Session: 03-01-01: Advanced Manufacturing in Aerospace Engineering

Paper Number: 167026

Understanding Solidification and Mechanical Behaviour of Al-Cu Binary Alloy Under Microgravity: A Molecular Dynamics Approach 

In-space welding and joining of metals has been around since 1960s [1]. There are many unique features of laser beam welding (LBW) for in-space welding (ISW) including absence of gravity, vacuum environment, and absence of natural convection heat transfer [1]. Ground-based research has advanced the knowledge on LBW and subsequent defects and microstructures. However, the physics of LBW process in microgravity, vacuum, and under extreme external temperature remains unclear. Solidification microstructure of alloys, especially dislocation distribution, exhibits interesting features that are not observed on ground. A major challenge to understanding the physics of solidification in microgravity is the scarcity of available data from in-space welding. Microgravity environments can be created using drop towers, parabolic flight or conducting experiments in the International Space Station [2]. While experimental tests are the most reliable method for generating credible data, they can be very expensive to conduct. Therefore, accurate and dependable simulated data serves as a more cost-effective and faster alternative.

 

In this work, the solidification behavior of Al 2219 alloy is studied for the first time mimicking the drop tower test at nanoscale using molecular dynamics simulation. To the best of the author's knowledge, this is the first attempt to understand the atomic level details of microstructure evolution during ISW using molecular dynamics. Outcome of the current research will enhance the understanding process-structure link in ISW. Moreover, the resulting dislocation evolution can be directly used as input to the mesoscale structure-property computations, such as crystal plasticity method. Current study can be used as a benchmark for generating simulation-based ISW data.

 

The study is performed considering a representative Al-Cu nanostructure similar to Al 2219. The structure is heated to 1300K temperature, reaching a melting state and cooled down to a solid state at 300K in different cooling rates of 0.01, 0.1, 0.5 and 5 K/ps. The heating and melting are performed in an NPT ensemble, where both temperature and pressure are controlled to allow volume relaxation. Later, the molten state is equilibrated in an NVT ensemble, where temperature is fixed while the system volume remains constant. The effect of drop tower is modeled by applying a variable gravitational force to the material system during cooling. This produces an effect comparable to the material falling freely in a vacuum. The simulation progresses to free fall in zero gravity using the NVE ensemble, which conserves energy and allows for natural motion of atoms without external forces. Finally, cooling occurs under the NPT ensemble again, with pressure control, while maintaining the free-fall dynamics to reach a stable low-energy state. Atomistic configuration of dislocations and grain structure evolution are observed with respect to time for different microgravity scenarios. The copper content in the material is also varied from 2, 6 and 8 percent by weight to examine the impact of both high and low copper concentrations during solidification in microgravity and a comparative is drawn with Al 2219. The results are then compared with the results from an identical system without the gravitational force.

 

Preliminary simulation results indicate that under zero gravity conditions, the grain boundary distribution is more homogeneous with minimal directional solidification. Variations in the dislocation motion and influences in the overall microstructure evolution during solidification are also observed as copper content increases.

Presenting Author: Apurba Sarker Virginia Tech

Presenting Author Biography: Apurba Sarker was born in Bangladesh. He completed his Undergrad bachelor degree from Bangladesh University of Engineering and Technology from the department of mechanical engineering. He will join Virginia tech Aerospace Engineering Department from Fall 25 for his PhD

Authors:

Apurba Sarker Virginia Tech
Sourav Saha Virginia Tech