Influence Mechanism of Thickness and Viscosity of Flat Energy Director to the Joints Quality by Ultrasonic Welding Jiale Lu1, Wei Dai2, Jun Wang2,3, Xianghui Gan4, Zhuang Wu4, Hang Wang4, Xujun Tian4, 1Princeton International School of Mathematics and Science, USA, 2School of Materials Science and Engineering, Wuhan University of Technology, China, 3Institute of Advanced Material Manufacturing Equipment and Technology, Wuhan University of Technology, China, 4China Ship Development and Design Center, China