Contribution of Different Strengthening Effects in Particulate-Reinforced Metal Matrix Nanocomposites Prepared by Additive Manufacturing

Laser assisted additive manufacturing (LAAM) is regarded as a complementary manufacturing method to traditional manufacturing technologies. Meantime, improving the mechanical performance of components fabricated by LAAM is an important research focus in recent years and it has drawn significant attention from both industrial and research aspects. In the present study, in order to obtain high-performance metal components by LAAM, nano-TiC particles are used to reinforce Inconel 718 and the mixed raw powder is processed by selective laser melting (SLM) technique. To investigate the effect of TiC amount on the property and performance of the composite, samples with four levels of nano-TiC addition (0, 0.4, 0.8 and 1.6 wt.%) are prepared, all other manufacturing parameters are set fixed. Furthermore, standard solid solution treatment at 980 °C for 1 hour is carried out to investigate its effect on the final properties. SEM observations are performed to analyze the microstructure of the composites. In addition, to understand the reinforcing mechanism of nano particles in LAAM-produced metal composites at both as-built and heat treated state, we consider four main strengthening mechanisms, (a) load-bearing effect, (b) enhanced dislocation density due to the residual plastic strain caused by the difference in the coefficients of thermal expansion (CTE) between the matrix and reinforcing particles, (c) Orowan strengthening effect, and (d) Hall-Petch strengthening. The effect of TiC nano particle amount on each of the four strengthening mechanisms is investigated separately and the results show that within the investigated range, the increase of reinforcement content leads to higher tensile strength. With 1.6 wt.% reinforcement, the ultimate tensile strength increases by 15%. At as-built condition, the composites have the maximum yield strength (YS) and ultimate tensile strength (UTS), while for solution treated samples, the tensile strengths are overall lower due to microstructure coarsening. Through quantitative investigation, it is found that both as-built and solution treated conditions, the load-effect strengthening effect is very small as compared with other contributors. Thermal mismatch strengthening effect is most significant at any volume fraction under as-built condition, mainly due to high SLM temperature. However, for solution-treated condition, CTE mismatch strengthening is weakened because solution treatment significantly equilibrates the thermal strain in the composite, and diminishes most strain-induced dislocations. However, Hall-Petch strengthening becomes dominating as large amount of nanoparticles effectively inhibit the grain coarsening during solution treatment.