1.1. Additive Manufacturing
The AlSi10Mg alloy belongs to the hypoeutectic aluminum–silicon group [
1]. This alloy has good fluidity accompanied by low shrinkage, making it an important cast alloy with excellent weldability. AlSi10Mg contains about 0.4 wt.% Mg, making the alloy a heat treatable material that responds to the precipitation hardening processes [
2]. Adding Mg to the Al-Si alloy allows the formation of Mg
2Si precipitations, which considerably strengthens the matrix without reducing other mechanical properties [
1]. The alloy is also hardened by rapid solidification, where high cooling rates are imposed, refining the microstructure [
3].
A high strength/weight ratio, lighter weights, and higher corrosion resistance of aluminum alloys enable their easy replacement for other materials in different engineering applications [
4]. Recently, additive manufacturing (AM) technologies have been applied for fabricating lightweight parts of simple and complex shapes to be used by different engineering sectors. Additive manufacturing has been used for various materials, including metals and their alloys. The overall AlSi10Mg alloy properties make the material ideally suitable for processing using the laser powder bed fusion (LPBF) AM technology and adoption of the alloy by many industries, including the aerospace and automotive sectors [
4,
5,
6]. To ensure the high quality of printed specimens, it is essential to understand the impact of both the LPBF process parameters and the material feedstock properties on the quality of the parts [
7], including the plastic anisotropic behavior; a notable lower ductility is typically observed for the vertical printed tensile samples when compared to their respective horizontal samples [
4,
5,
6,
7,
8]. Namely, the study of the unique LPBF solidification conditions and the material’s thermal history may lead to the understanding of the characteristic defects (porosity, hot cracking phenomena, and surface roughness), observed in AlSi10Mg alloy components produced by the LPBF technique [
6,
7]. The surface quality depends on the intrinsic material properties as well as on the scan strategies during printing, i.e., contour scans and skywriting scans. The roughness is typically caused by the parallel running welding beads and by stairs effect [
9].
So far, many studies have investigated the influence of the LPBF process parameters (i.e., thickness of the printed layer, laser power, scanning speed, build-strategy, chamber atmosphere etc.) on the quality of the LPBF parts. For example, Yang studied the influence of process parameters, such as laser power and scanning speed, on the vertical surface roughness of the AlSi10Mg parts fabricated by LPBF process [
9]. Steuben et al. 2019 discussed the use of a computational enriched analytical solution method (EASM) for AM modeling and simulation in order to predict the influence of process parameters on the properties and associated functional performance of the LPBF parts [
10]. Understanding the influence of main LPBF processing parameters on the surface roughness of AM components can be used for additional process optimization [
11]. Surface post-processing techniques, such as heat treatment, shot peening, polishing, sand blasting, and coatings [
12], and the resulting part qualities for the AlSi10Mg alloy have been discussed before [
13,
14,
15]. Yet, surface post-processing techniques increase the production time and cost [
12].
In this paper, we add more information on post-AM surface modification, presenting a novel finishing process and suggesting a methodology for electroless plating of silver, gold, and their alloys as a method to improve surface quality, both physical and ornamental.
1.2. Electroless Plating of Gold, Silver and Au–Ag Alloys
The quality of the surface finish of 3D-printed parts produced by the AM-LPBF is occasionally unsuitable for applications demanding, for example, low roughness and uniformity. Therefore, additional post-AM surface modification processes, such as machine finishing techniques and coatings, are frequently applied [
16]. Different metallic coatings are often used, improving electrical, thermal, optical, and mechanical properties, as well as erosion and corrosion resistance, and achieving a decorative appearance of the AM-printed parts. See, for example, the paper by Kuo et al. 2017, which specifically stated that improving the surface quality of additive manufactured parts by minimizing the roughness of the surface is a promising engineering topic, which can be achieved by coating the printed surface [
17]. The quality of the coating depends on various factors, including the parameters of the AM process, e.g., the position and orientation, and the presence of external defects, the part’s surface roughness, and the cleaning process of the surface before coating [
18].
Metal deposition of thin films is accomplished by either physical, chemical, or electrochemical methods, such as physical vapor deposition (PVD), chemical vapor deposition (CVD), and electrochemical deposition (ECD). Electroless deposition (ELD), which is an electrochemical method, is promising for printed metal surface finishing, since it is rather simple, relatively (to other methods) inexpensive and, most importantly, it yields good results [
19]. For instant, ELD may be used for the production of 3D-printed micro circuitries for structural electronic devices [
16].
The electroless plating process is based on the simultaneous metal reduction and reducing-agent oxidation on an activated surface, where the heterogeneous reactions continue in an autocatalytic form until the depletion of one of the components (or both), assuming the bath conditions, such as pH and temperature, are kept unchanged [
19]. The electrons for the chemical reduction are provided by the oxidation of a reducing agent [
20]. Hence, the electroless plating occurs without the use of an external current source, and therefore, it requires a relatively simple bath and control and support units. The electroless process occurs in an aqueous bath, which is relatively simpler than that of other metal and alloy deposition such as vacuum-based PVD and CVD equipment [
21,
22,
23,
24,
25]. In this self-initiating auto-catalytic process, a catalytic substrate is dipped into an aqueous solution containing metal ions (typically in a complex form), reducing agents, and pH adjustment components [
19,
23]. In a few cases, the bath may include also special additives in a minute amount for special functions such as brightening or leveling [
19]. ELD is a controlled method, which has been used industrially for many years, it operates at a moderately low temperature (<100 °C), and allows high selectivity depositions on both conductive and non-conductive surfaces [
20,
21,
22,
23]. The ELD technique provides relatively uniform thickness of metal and alloy deposits. For example, according to Shacham-Diamand et al. 2000 [
26] and Shukla et al. 2014 [
27], electroless silver deposition produces high-quality coating with a thickness of up to about 1 µm [
26,
27,
28]. This technique is particularly effective in the case of deep pores and rough surfaces [
28].
ELD manufacturing of nickel–phosphorus (Ni–P) was developed by Brenner and Riddell [
29] in the mid-1940s, and since then, it has been frequently used in numerous industries as a protective layer [
30]. For example, Ni–P film is commonly applied atop porous surfaces [
31]. Crystallization and phase transformation processes of ELD Ni–P film during thermal processing have roles in determining the material properties [
32]. By applying an ELD Ni–P layer on top of an aluminum alloy substrate, the material properties, such as corrosion resistance, wear resistance, and hardness may be significantly improved [
33,
34]. In addition, according to Asher et al. 2009, self-assembled monolayers (SAMs), such as functional silanes SAMs, may also be applied in order to improve the metal adhesion to oxidized surfaces [
21].
Gold and silver were among the first metals used by ancient societies; both are transition metals with shiny appearances [
35]. Gold and silver have excellent corrosion resistance, they are soft, ductile, and malleable metals with superb reflectivity and high electrical and thermal conductivity [
36,
37]. Electrum, a gold–silver alloy that usually contains 60 wt.%–80 wt.% Au and 20 wt.%–40 wt.% Ag [
38], was initially used during the first millennium BCE. The first known metal coins used in antiquity, which were dated to 630–620 BCE, were made of electrum [
35]. Some ancient electrum items contain more than 20 wt.%–40 wt.% silver [
39,
40]. For instance, according to Ashkenazi et al. 2017, the fourth century BCE electrum bar with granules from the Nablus Hoard was composed of 21.6 wt.%–64.5 wt.% Au and 35.5 wt.%–77.4 wt.% Ag [
35].
Applying gold, silver, and electrum coatings over objects made of aluminum alloys can efficiently shield the parts against corrosion in many severe environments, for instance, protection against pitting corrosion of Al in a surrounding of NaCl solutions [
36]. Silver deposition is often used to coat aluminum micro-mirrors when high reflectivity is needed in the visible light, near infrared light, and radio frequency wavelengths [
41].
There are different industrial gold and silver ELD processes employing various formulations. However, the associated published technological information is limited as a result of commercial confidentiality. In addition, industrial processes commonly use environmentally hazardous materials such as cyanide compounds [
42,
43]. A few studies have been published on the electrochemical deposition of Au–Ag alloy films [
44,
45,
46,
47]. However, no formulations of ELD for Au–Ag alloy coatings have been published in the open literature before Inberg et al. 2020 [
48].
Electroless gold, silver, and electrum coatings can be useful to improve the properties of AM-LPBF parts for many applications. For example, in some AM-LPBF AlSi10Mg items, additional coatings may be needed to improve electrical and thermal conductivity or to achieve a certain aesthetic appearance. The shiny and luxurious appearance of gold [
43], silver [
42], and electrum [
48] plating can be used in numerous applications, among them replicas of ancient objects, such as valuable coins for museum presentations [
42,
43]. ELD of gold [
49] and silver [
50] metals can also be applied over AM fused filament fabrication (AM-FFF) acrylonitrile butadiene styrene (ABS) polymer objects when an appropriate chemical pretreatment process of the surface is done [
49,
50].
The objective of this research is to study the use of a novel environmentally friendly ELD finishing process by gold, silver, and electrum, of AM-LPBF AlSi10Mg parts, as a part of their post-printing surface modification. In addition, based on previous work of Dresler et al. 2019 [
42] and Inberg et al. 2020 [
43,
48], this review aims to propose a methodology for effective Au, Ag, and Au–Ag ELD coatings of AM-LPBF AlSi10Mg objects. We should point out that one of the highlights of the technologies described here is the surface activation method. Since electroless plating is an autocatalytic process, it requires a high-quality catalytic surface to initiate the deposition. Any defect in the pre-deposition process will show up in the final coating. Hence, the SAM surface modification, which was developed by Osaka and his research group [
51], may contribute to the success of the proposed process. That method was developed originally for a silicon dioxide surface as a barrier layer. However, it can be applied for other oxidized materials, in our case to AlSi10Mg, allowing the formation of a stable bond between the SAM adhesion layer and the substrate.