Futuristic approach towards sustainable production of automobile structures

The focus of the automotive industries in the current era can broadly be divided into three aspects: cost effectiveness, safety of the passengers, and sustainable production. Aligned with these aspects, the FlexCrash European project aims at producing lighter, safer and circular crash structures for automobiles. The idea involves the production of such structures using recycled aluminium alloys processed by high pressure die casting. Now one might ask, “Why did FlexCrash select the said process and material?”. The answer is:

• High pressure die casting (HPDC) can produce complex-shaped, thin-walled parts of large dimensions, thereby decreasing the number of pieces that are usually required in a conventional method to produce the parts. Moreover, it can produce the parts in reduced cycle time. The reduction in the number of pieces used as well as the processing time makes the HPDC process more economic.
• Recycled aluminium alloys are obtained from recycling of scrap materials and can be able to exhibit mechanical properties within a range that are desirable in the structural parts, thus promoting sustainability and passengers’ safety at the same time.

HPDC, on one hand, develops a skin layer consisting of fine-grained microstructure at the surface of the cast components, which improves their mechanical properties. On the other hand, the process induces porosity in the components, which is considered the most influential factor in restricting their mechanical properties. At this point, you would anticipate that the usage of HPDC for manufacturing the structural parts from recycled Al alloy should impart a skin layer on the surface of the cast parts and their properties may be impeded by the porosity developed inside them. But is it really the case here? So, Luleå University of Technology in Sweden, one of the partners of FlexCrash project, endeavoured to address this question.

First, the HPDC processed recycled Al alloy is checked for the skin layer, which shows that the said layer is not formed continuously throughout on the surface of the cast parts. Moreover, at some locations, the skin layer followed a characteristic inhomogeneous nature as exemplified in Figure 1. Both the discontinuity and inhomogeneity in skin formation are resultant of the typical die geometry used in the HPDC process (the details of the die geometry and the scientific explanation behind the skin formation can be found here).

Then the cast parts are examined if the constituent recycled Al alloy used by FlexCrash can exhibit the required properties ensuring the passengers’ safety. The results manifest that although the alloy under study has the potential to achieve the strength and ductility within the required range, its properties are restricted by the inhomogeneous skin layer formed on the casting surface. More specifically, the restriction to the ductility of the alloy arises due to the abrupt failure of the material as the inhomogeneous skin layer acts as a crack initiation site owing to its weak bonding with the adjacent matrix. Figure 2 is an illustration of such fracture mechanism.

The reality behind the microstructure-properties relationship is further validated through investigation into the pores present in the cast parts and their effects on the tensile properties. And the results state that the porosity limits the ductility of the recycled alloy under study, however, it is not as detrimental as the inhomogeneous skin structure developed on the casting surface.

The inferences drawn from these investigations can be summarized as:

• The recycled Al alloy used by FlexCrash has the potential to exhibit the mechanical properties within the range that is required for the crash tolerant structures.
• Having said that, the parameters used for the HPDC process including the die geometry should be steered in a way to avoid the harmful inhomogeneity in the skin formation.