die-less forming process for low volume production. Achieving good accuracy using this process is a challenging task. Bending between clamped boundary and component opening, tool deflection, sheet spring-back and rigid body displacements are the major reasons for geometric inaccuracy in components formed using ISF process. In order to achieve desirable geometric accuracy, accurate prediction of sheet thickness, tool-sheet contact area and forming forces is important. In this article, modified analytical model is presented to accurately predict formed component thickness, contact area and forming forces during single and multi-stage incremental forming. Predictions of models presented in this work are compared with experimental work carried out during this work as well as experimental results available in the literature and they are in good agreement. Prediction models developed during the present work require very less computational resources compared to finite element analysis.

Experimental Studies in Multi Stage Incremental Forming of Steel Sheets

Incremental Sheet Forming (ISF) is a relatively new sheet metal forming process. It hasshown great diversity in its applications ranging from automotive to biomedical fields. However, the geometries having steep walls cannot bemanufactured in single stage incremental forming process due to the fact that the maximum wall angle that can be formed is limitedfor a given sheet material and thickness. This limitation can be overcome with Multi Stage Incremental Forming (MSIF) process. This paper presents some of the experimental studies in multi stage incremental forming of steel sheets to get steep wall angles.The effect of process parameters in forming cylindrical, square and spherical cups in MSIF process has been discussed.

Tool Path Planning for manufacturing of Asymmetric parts by incremental sheet metal forming by means of Robotic manipulator

Asymmetric parts have been in a great demand in industries and manufacturing of the same requires affluent dies. Single point incremental forming and two point incremental forming are the two methods which have been introduced to replace partially the use of expensive dies in introduction phases. But one of the major limitations of the existing methods of incremental sheet metal forming is the limiting wall angle, where a wall angle of greater than about 70° is difficult to obtain. The recently proposed solution of multi-stage incremental forming can enable obtaining wall angles close to 90° but it takes longer manufacturing time and may pose problems for materials that severely strain harden. Advance automation and robotics can enhance this task, productivity and accuracy of the part. One of the technologies evolving in manufacturing industry is Robot assisted incremental sheet metal forming. (RISF). 3 axis CNC machine and robotic manipulator can be integrated to make the process more flexible. Forming of any part with a robotic manipulator requires continuous relocation of supporting robots during machining. In this forming operation tool is given 3DOF and manipulator is given 2DOF.Tool head and manipulator sequentially relocate and affix themselves in such a way that final complex geometry can be formed. To follow the work piece geometry, adequate positioning of end effector is required. In this paper evolutionary path planning of robotic manipulator and CNC tool is presented.

Study on forming method of vertical wall cylinder parts formed by multi-stage incremental forming

In this paper, the deformation analysis, the tool path design, the experimental condition and the result analysis of the vertical wall cylinder forming in incremental forming process are presented. According to the sine law, the vertical wall shell cannot be finished at one stage in incremental forming process, it must be formed at many stages. Thus, three process methods are presented to make vertical wall shell parts quickly, and the experiment and analysis are made to verify it.

Thickness distribution of multi-stage incremental forming with different forming stages and angle intervals

Although multi-stage incremental sheet forming has always been adopted instead of single-stage forming to form parts with a steep wall angle or to achieve a high forming performance, it is largely dependent on empirical designs. In order to research multi-stage forming further, the effect of forming stages (n) and angle interval between the two adjacent stages (Δα) on thickness distribution was investigated. Firstly, a finite element method (FEM) model of multi-stage incremental forming was established and experimentally verified. Then, based on the proposed simulation model, different strategies were adopted to form a frustum of cone with wall angle of 30° to research the thickness distribution of multi-pass forming. It is proved that the minimum thickness increases largely and the variance of sheet thickness decreases significantly as the value of n grows. Further, with the increase of Δα, the minimum thickness increases initially and then decreases, and the optimal thickness distribution is achieved with Δα of 10°. Additionally, a formula is deduced to estimate the sheet thickness after multi-stage forming and proved to be effective. And the simulation results fit well with the experimental results.

Generation of Technological Model for Multi-Stage Incremental Forming

In order to the difficult forming sheet metal part with the vertical wall to be formed by the multi-stage incremental forming, the algorithm of the recognition and modify the difficult forming surface of the vertical wall is studied. The method for generation of the technological model for multi-stage incremental forming is also presented. The case studies show that the algorithm is feasibility.

Study on formability of vertical parts formed by multi-stage incremental forming

Multi-stage incremental sheet metal forming (MS-ISF) is a good solution to overcome the maximum forming angle of single-stage incremental sheet metal forming. In order to test the formability of MS-ISF, a novel method to get forming limit diagram (FLD) of MS-ISF was proposed based on changing forming strategies. Aluminum alloy AA1060-O sheet was employed to prove the feasibility of the method. Test results show that the method can provide FLD of MS-ISF which is affected by loading path.

Vertical Wall Formation and Material Flow Control for Incremental Sheet Forming by Revisiting Multistage Deformation Path Strategies

In this article, multistage deformation path strategies for single point incremental forming (SPIF) are revisited with the purpose of controlling material flow (improving sheet thickness distribution) and forming a vertical wall surface for cylindrical cups. It is noted that stretching and thinning are two main deformation modes during SPIF. How to control material flow in an optimal way is a key point for successful forming. Multistage incremental forming shows more advantages than single-stage forming, especially dealing with shapes with steep walls. In this study, three basic multistage deformation path strategies have been proposed, that is: A. incremental part diameter; B. incremental draw angle; and C. incremental part height and draw angle. Those strategies and their combinations have been evaluated in terms of formability and compared in order to understand the material allocation mechanism and optimize the multistage forming process. In addition, approximate plane-strain analysis models have been given to provide formability predictions between single-stage and multistage strategies, and between strategies B and C, respectively. The prediction results show good agreement with the experimental results. It is demonstrated that the strategic combination A + B is the optimal way to achieve the forming target.

Numerical simulation and experimental investigation of multistage incremental sheet forming

Multistage forming is usually adopted to form those parts which have steep angles or even vertical walls during incremental sheet forming (ISF) process. In order to study the multistage incremental forming further, based on a finite element method model which was experimentally verified, different forming strategies were adopted to form a frustum of cone with a wall angle of 30° to research the influence of the number of forming stages (n) and the incremental wall angle between the two adjacent stages (∆α) on the formability of ISF. The simulation results including the thickness distribution, the equivalent plastic strain, and the magnitude of springback were analyzed in detail. It was found that with the growth of n, the minimum thickness increases largely, and more uniform thickness distribution is achieved, but the quantity of springback becomes larger in contrast with a single-pass process because of the accumulation of springback during each forming stage. Furthermore, an expression to figure out the appropriate value of n was given. In addition, the maximum thickness reduction decreases initially and then increases as the value of ∆α grows. Meanwhile, it indicates that there is no relation between ∆α and the quantity of springback.

Hole-flanging by incremental sheet forming

Incremental forming of hole-flanges in sheet metal parts is an emerging process with a high potential economic payoff for rapid prototyping and for small quantity production. However, as with all new sheet metal forming processes, there is need for examining its deformation mechanics and describing the physics behind the occurrence of failure.How metal fails, how pre-cut holes influence strain and stress in single point incremental forming, and how these subjects can be brought together in order to understand the overall formability of hole-flanging by multi-stage incremental forming are still not well understood. However, they are of great importance for improving the performance and industrial applicability of the process.This paper attempts to provide a new level of understanding for the process by combining circle grid analysis and independent characterization of the mechanical properties and formability limits of the material with the fabrication of conical and cylindrical hole-flanges. Experimental observations, measured strain paths and material formability limits by necking and fracture allow concluding that hole-flanging by incremental forming gives rise to a new mode of deformation, not found in conventional incremental forming of sheet metal blanks without pre-cut holes, and to failure by fracture without previous localized necking.