Objective The 24CrNiMo alloy steel has good strength and toughness matching, as well as thermal stability, making it a suitable material for manufacturing high-speed train brake discs. Traditional alloy steel brake disc manufacturing techniques have a complex manufacturing procedure and a long processing cycle, which cannot meet the increasingly complex design requirements. Selective laser melting allows for the free design and production of parts with complex structures, high forming precision and good surface quality, significantly shortening the product development and production cycle. The fabrication of the 24CrNiMo alloy steel using advanced selective laser melting technology has some research value. The process parameters of the selective laser melting technology are critical to the brake disc’s performance. Scanning strategy is an important process parameter for selective laser melting (SLM) brake disc manufacturing. This study aims to analyse the effect of scanning strategy on the microstructure and thermal fatigue performance of SLM formed parts of 24CrNiMo alloy steel. During long-term service, high-temperature thermal fatigue will crack the brake disc. When the crack reaches a certain length, the brake disc will fail. It is necessary to investigate and evaluate the high-temperature performance of SLM formed parts made of 24CrNiMo alloy steel before they can be used in actual production.

Methods To analyse the microstructure and properties of SLM parts under different scanning strategies, EP-M250 selective laser melting equipment was used to fabricate 24CrNiMo alloy steel samples under four scanning strategies: 0° linear scanning, 45° rotating scanning, 90° rotating scanning, and 67° rotating scanning. The Archimedes drainage method was used to determine the density of samples under various scanning strategies. The optical microscope and scanning electron microscope were used to analyse the microstructure of the formed parts. For phase analysis of the formed parts using different scanning strategies, an X-ray diffractometer was used. The scanning strategy’s effect on the microstructure of formed parts was investigated. A thermal fatigue test device was used to evaluate the thermal fatigue performance of the formed parts with different scanning strategies on a flat specimen with a V-shaped notch.

Results and Discussions The grain orientation distribution reflects the effect of the laser scanning strategy on the microstructure of SLM samples. The grain growth under the 0° linear scanning strategy has a strong orientation when compared to the rotating scanning strategy. The grains growing along each orientation intersect at the centre of the molten pool during solidification, and the microstructure boundary is formed at the centre of the molten pool [Fig.7 (a) and (c)]. The rotation scanning strategy shifts the direction of heat dissipation between adjacent layers, disrupting grain epitaxial growth. The grain orientations are random, and the texture is poor [Fig. 7 (b) and (c)]. When the 0° linear scanning path is parallel to the direction of the thermal fatigue notch, the molten pool’s centre has high microstructure heterogeneity and becomes a weak area, and the thermal fatigue specimen has a high crack growth rate. Under rotating scanning strategy, the grain orientation is random, and the formed part had no obvious microstructure boundary and molten pool weak zone, which hinders thermal fatigue crack growth. Under different cycles, the rotating scanning strategy sample has a lower thermal fatigue crack growth rate than the 0° linear scanning sample (Fig. 10).

Conclusions In this study, the 24CrNiMo alloy steel was fabricated by SLM technology. The effects of scanning strategy on microstructure, phase composition and thermal fatigue properties of the formed parts were studied. The change of scanning strategy changes the morphology of the molten pool. Under different scanning strategies, the microstructure of SLM formed part of 24CrNiMo alloy steel consists of granular bainite, martensite, and residual austenite. The phase compositions of the formed parts using various scanning strategies are alpha-Fe with a trace of gamma-Fe. The grains have a strong orientation when using the 0° linear scanning strategy. The rotation scanning strategy has a crushing effect on the crystal grain’s prolonged growth, and the orientation is weakened. When the laser scanning path is parallel to the notch direction of the thermal fatigue samples, the crack length increases fastest and finally reaches 1162 μm. The molten pool’s centre has a high degree of microstructure heterogeneity, making it easy for the thermal fatigue crack to spread. Crack propagation is hampered by the random distribution of grain orientation in rotating scanning mode. Thermal fatigue crack propagation is caused by the combined action of thermal stress and high-temperature oxidation.