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Innovative Stamping Die for Aluminium Ribs Hot Stamping

Periodic Reporting for period 3 - Rib-ON (Innovative Stamping Die for Aluminium Ribs Hot Stamping)

Okres sprawozdawczy: 2020-06-01 do 2021-06-30

The overall aim of the “Rib-On” project was to develop and manufacture an innovative stamping die based on a modular/reconfigurable and low-cost approach to successfully produce different outer external wing rib models using new high performance aluminium alloys and tailored die steel and coating solutions.

The following challenges has been overcome for the success of the project: (i) Hot forming of aluminium is not a widespread technology. (ii) Die users allow little play to improvement by material selection. (iii) Modular/reconfigurable dies are infrequent. (iii) Integrating heat treatment and stamping in a single operation allows reducing costs but is difficult to achieve.

Thanks to work performed in Rib-ON project the hot stamping process developed has been proved as a suitable technology for the manufacturing of this type of parts. In this regard, the following conclusions have been obtained regarding the technical feasibility of the hot forming process applied to wing rib geometries:
- Shape tolerances can be achieved within ±0.4mm with hot stamping.
- Second operation of cold forming can be omitted, as the part tolerances are achieved with hot forming. In case narrower shape tolerances were needed (for instance in matting surfaces), a secondary cold forming operation could be used to restrike these areas.
- Minimum mechanical properties can be achieved after an aging treatment at 180°C.
- With the cycle time defined there is no need for a cooling system in the hot forming module. Both part and tool temperatures after hot stamping remain below 50°C.
From the beginning of the project until the end of the third (and last) period, the work has been focused on carrying out all the required activities to achieve the objective foreseen for the project. In this regard:

A material characterization of different aluminium series (5xxx, 6xxx and 2xxx) commonly used in hot stamping technology were performed during the first months of the project, getting insight on these families for potential application in Rib-On. The alloy finally selected by the Topic Manager was 2198.

FLD0s at different temperatures (room temperature, 200ºC and 450ºC) on 2198 sheets and with a thickness of 3.2 mm and 1.8 mm were performed, improving the knowledge of the limits of conformability of the material.

Considering the different wing rib geometry options provided, several hot forming process analyses were carried out with the aim of studying the thinning and appearance of wrinkles or defects, reporting drawbacks and suggesting new geometry options for the part to optimize the process.

With the aim of defining the material for the manufacture of the stamping die, a preliminary study of the friction and wear characteristics of three tool materials widely used in the construction of stamping tools were performed. Besides, a hot forming tooling design analysis was performed identifying the main elements of the stamping die and finding the most efficient process in terms of material usage, productivity and cost, and keeping in mind the modularity approach for the solution to be developed. Once the final part geometry was defined by the Topic Manager, a detailed design of the selected die concept was done.

The different elements of the stamping die (hot and cold module) were manufactured. Once the hot stamping module was obtained, hot stamping trials were performed at BATZ’s facilities. Corrective measures were implemented to ensure that the parts could be extracted without difficulties.

Additionally, solubilization studies considering the final thickness defined (1.8 mm) was performed to define the process window of the ribs in terms of heating condition prior to hot stamping step. In addition, an aging treatment study was performed on samples extracted from sheet blank and from hot stamped ribs to define the process window for aluminium sheet.

After the implementation of the corrective measures on the hot module, trials were performed at BATZ facilities with the aim of carrying out a full process validation of the behaviour of the tool (hot and cold modules). Although final try-outs with the hot forming module indicated that the cold forming module might not be needed to achieve shape tolerances defined for the wing rib, in general terms results obtained from the cold forming module indicated a slight improvement in part’s shape compared to the hot formed part. The angle of the side walls is more homogenous after the cold stamping. The cold forming tool acts as a restrike operation.

A rib manufactured using the die developed has been analysed through metallurgical characterization of critical zones of the rib and the evaluation of the mechanical properties. The results obtained has allowed to validate the hot forming process developed for the manufacturing of the wing rib geometry provided.

It is also important to point out that different modularity options have been evaluated to study the potential industrial exploitation of the project results.

Thanks to the project the following exploitable results have been obtained:
- Cost competitive non-conventional tool material choice criteria
- Aluminium hot stamping capability demonstration for 2xxx series, both for die manufacturing and for small to medium batch supply
- FLD0 testing capacity

In terms of dissemination activities, the work performed has led to the following scientific publication: Muro M., Aseguinolaza I., Artola G., “Die Material Selection Criteria for Aluminum Hot Stamping”. J. Manuf. Mater. Process. 2021, 5, 15. Additionally, it is foreseen to work on three publications: one presenting the results that let to discarding the PVD coating in the dies (Indexed Journal), one presenting the results from FLD0 (Conference Paper) and one presenting the mechanical property results (Indexed Journal). A paper disseminating the FLD0 results will be presented in CHS2 2022.
The development of the project has led to the following major innovations:

- Hot stamping process has been proved as a suitable technology for the manufacturing of wing ribs.
- The FLD0 testing performed to evaluate the formability of 2198 alloy has allowed to extend the datasheet in terms of the conformability limits presented by the 2198 alloy at room temperature, 200ºC and 450ºC.

- In terms of cost reduction, the conclusions on the material and manufacturing process of the die obtained are relevant:
• Raw material cost is cut by 88% by using cast iron instead of the standard hot work ERS forged steels that are currently used in similar applications.
• Heat treatment costs are saved, and the use of internal cooling systems are not necessary thanks to the long takt time allowed by the small batch sizes of the part typology that also reduce machining costs.
• Die coating are also avoided since back-to-back tests pf PVD coated dies didn’t show any significant advantage as galling was more severe than for uncoated cast iron.

Thanks to the project the following results has been achieved:
- Design and manufacture of a cost effective modular hot stamping die.
- Development and validation of in-die heat treatment technique for aluminium hot stamping.
- Design and build a hot stamping aluminium stamping die, whose material has been optimized for the application.
- Manufacture of a wing rib demonstrator by hot stamping, satisfying the geometrical, mechanical and metallurgical requirements of these structural components.
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