AA5754 H111 is the most common Non Heat Treatable (NHT) aluminium alloy and temper used in automotive to produce medium strength pieces, due to its high formability and good corrosion resistance.
This material has been produced keeping the hardening element, magnesium, at the same level as the standard alloy and decreasing the amount of iron and silicon. Iron is the most common impurity in aluminium; being its solubility in the solid state very low, most of the iron present in aluminium appears as an intermetallic second phase in combination with aluminium and silicon. The apparent volume fraction of second phase particles is almost a direct function of iron content. During deformation, second phase particles can fracture, creating voids and, ultimately, can contribute to sheet failure.
Edge cracking is a common issue in aluminium forming (see figure 1). Within the framework of the EU FormPlanet project, AA5754 H111 sheet samples supplied by Aludium have been evaluated in Eurecat using the Hole Expansion Test (HET) and the Half-Specimen Dome Test (HSDT), both able to reproduce industrial forming processes.
Figure 1. Edge cracking in automotive components
The Hole Expansion Test (HET) and the Half-Specimen Dome Test (HSDT) are already known methodologies. Eurecat, within the framework of the FormPlanet project, has optimised these methodologies to analyse the effect of different process’ parameters that can affect the behaviour during forming. These optimised tests provide a solution for a more accurate and cost-efficient evaluation of edge-cracking behaviour in advanced sheets, and will be offered as services on the FormPlanet Open Innovation Test Bed.
Non-standardised Hole Expansion Test (HET) and the Half-Specimen Dome Test (HSDT)
The Hole Expansion Tests (HET) performed in the project analyses non-standardised parameters affecting the Hole Expansion Ratio (HER) as clearance, tool wear, burr position and time from punch to expand. The test objectively measures crack initiation and propagation by obtaining reliable and repetitive values of HER.
On the other hand, the Half Specimen Dome Test (HSDT) permits to determine the edge cracking behaviour during flanging in metal sheets. Compared to hole flanging tests, HSDT is a more time and cost-saving test for evaluating edge cracking. Another advantage is that is unaffected by aspects related to the tools manufacturing for cutting the holes, as the coaxiality between punch and die.
HET and HSDT results for AA5754 H111 aluminium alloy
Eurecat has developped an experimental test configuration to measure crack inititation and propagation and obtain reliable value of HER and validate them in the Aludium AA5754 H111 aluminium alloy. Figure 2 shows the most relevant results of the tests carried out, considering HER versus clearence and burr position and versus punch radii.
Figure 2. Hole Expansion Ratio versus clearance and burr position (left), and versus punch radii (right).
The most relevant conclusions from this test is the positive effect of burr up position and punch radii 0.1 in the HER.
Regarding HSDT, the main information that can be extracted from this test are the fracture strains of the sheared edge in the previous stage to the appearance of the first crack through the specimen thickness. The test also allows to detect the effect of material anisotropy in edge-cracking. Thus, the maximun strain at fracture is obtained with specimen cut at 45º, followed by 90º and finally 0º.
This is an interesting output from HSDT, since can provide information about the best orientation for sheet blanking.
Conclusions from both tests, HET and HSDT, can be easily translated to customers producing critical forming parts, as well as they could be the appropiate tools to confirm the best edge craking performance of a new version of AA5754 developped by Aludium.
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.