Experimental investigation of friction coefficient in tube hydroforming

The friction coefficient between tube and die in guide zone of tube hydroforming was obtained. In hydroforming, the tube is expanded by an internal pressure against the tool wall. By pushing the tube through tool, a friction force at the contact surface between the tube and the tool occurs. In guiding zone, the friction coefficients between tube and die can be estimated from the measured axial feeding forces. In expansion zone, the friction coefficients between tube and die can be evaluated from the measured geometries of expanded tubes and FE analysis.

Classification and Analysis of Tube Hydroforming Processes with Respect to Adaptive FEM Simulations

Tube hydroforming process is a relative new process f or production of structural parts of low weight and high rigidity. The successfu lness of the process depends largely on the a proper selection of loading path w hich is axial feeding distance as related to the applied internal pressure. Due to the complicated nature of plastic deformation, a optimum loading path which w ill guarantee good hydroformed parts free of winkle and fracture has often to be determined by finite element analysis. In order to save trials and errors, adap tive FEM simulation method has been developed. To effectively apply the adaptive simulation method, we have to know the applicability of the method. In this pap er, a classification of tube hydroforming (THF) processes based on sensitivity to loading parameters has been suggested. Characteristics of the classification have been analyzed in terms of failure mode, dominant loading parameters and th eir working windows. It was discussed that the so called pressure dominant THF p rocess is the most difficult process for both simulation in FEM analysis and pra ctical operation in real manufacturing situation. To effectively find out the op timum loading path for pressure dominant THF process, adaptive FEM simulation st rategies are mostly needed.

Pressurization System in Low Pressure Tube Hydroforming

Advanced high strength steels are the group of material with high strength and good formability, because high strength lesser gauge thickness can be used without compromising the function of component. In terms of economic forming process, hydroforming is the manufacturing option which uses a fluid medium to form a component by using high internal pressure. This process gained steep interest in the automotive and aerospace industries because of its many advantages such as part consolidation, good quality of the formed part etc. The main advantage is that the uniform pressure can be transferred to whole projected part at the same time. Low pressure tube hydroforming considered an inexpensive option for forming these advanced high strength steel. This paper investigates the pressurization system used during the low pressure tube hydroforming cycle. It is observed that the usage of ramp pressure cycle during forming the part from low pressure tube hydroforming results in lesser die holding force. Also, the stress, strain and thickness distribution of the part during low pressure tube hydroforming are critically analysed.

Wrinkling behavior of magnesium alloy tube in warm hydroforming

In tube hydroforming with axial feeding,under the effect of coupled internal pressure and axial stress,wrinkles often occur and affect the forming results.Wrinkling behavior of an AZ31B magnesium alloy tube was experimentally investigated with different loading paths at different temperatures.Features of wrinkles,including shape,radius and width,were acquired from the experiments,as well as the thickness distribution.Numerical simulations were carried out to reveal the stress state during warm hydroforming,and then the strain history of material at the top and bottom of the wrinkles were analyzed according to the stress tracks and yielding ellipse.Finally,effects of loading paths on expansion ratio limit of warm hydroforming were analyzed.It is verified that at a certain temperature,expansion ratio limit can be increased obviously by applying a proper loading path and realizing enough axial feeding.

Analysis of the Internal Pressure in Tube Hydroforming and Its Experimental Investigation

The internal pressure of the process was studied theoretically and experimentally. The external load character and internal stress character of tube hydroforming were discussed first. Then, according to the characters, the function and classification of internal pressure were presented in general. Base on the stress analysis, its effect on the yield criterion and calculation formula were also researched and derived. To verify the correction of the theoretical analysis and derived formula, experiments with different internal pressures were carried out and the result was compared and discussed. It demonstrates that internal pressure plays an important role in tube hydroforming and theory and formula discussed and derived by this paper are feasible in practice.

Characteristics of thickness distribution of tailor-welded tube hydroforming

Both experimental and mechanical analyses were carded out to investigate the characteristics of thickness distribution for tailor-welded tube （TWT） hydroforming with dissimilar thickness. Then, the effects of weld-seam position and thickness difference were also revealed. A multiple-diameter tube was formed to reveal the characteristics and the regularity of thickness distribution during TWT hydroforming. It is indicated that there are obvious fluctuations in thickness distribution though the TWTs have the same expansion ratio. The thinning ratio of thinner tube is bigger than that of thicker tube especially in the zone closed to the weld-seam. The difference in thinning ratio between two tube segments can reach 9%. Consequently, sudden and large fluctuation of thickness appears in the zone nearby the weld-seam. The difference in thinning ratio between thinner and thicker tubes enlarges as the thickness difference increases, but improves as length ratio increases. Different strain states are the main reason to induce nonuniform thickness distribution. The difference in thickness is the main reason to induce different strain states on thinner and thicker tubes.

Warm hydroforming of magnesium alloy tube with large expansion ratio

Process of warm tube hydroforming was experimentally investigated for forming an AZ31B magnesium alloy tubular part with a large expansion ratio. Effects of temperature on the mechanical properties and formability were studied by uniaxial tensile test and hydraulic bulge test. Total elongation increases with temperature up to 250℃, but uniform elongation and maximum expansion ratio get the highest value at 175℃. Different axial feeding amounts were applied in experiments to determine the reasonable loading path. A preform with useful wrinkles was then realized and the tubular part with an expansion ratio of 50% was formed. Finally, mechanical condition to produce useful wrinkles is deduced and the result illustrates that useful wrinkles are easier to be obtained for tube with higher strain hardening coefficient value and tubular part with smaller expansion ratio.

State of the Art and Perspectives of Hydroforming of Tubes and Sheets

Hollow parts of high accuracy and high strength can be produced by forming methods using liquid media. Hydroforming of tubes has reached a high standard for small parts (volume some 1000 cm3) and is further developed for larger parts (volume some 10.000 cm3). Processes for hydraulic sheet metal forming are sometimes used for small parts from single sheets These pro-cesses are currently under intensive investigation, which is also true for the processing of double layered sheets Single sheets can be formed using membranes which separate the workpiece and the liquid. This results in interesting possibilities for a part and process integration in one step The forming performance of aluminum alloys can be enhanced by using a heated liquid media when forming without membranes.

Mechanism of weld-line movement within hydroforming of tailor-welded tube

To reveal the reason of weld-line movement in hydroforming of a tailor-welded tube (TWT) with dissimilar thickness,the stress ratio of axial stress to circumferential stress is derived by mechanical analysis and analyzed between the thicker and thinner tubes,as well as the property of the axial strain. During TWT hydroforming,tensile strain along axial direction happens on the thinner tube. On the contrary,compressive strain happens on the thicker tube. Experiments are conducted to varify the weld-line movement regularity and strain distribution. It indicates that the weld-line moves from the thinner part to the thicker during TWT hydroforming. The thinning ratio of the thinner tube is bigger than that of the thicker tube,especially in the zone near weldline. Stress ratio difference between the thicker tube and the thinner tube is the main reason of weld-line movement and non-uniform thinning ratio distribution.

Optimization for Hydroforming Loading Paths of Parallel Multi-branch Tubes Based on Grey System Theory

According to characteristic of hydroforming of parallel multi-branch tubes,multi-objective problems were transformed to single objective problem of relational grade comparison by grey system theory.Two different objectives were selected,according to the principle that process parameters were optimal which of grey relational grade were maximum,the optimal loading parameters under different objective condition were obtained,and loading paths were optimized.The results indicated that parallel multi-branch tubes hydroformed under loading paths optimized by grey system theory could meet with the requirement that objective was optimal.And the optimal loading paths under different objectives were different,and the appropriate objective should be selected according to forming characteristic.

Deformation and defects in hydroforming of Y-shaped tubes

Hydroforming process of a Y-shaped stainless steel tube was investigated through numerical simulation and experiments. The forming process and reasons of typical defects were analyzed with three different loading paths. Thickness distribution of formed Y-shaped tube was obtained. It is shown by numerical and experimental results that the transition regions are depressed in the forming condition of low inner pressure and wrinkles occur, while fracture occurs in the forming condition of high inner pressure. After forming, the thickness in left transition fillet region is the largest, that in fight transition fillet region is thinner, and the thinnest thickness is at the top of the protrusion. The original thickness line is below the top of the protrusion. The thinning area occurs above this line, while the thickening area is below this line. The maximum thinning rate is significantly increased as the calibration pressure increases, while the maximum thickening rate remains almost unchanged.

Hydroforming of AZ61A tubular component with various cross sections

The effects of temperature on the mechanical properties and elongation of AZ61A tubular part were derived by uni-axial tension tests at various temperatures.Warm hydroforming of an AZ61A tubular part for passenger car was then numerically and experimentally investigated.The complete processes including bending,pre-forming and hydroforming were analyzed and discussed. Microstructure at the corner of the typical section was observed before and after the final hydroforming process.It is shown that the yielding strength,tensile strength and total elongation increase as temperature increases,while the elongation before necking decreases.The temperature range from 225℃to 250 ℃is more suitable for hydroforming of theAZ61A magnesium alloy tube with various cross sections.Pre-forming and hydroforming with high strain values are feasible at elevated temperature.Grain refinement is observed at the corner of the part after warm hydroforming.Thinning ratio analysis illustrates that non-uniform deformation at elevated temperature should be considered in process optimization to avoid severe local thinning.

Optimizing loading path and die linetype of large length-to-diameter ratio metal stator screw lining hydroforming

In order to meet the high temperature environment requirement of deep and superdeep well exploitation, a technology of large length-to-diameter ratio metal stator screw lining meshing with rotor is presented. Based on the elastic-plasticity theory, and under the consideration of the effect of tube size, material mechanical parameters, friction coefficient and loading paths, the external pressure plastic forming mechanical model of metal stator screw lining is established, to study the optimal loading path of metal stator lining tube hydroforming process. The results show that wall thickness reduction of the external pressure tube hydroforming(THF) is about 4%, and three evaluation criteria of metal stator screw lining forming quality are presented: fillet stick mold coefficient, thickness relative error and forming quality coefficient. The smaller the three criteria are, the better the forming quality is.Each indicator has a trend of increase with the loading rate reducing, and the adjustment laws of die arc transition zone equidistance profile curve are acquired for improving tube forming quality. Hence, the research results prove the feasibility of external pressure THF used for processing high-accuracy large length-to-diameter ratio metal stator screw lining, and provide theoretical basis for designing new kind of stator structure which has better performance and longer service life.

Design of hydroforming process for an automobile subframe by FEM and experiment

Tube hydroforming technology has shown the attention of the automotive industry due to its advantages over conventional stamping and welding methods.In this study,the tube hydroforming process including tube bending,preforming and hydroforming process for an automobile subframe is analyzed and designed by the simulation software AutoForm of a finite element method (FEM) program.A parametric study is carried out to obtain the effect of the forming parameters such as initial tube size and loading path on the forming results.The simulation results are also compared with experiment results.The research indicates that the multiple forming operation of the tube hydroforming process can be simulated accurately by using the implicit code AutoForm,and the formability of tube hydroforming can be improved by designing suitable forming parameters.

Experimental and Numerical Research on Deformation Behavior of Thin-Walled and Large Expansion Ratio Guide Vane Liner in Hydroforming Process

Some tube hydroforming process tests and further research work were conducted to manufacture hollow guide vane liners( made of super alloy GH3030).The relative thickness( t0/ OD) of the tubular blank is approximately 0. 01,and the maximum expansion ratio( Dmax/ OD) of the needed part is more than 40%,and the length to diameter ratio of the expansion regionis more than 3. 0. It is very hard to manufacture this kind of ultra-thin-wall,curved axis and large expansion ratio tubular part without fracture and wrinkles. The success of the process is highly dependent on useful wrinkles with appropriate internal pressure and axial feeding. A simplified finite element model and a theoretical model are used for detecting the deformation behavior and forming laws. Further study results demonstrate that the useful wrinkles do not appear at the same time and middle-wrinkles need bigger axial force than tube-end-wrinkles and feeding-wrinkles. The wrinkles can transfer bigger axial force after its wave peak has come into contact with the die inner surface. The thickness thinning rate of the element at the peak is bigger than that at the trough. With the increase of the axial and hoop stress ratio,the critical buckling stress also increases. Microstructure examination results show that the grain size in the maximum thinning zone has been stretched and refined after the large deformation and annealing treatment.The process is feasible and the finished part is qualified.

面向壁厚均匀的液压成形三通管的坯料设计方法与参数优化

目的研究连续变厚度管液压成形过程中三通管的壁厚均匀性问题。方法首先,利用有限元软件分析得到了等厚管液压成形三通管过程中的成形规律,根据其成形规律,反向设计了用于液压成形三通管的连续变厚度管坯;其次,采用正交试验方法,研究了连续变厚度管厚区壁厚、过渡区长度对液压成形三通管成形质量的影响;最后,采用多岛遗传算法,对连续变厚度管的结构参数进行了优化验证,提高了液压成形三通管的成形质量。结果在等厚管液压成形三通管成形过程中,支管顶部壁厚值持续减小、轴向过渡圆角及直管底部中间位置处壁厚值持续增大;环向过渡圆角处等效应力最大,其他位置连续变化;轴向压应力使得三通管壁厚增大,拉应力使得壁厚减小;正交试验结果表明,当连续变厚度管厚区厚度、环向过渡区长度达到某一值时将出现较好的壁厚均匀性;在4种过渡区曲线线型中,直线型的壁厚均匀性最好;通过多目标优化得到最佳连续变厚度管结构参数,并通过有限元仿真进行验证,相对误差均在2%以内,使用优化后的连续变厚度管液压成形三通管相对于等厚管,壁厚均匀值减小了0.423 mm。结论可以采用厚度补偿或减薄的方式来提高成形三通管的壁厚均匀性;连续变厚度管坯相比于等厚管,显著提高了液压成形三通管的壁厚均匀性。

热处理对2219铝合金薄壁旋压管组织与性能的影响

本文对2219铝合金薄壁旋压管进行了退火、固溶及时效热处理,研究了热处理工艺对微观组织、拉伸性能和液压胀形性能的影响。结果表明:2219铝合金旋压管在400℃以下退火时以回复为主,温度高于450℃时发生再结晶。随退火温度升高,基体组织逐渐由形变组织向再结晶组织转变。固溶处理时随固溶时间延长,再结晶程度逐渐增加,位错密度降低。经退火和固溶处理后,管材的硬化率和延伸率显著提高,抗拉强度随着退火和固溶温度升高而逐渐增大,固溶处理后抗拉强度达到340 MPa,延伸率提高到25%。时效处理后管材的抗拉强度进一步提高到435.2 MPa。管材液压胀形极限随热处理温度升高而升高,固溶态2219铝合金旋压管具有较好的液压胀形能力,适合进行液压成形。

Formability determination of AZ31B tube for IHPF process at elevated temperature

Ring hoop tension test and tube bulging test were carried out at elevated temperatures up to 480 ℃to evaluate the formability of AZ31B extruded tube for internal high pressure forming （IHPF） process. The total elongation along hoop direction and the maximum expansion ratio （MER） of the tube were obtained. The fracture surface after bursting was also analyzed. The results show that the total elongation along hoop direction and the MER value have a similar changing tendency as the testing temperature increases, which is quite different from the total elongation along axial direction. Both the total elongation along hoop direction and the MER value increase to a peak value at about 160 ℃. After that, they begin to decrease quickly until a certain rebounding temperature is reached. From the rebounding temperature, they begin to increase rapidly again. Burnt structure appears on the fracture surface when tested at temperatures higher than 420 ℃. Therefore, the forming temperature of the tested tube should be lower than 420 ℃, even though bigger formability can be achieved at higher temperature.

某变径管状零件液压成形加载路径优化研究

针对现有管件液压成形加载路径优化方法实用性差的问题,提出了试错仿真分析与二分法相结合的加载路径优化方法。首先,通过分析变径管的几何数模、管材力学特性及成形工艺,设计了变径管液压成形模具。其次,进行变径管液压成形实验,与仿真结果进行对比,证明了有限元模型及仿真方法的正确性。最后,利用试错仿真分析与二分法相结合的加载路径优化方法,对变径管液压成形进行加载路径优化,得到了最优加载路径,为管材液压成形加载路径优化方法的工程化应用提供参考。