One of the most critical innovations surrounding the extension of cold-spray materials processing from a non-structural repair or coating acquisition into a structural repair and solid-state additive manufacturing (AM) is the development of thermal-preprocessing technology for the feedstock powder.

Worcester Polytechnic Institute (WPI) has developed techniques for metallography of individual powder particles and the heat treating of these powders. During the cold-spray process, the powders do not melt and retain the microstructure of the powder particles.

Specifically, the cold-spray solid-state deposition technique utilizes powder feedstock to create a consolidated part via supersonic particulate impact processing. Being a solid-state process, the feedstock powder microstructure is refined due to high strain rate impact-induced severe plastic deformation. Therefore, the resultant deposits microstructure can be tuned and controlled through thermal heat treatments prior to consumption during the cold-spray process.

Motivation for applying said heat treatments enables more homogeneous deposit microstructures, enhanced deposition efficiency, process parameter optimization and the like. This column will describe a few cases where thermal preprocessing and powder feedstock characterization and analysis can be applied for target applications and structural uses.

Fig. 1. Secondary electron micrographs of the (a) as-atomized powder, (b) 230°C heat-treated powder and (c) 385°C heat-treated powder. Images (d-f) present micrographs of the cold-sprayed 230°C heat-treated feedstock specimen at two different magnifications, while (e-g) present similar micrographs for the cold-sprayed 385°C heat-treated powders.^[1]

Thermal Preprocessing of Al F357 Powder

Given the significant focus on thermal preprocessing of Al alloys within the cold-spray community in the pursuit of structural strength-ductility synergy in uniaxial tension, recent work by Tsaknopoulos et al. considered the compatibility and tunability of gas-atomized Al F357 powder within cold-spray processes via heat treating the feedstock powder for the pursuit of achieving an overaged condition (230°C for 75 minutes) and the degassed state (385°C for 360 min).^[1] 

Fig. 1 presents cross-sectional scanning electron micrographs of the as-atomized, overaged and degassed Al F357 feedstock powders and the resultant microstructures achieved when the overaged and degassed feedstocks were deposited via high-pressure, He cold-spray processing.

While minimal particulate microstructural evolution relative to the as-atomized condition was observed in the overaged condition, the partly equiaxed and dendritic structure found in the as-atomized condition transitioned to script-like phase coarsening and phase ripening along the grain boundaries in the degassed condition. In addition, due to the solid-state nature of cold spray, the heat-treated microstructures are also clearly shown to have been refined and retained in the consolidated counterparts shown in Fig. 1.

Fig. 2. Uniaxial tensile test parameters were obtained for the thermally preprocessed Al F357 cold-sprayed consolidations.^[1]

Beyond microstructural analysis alone, mechanical characterization of the consolidated deposits – as a function of the thermally preprocessed condition of the feedstock employed – was also applied by Tsaknopoulos et al. By way of applying uniaxial tensile testing and constant processing parameters during cold spraying, Fig. 2 demonstrates the fact that more excellent elongation to failure percentage values were obtained when the powder was initially degassed.

In contrast, enhanced yield and ultimate tensile strengths were observed in the case of the overaged consolidation.^[1] Noticeable improvements in ductility (irrespective of the thermal preprocessing recipe applied herein) and comparable strengths (when compared to Al F357’s cast counterpart) were achieved through microstructural modification of the as-atomized powder prior to use in cold spray.

Conventionally Sprayed Material Systems: Al 6061 and 3xx Stainless Steel

Beyond Al F357, which has been employed less frequently to date than many other Al systems in the cold-spray community (such as Al 6061, Al 7075, Al 5056 and Al 2024, for example), Fig. 3 presents the effect of thermal-preprocessing temperature on the grain size of a gas-atomized Al 6061 feedstock as a function of particle diameter.^[2]

Fig. 3. Grain size versus particle size (i.e., cooling rate) for the gas-atomized Al 6061 powder in the as-atomized, 200°C for 24 hours and 230°C for 24-hour heat-treated conditions.^[2]

From the trends shown in Fig. 3, one can garner insights into the relationship between grain size and solidification rate, which is directly related to the size of a given droplet during atomization across an as-atomized condition, a 200°C heat-treated state and a 230°C thermal-preprocessed condition. By way of considering said relationships in the context of grain-size strengthening mechanisms, wherein more refined grains generally result in increased yield strengths, enhanced strain rates and work hardening can be achieved for specific applications by way of lowering the powders’ resistance to plastic deformation for a given cold-spray processing parameter recipe.

Fig. 4. Stainless steel powder hardness as a function of austenitization time coupled with cross-sectional micrographs.^[3]

Beyond the realm of alloyed Al systems, stainless steel powders have also been shown to achieve enhanced cold-spray processability through thermal preprocessing prior to consumption. In the case of Massar et al., decreased feedstock hardness values obtained through nanoindentation testing were found to decrease as a function of the thermal-preprocessing hold time (Fig. 4).

Fig. 5. Optical microscopy of the as-atomized and cold-sprayed and heat-treated and cold-sprayed specimens coupled with hardness and porosity measurements for each in the case of stainless steel powder.^[3]

In terms of the cold-sprayed specimens stemming from the application of the austenitization heat treatment relative to the as-atomized condition, the resultant consolidations maintained cracking and greater porosity when the as-atomized powder was employed (Fig. 5). Nevertheless, there is ample room for continued consideration of thermal preprocessing within the realm of cold-spray AM as structural applications and strength-ductility synergy in the as-deposited state remain ripe for continued consideration across numerous material systems and processing parameter combinations.

For more information: Contact Richard D. Sisson Jr., director of the Center for Heat Treating Excellence (CHTE), Worcester Polytechnic Institute, 100 Institute Road, Worcester, MA 01609; tel: 508-831-5335; e-mail: sisson@wpi.edu; web: https://wp.wpi.edu/chte/.

All graphics provided by the author

References

1. Tsaknopoulos, K., Grubbs, J., Siopis, M., Nardi, A., & Cote, D. (2021). Microstructure and Mechanical Property Evaluation of Aluminum F357 Powder for Cold Spray Applications. Journal of Thermal Spray Technology, 30(3), 643-654.
2. Sousa, B. C., Gleason, M. A., Haddad, B., Champagne Jr, V. K., Nardi, A. T., & Cote, D. L. (2020). Nanomechanical characterization for cold spray: From feedstock to consolidated material properties. Metals, 10(9), 1195.
3. Massar, C., Tsaknopoulos, K., Sousa, B.C. et al. Heat Treatment of Recycled Battlefield Stainless-Steel Scrap for Cold Spray Applications. JOM 72, 3080-3089 (2020). https://doi.org/10.1007/s11837-020-04259-5