Modification of Surface Tribological Properties by Microscale Pulsed Laser Processing

Download or read book Modification of Surface Tribological Properties by Microscale Pulsed Laser Processing written by Shixuan Chen and published by . This book was released on 2019 with total page 0 pages. Available in PDF, EPUB and Kindle. Book excerpt: When a microscale pulsed laser beam deposits localized energy on a metallic surface, the irradiated spot can melt and cool down rapidly. A scanned surface area experiences a cycle of rapid heating and cooling treatments, which in turn causes microstructural evolution. The microstructural evolution leads to changes in mechanical strength and tribological properties. Besides, microscale pulsed laser processing controls micro-scale surface features. Therefore, it is a promising technique to produce surfaces with improved tribological performance. The objective of this thesis is to investigate the microstructural changes and the multi-scale tribological performances of the microscale pulsed laser processed surfaces. The tribological performances are related to the microstructures to understand the underlying mechanisms of the enhancements. The thesis starts with applications of microscale pulsed laser processing on a couple of metallic alloys to study the microstructural changes in the processed area, such as phase transformation and chemical redistribution (Chapters 2 and 3). Initially, this method is applied on Ti6Al4V alloy to create continuous surface layers with altered phase components. Two laser powers are used to achieve low and high energy inputs. The obtained phases are predominantly martensitic Îł' and Îø phases due to the extremely rapid heating and cooling in the melt zone and heat affected zone. Higher laser power produces thicker martensitic Îł' surface layers several microns into the processed surface, exhibiting higher hardness compared to the surface processed by low power laser. The hard martensitic surface layer resulting from microscale pulsed laser processing facilitates superior resistance to abrasive wear and low cycle fatigue wear. Then, the influence of microscale pulsed laser processing on chemical distribution and wear resistance of A384 casting aluminum (Al) alloy is investigated. The large proeutectic silicon (Si) precipitates in the base material melt and disperse into the Al matrix during melting. Thanks to the rapid cooling, the dispersed Si lacks time to grow. Thus, the process results in a homogeneous remelted surface layer with well-distributed small Si particles. The hardness of the layer becomes more uniform compared to the base material, which is an expected consequence of elemental homogenization. Besides, the hardness and wear resistance of the homogeneous surface is higher than the unprocessed Al matrix. Microscale pulsed laser processing can also cause significant geometric and chemical changes in a multi-material system. Chapter 4 and 5 of this thesis study those changes on S7 tool steel surfaces dip-coated by nanoparticles and melted by microscale pulsed laser. The nanoparticles can either mix with the substrate or agglomerate on the surface, depending on the laser settings and nanoparticle properties. The magnitude of the fluid flow in the melt pool increases as the laser power increases. Higher laser powers result in stronger flow of the molten mix and help mixing of nanoparticles in the substrate. Besides, nanoparticles with good wettability, solubility and reactivity with the substrate can mix with the substrate and form alloyed regions; whereas nanoparticles with poor wettability agglomerate on the surface and form clusters. For instance, thanks to their good wettability and solubility in steel, SiC nanoparticles mix with the S7 tool steel during processing and form alloyed regions. The alloyed regions exhibit higher hardness and wear resistance compared to the substrate processed without nanoparticles. That enhancement should come from the strengthening effect of the nanoparticles dispersed in the metal matrix. In contrast, due to the poor wettability in liquid iron, Al2O3 nanoparticles form clusters on the S7 tool steel surface after microscale pulsed laser processing. The clusters exhibit higher hardness, denser structure and stronger bonding to the substrate compared to the as-coated layer. Therefore, the clusters can withstand severe scratching and exhibit excellent wear resistance. In this study, microscale pulsed laser processing is applied on metallic surfaces to achieve enhancement of the multi-scale tribological properties. The enhancement can be realized by altering the phase component, chemical distribution and introducing reinforcements. The method is flexible with the scale and the selection of the processed material, therefore has a great potential in micro device and small part manufacturing.