Effect Of Pulsed Laser Surface Treatment On Microstructure And Property Of Pure Ti And Ti-6Al-4V

In this work,a high-purity Ti,a high-purity Ti preplated with Cr and a hot-rolled Ti-6Al-4V(TC4)was surface-treated by pulsed laser at two different powers(100 W and 200 W),with microstructural features(like grain morphology and size,crystallographic orientation,grain boundary misorientation and local composition)in various laser-modified zones characterized by energy-dispersive spectrometry(EDS),electron channeling contrast(ECC)imaging,and electron backscatter diffraction(EBSD)techniques.Hardness variation induced by the laser surface treatment(LST)was also examined and correlated with these microstructural characteristics.The following conclusions are obtained:(1)A high-purity Ti was treated by the pulsed laser.After the LST,two completely different modification zones were obtained: melted zone(MZ)is composed of fine ? martensitic plates with nanotwins inside,and heat affected zone(HAZ)is composed of multiple sub-grains with irregular-shaped boundaries.Due to LST-induced rapid cooling,??? martensitic transformation occurs in the MZ,resulting in plate structures with scattered orientations.The subgrains in the HAZ are formed by thermal recovery of stress-induced dislocation activities.The absence of duplex-phase domain in high-purity Ti leads to sharp MZ/HAZ interface,while inhomogeneous high angle boundary migration leads to obscure HAZ/matrix interface.Remarkable hardness increase is noted in the MZ,which can be ascribed to both greatly refined grains(? plates)and nanotwins;there is only slight hardness increase in the HAZ.(2)A high-purity Ti sheet was treated by laser surface alloying(LSA)with Cr at two different powers(100 W and 200 W),with microstructural features and properties analyzed.At both laser powers,there are two modification zones with distinct microstructural characteristics: melted zone(MZ)near the surface,composed of martensitic ? plates supersaturated with Cr and dense nanotwins inside them;heat affected zone(HAZ)beneath the MZ,featured by irregular-shaped grains and substructures with varied sizes and little Cr in their interiors.Hardness measurements show that remarkable hardness increase(about three times that of the matrix)could occur in the MZ after the LSA treatments.This can be ascribed to combined contribution from grain refinement,presence of abundant nanotwins and solid solution of Cr.The subgrains in the HAZ,produced by dislocation recovery,have only marginal contribution to hardness increase.The hardness of the Ti(Cr)-200 W specimen is slightly lower than that of the Ti(Cr)-100 W specimen,which is related to the dilution of Cr and the reduced cooling rate associated with enlarged modification zone at higher power.After comparing with laser surface treatments without alloying,it is confirmed that the LSA with Cr in the present work is much more effective in hardening the surface of pure Ti.(3)A hot-rolled TC4 was treated by the pulsed laser at two different powers(100 W and 200 W),with microstructural features and properties analyzed.At both powers there are two modification zones with distinct microstructural characteristics: melted zone(MZ)near the laser beam center,completely composed of fine martensitic plates with dense {10-11} nanotwins inside them;heat-affected zone(HAZ)far away from the laser beam center,comprised of mixed structures of short-rod ? particles,martensitic plates and untransformed bulk ? grains,.Hardness measurements reveal that the hardness of the TC4 sheet can be markedly increased(especially in the MZ)after the LST.In-depth analyses suggest that the hardness increase in the MZ can be ascribed to combined contributions from grain refinement,the presence of nanotwins and solid solution of alloying elements while only the structural refinement by fine plate structures contributes to hardening in the HAZ.Comparisons between both the LSTed specimens reveal that increasing the laser power from 100 W to 200 W can effectively enlarge the laser-modified zones(both the MZ and the HAZ)and simultaneously refine plate structures,leading to further hardness increase.