Comparative Investigation of the Experimental Determination of AA5086 FLCs under Different Necking Criteria

The construction of a forming limit diagram (FLD) is a conventional approach to obtain limit strains and to evaluate the formability of sheet metal. Appropriate necking criteria should be applied to determine the forming limit curve (FLC) accurately. In recent years, deep research on the determination of the FLC has been carried out; meanwhile, several necking criteria have been proposed. However, the application of inappropriate necking criteria would cause deviations when determining FLCs. In this study, both Marciniak and Nakajima tests were carried out on the AA5086 aluminum sheet to make a comparative investigation of different necking criteria in the determination of FLCs. In the Marciniak test, four existing necking criteria were chosen to construct FLCs, and analyzed in detail. The well-performed time dependent and position dependent methods were selected for the Nakajima test. Meanwhile, the modified Wang method based on the height change of the adjacent points was proposed. The comparative results showed that the time and position dependent methods were relatively conservative in both experiments, while the modified Wang method could identify the onset of localized necking more accurately.

Revisiting flangeability in hole-flanging by single-stage incremental forming and conventional process

Recently, hole-flanging by single-stage incremental forming (SPIF) has been proposed as a suitable process to perform hole flanges for small- and medium-sized batches with high flexibility in shape. However, this incremental forming has many differences compared with the conventional press working operation in terms of strain and thickness distributions, failure mechanisms and flangeability measures. In fact, regarding the evaluation of the formability of the flanges, the classical Forming Limit Ratio (LFR) should be used with care to quantify this property in hole-flanging by SPIF. Additionally, the FLC (Forming Limit Curve for necking) and FFL (Fracture Forming Limit) curves, powerful tools for analysing sheet failure in practice, may also yield erroneous prediction of necking in conventional press working or fracture in SPIF. The aim of this work is to present a comparison and analysis of the formability of hole flanges performed by SPIF and press working in AA7075-O sheets. Two complementary parameters to the LFR to compare the flangebility in both operations are discussed, along with the influence of bending induced by the forming tool and the stress triaxiality in the evolution of the principal strains during the forming process. The results point out the limitations in the current practice.

Influence of the Laser Heat Treatment on the AA5754-H32 strain path during hydraulic bulge tests

The hydraulic bulge test represents an effective experimental method to characterise sheet metals since the equivalent strains before failure are much larger than those measured during tensile testing and there is nearly no frictional effect on the results. Recently this test has been proposed not only for extracting data concerning the equi-biaxial strain condition, but to determine the forming limit diagram (FLD) in the range of positive minor strains. In the proposed methodology, different strain paths can be obtained by merely using a test blank having two holes with a suitable geometry and position to be tested, without the need of dies with elliptical apertures. However, a carrier sheet is necessary, thus implying results may be affected by friction effects. This paper proposes a new methodology for the determination of the right side of the Forming Limit Curve (FLC), based on the adoption of local heat treatments aimed at determining different strain paths on the blank to be tested while using the classical circular die for bulge tests. In particular, the formability of the alloy AA5754-H32 was investigated; 3D Finite Element simulations were conducted setting different laser strategies and monitoring the resulting strain path. Results revealed that the proposed methodology supports obtaining many additional points in the right side of the FLC, thus being effective and friction free.

A Modified M-K Method for Accurate Prediction of FLC of Aluminum Alloy

Forming limit curve (FLC) is an important failure criterion for sheet metals in sheet metal forming, while the M-K model is widely used for the prediction of the FLC. In the M-K model, such prediction is greatly influenced by the initial thickness imperfection factor and material properties, from which the original M-K model’s theoretical derivation is proposed as a solution to the above mentioned issue in this paper. Then the relationship between the M-K model and Keeler’s empirical formula is then studied, from which a new method to predict FLC is proposed that combines the M-K model with Keeler’s empirical formula according to the previous analyses. It turns out that this new method can simplify the calculation procedure. Finally, the experimental results of two kinds of aluminum alloys, AA6016 and AA5182, have verified the effectiveness of the proposed method.