Ultrasonic pretreatment enhances stage-specific moisture migration and drying performance of Ailanthus altissima wood

Ultrasonic pretreatment can modify the internal pore structure of wood, potentially improving drying quality and performance by facilitating moisture migration. However, the stage-specific migration of free and bound water, as well as the existence of treatment thresholds, remains unclear. In this study, Ailanthus altissima blocks were pre-treated in a 40 kHz, 320 W bath for 1–5 h, then oven-dried at 60 ℃, and in-situ moisture loss was gravimetrically monitored hourly above and below the fiber saturation point (FSP). Key anatomical and physical changes, including pore structure, extractives content, density, cell morphology, and Shore hardness, were analyzed to identify the dominant factors influencing water migration. Results showed that ultrasonic pretreatment significantly improved the overall drying rate. Above the FSP, free water migration was primarily influenced by the extractive content and pore pathways (pits). Below the FSP, bound water migration was additionally constrained by cellular structure and reduced mechanical properties. An increased aspect ratio and lower strength hindered the mobility of bound water. Moreover, bound water was less responsive to ultrasound than free water, a threshold effect was observed, and longer treatment did not always yield better drying performance. This study provides new insights into the drying mechanisms of ultrasonically treated wood, bridging a knowledge gap in moisture migration theory. These findings advance understanding of ultrasound-assisted drying mechanisms and provide a scientific basis for optimizing ultrasonic pretreatment parameters for wood drying.