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Intelligent Hydroforming . . .

The demand for lighter component solutions remains a primary driver in the automotive industry. Besides increased use of Magnesium, Tube Hydroforming using Advanced High Strength Steel blanks (when combined with clever design and effective materials know-how), is one of the new enabling technologies that has the capacity to deliver cost effective mass reduced solutions.

Hydroforming has traditionally used tube made from standard steel grades on well run conventional tube mills and for the majority of applications will continue to do so, but as the technology matures, the potential for using Advanced High Strength Steel (AHSS) has led to opportunities for the Corus Hyfo Tubular Blanks isbeing explored.

This article summarise s d some of the issues, difficulties as well as benefits, that the use of AHSS is presenting. It draws upon the findings of a technical paper presented by Maarten Kelder and Kevin Edgar at the 2003 TPA/SME International Hydroforming Conference and has been used to promote the use of Tubular Blank technology. The paper drew on experience gained from the production of Tubular Blanks for an Engine Cradle project.

The benefits...

The weight benefits of using High Strength Dual Phase steel grades for structural parts such as engine cradles are clear. Wall thicknesses can be reduced, minimising overall weight even more so as material can easily be placed where it is most needed using a ‘Tailored Tubular Blanks’ produced by Corus Hyfo.

In terms of performance, correctly select ing ed these grades can greatly improve structural performance.

However taking this cradle as an example, developing a successful process for a hydroformed Dual Phase part in general, is not a straightforward task.

In principle the limits of Hydroforming High Strength Steels should be taken into account right at very early stages of the design process, so that problems associated with forming in particular are ‘designed out’. In addition, the sensitivit ies y to material property changes , induced by the tube welding process , are increased in the case of hydroformed Dual Phase parts. The hydroformed part in its pre-bent state will interact with the walls of its enclosing die, having an effect o n the flow of material during forming and therefore on the local elongation. ‘Hot spots’ can be an issue.

The challenges...

Formability

The basic properties of Dual Phase materials are a very high n-value at the beginning of the stress-strain curve (Figure 3), resulting in a rapid increase of the yield stress for small deformations (strain hardening), compared to traditional steels. However the n-value at higher strain levels is much lower such that it becomes more sensitive to strain localization around pre-existing defects or geometry constraints such as tight bends or changes in material property such as in local to the heat affected zone s near welds.

Therefore a careful evaluation of how different stages of the entire process, from Tubular Blank production through the hydroforming process , reduce the material formability are essential, as with Dual Phase steels unlike conventional materials such as mild steel, annealing cannot be used to recover formability after strain hardening since it is very costly, and would destroy the martensitic phase in the material.

Roll forming any conventional tube results in inducing strain hardening in both the longitudinal direction and the circumferential direction of a tubular blank caused mainly by the calibration stands in the tube mill. The effect of these strains can be estimated by modelling the tube making process using Finite Element models.

It is clear from work conducted in Corus RD&T on behalf of Corus Hyfo that a Tubular Blank, for any given size or material type, exhibits far less strain than a conventional tube. Finite Element simulation of the Tubular Blank forming process showed that an even strain distribution and a low maximum strain level of only 2% can be achieved. A roll formed tube generally shows a non-even strain distribution in combination with a maximum strain level of approximately 6%. Therefore making Tubular Blanks it very suitable for steels where the benefits of strength are required for performance but the need to preserve formability to complete the processing of the material are required. This has been proven through comparisons of tube, and tubular blank both through analysis and practical experimentation.

Prebending and Preforming of the tube (figure 4) also take their share of formability, so anything that can be saved in the tubemaking process will help in the success of the hydroform process.

Sensitivities to welding process

Heat-based welding processes induce changed material properties close to the weld line called ‘Heat Affected Zone’ (HAZ). Different welding proceses have a different impact. Figure 5 above, compares a traditional resistance weld with a Laser weld, showing that the Laser based process yields a narrower HAZ between weld and parent metal.

The Tubular Blank uses Laser welding. As previously mentioned, the fully formed hydroform often has strain levels in critical areas which would seek out any inherent weld weakness and hence we have seen benefits from using the Tubular Blank in combination with AHSS to minimise the effect of the HAZ. .

Conclusion

Dual Phase and other Advanced High Strength Steels can be used to in tubular blank form as the basis for cost-effective lightweight hydroforming components.

Knowledge of the full creation process from slab to the performance of the finished part in service is essential to take advantage of Advanced High Strength Steels in hydroformed automotive components. The material choice significantly affects the design of the component and the production process. Control of the parameters affecting the forming and joining of the component at each step is vital.

It is now a proven fact that the application of Advanced High Strength Steel can deliver performance-leading benefits when combined with appropriate tubular technology and a full understanding of the impact of the material’s behaviour at each stage of the process as described by modern simulation techniques.