Effect of temperature on powder flowability assessed with GranuDrum HT

Introduction

Numerous applications involve processing granular materials or powders at elevated temperatures. This can be due to requirements of the process or variations of the environmental conditions especially for production sites located around the world. Temperature elevation can induce different mechanisms that will change the powder properties such as moisture evaporation, change in particle properties (stiffness, shape roughness,…), or can induce oxidation… These modifications in the powder change the powder's cohesiveness which impacts its flowability [1]. Therefore, powder flowability must be evaluated as much as possible at a temperature close to the one in the process to provide reliable predictions.

In this study, we investigated the influence of temperature on three different powders generally used in powder bed fusion processes for Additive Manufacturing. In such methods, the powder is usually spread at a high temperature to form a powder bed layer, close to the sintering temperature, and then a laser or a plasma beam sinters or melts the powder at precise regions to print the part layer after layer. Therefore, it is important to perform measurements at a temperature close to the process to predict the spreadability. The flowability and the rheology of the powders are evaluated in a rotating drum geometry (GranuDrum HT, Granutools). The Dynamic Cohesive Index metric computed by the instrument [2] is used to evaluate the influence of temperature on the cohesive behavior of the materials. This Dynamic Cohesive Index is known to be correlated with powder spreadability in powder bed fusion processes for Additive Manufacturing. The larger the Dynamic Cohesive Index the lower the spreadability (ISO/ASTM TR 52952:2023). Therefore, the Dynamic Cohesive Index allows to assess the effect of temperature on spreadability.

A significant impact of the temperature elevation on the powder is observed. Different effects of the temperature on powder cohesion are observed depending on the investigated material, and attributed to different possible mechanisms.

Powder Materials

In this work, three powder materials were analyzed: a Polyamide powder (PA11), a Titanium alloy powder (Ti-6Al-4V), and an Aluminum alloy powder (AlSi10Mg). These powders are selected for their different behaviours with temperature to highlight the importance of characterizing powder at temperatures close to the manufacturing process.

Visual observation of layer homogeneity is usually the only way for operators to quantify the spreadability of powders during the recoating. However, relating the powder characteristics to its spreadability during the recoating process beforehand should provide a more cost-effective way to classify and select the optimal powder and recoating speed combinations.

The aim of this application note is to present how the characterization of the macroscopic properties of metallic powders can be related to their spreadability inside SLM printers. The spreadability of four metal powders have been determined with the GranuDrum, an automated rotating drum measurement method, whose cohesive index measurement has been shown to quantify the spreadability of the powder during the recoating process (see reference G. Yablokova et al. 2015). A new technique combining measurements inside a SLM printer and image processing have been developed to quantify the homogeneity of the powder bed layers during recoating.

The general principle of this study is summarized in figure 2: a low cohesive powder which show a smooth powder/air interface in the rotating drum will produce a more homogeneous layer inside the printer. On the opposite, a cohesive powder with bad flowing properties will exhibit an irregular interface in GranuDrum and non-homogeneous layer on the bed during the recoating.

GranuDrum High Temperature

The GranuDrum instrument is an automated powder flowability measurement method based on the rotating drum principle. A horizontal cylinder with transparent sidewalls called drum is half filled with the sample of powder. The drum rotates around its axis at an angular velocity ranging from 2 rpm to 60 rpm. A CCD camera takes snapshots (30 to 100 images separated by 1s) for each angular velocity. The air/powder interface is detected on each snapshot with an edge detection algorithm. Afterwards, the average interface position and the fluctuations around this average position are computed. Then, for each rotating speed, the Dynamic Angle of Repose αf is computed from the average interface position, and the Dynamic Cohesive Index (DCI) is measured from the interface fluctuations [2]

Figure 1: Sketch of GranuDrum measurement principle

In general, a low value of the Dynamic Angle of Repose corresponds to good flowability. It is influenced by a wide set of parameters: the friction between the grains, the shape of the grains, the cohesive forces (van der Waals, electrostatic, and capillary forces) between the grains. On the contrary, the Dynamic Cohesive Index is only related to the cohesive forces between the grains. A cohesive powder leads to an intermitted flow while a non-cohesive powder leads to a regular flow. Therefore, a Dynamic Cohesive Index close to zero corresponds to a non-cohesive powder. When the powder cohesiveness increases, the Dynamic Cohesive Index increases accordingly and the spreadability decreases.

The GranuDrum HT operates identically to GranuDrum with the difference that measurements are performed at a controlled temperature from room up to 250°C in a safe and easy way.

Figure 2: Picture of the GranuDrum HT (left) with a focus on the heating device (right)

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Dynamic Cohesive Index analysis

Experimental Protocol

The three powders were analyzed with the GranuDrum HT. For each powder, an increasing speed sequence from 2 up to 60 rpm was performed at different temperatures. The curves of the Dynamic Cohesive Index as a function of rotating speed were compared to assess the powder cohesion evolution with temperature and shear rate. In this way, the effect of temperature on powder cohesion and powder rheology can be evaluated.

For polymer powders, the process temperatures for 3D printing are generally lower than 200°C. For the case of PA11, the melting point is around 180°C and the process temperature is thus lower than this temperature. Therefore, the powder was tested at 80°C, 120°C, and 140°C and at room temperature to assess the evolution of the cohesion with temperature. For metal powders such as Titanium alloys or Aluminum alloys, the process temperatures are generally larger than 200°C. Therefore, the metal powders were measured at 200°C and room temperature for comparison.

Experimental results and interpretation

PA11

In the case of PA11, a significant increase in cohesion is observed with temperature as highlighted by the gradual evolution of the DCI curves. The shear-thinning observed at room temperature is conserved even at higher temperatures. This global increase in cohesion with temperature is important. Indeed, this implies that the spreadability of the powder during the process at high temperatures would be lower than the spreadability predicted by a measurement performed at room temperature. Therefore, it is preferable to characterize the powder at a temperature close to the process to assess correctly its spreadability. For the PA11, this spreadability could be improved by decreasing the temperature during the recoating step. Another possibility would be to increase the speed of the recoater since the shear-thinning behaviour is present at room and high temperatures.

This evolution in DCI may be due to particle sintering or partial melting of the polymer grains. Indeed, in polymer Additive Manufacturing, polymer powders are generally heated and spread close to the melting point (between 180°C and 189°C for PA11) to reduce the energy consumed by the selective laser (or plasma). Therefore, solid bridges can be generated, especially with amorphous materials like polymers, leading to sintering between particles. These solid bridges limit the mobility of particles and thus increase the cohesion of the powder.

Figure 3: Effect of temperature on PA11 cohesion

TITANIUM ALLOY

Figure 2 presents the Cohesive Index as a function of the GranuDrum increasing rotating speed, which can be related to the recoater speed (see appendix 1). The Cohesive Index is linked to the fluctuations of the interface (powder/air) position induced by cohesive forces (Van der Waals, Electrostatic and Capillary). Thus, as discussed in section I.2, it allows to quantify powder spreadability. For an ease of comparison, the three recoater speeds used in the in situ SLM printer measurements are also indicated in figure 2.

Figure 4: Effect of temperature on Titanium alloy powder

ALUMINUM ALLOY

In the case of Aluminum alloy, a change in the rheology is observed due to temperature. Indeed, the shear-thickening observed at room temperature is strengthened at 200°C. It results in a decrease in cohesion with temperature at low shear rates while the contrary is observed at large shear rates. Consequently, if the powder is spread at a low shear rate, the spreadability is expected to be improved with temperature while it is expected to decrease with temperature if the spread is done at a high shear rate. In addition, this highlights the importance of characterizing the powder rheology and the effect of temperature on it. For this case, the spreadability during the recoating step at high temperature is more sensitive to shear rate than what can be predicted by a measurement performed at room temperature.

Complex mechanisms could explain the strengthening of the shear-thickening for the Aluminum alloy powder. Changes in the surface properties of the particles like stiffness, shape, roughness, or oxidation may be in cause.

Figure 5: Effect of temperature on Aluminum alloy powder

Conclusions

In this study, the impact of temperature on powder cohesion is clearly evidenced thanks to the temperature control of the GranuDrum HT. Three different powders used in Additive Manufacturing were investigated and various behaviours were observed with temperature depending on the tested powder:

• The temperature can induce partial melting or sintering between particles, increasing the cohesion, especially in the case of polymer powders. → example of PA11
• The temperature can induce a drying of the powder, decreasing the cohesion coming from the capillary bridges. → example of Ti-6Al-4V
• The temperature can induce a change in the rheology of the powder due to a change in particle properties (shape, stiffness, oxidation,…). → example of AlSi10Mg

These changes in powder cohesion change the spreadability of the powder and consequently affect the quality of the powder bed layer in powder bed fusion technologies for Additive Manufacturing.

As a result, characterizing a powder at room conditions while the manufacturing process is done at an elevated temperature can lead to wrong conclusions.

Therefore, it is of huge importance to characterize powder properties at a temperature close to the process for a better processability assessment. For this purpose, the GranuDrum HT technology opens the way to spreadability assessment for Additive Manufacturing, integrating the temperature as a control parameter.

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References

• G. Lumay, F. Francqui, C. Detrembleur, N. Vandewalle, Influence of temperature on the packing dynamics of polymer powders, Advanced Powder Technology (2020), 4428-4435, 31(10).
• A. Neveu, F. Francqui, G. Lumay, Measuring powder flow properties in a rotating drum, Measurement 200 (2022), 111548.
• O. K. Radchenko, K. O. Gogaev, Requirements for Metal and Alloy Powders for 3D Printing (Review), Powder Metallurgy and Metal Ceramics (2022), 135-154, 61.

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