Surface stiffness gradient in Ti parts obtained by laser surface alloying with Cu and Nb

Titanium alloy components with a stiffness gradient have attracted technological interest for biomedical and structural applications. A stiffness gradient can be achieved by generating a chemical gradient or through localized phase transformation. The laser surface alloying technique can be used to fabricate parts with stiffness and hardness gradients by introducing a compositional gradient at the surface of the component. In this work, the surface of commercially pure titanium was modified by laser surface alloying, employing Nb or Cu as alloying elements. Single laser tracks were obtained using different heat inputs. The fusion zone of the laser tracks was characterized by scanning electron microscopy, X-ray diffraction and nanoindentation. The alloying content in the fusion zone decreased in response to increasing heat input. Independently of the heat input, the fusion zone of the Cu-alloyed laser tracks contained α phase dendrites, Ti2Cu, α/α′ phase lamellae, and the α-Ti+Ti2Cu eutectoid constituent. At a low heat input, the higher Nb content introduced on the modified surface sufficed to metastabilize the α″ and β phases, while α′ was predominantly present in the laser tracks produced at higher heat inputs. All the laser tracks presented significantly increased hardness, while the change in Young's modulus was dependent on the added alloying element and the processing parameters. The Young's modulus of the Nb-alloyed laser track obtained at 200W was 30% lower than that of the substrate, while all the other conditions produced a stiffer surface region. Hence, a harder but less rigid coating can be obtained by this route.