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Composite materials are coming into more widespread use and beginning to replace aluminium. Processing them requires moulds with specific physical characteristics, a notable one being a very low coefficient of expansion. We are starting to take advantage of new characteristics like localised heating of the mould cavity, which allows substantial energy savings. The materials that make it possible to meet these requirements are presented here.
(Published on April 2006 – JEC Magazine #24)
CHRISTIAN SERGENT, COMMERCIAL DIRECTOR, TOOL STEELS, THYSSENKRUPP MATERIALS FRANCE THOMAS GARAIX, COMMERCIAL MANAGER, AEROSPACE, FRANÇOIS BONJOUR, PRODUCT MANAGER, AEROSPACE, THYSSENKRUPP MATERIALS VDM
In many industries, more and more factors are promoting the use of composite materials. In aviation, as larger and larger aircraft are being designed, the structures become thicker, increasing the density. The quest for fuel savings is a constant one and airline companies are also looking to achieve aircraft autonomy over greater distances. With this as a given, it is no surprise that, for identical mechanical properties, the more lightweight materials are thriving. And in automotive, fuel savings and weight constraints, plus the objective of lowering the centre of gravity given the height of new vehicles constitute reasons to use composite materials. Other applications are also emerging, for example in the building and construction industry or in sports and leisure (boating).
Table 1: main grades: alloys with high dimensional stability, alloy steels, and stainless steels*.
* Average values obtained at ThyssenKrupp Materials France and ThyssenKrupp VDM plants.
How can one guarantee correct forming and geometry for composite parts? The answer is: through the right choice of mould. The typical specifications demanded these days require taking a temperature range of 20-250°C into consideration, and tomorrow it will be as much as 400°C. Different materials are chosen as a function of their expansion coefficients to allow meeting the required criteria and also guarantee the final part geometry with variations in temperature. In addition, their specific thermal conductivity coefficients allow perfect control over the heat fluxes within the material. Thanks to the electrical and magnetic properties of these materials, controlled induction heating can be used to control the temperature in the different parts of the mould optimally. RocTool’s innovative Cage System® moulding technology is one example of this.
The result is that materials such as alloys with high dimensional stability (iron-nickel, ironchromium- aluminium, or nickel-chromium), alloy steels, and stainless steels are being used alone or in combination (table 1).
By way of comparison, expansion coefficients commonly used in the industry range from -0.8 for a high-performance material like carbon to +23.8 for aluminium alloys when dimensional stability is not required (table 2).
Table 2: expansion coefficients of some materials commonly used in the industry.
Processing iron-nickel alloys
Given the excellent properties of Pernifer 36 iron-nickel alloy and its highly targeted use, it is interesting to explore the processing of this product a bit further. Pernifer 36 is processed with commonly used techniques, but with specific parameters that are in use in some shops, to make elements that are later assembled, welded and milled.
The materials are processed using standard techniques that are suitable for the large sizes required in the market.
Welding sheet iron
Welding is performed using the TIG process. The seam weld must have the same chemical and mechanical properties as the base material. Welding can be used to obtain large sized parts. The GMAW (Gas Metal Arc Welding), SAW (Submerged Arc Welding), plasma, electron beam and laser welding processes can also be used. When major deformations or machining are done, it is advisable to perform a stress relief anneal to eliminate stresses.
The stress relief anneal should be performed at a minimum temperature of 600°C, and the cycle time should be a function of the thickness of the material. Very slow cooling in the oven is recommended.
The coefficient of thermal expansion (CTE) depends on the processing operations – cutting, rolling, bending, or rough machining – and on the annealing temperature and successive heat treatments.
Machining parameters for Pernifer 36 iron-nickel alloy are not the same as for standard materials (table 3).
Table 3: optimised machining parameters for Pernifer 36 iron-nickel alloy.
Positive-rake TiN- or TiAlN-coated carbide tools produce the best surface finish. High-speed steels can also be used for good results, but only for short machining jobs, due to wear constraints. Lubrication is necessary for a good surface finish, whether you use micro-lubrication or heavy lubrication.