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Laser Joining
of Dissimilar Metals and Transparent Dielectrics
Laser Autogenous
Brazing of Dissimilar Metals for Biomedical Applications
The goal of
this project this project is to join the dissimilar metal pair of
nickel-titanium to stainless steel, for the application of implantable
medical devices. This is to exploit NiTi’s unique properties (superelasticity
& shape memory) while reducing cost by using stainless steel in
places where NiTi is not necessary. When joining these materials, the
biggest concern is the formation of brittle intermetallic phases at the
joint. These phases can be minimized by limiting the amount of heat input,
resulting in limited melting at the interface. By using a laser as the heat
source, we are able to precisely control which regions are heated and also
the temperature that is reached in these regions. We have been joining
micro-scale wires and tubes in a butt weld alignment along their axial
direction. Using a process we developed called 'autogenous laser brazing'
the size of the melt region is reduced to a width less than that of the
laser beam by taking advantage of the thermal accumulation occurring at the
interface. A thermal resistance exists across the interface of the two
materials, so as the laser scans towards the interface, heat builds up
within the irradiated wire, allowing for the interface to be the region of
the wire reaching the highest temperature.
Uniaxial
tensiel testing is one of the major characteristic techniques. Fracture
strengths of the joints higher than the yield strength of the base materials
has been achieved. Material characteristics have been analyzed via
techniques such as Energy Dispersive X-ray Spectroscopy (EDX) and Electron
Backscatter Diffraction (EBSD). |
