The article describes requirements for crash-relevant components, forming manufacturing methods such as cold and hot forming, as well as possible solutions concerning optimisation of component properties and manufacturing systems.
An important aim of the automotive industry is to decrease the fuel consumption by reducing the weight of the vehicle without loss of security to enlarge the agility. Lightweight construction and safety are two central issues as shown by the different developments in vehicle body manufacturing. The consistent evaluation of passenger vehicles using crash tests has led to the combination of new manufacturing alternatives and high strength materials representing an essential share of the body in white.
Essential improvements in forming technology could be achieved by utilising the results yielded by deformation science (crash test or deformation images resulting from crash test).
An even more consistent implementation of cross-linking between usage properties (crash) and manufacturing was achieved by the foundation of the Frank-Stronach-Institute (Graz). The institute’s Vehicle Safety Institute (VSI) and Tools and Forming (T&F) are collaborating in an interdisciplinary way. The aim is to optimise crash performance properties and lightweight construction on one side and reduce production costs on the other side. For this purpose, modern experimentation and measurement equipment has been made available, and by way of a close collaboration with different suppliers such as steel producers, machine producers (press and tooling), program developers of FE-software or optimisation software, know-how in this field is being improved continuously.
The main objective in vehicle safety is to provide sufficient protection for vehicle passengers in case of a crash. It is obvious, that for this reason higher and higher requirements are being placed on vehicle structure.
Therefore, complex sheet metal formed from parts made of high-strength steel grades is being used to prevent a collapse of the vehicle structure.
Different test regulations require automobile producers to see which parts need to endure forces, static as well as dynamic in order to avoid failure of the passenger cell [5].
The A-pillar could be mentioned as an example, since it is exposed to extreme strain during the FMVSS 216 test, the so-called roof crush performance test.
During this test, a force which corresponds to 1.5 times the vehicle weight is placed statically on the upper end of the A-pillar. The roof crush should not exceed 127 mm.
Assuming that this test is also run on a convertible which doesn’t have a roof main chassis beam as additional support element, the use of complexly deformed parts made of high strength steel grades in order to pass this test is unavoidable. There are, however, limitations to achieving the necessary stiffness due to certain geometrical forms, since on the one hand too wide A-pillars can reduce the driver’s field of view and on the other hand design specifications only permit a very small construction area.
As a further example, the application of force on the B-pillar can be mentioned. In this test, a crash sled weighing 950 kg is moved at a speed of 50 +/- 1 km/h against a vehicle. A barrier consisting of 6 connected and independent sections of aluminium honeycomb is mounted on the front of the sled. The lower edge of the barrier has a height of 300 mm.
The impact energy must be absorbed by the side structure of the vehicle, whereby the intrusions should remain at a minimum in order to protect the passengers.
The rigidity of the B-pillar is important in this test. The B-pillar is exposed to a bending strain, tensile strain and torsional strain. Additionally, belt redirecting elements are placed on the B-pillar, which in case of a crash must apply a force of 6 kN on a certain point.
Materials which can sustain this dynamic strain, which are weldable and which allow complicated geometrical forms to be manufactured represent great challenge for sheet-metal forming.
Figure 1
Today, there are mainly two proven sheet-metal forming methods for the serial production of “highest-strength” components. These are cold forming of high-strength materials and press hardening of boron-alloyed steels.
In cold forming, a distinction is made between single part manufacturing and the manufacturing of sequence composite tools from coil. The choice of the method depends on the drawing depth and on the used material grades [1]. Hot forming also distinguishes between two variants – direct and indirect press hardening – whereas the physical processes such as the targeted conversion of austenitic structure into martensite structure, is the purpose of both method variants.
Figure 2 Pictures: Muller Weingarten
The major difference between both methods lies in the used half-finished product. During direct press hardening, plain moulding blanks are heated to austenite temperature. The final geometry is given in a drawing tool and after forming, the guided cooling process for structure conversion into martensite is commenced.
In direct press hardening, uncoated as well as hot-dip aluminized steel can be used. A main advantage of using hot-dip aluminized sheet metal is the avoidance of cinder formation during heating of the material and during handling. This helps prevent cinder residues from forming in the tools, which can negatively influence the abrasion of the tools. Additionally, no inert gas needs to be used in the oven.
During indirect press hardening, cold formed components (until approx. final contour) are used as half-fabricated products for the hot forming process. The objective of this method is to reduce abrasion of the effective tool surfaces, which is caused by the strong relative motion between tool and blank during press hardening.
A decision, if a component is manufactured using hot or cold forming depends on whether a cost or weight objective is prioritised for the vehicle. If the cost objective is prioritised, then cold forming is used. However, with rising degrees of component complexity, only decreasing initial strengths of materials can be used, which can lead to an increase of the used sheet thickness, (Figure 2). In case of a weight objective in connection with a high demand on strength (concerning vehicle safety), the hot forming method predominates.
After separating and centering of the blanks, they are placed on the transport rolls of a continuous annealing furnace. In this continuous annealing furnace, the blanks pass through different heat zones and are heated to austenitizing temperature.
The choice of length for a serial oven depends largely on the spectrum of components to be manufactured. Often, it is not wise to plan a universally useable facility since the annealing furnace is planned according to the highest mass flow and therefore it is not economically feasible for small components (low mass flow) due to the higher invest for the heat treatment device. Alternative heating methods, such as coupled induction heating offer great potentials.
Austenitized sheets must be placed inside the forming tool as soon as they come off the discharge rollers of the heat treatment device, since thin blanks have a temperature loss of about 20 to 30°K/second. The forming must take place before the martensite start temperature is reached, otherwise crack formation due to higher strength and lower ductile yield of martensite takes place.
After forming, components are cooled by way of cooled tools in order to achieve a structural change of austenite into martensite. This cooling phase makes up most of the cycle time and therefore bears the greatest potential for cost reduction.
The serial production of press hardened boron-alloyed steels offers considerable potentials which can largely be utilised by way of development of innovative facility and tooling concepts.
Potentials lie in the reduction of investment costs and reduction of press hardening cycle time, which has a direct influence on component unit costs. A reduction of the cycle time by one second can yield a component price reduction of up to 5 %. Today, common cycle times lie in the range of 10 to 15 seconds, which includes handling.
An interesting possible solution is discussed [4] here. First, the steel coil is heated in an induction oven located prior to the press. Here the raw material is heated to austenite temperature. In the sequence composite tool the splitting and consequently the press hardening take place.
The locking-out of individual parts takes place after the splitting of the side strips. Advantages of this idea lie among other things in the short design, the low invest and the low space requirement.
The realisation of such concepts must be evaluated by way of feasibility studies.
Aside from work done on improving known tool concepts, the Institute for Tools & Forming is working on the development of new tool concepts (e.g. cooled tools consisting of laminated sheet-metal).
The optimisation of direct and indirect press hardening is done by consistently using FE-methods, with the intention to automatically network the individual computing types.
The computation of the hot forming tool is aimed at realising different cool rates within the tool, aside from optimising the cooling canal positioning and avoiding overstraining. Thereby, component properties can be influenced by way of directed adjustment of the structure (pure martensite or mixed structure). Measurements using a thermo-imaging camera validated these results as well as enabled a further development of currently available simulation tools.
The objective of a coupled forming process calculation and of the tool performance is the thermo-mechanic calculation of hot forming of boron-alloyed steel grades under consideration of micro structural development in the material during cooling and determination of the operating characteristics of the component.
In addition to these methods, a concept is currently being studied in which milling of the cooling with a subsequent coupling of the DMD-method is implemented (direct metal deposition). Further, there is a possibility of combining the soft core of heat-conducting material (e.g. copper with a hard abrasion protection layer on the surface).
New potentials due to innovative manufacturing processes and combinations as well as use of high strength grades should be achieved through close collaboration of vehicle safety with forming technology. Apart from the consistent use of FE-simulations combined with optimisation tools, the future collaboration of crash performance and formable light structures with optimal materials will be improved. Thereby, not only vehicle safety can be specifically influenced, but there is also the possibility of optimising the cost structure of design and manufacturing.
The Frank-Stronach-Institute has the possibility of running dynamic as well as static tests. Therefore, material strength properties of vehicle components can be optimised at an early stage in the development phase. The close collaboration of the Tools & Forming Institute as well as Vehicle Safety Institute will lead to an acceleration of the development process.
Keywords: Press hardening, Boron Steel Sheets, Sheet metal forming,
References:
[1] Kolleck, R.; Feindt, J.-A.; Lenze, F.-J.; Heller, T.: Manufacturing methods for safety and structure body parts for lightweight body design. IDDRG 2004, Stuttgart, Sindelfingen.
[2] Kolleck, R.; Feindt, J.-A.: Direktes und indirektes Presshärten borlegierter Stähle- Möglichkeiten und Grenzen, Internationale Konferenz “Neuere Entwicklungen in der Blechumformung”, 2004, Fellbach
[3] Kolleck, R.: Vom Halbzeug zum Bauteil – Aspekte wirtschaftlicher Fertigungssysteme. 9. Sächsische Fachtagung Umformtechnik. 8./9. Oktober 2002 in Dresden
[4] Deutsches Patent 10322928, TK Automtive: Verfahren zum Herstellen von Formbauteilen. Deutsches Patentamt München, 21.05.2003
[5] Steffan, H., Moser, A., Hoschopf, H., (2000) HWS Syndrom bei seitlicher Belastung. 9.Jahrestagung des Europäischen Vereins für Unfallforschung und Unfallanalyse (EVU), 14.-16. September 2000, Berlin, Germany pp. 421-432
