TPT January 2007

Predicting the properties of welded rollformed tubes for subsequent processes using the Finite Element Method By Mr Albert Sedlmaier, data M Software GmbH, Germany

These aspects alone illustrate the potential presented by the use of powerful analysing and optimising tools. If all the efficiency of rollforming is to be utilised in the production of tubes, methods and means must be applied in the tool design phase that will lead to the improvement of properties of a semi-finished tube product for its subsequent processing. The software solution presented here is suitable not only for steel but also for non-ferrous metals such as aluminium or copper. Expertise in roll tool development and design The Copra ® RF software program supports all steps in the development of tube cross-sections. This begins with the definition and design of the individual forming steps (flower pattern and roll passes), proceeding through the generation of technical documentation to quality control. A multi-stage concept accelerates both the design and analysis process. The forming sequence (flower pattern) is calculated either according to the designer’s wishes or based on pre-stored, corporate strategies (centre line, double radius, W-bend, linear, cage, etc). The geometry of the various roll tools is calculated by the design software according to predefined machine parameters stored in a database. Calculation of plastic strain values Rollforming is a continuous process with rotary tool motion. The sheet metal is bent into shape at several consecutive stations by vertically or horizontally aligned, mating rolls. For the most part, a change in the thickness of the metal is not intentional in rollforming, but results in practice through the process. Forming occurs not only at the direct points of contact between tool and workpiece, but also in the region ahead of the roll tool. Points on the strip edge travel further than points in the middle of the strip. In most cases this produces strain with a maximum that lies in the

Abstract Determining and optimising the properties of a semi-finished tube in advance of subsequent processes such as bending and hydroforming requires simulation of the entire process chain. In many cases – for example, in hydroforming – one starts by simulating the forming operation with the final tube, although major properties such as work hardening have already been defined by the production process in the tube welding plant. For this reason simulation can commence as early as the rollforming phase, considering the individual forming steps in the process chain, ie rollforming, preparation for welding and calibration. Investigations show that these steps have a substantial impact on the final properties of a tube, and can be influenced positively by adroit variation of the tube forming process. A computer program system is presented here, by which the entire tube making process chain – rollforming, welding, calibration and even verification of the tube properties by means of hydroforming – can be mapped. Introduction Given the variety of their applications, welded roll formed tube and profiles have gained increasing importance in recent years, making inroads into new sectors such as the automobile industry. The reasons for this include the introduction of new materials, and improved possibilities for designing roll tools. Advantages that arise from the overall process are the large choice of profile cross- sections, and the work hardening of the material that results from the forming operation. Although there are benefits, there are also a number of constraints. For example, the design and production of roll tools can often be time-consuming, as can the startup and tryout of tool sets. Unwanted deformation and strain in the end-product can also be time-intensive, particularly if the completed tube or profile is to be treated further in follow-on processes. For instance, this is the case in the bending or hydroforming of tubes.

region of the strip edge. Such strain can lead to process-specific problems like strip edge waviness or undesirable deformation of the tube. Deformation of this kind is the result of residual stress induced by plastic strip edge strain. The definition of the forming geometry, ie the tool design, must be aimed at preventing or at least minimising this residual stress. Residual stress is stress that remains in the part following plastic deformation when the load on the part is removed. It is produced by the elastic component of the deformation, creating resiliency after removal of the load.

fi Figure 1 : Finite element simulation: strip edge waves due to residual stress after first breakdown (left); strip edge buckling after fin passes in practice (right)

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J ANUARY /F EBRUARY 2007