Ullah Md Asmat, Li Wencai, Siebenbuerger Miriam, Savella Felipe, Palardy Genevieve
Department of Mechanical and Industrial Engineering, Louisiana State University, 3261 Patrick F. Taylor Hall, Baton Rouge, Louisiana 70803, United States.
Department of Mechanical Engineering, University of Michigan, 2350 Hayward Street, Ann Arbor, Michigan 48109, United States.
ACS Appl Eng Mater. 2025 May 1;3(5):1455-1467. doi: 10.1021/acsaenm.5c00281. eCollection 2025 May 23.
Ultrasonic welding (USW) is a fast and effective method for joining thermoplastic composites, offering excellent bonding strength that results in lightweight, durable structures, making it a cost-effective alternative to traditional joining techniques. The crystallinity at the weld interface impacts the mechanical properties and chemical resistance of the joint. The crystallization mechanisms at the bonded interface remain inadequately understood for the USW process, especially given its rapid cooling rates. This study investigates the use of polypropylene (PP) and multiwalled carbon nanotube (MWCNT)/PP films for ultrasonic welding of glass fiber (GF)/PP adherends, focusing on how process parameters influence the crystallinity degree, crystalline phases, crystallite size and spacing, lamellar structure and anisotropy, and molecular changes at the welded interface. Differential scanning calorimetry (DSC), scanning electron microscopy (SEM), Fourier transform infrared spectroscopy (FTIR), X-ray diffraction (XRD), and small-angle X-ray scattering (SAXS) were employed to gain a better understanding of crystalline structure at the interface. Four different sets of welding force and amplitude were tested: (1) 500 N, 38.1 μm, (2) 500 N, 54.0 μm, (3) 1500 N, 38.1 μm, and (4) 1500 N, 54.0 μm. The study demonstrated that despite fast cooling rates obtained during the process, higher welding force and amplitude significantly enhanced crystallinity, achieving 55% for welds with pure PP films and approximately 60% for MWCNT/PP films, compared to 35% and 41%, respectively, before welding. Notably, amplitude influenced the crystallinity at the welded interface more significantly compared to the force. SAXS experiments revealed that both pure PP and MWCNT/PP films exhibited isotropic structures prior to welding, but distinct anisotropy after welding. These findings suggest that strain-induced crystallization results from the welding process, with the degree of anisotropy correlating with the applied welding force and amplitude.
超声焊接（USW）是一种用于连接热塑性复合材料的快速有效方法，能提供优异的粘结强度，从而形成轻质、耐用的结构，使其成为传统连接技术的经济有效替代方案。焊接界面处的结晶度会影响接头的机械性能和耐化学性。对于超声焊接过程，粘结界面处的结晶机制仍未得到充分理解，尤其是考虑到其快速冷却速率。本研究调查了使用聚丙烯（PP）和多壁碳纳米管（MWCNT）/PP薄膜对玻璃纤维（GF）/PP被粘物进行超声焊接的情况，重点关注工艺参数如何影响结晶度、晶相、微晶尺寸和间距、片层结构和各向异性以及焊接界面处的分子变化。采用差示扫描量热法（DSC）、扫描电子显微镜（SEM）、傅里叶变换红外光谱（FTIR）、X射线衍射（XRD）和小角X射线散射（SAXS）来更好地了解界面处的晶体结构。测试了四组不同的焊接力和振幅：（1）500 N，38.1 μm，（2）500 N，54.0 μm，（3）1500 N，38.1 μm，以及（4）1500 N，54.0 μm。研究表明，尽管在该过程中获得了快速冷却速率，但较高的焊接力和振幅显著提高了结晶度，纯PP薄膜焊接后的结晶度达到55%，MWCNT/PP薄膜约为60%，而焊接前分别为35%和41%。值得注意的是，与焊接力相比，振幅对焊接界面处的结晶度影响更为显著。SAXS实验表明，纯PP和MWCNT/PP薄膜在焊接前均呈现各向同性结构，但焊接后呈现明显的各向异性。这些发现表明，焊接过程会导致应变诱导结晶，各向异性程度与施加的焊接力和振幅相关。
