Process Formability Research Articles

Enhancement of incremental forming process formability by using improved clamping and multi-stage deformation strategies

Enhancement of incremental forming process formability by using improved clamping and multi-stage deformation strategies

Porovnanie charakteristík tvárniteľnosti vysokopevných dvojfázových ocelí skúškou ťahom a LDH skúškou

The paper deals with the issue of formability evaluation of high-strength dual phase steels DP800 and DP1000 with a thickness of 1.6 mm. These steels are used for automotive body deformation zones. The material formability was evaluated by tensile testing with evaluation of mechanical properties, normal anisotropy ratio and strain hardening exponent. The process formability of steels was evaluated by LDH test, which is used for different deformation states modelled by the specimen width. The results of the LDH parameter achieved were correlated with the measured material formability parameters.

Emerging Trends in Single Point Incremental Sheet Forming of Lightweight Metals

Lightweight materials, such as titanium alloys, magnesium alloys, and aluminium alloys, are characterised by unusual combinations of high strength, corrosion resistance, and low weight. However, some of the grades of these alloys exhibit poor formability at room temperature, which limits their application in sheet metal-forming processes. Lightweight materials are used extensively in the automobile and aerospace industries, leading to increasing demands for advanced forming technologies. This article presents a brief overview of state-of-the-art methods of incremental sheet forming (ISF) for lightweight materials with a special emphasis on the research published in 2015–2021. First, a review of the incremental forming method is provided. Next, the effect of the process conditions (i.e., forming tool, forming path, forming parameters) on the surface finish of drawpieces, geometric accuracy, and process formability of the sheet metals in conventional ISF and thermally-assisted ISF variants are considered. Special attention is given to a review of the effects of contact conditions between the tool and sheet metal on material deformation. The previous publications related to emerging incremental forming technologies, i.e., laser-assisted ISF, water jet ISF, electrically-assisted ISF and ultrasonic-assisted ISF, are also reviewed. The paper seeks to guide and inspire researchers by identifying the current development trends of the valuable contributions made in the field of SPIF of lightweight metallic materials.

Reprint of: Residual stress inclusion in the incrementally formed geometry using Fractal Geometry Based Incremental Toolpath (FGBIT)

Single Point Incremental Sheet Forming (SPISF) is a well-known flexible alternative to conventional generative manufacturing processes. In SPISF, the geometry to be formed is fragmented into series of 2D slices and the plastic deformation is achieved through layer by layer movement of a Numerically Controlled (NC), hemispherical or ball end forming tool. The whole plastic deformation is the sum of all localized strains developed during each increment. Spiral, constant z incremental toolpaths, and their variants are common conventional toolpaths for SPISF. Several researchers have investigated these toolpaths extensively. Fractal Geometry Based Incremental Toolpath (FGBIT) is a recently developed toolpath for SPISF that improves the process formability and stress distribution. Unlike conventional toolpaths, FGBIT deforms the base region of the formed geometry which induces work hardening and residual stresses into the work piece. This may lead to the forming of high strength components. The residual stress distribution over the base region of the formed component (square cup) has been investigated in this study. Further, a comparison based on residual stress distribution between FGBIT and conventional incremental toolpaths is presented. Residual stresses have been measured by using nanoindentation technique. Pile up generation near the periphery of the indent is investigated for conventional and FGBIT based toolpaths. It has been observed from the experimental results that, the strength of the formed component increases due to induced compressive surface residual stresses while using FGBIT hence, metal components with high fatigue life and better strength-to-weight ratio can be formed.

An Infrared local heat-treatment method to improve local formability of forming and trimming processes

This paper presents a local heat-treatment method to improve the local formability for stamping and trimming processes by using focused infrared (IR) rays. Although the demand of ultra-high strength steel (UHSS) has been increasing in order to reduce car weights, the low formability of the UHSS has been a limitation. In order to resolve the formability problem, heat treatment methods have been often studied, and the authors also have developed a local heat treatment method by using focused infrared (IR) rays for a decade. The difference of this method from the author’s previous works is that the new heat treatment is completed by the supplier. The supplier of the materials completes the local heat treatment on target areas of blanks where to improve the local formability, then stamping companies can conduct cold forming and trimming. The target material of this paper is a 1.5GPa steel, CR 1480M steel supplied by POSCO. This paper discusses the local formability change caused by the presented IR heat treatment.

Residual stress inclusion in the incrementally formed geometry using Fractal Geometry Based Incremental Toolpath (FGBIT)

Abstract Single Point Incremental Sheet Forming (SPISF) is a well-known flexible alternative to conventional generative manufacturing processes. In SPISF, the geometry to be formed is fragmented into series of 2D slices and the plastic deformation is achieved through layer by layer movement of a Numerically Controlled (NC), hemispherical or ball end forming tool. The whole plastic deformation is the sum of all localized strains developed during each increment. Spiral, constant z incremental toolpaths, and their variants are common conventional toolpaths for SPISF. Several researchers have investigated these toolpaths extensively. Fractal Geometry Based Incremental Toolpath (FGBIT) is a recently developed toolpath for SPISF that improves the process formability and stress distribution. Unlike conventional toolpaths, FGBIT deforms the base region of the formed geometry which induces work hardening and residual stresses into the work piece. This may lead to the forming of high strength components. The residual stress distribution over the base region of the formed component (square cup) has been investigated in this study. Further, a comparison based on residual stress distribution between FGBIT and conventional incremental toolpaths is presented. Residual stresses have been measured by using nanoindentation technique. Pile up generation near the periphery of the indent is investigated for conventional and FGBIT based toolpaths. It has been observed from the experimental results that, the strength of the formed component increases due to induced compressive surface residual stresses while using FGBIT hence, metal components with high fatigue life and better strength-to-weight ratio can be formed.

Injection by sheet-bulk forming

This paper is focused on injection by upsetting of sheet-formed side walls. The presentation draws from fundamental aspects of material flow to typical failures caused by buckling and by the development of material flaws. Results show that the utilization of two-piece blank holders in conjunction with an adequate selection of the die cavity thickness and the creation of relief-holes to allow divided material flow are effective and easy-to-implement strategies to increase process formability and ensure the production of sound sheet-bulk parts. The work is carried out in two completely different materials (polyvinylchloride and aluminium AA1050) and its originality stands not only for the investigation on sheet injection but also for the utilization of thermoplastics in sheet-bulk forming. Numerical modelling with finite elements gives support to the overall results and discussion.

Analysis of formability in conventional hole flanging of AA7075-O sheets: punch edge radius effect and limitations of the FLC

Hole flanges are commonly manufactured in mass production by conventional press-working. In an industrial context, the process limits are usually evaluated by means of the Limiting Forming Ratio (LFR), which defines the maximum attainable diameter of the flange related to the initial pre-cut hole. Nevertheless, the LFR does not provide enough information about material deformation, thickness distribution or the expected onset and type of failure. In this sense, a systematic investigation using the Forming Limit Diagram (FLD) is needed. Under these circumstances, the aim of this work is to provide a better understanding of the deformation process in conventional hole flanging, analysing the effect of the punch edge radius as well as the influence of the strain distribution along the hole flange on formability. An experimental campaign on 1.6 mm thickness AA7075 annealed sheet was completed using cylindrical punches with different profile radii. In addition, numerical modelling of the process using finite elements was developed. The results allow better understanding of the physical mechanisms governing material deformation and process formability in very ductile materials. In this regard, this research work contributes to the current state of the art in conventional hole flanging by establishing the modes of failure that prevail in the process and the influence of the bending induced by the tool radius in the flange formability. In addition, the limitations of using conventional FLD for providing a limiting threshold for the strain states attained in the flanges deformed with different punch profiles are also identified.

Finite element modelling and analysis of the effect of frequency on the electromagnetic compression of tubes

Electromagnetic compression of the tube is a high strain rate forming process, in this process formability of the material can be increased by reducing the spring back effect developed in the process. High electrical conductivity materials like copper, aluminum are generally used for electromagnetic compression. In electromagnetic compression, frequency of the current pulse is an important parameter to be analyzed. In this work the effect of current pulse frequency on the electromagnetic compression of the tube has been studied. The tube and coil were modelled and analyzed using finite element software LS-DYNA. It has been found that the deformation obtained during tube compression were increases first with the increase in frequency, obtained a maximum value and then decreases with the further increase in the frequency. The change in frequency greatly influences the deformation obtained during the electromagnetic compression of tubes.

Experimental and numerical research on process formability in magnetic pulse forming of AZ31 magnesium alloy sheets

The plain strain of AZ31 magnesium alloy sheet in magnetic pulse forming was investigated by numerical simulation and experimental method. Combination of uniform pressure coil and Holmberg’s specimen was employed to evaluate the plain strain of AZ31 sheet. The numerical simulation for magnetic pulse plain strain of AZ31 sheet is performed by means of ANSYS FEA software. The magnetic flux density of uniform pressure coil was distributed uniformly, especially at the center of gauged area of AZ31 sheet directly in relation to the deformation behavior of AZ31 sheet. The velocity of typical point increases as increasing energy, and the more position closes to the center of sheet the higher velocity achieves. The forming height is increased with increasing discharge voltage. Compared with C=768 µF and C=1536 µF, the capacitance of 1152 µF is more effective for forming, which is confirmed by experiments. The peak velocity at the center of sheet is about 105 m/s. The major strains of magnetic pulse plane strain lie approximately in the strain ranges of 5.83-6.45%. However, the 3.22-3.82% (major strain) are the limit strains in quasi-static condition. The experimental results indicate that the major strain of AZ31 sheet improves about 75% compared with the quasi-static case.

Tool path strategies and deformation analysis in multi-pass incremental sheet forming process

Incremental sheet forming (ISF) is a new promising manufacturing technology with high potential to shape complex three-dimensional parts by using a relative small tool. However, ISF is presently limited due to non-uniform thickness distribution of formed parts, especially excessive thinning on severely sloped regions. This typically leads to fracture and hence limits process formability. This paper aims to tackle this issue by developing appropriate tool path strategies for multi-pass deformation design in ISF. More specifically, two more complex shapes are designed to validate the proposed multi-pass deformation design methodology by finite element simulations and experiments. The interactions of different tool path strategies on material thinning of the final part are evaluated in terms of process formability and thickness strain distribution given the same multi-pass design. Furthermore, the movement of material points is traced to explain the material flow mechanism in multi-pass forming compared to single-pass forming. It is concluded that both deformation passes (intermediate shapes) and tool path generation strategies would influence the thickness strain distribution as well as process formability. Appropriate tool paths should be devised to further reduce material thinning and improve the process formability by taking the geometrical features of the designed part into account. Additionally, the proposed multi-pass deformation design enables sufficient material to flow into the deformed region from the outside region so as to allow a compressive deformation state to develop and steeper shapes to be formed.

Numerical and experimental study of stretching effect on flexible forming technology

The flexible stretch forming technology (FSFT) is suitable for flexible manufacturing because it affords several advantages including applicability to various forming processes such as sheet metal forming, single curved surface forming, and quadratic curved surface forming. In this study, the formation of a quadratic curved surface with a saddle-type shape by the flexible stretch forming process is systematically investigated through a numerical simulation. A 4-mm-thick Al 3003-H14 aluminum alloy is used as the initial blank material. Urethane pads are defined based on a hyperelastic material model as a cushion for the smooth forming surface. The elastic recovery deformation behavior is also investigated to consider the exact result after the last forming process. The simulation indicates that the stretch forming process can be used to apply more stress to the blank and to reduce the elastic recovery effect. An experiment was then performed to confirm the process formability and reduction of the elastic recovery effect. A comparison of the objective surface between the simulation and the experimental results verified that the stretch forming process reduced the elastic recovery effect. This confirms that FSFT can be feasibly used to manufacture quadratic curved surfaces.

An Analytical Model for Deformation Path Design in Multistage Incremental Sheet Forming Process

Incremental sheet forming (ISF) is a new promising technology due to its flexibility and low-cost tooling properties compared with conventional forming processes. However, a common technical problem encountered in ISF is non-uniform thickness variation of formed parts, particularly excessive thinning on severely sloped regions, which may lead to the part fracture and limit the process formability. Design of multistage deformation paths (intermediate shapes or preforms) before the final part is a desirable and practical way to control the material flow in order to obtain more uniform thickness distribution and avoid forming failure. Based on the shear deformation and the strain compensation idea, an analytical model for designing multistage deformation paths and predicting the thickness strain distribution is proposed. The feasibility of the proposed model is validated by the finite element analysis (FEA) and experimental tests in terms of the comparison of prediction, simulation and experimental results on the thickness strain distribution and the process formability.

Multi-pass deformation design for incremental sheet forming: Analytical modeling, finite element analysis and experimental validation

Incremental sheet forming (ISF) is a promising rapid prototyping technology with high potential to shape complex three-dimensional parts. However, a common technical problem encountered in ISF is the non-uniform thickness distribution of formed parts; particularly excessive thinning on severely sloped regions. This may lead to fracture and limit the process formability. Design of multi-stage deformation passes (intermediate shapes or preforms) before the final part, is a desirable and practical way to control the material flow in order to obtain a more uniform thickness distribution and avoid forming failure. In the present paper, a systematic methodology for designing multi-stage deformation passes considering the predicted thickness strains given the design shape is proposed based on the shear deformation and the strain compensation mechanism. In this methodology, two analytical models (M1 and M2) are developed by taking into account; the global average thickness strain and only the material in the final part region used in the forming (M1), and the local weighted average thickness strain and the additional material around the final part region used in the forming (M2), respectively. The feasibility of the proposed design methodology is validated by finite element analysis (FEA) and experimental tests using an Amino ISF machine. The results show that a more uniform thickness strain distribution can be derived using M2. The incurrence of the highest strains can be delayed in the intermediate stages and the flow of material is allowed into the deformed region, thereby allowing a compressive stress state to develop and enabling steeper shapes to be formed. Therefore, the process formability can be enhanced via the optimized design of deformation passes.

On the relative performance of hole-flanging by incremental sheet forming and conventional press-working

The purpose of this article is to provide readers with a broad understanding on the deformation mechanics of hole-flanging by incremental sheet forming that will enable them to identify the major operating parameters, to determine the process formability window and to successfully design hole-flanged sheet metal parts. The methodology draws upon the utilization of circle grid analysis, measurement of the gauge length strains at the onset of fracture in a wide range of sheet formability tests, determination of the failure limits by necking and fracture and evaluation of the relative performance of hole-flanging by incremental sheet forming and conventional press-working. The presentation is illustrated with selected test cases obtained from a comprehensive experimental investigation. The overall content of this article widens and enhances previous research work by the authors in the understanding of the deformation mechanics and failure in hole-flanging by incremental sheet forming and includes new experimental data in stainless steel AISI304L.

Verification of the Effect of Temperature and Duration of Stamina for Heat Treatment of Zinc Coating on the Formability of Procedural AHSS Steels

Subject of presented contribution is process formability parameters evaluation of both side coated TRIP and HSLA steels in basic state and after its annealing in order to Fe-Zn coating creation. The samples were annealed at 450, 500 and 550°C for 10 and 60 seconds in argon shielding gas. Obtained values are compared with each other in order to determine whether the influence of temperature and duration after annealing changed the original class inclusion on a scale to measure the quality of steel sheets for deep drawing according to the value of limit drawing ratio. Limit drawing ratio was used as a process formability criterion and were reached by modified Swift cupping test. HSLA steel H220PD and TRIP steel RAK40/70 were used as experimental materials.