With the development of technology and the complication of the designs of machine parts and assemblies, the need for the manufacture of parts of complex geometry with inhomogeneous physical and mechanical properties is growing. Since known materials cannot fully provide all the physical and mechanical needs for such parts, it became necessary to manufacture heterogeneous parts, where individual parts of this part will be made of different materials. Even with conventional techniques, manufacturing multimaterial products (MMP) is a complex technological task. The main obstacles include time-consuming technological processes, production costs, and the properties of the parts. Therefore, it provokes a demand for new ways to solve these problems. Additive manufacturing, particularly laser-directed energy deposition (LDED), is considered to be a key technology for obtaining MMP and tackling the issues [
1,
2,
3,
4,
5,
6,
7]. In this technology, many authors note the importance of the powder-feed scheme [
8,
9]. It is concluded that the coaxial method is the most beneficial, since powder feeding and optical system are independent of moving parts or a nozzle, providing a stable LDED process [
10].
Today, additive manufacturing is developing rapidly, especially selective laser melting and LDED. New materials including ceramics are being introduced [
11,
12,
13,
14], improved laser optical schemes are being proposed [
15], and multimaterial additive manufacturing is developing. The authors of [
16] show the manufacture of bimetallic steel–copper products by selective laser melting. In spite of achieving a strong metallurgical bond, the authors emphasized that further studies are necessary in order to determine optimal processing parameters. However, being more productive, LDED is preferred in certain applications. This group of technologies includes several techniques capable of obtaining 3D objects without a powder bed, such as laser-directed energy deposition, welding, and cold gas dynamic spraying [
17,
18,
19]. Nevertheless, LDED is considered to be the most widespread and developing technology. In [
20,
21], the LDED process of 316L steel and a Co-based superalloy was investigated both theoretically and practically. It was recommended to coat the powder layer with different thicknesses for each material due to distinctions in thermal and physical properties. The authors of [
22,
23] studied the influence of LDED working parameters on the structure and properties of the materials. The authors of [
24,
25,
26] investigated the properties and showed the possibility of the practical application of the following pairs of metals: steel–copper, steel–aluminum, steel–titanium, and steel–nickel. For example, a steel–copper (bronze) pair can be used as heat exchangers (due to the high copper/bronze conductivity) or as a cam bushing (high tribological properties of copper/bronze), and a steel–aluminum pair can be used as adapters in oxygen regenerators or as parts of electrolytic refining equipment (high corrosion resistance of aluminum). The authors of [
27] deposited molybdenum powder (Mo 99%) of a fine fraction by the preliminary layer deposition method, under the following LDED parameters: V = 0.8 cm/s, do = 0.3 cm, PL = 2 kW. It has been calculated that 51% of the absorbed power is spent on evaporation. A homogeneous structure was obtained, which, according to the authors, should positively influence the provision of high-performance properties. In [
28], coatings from a high-entropy alloy (HEA) (CoCrFeNi)95Nb5) 100-xMox (x = 1, 1.5, and 2) were deposited on a substrate of steel 45 by LDED. The effect of laser radiation power and the percentage of molybdenum in the alloy on the microstructure and microhardness of coatings has been studied. The microstructure of the coating consisted of columnar dendrites which disappear with an increase in the molybdenum content; the grains are crushed and become more compact. The volume fraction of the interdendritic phase at a laser radiation power of 800 W was small and uneven. As the laser-beam power increased to 1000 W, the volume fraction of the interdendritic phase increased. At a laser-beam power of 1200 W, the volume fraction of the interdendritic phase decreased again. The coatings obtained at a power of 1000 W had the highest volume fraction of the interdendritic phase and a higher microhardness. The authors of [
29] carried out the deposition of T15M composite powder on a steel substrate at different laser-scanning velocities. It was found that the coating obtained at a deposition rate of 200 mm/min had better wear resistance. In [
30], it was shown that the microhardness and wear resistance of composite coatings based on Co on the titanium alloy Ti-6Al-4V increased with a decrease in the specific energy of laser radiation. Sun et al. deposited a composite Ni/Mo coating on the surface of a copper alloy using LDED [
31]. The laser power was 6000 W, the scanning speed was 5 mm/s, and the powder feed rate was 10 g/min. The surface layer consisted of three Ni layers and two Mo layers. Due to the flowability and nonequilibrium solidification of molybdenum in the molten state, pores and cracks along the grain boundaries were observed in the Mo layer. The surface hardness of the Mo layer ranged from 200 to 460 HV. In [
32], coatings from the Ni–Cr–Mo alloy with different Cr content on carbon constructional steel, obtained by LDED, were studied. An increase in the Cr content led to the formation of a dense protective passive film. Corrosion resistance first deteriorated with increasing Cr content from 18 wt.% to 22 wt.% and then improved with increasing Cr content from 22 wt.% to 26 wt.%. The authors of [
33] studied the LDED of powders of high-entropy FeCoCrNi and FeCoNiCrMo0.2 alloys on the surface of stainless steel 304. It was found that the addition of Mo leads to a significant increase in the size of the dendrites in the middle region of the FeCoCrNiMo0.2 coating. The FeCoCrNiMo0.2 coating has a high corrosion resistance: a low current density in a NaCl solution with a concentration of 3.5 wt.%. A study of LDED using Ni–Mo–Si powder mixtures on an austenitic stainless steel substrate shows that the coating microstructure consists of primary dendrites and interdendritic eutectic [
34]. Due to the presence of a large amount of hard and wear-resistant phase, the laser-deposited composite coating containing metal silicides has excellent wear resistance under conditions of sliding friction at high temperatures.
The review shows that the development of multimaterial LDED is promising. However, several highly demanded material systems for LDED, such as molybdenum and low-carbon steel, corrosion-resistant steel, and nickel heat-resistant alloy, still require further research to meet industry needs. So far, no successful industrial-scale experience of LDED of these materials has been reported in the literature.
This work aims to study the possibility of manufacturing bimetallic products for specific industrial applications by LDED.