A numerical study of the effect of loading conditions on tubular hydroforming

In this work, the mechanics of tubular hydroforming under various types of loading conditions is investigated. The main objective is to contrast the effects of prescribing fluid pressure or volume flow rate, in conjunction with axial displacement, on the stress and strain histories experienced by the tube and the process of bulging. To this end, axisymmetric finite element simulations of free hydroforming (without external die contact) of aluminium alloy tubes are carried out. Hill’s normally anisotropic yield theory along with material properties determined in a previous experimental study [A. Kulkarni, P. Biswas, R. Narasimhan, A. Luo, T. Stoughton, R. Mishra, A.K. Sachdev, An experimental and numerical study of necking initiation in aluminium alloy tubes during hydroforming, Int. J. Mech. Sci. 46 (2004) 1727–1746] are employed in the computations. It is found that while prescribed fluid pressure leads to highly non-proportional strain paths, specified fluid volume flow rate may result in almost proportional ones for the predominant portion of loading. The peak pressure increases with axial compression for the former, while the reverse trend applies under the latter. The implication of these results on failure by localized necking of the tube wall is addressed in a subsequent investigation.

Experimental and numerical investigation of low pressure tube hydroforming on 409 stainless steel

Tube hydroforming has been widely used to produce automotive structural components due to the superior properties of the hydroformed parts in terms of their light weight and structural rigidity. Compared to the traditional manufacturing process for a closed-section member including stamping and followed by welding, tube-hydro forming leads to cost savings due to reduced tooling and material handling. However, the high pressure pumps and high tonnage press required in hydroforming, lead to increased capital investment reducing the cost benefits. This study explores low pressure tube hydro forming which reduces the internal fluid pressure and die closing force required to produce the hydroformed part. The experimental and numerical analysis was for low pressure hydro formed stainless steel tubes. Die filling conditions and thickness distributions are measured and critically analysed.

Numerical investigation of high and low pressure tube hydroforming

Hydroforming is one option to reduce vehicle weight while increasing component stiffness and rigidity. This typically involves using a fluid to form a component with high internal pressure. Tube hydroforming has gained increasing 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 part at the same time. In low pressure hydroforming, the internal pressure is significantly and the hydroformed section length of line stays almost the same as the circumference of the blank tube. This paper details the comparison between high and low pressure hydroforming. It is shown that the internal pressure and holding force required for low pressure hydroforming process is much less than that of high pressure. Also stress and thickness distribution are more uniform and the process is highly suitable for the forming of advanced high strength steels.

FEA comparison of high and low pressure tube hydroforming of TRIP steel

The increasing application of hydroforming for the production of automotive lightweight components is mainly due to the attainable advantages regarding part properties and improving technology of the forming equipment. However, the high pressure requirements during hydroforming decreases the costs benefit and make the part expensive. Another requirement of automotive industries is weight reduction and better crash performance. Thereby steel industries developed advanced high strength steels which have high strength, good formability and better crash performance. Even though the thickness of the sheet to form the component is reduced, the pressure requirement to form the part during expansion is still high during high pressure hydroforming. This paper details the comparison between high and low pressure tube hydroforming for the square cross-section geometry. It is determined that the internal pressure and die closing force required for low pressure tube hydroforming process is much less than that of high pressure tube hydroforming process. The stress and thickness distribution of the part during tube crushing were critically analysed. Further, the stress distribution and forming mode were studied in this paper. Also friction effect on both processes was discussed.

An investigation of complex shapes during low pressure tube hydroforming

Hollow structures made of Advanced High Strength Steel (AHSS) are increasingly used in the automobile industry for crash and structural components. Generally high pressure hydroforming is used to form these tabular parts, which is a costly manufacturing process due to the high pressure equipment and large tonnage presses required. A new process termed low pressure hydroforming, where a pressurized tube is crushed between two dies, represents a more cost effective alternative due to the lower pressures and die closing forces required.

In this study the low pressure tube hydroforming of one simple and two different complex hollow shapes is investigated. The complexities of the pat1S compared to simple shapes are critically studied and the die filling conditions are investigated and discussed. FUl1hennore the thickness distributions over the circumference of the part during forming are analyzed.

Crash investigation of side intrusion beam during high and low pressure tube hydroforming

Advanced high strength steels (AHSS), in particular, are an attractive group materials, offering higher strength for improved energy absorption and the opportunity to reduce weight through the use of thinner gauges. High pressure tube hydroforming (HPTH) has been used to produce safety components for these steels, but it is expensive. Low pressure tube hydroforming (LPTH) is a lower cost alternative to form the safety components in the car. The side intrusion beam is the second most critical part after front rail in the car structure for passenger safety during crash. The forming as well as crash behaviour of a square side intrusion beam from both processes was investigated using numerical simulation. This paper investigated the interaction between the forming and crash response of these materials in order to evaluate their potential for use in vehicle design for crashworthiness. The energy absorption characteristics of the different tubes were calculated and the results from the numerical analyses compared for both hydroforming process.

Analysis of a Non-Traditional Micro Tube Hydroforming Process

As the demand for miniature products and components continues to increase, the need for manufacturing processes to provide these products and components has also increased. To meet this need, successful macroscale processes are being scaled down and applied at the microscale. Unfortunately, many challenges have been experienced when directly scaling down macro processes. Initially, frictional effects were believed to be the largest challenge encountered. However, in recent studies it has been found that the greatest challenge encountered has been with size effects. Size effect is a broad term that largely refers to the thickness of the material being formed and how this thickness directly affects the product dimensions and manufacturability. At the microscale, the thickness becomes critical due to the reduced number of grains. When surface contact between the forming tools and the material blanks occur at the macroscale, there is enough material (hundreds of layers of material grains) across the blank thickness to compensate for material flow and the effect of grain orientation. At the microscale, there may be under 10 grains across the blank thickness. With a decreased amount of grains across the thickness, the influence of the grain size, shape and orientation is significant. Any material defects (either natural occurring or ones that occur as a result of the material preparation) have a significant role in altering the forming potential. To date, various micro metal forming and micro materials testing equipment setups have been constructed at the Michigan Tech lab. Initially, the research focus was to create a micro deep drawing setup to potentially build micro sensor encapsulation housings. The research focus shifted to micro metal materials testing equipment setups. These include the construction and testing of the following setups: a micro mechanical bulge test, a micro sheet tension test (testing micro tensile bars), a micro strain analysis (with the use of optical lithography and chemical etching) and a micro sheet hydroforming bulge test. Recently, the focus has shifted to study a micro tube hydroforming process. The intent is to target fuel cells, medical, and sensor encapsulation applications. While the tube hydroforming process is widely understood at the macroscale, the microscale process also offers some significant challenges in terms of size effects. Current work is being conducted in applying direct current to enhance micro tube hydroforming formability. Initially, adding direct current to various metal forming operations has shown some phenomenal results. The focus of current research is to determine the validity of this process.

Formability and forming of advanced steels

The thesis identified how advanced high strength steels perform compared to conventional steels in terms of weight reduction and crash performance for automotive bodies. The novel production method of low pressure tube hydroforming was applied to form these advanced steels to reduce the press tonnage and fluid pressure compared to the conventional high pressure process. In addition analytical models were developed to predict the force and pressure in the low pressure process.

Review of approximate analyses of sheet forming processes

Approximate models are often used for the following purposes: in on-line control systems of metal forming processes where calculation speed is critical; to obtain quick, quantitative information on the magnitude of the main variables in the early stages of process design; to illustrate the role of the major variables in the process; as an initial check on numerical modelling; and as a basis for quick calculations on processes in teaching and training packages. The models often share many similarities; for example, an arbitrary geometric assumption of deformation giving a simplified strain distribution, simple material property descriptions - such as an elastic, perfectly plastic law - and mathematical short cuts such as a linear approximation of a polynomial expression. In many cases, the output differs significantly from experiment and performance or efficiency factors are developed by experience to tune the models. In recent years, analytical models have been widely used at Deakin University in the design of experiments and equipment and as a pre-cursor to more detailed numerical analyses. Examples that are reviewed in this paper include deformation of sandwich material having a weak, elastic core, load prediction in deep drawing, bending of strip (particularly of ageing steel where kinking may occur), process analysis of low-pressure hydroforming of tubing, analysis of the rejection rates in stamping, and the determination of constitutive models by an inverse method applied to bending tests.