Growth seen in high-volume automotive structural parts
January 24, 2002
This article examines hydroforming in Germany, focusing on the advancement of the technology. It specifically discusses growing automotive uses, a new type of hydroforming press, material quality requirements, cost factors, new testing methods, and simulation software.
"We started to use the hydroforming process for the production of structural parts in 1999, and our current timetable schedules an annual production volume of 2.5 million hydroformed parts by 2005," explained Dipl.-Ing. Matthias Schroeder, calculation specialist in the hydroforming preseries design department of DaimlerChrysler's Hamburg factory, during the lecture he delivered at the Steel Symposium on Hydroformed Steel Parts last year in Dsseldorf. In weight, 2.5 million parts is equivalent to about 3,000 metric tons of steel.
By 2005 the Hamburg site will have five presses and two production lines to make hydroformed frame and body parts. This decision to increase use of hydroforming underscores that the Hamburg location, where experience with the technology has been growing since 1993, when niche production of high-alloy stainless steel exhaust manifolds began, has become the top German center for this technology within the group.
During recent years DaimlerChrysler specialists focused mainly on optimizing process productivity and were able to achieve cycle times in the 30-second range. Most parts are made of higher-strength steels, but high-alloy stainless steel and aluminum profiles are processed too.
Forming tubular structures by applying water under high pressure in a two-part steel mold until they assume the shape defined by the contours of the tool is the best-known variant of a fast-evolving family of forming internationally called hydroforming. Some specialists such as Prof. Dr.-Ing. Matthias Kleiner, who holds the Forming Technology chair at the University of Dortmund, Germany, prefer the designation forming with working media. In past years many technology variants and combinations with other forming processes have emerged, but in industrial applications, hydroforming technology clearly still leads the way.
Hydroforming also can be used to shape more complex structures, for example, an auxiliary support frame for the Audi A4® (see Figure 1). Another process variant gaining ground is the high-pressure forming of steel sheet, which involves forming flat sheet material into the cavity of a tool by applying water pressure from one side. Because such parts have large surfaces, this implies the application of enormous forces of up to 100 meganewtons (10,000 metric tons) must be applied.
A new type of press has been developed in which the developing pressure is withheld by a winding of high-strength steel cable (see Figure 2) instead of the massive steel columns used with conventional technology. A prototype was put into operation at Dortmund University in fall 2001.
Advantages of hydroforming are excellent geometric and dimensional part accuracy, an exceptionally high level of usage of the formability of material, and a more even distribution of residual stresses. The process yields solutions that could not be obtained by conventional forming technologies, especially for intricate hollow structures such as fuel tanks (see Figure 3).
"In the hydroforming process, the quality of the material you use is of prime importance," said Dr.-Ing. Jrgen Oswald, CEO of the Oswald Hydroforming GmbH & Co. KG in Crimmitschau, Germany. The main parameter for material is elongation without necking, or the maximum plastic elongation the material will sustain before deformation becomes nonuniform.
Exceeding this limit results in a bad part because of uneven deformation distribution within the workpiece. High elongation without necking is required to achieve a high degree of plastic deformation and thus optimized flexibility for part design. For safety parts, designers have to take care to remain well below this limit, leaving a residual formability of 5 to 10 percent as plastic deformation reserve for crash situations.
Of course, the price of the primary material also plays an important role. But because designers often have to go to the very limits of the material properties, standard primary material may be unsuitable. In such cases it becomes vital to have reliable vendors who can supply material with properties that are tailored to meet the specific requirements of the application (see Figure 4). If the proper consideration is not given to material, a company may find, after investing thousands of dollars in a tool, that it cannot obtain material that can be processed in it with acceptable results.
Besides the standard properties of a batch of sheet or tube material, its professional record influences the outcome of the hydroforming process to such an extent that the fabricator is well-advised to acquire an in-depth knowledge of the material's source.
"Specifically for safety-relevant parts for coups and cabriolets, the attractiveness of hydroformed variants becomes quite tempting," said Dipl.-Ing. Peter Freytag of Salzgitter AG, who presented the results of a thorough profitability analysis using a cost calculation program developed by the Massachusetts Institute of Technology. For an in-depth technical and economical assessment jointly carried out by Salzgitter and Porsche, this calculation model was specifically adapted to hydroforming.
The analysis was performed on parts used for A columns such as the one in the new Model 3 BMW® cabriolet. These safety parts support the side of the windshield and must withstand even a rollover without buckling. The costs of a conventional sheet shell variant and a hydroformed variant were compared for two scenarios —30,000 cars per year and 200,000 cars per year.
Although in both cases the hydroformed variant cost between 10 and 20 percent more, noncalculable aspects of the worth assessment revealed clear technical advantages, such as better stiffness combined with optimized space usage and reduced weight. Furthermore, it can be expected that the higher cost will be reduced by future optimization of the process. It also should be noted that the hydroformed variant leads to cost reductions downstream in the manufacturing chain, such as elimination of parts and of multilayer sheet welding operations, reductions in transportation volume, and reductions in seam-sealing expenditures.
"The usual testing methods for tubes are not suited to assess the material properties vital for the hydroforming process," explained Prof. Dr.-Ing. Peter Groche from the Institute for Production Technology and Forming Machines of the Technical University in Darmstadt, Germany. He therefore developed a free tube-bursting test whose results will help to characterize a material's properties, particularly material flow and breakage characteristics. Schuler Hydroforming GmbH & Co. KG has developed a fully automated test stand based on this test method that can be used for different tube diameters by using a set of adapters.
The computerized simulation of forming characteristics plays an important role in hydroforming. Most lecturers agreed such simulations are valuable tools in limiting risks in the design phase of new parts. But it also became evident that the simulation tools currently available do not fully meet expectations.
"When performing computer simulations, we sometimes find wrinkling effects that we cannot describe by software within the limits of economical viability mainly imposed by computer calculation time restrictions," said Schroeder.
And Oswald reported that computer simulations predicted failures in parts that have been successfully produced for years. It seems the producers of these programs need to do some more homework on adapting their models to real practice.