Microstructures and mechanical properties of multi-layered materials composed of Ti-6Al-4V, vanadium, and 17-4PH stainless steel produced by directed energy deposition

Abstract

The joining of dissimilar Ti alloy and stainless steel (SS) is challenging but necessary for the extensive appli-cations of Ti alloy/SS hybrid materials. Consequently, joints formed by the sequential laser deposition of V and 17-4PH SS on Ti-6Al-4V and V and Ti-6Al-4V on 17-4PH SS were investigated. The effect of the laser power (150, 180, 210, and 300 W) on the properties of the joints was also investigated to determine the optimal conditions for the deposition of the V interlayer. The microstructures and mechanical properties of the joints were compared with those of similar Ti-6Al-4V/V/17-4PH SS joints produced by welding. The joints produced on Ti-6Al-4V substrates using laser powers of 180 (VTi180) and 210 W (VTi210) were crack-free and consisted mainly of body-centered cubic phases throughout their Ti-6Al-4V/V/17-4PH SS microstructures. The joints produced on 17-4PH SS substrates using laser powers of 180 (VFe180) and 210 W (VFe210) had regions with cracks near their 17-4PH SS/V interfaces, owing to the presence of the brittle Fe-V sigma phase. The joints produced with V deposited using a low laser power of 150 W or a high laser power of 300 W delaminated. The tensile strengths of VTi180, VTi210, VFe180, and VFe210 were 575, 700, 520, and 470 MPa, respectively, markedly exceeding those of most similar reported welded joints. Narrow fusion zones, negligible heat-affected zones, and the presence of compositionally graded interlayers that cause interlayer strengthening accounted for the high strengths of the additively manufactured joints. The comparatively high Ti content of the V interlayer of the VTi210 joint accounted for its extraordinary tensile strength (700 MPa) and relatively high hardness. The lower strengths of VFe180 and VFe210 were attributed to the presence of brittle interlayer regions formed by the incorporation of Fe. Therefore, the use of V in the Fe-V-Ti sequence is not recommended.