Abstract
Ultrasonic welding (USW) is considered one of the most suitable methods to join semi-crystalline carbon fiber-reinforced thermoplastics (CFRTPs). The degree of crystallinity (DoC) of the semi-crystalline resin will affect the ultrasonic welding process by affecting the mechanical properties of the base material. In addition, ultrasonic welding parameters will affect the joint performance by affecting the DoC of the welded material at the welding interface. This paper investigates the effect of DoC of carbon fiber-reinforced PA6 (CF/PA6) base material and welded material on its ultrasonically welded joints' performance. Distinct pre-welding heat treatments are conducted on the base material before welding. The DoC is calculated by DSC, while the crystalline phases (α and γ phases) and crystallize size are determined using XRD. The results demonstrate that the heat treatment process of heating temperature of 180 °C and cooling with an oven (180-O) could increase the DoC of CF/PA6 from 27.2% to 33% and the ratio of α/γ from 0.38 to 0.75. The joint strength of 180-O sheets reached 17.7 MPa, which is 26.7% higher than that of the as-received sheets. The DoC of the welded material at the welding interface obtained with different combinations of welding parameters is characterized. Higher welding force and amplitude result in faster cooling rate at the welding interface and more effective strain-induced crystallization, leading to higher DoC of the welding interface.