Improved crash characteristics of high-strength aluminum safety components
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Safety components made of aluminum have the highest requirements in vehicle design. Leichtmetallkompetenzzentrum Ranshofen and Hammerer Aluminium Industries Extrusion have improved the crash properties of an extruded profile by means of alloy modification and geometric adaptation of the cross section. An increase in energy absorption and reduction of cracking has been achieved.
Crash Properties of Aluminum
In recent decades, aluminum has become more and more important as a lightweight material. However, further improvements of the properties are still essential to meet the increasing demands in the field of lightweight design in the future. This article describes how the crash characteristics of an extruded two-chamber profile were improved. The profile is used as a safety component in vehicles in the load-bearing structure of the body. In order to increase the energy absorption of the profile in the event of an impact, the yield strength of the material is increased. While this may improve the lightweight design of the component, the change to the alloy can adversely affect the crash characteristics. Here an approach is presented which addresses the geometry as well as the microstructure to improve the profile. The crash properties were determined by standardized, quasi-static compression tests.
Identification of Critical Locations
The identification of the critical points of the two-chamber profile stands at the beginning of the investigation. These are the areas where material failure occurs due to deformation.
The quasi-static compression of the profile is simulated using the finite element (FE) method in LS-Dyna. For this purpose, the profile is discretized with solid elements, enabling a very detailed representation of the geometry.
In addition to the von Mises flow rule for stress analysis, the use of a failure model allows the prediction of material failure in the profile. The aim is to make specific geometrical changes of the cross-section based on the results from the simulation of the original profile, which should reduce the loads and thereby minimize the risk of fracture during compression. In addition, the analysis provides insights for materials engineering, where special attention must be paid to the microstructure due to the increased stress.
▸ microstructural optimization
▸ reduction of notch stresses.
However, boundary conditions for solving the problem result from the fact that the changes to the material and the geometry may only move within the customer specifications. This means that no significant changes to the profile which have an influence on further processing steps may be undertaken. For example, the dimensional tolerances must be complied with.
The main alloying elements in aluminum alloys of the 6000 series (“AA6xxx”), which determine the strength of the profiles, are magnesium, silicon and, in minor additions, copper. These alloying elements form precipitates which increase strength during the heat treatment following the extrusion. It is crucial for these process steps that the profile is cooled immediately after extrusion within a very brief time. The higher the amount of alloying elements of the alloy, the higher the cooling rate must be. For high-strength alloys such as AA6082, this requires quenching with the use of water spray cooling or water baths. The process step of cooling is a process-critical step: If the cooling capacity is too high, the profile deforms. If the cooling capacity is too low, the required mechanical strength cannot be reached after the heat treatment . In both cases, the profile cannot be used. Therefore, the optimum cooling conditions and strength must be determined individually for each profile cross section. In the case of crashworthy profiles, the subject becomes even more crucial; only high cooling capacities result in good folding without fracture.
The dispersoids are formed during heating to the homogenization temperature. In the further course of homogenization, these dispersoids coarsen . The larger the dispersoids, the lower their grain boundary pinning effect (low recrystallization inhibition) and thus their positive influence is reduced. It should be pointed out that the process step “heating up to homogenization temperature” is the first in the entire process chain after casting and is one of the largest factors influencing the crash behavior.
In summary, it can be said that the strength of the profiles is adjusted by the main alloying elements silicon and magnesium. By addition of elements such as manganese and chromium, the microstructure can be positively influenced in a way that no or only slight superficial fracture in the event of a crash arises.
Reduction of Notch Stress
In further simulations, the improved geometry is compared with the original profile. The occurring stresses, energy absorption and folding pattern are investigated. Subsequently, compression tests with extruded profiles are carried out to validate the simulation.
The result shows that the introduction of the changed radii leads to a mitigation of the critical points. Specifically, the stresses can be reduced by 4.7 %. Furthermore, the simulation of the profile with the tensile triangle design on the T-sections exhibits fewer eroded elements, suggesting less material failure in the real profiles. This reduction in material failure results in an increase in specific energy absorption of 2.7 %.
The simulation also shows that the change has no influence on the folding pattern of the profile. The additional material compared to the original design, Figure 4, does not prevent the profile from the asymmetrical folding of the two chambers. The extra weight of the profile is about 17 g/m due to the geometric change which is negligible.
For the validation of the simulation results, an extrusion die is produced which possesses the numerically determined modified radii at the T-section. In order to obtain comparable material properties, the same material is used for the profiles with regular radii and improved tensile triangle design and also the extrusion parameters and the heat treatment are kept constant.
For testing, the profiles are compressed with a length of 300 mm on a universal testing machine with a speed of 100 mm/min. The resulting compression force and the traverse displacement are recorded during the experiment. The profile is not clamped or fixed during the experiment.
A different result was achieved for the profile with the tensile triangle designs in the T-sections. The folding pattern is regular and the tendency to crack is reduced, Figure 6 (right). The different folding pattern leads to a 5.4 % increase in energy absorption in the profiles with tensile triangle radii in the T-sections. Thus, the result of the simulation is exceeded by the tests.
In the presented work, the crash behavior of a high-strength two-chamber extruded profile was improved by means of alloy modifications and geometrical adjustments. The critical parts of the cross-section were determined by FEM simulations of the compression behavior. For this purpose, a detailed model of the profile with volume elements in LS-Dyna was established. The T-sections which connect the ligament dividing the profile to the outer walls are the most critical points of the cross section. These T-sections are also critical regions during production of the profiles since the achievable cooling rates are lower there which may lead to recrystallization.
For the crash properties of the material, it is important that it shows a very fine fibrous structure. This can be achieved by preventing recrystallization before and during extrusion of the profile. In addition to optimized cooling, so-called dispersoid-forming elements such as manganese, chromium, iron, vanadium and zirconium were added.
Another approach to improving crash properties is to reduce the notch effect in the T-section area during folding. For this purpose, the occurring stresses in the simulation were analyzed and a notch shape optimization was introduced via tensile triangle design according to Mattheck. In the simulation and in the compression test (quasi-static), the modified profiles exhibit a reduction in the critical stresses, thus a decrease of the tendency to fracture and an improvement of the entire folding pattern. As a result, the energy absorption can be .improved by 2.7 % in simulation or by 5.4 % in experiments without without exceeding the dimensional tolerances of the cross section or significantly increasing the weight.
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