Cold-Spray Coating of AlCrCoFeNi High Entropy Alloy on F-0008 Powder Metal Part

High-entropy alloys (HEAs) are presently of great research interest in materials science and engineering. HEAs typically contain at least five elements with equimolar or near equimolar concentrations. Because of their high-entropy effects, HEAs have the potential to be used in many applications such as high temperature materials, cryogenic materials, wear resistance, diffusion barriers, etc. [1-3] HEAs typically contain at least five elements with equimolar or near equimolar. Main methods for strengthening high entropy alloys are displacement solution strengthening and interstitial solution strengthening. Adding elements with large atomic radius results in lattice distortion and therefore presents displacement solution strengthening. However, the increase of lattice strain and lattice defect leads to toughness decrease of the alloy. Interstitial solid solution can improve the strength and hardness of high entropy alloy through forming solid solution of small atomic elements in the alloy. Due to the small size of interstitial atoms, the lattice distortion is larger than that of displacement solution strengthening [4,5]. Based on their considerable structural and functional potential as well as richness of design, HEAs are promising candidates for applications in powder metallurgy (PM). In this study, cold-spray coating was used for surface coating of AlCrCoFeNi onto iron-based PM part, F-0008, for surface modification and corrosion protection. Cold spray (CS) process is a solid-state deposition process in which particles at temperatures well below their melting point are accelerated to supersonic velocities by pressured gas and impinge on a substrate and/or previously deposited particles to develop coatings/parts [6-8]. The bonding relies on the kinetic energy of the particles rather than thermal, as is the case in thermal spray processing [9,10]. The cold spray deposition of the AlCoCrFeNi2.1 high entropy alloy (HEA) was performed using a WarpSPEE3D system developed by SPEE3D (Melbourne, Australia). The deposition work was conducted at HAMR Industries LLC (Pittsburgh, PA), an industry partner supporting this university-led project. This high-throughput additive manufacturing machine is capable of supersonic deposition via cold spray, using a De Laval-type nozzle to accelerate powdered feedstock in a heated compressed gas stream. The machine includes a 6-axis robotic arm that enables complex tool path programming and precise deposition over varied geometries. Three distinct tool path strategies were evaluated to study their effects on coating uniformity, edge coverage, and microstructural consistency. These were programmed within SPEE3D’s proprietary software, TwinSPEE3D. Process parameters such as hatch spacing (0.50 – 1.00 mm), traverse speed (16-100 mm/s), and nozzle standoff distance (10-25 mm) were optimized for coating effectiveness and quality. For this study, compressed air at 700 ˚C and 3 MPa pressure was used for propelling the powder particles. The substrates used were F-0008 powdered metal rectangular bars. Prior to deposition, the specimens were prepared by lightly abrading the surface with a Scotch-Brite™ general-purpose hand pad containing aluminum oxide, followed by cleaning with acetone and drying with compressed air. These steps ensured a clean, mechanically interlocked surface for improved coating adhesion. The feedstock used was spherical powder of equimolar AlCoCrFeNi2.1 high entropy alloy with particle size ∼25 μm. Cold sprayed parts/substrates were heat treated at 900 ˚C for 2 hours to improve structural integrity of the coating. The specimens for microstructural characterization were mechanically polished and etched with 2% Nital. The specimens were also microstructurally characterized using a Joel JSM-7200F Field emission scanning electron microscope (SEM) and energy dispersive spectroscopy (EDS). Fig. 1 shows the through-thickness microstructure across the interface between the HEA coating and the substrate (F-0008). Although there was a clear bonding adhesion at the interface, the microstructure morphology at the interface was quite different from that of the coating. The EDS analysis confirmed metallurgical bonding at the interface as shown in Fig. 2b. From the EDS mapping in Fig. 2b, it can be deduced that the rough morphology of the interface is due to the melting of the Aluminum from the HEA. Aluminum with a low melting temperature (660 ˚C compared to other elements (∼1,500 ˚C) melts, due to a high gas temperature (700 ˚C) and impact energy, forming oxide at the interface. This is the reason for a high Al content (Fig. 1b) at the interface compared to other areas of the coating. Additionally, the larger atom size of Al (0.143 nm) compared to Fe and other elements (∼0.125 nm) may contribute to weak and sluggish diffusion of Al resulting to its higher content at the interface. The results from this study have clearly shown that cold-spray coating is a potential surface coating method for powder metal parts, and may provide a viable solution for clean and environmentally friendly production methods for applications such as corrosion prevention, wear resistance surfaces, etc. However, optimal processing parameters/conditions such as gas temperature, traverse speed, nozzle standoff distance, have to be determined to ensure better coating quality [11]. SEM and EDS characterization of the coating showing (a) through-thickness microstructure and morphology of the interface, and (b) elemental mapping across the interface.