External inversion of thin-walled tubes using a die: experimental and theoretical investigation

External inversion of thin-walled tubes using a die is used for producing double-walled parts that are difficult to manufacture by any other technique. Previously published works in the field generally provide the main geometrical and material parameters that govern the process. However, insufficient knowledge can still be found in understanding the influence of interface friction on the material flow and of the strain path and material damage on the occurrence of fracture. Moreover, although the analytical conditions for the occurrence of buckling are easily available in the literature its numerical modelization is usually not commonly available.This paper presents a comprehensive numerical and experimental modelling of the external inversion of thin-walled tubes with the purpose of reexamining the process mechanics and obtaining a better understanding of the deformation modes associated with its formability limits. Fundamental research is undertaken by using virtual prototyping modelling techniques based on the finite element method. The experimental work is performed on Aluminium Al6060 industrial tubes and it is mainly utilized for supporting and validating the theoretical investigation.     

Expansion and reduction of thin-walled tubes using a die: Experimental and theoretical investigation

The production of sound thin-walled tubular parts by expansion and reduction using a die is generally limited to components having geometrical features within a compact range. Basic design rules, providing the geometrical and material parameters that govern the process, are mainly derived from the accumulated experience of both manufacturers of tubular parts and suppliers of machine-tools. However, no information is generally available on the influence of interface friction on material flow and on the effect of material damage and strain path on the occurrence of fracture, wrinkling and local buckling. The aim of this paper is to refresh and extend the fundamentals of tube expansion and reduction using a die by means of a comprehensive theoretical and experimental investigation. The emphasis is focused on understanding modes of deformation and on establishing formability principles for the benefit of those who design tubular parts in daily practice. The theoretical investigation is accomplished by the utilization of virtual prototyping modelling techniques based on the finite element method and the experimental work is mainly utilized for supporting and validating the theoretical investigation.     

Cold expansion and reduction of thin-walled PVC tubes using a die

The production of sound thin-walled tubular parts by expansion and reduction using a die is generally limited to metals. Current research work in the field follows the same trend as that of industry and, therefore, there is no accumulated experience, no practical design rules and no information available in the specialized literature concerning the utilization of polyvinyl chloride (PVC) tubes (as well as other polymers). As far as authors are aware no technology validation studies have ever been performed with the objective of employing cold expansion and reduction forming processes for shaping the end of thin-walled PVC tubes.This paper is concerned with the abovementioned lack of knowledge and is a first step towards the understanding of the mechanics of deformation of tube end forming applied to PVC (and polymers, in general). The presentation addresses the influence of major process parameters on the formability limits of the expansion and reduction of thin-walled PVC tubes using a die and proposes an innovative extension of the flow formulation that is capable of modelling cold plastic deformation of pressure-sensitive polymers under a non-associated flow rule.The overall investigation is assessed with experimental data obtained under laboratory-controlled conditions and results show that free expansion and reduction of thin-walled PVC tubes using a die can be successfully utilized for producing custom and specific shapes at the tube ends.     

管坯推压缩径端部翘曲机理及其影响因素分析

基于对管坯推压缩径的理论分析、试验研究和数值模拟,揭示出缩径时管坯端部存在翘曲的主要原因:管坯内外层缩径量不均匀,使得内层的轴向变形大于外层,导致变形后的管坯端部产生刚体转动,使其外径大于凹模出口处内径,产生翘曲,而且翘曲区的长度近似等于最初位于凹模锥口部分坯料流出后的长度;并分析了缩径率、缩径凹模半锥角、摩擦系数、缩径管坯初始壁厚对管端翘曲和轴向伸长率的影响规律。     

Numerical Investigation of Process Parameters on External Inversion of Thin-Walled Tubes

In this paper, a computational study of development of external inversion mode of deformation of round-metallic aluminum tubes over a fixed profile die is presented. Inversion mode of deformation is analyzed in detail by using a finite element code FORGE2. The proposed finite element model for this purpose idealizes the deformation as axisymmetric. Six-noded triangular elements are used to discretize the domain. The material is modeled as rigid-viscoplastic. Typical variations of the equivalent strain rate and equivalent strain along the length of the deforming tube are studied to predict the development of inversion mode of deformation. The influence of the friction present at the contact interface between the tube and the die is also examined to suggest a successful inversion of tube. Energy absorbed in overcoming the frictional stresses between the tube-die interfaces is compared with the total energy required in the inversion of tube. A few predicted results which include the geometry of inverted tube load-compression variation during inversion process are compared with their experimental counterparts to validate the computational model.     

汽车发动机排气歧管的内高压成形技术

研究了Y型三通管的内高压成形工艺过程,分析了成形过程中过渡区起皱及支管破裂等缺陷产生的原因,从而为实际生产中的Y型三通管内高压成形工艺设计提供了相关指导。利用所成形的Y型三通管制造了汽车发动机排气歧管样件,为内高压成形技术在汽车行业的推广应用奠定了理论与实验基础。     

DOE applied to optimization of aluminum alloy die castings

This paper discusses the application of a design of experiments (DOEs) experimental method for analysing the influence of three injection parameters (slow shot, fast shot and up set pressure) on the internal quality of die casting SAE 305 alloy parts. Initially, the experimental methods applied to the manufacturing process are described. An experimental design was followed: it employed different combinations of injection parameters and aimed to assess the presence of porosity in the die casting parts. The quality assessment of the die casting parts was based on density measurements and qualitative image analysis. Results were evaluated by means of variance analysis, which assessed how the variation in the three different injection parameters influenced the integrity of the components.     

The effect of strain hardening in the extrusion of bimetallic tubes of porous internal layer

The effect of strain-hardening features of porous materials treated by cold extrusion through a conical die is studied theoretically in the present paper. The theoretical model developed by using the fundamentals of the Plasticity Theory of Real Porous Materials investigates all technological parameters affected in the case of extruding bimetallic tubes with a porous internal layer. Comparative analysis between strain-hardened and ideally plastic materials used in the fabrication of bimetallic tubular components, as far as their effect on various technological extrusion parameters is concerned, such as variation of porosity, applied load, level of total deformation, etc., has been conducted successfully, providing, in this manner, with useful concluding remarks from engineering point of view.     

铝合金小弯曲半径管内压推弯成形过程的数值模拟

介绍了小弯曲半径管内压推弯成形的基本原理,并采用有限元法对成形过程进行了模拟,获得了内压力和摩擦条件等主要工艺参数对成形过程的影响规律,对可能出现的成形缺陷进行了预测,并深入分析了引起缺陷的内在原因,所得结果与试验吻合较好。     

Development of an abrasive jet machining system assisted by two fluidized beds for internal polishing of circular tubes

This paper deals with the definition of a relatively novel machining technology to finish the internal part of narrow and long tubular parts made from high resistance stainless steel.A hybrid technology, namely, fluidized bed assisted abrasive jet machining (FB-AJM), was developed and a thorough experimental investigation was concurrently performed.First, a systematic approach, based upon design of experiments, was used to examine the influence of leading operative variables on process. Surface roughness and material removal trends consistent with theoretical expectations were found. Subsequently, the machining mechanisms were analyzed in terms of the evolution of roughness and waviness profile. FB-AJM was found to be a not pressure-copying machining technology. Lastly, the uniformity and the precision of machining all around the internal circumferences of the workpieces were checked out to assure the applicability of FB-AJM to process an ever-growing variety of complex shaped components.     

Nosing process of empty and foam-filled circular metal tubes on semispherical die by theoretical and experimental investigations

Nosing process of circular metal tubes in empty and polyurethane foam-filled conditions on a semispherical rigid die was analyzed by theoretical and experimental methods. A new theoretical model of plastic deformation of circular metal tubes was demonstrated during the nosing process on a rigid semispherical die. Based on the analytical model, some theoretical relations were calculated to estimate instantaneous forming load and dissipated energy of empty and foam-filled circular metal tubes versus axial displacement. Some circular brazen and aluminum tubes were prepared and shaped into semispherical nosed nozzles to verify the present theory. Comparison of theoretical predictions and the corresponding experimental measurements reveals that predicted load-displacement and dissipated energy-displacement diagrams by theoretical formulas have a good correlation with the corresponding experimental curves and it proves verity of the theory. Also, the present theory shows that dissipated energy and axial load of empty tubes depend on material type, wall thickness and diameter of the tubes and they are independent of tube initial length. Furthermore, the experimental results show that the presence of polyethylene Teflon-constraints increases ultimate axial displacement of the forming process.     

芯棒位置对冷拔内螺旋凸筋管成型的影响

本文借助于有限元分析软件Marc对冷拔内螺旋凸筋管进行了有限元模拟,分析了芯棒与凹模的轴向相对位置对冷拔内螺旋凸筋管成型的影响.     

翻转管吸能特性及翻管成形工艺的研究

介绍了一种塑性变形吸能的金属翻转管 ,并对其吸能特性进行了研究。这种翻转管可以通过在圆角模上轴压圆管进行翻制 ,讨论了与之相联系的各种变形模式。研究表明 ,采用优化的圆角半径可稳定地翻转 ,而不会屈曲和撕裂 ,并对翻转变形进行了简单的理论分析。     

圆角模翻管特征的研究

本文根据管坯在锥头模上翻卷变形，设计研制了圆角翻管模，用这种模具翻管，可提高翻管制件的几何精度。通过对圆角模翻管变形特征的分析，论述了圆角翻管模的圆角半径ｒ的翻管力，翻管件质量和翻管时管坯失稳的影响，应用能量法则，建立了圆角翻管模翻管的变形模式，导出了这种模具翻管时重要工艺参数的计算式，通过对ＬＦ２１Ｍ铝合金管坯翻管的工艺计算和实验结果的比较，证明本文所导出的关系式可用作设计圆角翻管模时的工艺计算     

内压对Y型三通管内高压成形影响研究

利用数值模拟对Y型三通管内高压成形过程进行了研究,研究了87MPa～145MPa范围内5条不同内压的加载路径的成形过程,分析了过渡区内凹、支管高度不足等缺陷产生的原因和内压为116MPa时零件成形过程中典型位置的壁厚变化,以及内压对零件壁厚分布的影响。数值模拟结果表明,106MPa～126MPa为成形Y型三通管合适的压力区间,但不同内压成形的零件最小壁厚不同。     

Two-Point Incremental Forming with Partial Die: Theory and Experimentation

This paper proposes a new level of understanding of two-point incremental forming (TPIF) with partial die by means of a combined theoretical and experimental investigation. The theoretical developments include an innovative extension of the analytical model for rotational symmetric single point incremental forming (SPIF), originally developed by the authors, to address the influence of the major operating parameters of TPIF and to successfully explain the differences in formability between SPIF and TPIF. The experimental work comprised the mechanical characterization of the material and the determination of its formability limits at necking and fracture by means of circle grid analysis and benchmark incremental sheet forming tests. Results show the adequacy of the proposed analytical model to handle the deformation mechanics of SPIF and TPIF with partial die and demonstrate that neck formation is suppressed in TPIF, so that traditional forming limit curves are inapplicable to describe failure and must be replaced by fracture forming limits derived from ductile damage mechanics. The overall geometric accuracy of sheet metal parts produced by TPIF with partial die is found to be better than that of parts fabricated by SPIF due to smaller elastic recovery upon unloading.     

FEM numerical simulation of tube axial drawing process

Tube inversion by the axial drawing is an advanced forming process for manufacturing double-walled tubes with high quality, high efficiency, and low consumption. However, to realize forming process depends on producing the tearing in deforming zone, which has a close relationship with forming load. So in this paper, the influence of forming condition parameters on the deforming force and the process is investigated by rigid-plastic FEM numerical simulation. The results show that: (1) during the whole forming process, the shape of the tube remains unchanged when the radius of the core die is larger than a certain value, so a precision forming can be easily realized; (2) for a given r/d₀ or t₀/d₀, the deforming force depends mainly on the value of r/t₀, and there is a critical parameter k, when r/t₀ < k, the steady forming force decreases with an increase of r/t₀; on the contrary, when r/t₀ > k, the steady forming force increases with an increase of r/t₀; (3) the material hardening exponent n of the tube and the friction coefficient μ have a remarkable influence on the deforming force. The smaller the value of n and the larger the value of μ the larger the forming force.     

Investigation into the pre-forming's effect during multi-stages of tube hydroforming of aluminum alloy tube by using useful wrinkles

Tube hydroforming is a process for manufacturing of lightweight components, especially automotive parts, with advantages of cost and weight reduction, better structural integrity and increased strength and stiffness over the conventional stamping process. Generally, the hydroformed parts can be obtained by multi-stages made up of bending, pre-forming and finishing. Based on the characters of FEM and using the useful wrinkles, the process of multi-stages was investigated by using an aluminum alloy. For avoiding the typical failure modes such as folding back, wrinkling, buckling and fracture due to unreasonable selection of the internal pressure and the axial punch feeding, the optimization strategy was created and the process parameters were optimized by using “useful” wrinkles instead of “harmful” wrinkles in the pre-forming stage and in the finishing stage. The suitable pre-forming die cavity shape was discussed. The results from simulation keep a reasonable agreement with that from experiment.     

拉伸翻管模的研究

本文分析了在轴压式翻管模上翻管，翻管件精度不高的原因，设计研制了拉伸翻管模。介绍了拉伸翻管模的特点和特点和工作原理及设计要点。通过理论分析，确定了拉伸翻管的几何条件和力学条件，并导翻管力的计算式。对ＬＦ２１Ｍ铝合金管坯实例工艺计算和实验结果比较，证明本文所导出的工艺参数的关系式是合理的。在所设计的模具上进行翻管，能获得高质量的翻管制件。本文的结果可作为拉伸翻管工艺分析和模具设计的依据。     

Cold Heading of Cylindrical PVC Billets: An Experimental and Theoretical Investigation

The design of upset forged metal parts by cold heading is presently well established. Major operating parameters are identified and the typical modes of deformation that may occur during the process are fully characterized. In contrast to metals, there is no research work and no practical design rules in the specialized literature concerning the cold heading of polymer parts. This paper is concerned with the abovementioned lack of knowledge and is a contribution toward the application of cold heading to polymers. The experimental research work makes use of polyvinyl chloride (PVC) and the overall findings are interpreted in the light of an innovative extension of the flow formulation that is capable of modeling cold forming of pressure-sensitive materials under a nonassociated flow rule. Emphasis is placed on the deformation mechanics of cold heading applied to PVC and on the influence of the major operating parameters on the overall formability limits of the process. Results show that cold heading of PVC can be successfully utilized for producing upset forged parts.