_Applied_  
_sciences_  
_Article_  
Mapping Spatial Distribution of Pores in an Additively  
Manufactured Gold Alloy Using Neutron Microtomography  
**[PERSON] 1,*\({}^{\text{\textregistered}}\)\({}^{\text{\textregistered}}\), [PERSON] 2,*\({}^{\text{\textregistered}}\), [PERSON] 1, [PERSON] 2, [PERSON] 2, [PERSON] 1\({}^{\text{\textregistered}}\)\({}^{\text{\textregistered}}\) and [PERSON] 1\({}^{\text{\textregistered}}\)\({}^{\text{\textregistered}}\)**  
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Conventionally, porosity in a bulk material can be investigated using the Archimedes method, microscopy, or X-ray tomography. However, all of these methods are of limited use for AM-produced gold alloys. The Archimedes method [8] has been used in different studies to evaluate the relative density of parts [9; 10]. In L-PBF processing, due to the geometrical constraints and/or partial consolidation of powder particles, there might be particles left inside of pores after build completion [11]. Since the used immersion liquid cannot wet rough surfaces efficiently and also does not take these internally trapped powders into consideration, the Archimedes method is not a very reliable technique to precisely measure the porosity content in L-PBF samples. Furthermore, as the size of the precious metal samples is usually limited, the confidence level of the Archimedes method is low. Above all, the Archimedes method does not provide any information about the pore size and spatial distributions within the material, which are important parameters with respect to the abovementioned requirements [12]. Likewise, the high cost associated with material loss also seriously limits the use of optical microscopy on mechanical sections of representative volumes. [PERSON] et al. [7] conducted a full process study on the L-PBF of yellow gold alloys and analyzed the porosities in the printed samples by mechanical polishing. The effects of the heat treatment and alloying addition (Fe and Ge) on the optical reflectivity and, as a result, on the final porosity were analyzed. The average porosity using untreated powder particles was in the order of 3% to 4%. With the use of thermal heat treatment and alloying, [PERSON] et al. could decrease the final porosity to 0.7% and 0.1-0.3%, respectively. They used the addition of Ge to the yellow gold chemical composition to lower the solidus temperature, which provided easier conditions to melt the material. Moreover, the addition of Fe increased the oxidation of the powder particles and, as a result, decreased their reflectivity. In general, quantitative investigations can be performed using X-ray tomography [13; 14; 15; 16; 17; 18]. For example, [PERSON] et al. [19] observed the internal structure of AISi10 Mg L-PBF-printed samples to investigate the effect of the process parameters on the final porosity content, and to provide knowledge to control the quality of L-PBF parts. However, the penetration depth of X-rays in gold alloys is rather limited [20], which makes the investigation of representative volume elements hardly possible. Neutron tomography provides a viable alternative to visualize the distribution of pores in additively manufactured samples [21]. Here, recent advances in the available instrumentation have been utilized [22; 23] that allow resolution of features with dimensions of a few micrometers [24]. The neutron cross-section of gold and, hence, the penetration depth of thermal neutrons for gold alloys is such that it provides excellent transmission imaging contrast conditions for samples in the range of several millimeters in thickness. Thus, imaging with neutrons has evolved to be a highly suitable experimental technique for studying the relationship between porosity and L-PBF processing parameters in samples opaque to X-rays.