Direct energy deposition for buildup lightweight crash tolerant structures

In automotive construction, the safety of passengers during a crash is a top priority. For this reason, crash-relevant structures in cars have become increasingly stiffer and heavier over time to enhance occupant protection. However, this strategy contradicts the goal of reducing vehicle weight to lower CO₂ emissions. The challenge is to develop lighter structures that still provide excellent occupant protection by the project partner Stellantis’s CRF research centre and Gemmate Technologies.

To tackle this issue, the FlexCrash partner Fraunhofer IWS is focusing on direct energy deposition (DED), to develop process parameters and build up first lightweight Aluminum structures. DED has the advantage of being easily combined with existing conventional manufacturing processes. This property allows the application of stiffening structures using DED on prefabricated components, thus tailoring the mechanical properties of the parts locally while keeping the component weight low.

The experiments carried out in the FlexCrash project were based on DED by means of laser technology and AlSi10Mg powder filler material (Figure 1). When processing aluminum alloys using DED, several challenges must be considered in advance, such as the oxidation of the hot melt pool surface and hydrogen embrittlement. Therefore, all experiments were conducted in an inert atmosphere. The low melting temperature (about 600°C) of the alloy is also difficult to handle. The associated long cooling time combined with the low surface tension of the melt results in very thin and wide weld beads.

Based on initial parameter studies, a suitable process window could be determined. However, it became evident that when building multi-layered, thin-walled structures, the heat dissipation into the base body undergoes significant changes, especially in the first layers. Therefore, an adjustment of the build strategy was made to compensate for the effect of heat dissipation and to adjust the volume deposition (Figure 2).

But what improvements do these stiffening structures bring about in testing? To address this question, bending samples were created. Straight stiffening structures were built on sheet samples using DED (Figure 3), stress-relieved, and subjected to a three-point bending test.

These findings indicate that DED-applied reinforcements can enhance crash resistance while maintaining a lightweight design.

The results of the tests were significant (Figure 4). It can be seen that the applied stiffening structures lead to a significant increase in the introduced deformation energy:

• When the structures are applied on the compression side of the test, the required deformation energy increases by 71%.
• On the tension side, an increase of 26% is observed compared to the unstiffened sheet sample.

Based on these initial results in the FlexCrash project next phases involve:

• Further simulation designs carried out by the project partner Gemmate Technologies.
• Optimising the most effective reinforcement structures (Stellantis’s CRF research centre and Gemmate Technologies).
• The best of these optimised structures will then be hybrid manufactured and tested for their crash behaviour in test stands Stellantis’s CRF research centre and Gemmate Technologies and Fraunhofer IWS).

The results of these tests will, in turn, contribute to the optimisation of the simulations (Gestamp), leading to safer and more efficient vehicle designs. By combining lightweight materials with smart reinforcement strategies, FlexCrash project is paving the way for greener, safer cars without compromising passenger protection.