Laser surface melting of TC21 (α + β) titanium alloy with 80 % track Overlap: Microstructural evolution and hardness Enhancement

TC21 titanium alloy is broadly used in aerospace, automotive, and petrochemical applications due to its extraordinary properties. Laser surface melting (LSM) provides a practical approach to enhancing its surface characteristics. This study evaluates the influence of LSM on microstructure and hardness of TC21 titanium substrate using a fiber-coupled diode laser operating in continuous-wave mode at laser powers from 500 to 1000 W and scanning speeds of 250 and 500 mm.min −1 , corresponding to six different laser fluences (15 to 60 J.mm −2 ). Nine tracks with 80 % overlap, the first such configurations applied to TC21 alloys, were performed to achieve a uniform remelted layer and minimize surface defects. Laser-treated and sub-surfaces regions were characterized using FESEM, EDX, and XRD techniques. Comparative analysis with conventional treatments was performed. Results revealed that at the lowest fluence (15 J.mm −2 ), incomplete phase transformation occurred, leaving dendritic β-phase and achieving a hardness of 750 HV 0.3 . At 22.5 J.mm −2 , nearly complete transformation to martensite α -phase with transformed β-phase and prior α-phase yielded hardness of 1042 HV 0.3 . The intermediate fluences (30 to 45 J.mm −2 ) produced a composite like a metal matrix comprising spheroidal transformed β-phase, fragmented acicular and globularized martensite α -phase, and secondary α-lamellae embedded within the residual β-matrix. The highest fluence (60 J.mm −2 ) resulted in a remarkably high volume fraction of the secondary α-lamellae and a 3 mm deep treated layer with a peak hardness value of 1624 HV 0.3 . The optimized overlap ratio and laser fluence were crucial in achieving defect-free surfaces with significantly enhanced mechanical properties.