Conventional tool try-out in the sheet forming industry often means many loops of die spotting, a high effort to compensate the tool surface and to rework specific areas of the tool. The try-out procedure can take up to a couple of weeks for complex tooling applications which has a high impact on cost and lead time. FEM sheet metal forming simulations can be used to predict the forming behaviour of the sheet metal and can give a good indication of thermal distortion and springback of the final part. The accuracy of the used simulation models is determined by the degree of idealization of the applied models and the simulation input parameters.
Within the FormPlanet project, AP&T has worked on setting up a FEM sheet forming simulation model, based on measured process data obtained during the hot forming process in order to establish a digital twin, which can be used to guide the try-out of forming dies.
Input data such as the speed of the press slide, the force of the cushion, the temperature loss during blank transfer and 3D-scanned tool surfaces were used to enhance the performance of the simulation. A reduction of one-week try-out time is expected by implementing a digital twin during the tool try-out process for hot forming applications.
Practical forming trials
The forming tests were executed at AP&T by using an aluminium test line. The test line is equipped with a convection multi-layer furnace (MLF), designed for aluminium heating, to heat up the test samples to a uniform solutionising temperature. The test line also includes a servo hydraulic press (600 tons) which enables flexible programming of the press kinematics. To execute the trials, a scaled door ring test tool was used to form AA6010 and AA7075 materials with a sheet thickness of 2 mm. The temperature evolution of the hot sheet was tracked with thermocouples mounted on the test sample.
The following test procedure was implemented:
- The tool surfaces were lubricated prior to forming to reduce friction and potential galling on the tool surfaces.
- The blank was heated in the MLF to a solutionising temperature of 560°C (AA6010) and 480°C (AA7075).
- The blank was transferred from the MLF to the test tool, seen in Figure 1, by using linear automation.
- The part was formed in the servo hydraulic press at an average speed of 100 mm/s.
- After the forming operation the part was quenched in the closed tool for 10 seconds (600 tons).
Figure 1. Test tool used in the project. The tool consists of a punch, cushion and die.
Obtained input parameters to implement in the FEM simulation model
Throughout the practical trials, various parameters were measured which can be implemented in the FEM sheet forming simulation. These parameters are describing the press kinematics and the thermal behaviour of the sample. Additionally, the tool surfaces were 3D-scanned to enable modelling of geometrical differences between the nominal tool surface and the manufactured tool. Two of the most crucial parameters that were gained during the forming are the slide speed and the cushion force. As both parameters are constant set values to control the press, the actual speed of the press slide and force of the cushion yield a non-linear result during the cycle, as seen in Figure 2 and Figure 3. The movement of the slide and cushion is divided into a closing and stamping stage.
Two of the most crucial parameters that were gained during the forming are the slide speed and the cushion force. As both parameters are constant set values to control the press, the actual speed of the press slide and force of the cushion yield a non-linear result during the cycle, as seen in Figure 2 and Figure 3. The movement of the slide and cushion is divided into a closing and stamping stage.
Figure 2. Press output data for the cushion force.
The closing and stamping stage needs to be modelled accordingly in the simulation model. The individual stages are not only interesting due to the speed change of the slide itself, but also affects how much the sample will be cooled while lying on the cushion prior to the forming stage. As seen in Figure 3, the time during closing is 3.4 s and during forming 2.3 s which yields a total cycle time of 15.7 s if the quenching of 10 s is included.
Figure 3. Press output data for the slide speed.
The thermal behaviour of the blank is vital for hot forming applications of aluminium materials, especially the initial forming temperature has a high influence on the accuracy of the forming simulation result. To consider the cooling of the blank during the transfer from the MLF to the press, the heat transfer coefficient (HTC) of the blank and ambient must be determined. Therefore, the temperature evolution of the blank was measured, shown in Figure 4. The transfer time for the sample from the MLF to the tool was measured manually to be 32 s, which yields a temperature at the start of the closing stage of roughly 432°C (705 K) for AA7075.
Figure 4. Graph showing the cooling of the AA7075 sample during the experiment.
Further important elements to be implemented into the FEM simulation are data about the geometry of test tool. This is done by performing a 3D-scan of the tool surfaces using Polyworks, as shown in Figure 5. The measurement data will be meshed and imported into the simulation software. The goal of the procedure is to consider the deviations of real tool surfaces to the nominal geometry from CAD. Using surface data from the 3D-scan in the simulation model will lead to a more accurate contact distribution in the model.
Figure 5. Scanning the tool surface with a Romer Absolute 7730Si and using Polyworks to align top and lower halves to the same coordinate system.
Deviations in the tool surface affects the contact between blank and tool which can result in unforeseen temperature profiles and draw-in of material. Furthermore, it can also influence the thinning and thickening behaviour of the blank. Once the scanned surfaces are meshed and imported into PamStamp, the distance between punch and die can be measured and the deviations can be visualised, see Figure 6.
Figure 6. 3D-scan of the tool surfaces – distance between punch and die surface when upper and lower tool unit is closed.
Material characterisation data needed to formulate a material card
Apart from the temperature measurement data, press kinematics parameters and 3D-scans of the tool surfaces obtained during the practical forming trials, the FEM sheet forming simulation model would be incomplete without a proper material card. For this purpose, AP&T defined a material characterisation matrix for AA7075 and AA6010, respectively. From previous work, AP&T already has access to the heat transfer coefficients depending on gap and pressure for the alloys but lacks flow curves and anisotropy values. In order to define these material parameters, hot tensile tests were performed for each aluminium alloy by research institutes COMTES FHT a.s. and Fraunhofer IWU. Fraunhofer IWU performed the tests for AA7075 and AA6010. COMTES FHT performed tests for AA6010.
For Fraunhofer IWU, the test matrix at different strain rates and temperatures was defined as seen in Table 1 for the hot tensile tests.
Table 1. Hot tensile test matrix for AA7075 and AA6010 from Fraunhofer IWU.
In addition to the hot tensile tests, Fraunhofer IWU also performed tensile tests in three different load states to describe the forming limit characteristics (FLC) of the alloys. These tests were conducted for uniaxial strain, plane strain and biaxial strain at temperatures 280°C, 360°C and 400°C for AA7075 and 300°C, 350°C, 400°C and 450°C for AA6010.
COMTES FHT also performed hot tensile tests for AA6010 using the test procedure illustrated in Figure 7 at different strain rates and temperatures, defined in Table 2.
Figure 7. Scheme of tensile tests process.
Table 2. Hot tensile test matrix for AA6010 from COMTES FHT.
Thickness measurement to validate the FEM simulation performance
To validate the FEM simulation performance the part thickness of the formed parts will be used as reference. Thus, 3D-scanning of the formed part was executed to visualise the sheet thickness distribution as seen in Figure 8.
Figure 8. Formed part (left) and 3D-scan of formed part (right) showing the thickness of the part.
There are several critical areas of interest in the part that can be used to compare the thickness of the formed part to the simulation result. The critical areas are marked in red in Figure 8. These regions exhibit different load states which enable a meaningful validation of the material input data used in the simulation model.
Moving forward, the next step is to set up the FEM model in PamStamp using all the parameters from the practical trials. The following simulations stages are planned:
In addition, to improve the quality of the scanned tool surfaces it will be required to enable a high mesh quality of the tool surfaces and to reduce the number of elements. In order to set up the simulation model as accurate as possible, the material cards to describe the forming behaviour of the aluminium alloys will be completed with flow curves and anisotropy values based on the experimental results of the hot tensile tests performed by Fraunhofer IWU and COMTES FHT.
Once the simulation model is finalised the thickness results will be used in order to validate the performance of the simulation model.
Master’s degree in mechanical engineering at Lund University.
Part of the R&D team at AP&T’s department Forming Technologies as a Research and Test Engineer.
Main research topics and development areas are within aluminium and composite materials, forming simulations and forming techniques.