Investigation of Ultrasonically Welded Thermoplastic Composite Joints Using Netlike Energy Directors

Abstract

This paper proposes a netlike energy director (ED) made of carbon fiber-reinforced thermoplastic (CFRP) composites to improve the ultrasonic welding energy efficiency. To explore the benefits of using netlike EDs, the joint qualities with flat EDs and without EDs are included for references. Also, the influence of the netlike ED geometry (i.e., hexagonal nets with different side lengths and thicknesses) on weld attributes and joint quality are investigated in terms of joint cross-sectional microstructure, fracture morphology, and maximum shear load. It is found that for both the netlike and flat EDs, the welding process can be accelerated compared to the one without EDs, which is caused by the concentration of welding energy into the expected welding region and the avoidance of welding edges introduced by surface curvature. Meanwhile, compared with flat EDs, the maximum shear loads of the joints with netlike EDs are improved, introduced by the decrease of the contact area and consequent higher cyclic strain, resulting in more melted materials during the ultrasonic welding process. However, with the increasing of the netlike ED thickness, more porosities are generated in the welding layer leading to reduced bonded region and decrease of the maximum shear load. From the joint fracture morphology analysis, it is found that the netlike EDs introduces carbon fibers in the welding layer, and the fracture modes include fiber-matrix debonding and fiber pull-out in addition to polymer fracture, confirming the feasibility of improving joint quality by introducing CFRP netlike EDs.