The GranuDrum High Temperature instrument's temperature control feature allows you to learn more about the impact of temperature on powder cohesion.
Introduction
A number of applications involve processing granular materials or powders at high temperatures. This may be driven by manufacturing process requirements or variations in environmental conditions, particularly in production facilities located worldwide.
Temperature elevation can trigger various mechanisms that alter powder properties, including moisture evaporation, changes in particle characteristics (e.g. stiffness, shape roughness), and oxidation. These changes affect the cohesiveness of the powder, which, in turn, influences its flowability.1 Therefore, evaluating powder flowability at temperatures similar to manufacturing is essential for providing reliable predictions.
This study investigates the impact of temperature on three different powders commonly utilized in powder bed fusion processes for Additive Manufacturing. In these methods, the powder is typically spread at an elevated temperature to form a powder bed layer near the sintering temperature. A laser or plasma beam then sinters or melts specific powder regions to produce the part layer by layer.
Performing measurements at temperatures near those used in manufacturing is critical for predicting spreadability.
Powder flowability and rheology are assessed in a rotating drum geometry (GranuDrum HT, Granutools). The Dynamic Cohesive Index metric, calculated by this tool, is utilized to assess the impact of temperature on the cohesive behavior of the materials2
This index correlates with powder spreadability in powder bed fusion processes for Additive Manufacturing. A higher Dynamic Cohesive Index indicates reduced spreadability (ISO/ASTM TR 52952:2023). Thus, the Dynamic Cohesive Index serves as a valuable tool for assessing the effect of temperature on powder spreadability.
Temperature elevation has been shown to significantly impact powder properties. Its effects on powder cohesion vary depending on the investigated material and may be attributed to different mechanisms.
Powder Materials
In this study, three powder materials were examined: a polyamide powder (PA11), a titanium alloy powder (Ti-6Al-4V), and an aluminum alloy powder (AlSi10Mg). These powders were selected for their different behaviors with temperature to emphasize the importance of characterizing powders at temperatures similar to those used in the manufacturing process.
Typically, operators assess powder spreadability during recoating by visually observing the homogeneity of the powder layer. However, establishing a relationship between powder characteristics and spreadability before recoating could provide a more economical approach for identifying and selecting the best powder and recoating speed combinations.
This application note aims to demonstrate how characterizing the macroscopic properties of metallic powders can correlate with their spreadability inside SLM printers. Four metal powders were spreadable using the GranuDrum, an automated rotating drum measurement method. The cohesive index measured by this method has been shown to quantify powder spreadability during recoating(see reference G. Yablokova et al. 2015).
A novel method combining measurements inside an SLM printer with image processing has been engineered to quantify the homogeneity of the powder bed layers during the recoating process.
Figure 2 outlines the general principle of this study: A powder with low cohesion that exhibits a smooth powder/air interface in the rotating drum will produce a more homogeneous layer inside the printer. In contrast, a cohesive powder with poor flow properties will display an irregular interface in the GranuDrum and produce a non-homogeneous layer on the bed during recoating.
GranuDrum High Temperature
The GranuDrum instrument is an automated method for measuring powder flowability, based on the rotating drum principle. This system consists of a horizontal cylinder with transparent sidewalls, called the drum, which is half-filled with a powder sample. The drum rotates around its axis at an angular velocity ranging from 2 rpm to 60 rpm. A CCD camera captures images (30 to 100 images separated by 1 second) for each angular velocity. An edge detection algorithm detects the air/powder interface in each snapshot.
Next, the average interface position and the fluctuations around it are calculated. For each rotating speed, the Dynamic Angle of Repose αf is calculated from the average interface position, and the Dynamic Cohesive Index (DCI) is determined from the fluctuations in the interface.2
Figure 1. Sketch of GranuDrum measurement principle. Image Credit: Granutools
In general, a low Dynamic Angle of Repose indicates better flowability. This value is influenced by various factors, including friction between the grains, grain shape, and the cohesive forces (van der Waals, electrostatic, and capillary forces) between them.
In contrast, the Dynamic Cohesive Index is solely related to the cohesive forces between the grains. A cohesive powder results in intermittent flow, while a non-cohesive powder results in regular flow.
Therefore, a Dynamic Cohesive Index close to zero indicates a non-cohesive powder. An increase in powder cohesiveness leads to a higher Dynamic Cohesive Index and a decrease in spreadability.
The GranuDrum HT functions almost identically to the GranuDrum, with the key difference being that measurements are performed safely and smoothly at controlled temperatures ranging from room temperature to 250 °C.
Figure 2. Picture of the GranuDrum HT (left) with a focus on the heating device (right). Image Credit: Granutools
Dynamic Cohesive Index Analysis
Experimental Protocol
The three powders were assessed using the GranuDrum HT. For each powder, a sequence of increasing speeds, ranging from 2 to 60 rpm, was performed at various temperatures. The curves of the Dynamic Cohesive Index as a function of rotating speed were compared to evaluate powder cohesion evolution with temperature and shear rate. This approach enables the assessment of the impact of temperature on powder cohesion and rheology.
Polymer powders' processing temperatures for 3D printing typically remain below 200 °C. The processing temperature is set lower than its melting point for PA11, which has a melting point of approximately 180 °C.
As a result, the powder was tested at 80 °C, 120 °C, and 140 °C, as well as at room temperature, to evaluate the evolution of cohesion with temperature. For metal powders, such as titanium alloys or aluminum alloys, the processing temperatures are typically higher than 200 °C. Therefore, the metal powders were measured at 200 °C and room temperature for comparison.
Experimental Results and Interpretation
PA11
For PA11, a notable increase in cohesion is observed with temperature, as demonstrated by the gradual evolution of the DCI curves. The shear-thinning observed at room temperature persists even at higher temperatures.
This overall rise in cohesion with temperature is significant, as it indicates that the powder's spreadability at high processing temperatures would be lower than predicted based on room temperature measurements. Therefore, to accurately assess spreadability, it is preferable to characterize the powder at temperatures similar to those used in manufacturing.
For PA11, spreadability could be improved by lowering the temperature during the recoating step. Alternatively, increasing the speed of the recoater may also enhance spreadability, as shear-thinning behavior is present at room and elevated temperatures.
The evolution of the DCI curves could be attributed to particle sintering or partial melting of the polymer grains. In polymer Additive Manufacturing, polymer powders are typically heated and spread near their melting points (between 180 °C and 189 °C for PA11) to reduce energy consumption by the selective laser (or plasma).
Therefore, solid bridges can be generated, especially with amorphous materials such as polymers, resulting in sintering between particles. These solid bridges restrict particle mobility, increasing the powder’s cohesion.
Figure 3. Effect of temperature on PA11 cohesion. Image Credit: Granutools
TITANIUM ALLOY
Figure 2 displays the Cohesive Index as a function of the GranuDrum’s increasing rotating speed, which can be related to the recoater speed (see Appendix 1). The Cohesive Index is associated with fluctuations in the interface (powder/air) position caused by cohesive forces, such as Van der Waals, electrostatic, and capillary forces.
As outlined in section I.2, this index quantifies powder spreadability. Figure 2 shows the three recoater speeds utilized for the in situ SLM printer measurements for ease of comparison.
Figure 4. Effect of temperature on Titanium alloy powder. Image Credit: Granutools
ALUMINUM ALLOY
In the case of aluminum alloy, a change in rheology is observed due to temperature. Specifically, the shear-thickening observed at room temperature becomes stronger at 200 °C. This reduces cohesion with temperature at low shear rates, while the opposite is observed at large shear rates.
As a result, when the powder is spread at a low shear rate, its spreadability is expected to improve with temperature, whereas, at a high shear rate, it is expected to decrease with temperature. This underlines the importance of characterizing the rheology of the powder and the impact temperature has on it.
In this scenario, spreadability during the recoating step at elevated temperatures is more sensitive to shear rate than would be predicted from measurements taken at room temperature.
Complex mechanisms could explain the increased strength of the aluminum alloy powder's shear-thickening. Changes in particle surface properties, including stiffness, shape, roughness, or oxidation, may be contributing factors.
Figure 5. Effect of temperature on Aluminum alloy powder. Image Credit: Granutools
Conclusions
This study demonstrates the impact of temperature on powder cohesion, thanks to the temperature control feature of the GranuDrum HT. Three different powders commonly used in Additive Manufacturing were investigated, and various behaviors were observed with temperature depending on the powder tested:
- Temperature can cause partial melting or sintering between particles, increasing cohesion, especially for polymer powders (e.g. PA11).
- Temperature can lead to powder drying, reducing cohesion from capillary bridges (e.g. Ti 6AI 4V).
- Temperature can alter the rheology of the powder due to changes in particle properties such as shape, stiffness, and oxidation (e.g. AISi10Mg)
These changes in powder cohesion alter its spreadability, affecting the quality of the powder bed layer in powder bed fusion technologies for Additive Manufacturing.
Consequently, evaluating a powder at room temperature while manufacturing occurs at an elevated temperature can result in inaccurate conclusions.
Therefore, it is crucial to evaluate powder properties at temperatures close to those used in manufacturing to better assess processability. For this purpose, the GranuDrum HT technology facilitates the assessment of spreadability for Additive Manufacturing, integrating temperature as a control parameter.
References and Further Reading:
- Lumay, G., et al. (2020). Influence of temperature on the packing dynamics of polymer powders. Advanced Powder Technology, 31(10), pp.4428–4435. https://doi.org/10.1016/j.apt.2020.09.019.
- Neveu, A., Francqui, F. and Lumay, G. (2022). Measuring powder flow properties in a rotating drum. Measurement, 200, p.111548. https://doi.org/10.1016/j.measurement.2022.111548.
- Radchenko, O.K. and Gogaev, K.O. (2022). Requirements for Metal and Alloy Powders for 3D Printing (Review). Powder Metallurgy and Metal Ceramics, 61(3-4), pp.135–154. https://doi.org/10.1007/s11106-022-00301-0.
This information has been sourced, reviewed and adapted from materials provided by Granutools.
For more information on this source, please visit Granutools.