The first problem faced by German Volkswagen (VW) Group was that it was necessary to perform countless correction cycles on mold during production process, and there was a long way to go before component met drawing specifications. In order to ensure dimensional accuracy, expected warpage of structural parts can be reserved in advance in simulation. Retention rate means that structural part geometry is deliberately warped in mold cavity so that finished casting is consistent with target geometry after demolding. This greatly reduces cost of correction process and all uncorrectable scrap. At the same time, number of mold corrections after first die casting is also greatly reduced. Therefore, not only resources are saved, but also workload is greatly reduced.

Until now, information on die warpage retention rate has been based on empirical values or measurement results of already manufactured castings, for which die casting molds already existed. With possibility of simulating casting process, this information can be determined at an early stage, without need to produce die casting molds. In following case study, we use an aluminum structural component from Volkswagen (Figure 1) to show how to take warpage retention rate into account in die casting mold. Rocker beam is installed in rear structure of Volkswagen Touareg and Porsche Cayenne.

 

Figure 1: Die geometry of structural parts
This die casting technology for thin-walled structural components is designed for heat-free, tough aluminum alloys. Advantage of this is that expensive heat treatment and subsequent straightening steps can be omitted.

Representing die filling and solidification processes as realistically as possible is important for reliable prediction of structural component warpage. For this purpose, in addition to die casting mold layout, the entire temperature control and multiple heating cycles with realistic process times are taken into account in simulation model. Component warpage in die casting mold was calculated before and after demolding, after quenching in cooling pool, after separation of casting runners and overflows. Warpage results from die casting process simulation were used as retention rate for mold.
Based on empirical values of process experts, result factor derived from die casting process simulation was -0.75 (-1 represents a 100% retention rate, so -0.75 is equivalent to 75%). Warpage geometry was then scaled to cavity dimensions. Mold manufacturing department then used revised warpage results as a 3D solid model for design mold plate. Based on virtually determined mold deviation dimensions, mold (component, gating runners and overflows, and mold inserts) needed to be redesigned.
Subsequently, a complete process simulation was performed using corrected (retained) die casting model to verify retained dimensions. To assess process stability, Volkswagen used Magmasoft to analyze influence of key process parameters such as cooling temperature and mold opening time on warpage behavior of connection between sill longitudinal beams. In final simulation, virtually corrected mold kept most of component areas within permissible tolerance range.

After optically measuring die casting mold to exclude errors in die casting mold production process, Volkswagen (VW) die cast the first blanks and then compared actual geometric deviations with virtual warpage predictions. Optical measurement results of actual production components were then compared with simulation results (from retained casting mold). At the same time, stability of selected process parameters of actual existing die casting process was analyzed. Aim was to examine influence of different mold opening times and temperature control on component warpage.

 

Figure 2 Temperature distribution when component is removed from mold
Warpage of structural components during die casting process consists of springback during component demoulding (stored elastic stresses released when component is removed from mold) and subsequent (free) warpage during cooling to ambient temperature. During cooling, hot local areas "pull" more than colder areas, which leads to hot warpage. Figure 2 shows temperature distribution when component is removed from mold.

 

Figure 3. Evaluation of warpage after casting from Magmasoft measurement perspective. Deviation in the middle area is approximately -0.6 mm, and maximum deviation from target geometry at the left end of longitudinal beam is approximately +2.3 mm.
Figure 3 shows measurement of virtual warpage results without insulation. Maximum deviation from target geometry is about +2.3 mm at left end of longitudinal beam. Measurement is carried out by positioning with a reference point system (RPS). Task of reference point system (RPS) is to clearly position component in free space. After simulated warped component is transferred to measurement value evaluation system used by VW, a virtual measurement report can be created, similar to quality check of real part during series production. In this way, calculated component warpage can be analyzed in necessary detail and in compliance with specifications. In line with evaluation of measurement angles (see Figure 3), maximum geometric deviation at left end of longitudinal beam is also clearly visible (RPS positioning).

 

Fig. 4 Warpage evaluation of the longitudinal beam: a) without retention, b) calculated warpage result from casting simulation with retention in RPS positioning; c) actual measurement result of cast sill longitudinal beam
Figure 4a shows warpage result calculated from simulation without retention. Figure 4b shows warpage result calculated from simulation with retention, also with RPS positioning. It can be seen that component warpage has been significantly reduced. Almost all critical areas are within specified tolerance of +/- 0.7 mm for connection and contact surfaces. Figure 4c depicts actual measurement result of sill longitudinal beam with same parameters as used in warpage prediction. Component is well within specified tolerances and corresponds qualitatively and quantitatively to geometry predicted by simulation.

Predictions of casting process simulation using simulation analysis for sill longitudinal beam of connecting part show good agreement with actual retention rate measurements. Mold corrected according to simulation results means that most of component areas are within tolerance and no additional correction grinding of mold is required after first die casting. Previously, warpage retention rate of mold had to be determined through multiple die casting tests. This method is expensive and cumbersome. For Volkswagen (VW), new virtual method is no longer a one-way street: large castings can be die-cast without detours and, most importantly, without warpage.