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Hybrid ex-situ/in-situ reinforced titanium matrix composites (TMCs) were fabricated by selective laser melting (SLM). The optimized pre-processed 5 wt% B 4 C/Ti-6Al-4V composite powder feedstock and the un-reinforced Ti-6Al-4V powder were consolidated using energy densities in the range of 50–75 J/mm3. Despite the full melting of the powder particles in the monolithic Ti-6Al-4V system, complete melting of the host Ti-6Al-4V constituent in the composite case took place by energy densities exceeding 62.5 J/mm3. Presence of the guest B 4 C particles surrounding the un-melted/partially melted host particles gave evidence of the non-efficient guest-to-host heat transfer. In-situ formation of (TiB + TiC) reinforcements was discussed based on a mechanism proposing dissolution rather than melting of the guest particles. The degree of dissolution was a significant function of the energy density and the guest particle size. Microstructural evolutions during SLM of 5 wt% B 4 C/Ti64 composite were studied, and the non-equilibrium solidification sequence was suggested based on the microstructural observations and the equilibrium solidification path. High cooling rates during SLM inhibited some of the liquid and solid-state transformations in the TMCs. This was confirmed by microstructural observations of the arc-melted parts fabricated by the same composite feedstock. The SLM processed TMCs showed 30–80% enhancement in microhardness compared to the unreinforced Ti64. Unlabelled Image • SLM of 5wt.% B 4 C/Ti-6Al-4V composite powder was studied and compared with the monolithic unreinforced Ti-6Al-4V. • TiB and TiC reinforcements were formed through the dissolution of guest B 4 C particles rather than their melting. • Non-equilibrium solidification sequence showed deviation from equilibrium conditions. • The fabricated composites showed 30–80% enhancement in microhardness depending on the laser energy density. [ABSTRACT FROM AUTHOR]

In this study, fatigue, corrosion fatigue and electrochemical behaviors of butt-welded A106-B steel pipes were investigated. These pipes were subjected to mechanical and thermal cyclic stresses due to the internal pressure fluctuations and cooling–heating cycles. Residual stress measurements were carried out for three different depths through the thickness and were correlated to the microstructural observations and microhardness measurements. Results showed that the maximum tensile residual stresses existed at the boundary of weld metal and base metal because of the contraction–expansion effects resulting from phase transformations and cooling rate differences. Fatigue fractures occurred at regions experiencing the maximum tensile residual stress under fatigue and corrosion fatigue tests. The change in frequency had no significant effect on the atmospheric fatigue. However, lower frequencies were associated with shorter corrosion fatigue lives in the corrosive environment containing Cl − ions. The corrosion potentials for different regions of weld zone were close to each other while corrosion current density and corrosion rate varied significantly. [ABSTRACT FROM AUTHOR]

This study aims at a comprehensive investigation of residual stresses in laser powder bed fusion processed H13 tool steel through experiment and part-level simulation. Twin-cantilever beams and cubic coupons were printed in a wide range of process parameters to characterize residual stresses under preheated and non-preheated conditions. Results revealed the significant role of martensitic phase transformation, process parameters, densification level, and preheating condition on the residual stress regime and beam deflections. The residual stress regime was governed by the competition between the tensile and compressive stresses caused by the solidification/cooling contraction and martensitic phase transformation, respectively. The compressive residual stresses on the top surface turned into tensile immediately below the last consolidated layer. Higher heat inputs led to the increased magnitude of compressive residual stresses on the top surface of non-preheated samples. For parts having relative densities higher than 97%, the residual stresses were found to be independent of the densification level. Deflection modeling results indicated that for steels experiencing martensitic phase transformation, the martensite start temperature needs to be set as the relaxation temperature to accurately predict the beam deflection. This study proves that 200 °C of preheating eliminates not only cracks but also the need for stress-relief post-processing. [Display omitted] • Martensitic transformation left behind compressive residual stresses which turned into tensile upon adding next layer. • Top surface residual stresses grew more compressive by increasing the volumetric energy density in the non-preheated case. • A great agreement between experimental and simulation results was achieved upon selecting a proper relaxation temperature. • 200 °C preheating was found to eliminate the need for costly and time-consuming stress-relief post-processing treatments. [ABSTRACT FROM AUTHOR]

This study investigates the feasibility of fabricating defect-free functionally graded bi-materials (FGMs) with enhanced wear resistance via incorporation of vanadium carbide (VC) into H13 tool steel. Three distinct composite powders containing 1, 3, and 5 wt%VC were prepared through ball-milling and subjected to laser powder bed fusion (LPBF) process to print different composites on top of monolithic H13 in a wide range of process parameters. Almost fully-dense parts were achieved (maximum of 99.8, 99.8, and 99.5% for 1, 3 and 5 wt%VC composite systems, respectively); however, the increase in VC content narrowed down the processability window range from 60 J/mm3 for 1, and 3wt%VC systems to 30 J/mm3 for 5 wt%VC system. The mechanical properties of optimum samples were characterized through microhardness, nanohardness, and wear tests. The incorporation of VC significantly improved the mechanical properties, 17–40% in microhardness, 10–40% in nanohardness, and 20–53% in wear resistance. The underlying reasons behind such an improvement were correlated to the dissolution of VC during the heating stage of the LPBF process and the formation of (V + C)-supersaturated solid solution in large extents as a result of extremely high cooling rates. This study introduces LPBF-processed FGMs as promising candidates for applications in which wear resistance is paramount. Unlabelled Image • Improving the wear resistance of H13 tool steel by incorporation of VC was thoroughly investigated. • Almost defect-free functionally graded VC-H13 containing 1 to 5wt.% VC was successfully fabricated via LPBF process. • 17-40%, 10-40%, and 20-53% improvement in microhardness, nanohardness, and wear resistance, respectively were achieved. • Dissolution of VC increased supersaturation of V and C in the matrix and significantly enhanced the wear resistance. [ABSTRACT FROM AUTHOR]