- Researchers used neutron diffraction line-scanning method to examine the residual stress in automotive parts.
- They used VULCAN instrument to investigate high-strength steel in different conditions.
The demand of lightweight and stronger material for automotive structures is increasing at an enormous rate. Among several reliable materials, advanced high-strength steels (AHSS) have shown huge potential, the market share of which are projected to raise exceptionally in the near future.
In the past decades, some new classes of AHSS have been introduced to increase the strength and sustain good ductility of automotive components. Common metallurgical AHSS classes, such as martensitic, dual phase, complex phase, and transformation induced plastic-assisted steels have helped reduce maintenance costs, extended gas efficiency and save lives.
Now, scientists at Oak Ridge National Lab’s Spallation Neutron Source and United States Steel Corporation have used neutrons to investigate the characteristics of hydroformed AHSS and how it reacts to residual stress applied during manufacturing process.
Since AHSS contains retained austenite (a solid solution of mostly iron and carbon), understanding how it reacts to a variety of manufacturing processes like hydroforming or stamping could help scientists to validate models that are stronger, lighter, more durable, and easier to design and build.
To spatially resolve the residual stress in modules during synthesis, processing, and service, scientists used VULCAN instrument at Spallation Neutron Source. It allows to scrutinize materials in different conditions, including texture changes, phase formation, temperature distribution, stress development, precipitation and any kind of deformation.
VULCAN instrument | Credit: Oak Ridge National Lab
Why Neutron Diffraction?
Previous studies had used the neutron diffraction line-scanning method to examine the residual stress in welded components. Researchers discovered that it’s also applicable for the components built with AHSS.
Neutron diffraction is basically an application of neutron scattering to analyze atomic and magnetic structure of a material. It’s quite similar to X-ray diffraction, but since their scattering characteristics are very different, X-ray and neutrons provide complementary data.
In particular, X-rays are used for superficial examination and strong X-rays are used for thin specimens or shallow depths, while neutrons are advantageous for the localization of light atoms as well as determination of magnetic ordering. They have high penetration depth, and thus can be used for bulk samples.
With neutron diffraction, it’s possible to analyze the intrinsic properties of the hydroformed parts of the AHSS, and study their emergence across lattices and multiple cross sections in details.
Scientists used this neutron data of residual stress distribution to generate computer simulations. These simulations help them improve the model for designing and building automotive parts.
A vehicle gradually undergoes cyclic force, or fatigue over a course of time. Residual stress in the as-formed component could affect its fatigue performance, which is not covered in the simulation due to lack of validated material model and residual stress data.
However, these tests and simulations could help engineers analyze the fatigue performance of components and how it is affected by various manufacturing processes. According to the researchers, the residual stress data obtained in this study may set a new exemplar of specialization and preciseness for selection and validation of material models.