Titanium alloys exhibit excellent biocompatibility, corrosion resistance, and specific strength, making them ideal materials for medical implants and prostheses such as dental implants and joint replacements. However, titanium alloys application in severe environments is limited owing to their poor surface wear resistance. Nitriding is a common method to enhance the performance of titanium alloys. However, for complex structural parts, the traditional nitriding process is cumbersome and complex, and the thickness of the resulting nitrided layer makes it difficult to achieve the desired effect. To meet the demand for improving the wear resistance of TC4 alloy in the fields of aerospace, medical devices, and bio-implants, some scholars have proposed a new method of simultaneous nitriding in additive manufacturing. By combining the advantages of additive manufacturing and laser gas nitriding, high-performance titanium alloys with larger nitrided areas can be obtained while increasing the processing efficiency. At present, research on synchronous nitriding in the additive manufacturing process has achieved some results; however, there have been few studies on the effect of nitrogen volume fraction on the organization and properties of titanium alloys during the forming process of selective laser melting (SLM). As a result, simultaneous SLM nitriding is achieved through selective laser melting under atmospheres with different nitrogen volume fractions. This microstructure and properties of SLM-formed TC4 alloys with different nitrogen gas volume fractions are systematically analyzed ,and the role of nitrogen in the SLM process of TC4 alloy is explored, providing a research basis for further nitriding technology development.

The substrate is sanded and cleaned to remove surface impurities. Subsequently, high-purity nitrogen and argon are passed into the high-pressure gas supply proportioning system in different ratios for mixing, and the mixed gases are then passed into the atmosphere protection box to provide a processing environment with different nitrogen volume fractions for the subsequent selective laser melting. The morphology, organization, physical phase, and elements of SLM-formed specimens are analyzed using optical microscope (OM), scanning electron microscope (SEM), X-ray diffractometer (XRD) and energy dispersive spectrometer (EDS) to determine the effect of the nitrogen volume fraction on the microstructure of the formed specimens. Based on this, the nitrogen mechanism in the SLM process is discussed.

In this study, the effect of different nitrogen volume fractions on the microstructure of SLM-formed TC4 alloys is investigated. The experimental results show that when the nitrogen volume fraction is 25%, the microstructure does not show obvious changes, and the nitrogen element primarily exists in the form of a solid solution. With a further increase in nitrogen volume fraction, the titanium nitride organization appears and gradually increases, and its morphology is transformed into dendritic crystals (Fig. 4). Subsequently, the microhardness, wear resistance, and corrosion resistance of the formed specimens with different nitrogen volume fractions are analyzed, and it is discovered that the microhardness gradually increases with increasing nitrogen volume fraction (Fig. 8). However, its abrasion resistance does not increase with increasing nitrogen volume fraction, and its abrasion resistance under 100%N₂, 25%N₂, 50%N₂,75%N₂, and 0%N₂ is from the largest to the smallest (Fig. 9). The corrosion resistance is analyzed, and it is discovered that the corrosion current density of the specimens with nitrogen volume fractions of 25% and 50% is reduced by 29.4% and 56.1%, respectively, compared to that without nitrogen, indicating an improvement in the corrosion resistance. When the nitrogen volume fraction is increased further, the corrosion resistance begins to decrease (Fig. 11), owing to its higher hard phase content, which increases material brittleness and causes a large residual stress. In the process of electrochemical corrosion, microcracks are produced, and autocatalytic corrosion emerges. Finally, the mechanism of nitrogen action in the forming process is analyzed. It is shown that the pre-positioned TC4 alloy powder is nitrided under the action of the temperature field generated by SLM molding, and the pre-nitrided TC4 alloy powder melts into the specimen. In addition, nitrogen gas permeates from the gas/liquid interface to the molten pool surface, where it reacts with titanium to form nitrides. Driven by convection within the molten pool, these nitrides are subjected to mass transfer towards the interior of the molten pool (Fig. 14).

In this study, synchronous nitriding is achieved during SLM of a TC4 alloy in atmospheres with varying nitrogen volume fractions. Subsequently, an in-depth analysis of the microstructures, physical phases, and properties of the specimens fabricated with different nitrogen volume fractions is performed. In addition, the role of nitrogen gas during the SLM formation process is investigated. The results show that as the nitrogen volume fraction increases from 0 to 25%, the β-pillar crystals gradually decrease, and the nitrogen element mostly exists in the form of a solid solution. When the nitrogen volume fraction is increased from 50% to 100%, the titanium nitride organization appears and gradually increases, and the morphology changes to dendritic. In terms of properties, the TC4 alloy microhardness gradually increases with increasing nitrogen volume fraction. A moderate increase in nitrogen volume fraction improves the wear resistance of SLM-formed TC4 alloys, whereas an excessively high nitrogen volume fraction reduces their corrosion resistance. To ensure that the SLM-formed TC4 alloy has good wear and corrosion resistance, the nitrogen volume fraction should be controlled in the interval of 25%?50%. Nitrogen plays a role in the SLM forming process of the TC4 alloy in two ways: first, under the influence of the forming temperature field, it causes the pre-positioned TC4 alloy powder to undergo nitriding and the pre-nitrided powder melts and enters into the alloy; second, it permeates from the gas/liquid interface into the melt pool, achieving inward mass transfer with the help of the melt pool convection.