Papers by Author: Kathleen Siefert

Abstract: This paper presents a new technique to model the effect of intermediate induction heat treatment (IIHT) on pre-strained aluminium sheets, predominantly AA5182. IIHT is a heat treatment technique carried out between two conventional cold forming steps, which eventually lead to enhanced formability of aluminium alloys. The aim of IIHT is to alleviate the strain hardening of the material which is introduced in the first cold forming step and there by reducing the yield limit and increasing the hardening modulus for subsequent forming steps. As a result, a remarkable increase in formability can be achieved in the subsequent forming steps at room temperature. The scientific aspect of the IIHT process is demonstrated by defined pre-strained tensile test specimens at different object temperatures to establish a process window. To accurately model the effect of IHTT in simulations, it is necessary for the material model to consider the plastic recovery that the material undergoes during heat treatment. To this effect, material model Mat133 (Barlat_YLD2000) in LS-Dyna has been enhanced to account for the effect of intermediate heat treatment. The numerical simulation is carried out in four steps namely pre-forming, springback simulation to account for residual stresses, thermo-mechanical coupled simulation for heat treatment, and final forming with enhanced material model. To validate this model, experiments have been carried out on a simple cross-die deep drawn cup and compared with simulation results.

Abstract: To improve the formability of commercial aluminium alloy AA5182, a new heat assisted forming method is used. The process sequence of this method is; cold forming (pre-forming, 90 to 95% of final shape), heat treatment and cooling it down to room temperature and final cold forming. AA5182 is a non-heat treatable alloy and hence heat treating a strain hardened non-heat treatable aluminium alloy leads to lose in strength and gain in plastic recovery. Therefore by heat treating a preformed part and then following it up by further forming stages, it not only gains the lost strength back but also shows increased formability. This behavior is particularly useful in forming more complex automotive interior body parts. To accurately simulate this method, modeling the effect of heat treatment is important. Initial investigations on tensile tests showed that the degree of pre-forming in combination with heat treatment is directly proportional to plastic recovery. Which means, the more the pre-strain is the more the recovery becomes viable. Based on this, a new algorithm has been developed and implemented in LS-DYNA to capture the effect of heat treatment. Finally experimental investigations were carried out on a cross die deep drawn cup to validate the developed simulation model.

Abstract: This paper presents a new procedure for a heat treatment embedded between two cold forming steps. A first cold forming step induces a defined strain hardening in the material. The following step is the heat treatment which takes place in a furnace at various temperatures and for certain durations. The application of such an intermediate heat treatment reduces the strain hardening of the material and enhances the elongation. This allows a higher degree of deformation in the second cold forming operation. The achievable properties of the aluminum alloy AlMg4.5Mn (AA5182) were discussed in detail. Further investigations using Nakajima test setup revealed an increased formability of the material. First the Nakajima samples were pre-strained along different linear strain paths to a predefined strain value. Afterwards the samples were heat treated without allowing the aluminum alloy to recrystallize. After cooling down the samples to room temperature, the tests are continued until the material’s fracture. As a result heat treatment dependent forming limit curves (FLC) are obtained. In comparison with a measured FLC at room temperature the support of the intermediate heat treatment on enhanced formability were shown. Furthermore the method is not restricted to AA5182 aluminum alloys.