AA5657 is an aluminum alloy largely used in luxury perfumery due to its high formability and good response to anodizing process. Often, new fashion designs need special tempers to be able to form bright finish sheets, maintaining the optimal surface quality. Aludium final customers produce pieces in transfer or progressive presses fed by narrow coils, usually 35-70mm wide (figure 1).
The limited size of samples available, usually closures or narrow flat sheets, is an additional obstacle to evaluate properly the formability behaviour of new developments, or even to understand the unexpected cracking of some pieces produced with coils manufactured with the same process as those that do not exhibit cracking defects during forming operations.
Figure 1.AA5657 Bright finish coil, supplied in slitted narrow bands.
Miniaturised tensile test in cosmetic pieces
The Miniaturised Tensile Test (MTT) method, successfully optimized by COMTES FHT in FormPlanet project, can be applied to evaluate local properties of real parts or when the insufficient amount of experimental material is available for testing.
The MTT permits to determine mechanical properties such as yield strength, ultimate tensile strength, uniform and total elongation, and reduction of cross-sectional area. In addition, this testing method can be utilised to investigate the material’s anisotropy, strain rate sensitivity, and other material properties.
Figure 2.The geometry of miniaturised tensile test specimen.
AA5657 H22 real pieces with and without cracks, as well as bright flat sheet samples, have been evaluated in COMTES FHT via MTT to investigate the local mechanical properties. Specimens were extracted from three different areas (1, 2, 3 as marked in figure 3) in the three main rolling directions.
Figure 3. Cosmetic piece showing orientations of the specimens’ extraction.
Based on the recorded data, the yield strength, ultimate tensile strength, elongation and reduction in cross-sectional area were evaluated, see figure 4.
Figure 4. Engineering stress-strain curves. Flat sheet (left), piece w/o cracks (centre), piece with cracks (right).
Results show that real cracked pieces show the highest residual plasticity after drawing in the 1st area. The results of flat sheet testing are in very good alignment with the 1st area results. It points out that the 1st area is the least affected by the drawing process.
On the other hand, the 3rd area showed the lowest residual plasticity caused by the distinct deformation strengthening. The 2nd and 3rd areas were shown to be the most affected by the drawing process, which resulted in deformation strengthening of the areas and depletion of plasticity, causing cracking.
MTT results of pieces without cracks presented very low strengthening after the drawing process, especially for the 2nd area, which is characterised by higher elongation values and compromised Rp0.2 and Rm values comparing to pieces with cracks, which might have been the reason, why there was no crack evolution observed during the drawing process.
The main conclusion that can be sorted from this study is that the miniaturised tensile tests permit to evaluate local mechanical properties to develop new pieces/prototypes, as well as to detect any punctual deviation on material homogeneity of pieces versus the flat rolled sheet.
Aludium
Aludium is a leading supplier of aluminium solutions for the industrial, distribution and automotive markets in Europe. Together with its research centre CINDAL, Aludium will contribute to the project with its high expertise in aluminium production, functionalization and characterisation, validating and adjusting microstructural studies, mechanical and formability tests.
