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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 NiTiís 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).

 

Laser Joining of Transparent Dielectrics (e.g., Glasses) via Nonlinear Absorption

Laser beam welding is considered to be a highly flexible technique with potential for joining glasses in applications such as flat panel displays, lab-on-a-chip, OLED lighting. Glass-glass joining by conventional lasers requires an interlayer or opaque material between the top and bottom pieces to absorb the laser energy. However, due to the nonlinear absorption characteristics of femtosecond lasers, localized joining of transparent materials without intermediate absorbing layer is possible when laser beam is tightly focused on the interface.

Transmission welding using femtosecond laser at a low repetition rate has been done to investigate the morphology and mechanical properties of the weld zone. The role and effect of the gap at the interface between two transparent material plates on the welding and joining mechanisms have also been investigated. From indentation fracture analysis and obtained geometry of the weld seam, it suggests that this technique is promising for spatially selective joining of glasses.

 

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