During deposition of coating on a substrate using welding leads to the formation of equi-axed grains in the interface, long columnar dendrites in the middle and multi directional fine dendrites near the coating surface is common. what are morphological changes across the depth direction is expected when a moving heat source like laser treatment is applied at the top of the coating allowing the surface alone to be re-melted?
In fast superficial remelting by laser, large dendrites will melt and recrystallise in the form of much smaller dendrites. The smaller the melt, the smaller the crystals become.
thank you Dr. Vadim Verlotski . will it affect/promote the grains/grain growth in the middle of the coating due to conduction? say if the remelting thickness is 70 micron of a coating thickness of 2mm?
Dr. Vadim Verlotski Is it valid even if the region (subsurface) is heated above the re-crystallization temperature of a material ?
Recrystallization in the solid state takes minutes and hours. With an extremely fast laser remelting, material only stays hot enough for a few seconds and there is not enough time for recrystallization.
Deenadayalan
You have not said anything on coating layer or on laser. When surface is being treated with laser, the surface goes under morphological changes and these changes depend upon the type of laser being used, wavelength rate of heating and cooling of the surface and energy input.
In laser treatment thin layer of the coating is melted, and convection current within the melt zone would cause mixing of liquid before rapid solidification and laser treated surface layer is uniform and most probably amorphous or extremely nanograined well bonded and generally free from cracks and pores.
Some years ago we carried some work on Al-Si alloys and in laser processing we found melting of eutectic region without affecting underlying primary aluminum dendrites, Hope this helps.
Dr. K
- Badges
- Science topic
- Similar topics
- Engineering
- Materials Engineering
- Coating