Metal-Polymer Adhesive Bond Characterization in an Additive Manufacturing Environment

Recently, the aerospace industry has turned the focus of its manufacturing efforts towards additive methods. For many aerospace applications, however, hybrid materials are preferred for their ability to combine optimal properties from various material sets, and these materials are not yet compatible with large-scale additive manufacturing. To fix this lack of compatibility, new additive methods must be developed that can print dissimilar hybrid materials on one print bed at a large scale, which will require a reliable dissimilar material joining method.Among current joining techniques, one of the most promising for this application is adhesive bonding. Typically, adhesive bonding requires optimizing the conditions of bond surfaces by sanding and/or machining. This is inconvenient for gantry-based additive manufacturing systems, as the extra weight of any tools must be accounted for. For this study, the case of adhesively bonding additively manufactured Grade 5 Ti-6Al-4V with carbon fiber-reinforced PPS without any surface modifications is investigated. The flatness of the surface profiles of all the printed PPS samples were measured by a laser profilometer, and a computational model was developed to characterize these surfaces. Small double lap joints of Ti and PPS were bonded using two different commercially available epoxy adhesives. Two different bead orientations and two different bead thicknesses of PPS samples were used. The double lap samples were tested, and the shear strength of each bond was determined.Due to large variations in the surface flatness of the PPS material, as demonstrated by the laser surface characterization results, the bonded area changed significantly from sample to sample, thus producing a large variation in the measured shear strengths. These bonds, however, were stronger than the ones formed with smooth machined surfaces. It is thus concluded that the poor resolution produced by large-scale extrusion additive manufacturing processes is currently sub-optimal for bonding but shows promise and should be investigated further.