What is in this article?:
- Pioneering a New Forming Concept
- Cross-Wedge Rolling within the Warm Temperature Range
- Test site for Parameter Studies
Longitudinal axis symmetrical pre-forms
Three types of defects
EC funds research
Cross-wedge rolling is a well-established process used by forgers to produce cylindrical pre-forms, with very efficient material utilization. To prevent the appearance of material defects during forming, the wedge geometry of the tools is adjusted to the designated pre-form.
| After cross-wedge rolling, a workpiece is shown with the wedge tool. |
Today, cross-wedge rolling is executed within the hot temperature range. To analyze warm cross-wedge rolling, the scientists at IPH — Institut für Integrierte Produktion Gemeinntzige GmbH developed a test site for cross-wedge rolling experiments.
Cross-wedge rolling is used to convert cylindrical billets into longitudinal axis symmetric pre-forms. The billet is formed by two wedges, moving tangentially to the work piece. The method is well suited for forming conical axles. A cross-wedge roll consists of two or three tools that are clamped onto the rollers. The work piece is inserted into the roll gap in the longitudinal direction. Figure 1 shows a conventional cross-wedge rolling machine with round tools.
The test site used in the IPH development uses flat tools, because these are easier to design and manufacture. A rolling wedge may consist of up to four zones: In the first zone – the knifing zone – a slit-shaped cut is centered upon the billet. The second zone is used as a guide zone, where the billet rotates around its axis to generate an even cut. Within the subsequent stretching zone, the central cut is dilated to a designated value and the billet is stretched in longitudinal direction. Within the terminal sizing zone the billet is, at least once, rotated around its longitudinal axis [Doe07]. Thus, an evenly formed work piece is produced. Compared to more conventional production methods, for example, turning, die forging or casting, cross-wedge rolling offers many advantages due to its
• High productivity,
• High material efficiency,
• Low operational costs,
• Easy machine concept,
• Low energy consumption, and
• No lubrication and coolants [Her97].
| Figure 1: Conventional cross wedge rolling machine with round rollers and tools. [Las10] |
Despite these advantages, cross-wedge rolling is not widely used for industrial purposes because the design and construction of the base tools for cross-wedge rolling requires expert knowledge. The challenge is to prevent possible failure mechanisms during rolling.
Johnson and Mamalis discern three types of part defects: surface defects, internal defects and imperfect shape [Joh77]. The formation of helical dents or necks and folds are among the surface defects. Internal cavities and part-fractures are considered to be internal defects. Imperfect shapes are caused mostly by a defective wedge design, which results in a deviation of the finished part geometry from the required net shape. These defects can lead to sudden part-failure during operation.
Failure mechanism generation and its impact on parts during cross-wedge rolling were investigated using practical tests and the finite-element method for calculations. Based on numeric and experimental testing-methods, the development of defects can be construed as a function of raw material properties and the three primary parameters of cross-wedge rolling: forming angle, extension angle and reduction of diameter.
Another influential factor is slippage between the part and tool during rolling [Gla98, Pat98]. If the slippage is too great, the work piece is extruded between the wedges without rotating, and thus its axial formation is incomplete. The friction between work piece and tool is directly tied to the slippage. The higher the friction, the smaller this slippage will be.