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

• High laser fluence and increased track overlap significantly improved the surface properties of TC21 alloy. • Eighty percent track overlap produced uniform defect-free remelted layers. • A novel composite-like microstructure was formed, greatly enhancing the surface hardness. • Surface hardness exceeds 1000 HV, substantially surpassing that of the untreated alloy. 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.