Analytical prediction of formed geometry in multi-stage single point incremental forming

Single Point Incremental Forming (SPIF) is a die-less forming process that can be economically used for low volume production of sheet metal components. One of the limitations of SPIF is the maximum wall angle that can be formed in a single stage. To overcome this limitation, Multi-stage Single Point Incremental Forming (MSPIF) is used to form components with large wall angles. When the tool is moved from out-to-in during any stage, material present ahead of it (towards the centre of the component) moves down rigidly. If this rigid body displacement is not considered during tool path generation for MSPIF, it leads to stepped/unwanted features. Predicting the component geometry after each stage helps in monitoring the shape being developed and in turn is useful in designing intermediate stages to form required final geometry with desired accuracy. In the present work, a simple methodology is proposed to predict rigid body displacement based on tool path and process parameters (tool diameter, incremental depth, sheet thickness) used. Tool and sheet deflections due to forming force are also considered to predict final geometry of the component. Proposed methodology is validated by comparing predicted profiles with experimentally measured profiles of high wall angle axisymmetric components formed using different materials and sheet thicknesses. Predicted profiles are in good agreement with experimental results.     

Incremental forming of Cu-35Zn brass alloy

Incremental Sheet Forming (ISF) and particularly its Single Point (SPIF) variant has been studied intensively over the last years given the potential for low-cost prototyping and small batches production. Numerical and experimental works have been covering a widespan of materials and geometries. This paper fills an important gap regarding studies of the SPIF process applied to brass alloys, and particularly the commonly used Cu-35Zn brass alloy. Despite being a material widely used in industry for centuries, with excellent cold formability and innumerous applications, there is still no relevant information on the mechanical response and properties of this material under SPIF. This research is based in SPIF experiments with brass alloy sheets with different thicknesses (0.50, 0.70 and 1.00 mm), to obtain data such as forming forces or forming fracture lines to be compared against standard forming limit diagrams or against other materials under ISF. Other data like friction during the process was evaluated as well. Fifteen sets of experiments were conducted, using different values of step down (0.10, 0.50 and 1.00 mm) and two forming tools with diameters 10 and 15 mm.     

Enhancing time efficiency on single point incremental forming processes

Single Point Incremental Forming (SPIF) is a promising manufacturing technology concerning the production of customized products, low batches or prototyping of ready-to-use parts, given its easy implementation and absence of dedicated tooling. The range of application is wide, covering many materials and virtually unlimited geometries. Indeed, nowadays’ process downsides are more related to high forming times and dimensional inaccuracy. There are many processing parameters that can be optimized to circumvent such problems. In this work, focus is given on the effect of increasing tool feed rate. To this end, a dedicated SPIF machine is employed. After forming steel and aluminum sheets, parameters like forming force, maximum wall angle and formability are assessed for a range of velocities from 1500 to 12,000 mm/min. Parameters like step down or tool diameters are kept constant for a clear comparison. It will be shown how the process can be fastened up without seriously compromising its feasibility.     

Strain evolution in the single point incremental forming process: digital image correlation measurement and finite element prediction

Incremental Sheet Forming (ISF) is a relatively new class of sheet forming processes that allow the manufacture of complex geometries based on computer-controlled forming tools in replacement (at least partially) of dedicated tooling. This paper studies the straining behaviour in the Single Point Incremental Forming (SPIF) variant (in which no dedicated tooling at all is required), both on experimental basis using Digital Image Correlation (DIC) and on numerical basis by the Finite Element (FE) method. The aim of the paper is to increase understanding of the deformation mechanisms inherent to SPIF, which is an important issue for the understanding of the high formability observed in this process and also for future strategies to improve the geometrical accuracy. Two distinct large-strain FE formulations, based on shell and first-order reduced integration brick elements, are used to model the sheet during the SPIF processing into the form of a truncated cone. The prediction of the surface strains on the outer surface of the cone is compared to experimentally obtained strains using the DIC technique. It is emphasised that the strain history as calculated from the DIC displacement field depends on the scale of the strain definition. On the modelling side, it is shown that the mesh density in the FE models plays a similar role on the surface strain predictions. A good qualitative agreement has been obtained for the surface strain components. One significant exception has however been found, which concerns the circumferential strain evolution directly under the forming tool. The qualitative discrepancy is explained through a mechanism of through-thickness shear in the experiment, which is not fully captured by the present FE modelling since it shows a bending-dominant accommodation mechanism. The effect of different material constitutive behaviours on strain prediction has also been investigated, the parameters of which were determined by inverse modelling using a specially designed sheet forming test. Isotropic and anisotropic yield criteria are considered, combined with either isotropic or kinematic hardening. The adopted constitutive law has only a limited influence on the surface strains. Finally, the experimental surface strain evolution is compared between two cones with different forming parameters. It is concluded that the way the plastic zone under the forming tool accommodates the moving tool (i.e. by through-thickness shear or rather by bending) depends on the process parameters. The identification of the most determining forming parameter that controls the relative importance of either mechanism is an interesting topic for future research.     

Experimental investigations on the single point incremental forming of a titanium alloy component combining static heating with high tool rotation speed

The present work focuses the attention on the Single Point Incremental Forming (SPIF) process of a scaled car door shell made by Titanium alloy (Ti6Al4V). The effect of a temperature increase contemporary due to electric static heating and tool rotation speed was investigated.Preliminary tensile tests allowed to define the temperature level to be assured on the sheet in order to determine a consistent flow stress reduction. SPIF tests were carried out adopting rotation speed in the range 800–1600 RPM, while simultaneously changing the pitch value in the range 0.5–1.0 mm. Temperature during the forming process was continuously measured in the central area of the blank using a pyrometer. In addition a digital image correlation system was used for measuring the strain distribution over the formed part.The combination of the two approaches (heating by both electric bands and high tool rotation speed) revealed to be a feasible solution for manufacturing hard to work materials like Ti alloys, since the investigated case study was successfully formed by SPIF. In addition, a positive effect of the tool rotation speed in stabilizing the necking (thus allowing to reach higher level of stretching) was recognised.     

Experimental and numerical study of a new hybrid process: multi-point incremental forming (MPIF)

Multi-Point Incremental Forming (MPIF) process is a new hybrid process that combines two common manufacturing methods. These are Multipoint Forming (MPF) and Incremental Sheet Forming (ISF) processes. In this study, an experimental set-up, based on a MP reconfigurable die, was designed and manufactured to explore the flexibility of this innovative process and its potentialities to produce complex parts using the same tools. The obtained results have indicated that this novel technique, that doesn’t require costly equipments, is an effective approach to manufacture multitude of parts with different shapes. Moreover, it has been shown that the parts geometrical accuracy as well as thickness distribution are enhanced compared to the conventional ISF process and that the geometrical defects, called ‘dimples’ and caused by the pins’ ends, are significantly reduced and almost eliminated after the insertion of a rubber piece between the reconfigurable die and the blank sheet. On the other hand, the effect of the size and geometry of the rectangular pins on the geometrical accuracy and the dimpling defect has been studied using a finite element analysis.     

Improving geometrical accuracy for flanging by incremental sheet metal forming

Incremental Sheet Forming (ISF) is a manufacturing technology for individualized and small batch production. Among the opportunities this technology provides there is the possibility of a short ramp-up time and to cover the whole production chain of sheet metal parts by using a single reconfigurable machine set-up. Since recent developments proved that manufacturing of industrial parts is feasible, finishing operations such as flanging and trimming gain importance, which are an integral part of manufacturing process chains of many sheet metal parts. This paper analyses the technological capabilities of performing flanging operations by ISF. Due to the localized forming zone and the absence of surrounding clamping devices, ISF exhibits a different material flow than conventional flanging processes. In this paper, the influence of the tool path characteristics, the flange length as well as the flange radius is analysed in order to establish a process window and to compare it to the process limits of conventional flanging operations. Since geometrical deviations occur when flanging operations are performed by ISF, a new adaptive blank holder is developed, which acts in the vicinity of the forming tool and reduces unwanted deformation outside the primary forming zone. The experimental results show the benefits of the adaptive blank holder with respect to geometric accuracy. The established process window and the adaptive blank holder hence contribute to the applicability of incremental flanging operations, such that ISF can be used for all forming and flanging operations along the process chain.     

Thermal modeling in electricity assisted incremental sheet forming

A thermal model was built to account for the effects of geometrical parameters of sheet specimen, process parameters and material parameters on the temperature increase of the sheet specimen in Electricity-Assisted Incremental Sheet Forming (EISF). In the EISF, the local area of sheet specimen contacting with a forming tool is heated by direct current, which flows through the forming tool to the sheet specimen. EISF experiments of two high strength steel sheets were carried out to validate the thermal model. The thermal model can be integrated into the control program of EISF system to achieve more accurate temperature control.     

Development of tooling concepts to increase geometrical accuracy in high speed incremental hole flanging

Incremental Sheet Forming (ISF) has been developed as a flexible manufacturing technology for small batch production and prototyping. ISF can also be used to form additional features or stiffening elements such as hole flanges. Incremental Hole Flanging (IHF) operations seem to be a promising alternative to conventional hole flanging. If it was possible to exploit the extended formability of ISF while achieving accuracy and process times of conventional hole flanging, IHF could substitute conventional flanging operations in many cases. However, the long process times and limited geometrical accuracy hinder industrial take-up. In this work, two different tooling concepts which allow incremental hole flanging operations at high speeds are investigated. The first tool is designed as a single forming tool that offers high flexibility and a comparison to conventional Incremental Hole Flanging. The second tool consists of four forming tools to improve the geometrical accuracy of hole flanges. In order to achieve high speeds, the experimental setup is installed on a turning machine. Compared to hole flanging with a conventional CNC machine, the forming time to expand a hole from 50 mm to 100 mm could be reduced from 1680 s to 15.7 s. The geometrical accuracy of the parts formed with the second tool concept could be improved significantly (up to 3 times regarding to the mean surface deviation to at maximum speed). Furthermore, it is shown that forming at high speeds has no significant influence on the characteristics of sheet thickness, strain, forces or geometrical accuracy.     

特种陶瓷成型方法

成型技术是制备陶瓷材料的一个重要环节.特种陶瓷成型方法总的来说可分为干法成型和湿法成型两大类,干法成型包括钢模压制成型、等静压成型、超高压成型、粉末电磁成型等;湿法成型大致可分为塑性成型和胶态浇注成型两大类;近些年来固体无模成型技术在特种陶瓷的成型研究中也取得了较为快速的发展.对特种陶瓷的这些成型方法进行了简要介绍,指出了各种成型方法的优缺点,并展望了特种陶瓷成型方法的发展趋势.     

Multi-objective Optimization of Sheet Metal Forming Die Using Genetic Algorithm Coupled with RSM and FEA

Present study describes the approach of applying response surface methodology (RSM) with a Pareto-based multi-objective genetic algorithm to assist engineers in optimization of sheet metal forming. In many studies, finite element analysis and optimization technique have been integrated to solve the optimal process parameters of sheet metal forming by transforming multi-objective problem into a single-objective problem. This paper aims to minimize objective functions of fracture and wrinkle simultaneously. Design variables are blank-holding force and draw-bead geometrical parameters (length and diameter). RSM has been used for design of experiment and finding relationship between variables and objective functions. Forming limit diagram has been used to define objective functions. Finite element analysis applied for simulating the process. Proposed approach has been investigated on a fuel tank drawing part and it has been observed that it is more effective and accurate than traditional finite element analysis method and the “trial and error” procedure.     

Flow forming of thin-walled precision shells

Flow forming is an innovative form of cold and chipless metal forming process, used for the production of high precision, thin-walled, net-shaped cylindrical components. During this process, the length of a thick walled tube, commonly known as a preform, is increased with a simultaneous decrease in the thickness of the preform without any change in the internal diameter. Forming of the preform is carried out with the help of one or more rollers over a rotating mandrel. By a pre-determined amount of thickness reduction in one or more number of forming passes, the work material is plastically deformed in the radial direction by compression and made to flow in an axial direction. The desired geometry of the workpiece is achieved when the outer diameter and the wall of the preform are decreased, and the available material volume is forced to flow longitudinally over the mandrel. Over the last three and a half decades the flow forming technology has undergone several remarkable advancements. The versatility of the process makes it possible to produce a wide variety of axi-symmetric, nearer to the net-shape tubular parts with a complex profile using minimum tooling changes. In this review article, process details of flow forming have been elaborated. The current state-of-the-art process has been described, and future developments regarding research and industrial applications are also reviewed.     

Experimental investigations and optimization of forming force in incremental sheet forming

Incremental sheet forming process has been proved to be quiet suitable and economical for job and batch type production, which exempts expensive and complex tooling for sheet forming. Investigation of forming forces becomes important for selecting the appropriate hardware and optimal process parameters in order to assure perfection and precision of process. Moreover, lack of available knowledge regarding the process parameters makes the process limited for industrial applications. This research paper aims at finding out effects of different input factors on forming forces in single-point incremental forming (SPIF) process. For operation sustainability and hardware safety, it becomes critical to optimize forming forces for a given set of factors to form a particular shape. In this study, optimization of input factors has been performed to produce conical frustums with helical tool path using Taguchi analysis as design of experiment (DOE) and analysis of variance (ANOVA). The optimal experimental conditions for forming forces have been calculated as sheet thickness (0.8 mm), step size (0.2 mm), tool diameter (7.52 mm), tool shape (hemispherical), spindle speed (1000 rpm), feed rate (1000 mm/min) and wall angle (50^o). Effects of tool shape and viscosity of lubricants have also been investigated. An intensive understanding of the mechanism of forming forces has been presented, which shows that force trend after peak values depends upon instant input factors that can be categorized as a safe, severe and crucial set of parameters.     

弹体毛坯热冲拔工艺

目的设计弹体毛坯热冲拔成形工艺方案。方法将冲子压入加热后的钢坯,使钢坯在压力作用下充满模腔,压成盂形,然后套在引伸冲子通过一串直径逐渐缩小的模圈,使弹体拉长、直径减小、壁厚减薄,获得合理的弹体毛坯尺寸,生产出合格毛坯。分析弹体毛坯冲拔成形原理、温度控制、工装设计及冲拔过程中的金属流动规律,根据体积不变原理,计算出冲拔毛坯尺寸。结果通过批量试制,根据冲拔过程中常见问题的控制措施,优化了冲拔工艺参数,实现了弹体毛坯热冲拔成形。结论通过冲拔原理分析、毛坯及工装设计,形成了热冲拔设计规范,该弹体毛坯热冲拔工艺可行。     

热变形参数对微型齿轮成形影响的实验研究

为了研究凸模速度、成形温度和成形载荷等热变形参数对5A02铝合金微型齿轮成形的影响规律,设计制造了微型齿轮模具,在自行研制的微塑性成形设备上,将压入高度比作为填充程度的评价参数进行了成形实验,成功获得了质量良好的节圆直径为Ф1 mm的5A02铝合金微型齿轮.研究结果表明,压入高度比可以很好地定量表示微型齿轮闭式模锻的填充程度,成形载荷是影响微型齿轮填充程度的主要因素,降低凸模速度、提高成形温度有利于提高微型齿轮的填充程度.     

Establishing a new Forming Limit Curve for a flax fibre reinforced polypropylene composite through stretch forming experiments

In developing an understanding of the failure in natural fibre reinforced polymer composites, the failure limits of this class of the material system are required. It is found that the conventional Forming Limit Curve is not suitable to predict the failure initiated in the natural fibre composite as principal strains cannot differentiate the strain on the flax fibres and the polypropylene matrix. This study proposes a new Forming Limit Curve for the composite which expresses limiting fibre strain as a function of forming mode depicted by the ratio of minor strain to major strain. The new Forming Limit Curve, along with the Maximum Strain failure criterion have been successfully implemented in FEA simulations, and numerical simulations suggest that the former is more accurate. The current work provides an innovative method to predict the onset of failure in natural fibre composites, which can be applied in composite forming and structural design.     

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.     

Optimised tool-path design to reduce thinning in incremental sheet forming process

Rising energy prices and customers’ increasing ecological awareness pushed energy efficient manufacturing to the top position in industrial interests. Actually, companies want to identify the most effective measures to increase energy efficiency in manufacturing processes looking at the sustainability of their product as a point of strength and not only as an extra-cost according to an ancient production vision. For the above considerations, the scientific community introduced in the last years newer technological alternatives to improve the global efficiency in production processes. Incremental Sheet Forming belongs to this family and can be classified as a flexible solution for the modern market requirement. Nowadays, if the points of strength of the above process are widely recognized, more efforts are still necessary to enhance the product performance allowing a wider industrial suitability. In particular, a significant problem which penalizes the quality of the manufacture parts, is the not homogeneous thickness distribution. The research here presented can be placed in this frame: a promising analytical model is highlighted and a user friendly procedure is set up to simplify the design phase with the aim to optimise the thickness distribution along the profile. Satisfactory experimental results which validate the proposed technique are also presented.     

材料超塑性和超塑成形/扩散连接技术及应用

大量的工程材料都具有超塑性,以材料超塑性为理论基础的超塑成形/扩散连接技术是先进制造技术的一种,在航空航天等许多工业部门得到了越来越多的应用.分析了材料超塑性现象,超塑性变形机理研究进展,超塑成形/扩散连接技术的理论基础.以及超塑成形/扩散连接复合工艺的技术优势、研究进展和应用现状,并展望了超塑成形/扩散连接技术的发展趋势.     

On the use of Back-drawing Incremental Forming (BIF) to improve geometrical accuracy in sheet metal parts

Industrial interest about Incremental Sheet Forming (ISF) process is growing in the last years. Up to a few years ago, two main investigation ways were proposed, the former aimed at analysing the process mechanics, the latter at reproducing some ??case study?? geometries. In industrial applications, if the long cycle-time can be neglected in small batches manufacturing, geometrical accuracy represents a relevant drawback, especially when the product has to be coupled to one another. For this reason, in the opinion of the authors, the low accuracy is the most relevant defect of ISF processes today. Among the techniques already set-up to reduce inaccuracy, the use of different material supports or the use of ??arbitrarily modified?? tool trajectories are probably the most known. In this paper a simple approach is proposed, based on the process self capability to correct inaccuracy when different steps of Incremental Sheet Forming are carried out on both the part surfaces. In particular, it is demonstrated that a relevant increasing in accuracy is obtainable at the second repeated step, while new ones do not reduce the inaccuracy sensitively. The above approach builds a new scenario since it allows to keep the basic equipment (without any support) and does not require any further knowledge concerning the material behaviour after the punch action. These aspects are deeply discussed in the next chapters.