Impact of laser process parameters in direct energy deposition on microstructure, layer characteristics, and microhardness of TC21 alloy

Abstract

In the present study, layers consisting of 40% Stellite-6 and 60% WC were deposited on Ti-6Al-3Mo-2Sn-2Zr-2Nb-1.5Cr-0.1Si (TC21) alloy by means of direct energy deposition (DED) technology aiming to improve the microstructure and microhardness. Five powder feeding rates ranging from 20 to 100 ɡ min−1 were applied using CW fiber-coupled diode laser with 4 kW output power. The deposited layers were analyzed via scanning electron microscopy (SEM), energy-dispersive X-ray analysis (EDX), and X-ray diffractometry (XRD). The results show that WC particles are dispersed in a heterogeneous manner in the deposition zone, especially at the rates 20, 40, and 60 ɡ min−1. In addition, microcracks appeared in the interface zone particularly at 100 ɡ min−1 due to the higher induced residual stresses caused by the difference in the coefficient of thermal expansion between Stellite-6, WC particles, and TC21 substrate alloy. Several complex carbides and intermetallic compounds such as W2C, TiC, Cr7C3, Co3W3C, and Co25Cr25W8C2 were detected in the deposited layers depending on the powder feeding rate. With further increase in the powder feeding rate, the fractions of W2C and the bulk (unmelted) WC particles were increased and that of the TiC particle was reduced correspondingly due to the thermal diffusion. The layer thickness increased from 1.3 to 2.7 mm when the powder feeding rate increased from 40 to 100 ɡ min−1, while the dilution ratio decreased from 23 to 5.3% as a result of the thermal diffusion of the laser energy. The microhardness of the composite was found to be three times higher than that recorded for the TC21 substrate (1020 vs. 340 HV0.05). The results revealed that the best homogeneous microstructure with the highest microhardness was achieved at the powder feeding rate of 100 ɡ min−1 whereas microcracks free layers were accomplished at 40 ɡ min−1.