The effect of low-pressure plasma treatment on the lap shear strength of laser assisted joining of polyamide 6.6 - AISI 304 hybrid structures

Christian Timmer, TU Dortmund University, Dortmund, Germany
Wolfgang Tillmann, TU Dortmund University, Dortmund,
Lukas Wojarski, TU Dortmund University, Dortmund,
Christian Hopmann, RWTH Aachen, Aachen,
Patricia Fatherazi, RWTH Aachen, Aachen,

Polymer-metal hybrid structures are gaining importance due to the combination of highly diverse functional, chemical and mechanical aspects with a high potential as lightweight assemblies. Laser-assisted metal and plastic (LAMP) joining is a promising method to join these dissimilar materials with the absence of adhesives or mechanical fasteners. In this work, the semi-crystalline thermoplastic polyamide 6.6 and the stainless steel AISI 304 are bonded by the means of direct and indirect laser joining. In order to determine the optimal joining parameters for the maximum lap shear strength a one-factor-at-a-time design of experiments was used, in which the laser power and scan speed has been varied. Furthermore, a low pressure plasma treatment was applied to improve the strength of the adhesion based polymer - metal hybrid joints. The plasma interacts with the surface and removes all remaining impurities especially hydrocarbons and activates the surface improving the surface free energy. As plasma gases O2, H2O and Ar were utilized. Additionally, the surface free energy changes of the plasma treated surfaces were measured according to the OWRK-Method with water and diiodomethane as testing liquids. Finally, the plasma treated samples were joined and the lap shear strength was obtained. It could be observed, that samples manufactured by direct laser joining obtained lower strength values compared to indirect laser joined samples. This is most probably caused by residual stresses due high cooling rates in this joint configuration. Furthermore, the plasma treatment of the AISI 304 substrates lead to a reduction of the joint strength regardless of a possible treatment of the polyamide 6.6 substrates. The shear strength could be increased by 50% by a plasma treatment of the polyamide 6.6 in a H2O plasma for 180s.

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