TECHNICAL ARTICLE:
| Manufacturing Tubes For Hydroforming Applications | ||||||||||
| By: F.J. Rípodas Agudo, Aceralia Tubes (Aceralia Transformados), Arcelor Group & Professor of Manufacturing Technologies, Universidad Pública de Navarra |
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Introduction The automotive industry is involved in a continuous process to minimize cost, but always taking into account the optimisation of its products concerning weight, strength characteristics and rigidity. The metal processing industry is working in this way in research and development for alternative materials and production processes. The siderurgic industry has been involved in projects like ULSAB (Ultra Light Steel Auto Body), ULSAC (Ultra Light Steel Auto Closure) and others with these concepts. The results have demonstrated that by using steel as the material of choice - in conjunction with the utilization of advanced manufacturing processes - pieces can be produced that achieve a mass reduction, while increasing structural performances, and at no cost penalty. To compound this benefit, steel is the most recyclable material in the world. Hydroforming technology has therefore been called to offer an interesting technical and economic potential.
Tubular hydroforming is achieving increasing acceptance in the automotive industry for making a wide variety of components. Current applications include suspension frame, body structure, power-train components and exhaust pipes. The major advantages of tubular hydroforming that have initiated these applications are cost reduction and weight savings, improved dimensional stability, improved integrity and increased strength and stiffness of the components.
Depending on the part design, pre-bending and pre-forming operations could be essential before the start of the tubular hydroforming process. In the tubular hydroforming process, a tube is first placed in the closed cavity of a forming die. Once the ends of the tube are sealed, the tube is filled and pressurized with hydraulic fluid. The internal pressure forces lead the tube to form into the shape of the tool cavity. Most hydroforming processes also use axial force-feeding at the tube ends to feed material into the tool during forming. With the application of axial force-feeding, higher limits at the end of the part can be achieved.
Source: Krupp Hoesch Automotive Hydroforming provides several advantages versus traditional stamped and welded structures, including:
When used with high strength steels hydroforming is able to produce structurally superior parts with thinner sections at a reduced mass. Hydroformed Tubes in ULSAB and ULSAC Program Tubular hydroforming was used in the ULSAB and ULSAC programs to improve structural integrity, reduce costs and achieve mass reduction. In the ULSAB Project Tubular hydroforming and its cold working effect produces high dimensional stability and increases effective yield strength in any component. The part making process incorporates four steps:
Rail side Roof (LH) Source: Porsche Engineering Services, Inc. – ULSAB Consortium ULSAB’s hydroformed side roof rail provides an essential load path for structural performance and crash energy management from the top of the ‘A’ pillar along the roof, into the ‘B’ and ‘C’ pillars and into the rear of the structure. The hydroformed side roof rail reduces the total number of parts and maximizes section size, allowing for both mass and cost savings.
The raw material for the side roof rail is a welded, high strength steel tube of thickness 1mm and outside diameter of 96mm. The yield strength is 280MPa.
In the ULSAC Project In ULSAC, hydroforming is used to produce the latch and hinge tubes, adding stability to the door structure and allowing for integration of additional functions, such as the hinge attachment, latch attachment, and bushings.
Although both the hinge and latch tubes were manufactured with similar hydroforming process steps, the latch tube's smaller wall thickness and three-dimensional curves – made it a more complicated part to manufacture. The part making process for latch tubes incorporates four steps:
Requirements – Tubes for Hydroforming Front Door Hinge Tube/Latch Tube: Quality
Dimensions and Tolerances
Appearances of Tube
Welding Requirements
Material Selection The material selection was done in material group meetings, attended by expert representatives of the Consortium member companies. The material types and grades selected for manufacture of the test door structure were according to the design material thickness specification.
The tubes can be manufactured in different ways. From the first tubes manufactured by forging, to the actual systems, there is an historical evolution of the technologies: Different Processes of Forming (Discontinuous and Continuous):
Processes of Junction:
The manufacturing tube processes for hydroforming applications must match the quality exigencies of this industry and, yet, have a cost as low as possible. The Most Common Forming and Welding Processes are:
Continuous Rollforming Process A steel strip is continuously roll formed into a tube shape and the longitudinal gap is continuously welded by applying high frequency welding process. The welding process is a result of inductive heating and compressing the edges of steel strip without supplementary material. The following sizing leads to exact tolerances of the tube for the hydroforming process. The external bead of the welded area is always removed, and the internal only when it is necessary.
The second process is the use of laser welding for joining. Laser-welding eliminates the bead. Compared to the high frequency welding, there is a much smaller heat-effected and decoated welded zone (if the raw material is coated).
Characteristics of the Continuous Roll Formed Tubes:
Discontinuous Forming Process A metal sheet is folded and bended into a tube shape piece by piece. Then, the longitudinal gap is welded by application of a laser welding process. Blanks can be produced in a tailored way, in conical form to obtain the subsequent tube. The Characteristics of Discontinuous Forming and Laser Welding Tubes:
Cold Drawing Tubes A HF roll formed tube or seamless tube is drawn into a tube. This tube has been reduced to a new final diameter and thickness. The process is made by means of both an external and an internal mandrel. The original tube goes through, at least, these processes:
The Characteristics of the Cold Drawing Tubes:
Today, longitudinally HF welded tubes constitute the most cost-effective alternative both to the laser welded tubes and to the cold drawn tubes. Optimized manufacturing methods led to the fact that it is possible to manufacture tailored tubes. The use of tailored tubes is the consequent development of welded profiles, which unlike standard tubes are capable of creating suitable hydroforming conditions. Every kind of these tubes can be supplied with final conditions like stress relieving, annealing or normalizing. Requirements on Tubular Products Hydroforming is defined as a process of manufacturing complex hollow components, in most cases using circular tubes as source material. It is vital to carefully check and ensure that the tube to be hydroformed retains the highest amount of formability and residual elongation factors, crucial to the effective use of this technology. Hydroformed parts distinguish themselves by a complex cross section geometry, which varies considerably above the longitudinal axis tube and in many cases even show punches, and so on. The rest of the main requirements, in geometry and appearance, placed on tubular products can be summarized as:
Furthermore, there are some additional requirements on the material that is used to form the tube:
The deformation capability of a tube is restricted by buckling, wrinkling and bursting.
Tubes Manufactured for Hydroforming Applications - Examples Different tubes are being manufactured at the moment, for several hydroforming applications. We will analyse two different tubes that have been designed according to the concepts previously described. Tube 73 x 1,3 This tube (known as crossmember) is intended to work as an instrument panel support after being reprocessed using the hydroforming technique. The raw material used is cold rolled DC03 (EN 10130). The flower design for this tube is as follows:
As stated before, the process of manufacturing must be carefully studied. Thus, specific software is used to calculate the elongation/compression and strains of the material through the forming process, in order to get the optimal design.
In order to complement this information, a set of technical data about this tube follows: Chemical composition of the raw material (coil)
Mechanical characteristics of the coil
Mechanical characteristics of the tube
Tube 60 X 2 This tube (known as subframe) is intended to work as an engine cradle after being reprocessed using the hydroforming technique. The raw material used is a DD13 (EN 10111). There is a possibility to manufacture this tube in different mills, and each one of them requires its own flower and tool design. The software above mentioned can handle this possibility with ease. Mechanical characteristics of the coil
Mechanical characteristics of the tube
The forming processes for both tubes are soft enough not to significantly change the mechanical properties from the coil to the final tube.
Conclusions Longitudinally welded steel tubes meet the high requirements placed on tubular products during hydroforming. Adapting materials, forming processes and welding methods, the manufacturing of tubes for hydroformed parts is optimised, and becomes a really cost-effective process with high standards in quality and complexity of the products than can be manufactured. Besides the experiences on tube manufacturing gained from the decades of activity of Aceralia Tubes, additional sources of information have been used to complete this article. References and Bibliography Taylan Altan (Ohio State University Professor), Metal Forming Handbook (1998) Additional Texts on Hydroforming: Leitloff, F-U, Innenhochdruckumformen mit dem Schäfer-ASE-Verfahren, Stahl (1995) 5. Additional Information on ULSAB and ULSAC Projects: http://www.worldautosteel.org/ Other Resources http://www.hydroforming.net/
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