Investigation of new volumetric nondestructive techniques to characterise additive manufacturing parts
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Abstract
The assessment of the potential of various nondestructive methods to characterise additive manufactured dense and lattice parts was investigated. The Archimedes’ and gas pycnometric methods for density measurements of both types of structures are presented as well as the percentage of cells in lattice structures. A multifrequency eddy current method for electrical conductivity measurements of lattice structures is also presented. Finally, Cscan ultrasound method for the characterisation of dense parts was investigated. The advantages and limitations of each method are underlined.
Keywords
Additive manufacturing Volumetric nondestructive techniques (NDTs) Archimedes’ method Gas pycnometric method Multifrequency eddy current method Cscan ultrasound method1 Introduction
In contrast to traditional manufacturing techniques, additive manufacturing (AM) is a technology which enables the fabrication of very complex geometry parts (internal structures and lattice structures). Furthermore, AM facilitates the mass production of highly customised parts, e.g., in relation to individualised medical implants. Considering these two points, characterisation techniques which were formerly used for parts manufactured with conventional methods have to be reconsidered. New nondestructive techniques (NDT) [1] as well as volumetric techniques have to be investigated. Xray tomography is presently the most appropriate technique [2, 3, 4] considering that it is a noncontact technique providing a high spatial resolution. However, it is a costly and timeconsuming technique which may be unsuitable for mass production.
The French National Metrology Institute, LNE, in collaboration, with NDT companies, Sciensoria and Eurosonic, have investigated two of the new NDT technologies that they are proposing: multifrequency eddy current and Cscan ultrasound respectively. In parallel, at LNE, we are applying density measurements to characterise the repeatability as well as the reproducibility of AM parts. This quantity is measured by two different methods: Archimedes’ and gas pycnometric methods. For a first exploration of these four methods, several dense specimens with different a priori defined defects and lattice specimens were studied.
In this paper, we will present the measurement outcomes of these AM specimens characterised by the abovementioned methods, compare the results, and highlight the advantages and the limitations of each method.
2 Density and percentage of lattice cell measurements
2.1 Archimedes’ method
2.1.1 Measurement system
2.1.2 Measurement protocol
The classical principle of the hydrostatic method involves measuring the apparent mass of the sample in air, then in twicedistilled water to deduce its density. However, in the case of lattice structures, in water, bubbles form at the interface airlattice preventing the water to penetrate deep inside the lattice because the surface tension of water is too high (72.8 × 10^{−3} N m^{−1} at 20 °C). In order to avoid this phenomenon, which distorts the measurements, absolute ethanol was used. In that case, a good penetration of the liquid into the lattice structure was observed as the surface tension of alcohol (22.27 × 10^{−3} N m^{−1} at 20 °C) is much lower than the one of water, preventing bubbles to disturb the measurement.
 1.
The balance was tared to zero.
 2.
Weighing in air (P_{a}): the sample was placed on the balance and compared with mass standards according to Borda’s doublesubstitution method.
 3.
The density of air (ρ_{a}) was calculated taking into account the temperature (T_{a}), the pressure, and the humidity in the laboratory during measurements of the sample in air.
 4.
The sample was removed from the balance.
 5.
An eventual derivation of the tare was checked. If a derivation is observed, the weighing is done again.
 6.
The balance is tared to zero
 7.
Weighing in absolute ethanol (P_{l}): the sample was placed in the absolute ethanol on the suspension device. Once the sample and the temperature were stabilised, the mass of the sample was compared with mass standards placed on the weighing pan according to Borda’s doublesubstitution method. This comparison in absolute ethanol was carried out twice. In between each measurement, an eventual derivation of the tare was checked and then the balance is tared to zero if no derivation is observed.
 8.
The temperature of the absolute ethanol inside the container was taken (T_{l}) in order to take into account the variation of temperatures on the density of alcohol. Indeed, the density of the ethanol (\( {\rho}_{l,{T}_0} \)) was preliminary evaluated by a pycnometric method at the reference temperature of T_{0} = 20 °C.
 9.
The density of air (ρ_{a,l}) was calculated taking into account the temperature (T_{a}), the pressure, and the humidity in the laboratory during measurements of the sample in ethanol.
 10.
Weighing in air (P_{a}): the sample was placed for the second time on the balance and compared with mass standards according to Borda’s doublesubstitution method.
 11.
The final result (ρ) is expressed as the mean of the two measurements carried out using the following relation:
This equation takes into account the air buoyancy correction.

Balance

Mass standards

Comparisons with mass standards

Density of the water

Density of the air

Volume expansion of the solid

Reproducibility of the measurements
Then, comparing the measured volume by the hydrostatic method with the one measured with a digital calliper, the percentage of material relative to the lattice was also determined.
2.1.3 Results
As can be observed from Figs. 2 and 3, the AM process is not repeatable as two similar specimens have different densities. Furthermore, the density should decrease as the size of the pores and canals are decreasing. This is not the case so the parts are not in compliance with the specifications and the process has to be reviewed. Other samples will be manufactured with a reviewed process in order to gain repeatability and confidence into the AM process.
All these specimens were fabricated during the same AM process and as can be shown in Fig. 4, their density is within the uncertainty bars so one can conclude that the AM process is repeatable. Figure 5 gives the evaluation of the percentage of lattice cells. The difference in the percentage of lattice cells is noticeable in between two different specimens and the repeatability of the AM regarding this aspect process seems good. There is an offset from 2 to 4% in between the theoretical and effective percentage of lattice cells.
2.1.4 Benefits and limitations of the method
The Archimedes’ method enables to perform density measurements of dense structures but also of lattice structures with a good accuracy. It also provides the percentage of lattice cells. The knowledge of these two quantities allows verifying the compliance with the part specifications. Furthermore, comparing the density of an AM part with the density of a part fabricated by classical methods with the same material provides a quantification of the material in terms of internal porosities. Moreover, comparing similar parts fabricated during the same AM process and similar parts fabricated in different AM processes enable to study the repeatability and the reproducibility, respectively, of the AM processes. However, the measurements turn out to be rather timeconsuming (20 min per part).
2.2 Gas pycnometric method
The density of the same cubic lattice specimens in TiAl6V, with different percentage of cells, measured previously by the Archimedes’s method, was also evaluated by the gas pycnometric method.
2.2.1 Measurement system
This system is provided with three sample cells. The larger one of 135 cm^{3} in volume (with an internal diameter of 49 mm and an internal depth of 75 mm) was used for the present measurements as well as nitrogen gas.
 1.
The device under test is placed in the pycnometer sample cell at the initial pressure of P_{atm}. The valve is closed.
 2.
The pressure is increased until P_{1} in the sample cell.
 3.
The pycnometer valve is open.
 4.
The pressure P_{2} is measured when the equilibrium between the two cells is reached.
 5.
The apparent mass of the sample (m) is measured with a precision balance.
 6.
The density is then calculated using the following equation:
2.2.2 Measurement protocol
Before the pycnometric measurements, the cells are calibrated in volume with calibration spheres. Then, ten successive measurements, at a pressure P_{1} of about 225 kPa, and with a 5min purging time, are performed. Finally, the sample is weighted.
2.2.3 Results
As can be shown in Fig. 7, the variation of the pycnometric measurements agrees with the ones performed with the Archimedes’ method. Nevertheless, the estimated uncertainties on the pycnometric measurements are under evaluated and have to be reviewed to be in accordance with the standard Archimedes’ method.
2.2.4 Benefits and limitations of the method
The gas pycnometric method enables to perform volume measurements of dense structures but also of lattice structures. Then, associated with mass measurements, the density can be calculated. The percentage of lattice cells can also be provided. The knowledge of these two quantities allows verifying the compliance with the part specifications. Furthermore, comparing the density of an AM part with the density of a part fabricated with the same material by classical methods provides a quantification of the material in terms of internal porosities. Moreover, comparing similar parts fabricated during the same AM process and similar parts fabricated in different AM processes enables to study the repeatability and the reproducibility, respectively, of the AM processes. Finally, in comparison to the Archimedes’ method, the measurements using the gas pycnometer are much faster, so the method is convenient for routine control. However, the uncertainty on measurement is not as good as with the Archimedes’ method and the sample cell of the pycnometer is small (around 135 cm^{3}) which prevents measurements of larger parts.
3 Multifrequency eddy current method
The same cubic lattice specimens in TiAl6V, with different percentage of cells, measured previously by the Archimedes’ and pycnometric methods, were also tested by an eddy current method.
3.1 Measurement system
The transducer is configured in emitter/receiver mode and operates on a wide frequency bandwidth (10 kHz–40 MHz). The computer software performs analysis on the received signal and displays the complex impedance plane Z = R + jX. Each Z curve represents several single measurements performed on a sample at a different distance. The higher the slope of a curve, the higher the electrical conductivity of the sample. However, the curves are not completely linear; consequently, their slopes are not constant. The Sciensoria’s computer software makes use of an advanced signal analysis technique to attribute a unique conductivity value to each curve with very high precision. The system enables noncontact measurements and measurements are independent of the probe position.
3.2 Measurement protocol
A 12mmdiameter transducer at 1 MHz was used. Then, three successive measurements on each side of the cubic lattice specimens were performed. Before each measurement, a reference measurement in air was taken to get rid of the derivative of the system.
3.3 Results
Figure 10 highlights that the measurement method used is repeatable as well as the AM process used to fabricate the lattice specimens.
Figure 11 shows that the measurement method enables to distinguish differences between the surfaces of the lattice specimen. The base surface is the one which was directly in contact with the AM platform which significate that it had to be mechanically detached from it.
Figure 12 demonstrates that the measurement method enables to distinguish differences between two percentages of cells in lattice specimens and confirms that higher percentage of cells, i.e., less metal, has a lower electrical conductivity (the slope of the quasilinear curve of 79% cells is higher).
3.4 Benefits and limitations of the method
The multifrequency eddy current instrument ZScope*7 from Sciensoria enables to measure the apparent electrical conductivity of the samples which can be used to characterise a dense structure as well as a lattice one. The roughness of the sample surface does not prevent measurement as the probe is not directly in contact with the sample and even the method enables to distinguish two different surface textures as well as two different pore size lattice structures. Furthermore, comparing similar parts fabricated during the same AM process and similar parts fabricated in different AM processes enables to study the repeatability and the reproducibility, respectively, of the AM processes. Finally, the measurements are fast so the method is convenient for routine control. However, the measurements are limited to a depth close to the surface so does not characterise the volume in its entirety and is not appropriate for parts less than 1 cm^{3} in volume.
4 Cscan ultrasound characterisation method
4.1 Measurement system
4.2 Measurement protocol
4.3 Results
4.4 Benefits and limitations of the method
The Cscan ultrasound method from Eurosonic provides 3D images of a sample surface but also of the inside features of a sample. From these 3D images, nondestructive quality controls can be performed as well as dimensional measurements. Consequently, the compliance with the part geometrical specifications can be checked. Furthermore, the measurements are much faster than with the Xray tomography (XCT) so the method is convenient for routine control. However, the method is not suitable for complex geometry parts and less accurate than XCT.
5 Conclusion
Different volumetric methods were investigated in order to determine their capabilities for the characterisation of additively manufactured parts. First of all, an Archimedes’ and pycnometric methods were investigated to perform density measurements on dense and lattice structures. Then, an eddy current method to determine the electrical conductivity via complex impedance measurements of lattice structures was presented. Finally, a Cscan ultrasound method for the characterisation of dense parts was explored. For each of these methods, the advantages and limitations were underlined.
Notes
Funding information
The work was supported by the project “MetAMMI” and has received funding from the EMPIR programme cofinanced by the Participating States and from the European Union’s Horizon 2020 research and innovation programme.
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