Controlling grain structure in metallic additive manufacturing using a versatile, inexpensive process control system

Additive manufacturing (AM), commonly termed 3D printing, is a revolutionary manufacturing technology with great industrial relevance in the aerospace, medical and automotive sectors. Metallic AM allows creation of complex intricate parts and repair of large components; however, certification is currently a concern due to lack of process consistency. A versatile, inexpensive process control system was developed and integrated, reducing variability in melt pool fluctuation and improving microstructural homogeneity of components. Remnant microstructural variation can be explained by the change in heat flow mechanism with geometry. The grain area variability was reduced by up to 94% at a fraction of the cost of a typical thermal camera, with control software written in-house and made publically available. This decreases the barrier to implementation for process feedback control, which can be implemented in many manufacturing processes, from polymer AM to injection moulding to inert-gas heat treatment

Phase stability of the AlxCrFeCoNi alloy system

The addition of Al to the A1 CrFeCoNi alloy has been shown to promote the formation of intermetallic phases, offering possibilities for the development of alloys with advantageous mechanical properties. However, despite numerous experimental investigations, there remain significant uncertainties as to the phase equilibria in this system particularly at temperatures below 1000°C. The present study makes a systematic assessment of the literature data pertaining to the equilibrium phases in alloys of the AlxCrFeCoNi system. Two alloys, with atomic ratios, x = 0.5 and 1.0, are then selected for further experimental investigation, following homogenisation (1200°C/72 h) and subsequent long-duration (1000 h) heat-treatments at 1000, 850 and 700°C. The Al0.5 alloy was found to be dual-phase A1 + B2 in the homogenised condition and following exposure at 1000°C but D8b phase precipitates developed following heat-treatment at the lower temperatures. In the Al1.0 alloy, B2, A2 and A1 phases were identified in the homogenised condition and at 1000°C. At 850 and 750°C, the A2 phase was replaced by the D8b phase. These experimental observations were used alongside literature data to assess the veracity of CALPHAD predictions made using the TCHEA4 thermodynamic database

Design and selection of high entropy alloys for hardmetal matrix applications using a coupled machine learning and CALPHAD methodology

This study aims to utilize a combined machine learning (ML) and CALculation of PHAse Diagrams (CALPHAD) methodology to design hardmetal matrix phases for metal-forming applications that can serve as the basis for carbide reinforcement. The vast compositional space that high entropy alloys (HEAs) occupy offers a promising avenue to satisfy the application design criteria of wear resistance and ductility. To efficiently explore this space, random forest ML models are constructed and trained from publicly available experimental HEA databases to make phase constitution and hardness predictions. Interrogation of the ML models constructed reveals accuracies >78.7% and a mean absolute error of 66.1 HV for phase and hardness predictions respectively. Six promising alloy compositions, extracted from the ML predictions and CALPHAD calculations, are experimentally fabricated and tested. The hardness predictions are found to be systematically under- and overpredicted depending on the alloy microstructure. In parallel, the phase classification models are found to lack sensitivity toward additional intermetallic phase formation. Despite the discrepancies identified between ML and experimental results, the fabricated compositions show promise for further experimental evaluation. These discrepancies are believed to be directly associated with the available databases but, importantly, have highlighted several avenues for both ML and database development

Microstructural evaluation of thermal-sprayed CoCrFeMnNi0.8V high-entropy alloy coatings

The aim of this work is to improve the understanding of the effect of the cooling rate on the microstructure of high-entropy alloys, with a focus on high-entropy alloy coatings, by using a combined computational and experimental validation approach. CoCrFeMnNi0.8V coatings were deposited on a steel substrate with high velocity oxy-air-fuel spray with the employment of three different deposition temperatures. The microstructures of the coatings were studied and compared with the microstructure of the equivalent bulk high-entropy alloy fabricated by suction casting and powder fabricated by gas atomization. According to the results, the powder and the coatings deposited by low and medium temperatures consisted of a BCC microstructure. On the other hand, the microstructure of the coating deposited by high temperature was more complex, consisting of different phases, including BCC, FCC and oxides. The phase constitution of the bulk high-entropy alloy included an FCC phase and sigma. This variation in the microstructural outcome was assessed in terms of solidification rate, and the results were compared with Thermo-Calc modelling. The microstructure can be tuned by the employment of rapid solidification techniques such as gas atomization, as well as subsequent processing such as high velocity oxy-air-fuel spray with the use of different spray parameters, leading to a variety of microstructural outcomes. This approach is of high interest for the field of high-entropy alloy coatings

Bending bad—testing caramel wafer bars (#TestATunnocks)

During the coronavirus pandemic, there have been significant challenges in the remote teaching and demonstration of experiments, especially those that require laboratory testing equipment. With a desire to give students a feel for our materials laboratory on open days and allow them to gain a deeper understanding of what materials science and engineering is about, we have designed an experiment focused on composite materials that can be performed remotely and without specialist equipment. This enabled students to experience a bend test sensorily through seeing, hearing and feeling it, creating a strong link to then being able to relate it to the pre-prepared experimental data taken in the laboratory. This fun, easy-to-run and engaging experiment allowed a shared experience and encouraged a discussion about students' observations, differences in results and implications of the bend strength of sandwich composites. We have found it not only works well universally by all ages but can be used with younger children to think about words such as 'stronger', 'stiffer' and 'flexible' and how materials can be different in different directions

The microstructural evolution of CM247LC manufactured through laser powder bed fusion

Numerous challenges persist with the additive manufacturing of high γ′ containing Ni-based superalloys such as CM247LC. Currently, significant cracking occurs during deposition of CM247LC components using laser powder bed fusion and during post-processing. Whilst post-deposition procedures seek to eliminate or minimise cracks, current procedures do not produce a microstructure suitable for service. This study systematically investigates the microstructural evolution of CM247LC manufactured using laser powder bed fusion following multiple post processing treatments. Phase and textural changes after each processing step were consistent with previous studies, although an additional Hf-rich and Cr-depleted segregation zone was identified along intercellular boundaries in the as-deposited condition, believed to be associated with the cracking propensity. Compositional modification of CM247LC including removal of Hf, reduction of C and addition of Nb eliminated the segregation zone but these changes were associated with an increased susceptibility to solidification and liquation cracking

On the Effect of Nb on the Microstructure and Properties of Next Generation Polycrystalline Powder Metallurgy Ni-Based Superalloys

Abstract The effect of Nb on the properties and microstructure of two novel powder metallurgy (P/M) Ni-based superalloys was evaluated, and the results critically compared with the Rolls-Royce alloy RR1000. The Nb-containing alloy was found to exhibit improved tensile and creep properties as well as superior oxidation resistance compared with both RR1000 and the Nb-free variant tested. The beneficial effect of Nb on the tensile and creep properties was due to the microstructures obtained following the post-solution heat treatments, which led to a higher γ′ volume fraction and a finer tertiary γ′ distribution. In addition, an increase in the anti-phase-boundary energy of the γ′ phase is also expected with the addition of Nb, further contributing to the strength of the material. However, these modifications in the γ′ distribution detrimentally affect the dwell fatigue crack-growth behavior of the material, although this behavior can be improved through modified heat treatments. The oxidation resistance of the Nb-containing alloy was also enhanced as Nb is believed to accelerate the formation of a defect-free Cr2O3 scale. Overall, both developmental alloys, with and without the addition of Nb, were found to exhibit superior properties than RR1000.This work was supported by the Rolls-Royce/EPSRC Strategic Partnership under EP/H022309/1, EP/H500375/1 and EP/ M005607/1

The influence of Fe variations on the phase stability of CrMnFexCoNi alloys following long-duration exposures at intermediate temperatures

The equiatomic CrMnFeCoNi alloy exhibits many desirable properties, but its susceptibility to the formation of embrittling intermetallic phases, makes it unsuitable for structural applications at elevated temperatures. As a result, there has been increasing interest in developing alternative alloys from the CrMnFeCoNi system that avoid this limitation. Here we present a detailed study of phase stability in a CrMnFexCoNi series of alloys, where x = 0, 0.5, 1.5 (in atomic ratio), following long-duration heat treatments of 1000 h at 900 and 700 °C, and up to 5000 h at 500 °C. Each alloy was single phase fcc following homogenisation. After exposure at 900 °C, large σ phase precipitates were present in the CrMnCoNi alloy, but alloys containing ≥0.5 Fe remained single phase fcc. At 700 °C, the alloys investigated all contained the σ phase. Cr-bcc precipitates were also present in the CrMnCoNi and CrMnFe0.5CoNi alloys and Cr carbide precipitates featured in the CrMnFe1.5CoNi alloy. Heat-treatment of the CrMnCoNi alloy at 500 °C caused a partial bulk decomposition of the fcc matrix, which produced a fine-scale intergrowth of phases: σ, NiMn-L10, Cr-bcc and a secondary solute-depleted fcc phase. In the alloy containing 0.5 Fe, cellular regions consisting of a NiMn-L10, Cr-bcc and solute-depleted matrix phase, developed along the grain boundaries. NiMn-L10 and Cr-rich precipitates also formed on grain boundaries in the 1.5 Fe alloy. From these experimental observations, it was clearly established that Fe stabilises the fcc matrix relative to the σ and bcc phases

Surveying the effects of aging a high C-containing co-based superalloy from the As-cast and solution heat-treated conditions

The microstructure of the high carbon-containing cobalt-based superalloy, Co-101, has been studied in the as-cast state and following a variety of heat treatments. In the as-cast state both M7C3 and Mo-rich M23C6 carbides were observed in the interdendritic regions. After thermal exposure at temperatures between 1000 °C and 1250 °C for 1, 10, and 100 hours, the M7C3 interdendritic carbide network was observed to transform into a Mo-lean M23C6 carbide. These changes were rationalized with thermodynamic calculations. The carbide transformation liberated carbide-forming elements that resulted in the precipitation of intragranular carbides in the dendritic regions at temperatures below 1150 °C. These carbides in the cast-aged material preferentially formed at the dendrite peripheries early during exposure, leading to wide particle size distributions. Peak hardness in the cast-aged material was attained within the first 10 hours of exposure and softening was observed thereafter. After solution heat treating at 1250 °C for 10 hours, the microstructure of Co-101 comprised an M23C6 interdendritic carbide network and solid solution dendrites supersaturated with carbide-forming elements. Subsequent aging of this microstructure for 100 hours at 900 °C led to a high number density and narrow particle size distribution of intragranular carbides. The characteristics of these carbides in the solution-aged material resulted in greater hardness, which was retained for longer durations of exposure, than the cast-aged specimens

Tools for the assessment of the laser printability of nickel superalloys

Additive Manufacturing (AM) is a revolutionary technology with great interest from the aerospace sector, due to the capability of manufacturing complex geometries and repairing of damaged components. A significant volume of research is being conducted with high-temperature alloys, particularly nickel superalloys. However, the high-temperature properties of nickel superalloys are derived from the high fraction of strengthening precipitates, which in turn, lead to poor amenability to additive manufacture. Various cracking modes are common in nickel superalloys, primarily as a result of the high level of alloying and the extreme thermal conditions experienced in AM. Herein, crack susceptibility calculations from literature were critically analyzed and combined, resulting in a simple failure susceptibility that correlates with literature. Currently, the range of alloys which have been tested in AM and reported in literature is limited and lacks a standard methodology, making accurate assessment of printability difficult. Scheil solidification calculations were performed, testing solute trapping (ST) and back diffusion models for both the cooling rates associated with laser powder bed fusion (L-PBF) and laser-directed energy deposition (L-DED). The results confirm that L-PBF exhibits cooling rates that can result in ST, unlike in L-DED. These differences mean that alloys cannot be developed more generally for AM, but must be developed with a specific AM process in mind