<p>Real-time measurement and quantitative analysis on the friction effect during the corner filling process in tube hydroforming present significant challenges due to the closed structure of the die. In this research, the theoretical unloading condition of tube blanks during the corner filling process has been deduced, and an experimental device combined with the digital image correlation technique was developed for visualizing this process. Subsequently, pure lead tubes (ideal rigid plastic model) and aluminum alloy tubes (hardening model) were used to investigate how friction and tube properties affect the deformation behavior. Results indicate that friction causes rapid unloading of the pure lead tube after a very small deformation (&lt; 0.02) upon contact with the die. Through lubrication and hardening, the impact of friction can be mitigated, and the strain distribution becomes more uniform. Specifically, after the observed area of the tube contacts the die, the maximum circumferential strain increases from 0.02 to 0.11, and the non-uniformity coefficient of thickness distribution decreases from 79 to 12%. This experimental device first enables the real-time quantitative measurement of the corner filling process, offering a reference for quantitative analysis and control of friction effect on corner filling behavior in tube hydroforming.</p>