Characterization and tailoring of powder used in additive manufacturing and plasma spheroidization

There are many processes that use metal powder as the starting material for the production of parts. With the growth of these manufacturing techniques, more critical part applications are being considered. In order to fully understand the process and create consistent parts, powder properties need to be well understood. Selective laser melting (SLM) is a powder bed-based additive manufacturing process. During processing, heat-affected powders are generated and can deposit within the build area. The current work investigated the characterization of heat-affected 304L stainless steel powder using techniques such as scanning electron microscopy, x-ray diffraction and x-ray photoelectron spectroscopy to detect differences in the heat-affected powder and to determine the best way to detect them. This heat-affected powder can also have an influence on the amount of times that the powder can be reused. A methodology was proposed where a fast, miniature powder recycling study was conducted. Area fractions and part spacing where deterioration of powder was observed can then be used to design a more in depth recycling study. The use of SLM for processing of more exotic materials such as metallic glasses was also of interest. However, the acquisition of powder forms of these materials that are suitable for processing via SLM is difficult and expensive. The work in this thesis aimed to use plasma spheroidization to tailor inert ground, angular Vitreloy 106A metallic glass powder and spheroidize it so that it was suitable for use in additive manufacturing processes. Several powder characterization techniques were used to evaluate the success of the process including x-ray diffraction, differential scanning calorimetry and Raman spectroscopy --Abstract, page iv

Formation of chromium-iron carbide by carbon diffusion in AlₓCoCrFeNiCu high-entropy alloys

Effect of the addition of carbon on phase formations in AlxCoCrFeNiCu (x = 0.3, 1.5, 2.8) high-entropy alloys (HEAs) was studied. Free diffusion of carbon from graphite crucible resulted in the partitioning of the entire Cr from the matrix and the formation of the (Cr,Fe)23C6 phase in all HEAs. No other metal-carbide phase was detected. The formation of (Cr,Fe)23C6 enhanced the overall hardness of the HEAs. By increasing the amount of Al, the Cr amount decreased resulting in the reduction of carbon diffusion and volume fraction of the (Cr,Fe)23C6 phase in HEAs. The hardness of matrix phases and the overall hardness of HEAs increased with an increase in the amount of Al

Recyclability of 304L Stainless Steel in the Selective Laser Melting Process

During part fabrication by selective laser melting (SLM), a powder-bed fusion process in Additive Manufacturing (AM), a large amount of energy is input from the laser into the melt pool, causing generation of spatter and condensate, both of which have the potential to settle in the surrounding powder-bed compromising its reusability. In this study, 304L stainless steel powder is subjected to five reuses in the SLM process to assess its recyclability through characterization of both powder and mechanical properties. Powder was characterized morphologically by particle size distribution measurements, oxygen content with inert gas fusion analysis, and phase identification by X-ray diffraction. The evolution of powder properties with reuse was also correlated to tensile properties of the as-built material. The results show that reused powder coarsens and accrues more oxygen with each reuse. The effects of powder coarsening and oxygen increase on the tensile properties of fabricated parts are being investigated

Long-Term Effects of Temperature Exposure on SLM 304L Stainless Steel

Austenitic stainless steel is extensively used in industries that operate at elevated temperatures. This work investigates the high-temperature microstructure stability as well as elevated-temperature properties of 304L stainless steel fabricated using the selective laser melting (SLM) process. Significant microstructural changes were seen after a 400°C aging process for as little as 25 h. This dramatic change in microstructure would not be expected based on the ferrite decomposition studied in conventional 304L materials. The as-built additively manufactured alloy has much faster kinetic response to heat treatment at 400°C. An investigation of the structures which occur, the kinetics of the various transformations, and the mechanical properties is presented. The impact of this on the application of SLM 304L is discussed

On the Current State of Powder Characterization

Modern powder metallurgy (PM) has been a prominent field since the 1920s when press-and-sintering offered a fast, efficient way to produce mass quantities of parts [1]. Characterization of the starting powder has always been of significant importance, because the powder properties influence its behavior during processing and, therefore, the final part. There has been unprecedented growth in PM in the past two decades due to new technologies such as metal injection molding (MIM) and additive manufacturing (AM). These new technologies have attracted the interest of different industries, such as automotive and aerospace, who are considering the use of PM for more critical parts. Thus, more strenuous characterization of the starting powder material as well as the built parts is required

Formation of chromium-iron carbide by carbon diffusion in AlXCoCrFeNiCu high-entropy alloys

Effect of the addition of carbon on phase formations in AlxCoCrFeNiCu (x = 0.3, 1.5, 2.8) high-entropy alloys (HEAs) was studied. Free diffusion of carbon from graphite crucible resulted in the partitioning of the entire Cr from the matrix and the formation of the (Cr,Fe)23C6 phase in all HEAs. No other metal-carbide phase was detected. The formation of (Cr,Fe)23C6 enhanced the overall hardness of the HEAs. By increasing the amount of Al, the Cr amount decreased resulting in the reduction of carbon diffusion and volume fraction of the (Cr,Fe)23C6 phase in HEAs. The hardness of matrix phases and the overall hardness of HEAs increased with an increase in the amount of Al. Impact statement The detailed phase analysis reveals that C addition to AlxCoCrFeNiCu HEAs leads to the formation of the (Cr,Fe)23C6 phase. The overall hardness can be controlled by the amount of C and/or Al

Powders for Additive Manufacturing Process: Characterization Techniques and Effects on Part Properties

Powder-bed based Additive Manufacturing is a class of Additive Manufacturing (AM) processes that bond successive layers of powder by laser melting to facilitate the creation of parts with complex geometries. As AM technology transitions from the fabrication of prototypes to end-use parts, the understanding of the powder properties needed to reliably produce parts of acceptable quality becomes critical. Consequently, this has led to the use of powder characterization techniques such as scanning electron microscopy (SEM), laser light diffraction, x-ray photoelectron spectroscopy (XPS), and differential thermal analysis (DTA) to both qualitatively and quantitatively study the effect of powder characteristics on part properties. Utilization of these powder characterization methods to study particle size and morphology, chemical composition, and microstructure of powder has resulted in significant strides being made towards the optimization of powder properties for powder-bed based AM processes. This paper reviews methods commonly used in characterizing metallic AM powders, and the effects of powder characteristics on the part properties in these AM processes.Mechanical Engineerin

Characterization of Heat-Affected Powder Generated during Selective Laser Melting of 304L Stainless Steel Powder

The selective laser melting (SLM) process is an Additive Manufacturing (AM) technique that uses a laser to fuse successive layers of powder into near fully dense components. Due to the large energy input from the laser during processing, vaporization and instabilities in the melt pool occur causing the formation of condensate and laser spatter, collectively known as heat-affected powder. Since heat-affected powder settles into the powder bed, the properties of the unconsolidated powder may be altered compromising its reusability. In this study, characterization of 304L heat-affected powder was performed through particle size distribution measurements, x-ray diffraction, metallography, energy-dispersive spectroscopy mapping, and visualization of grain structure with the aid of a focused-ion beam. The results show morphological, microstructural, and surface chemistry differences between the starting powder and heat-affected powder formed during processing which aid in the understanding of laser spatter and condensate that form in the SLM process

Powders for Additive Manufacturing Processes: Characterization Techniques and Effects on Part Properties

Powder-bed based Additive Manufacturing is a class of Additive Manufacturing (AM) processes that bond successive layers of powder by laser melting to facilitate the creation of parts with complex geometries. As AM technology transitions from the fabrication of prototypes to end-use parts, the understanding of the powder properties needed to reliably produce parts of acceptable quality becomes critical. Consequently, this has led to the use of powder characterization techniques such as scanning electron microscopy (SEM), laser light diffraction, x-ray photoelectron spectroscopy (XPS), and differential thermal analysis (DTA) to both qualitatively and quantitatively study the effect of powder characteristics on part properties. Utilization of these powder characterization methods to study particle size and morphology, chemical composition, and microstructure of powder has resulted in significant strides being made towards the optimization of powder properties for powder-bed based AM processes. This paper reviews methods commonly used in characterizing metallic AM powders, and the effects of powder characteristics on the part properties in these AM processes