A Comprehensive Review of Laser Powder Bed Fusion in Jewelry: Technologies, Materials, and Post-Processing with Future Perspective

Abstract

In recent years, additive manufacturing (AM) has played a significant role in various fashion industries, especially the textile and jewelry manufacturing sectors. This review article delves deeply into the wide range of methods and materials used to make intricately designed jewelry fabrication using the additive manufacturing (AM) process. The Laser Powder Bed Fusion (L-PBF) process is examined for its suitability in achieving complex design and structural integrity in jewelry fabrication even with respect to powder metallurgy methods. Moreover, the review explores the use of precious materials, such as gold, silver, copper, platinum, and their alloys in additive manufacturing. Processing precious materials is challenging due to their high reflectivity and thermal conductivity, which results in poor densification and mechanical properties. To address this issue, the review article proposes three different strategies: (i) adding alloying elements, (ii) coating powder particles, and (iii) using low-wavelength lasers (green or blue). Finally, this review examines crucial post-processing techniques to improve surface quality, robustness, and attractiveness. To conclude, this review emphasizes the potential of combining additive manufacturing (AM) with traditional craftsmanship for creating jewelry, exploring the potential future directions and developments in the field of additive manufacturing (AM) for jewelry fabrication.

1. Introduction

Even though plenty of manufacturing processes are available for making jewelry components, additive manufacturing processes are preferred to subtractive manufacturing processing to reduce material wastage. Additive manufacturing (AM) is also known as 3D printing, rapid prototyping, layer-by-layer manufacturing, digital manufacturing, or solid-free form manufacturing processing, and it is used for the fabrication of intricate geometries from 3D model data [1]. AM was developed as a potential tool of fabrication as compared to the traditional manufacturing (TM) method because it has design flexibility, customization features, the ability to create complicated shapes, and wastage reduction. This technology uses layer-by-layer printing to create items with robust mechanical quality [1,2]. According to ISO/ASTM 52900:2015(E), there are two main types of AM processes: (i) single-step AM processes and (ii) multi-step AM processes [3]. In the first case, independently of the techniques, the work material may achieve its fundamental form and expected qualities in a single phase. Layer after layer, parts in metal, polymer, and composite materials can be built up over time. The adhesion principle is generally used in multi-step AM processes to create the intended part geometry, and then additional stages are used to create the part’s desired qualities. For instance, it is possible that the geometry was first acquired through the joining of metallic, ceramic, and composite elements. They could go through further processes, like sintering or infiltration, to obtain the desired qualities later [4,5,6]. A thorough analysis of the AM process was provided by Parupelli and Desai, which focused on the recent advancements made by numerous researchers and industries. A summary of the features, benefits, and drawbacks of each AM printing method was reported and presented in their work. The key advancements in AM technology in various industries, including electronics, medical, aerospace, automotive, and construction industries, were also examined [7].

The main focus of this review is on jewelry production, to emphasize, on the basis of published work on the subject [8], the potential that these new technologies may have for the jewelry industry compared to traditional methods (TM). AM offers the advantage of allowing greater design freedom, creating innovative and complex designs that would otherwise be challenging to construct using conventional techniques [9]. This is not the only important aspect. A clear example of the potentiality of AM is presented in Figure 1, Figure 2 and Figure 3. A gold pendant piece “the Ojo” was constructed using Laser Powder Bed Fusion (L-PBF) with the help of a support structure [10]. The supporting structure for making the jewelry was designed using CAD software (Figure 1). Figure 2 reports the fabrication step (processing) of the Au jewelry while Figure 3 underlines the surface finishing that can be obtained by additional polishing treatment (post-processing) [10]. The complexity of the part and its manufactured cost are directly correlated in the current standard jewelry manufacturing process; however, by using L-PBF methods, this relationship can be greatly reduced compared to the conventional method.

In the L-PBF production, in fact, around 30% of extra material was processed to create the support structure (in some geometries, this amount can be decreased to virtually 0%). In the TM, such as lost wax castings, the extra material is close to 60% and even if this value can be decreased by optimizing the casting piece ratio, it can never reach L-PBF values. Telma Ferreira et al. reported that L-PBF was an alternate and more efficient technique to fabricate jewelry components while reducing material costs and production time as compared to TM. In fact, comparing the processing time of silver and stainless-steel jewelry fabricated by TM and AM, AM jewelry (3 weeks) was made faster than TM jewelry (more than 10 weeks) [11,12,13].

Based on previous literature, this review discussed the current state of the art of AM for jewelry applications. Furthermore, the future direction of AM for Industry 4.0 jewelry manufacturing is outlined (Figure 4).

2. Powder Metallurgy (PM) and Advanced Manufacturing (AM) Techniques Used for Jewelry Fabrication

Traditional jewelry manufacturing incorporates established methods that perfectly blend timeless craftsmanship with precision techniques. The process involves traditional steps such as design conception, material selection, and model creation. Essential activities such as casting, wax carving, and handcrafting play a key role in ensuring meticulous detail.

Without dwelling on these traditional methods, which have very long lead times, this paragraph discusses the advantages and disadvantages of powder metallurgy and AM techniques for metal jewelry fabrication (Figure 5).

Both PM and AM technologies depend on powdered materials with well-defined properties and morphology to ensure the quality and performance of the final products. In these technologies, the powder characteristics, such as particle shape, particle size distribution, apparent and packing density, flowability, and morphology are crucial. These factors impact the powder behavior during processing, thereby affecting the mechanical properties and accuracy of the finished part. In particular, spherical particles (Figure 6) of well-controlled grain size are generally preferred due to their better flowability and packing density. To obtain these powder characteristics, gas atomization is a widely used method for producing metal powders with the desired properties for both PM and AM. In this process, molten metal is dispersed into fine droplets by a high-pressure gas stream, typically argon or nitrogen, which solidify into powder particles as they cool. Gas atomization produces highly spherical powders with controlled particle size distribution, and minimal contamination, making it a suitable technique for generating high-quality powders for these technologies [14,15].

2.1. Powder Metallurgy Method Used for Jewelry Fabrication

In addition to the traditional techniques, powder metallurgy (P/M) refers to the net-shape manufacturing of components using powdered metal. Various techniques within powder metallurgy, such as press and sinter (P&S), metal injection molding (MIM) and hot and cold isostatic pressing (HIP and CIP) were recently investigated [16,17]. In particular, powder metallurgical processing finds widespread applications in jewelry which show a quality comparable to cast parts. The primary advantage lies in the reduced part cost due to minimized material waste and processing expenses. Consequently, the direct application of powder metallurgy in jewelry fabrication offers significant benefits [16,17]. Peter M. Raw first presented the pioneering development of a powder metallurgical method for large-scale ring manufacturing. Initially focused on carat-gold rings and later extended to platinum, the ring fabrication process involved steps such as powder atomization, cold pressing, sintering, and rolling to achieve the desired ring dimension. The rings produced by powder metallurgy have a fine grain microstructure, which contributes to improved wear resistance and strength. The fine grain size, often observed in powder metallurgical products, was attributed to the existence of an extremely fine, sub-microscopic porosity that has the potential to stabilize grain boundaries. Consequently, the method has both economic and technical advantages, marking a significant milestone in ring manufacturing [18]. J. T. Strauss explored P/M technologies demonstrating their ability to create high-quality jewelry at reduced costs compared to conventional casting and machining processes. Despite this potential, the use of P/M in the jewelry industry has remained limited. The current status of P&S and MIM in the jewelry production industries was also examined by Titan and Hilderbrand, that employed press and sinter (P&S) and MIM techniques to fabricate Tanishq pure gold coins and 18-karat gold 3N pieces (Figure 7) [19,20]. Sintering, a heat treatment process that involves the consolidation of powdered materials under an optimized thermal cycle, offers numerous methods for seamlessly combining a variety of colors. K. Wiesner reported [21] that the sintering method has already been firmly established in the manufacturing of wedding rings (Figure 8).

The major disadvantage of these techniques is, of course, the simplicity of design that can be achieved by having to go through a pressing process. To enhance the process mechanism as much as possible, the optimal process parameters, such as compacting pressure and temperature, have to be identified.

2.2. Additive Manufacturing (AM) of Jewelry

In this section, the production of jewelry by means of Laser Powder Bed Fusion technology (L-PBF) is discussed in detail, bearing in mind that, in the bibliography, this technology can also be referred to as selective laser melting (SLM, a term introduced by the Fraunhofer Institute in 1995), direct metal laser melting (DMLM, a process patented in 1994 by EOS) or, finally, as laser powder melting (PLM).

L-PBF operates by employing a high-power laser to selectively melt and fuse metal powder, thereby incrementally building up parts layer-by-layer [22]. L-PFB technology is characterized by several variables that have to be optimized to increase the mechanical and physical properties of finished goods. The main factors include laser power, scanning speed, and powder layer thickness that are corrected by the Equation (1):

$$VED={\displaystyle \frac{p}{vhl}}$$

(1)

where p is the power, v is the scanning rate, h and l are the line offset and the layer thickness, respectively.

However, the selection of suitable L-PBF process parameters for jewelry alloys posed a considerable challenge due to the notably high thermal conductivities and reflectivities inherent in these metals and their alloys. Figure 9a provides a comparative analysis of the reflectivity of polished metal surfaces across various metal families and laser wavelengths. Reflectivity values for Au, Ag, and Cu exceeded 90% within the YAG laser wavelength (~1 μm), while cobalt, nickel, and iron fell within the 60–70% range. Although these specific reflectivity values pertained to polished solid-material surfaces, molten metal surfaces and powder particles would exhibit similarly absorption values. Consequently, in the case of Au-, Ag-, and Cu-based materials, a considerably larger percentage of the laser beam energy was reflected back rather than being absorbed as heat, as depicted in Figure 9b [22].

The local absorption of heat and energy by the powder was facilitated by the reflection of some of the incident laser energy between the surfaces of the adjacent powder particles. Subsequently, the locally absorbed heat from the powder was conducted into neighboring regions, comprising both the previously melted layers and the unmelted powder (Figure 9b). In order to achieve satisfactory processing of Au-, Ag- and Cu-based alloys with minimal residual porosity, combinations such as higher YAG laser power, lower scanning speed, lower layer thickness and shorter hatching distance had to be carefully considered [22].

In the following sections, the results that can be obtained using L-PBF on gold, silver, platinum and copper-base alloys, were discussed highlighting which strategies were used to increase laser absorption and thus achieve maximum densification.

2.2.1. AM of Gold Material

Additive manufacturing, especially L-PBF, was most suitable for fabricating gold jewelry to achieve complex and customizable designs that are otherwise too time-consuming and challenging to fabricate with the powder metallurgy method. L-PBF selectively melts the metal powder layer-by-layer to produce the bulk structure, which is guided by the 3D CAD models. Fischer-Buehner et al. fabricated 18 K yellow-gold (18 K YG) jewelry with different process parameter combinations using L-PBF. Figure 10 illustrates the impact of laser power on 18 K yellow-gold powder. The laser power was varied between 50 and 95 W, with a layer thickness of 20 μm and scanning rates ranging from 200 to 400 mm/s. In Figure 10a, it is evident that a 50 W laser power, with parameters similar to those commonly used for cobalt-chrome or steel, results in poor densification. However, the images in Figure 10b–d demonstrate that the increase in the laser power leads to an improved densification. A final laser power of 95 W yields low residual micro-porosity levels. Using the optimized parameters, the authors printed jewelry in 18 K YG as shown in Figure 10e–h. Metallographic cross sections confirm that excellent surface quality can be achieved, even with a low residual amount of microporosity [22].

Mushtaq Khan et al. presented the use of L-PBF with 24 K gold powder. Au cubes were created using a set of optimized parameters (laser powder of 50 W, hatch distance of 80 μm, layer thickness of 100 μm, and scanning speed of 65 mm/s). The hardness and the porosity of the gold cubes were examined but the analysis underlined that, even after the optimization process, a full densification cannot be obtained due to the decreased energy absorption by the gold powder, which prevents full fusion with the preceding layer, leaving inter-layer porosity. Reducing the layer thickness from 100 to 50 μm or changing the hatch distance did not significantly affect the density of the gold cubes [24]. Similarly, Zito et al. [25,26] reported the fabrication of 18 K gold alloy by L-PBF and the outcome was a reduction in strength, hardness, and high roughness compared to conventional casting processes. In particular, the hardness of 18 K palladium white gold is 174 ± 5 Hv (conventional) versus 154 ± 2 Hv (L-PBF); the ultimate tensile strength is 480 ± 25 MPa (conventional) versus 460 ± 39 MPa (L-PBF), and elongation to failure is 33 ± 9% (conventional) versus 21 ± 8% (L-PBF). However, the authors also reported that the production time for one piece is 34 h (conventional) versus 2 h (L-PBF); for ten pieces, 34.5 versus 8 h, respectively; for one hundred pieces, 37.5 versus 78.5 h, respectively underlining the main field of application of AM which becomes competitive for small batches of high geometric complexity. For mass production, the high productivity time that characterized L-PBF also brings a higher market price per gram (EUR 4–12) as compared to the conventional method (EUR 0.2–1). Also, the surface quality (Rt) of traditional casting and direct casting methods show a surface quality (Rt) of 22.0 µm and 27.3 µm, while surface quality of L-PBF is 31.3 µm. Additionally, for 18 K yellow gold, the investment casting using a wax pattern results in an ultimate tensile strength of 414.9 MPa and 42.4% elongation, whereas a resin pattern yields 395.9 MPa and 38.5% elongation. L-PBF outperforms both with an ultimate tensile strength of 474.2 MPa and 33.5% elongation. These comparisons highlight the variations in mechanical properties, surface quality, and costs associated with different manufacturing techniques (conventional and additive) that can be considered as a reference for the production of precious materials in the jewelry industry [25,26].

2.2.2. AM of Silver Material

Due to its high reflectivity, sterling silver also posed a significant challenge for L-PBF (Figure 9a). The poor laser absorption of silver led to the incomplete melting of the powder, which caused defects such as lack of fusion, porosity, and keyholes, as shown in Figure 11. Additionally, the high thermal conductivity makes the processes more challenging due to the rapid dissipating of heat from the laser interaction area. Therefore, the L-PBF process parameters have to be optimized to reduce the defects. In particular, high laser power (p), low scanning speed (v), low hatch distance (h), and low layer thickness (l) were utilized to enhance the performance. Laser powder between 350 and 370 W was identified for producing dense pure Ag structures by John Robinson et al. [27]. The influence of various process factors was examined in the formation of 3D silver structures revealing that specific parameters, namely layer thickness, scan speed, and hatch distance set at 30 μm, 400–500 mm/s, and 0.15 mm, respectively, resulted in a relative density of 97%. Also, two different substrates such as copper and stainless steel were utilized to build the pure Ag. Due to the high thermal conductivity of Cu, energy absorption is insufficient; therefore, Cu is not suitable for depositing silver, which can, instead, be printed on a stainless-steel substrate [27].

The effect of L-PBF process variables for producing Ag jewelry on the material microstructure was discussed by Korium et al. [28]. A silver ring obtained by L-PBF is represented in Figure 12. Ring support structure characteristics and proper geometry were established. The microstructures of silver samples had internal porosities and defects with a laser power of 50 W [28,29], achieving a relative density of 91.06%. Additionally, optimal process parameters are further required to fabricate the dense part with a reduction in porosity and other defects [30].

2.2.3. AM of Platinum Material

Additive manufacturing of platinum presents several challenges due to its unique material properties, which require precise control of laser process parameters to avoid defects such as porosity and cracks. Yadroitsava et al. explored the use of L-PBF for processing pure platinum with optimized laser power (300 W), scan speed (800 mm/s), and layer thickness (30 µm) to produce platinum parts with densities up to 99.98% and minimal porosity. However, achieving fine, uniform grain structures necessary for high-quality platinum parts demands a delicate balance between build speed and part quality. The high thermal conductivity can lead to rapid heat dissipation, making it difficult to maintain the melt pool stability and achieve uniform layer fusion [31]. Figure 13 shows a platinum ring with a defective surface in the right-side ring shank due to an irregular melt and unsupported structure. This defective surface was analyzed by SEM (Figure 14), which shows a highly porous surface due to the balling effect of the melt pool and the adhesion of the powder particles (Figure 15). Klotz et al. fabricated a platinum ring with high relative density (99.8%) with the optimized process parameters of 500 mm/s as the scanning speed, 63 µm of hatch distance, and 95 W of laser power (using a Nd:YAG laser with a spot size of 30 µm and a wavelength of 1064 nm) [32].

Vorobyev and Guo [33] investigated femtosecond laser processing of platinum to address some of these difficulties. The study demonstrated that femtosecond laser blackening significantly enhanced absorptance, achieving about 95% over a broad wavelength range (250–2500 nm). Despite these advancements, maintaining high absorptance and reducing reflectance (below 5%) across such a wide spectrum involves meticulous control over the laser parameters and processing environment. Blackened platinum surfaces were produced without chemical changes, underscoring the importance of physical modifications to overcome the material’s natural reflectivity [33].

2.2.4. AM of Pure Copper Material

The fabrication of pure copper via laser-based additive manufacturing has huge challenges due to the high reflectivity, high thermal conductivity, low laser absorption, high electrical conductivity, and oxidation sensitivity of copper with the 1070 nm wavelength infrared laser (Figure 16a) [34,35]. Due to the low absorption behavior of pure copper, the fabricated parts, in general, exhibit various defects such as low density, high porosity, balling effect, layer delamination, and crack formation (Figure 16c–f) [36,37]. Also, the high reflectivity of copper substrate can lead to damage in the optical mirror [38], as shown in Figure 16b [39,40]. The high reflectivity and thermal conductivity of copper require the use of higher levels of laser power and careful control of process parameters to achieve the desired material properties. A bi-directional scanning strategy with a 90° rotation between subsequent layers, using a laser power of 725 W and a scan speed of 400 mm/s, can be preferably used; however, despite these efforts, achieving uniform and fully dense parts was challenging due to the inherent properties of copper [41].

For example, a relative density of 99% was achieved with a laser power of 500 W and a volumetric energy density range of 230–350 J/mm³. The L-PBF fabricated parts demonstrated tensile properties with a yield strength of 122 ± 1 MPa, an ultimate strength of 211 ± 4 MPa, and an elongation of 43 ± 3% [42]. Further, the densification and the thermal and electrical conductivity of the parts were enhanced by high laser power [43,44]. In the future, copper jewelry could be printed using non-thermal AM techniques such as cold spray additive manufacturing. This technique is similar to the traditional powder metallurgy route, though it lacks the flexibility and customization capabilities provided by other 3D printing methods [45].

2.3. Strategies to Improve the L-PBF Processability of Precious Metals

Although laser additive manufacturing technology has considerable advantages, there are some problems that need to be eliminated. As already reported, the high reflectivity of materials such as gold, silver, platinum, and copper means that a significant portion of the laser energy is reflected from the material surface. This reflection reduces the efficiency of the melting process, making it difficult to achieve the desired density and uniformity in the fabricated parts. High reflectivity can also damage the laser optical system, further complicating the manufacturing process. This low absorption rate requires the use of higher laser power, which can increase the risk of thermal distortion and residual stresses in the final product.

The methods suggested in the bibliography to overcome these problems are covered below. These methods involve the use of various alloying elements, powder coating, or the use of low wavelength lasers to enhance the metal absorbance. Alloying elements like gallium, germanium, and silicon can reduce reflectivity and improve laser absorption, leading to better fusion and reduced porosity. Powder coating can further enhance laser energy absorption and melting efficiency. Low-wavelength lasers (i.e., green or blue lasers) provide higher absorptivity, enabling the fabrication of high-quality components with improved mechanical properties (Figure 17).

2.3.1. Dopant Addition Strategy

The state of the art in the selection and analysis of the influence of doping of precious materials is illustrated in the following sections, broken down by the type of base metal.

Gold-Based Alloys

L-PBF of gold alloys has made significant advancements in achieving higher densification, reduced porosity, improved mechanical properties, and enhanced hardness. These improvements are largely attributed to the careful selection and incorporation of alloying elements, which increase the absorptivity of gold (Figure 18).

Zito et al. reported several innovative materials that can be used to create a wide range of jewelry pieces (Table 1). An element that can increase the gold absorption, i.e., is gallium. The authors reported two different gold alloy powders (Au:Ag:Cu:Ga = 18.8:1:5.2:0 and Au:Ag:Cu:Ga = 37.6:2:9.4:1) successfully manufactured using L-PBF. The addition of gallium resulted in low surface tension, which plays a key role in achieving the proper wettability for better adhesion between the deposition layers to produce defect-free AM components [46]. The improvement of surface quality and the reduction of porosity was also obtained by the addition of Ge and Si (Table 1). Figure 19 shows the gold alloy ring fabricated by L-PBF with two different set of parameters (standard and optimized) which differ mainly for the overlapping rate. Due to the higher laser overlapping rate, the jewelry fabricated with the optimized parameters (65 W, 330 mm/s, layer thickness 20 μm, and 75% of overlapping) ensured high densification with less defects and higher resolution with respect to the standard one (76.2 W, 250 mm/s, layer thickness 30 μm, 50% of overlapping). Moreover, the use of finer particle sizes (<15 μm) allowed for obtaining higher resolutions and smoother surfaces of the jewelry parts (Figure 20) [47].

The surface finish plays a key role in jewelry production. D. Zito et al. reported that the surface roughness is reduced from 72 to 55 µm with the addition of germanium in 18 K red gold alloy. Moreover, a better ductility was obtained with the addition of germanium between 0.2 and 0.4 wt%, while above 1.5–2 wt%, the jewelry became unacceptably brittle. In this case, the optimal process parameters for achieving an improved surface finish and reduced porosity are a laser power of 72.5 W, layer thickness of 140 μm, and a scanning speed of 0.33 m/s.

The authors also compared the effect of the addition of niobium and of both niobium and titanium in a gold–silver matrix (Table 1) and observed that almost equivalent hardness and significantly lower density were obtained: approximately 25 and 18% less than an equivalent alloy with palladium and the traditional white gold-nickel alloy [48].

The same authors [49] demonstrated the possibility of increasing the absorption of laser radiation by adding a small concentration of semiconductor elements (Si, Ge) that increase the electrical resistivity and can reduce the thermal conductivity of the red gold alloy, thus improving the L-PBF behavior. The presence of semiconductor elements reduces the porosity and improves the process stability [49,50].

Klotz et al. achieved an improvement in surface quality by optimizing powder composition (Table 1) with Ge, Fe or Ti. Microstructural analysis showed fine grains and low porosity [51] giving the possibility to create intricate and high-end jewelry [52].

Table 1. Representation of the elemental composition of yellow, red, and white gold (18, 14, 10, and 9 carats) powder alloy by Damiano Zito et al. and U.E. Klotz et al. [47,48,51,53].

Element Compositions:	Alloy	Gold (Au) wt%	Copper (Cu) wt%	Palladium (Pd) wt%	Nickel (Ni) wt%	Silver (Ag) wt%	Germanium (Ge) wt%	Silicon (Si) wt%	Niobium (Nb) wt%	Zinc (Zn) wt%	Titanium (Ti) wt%	Iron (Fe) wt%	Pd: Ag wt%	Cu: Ni wt%	Cu:Ag wt%
18 K Yellow gold powder alloy [47,53]	1	75.2	12.4	-	-	12	0.4	-	-	-	-	-	-	-	1:1
Gold alloy (Au, Ag, Pd, Cu, Si) [48]	12	76.26	13.5	1.93	-	4.69	-	3.62	-	-	-	-	-	-	-
Gold alloy (Au, Nb, Zn) [48]	13	75.2	-	-	-	-	-	-	18.5	6.3	-	-	-	-	-
Gold alloy (Au, Ag, Ti) [48]	14	75.2	-	-	-	8.4	-	-	-	-	16.4	-	-	-	-
3N [51]	15	75.1	12.5	-	-	12.4	-	-	-	-	-	-	-	-	-
Fe003 [51]	16	75.0	13.0	-	-	11.3	-	-	-	-	-	0.7	-	-	-
Ge001 [51]	17	75.1	12.8	-	-	11.8	0.4	-	-	-	-	-	-	-	-
Ti001 [51]	18	75.2	12.8	-	-	11.7	-	-	-	-	0.3	-	-	-	-
18 K Red gold powder alloy [53]	2	75.2	21	-	-	3.6	0.2	-	-	-	-	-	-	-	5.5:1
14 K Red gold powder alloy [53]	3	58.7	33.45	-	-	7.45	0.4	-	-	-	-	-	-	-	4.5:1
10 K Red gold powder alloy [53]	4	41.9	47.06	-	-	10.64	0.4	-	-	-	-	-	-	-	4.5:1
9 K Red gold powder alloy [53]	5	37.7	50.46	-	-	11.44	0.4	-	-	-	-	-	-	-	4.5:1
18 K White gold powder alloy [53]	6	72.5	-	12.4	-	12.2	0.2	-	-	-	-	-	1:1	-	-
14 K White gold powder alloy [53]	7	58.5 to 59.5	-	15 to 25	-	15 to 25	0.01 to 1	-	-	-	-	-	0.75:1 to 1:0.75	-	-
10 K White gold powder alloy [53]	8	41.7 to 42.5	-	25 to 45	-	25 to 45	0.01 to 2	-	-	-	-	-	0.75:1 to 1:0.75	-	-
18 K White gold powder alloy [53]	9	72.5	17.1	-	7.5	-	0.2	-	-	-	-	-	-	2:1 to 3:1	-
14 K White gold powder alloy [53]	10	58.5 to 59.5	23 to 35	-	8 to 20	-	0.01 to 1.5	-	-	-	-	-	-	2:1 to 3:1	-
10 K White gold powder alloy [53]	11	41.7 to 42.5	35 to 50	-	12.5 to 35	-	0.01 to 2	-	-	-	-	-	-	2:1 to 3:1	-

Table 1. Representation of the elemental composition of yellow, red, and white gold (18, 14, 10, and 9 carats) powder alloy by Damiano Zito et al. and U.E. Klotz et al. [47,48,51,53].

Element Compositions:	Alloy	Gold (Au) wt%	Copper (Cu) wt%	Palladium (Pd) wt%	Nickel (Ni) wt%	Silver (Ag) wt%	Germanium (Ge) wt%	Silicon (Si) wt%	Niobium (Nb) wt%	Zinc (Zn) wt%	Titanium (Ti) wt%	Iron (Fe) wt%	Pd: Ag wt%	Cu: Ni wt%	Cu:Ag wt%
18 K Yellow gold powder alloy [47,53]	1	75.2	12.4	-	-	12	0.4	-	-	-	-	-	-	-	1:1
Gold alloy (Au, Ag, Pd, Cu, Si) [48]	12	76.26	13.5	1.93	-	4.69	-	3.62	-	-	-	-	-	-	-
Gold alloy (Au, Nb, Zn) [48]	13	75.2	-	-	-	-	-	-	18.5	6.3	-	-	-	-	-
Gold alloy (Au, Ag, Ti) [48]	14	75.2	-	-	-	8.4	-	-	-	-	16.4	-	-	-	-
3N [51]	15	75.1	12.5	-	-	12.4	-	-	-	-	-	-	-	-	-
Fe003 [51]	16	75.0	13.0	-	-	11.3	-	-	-	-	-	0.7	-	-	-
Ge001 [51]	17	75.1	12.8	-	-	11.8	0.4	-	-	-	-	-	-	-	-
Ti001 [51]	18	75.2	12.8	-	-	11.7	-	-	-	-	0.3	-	-	-	-
18 K Red gold powder alloy [53]	2	75.2	21	-	-	3.6	0.2	-	-	-	-	-	-	-	5.5:1
14 K Red gold powder alloy [53]	3	58.7	33.45	-	-	7.45	0.4	-	-	-	-	-	-	-	4.5:1
10 K Red gold powder alloy [53]	4	41.9	47.06	-	-	10.64	0.4	-	-	-	-	-	-	-	4.5:1
9 K Red gold powder alloy [53]	5	37.7	50.46	-	-	11.44	0.4	-	-	-	-	-	-	-	4.5:1
18 K White gold powder alloy [53]	6	72.5	-	12.4	-	12.2	0.2	-	-	-	-	-	1:1	-	-
14 K White gold powder alloy [53]	7	58.5 to 59.5	-	15 to 25	-	15 to 25	0.01 to 1	-	-	-	-	-	0.75:1 to 1:0.75	-	-
10 K White gold powder alloy [53]	8	41.7 to 42.5	-	25 to 45	-	25 to 45	0.01 to 2	-	-	-	-	-	0.75:1 to 1:0.75	-	-
18 K White gold powder alloy [53]	9	72.5	17.1	-	7.5	-	0.2	-	-	-	-	-	-	2:1 to 3:1	-
14 K White gold powder alloy [53]	10	58.5 to 59.5	23 to 35	-	8 to 20	-	0.01 to 1.5	-	-	-	-	-	-	2:1 to 3:1	-
10 K White gold powder alloy [53]	11	41.7 to 42.5	35 to 50	-	12.5 to 35	-	0.01 to 2	-	-	-	-	-	-	2:1 to 3:1	-

Silver-Based Alloy

The unique properties of silver, such as its high thermal and electrical conductivity with low absorptivity, lead to porous L-PBF components characterized by low mechanical properties. Therefore, the addition of alloying elements, such as Cu and La₂O₃, to enhance densification, has been widely explored.

The normal sterling silver alloy powder, primarily composed of Ag and Cu, produced unsatisfactory results (Figure 21a) as reported by Fischer-Buehner et al. [22]. Figure 21b–d show similar results for three distinct 925 silver alloy powders, characterized by proprietary additives at the expense of copper. The composition of the additives is not disclosed by the author because the project is still in the research stage. Figure 21e–h display jewelry pieces in 925 Ag, demonstrating that finished surfaces can achieve exceptionally high quality, and are challenging to replicate with even the best casting techniques [22]. The 925 Ag alloy was processed with a laser power of 180 W, a scanning speed of 600 mm/s, a layer thickness of 30 µm, and a hatch spacing of 0.060 mm, reaching a relative density of 96.56% [30]. The microstructure reveals both columnar and elongated grains as shown in Figure 22. Wang et al. and Xiong et al. highlighted that the copper content in 925 silver alloy increased laser absorption, making it easier to process if compared to pure silver [30,54].

Zhao et al. [55] explored the addition of La₂O₃ to Ag–Cu alloys to further enhance their performance. The addition of La₂O₃ not only enhanced laser absorption but also promoted grain refinement, thereby improving both the mechanical properties and the corrosion resistance of the alloy. In the research, silver with 5.626% of copper was produced by varying the content of La₂O₃ (0, 0.4, 0.8, and 1.2%). The best results were obtained with 60 W of laser power, a scanning speed of 1000 mm/s, and a layer thickness of 20 µm. The addition of 1.2% of La₂O₃ resulted in the highest density (9.16 ± 0.55 g/cm³) and significantly improved the mechanical properties, including a nano-indentation hardness of 2.88 GPa with a decrease in porosity as La₂O₃ was increased (Figure 23a–e) [55].

Platinum-Based Alloy

The addition of semiconductor materials to platinum material significantly enhances their performance in the additive manufacturing process. For example, 800, 850, 900, and 950 Pt alloys were discussed by Corti et al., and the addition of gallium with Pt alloys was studied to enhance the densification and minimize the porosity [56]. Zito et al. investigated a 95PtGaInCu alloy using L-PBF with a 100 W fiber laser, collimated in a 10 µm diameter spot. The optimal processing parameters included a laser power range of 72.5–80 W, a layer thickness of 20 µm, a scanning speed of 0.25 and 0.33 m/s, and a spot distance of 40 µm. The as-built samples exhibited a porosity of 0.06 µm, surface roughness of 40 ± 13 µm, hardness of 222 ± 4 HV, ultimate tensile strength (UTS) of 582 MPa, and elongation of 14.5% [57]. Heat treatment improves hardness and ductility by 16% and 50% as compared to the as-built sample [57].

Figure 24 shows the effect of different element addition on the processability, assessed by single scan tracks (SSTs), of platinum alloy. For example, 2% of Ge has a strong effect with an increase in the track depth; Ga addition, instead, lead to hot cracking. These SSTs indicated that different alloy elements strongly affect processability, emphasizing the importance of optimizing compositions and process parameters.

Regarding rhodium (PtRh 90-10), it not only increases the material absorption capacity, but also allows for a more uniform and denser microstructure, comparable to conventionally manufactured parts, although the addition of 10% rhodium showed a higher electrical resistivity due to internal vacancies [58]. Wermuth et al. characterized L-PBF samples of platinum rhodium alloy (PtRh 80-20), achieving full density with minimal porosity [59].

Overall, the integration of semiconductor elements like gallium, indium, tin, germanium, zinc or rhodium in platinum alloys significantly improves their performance in additive manufacturing [60]. These enhancements include higher density, reduced porosity, better mechanical properties, and increased suitability for complex geometric designs, making these alloys highly valuable for additive manufacturing in jewelry.

Cu-Based Alloy

Pure copper shows high reflectivity and low absorptivity during the melting process by laser radiation. The high reflectivity represents a high risk due to the increase in the thermal distortion and residual stress of the build components. Therefore, to reduce the reflectivity and increase the absorptivity, copper alloy powders were developed. The addition of pre-alloy powder to pure copper led to reducing the reflectivity from 95 to 55.3% for Cu-15Sn, to 90% for Cu + 2% CNTs (carbon nanotubes), to 32% for Cu + 0.1C and to 44.2% for CuCr0.3 + 0.05C, as shown in Figure 25a–d [61,62,63,64]. Held et al. reported that the addition of tin to copper alloy reduces reflectivity, surface tension, and electrical and thermal conductivity to 80.8%, 1061 mN/m, 12.1% IACS, and 48.5 W/mK. As a result, the porosity decreased, and an enhancement of surface quality can be observed [65].

Ayub et al. studied the effect of the addition of different concentrations of CNTs, underlining that CNTs significantly enhance the thermal absorption of copper, allowing for lower laser power levels (<40%), reducing the reflectivity, and enhancing laser absorption. In addition, the densification of the sample with 1.5% CNTs is greater than for pure copper and samples with fewer CNTs, as shown in Figure 26a–d [62].

Jadhav et al. reported that the addition of carbon leads to an in situ deoxidation during the process, although the produced copper samples have a tensile strength of 125 MPa, a ductility of 3%, and an electrical conductivity of 22.7 × 10⁶ S/m, limited by the segregation of carbon nanoparticles and impurities, such as oxygen and phosphorus, at the grain boundaries. Moreover, microstructural analyses showed that defects, such as surface cracks, are primarily owing to the segregation of carbon at the grain boundaries (Figure 27a–d) [63].

Jadhav et al. studied the L-PBF behavior of both virgin and carbon-mixed CuCr0.3 powders. Additionally, in this case, carbon nanoparticles facilitate the in situ deoxidation of copper and chromium oxides. The mechanical properties were improved due to the precipitation of nanometric metallic chromium precipitates in the copper matrix (Figure 28c,f). Figure 28a,b show the microstructure of samples with epitaxial columnar grains growth, parallel to the build direction with carbon segregation at the grain boundaries (Figure 28d,e). These results confirm that carbon addition enhances optical absorption without adversely affecting powder properties, and significantly improves mechanical and electrical properties through deoxidation and subsequent precipitation strengthening [64,66].

Robinson et al. studied the effect of the addition of silver on the mechanical and thermal properties of pure copper by L-PBF, observing a significant improvement in mechanical properties [67]. For instance, the CuAg10 alloy, consisting of 92.51% Cu and 7.49% Ag, exhibited a yield strength of 223.5 MPa and an ultimate tensile strength (UTS) of 277.7 MPa. As the silver content increased, the mechanical properties further improved, with CuAg30 (64.20% Cu, 35.80% Ag) showing a yield strength of 330.3 MPa and a UTS of 381.8 MPa [68]. The thermal properties of these alloys were also noteworthy. CuAg alloys maintained high thermal performance with minimal variation despite increasing silver content, showcasing their potential for applications requiring excellent thermal management [68].

In Figure 29a–d, a tin-bronze alloy with a laser power of 95 W, a layer thickness of 20 μm, and varying laser scan speeds from 50 to 400 mm/s was reported. Minimizing the scan speed results in low residual levels of micro-porosity even at the lowest scan speed of 50 mm/s because it greatly enhances the energy available for local melting per volume unit. Instead, decreasing the laser scan speed leads to an increase in processing time and the final L-PBF fabricated product (Figure 29e–h) needs a surface finish post-processing treatment [22].

2.3.2. Powder Coating Strategy

A novel approach to overcome the challenges (high reflectivity and thermal conductivity) has been experimented by modifying the powder surface by coating it with less reflective materials. This approach has been used mainly for copper. Coated powders, such as nickel-coated copper powder (Figure 30a–c), have a higher absorption of laser energy (Figure 31a), thus improving the melting efficiency and leading to higher densification and improved mechanical properties. The nickel coating of copper powder particles was achieved by the immersion plating technique. The uncoated copper powder required high energy density from 200 to 500 J/mm³ to melt the powder while the coated powders achieve higher densities with 236 J/mm³ (Figure 32a–d) [69,70,71].

A different approach has been proposed for a CuCr1 powder. The method involves the outward diffusion of chromium in a nitrogen atmosphere, which allows for the formation of a chromium-nitride coating which significantly reduces the optical absorption of the powders (Figure 31b). Moreover, the surface-modified powder exhibited enhanced flowability and minimized oxygen uptake during storage, crucial for maintaining consistent L-PBF performance and part quality. For these reasons, the surface-modified powders have higher densification as compared to unmodified powders (Figure 33a–d) [72].

The highly conductive CuSn0.3 alloy was fabricated via L-PBF using tin-coated copper powder. The copper powder particles were coated with the immersion plating technique that involves a powder pretreatment to remove contaminants (alkaline solution at 80 °C for 5 min) and oxides (0.1 M HCl solution for 1 min), followed by coating and drying. Moreover, the research underlines the detrimental impact of high sulfur content (0.091 wt%) on the L-PBF process, which leads to solidification cracks and porosity, emphasizing the importance of limiting sulfur content to below 0.0025 wt% for optimal results. The coating facilitated the production of crack-free, fully dense copper parts with high thermal conductivity (334 ± 4 W/(m·K)), electrical conductivity (80 ± 1% IACS), tensile strength (256 ± 14 MPa), yield strength (203 ± 4 MPa), and ductility (21 ± 2%) using a fiber laser at 500 W. [73].

Tin and nickel coatings on copper powders for L-PBF was reported by Lindström et al. [74]. The tin and nickel coatings were performed by the immersion deposition method. The coated powders exhibited significantly lower reflectivity (~40%) compared to pure copper (~70%), resulting in lower porosity and better densification (Figure 34) [74].

Wadge et al. examined the impact of titanium (Ti) coatings on silver powders to improve absorptivity (from 27% to 45%). A thin and uniform Ti-coating was achieved by physical vapor deposition (pulsed DC magnetron sputtering) optimizing the process parameters (50 W, 3 h). The titanium-coated silver powders demonstrated a significant enhancement over uncoated powders that allow also to reduce the laser power (280 W for coated versus 320 W for uncoated), indicating a more efficient melting process. The enhanced absorptivity facilitated more uniform densification of the powder bed at lower energy densities, resulting in reduced porosity (0.7 ± 1.0% compared to 7.1 ± 2.0% for uncoated powder at 133 J·m⁻¹) [75].

Therefore, the various coated metal powders, including steels, titanium, aluminum, and copper, were examined for their performance in LPBF by Bidulsky et al. The coatings were obtained by several coating methods such as mechanical methods (ball milling, Turbula mixing) and non-mechanical methods (powder immersion, physical vapor deposition, chemical vapor deposition). The mechanical methods are simple and cost-effective but they affect the powder morphology, whereas non-mechanical methods allow one to obtain uniform coatings with methods that are more costly and complex [76]. Finally, titanium diboride (TiB₂), aluminum oxide (Al₂O₃), graphene oxide (GO) and carbon nanotube (CNT) coatings improved hardness, wear resistance, and mechanical properties, achieving relative densities close to the theoretical values [76,77].

To conclude, the use of coated powders in L-PBF processes enhances the absorptivity, flowability, and overall quality of the final parts.

2.3.3. Use of Laser with Lower Wavelength

A very interesting technological approach is to process highly reflective metals using blue laser (wavelength 450 nm, developed by group of researchers led by Prof. Masahiro Tsukamoto at Osaka University’s Joining and Welding Research Institute, in collaboration with Shimadzu Corporation, Nichia Corporation and Furukawa Electric Co., Ltd., in Japan) or green laser (wavelength 515 nm, developed by Fraunhofer Institute of Laser Technology (ILT), Aachen, Germany) characterized by high power and shorter wavelengths than common red lasers. At these wavelengths, in fact, highly reflective materials have a higher absorption of the laser, as shown in Figure 35 [61,78].

Green Laser

The use of green lasers, specifically at a wavelength of 515 nm, has been shown to significantly improve the performance of the laser-based additive manufacturing of pure copper with absorption in the range of 40–60% [79,80]. In particular, green lasers reduce reflective losses resulting in parts with better density, lower porosity, improved surface finish, less spatter, and higher productivity (Figure 36) [81]. The pure copper powder, obtained with a green laser, achieved relative densities above 99.95% and electrical conductivity up to 100% IACS, showing superior performance compared to infrared lasers [82,83].

De Terris et al. reported the fabrication of pure copper via L-PBF using both green and infrared laser beams. The green laser, which involves the complete melting of the material, produces superior properties in terms of mechanical, electrical, and thermal performance as compared to the infrared laser. The lack of fusion and keyholes that characterized the microstructure of samples obtained by infrared laser (Figure 37a–c) are completely absent in the samples obtained by the green laser (Figure 37d–f) [84]. The use of a green laser was found to potentially eliminate the need for oxidation or other powder treatments to produce materials with a high thermal conductivity. Porosity was minimized with specific combinations of laser power and scanning speed, achieving a high relative density of up to 99.9% [85,86].

Blue Laser

The application of blue lasers, particularly at a wavelength of 450 nm, has significantly improved the performance of additive manufacturing processes for precious materials due to their high absorptivity (Figure 35c) [78]. Published papers on the use of blue lasers in L-PBF are scarce. Wu et al. reported [87] that the increased absorption allows one to obtain higher densification and reduce the porosity in the final product. The manufactured parts exhibit an almost complete density, up to 99.6%, which enhances the mechanical properties and structural integrity of the material [87]. Other papers were found if weldability is considered. Recently, Pasang et al. further explored the mechanical properties and weldability of pure copper foil using blue diode lasers [75]. Blue diode lasers are absorbed the most (60%), significantly outperforming IR (5%) and green lasers (50%) [88].

Therefore, blue laser processing offers substantial benefits for copper welding and additive manufacturing. The high absorption efficiency of blue lasers at 450 nm leads to better energy utilization and favorable microstructural characteristics [89]. The findings underscore the potential of blue laser technology to overcome the limitations of traditional IR lasers.

2.4. AM of Precious Materials: Conclusion

Additive manufacturing has revolutionized the production of precious metals such as gold, platinum, silver, and copper. It enables intricate designs and complex structures with unprecedented precision. Researchers are actively investigating precious materials with improved properties such as strength, conductivity, and durability through the modification of materials (the addition of specific elements or coatings) and technologies (green or blue lasers).

Table 2. Summary of precious materials used in L-PBF of jewelry applications with process parameters and performance characteristics.

Materials	Composition/Strategy	Laser Power (W)	Layer Thickness (μm)	Scanning Speed (mm/s)	Hatch Distance (μm)	Relative Density (%)	Porosity (%)	Ref.
Gold	Yellow-gold alloy (75Au12.5Ag12.5Cu)	95	15–25	250–450	-	-	±0.2	[23]
	24-carat gold	50	100	65	80	-	<10.4	[24]
	750% yellow-gold alloy	65	30	330	35	-	-	[47]
	Au, Ag, Ti	50	-	250	150	-	0.22 ± 0.04	[48]
	750‰ red-gold alloy addition germanium	72.5	140	330	-	-	<0.5	[49]
	3N YG + Fe003	90	15	250–450	36	97	<0.5	[51]
	3N YG + Ge001	90	15	250–450	36	99.7	<0.3	[51]
	Fe003 + Ge001	90	15	250–450	36	99.9	<0.2	[51]
	Gold alloy: Table 2	72.5	140	330	-	-	<0.5	[53]
	Ag–Cu alloys with La2O3 of 0%	600	20	1000	-	84.7	-	[55]
	Ag–Cu alloys with La2O3 of 0.4%	600	20	1000	-	88.8	-	[55]
	Ag–Cu alloys with La2O3 of 0.8%	600	20	1000	-	93.3	-	[55]
	Ag–Cu alloys with La2O3 of 1.2%	600	20	1000	-	99.9	-	[55]
	Gold alloy	72.5	140	330	-	-	0.01	[65]
Silver	99.9% Pure silver	370	30	550–800	150	97	2.72–34.01	[27]
	Silver	350–370	30 and 60	400–500	150	97	±3	[28]
	99.9% Pure silver	400	30	400 and 800	150	99.8	-	[29]
	99.9% Pure silver	430	30	400	80	91.06	-	[30]
	925 silver	180	30	600	60	96.56	-	[30]
	Ag7.5Cu alloy	120	30	400	-	96.7	-	[54]
	Pure Silver (>99%)	370	30	400	140	89.9	-	[67]
	Uncoated silver powder	320	-	500	-	-	3.9	[75]
	Titanium (Ti) Coated silver powder	320	-	500	-	-	0.2	[75]
Platinum	Pure Platinum (Pt)	175	30	600	80	99.98	0.01	[31]
	950Pt alloy (950Pt-Ga-Cu)	95	20	500	63	>99.8	<0.1	[32]
	950‰ platinum alloy	62.5	140	250–330	-	-	<0.06	[49]
	950‰ Platinum	72.5	140	250–330	-	-		[50]
	95PtGaInCu alloy	72.5–80	20	250–330	-	-	0.06	[57]
	950Pt with Au, In and Ru	90	15	-	-	99.97	-	[60]
	80Pt20Ir	90	20	-	-	99.97	-	[60]
	50Pd50Rh	90	20–50	-	-	99.8	-	[60]
	50Pt50Rh	90	20–50	-	-	98.5	-	[60]
Copper	Pure copper 99.9%	350–1000	30–50	400–600	80	>99	-	[34]
	Pure copper powder 99.9Cu0.08O0.15P	600–1000	50	1000–5500	70	99.1	-	[35]
	Electrolytic Tough Pitch copper > 99.90%	400–500	30	400–600	-	99.82	-	[36]
	Pure copper	600–800	30	200–400	70–90	98	-	[38]
	Pure copper	300–800	30–50	200–1500	20–120	99.1	-	[39]
	Copper (Cu)	400	30	600	100	99.82	-	[40]
	99.8 wt% Cu	175	20	-	-	98.1	-	[41]
	Pure copper (Cu)	500	30	800	90	99.3 ± 0.2	-	[42]
	Pure copper (Cu)	370	30	1000	100	99.5	-	[43]
	Virgin CuCr0.3	600	30	300 and 400	90	98.5	-	[64]
	Carbon-mixed CuCr0.3: 0.05% Carbon	600	30	300 and 400	90	98.5	-	[64]
	Carburized CuCr1 powder	500	30	700	90	99.2	-	[66]
	Heat treated	500	30	700	90	99.3	-	[66]
	Pure Copper (>99%)	370	30	400	140	99.9	-	[67]
	CuAg10: 92.51% Cu, 7.49% Ag	370	30	400	140	100	-	[67]
	CuAg20: 72.50% Cu, 27.50% Ag	370	30	400	140	100	-	[67]
	CuAg30: 64.20% Cu, 35.80% Ag	370	30	400	140	100	-	[67]
	CuAg10: 92.51% Cu, 7.49% Ag	400	30	400	140	87	-	[68]
	CuAg20: 72.50% Cu, 27.50% Ag	400	30	400	140	83	-	[68]
	CuAg30: 64.20% Cu, 35.80% Ag	400	30	400	140	99	-	[68]
	Coated materials: Sn, Ni, and CrZr, etc.	300	30	800	100	>99	-	[76]
	IR laser	425–750	30	1000	-	>99	-	[81]
	Green laser	425–750	30	1000	-	>99	-	[81]
	Deoxygenated oxygen-free pure copper (Cu-OF)	500	30	-	120	99.8	0.017 ± 0.024	[82]
	Oxygenated electrolytic tough pitch copper (Cu-ETP)	500	30	-	120	99.8	0.013 ± 0.009	[82]
	IR laser	500	30	-	-	99.8	-	[84]
	Green laser	500	30	-	-	99.8	-	[84]
	Pure copper (Cu-OFE, C10100)	500	30	250–350	100	99.9	-	[85]
	Blue laser (450 nm)	650	-	1000	-	99.6	-	[88]
	Pure copper (99.9 wt%)	370	40	500	100	96	-	[90]
	Pure copper	500	30	400	90	97.5	-	[91]
	Virgin CuCr1 alloy	500	30	200	90	98.64	-	[92]
	Surface-modified CuCr1 alloy	500	30	800	90	99.1	-	[92]
	Pure copper	500	30	500	90	99.6	-	[93]

provides a summary of the main materials used in the literature for AM jewelry manufacturing, indicating process parameters and obtained properties.

Despite the potential benefits, the proposed methods may face challenges, such as the need for precise control over alloy compositions and coating processes to achieve consistent results. Furthermore, the implementation of low-wavelength lasers may require modifications to existing equipment and processes. Optimizing the parameters for each specific material will be crucial to achieving the desired performance outcomes.

3. Surface Finishing and Post-Processing in Jewelry Fabrication

This section discusses the post-processing methods for AM jewelry (Figure 38 and Table 3).

AM of precious metal jewelry is gradually gaining ground in the global jewelry business, with many industries already adopting or considering the technology. Two main elements still need to be improved to accelerate technology adoption: (1) AM jewelry design; and (2) AM jewelry polishing and finishing. Fletcher and Cooper analyzed the different polishing and finishing processes for AM precious metal jewelry [94,95,96]. The advantages of the various polishing methods such as Dish Finishing (DIF), Drag Finishing (DF), Stream Finishing (SF), Tumbling or Barreling (TU), Centrifugal Tumbling (CF), Vibratory Bowl Finishers (VBF), Laser Polishing (LP), electrolytic polishing, Electropolishing (EP), Electro-Mechanical Finishing (EMF), and Plasma Electrolytic Polishing (PEP) were discussed. Recent advances have opened up new possibilities for machine-assisted surface treatment techniques, which are becoming increasingly effective. The authors emphasize the many advantages of machine-assisted processing over manual processing, which have contributed greatly to the renaissance of the watch and jewelry industry. Modern finishing technology, in fact, aims to reduce waste, increase value, improve quality and create attractive surfaces, as well as reduce processing times.

For example, several mechanical polishing methods have demonstrated their ability to significantly reduce the time and effort required to achieve an adequate surface finish for an 18-carat AM jewelry article. A comparison of the processing times of various mechanical polishing procedures is illustrated in Figure 39, using hours as the unit of measurement. The reported data indicates that Stream Finishing (SF) is faster than other polishing methods [94].

The surface of an L-PBF part consistently shows a rougher texture than that of a cast part, regardless of the adjustments made to the process parameters. Consequently, pre-finishing operations are always necessary before starting regular finishing procedures to ensure that the surfaces achieve the desired smoothness, similar to that of cast parts. This aspect is particularly crucial in the context of surface finishing designs characterized by hollow and perforated structures, such as in the case of bracelets. If L-PBF production were scaled up to production volumes, the problem of as-built surface roughness pre-finishing would have to be addressed, although L-PBF production is currently not considered suitable for mass production, where mass finishing processes are considered essential [22].

Sandblasting, typically considered an exposed process, was able to effectively smooth and brighten L-PBF surfaces, even in the case of complex designs that are usually difficult to access. Figure 40 summarizes the results of the initial experiments on bronze L-PBF parts. Starting with the surface roughness in the state of construction (Figure 40a), dry blasting with corundum sand rapidly polished the surface in 1–2 min (Figure 40b,c); subsequently, a wet micro-sandblasting step with <50 μm glass microspheres further improved the smoothness and brightness in a further one to two minutes (Figure 40d,e). The resulting bronze ring had an excellent surface quality after hand polishing (Figure 40f). In addition, the author used electropolishing to improve gloss in hard-to-reach areas, surpassing traditional finishing techniques [22].

Electropolishing, an electrochemical material removal technique mainly used for metal parts, uses an external electric current source. In this process, the material is extracted from the workpiece (anode) and immersed in an electrolyte suited to the chemical composition of the metal, which leads to a significant reduction in surface roughness. The electropolished surface is not affected by the process, resulting in a smooth and highly reflective surface [94].

When metal is subjected to laser polishing, the surface of the metal punctually melts, and the smoothing effect of the liquid phase thus formed is highly dependent on its surface tension. To guide the laser beam, patterns are used that correspond to the contours of the surface. The innovation in laser polishing is rooted in the principle of active remelting, which differs fundamentally from traditional grinding and polishing methods that rely on abrasion and removal. This technique proves effective on hard-to-reach surfaces that are often difficult to clean manually or with machines (Figure 41) [94].

In electromechanical polishing, the metal object is coated with a gel-like substance and immersed in an appropriate electrolyte solution. During polishing, the microscopic particles of the gel are brushed against the metal surface, making it possible to treat articulated parts such as necklaces or interior walls inaccessible by mechanical means (Figure 42). Electroplasma polishing is a new surface treatment that produces exceptionally smooth and highly polished surfaces. Although PEP is considered an electropolishing process, it uses environmentally friendly, non-acidic electrolytes and operates at relatively high temperatures and voltages to achieve a better surface finish [94].

The application of mechanical polishing to additive manufacturing (AM) jewelry has proven effective in reducing manual finishing and polishing activities [94]. Ready-to-sell surfaces have been created by mass-finishing machines (MFM). Even though these MFMs use the prescribed finishing medium, it is often possible to improve the results by making some minor modifications to process variables such as speed, duration, compound, etc. The process parameters of the mass finishing machine to achieve better results have been evaluated and modified by Moser [97]. Overall, the polishing sequence, such as sandblasting, electropolishing, and hand polishing, is necessary to achieve a better surface finish (Figure 43a–c).

In summary, despite these advances, further research is needed to identify the most suitable mechanical finishing process that offers an optimal reduction in processing time. It is important to note that many of these processes are still at an early commercialization or experimental stage. Furthermore, extensive investigation and testing are required before incorporating them into the jewelry industry.

Table 3. Post-processing of AM for jewelry application.

Fabrication Method	Post-Processing Technique	Principle	Material Used	Feature Size (µm)	Advantage	Disadvantage
Traditional	Disc polishing [94,95,97]	Material is removed and smoothed from the workpiece surface using rotating and an abrasive slurry.	Abrasive materials are alumina, silicon carbide and diamond were used	0.1–1.0	High-quality surface finish, Mirror finish surface, Adaptability, Suitable for complex features	Time-consuming process for larger components, larger waste generation, and multiple steps were required
	Drag Polishing [94,95]	Utilizes a revolving drag plate and abrasive slurry to polish and sculpt the surface of a workpiece.	Diamond was used as an abrasive material and water as a carrier fluid	0.05–0.5	Precise and surface finish and material removal, efficient for intricate shape and adaptable for several materials	Knowledgeable operators are required, Limited to specific geometries, and produce a waste slurry.
	Tumbling or barreling [94]	Surface polishing is accomplished by friction and impact in a spinning barrel or drum packed with abrasive material.	Various materials, such as steel pins, ceramic beads, or abrasive stones, were used as abrasive media.	5–50	Efficient for edge radiusing, polishing and deburring, mainly utilized for batch production	It may take more stages to achieve a mirror-like quality, making it unsuitable for highly delicate or detailed components.
	Centrifugal Tumbling [94]	It uses a fast-revolving drum or bowl to polish surfaces by contacting them at high speeds while exposing workpieces and abrasive material to centrifugal forces.	Abrasive media such as steel, plastic and ceramic were used	1–20	Surface finishing that is effective and reliable, suitable for deburring, polishing, and edge radii, and ideal for different materials.	Better options for delicate or detailed components may exist, batch processing is only possible, and appropriate medium and compound selection is needed.
	Vibratory Bowl Finishers (VBF) [94,95]	It uses a vibratory bowl to transfer vibrational energy to combine workpieces, abrasive media, and a liquid compound, aiding surface finishing by relative motion and friction.	Abrasive media such as steel, plastic and ceramic were used	1–30	Deburring, polishing, and surface refinement techniques that are successful in batch processing, suited for various materials, and suitable for a wide range of materials.	It needs careful medium and compound selection, may not be the best option for highly delicate or detailed pieces, and makes noise while in use.
Non-Traditional	Laser Polishing (LP) [94]	Utilizes a powerful laser beam to melt and re-solidify the material’s surface, producing a polished and smooth finish.	No abrasive materials used (Laser beams), depends on the laser absorption and interaction properties.	0.01–0.1	Minimum material removal, provides precise control and the capacity to focus on specific areas, producing high quality finishes.	Limited to certain materials, it could be needed for post-processing processes and might be sluggish for larger components.
	Electro polishing (EP) [94]	The controlled removal of a layer of material from a metal’s surface using an electrolytic bath and an electric current to produce a polished and smooth finish.	Electrolytes used (for example, sulfuric acid, phosphoric acid)	0.1–10	Enhances corrosion resistance, offers a high-quality, mirror-like surface, and reaches intricate details and complicated geometries.	Limited to metals, could need specialist equipment, and the procedure can be relatively slow for larger components.
	Electro- mechanical polishing [94,98]	Combines mechanical and electrochemical abrasion to remove material from the workpiece’s surface, improving flatness and quality.	Combination of electrolytes and abrasives (For example, electrolytic solutions and diamond paste)	0.05–1.0	Produces surfaces with an optimal flatness and smoothness, appropriate for the manufacturing of semiconductors, and is highly automatable.	Limited to certain materials and applications, can be sluggish for larger components, and may need specialist equipment.
	Plasma Electrolytic polishing (PEP) [94]	Polish and remove material from a metal’s surface using a mix of electrolytic action and plasma discharge to produce a smooth and polished finish.	Electrolytic solutions (For example, alkaline solutions)	0.1–10	Provides a mirror-like polish, improves surface cleanliness, and increases corrosion resistance; it is suited for many metal alloys.	Restricted to metals, it may demand specialized machinery and can be mostly sluggish for large components.

Table 3. Post-processing of AM for jewelry application.

Fabrication Method	Post-Processing Technique	Principle	Material Used	Feature Size (µm)	Advantage	Disadvantage
Traditional	Disc polishing [94,95,97]	Material is removed and smoothed from the workpiece surface using rotating and an abrasive slurry.	Abrasive materials are alumina, silicon carbide and diamond were used	0.1–1.0	High-quality surface finish, Mirror finish surface, Adaptability, Suitable for complex features	Time-consuming process for larger components, larger waste generation, and multiple steps were required
	Drag Polishing [94,95]	Utilizes a revolving drag plate and abrasive slurry to polish and sculpt the surface of a workpiece.	Diamond was used as an abrasive material and water as a carrier fluid	0.05–0.5	Precise and surface finish and material removal, efficient for intricate shape and adaptable for several materials	Knowledgeable operators are required, Limited to specific geometries, and produce a waste slurry.
	Tumbling or barreling [94]	Surface polishing is accomplished by friction and impact in a spinning barrel or drum packed with abrasive material.	Various materials, such as steel pins, ceramic beads, or abrasive stones, were used as abrasive media.	5–50	Efficient for edge radiusing, polishing and deburring, mainly utilized for batch production	It may take more stages to achieve a mirror-like quality, making it unsuitable for highly delicate or detailed components.
	Centrifugal Tumbling [94]	It uses a fast-revolving drum or bowl to polish surfaces by contacting them at high speeds while exposing workpieces and abrasive material to centrifugal forces.	Abrasive media such as steel, plastic and ceramic were used	1–20	Surface finishing that is effective and reliable, suitable for deburring, polishing, and edge radii, and ideal for different materials.	Better options for delicate or detailed components may exist, batch processing is only possible, and appropriate medium and compound selection is needed.
	Vibratory Bowl Finishers (VBF) [94,95]	It uses a vibratory bowl to transfer vibrational energy to combine workpieces, abrasive media, and a liquid compound, aiding surface finishing by relative motion and friction.	Abrasive media such as steel, plastic and ceramic were used	1–30	Deburring, polishing, and surface refinement techniques that are successful in batch processing, suited for various materials, and suitable for a wide range of materials.	It needs careful medium and compound selection, may not be the best option for highly delicate or detailed pieces, and makes noise while in use.
Non-Traditional	Laser Polishing (LP) [94]	Utilizes a powerful laser beam to melt and re-solidify the material’s surface, producing a polished and smooth finish.	No abrasive materials used (Laser beams), depends on the laser absorption and interaction properties.	0.01–0.1	Minimum material removal, provides precise control and the capacity to focus on specific areas, producing high quality finishes.	Limited to certain materials, it could be needed for post-processing processes and might be sluggish for larger components.
	Electro polishing (EP) [94]	The controlled removal of a layer of material from a metal’s surface using an electrolytic bath and an electric current to produce a polished and smooth finish.	Electrolytes used (for example, sulfuric acid, phosphoric acid)	0.1–10	Enhances corrosion resistance, offers a high-quality, mirror-like surface, and reaches intricate details and complicated geometries.	Limited to metals, could need specialist equipment, and the procedure can be relatively slow for larger components.
	Electro- mechanical polishing [94,98]	Combines mechanical and electrochemical abrasion to remove material from the workpiece’s surface, improving flatness and quality.	Combination of electrolytes and abrasives (For example, electrolytic solutions and diamond paste)	0.05–1.0	Produces surfaces with an optimal flatness and smoothness, appropriate for the manufacturing of semiconductors, and is highly automatable.	Limited to certain materials and applications, can be sluggish for larger components, and may need specialist equipment.
	Plasma Electrolytic polishing (PEP) [94]	Polish and remove material from a metal’s surface using a mix of electrolytic action and plasma discharge to produce a smooth and polished finish.	Electrolytic solutions (For example, alkaline solutions)	0.1–10	Provides a mirror-like polish, improves surface cleanliness, and increases corrosion resistance; it is suited for many metal alloys.	Restricted to metals, it may demand specialized machinery and can be mostly sluggish for large components.

4. Future Directions of Additive Manufacturing for Jewelry Industries

Although many jewelry industries still use traditional methods to produce jewelry with huge, attractive parts [99], in recent times, the jewelry industry has been impacted by the additive manufacturing technique and research is still ongoing to produce high-quality jewelry parts with cost-effective methods. Initially, additive manufacturing was integrated with traditional craftsmanship (Figure 44). Integration has improved the design and post-processing of jewelry products, such as surface finishing, stone setting, and assembly, which are related to quality assurance [100,101].

Integrating additive manufacturing technologies into the jewelry industry involves several challenges. The transition from traditional methods to AM requires significant changes in workflow, which often meet with resistance from skilled craftsmen. The initial investment in AM equipment and staff training is considerable, especially for smaller companies, and difficult to justify without immediate returns. In addition, ensuring consistent quality and expanding the range of suitable materials, particularly precious metals, remains an ongoing problem that requires robust quality control and further research [100].

Despite significant advancements, additive manufacturing technologies are still maturing and facing significant research gaps and challenges. The process variables in the AM process, such as laser power, scanning speed, hatching distance, layer thickness, etc., are not yet fully understood, which affects product quality and consistency. The lack of standardized processes and materials leads to production inconsistencies, highlighting the necessity of establishing industry standards. Additionally, extensive post-processing is often required, emphasizing the need for research to streamline these steps. Studies on the long-term durability of AM-produced jewelry under everyday wear are also essential. Finally, developing hybrid manufacturing approaches that integrate AM with traditional techniques is crucial for leveraging the strengths of both methods [101].

In particular, digital design of jewelry parts will improve the jewelry industry productivity and design freedom to realize complex structures with greater precision.

In order for craft enterprises to adapt to technological developments, the following three strategic directions need to be pursued: (1) embracing the use of digital technologies rather than seeing technology as a threat to the enterprise’s future, (2) involving customers in the design and production processes and expanding the enterprise’s network, and (3) offering craft products through a wide range of services. The following strategic paths will help craft enterprises to survive, innovate, and increase their competitive advantage by exploiting the possibilities offered by innovative technologies and new production methods [102,103].

On the other hand, however, there are still many challenges to be faced when considering the future direction of the jewelry industry, particularly with regard to material choice, design complexity, surface finish, production speed, and efficiency (Figure 45). Moreover, the future perspective of additive manufacturing of jewelry will focus on the introduction of different types of innovative alloys, hybrid AM (manufacturing with post-processing), real-time monitoring, machine learning (ML), deep learning (DL) and Internet of Things (IoT) to improve the accuracy of jewelry pieces.

As far as materials are concerned, some jewelry applications still require more consumer-friendly alloys, such as harder alloys with higher tarnish resistance. Further research is needed to determine whether new alloys, such as shape memory alloys and bulk metallic glasses, are usable. Furthermore, with regard to nickel in white golds and cadmium in welds, future health and safety regulations may impose further restrictions on alloy compositions and encourage their further development [104]. Finally, the addition of semiconductor materials in precious metals and composite materials, such as precious metal matrix composites (PMMCs), could be explored in the future [104].

Next, the automation of additive manufacturing with post-processing (surface finishing) needs to be evaluated in order to increase the productivity of jewelry products that can become cost-effective (Industry 5.0). Real-time monitoring systems for additive manufacturing can enable the automatic identification of defects on the line and in situ, saving time and material [105]. The use of, among other things, machine vision to assess the quality of a 3D print [106,107] can make processes more efficient and reduce scrap [108,109,110]. For example, IoT closely monitors the scanning process, providing recommendations for optimal scanning for superior results. This integration addresses challenges related to material parameters, surface topology, data integration, and communication between scanning and additive manufacturing devices. Actuators, sensors, and other IoT connectivity devices play a crucial role in improving coordination between these devices. In addition, the use of artificial intelligence (AI) for 3D scanning is envisioned, foreseeing a future where the process is as simple as shooting a movie with a smartphone [111,112].

5. Conclusions

This article represents a comprehensive review on the use of AM for jewelry making. Nowadays, jewelry design has received new life through the incorporation of AM. As personal adornment is redefined through the lens of technology, this synergy goes beyond mere aesthetics. Using additive techniques (such as L-PBF), objects are created with precious materials such as gold, silver, and platinum. The main challenge in fabricating precious materials with L-PBF is their low absorbency and high thermal conductivity. This paper explores potential solutions to overcome these problems and improve the properties of precious materials produced by L-PBF. In addition, post-processing techniques improve surface finish and reduce porosity. Each piece of jewelry represents creativity and craftsmanship, allowing for the tangible manifestation of one’s unique talent. In addition to simplifying production, this combination of technology and craftsmanship ushers in a new era of cutting-edge jewelry that combines tradition and innovation. From the literature review, the following observations emerged:

• L-PBF is a good alternative for producing high-quality jewelry because of its superior mechanical strength, accuracy, and surface finish quality compared to powder metallurgy processes.
• Enhancing material-use efficiency in L-PBF for jewelry is essential for overcoming challenges like oxidation, agglomeration, and contamination. This can be achieved by better handling sensitive powders and optimizing recycling processes, thereby improving the sustainability and cost-effectiveness of this advanced manufacturing (L-PBF) method than the traditional method.
• This review highlights various strategies to optimize the performance of precious jewelry materials in L-PBF. In particular:
• The addition of alloying elements to precious materials can significantly increase mechanical strength, thermal stability, and overall durability by improving absorption at the common laser wavelength.
• Powder coating in precious metal fabrication using L-PBF improves laser absorption and reduces defects, resulting in higher densification and better surface quality. This approach is particularly beneficial for intricate jewelry designs, providing superior surface finish and structural integrity.
• The use of low-wavelength lasers (green or blue lasers) can be a technological approach to improve the properties of precious metals fabricated by L-PBF. These lasers improve energy absorption and reduce thermal gradients, leading to higher material density and reduced porosity.
• LP excels over other surface polishing techniques in producing accurate surface qualities, making it an excellent choice for high-end jewelry with complex patterns that require precise surface finishes.
• The integration of AM with traditional craftsmanship in jewelry production improves design flexibility, quality control, customization, and efficiency.