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In the current realm of materials science, there is a limited amount of data in understanding the exact way in which metallic nanoparticles behave upon high velocity collisions with each other.
While these collisions can occur as a result of both natural and/or artificial environments, the effects upon a collision of such particles onto an impacted surface can cause harmful, as well as several beneficial effects.
Despite the precision that certain computational software systems offer in estimating the way in which extreme collision events can affect the particles’ impact velocity, mass, temperature and shape, there remains a clear lack of experimental models that are capable of addressing this challenge.
In an effort to combat this issue, a group of researchers led by Sinan Muftu and Jae-Hwang Lee from the University of Massachusetts, Amherst analyzed the collision dynamics and nonlinear material characteristics of primarily aluminum (Al) nanoparticles through an advanced technique known as laser induced projectile impact testing (α-LIPIT).
Cold spray (CS) supersonically acceleration is a manufacturing technique that has become an increasingly popular method used in the additive manufacturing of metals. Within the CS equipment, powder is rapidly pushed through a supersonic gas jet before it impacts the substrate material.
Upon collision of the particles and the substrate, a significant amount of plastic deformation occurs, which is a process that describes the effect temperature has on material properties, that also allows the particles to localize onto the interface of the material. Primarily used as a coating technique to deposit pure metals, such as copper and aluminum, the CS method is a “cold deposition” technique that significantly reduces the amount of thermally induced stresses added to the substrate material1.
As this is a technique that is primarily applied for bulk processing, there is a lack of information provided on how single particle impacts can affect the final product.
In their study, the researchers primarily looked at a polycrystalline aluminum alloy that was composed of 97.5% aluminum, as well as other trace elements. The Al alloy particle size distribution was measured at 19.3 ± 5.3 μm.
Due to this factor, the researchers performed diameter (D0) measurements of each Al particle prior to conducting the α-LIPIT experiment by use of a long-working-distance optical microscope2. By utilizing two target substrates of sapphire and aluminum, which are realistic environments in which such a type of nanoparticle could undergo a collision event with, the collision events occurred over velocities ranging from 50-950 m/s.
The researchers discovered that Al particles that collide with a given substrate at a velocity measured to be below 530 m/s were able to maintain their distinct spherical shape, however, at higher velocities up to 660 m/s, the particles exhibited a much more flattened and deformed shape.
Further investigation into the impact effect of an Al particle at 530 m/s as compared to that of a higher velocity was studied by taking a cross-sectioned plane of the particles at both velocity points2. The 660 m/s Al particle showed a drastic change in its deformation characteristics as compared to the 530 m/s particle, in which almost all of the equiaxed grains present within the particle were significantly flattened.
At velocities greater than 550 m/s, the Al particle was found to undergo a significant collapse in its structure as a result of the induced pressure to the particle that existed in the absence of any resistance to this impact at this level2.
The experimental results obtained in this study were extremely accurate as compared to the numerical simulations conducted prior to the experiment, which therefore confirm the accuracy of these employed techniques.
These techniques are applicable for a wide range of alloys, polymers and biomaterials, which can provide for an advantageous tool in understanding the dynamic conditions present in high strain rate response tests, as well as other impact processes.
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References:
- “Laser assisted Cold Spray” – Industrial Laser Solutions for Manufacturing
- “Dynamics and extreme plasticity of metallic microparticles in supersonic collisions” W. Xie, A. Alizadeh-Dehkharaghani, et al. Scientific Reports. (2017). DOI: 10.1038/s41598-017-05104-7.
- Image Credit: Shutterstock.com/VAlex
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