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Bonding Composites to Metals for Medical Applications

By Rebecca Gurk on Tuesday, May 25th, 2010

Materials that cannot be welded must be joined with mechanical fasteners, solder, braze, or adhesives. For extended service above 450oF, a solder or braze is required. At lower operating temperatures, adhesive bonding is used.

Adhesives can bond many diverse material combinations and offer the best approach for joining dissimilar materials in corrosive environments. Fasteners, brazes, and sol- ders require additional corrosion protection, while adhesives form an internal corrosion barrier. Brazing and soldering are not options for joining plastics-to-metals or composites-to-metals, but the maximum use temperatures for plastics and compos- ites overlap those of many adhesives.

Depending on materials, adhesive bonding typically provides bond strengths of 1500- 6000 psi. The goal of good adhesive bond- ing practice is to drive the failure away from the bondline. When bonding plastics or composites, failure is often transferred into the bondedmaterial, making the plastic or composite itself the strength-limiting factor.

Adhesives tolerate joint gaps and mis- matches, adapting readily to bondlines of 0.001-0.03″. This reduces the machining and molding demands for metals and plas- tics or composites which reduces manufac- turing cost. Applied adhesive thickness accommodates uneven gaps and can address CTE mismatch or residual strain from thermal cycling. Adhesives can also impart better fatigue performance, impact resistance, and sound dampening.

While only adhesive bonding is practical for bonding metals-to-composites, combining adhesives bonding and welding methods, such as resistance welding, laser welding, and ultrasonic welding is a tremendous advantage in producing weldbonded metallic structures. Weldbonded metallic structures can integrate with bonded metal-to-composite substructure. EWI has worked with bonding fiberglass- and carbon-based composites to steel and stainless steel along with bonding programs for titanium-to-aluminum, rubber-to-titanium, ceramic-to-titanium, and SMC-to- aluminum applications.

Good surface preparation is required to maintain bond integrity and EWI has imple- mented several preparation methods specific to materials. EWI has worked to integrate joint design, modeled strain prediction, NDE of bonded joints, static and fatigue testing, and environmental exposures to provide a complete bonding solution based on worst case predictions.

Combining composites and metals can offer the maximum in materials-based weight reduction designs. Adhesive bonding is certainly a worthwhile approach for joining composites, plastics, and metals to provide the most efficient structure. EWI draws from many internal resources to produce reliable bonded structures.

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