CoCrFeNi-based high-entropy alloys have received extensive research attention owing to their outstanding performance. By adding Al elements, the alloy properties can be further optimized to produce lighter high-entropy alloys, which show great potential for research and application in military, aerospace, nuclear industry protection, and many other fields. However, harsh acidic environments, such as acidic wastewater and acid rain, cause significant industrial corrosion, severely threatening the service performance of materials. Therefore, it is essential to enhance the corrosion resistance of the material to acidic conditions while ensuring its strength. In recent years, selective laser melting (SLM) technology has been widely used to prepare high-entropy alloy components towing to its desirable characteristics such as rapid forming, high precision, and good density. SLM is a layer-by-layer construction process that involves rapid cooling and heating cycles, which can easily lead to the accumulation of residual thermal stresses within the material. Studies have shown that heat treatment can improve the residual stress in alloys, thereby enhancing their mechanical and corrosion resistance properties. In this study, an AlCoCrFeNi high entropy alloy was prepared via SLM and post-processed under different heat treatment temperatures to reveal the influence of different heat treatment temperatures on the microstructure, mechanical properties, and corrosion resistance in H₂SO₄ solution with concentration of 0.5 mol/L.

Pre-alloyed AlCoCrFeNi high-entropy powder, with a near-atoms ration, was prepared via aerosol atomization. The stainless steel measuring 200 mm×200 mm×200 mm was selected as experimental substrate. The SLM forming AlCoCrFeNi high entropy alloy experiment used a laser system. Based on the preliminary process exploration, the determined optimal experimental parameters were as follows: laser power of 200 W, scanning speed of 1000 mm/s, layer thickness of 50 μm, spot diameter of 85 μm, scan rotation angle between adjacent layers of 67°, scan spacing of 80 μm, overlap rate of 50%, and the chamber was filled with protective gas (argon) during processing. A resistance furnace was used for the heat treatment process of the samples; a diffractometer was employed to analyze the microstructure composition of the alloy; a scanning electron microscope (SEM) was utilized for microscopic characterization of the samples; an energy dispersive spectrometer (EDS) was used to characterize and analyze the elemental distribution and composition in the alloy structure; the fully automatic Vickers hardness tester was used to measure the surface hardness of the samples before and after heat treatment; the diffractometer was used to analyze the residual stress values of the alloy samples; at room temperature, an electrochemical system workstation was used to perform electrochemical corrosion tests on the samples in a H₂SO₄ solution with concentration of 0.5 mol/L as the corrosive medium, and the SEM was used to observe the corrosion morphology.

The alloy exhibits good phase stability at 650 ℃. When the heat treatment temperature is increased to 850 ℃, the phase transitions from a single body-centered-cubic (BCC) phase to a BCC+ face-centered-cubic (FCC) biphasic solid solution structure. As the heat treatment temperature rises, the intensity of the FCC phase diffraction peak gradually increases, and following high-temperature heat treatment, the diffraction peak shifts toward smaller angles (Fig.2) Following high temperature heat treatment, the microstructure of the alloy is composed of a BCC phase and precipitated phase FCC (Fig.3). The elements exhibit the enrichment phenomenon (Fig.4). As the heat treatment temperature increases, the hardness and residual stress of alloy show a more obvious downward trend (Fig.6). The Nyquist curves of the original alloy, 650 ℃, and 850 ℃ heat-treated alloys basically show complete capacitive arcs, while those of 1050 ℃ and 1250 ℃ heat-treated alloys exhibit incomplete capacitive arcs. As the heat treatment temperature increases, the impedance value and phase angle of the alloys gradually increase, enhancing their corrosion resistance (Fig.7). White corrosion products and pitting are evident on the surface of the original alloy and 650 ℃ heat-treated alloy. As the heat treatment temperature increases, the corrosion products on the surface of the alloy gradually decrease, and only a large number of uniform pitting exists (Fig.9).

This study employs selective laser melting technology to prepare AlCoCrFeNi high entropy alloys and investigates the effects of heat treatment temperature on the microstructure, mechanical properties, and electrochemical corrosion behavior in a H₂SO₄ solution with concentration of 0.5 mol/L. Microcracks are present within the melt pool and at the boundaries of SLM-formed AlCoCrFeNi high entropy alloys, which consist of a single BCC phase. The alloy exhibits good phase stability following heat treatment at 650 ℃. As the heat treatment temperature increases, the SLM-formed AlCoCrFeNi high entropy alloy transitions from a single BCC phase to a biphase solid solution structure of BCC+FCC, with the surface microhardness decreasing to 336 HV and residual stresses being relieved. The alloy treated at 1250 ℃ has optimal corrosion resistance.