Formability In Single Point Incremental Forming Research Articles

On the Manufacturing of a Cranial PEEK Implant Using SPIF

This article presents an experimental investigation of biocompatible Polyether ether ketone (PEEK) polymeric sheet deformed using Single Point Incremental Forming (SPIF) at room temperature, with the objective of manufacturing a cranial medical implant. The investigation was performed in terms of formability and failure within the principal strains space, being the material Forming Limit Diagram (FLD) assessed by means of Nakajima tests. This material characterization allowed to obtain the formability limits at necking and at fracture of PEEK polymeric material. In addition, an experimental work plan in SPIF was performed in terms of spifability (or formability in SPIF) with the aim of determining the effect of the main process parameters on the spifability, modes of failure, and temperature, among others. As a result, an optimum set of parameters along with the established methodology were used for manufacturing a cranial medical implant made of this high-performance and biocompatible polymeric material. The work primarily shows the feasibility of manufacturing PEEK medical prostheses and implants using SPIF.

Exploring deformation mechanics of temperature assisted incremental forming with hybrid heating

The single point incremental forming (SPIF) process is a die-less rapid prototyping sheet metal forming method, extensively researched for over two decades. SPIF shows higher formability compared to conventional sheet forming methods. Deformation of materials such as magnesium (Mg) is favored at elevated temperatures due to their poor room temperature formability. Past studies have explored heat-assisted forming techniques to achieve improved formability in SPIF. However, the underlying mechanics of deformation is sparsely explored. The present work explores the mechanism of formability improvement based on thermal gradients for SPIF in hybrid heating. The SPIF experiments are conducted for a combination of local and global heating conditions achieved using tool contact friction and cartridge heaters. The local temperature, stress distributions, and limiting strains are obtained numerically. Fracture-forming limit diagrams (FFL) are developed, and numerical predictions are validated using experimental strain measurements under different forming conditions.

Analysis of Flangeability by Single-Stage SPIF and Press-Working in AA7075-O Sheet

Abstract Recently, the research interest of hole-flanging has turned from conventional press-working to single-point incremental forming (SPIF) as a viable process for small- and medium-sized batches. Both technologies have been studied separately using different approaches and, therefore, most studies cannot be easily compared. Besides, some studies that measured the formability in SPIF using the classical limiting forming ratio (LFR) showed conflicting results that still need to be clarified. Under these circumstances, the aim of this work is to provide a better understanding of the deformation process and the material formability in hole-flanging by critically comparing both forming processes. To this end, a series of experimental tests on AA7075-O sheet of 1.6-mm thickness by press-working and single-stage SPIF, using forming tools with different profile radii, are analyzed. The material formability and flange geometry are compared and discussed in detail. The process limits are analyzed by using both the Forming Limit Diagram (FLD) and the LFR. The failure modes by necking and fracture are clearly identified and assessed on both processes along with the influence of the bending induced by the tools during the flange forming. Results conclude that the LFR is not an adequate parameter to compare formability between processes other than press-working and, accordingly, two additional variables based on either the flange height or the average thickness reduction are proposed to successfully analyze flangeability.

Formability of Materials with Small Tools in Incremental Forming

Single point incremental forming (SPIF) is an innovative sheet forming process with a high economic pay-off. The formability in this process can be maximized by executing forming with a tool of specific small radius, regarded as threshold critical radius. Its value has been reported as 2.2 mm for 1 mm thick sheet materials. However, with a change in the forming conditions specifically in the sheet thickness and step size, the critical radius is likely to alter due to a change in the bending condition. The main aim of the present study is to undertake this point into account and develop a relatively generic condition. The study is composed of experimental and numerical investigations. The maximum wall angle (θmax) without sheet fracturing is regarded as sheet formability. A number of sheet materials are formed to fracture and the trends correlating formability with normalized radius (i.e., R/To where R is the tool-radius and To is the sheet thickness) are drawn. These trends confirm that there is a critical tool-radius (Rc) that maximizes the formability in SPIF. Furthermore, it is found that the critical radius is not fixed rather it shows dependence on the sheet thickness such that Rc = βTo, where β varies from 2.2 to 3.3 as the thickness increases from 1 mm to 3 mm. The critical radius, however, remains insensitive to variation in step size ranging from 0.3 mm to 0.7 mm. This is also observed that the selection of tool with R < Rc narrows down the formability window not only on the higher side but also on the lower side. The higher limit, as revealed by the experimental and FEA results, diminishes due to excessive shearing because of in-plane biaxial compression, and the lower limit reduces due to pillowing in the bottom of part. The new tool-radius condition proposed herein study would be helpful in maximizing the formability of materials in SPIF without performing experimental trials.

Enhancement of fracture forming limit by crystallographic texture reformation of AA1050 sheets in Single Point Incremental Forming

Single Point Incremental Forming (SPIF) process that results in the formation of sheet metal components leads to higher forming capabilities in comparison to conventional forming strategies. However, the formability in SPIF is critical and limited by the rapid/sudden failure in the sheet material (without material localization) for the components with high wall inclination angles. The state of stresses and strains plays a huge role in determining the forming limits in SPIF, which is complexly governed by various factors such as sheet thickness, lubrication, incremental step depth, tool rotational speed, part geometry, tool dimensions, and geometry, to name a few. The present work focuses on investigating formability of the sheets from its microstructure and crystallographic texture point of view. Preliminary experimentation led to the conclusion that SPIF of the original commercially supplied AA1050 H14 sheets results in early and premature failure. Microstructural investigation of the undeformed sheet showed that the commercially supplied sheets undergo strain hardening due to cold rolling and therefore, results in dislocated and disorientated microstructure and texture. The present work attempts to reform the microstructure of the sheet by preheating at the optimized temperature. Finite element analysis (FEA) of the process led to the observation that the reformed microstructures result in enhanced fracture forming limit strain and reduced stresses in SPIF. This may be attributed to the decrease in the number of dislocations and dislocated grains, and decrease in the surface area of grain boundary per grain with preheating. Improved fracture forming limits due to preheating were experimentally validated by utilizing digital image correlation technique (DIC) for acquiring true strain values subjected to fractured original and different preheated samples.

Microstructure and texture based analysis of forming behavior and deformation mechanism of AA1050 sheet during Single Point Incremental Forming

In case of Single Point Incremental Forming (SPIF) process, the deformation mechanism, absence of material localization prior to fracture and enhanced formability are still not satisfactorily explained. The objective of this work is to understand the sheet deformation characteristics, forming behavior, and dominant deformation mechanism of SPIF process. Process deformation characteristics such as dimensional accuracy, thickness distribution, true surface strain, von Mises stress and equivalent plastic strain, evolved at different forming stages, were evaluated through experimental investigation and Finite Element Analysis (FEA) of the formed parts. The evolved microstructural features for different stages and modes of strains have been analyzed by Electron Back Scatter Diffraction (EBSD) and X-Ray Diffraction (XRD) techniques. Analysis has also been carried out to identify the reason of typical failure under biaxial strain mode. An abundance of {001} <100> Cube texture component in the early forming stage indicates the occurrence of dominant shearing in this stage. The intermediate forming stage develops dominant {011} <111 > P texture component, which remains intact up to the final forming stage. This shows that Particle Stimulated Nucleation (PSN) has taken place due to excessive strains occurring in the sheet material during SPIF, causing grain refinement and shear band formation, and simulating kind of recovery or recrystallization process. Enhanced formability in SPIF can be attributed to the depletion of {001} <100> Cube texture, and development of {011} <111 > P texture and {011} <211> Brass texture component with the progression of SPIF.

An investigation on flaring process of thin-walled tubes using multistage single point incremental forming

The flaring of a thin-walled tube is conventionally carried out by pressing the tube end using dedicated die sets. As an alternative process with the high flexibility and low complexity, in this research, the utilization of single point incremental forming (SPIF), which is the simplest variant of incremental sheet metal forming (ISMF) processes, was assessed and investigated experimentally and numerically for flaring of the tube end. Results predicted by the finite element (FE) model through the commercial code Abaqus/Explicit showed a reasonable agreement with the experimental results. In the present research, the formability in SPIF was compared with that in pressing process and it was concluded that using multistage SPIF, the maximum semi-cone angle achievable before fracture can be improved at least 100%. The results showed that the thickness distribution in the conical flaring of the tube end includes thinning and thickening regions affected by the deformation strategy of multistage SPIF. In this regard, the formability of the tube end and the material deformation were studied under various forming strategies. Finally, for non-axisymmetric flaring of the tube end in the form of a pyramid with the semi-angle of 20° using SPIF, different three-stage forming strategies were designed and successfully implemented. The thickness distribution and the geometric accuracy under designed three-stage strategies were investigated.

Numerical prediction of failure in single point incremental forming using a phenomenological ductile fracture criterion

Besides the high simplicity and flexibility, excessive thinning of the sheet metal is one of the limitations of single point incremental forming (SPIF). To overcome this drawback, a finite element (FE) simulation can be implemented for determination of an appropriate deformation strategy. Since the formability in SPIF is often limited by fracture failure, the prediction of ductile fracture using a FE simulation is of the significant importance. In this paper, by implementing the phenomenological modified Mohr-Coulomb (MMC3) model through an appropriate user subroutine within the commercial FE code Abaqus/Explicit, the ductile fracture in SPIF is investigated. Based on the designed tensile tests, the MMC3 criterion is calibrated for AA6061-T6 aluminum alloy sheet using an inverse approach. The stress and strain states of the localized deformation field in SPIF are comprehensively analyzed. With the aid of stress triaxiality and normalized Lode angle parameter, it is shown that the loading path in SPIF is highly nonlinear. Accordingly, a nonlinear damage accumulation rule is utilized to predict the onset of fracture. The comparison of predicted fracture depths of SPIF parts with experimental ones demonstrates an average discrepancy of 10% for utilizing the MMC3 criterion. A good agreement of the fracture location can also be found between experiments and FE simulations. Numerically computed fracture strains show that the fracture forming limit diagram (FFLD) under a proportional loading, like a tensile test, differs in shape and value from the one obtained under a non-proportional loading like SPIF in which the FFLD is higher.

High-Speed Incremental Forming Process: A Trade-Off Between Formability and Time Efficiency

Making use of “optimal experimental design,” the paper attempts to investigate individual and interactive effects of predictor parameters, namely tool size, pitch size, feed rate, spindle rotational speed, and blank thickness, on sheet formability in single point incremental forming (SPIF) process. For the sake of precision, a novel sensor system was developed and employed to detect crack as it initiates on SPIF test specimens. A novel benchmark for formability in SPIF was established by addressing normal strain along sheet thickness, maximum attainable forming angle, and the rate of variation in forming angle. The process was finally optimized in terms of maximum achievable formability and minimum processing time. Accordingly, high-speed forming (with forming speed of at least 5000 mm/min) was realized to be perfectly viable, whereas the sheet formability remains quite satisfactory (over 90% of the maximum value). The key role of high spindle speed (up to 3000 rpm) was also highlighted in this regard.

Threshold tool-radius condition maximizing the formability in SPIF considering a variety of materials: Experimental and FE investigations

In the current study, a new level of understanding on the influence of using small tool radii on the formability (θmax) is identified for single point incremental forming (SPIF). The relative value of tool radius and blank thickness (i.e., R/TB, where R is the tool radius and TB is the blank thickness) was varied over a range (from 1.1 to 3.9), and a formability diagram in the R/TB–θmax space was obtained. The formability was observed to show an inverse V-type pattern which revealed that there is a critical radius of tool (Rc) that maximizes the formability in SPIF. Further, this radius which was found to be independent of the material type (or property) is a function of blank thickness related as, Rc≈2.2TB. This radius was termed as threshold radius. The formability, in agreement with general opinion in the literature, was noticed to increase with the decrease in the tool radius above the threshold value. However, contrarily it reduced with the decrease in the tool radius below the threshold value. In fact, undue surface cutting and metal squeezing was detected when the tests were performed with pointed tools, i.e., below threshold radius. This unstable deformation, which according to the FE analyses was found to be an outgrowth of in-plane compression under the tool center, increasingly weakened the material by inducing corresponding increase in damage (quantified by stress triaxiality) with the decrease in the tool radius. On the other hand, the damage was also observed to increase due to decrease in compression with the increase in the tool radius above the threshold value. This revealed high compression with low damage constitutes the most conducive condition that maximizes the formability in SPIF, which is realized when R≈2.2TB.

Critical analysis of necking and fracture limit strains and forming forces in single-point incremental forming

Single-Point Incremental Forming (SPIF) is an emerging manufacturing process especially suitable to produce small batches of metal parts. Moreover, the enhanced formability of metal sheets deformed by SPIF makes this technology useful to those industrial applications requiring high deformation levels. In this sense, the precise setting of limit strains in SPIF in relation to the conventional formability limits of the material, as well as the influence of the process parameters on these strains, are essential variables to understand how and how much can be deformed the metal sheets in real production. On the other hand, the forming force in SPIF is an essential variable, especially for the design of dedicated equipment or for the safe use of adapted machinery. This paper revisits failure in SPIF by means of an experimental analysis of the influence of process parameters, such as the tool diameter, the spindle speed and the step down, on the formability in SPIF (spifability) of AISI 304 metal sheets, studied in the light of circle grid analysis. The work also involves the independent determination of conventional formability limits by necking and fracture under laboratory conditions by using stretching tests (Nakazima tests), in conjunction with stretch-bending tests performed in order to quantify the influence of the bending induced by the tool radius. Failure strains are experimentally obtained and compared in stretch-bending and SPIF tests, being the failure mode discussed in each case. Finally, the axial forming force evolution was recorded with the aim of analyzing the range of process parameters that would guarantee the safely utilization of the non-dedicated process equipment.

The pillowing tendency of materials in single-point incremental forming: Experimental and finite element analyses

The pillow is a defect that adversely affects the geometrical accuracy as well as the formability in single-point incremental forming. With a main objective to control this defect, the effects of mechanical properties of material on pillowing are examined in this work. To identify the mechanical property that significantly affects pillowing, single-point incremental forming tests are conducted using a variety of materials (i.e. 11). It is found that a property (i.e. area reduction at tensile fracture) that controls the formability of a material in single-point incremental forming does not have any significant effect on its pillowing tendency. Interestingly, hardening exponent (i.e. a property that has controlling influence on the stretch-ability of material) appears to be the most influential property that determines the pillowing tendency of sheet metals in single-point incremental forming. Furthermore, the pillowing tendency of a material decreases with the decrease in this particular property. This, according to finite element analysis, occurs because strain localization around the tool/sheet contact correspondingly increases. To select and rank materials with respect to the pillowing behavior, a formula describing the property–pillowing relationship is proposed. As a secondary objective, the correlation between pillowing and forming depth is also investigated in this work. It is shown that initially the pillow progresses as the forming depth increases. However, after forming has been carried out to a certain depth, the pillow begins to regress, most likely due to strain hardening of sheet metal. In conclusion, it is suggested to lower the hardening exponent of sheet metals in order to control pillowing in single-point incremental forming.

Guidelines for Tool-Size Selection for Single-Point Incremental Forming of an Aerospace Alloy

Single-point incremental forming (SPIF) is relatively a novel sheet forming process to produce small lots with low cost. In this article, guidelines for tool size selection for SPIF of an aerospace grade alloy (AA-2024) are presented. The role of tool size, with respect to sheet thickness employed, on the formability in SPIF (i.e., spifability) is clarified. The response surface method is employed in order to study the significance of sheet thickness (t o) and tool radius (r) on the spifability. The results of analysis of variance (ANOVA) show that the sheet thickness and tool radius are not significant individually, however, a strong interaction between these parameters exists. Moreover, for a particular sheet thickness, there exists only one r/t o value at which the spifability can be maximized under a given set of conditions. Furthermore, for a given sheet thickness, if a tool with radius lesser than a particular value is used (i.e., r/t o < 2) the material squeezes out from the tool/sheet interface. And, such a condition adversely affects the material spifability. FEA using LS-DYNA has also been carried out.

Forming Parameters and Forming Defects in Incremental Forming of an Aluminum Sheet: Correlation, Empirical Modeling, and Optimization: Part A

Single point incremental forming (SPIF) has potential to be employed commercially and can replace conventional sheet stamping process if its defects, such as poor geometrical accuracy, can be overcome. In the present article, three forming defects, namely, squeezed out wall formation, corner fold corner, and bulge height, are introduced. It is demonstrated that the formation of these defects has adverse effect on the formability in SPIF. In order to eliminate, if possible, the formation of these defects, the potential effect of operating (i.e., tool radius and step size), geometrical (i.e., wall angle), and material (i.e., sheet thickness and material property) parameters and their interaction on the defect formation and growth has been quantitatively investigated by employing response surface design method. With the help of proposed empirical models, the investigated defects can be overcome through parametric optimization.

Role of Material Properties in Improving Sheet Formability in SPIF Process

Single point incremental forming (SPIF) is a novel sheet metal forming process. Owing to unique deformation mechanism, this process improves the sheet formability as compared to the conventional stamping process. In the current paper, the mechanical properties and spifability (i.e. formability in SPIF) of a wide range of materials were tested. The mechanical properties were mainly determined through tensile testing and the spifability was evaluated using Varying Wall Angle Conical Frustum (VWACF) test. Each mechanical property was drawn against the improvement in sheet formability (i.e. difference of spifability and stampability) and the sole most influential property was identified. It was found that the improvement in formability increases with the increasing of true thickness strain at tensile fracture.

Empirical modelling of the influence of operating parameters on the spifability of a titanium sheet using response surface methodology

Single-point incremental forming (SPIF) has the potential to replace conventional sheet forming processes for customized and small batch size sheet metal products. In this research work, the operating parameters (i.e. step size, tool size, and forming speed) which can affect the formability during SPIF process were altered over wide ranges and their effect on the formability of a titanium sheet was investigated. A six-dimensional hyper-surface showing the influence of investigated parameters on the formability was developed by using a response surface methodology. The capability of the SPIF process to enhance the cold formability of titanium sheet was also examined. This was carried out by comparing the sheet formability in SPIF and stamping processes.

An experimental study on some formability evaluation methods in negative incremental forming

In single point incremental forming (SPIF), the final thickness of a deformed sheet can be predicted by the sine law. Therefore, the formability in SPIF can be expressed as the maximum wall angle ( θ max) that a sheet would endure without fracturing. In the present study, two tests were carried out in order to evaluate the formability of an aluminum sheet. In the first test, conical frustums and square pyramids, a set of each, were produced by systematically varying the wall angle in order to investigate θ max. In the second test, four conical frustums, each having varying wall angle, designed by revolving different curved lines were formed to fracture. The results revealed that the value of θ max obtained from the former test was smaller than those obtained from the latter one. Moreover, a variation among the values of θ max obtained from the parts of the second test was also found. Since the first test shows the minimum possible value of θ max, it should be employed in combination with the second test so as to minimize the number of experiments required.