Current research and industrial application of laser powder directed energy deposition

Additive Manufacturing (AM) technologies are recognized as the future of the manufacturing industry thanks to their possibilities in terms of shape design, part functionality, and material efficiency. The use of AM technologies in many industrial sectors is growing, also due to the increasing knowledge regarding the AM processes and the characteristics of the final part. One of the most promising AM techniques is the Directed Energy Deposition (DED) that uses a thermal source to generate a melt pool on a substrate into which metal powder is injected. The potentialities of DED technology are the ability to process large build volumes (> 1000 mm in size), the ability to deliver the material directly into the melt pool, the possibility to repair existing parts, and the opportunity to change the material during the building process, thus creating functionally graded material. In this paper, a review of the industrial applications of Laser Powder Directed Energy Deposition (LP-DED) is presented. Three main applications are identified in repairing, designed material, and production. Despite the enormous advantages of LP-DED, from the literature, it emerges that the most relevant application refers to the repairing process of high-value components.


Introduction
The improvement of Additive Manufacturing (AM) systems and the increasing knowledge on the properties of the produced parts drive the use of AM processes for the production of final components. Automotive [1,2], medical [3,4], and aerospace [5][6][7] are only a few of the sectors in which AM processes are successfully used [8]. The common factors of these sectors are the geometrical complexity and the small to medium batch production which makes production by AM economically convenient [9][10][11].
Considering the processes that use metal alloys as feedstock material, according to ASTM [12], the main metal AM processes are Powder Bed Fusion (PBF) processes, which include Laser-Powder Bed Fusion (L-PBF) and Electron-Powder Bed Fusion (E-PBF), and Directed Energy Deposition (DED) processes.
From the literature, it is possible to observe that, despite the high level of industrialization of powder bed processes, they can be used only for the production of small/medium components, with a maximum size of 400 mm [13,14]. DED processes allow overcoming this problem since the deposited dimension can reach up to 3000 mm in size. The main DED systems are listed in Table 1, with the indication of the building volume. Another advantage of the DED process with respect to the other metal AM processes is that the substrate could coincide with the surface of an existing component, and this characteristic is very powerful for the repair application. Moreover, it is also possible to change the material during the deposition process, thus obtaining components that are characterized by different properties in different areas.
Conversely to PBF processes, the DED process is not a consolidated process and therefore is considered premature for industrial production [15][16][17]. In addition, also the number of DED systems installed around the world is still low, as depicted in the graph in Fig. 1. This graph illustrates the industrial distribution of metal AM systems sold and shows that the metal AM market is dominated by the powder bed fusion processes with a percentage of 82% [18,19]. Directed energy deposition is the second technology for number of systems sold with a percentage of only 8% (ten times lower with respect to PBF processes).
However, it should be noted that the industrial interest regarding DED processes is growing exponentially [20,21] and it is therefore important to highlight the main industrial achievement obtained nowadays and identify the future trends of this technology.
Several reviews are available regarding the DED process on the physics of the process [15,22,23], on the repair operations [24], on the monitoring technologies [25], and the alloys processed by DED [26][27][28][29]. However, all of them focused on the characterization of the parts and the effect of process parameters. This review aims to illustrate the technological readiness level of the DED technology and the feasibility of applying DED in industrial production. Thus, current successful industrial cases are described, where the DED process was applied. The review could guide researchers and manufacturers to the next steps for more in-depth industrialization of the technology.
There are several DED processes that differ in the feedstock material, which can be in the form of powder or wire, and for the energy source that can be a laser or an electron beam. Nevertheless, powder material and laser energy source are the most common equipment used in the industry [30]. As a consequence, the primary scope of this work is to describe the main industrial applications of directed energy deposition processes that use powder as feedstock and laser as energy source, and this technology in the following is referred as Laser Powder Directed Energy Deposition (LP-DED) process. Hence, firstly, a brief description of the LP-DED process and system is presented. Then, the applications are described focusing on repairing operations, production of functionally graded and designed materials, and production of near net shape components.

LP-DED process
A schematic representation of an LP-DED system is depicted in Fig. 2. It is composed of four fundamental elements: the laser, the motor, the feedstock feed mechanism, Table 1 Main DED manufacturer and systems considering the process and the building volume [13]  in turn composed by the powder feeder and by the deposition head, and the control unit [31].
In the LP-DED process, a focused laser beam is used to produce a melt pool on a substrate or building platform. Then, a nozzle, or deposition head, feeds the powder material into the generated melt pool by means of carrier gas. When the powder material enters into the melt pool, it melts instantaneously increasing the volume of liquid material. When the laser moves away, the molten material solidifies quickly and a raised track is obtained [22]. The working area, near the generated melt pool, is protected from oxidation using a shielding gas [32]. Both the carrier gas and the shielding gas are typically argon [30]. The relative movement is achieved by moving the deposition head or by moving the substrate [22,30,32]. Repeating this operation several times, it is possible to obtain three-dimensional complex components. Some systems are equipped with additional axes that allow to tilt and rotate the building platform, making possible the production of overhanging features [5,33].
Based on the physics of the LP-DED process, several technologies have been developed [22,30,32,35,36]. The most used LP-DED technologies are the Directed Light Fabrication (DLF) process developed at Los Alamos National Laboratory [37,38], the Laser Engineered Net Shaping (LENS®) process developed at Sandia National Laboratories [39], and the Direct Metal Deposition (DMD™) process developed by POM Group [40][41][42][43]. Differences between DLF, LENS, and DMD processes include laser power, laser spot size, laser type, powder delivery method, inert gas delivery method, feedback control system, and the motion control used [30]. It was demonstrated that DLF is a suitable production technology for the production of near net shape complex components of refractory metals [44] and stainless steel alloys [45]. In DLF, a glove box is used to maintain the level of oxygen under 10 ppm to avoid problems related to oxidation phenomena. DLF uses a controlled sequence of tool paths derived from the CAD data to define the motion of the substrate. This approach is similar to that used in Computer Numerical Control (CNC) programming; however, in DLF, a postprocess is required to convert the path of the deposition head and the control of the powder feed into a G-code [33,37,46]. Differently from DLF, in the LENS process, the motion path derives from the STL file as in common AM processes [22,33]. In the LENS process, the powder is delivered into the deposition zone using a deposition head with four axisymmetric nozzles [15,22]. The DMD process uses a patented concentric nozzle [41] to deliver the metal powder into the working area. Furthermore, the DMD system is equipped with a feedback system that provides a closed-loop for the control of the dimensions of the deposited part [40,43].

Repair and maintenance
The damage of a component could be induced by several phenomena like corrosion, thermal stress, and variable thermal cycle and impact [5,32]. The damaged components are typically replaced by new parts; however, under certain circumstances, it is more convenient to repair them. This is the case when the repaired components are characterized by high economic value. This high value derives from the complex operations required to produce the component and from the valuable material used [47]. For this reason, repairing these components could mean significant cost savings [5,47]. For example, the US Army's Anniston Deport (ANAD) estimated an annual cost saving up to $5 million repairing Honeywee gas turbine components for the M1 Abrams tank [48].
In the industry, Tungsten Inert Gas (TIG) welding was the first technique used for repairing damaged components [5,49]. Despite its relatively simple applicability, the TIG process induces a huge amount of heat in the repaired Fig. 2 Representation of a general laser powder directed energy deposition (LP-DED) system [34] component, and this leads to high residual stresses and distortions [50]. On the other hand, Plasma Transferred Arc Welding (PTAW) and Electron Beam Welding (EBW) processes satisfy the requirement of a low heat input; however, their equipment is more complex and expensive [49,51,52].
To date, among other technologies, the LP-DED process is one of the most used processes to repair damaged components [53,54] thanks to the less heat input [55][56][57], less warpage and distortions [24], and higher precision [58,59] compared to conventional processes. Also, the mechanical properties of the repaired parts are promising and attractive in fact, as reported in Table 2, the mechanical performance in terms of yield strength and ultimate tensile strength are comparable with the bulk material; however, the elongation is quite lower. Hence, further researches are necessary.
Moreover, the repair operation using the LP-DED process requires a huge interaction between different disciplines, such as CAD/CAM software and inspection systems, and a high level of operators' knowledge [62].
According to Ruiz-Salas et al. [62] and Yilmaz et al. [63], the repair operation of a damaged component follows these different steps: • Obtaining geometry of the damaged part through 3D digitalization • Comparison of the nominal and the actual geometry to highlight the damaged area • Evaluation of the repaired area and surface preparation • Material characterization, optimizing the process parameters • Tool path definition through CAM software • Affected area reparation • Machining the repaired area • 3D digitalization of the repaired part • Comparison of the repaired geometry with the original CAD model to validate the reparation In the following paragraphs, the main achievements of repair operation using the LP-DED process are described, highlighting the industrial sector that are the aerospace sector, the tooling sector, and the other sectors.

LP-DED repair in the aerospace sector
The aerospace sector is characterized by components produced using high-performance materials, such as Ti6Al4V and Inconel, that are very expensive due to difficulties in manufacturing and due to the complex geometry [64]. The possibility to drastically reduce the cost by repairing the damaged components instead of replacing them is the driving force of the repair application in this sector [5,65]. The LP-DED process thanks to its high precision and to the minimal distortion induced in the repaired component [24] allows obtaining acceptable results in terms of dimensional deviation and metallurgical bonding and hence turns out to be the optimal repair process in the aerospace sector [65]. Moreover, the use of the LP-DED process in the aerospace sector could lead to environmental benefits due to less waste of material as demonstrated by Wilson et al. [53] during the repair process of a damaged turbine blade (Fig. 3). In their work, an Optomec LENS 750 machine was used to repair a 316L stainless steel blade damaged in the tip portion. The repaired blade showed good results with an accuracy of about 0.03 mm with respect to the nominal geometry. Moreover, the Life Cycle Assessment (LCA) showed the effectiveness of using the LP-DED process in repair operations [66]. In detail, when the repair volume was of about 10%, the use of the LP-DED process resulted in an improvement in the carbon footprint of 45% and a total energy saving of about 36% with respect to replacing with a new one. Tensile tests were also performed on undamaged and repaired samples. Results showed that the Ultimate Tensile Strength (UTS) of the repaired and the undamaged sample are 793 MPa and 815 MPa respectively and these values are consistent with the annealed bar. In addition, it was noted that both repaired and undamaged tensile samples broke away from the central and the repaired region.
However, in the aerospace sector, it is well known that the quality of the part is the priority; hence, tens of feasibility researches were performed and applied. For this reason, the feasibility of the repairing application of the LP-DED process has been studied by different researchers, both at academic and industrial levels, by testing case studies of industrial interest. For instance, Optomec [67] used the LP-DED process to repair an AM355 steel T700 blisk damaged due to the effect of erosion on the airfoil leading edges (Fig. 4). The repair operation was performed using a cobaltbased wear-resistant material and showed an attractive return of investment. The repair was mechanically verified with 50,000 cycles of low cycle fatigue spin test [68] and with 60,000 rpm spin test [67,68]. Gasser et al. [54] illustrated the main results obtained into one of the major European projects that is FANTASIA, regarding the use of the LP-DED process in repair and maintenance in turbo-engine applications. The main aspect faced in this project is the accuracy during the deposition. In addition to the accuracy, the results showed that the deposited material is characterized by adequate static and dynamic characteristics thanks to the obtained microstructure. Figure 5a depicts a damaged HPT shroud of Rene N5 CFM56. The challenges during the repair operation were to spare the cooling holes and to avoid melting of the thin edges obtained in FANTASIA. In order to obtain the repaired surface, the damaged component was firstly scanned and then the data were used to generate the deposition path. The result, depicted in Fig. 5b, showed that LP-DED could be used to repair HPT shrouds with low dilution and good accuracy, lower than 0.15 mm [69].
The use of the LP-DED process was also certified by Rolls-Royce Deutschland for 15 different repair operations [54,70]. Two of the most significant examples are depicted in Figs. 6 and 7 respectively. Figure 6 shows a BR715 HPT case produced using a nickel alloy. During functioning, some features of this part such as bosses, flanges, and brackets are subjected to wear. LP-DED was successfully used to repair a nickel-based alloy Nimonic PE16 worn flange using Ni625 alloy. During the process, the oxidation phenomena were avoided by means of the shielding gas. Figure 7 shows the damping wire grooves of a BR715 HPC front dump made of Ti6Al4V alloy. In this application, the component was repaired by local reconditioning of   [54] the groove wall. The application was characterized by two critical issues. The first is that the wall in front of the worn one must not be affected by the process. Moreover, it is necessary to consider that the working area is restricted due to the presence of the groove geometry. The LP-DED process allowed to overcome these issues; however, the quality of the repaired part has to be improved through the optimization of process parameters [71]. For example, Liu et al. [72] performed a preliminary work in order to investigate the capability of the LP-DED process to repair aluminium alloy aircraft structures. Results showed that good metallurgical bonding without cracks could be obtained with the proper combination of process parameters. However, due to the weak interface between the deposited material and the substrate, both the tensile strength and the fatigue life were lower compared to the properties of the substrate material. Hence, further investigations are required [72]. In addition to the process parameters, the deposition strategy is another factor that affects the part quality [16,73]. Petrat et al. [56] optimized the deposition strategy in order to minimize the dimensional deviation and to obtain a regular geometry during the repair operation of a gas turbine burner of Inconel 718. The repaired component was characterized by a circular geometry and the deposition strategy consists of circular paths with a variable starting point and, in the last layer, an inner and an outer circle track. Cobalt-based alloy was successfully used by Zhang et al. [74] to repair H-13 tool steel samples. In their study, the distortion of the components was minimized using a continuous deposition strategy eliminating the accelerations and the decelerations of the deposition head. Results showed a good metallurgical bond between the substrate and the deposited material. Moreover, the repaired component exhibited a higher UTS and a lower value of ductility compared to those of the base component. Kistler et al. [75], repairing Ti6Al4V samples, analysed the effect of part thickness, deposition strategy, number of deposited layers, initial temperature, and interlayer dwell time on the quality of the repaired part. Results showed that the HAZ was mainly influenced by the part thickness; in particular, thick parts were characterized by a smaller HAZ with respect to thin parts. Porosity was only slightly influenced by the number of the deposited layers and increased with this number. Then, it was observed that the hardness was influenced by the initial temperature of the substrate and higher temperatures led to a lower value of hardness due to the smaller thermal gradient.
Qi et al. [76] in order to repair turbine engine compressor and blisk airfoils of Inconel 718 developed a geometrybased adaptive deposition strategy method. This method was based on analytical relationships that were introduced into the G-code in order to relate the movements of the deposition head to the part geometry. In particular, the analytical relations relate the process parameters to the deposited track. Therefore, on the basis of the geometry of the part to be repaired, the process parameters and the hatching distance were varied during the repair process. Nowotny et al. [58] repaired titanium blades on a rotor of an aircraft engine (Fig. 8a). The repair process was performed in a closed inert gas chamber in order to reduce the oxidation. The result of the repair operation is depicted in Fig. 8b. The selection of appropriate process parameters and deposition strategy allowed obtaining a fully dense and fine crystalline microstructure. The mechanical performances, in terms of tensile and fatigue strength, were at least equal to the base material. Finally, Nowotny et al. [58] showed that an integration between different systems, such as LP-DED and 10 mm Fig. 6 Reparation of a BR715 HPT case flange using laser metal deposition process [54] (a)  Fig. 7 a Ti6Al4V groove wall repair using LP-DED process and b cross-section of the repaired groove [54] 6898 The International Journal of Advanced Manufacturing Technology (2022) 119:6893-6917 milling, could lead to economic advantages [58]. The integration of different processes into a single fully integrated repair solution was analysed by Jones et al. [77]. This integration was achieved with the RECLAIM (REmanufacture of high-value products using a Combined LAser cladding, Inspection and Machining system) approach by designing a system with a modular tool holder capable of adapting to different tools. The validation was performed repairing a titanium turbine blade. The results showed that integrating LP-DED, scanning system and machining in a single machine, could open new repair possibilities and expand the success rate with minimum capital investment. Moreover, they showed that this approach simplifies the logistic by reducing the part transportation and that it could be used not only for turbine blade repairing but also for solving other additional defects.

LP-DED repair of tools and moulds
On dies and moulds, during their use, different types of defects, such as heat cracks and wear, could occur [78], and for this reason, they have a limited in-service lifetime [79]. One of the most challenging issues during the repair operation of dies and tools is related to the low weldability of the materials commonly used for their production [80]. In addition, due to the high carbon content and alloying elements, there is a high tendency to the formation of brittle phases [81].
The minimization of cracks was obtained by studying the effect of process parameters and the surface preheating. For example, Borrego et al. [82] depositing P20 alloy on H-13 tool steel substrate showed that the numbers of cracks and defects could be reduced through an optimization of the process parameters. In addition, they showed that the fatigue resistance is mainly influenced by the residual stresses. In fact, during the fatigue test at R = 0, the repaired samples exhibited a significantly lower resistance. Instead, when the fatigue test was conducted at R = 0.4, the non-repaired and the repaired samples showed a similar fatigue behaviour, and this was attributed to the high level of tensile residual stress. Then, Kattire et al. [83] repaired H-13 tool steel with CPM 9 V steel powder analysing the influence of process parameters on track geometry and track quality. Results showed that the main process parameters that affect the track geometry were laser power, travel speed, and powder feed rate; instead, the dilution was mainly affected by powder feed rate and gas flow rate. The mechanical characterization of the samples showed that in the deposited layer, compressive residual stresses were generated. Leunda et al. [84] varying the process parameters repaired heat cracks, caused by repetitive heating and cooling cycles [85], on Vanadis 4 die using Vanadis 4 and CPM 10 powder. Results showed that a good metallurgical bonding was obtained without defect, and this suggests that LP-DED can be successfully used to repair heat cracks. The microhardness of the repaired area was of about 700 HV 0.3, resulting slightly lower with respect to that of the base material. This was attributed to the similar microstructure observed in the repaired area and the base material; hence, Leunda et al. [84] suggested that LP-DED process can be used to repair tool steel die without modifying the mechanical characteristics. Lestan et al. [86] during the deposition of Metco 15 E, Colmony 88, and VIM CRU 20 powders on cast iron substrate showed that the formation of cracks was reduced by preheating the substrate.
Ren et al. [87] in order to improve the accuracy and the reliability of repair operations combined the use of adaptive zigzag tool path pattern with 3D alignment technology.
The proposed repair strategy was tested on the die repair for Spartan Light Metal LLC. Figure 9 shows the damaged die core (left), the die after the deposition process (centre), and the die after the finishing operation (right). In their work, they demonstrated the capability of the LP-DED process for repair operation; moreover, a high thermal conductivity in the component produced by LP-DED was obtained compared to the original component and the component repaired using welding technologies.
In addition to the feasibility of the repair operation, a lot of works were focused on the sustainability and the environmental impact that derives from repair operation. The sustainability and the environmental impact were measured by different factors such as energy consumption, pollution, material waste, lead time, and cost [79,80,[88][89][90]. The impact of these factors is usually investigated by Life Cycle Analysis (LCA) [91]. From an economic point of view, it was widely demonstrated the economic advantage of repairing them [24,78]; moreover, InssTek Inc. repaired a hot forging die (Fig. 10) and showed that the service life of the repaired die was 2.5 times higher with respect to the original die [88].
Bennett et al. [92] repaired an automotive steel die, illustrated in Fig. 11a, and showed that the repaired die, shown in Fig. 11b, was characterized by the same life as the original die life. On the contrary, the life of the die repaired using the traditional process varied between 12.5 and 29.2% with respect to the original die life. Moreover, the results of LCA showed that the LP-DED repair process has a lower environmental impact compared to traditional repair processes.

LP-DED repair in other sectors
The sustainability interest in the automotive industry is becoming more and more important [93,94]. Steel and grey cast iron are two of the most used materials in the production of car components [95]; hence, repairing these components could lead to an extension of end of life [94]. However, these materials are not easily repairable due to their tendency of crack formations. At this scope, studies were performed in order to optimize the process. For example, Bennett et al. [96] used LP-DED process to repair a grey cast iron diesel engine component with 316L stainless steel. Figure 12 depicts the component before and after the repair process. Cracks were reduced using a spiral deposition strategy. Moreover, before and after the deposition, preheating and postheating phases were used to control the heating and the cooling rate. The mechanical properties of components repaired using stainless steel were evaluated by Yu et al. [97]. They showed that comparing the traditional process with the LP-DED process, the HAZ was reduced from 2.2 to 0.1 mm. Both the traditional and the LP-DED processes caused a reduction of the UTS of about 16% compared to the original part. However, it was observed that the repair process highly influenced the value of the elongation. Using the traditional process, the elongation decreased of about 20% compared to the undamaged part instead, and using LP-DED process, it was observed that the elongation increased of about 60%. Piasecki et al. [98] repaired a crankshaft pulley hub using Stellite 6. The repaired part was analysed using dye penetrant inspection. The result showed that no cracks, discontinuities, porosity, and other surface defects were present.
Due to the harsh environmental condition, components in the marine sector are usually subjected to corrosion, erosion, and oxidation [99] that reduce the operational cost [99,100]. These components are usually repaired using welding techniques such as MIG and submerged arc; however, these techniques led to distortions, a large HAZ, and a poor repeatability [101]. On the other hand, Kampanis and Hauer [101] repaired a propeller shaft 11.2 m long and with a diameter  [96] of 650 mm using the LP-DED process and showed that LP-DED was a suitable process for a fast, efficient, and safe repair operation. For example, the in-service life of bulkheads was significantly affected by corrosion [102]. In naval ship and submarine sectors, HSLA-100 steel is the material widely used for bulkhead production [103] due to its high strength, good low-temperature impact, and good weldability [104]. Sun et al. [105] demonstrated that HSLA-100 steel plate could be successfully repaired using the LP-DED process. Repaired grooves were defect-free and a good metallurgical bonding was obtained. Results showed that conversely to cold spray, thermal spray, plasma spraying, and arc welding, using the LP-DED process, no cracks due to lower residual stress were observed. Moreover, the deformation of the repaired parts was very limited. Moreover, components in the marine sector are usually characterized by huge dimensions of over 400 mm [99][100][101]. Using conventional repair processes, such as TIG or PWAG, it is not possible to repair these components onboard and it is necessary to transport them to workshops or laboratories and this activity is time-consuming and expensive [99]. Hence, it is important to repair these components in situ without removing them from the case structure. KIMI successfully repaired marine pistons (Fig. 13) demonstrating the economic benefits of LP-DED process. Additionally, they demonstrated that the repaired components exhibited a higher hardness and corrosion resistance [24]. Hence, using the LP-DED process, the piston's service life was prolonged [106]. A marine diesel engine crankshaft was repaired by Koehler et al. [107]. Using the LP-DED process, a good metallurgical bonding between the deposited material and the base material was obtained. Figure 14 shows the top and the bottom views of the repaired crankshaft. The repaired width was of about 50 mm and during the repair operations, it was demonstrated that the functional aspects of crankshaft such as the oil bore were not altered or damaged. Later, Torims [108] used the LP-DED process as an in situ repair process for a marine crankshaft with a diameter of 450 mm. The deposition head was placed onto the crankshaft journal grinding equipment. This solution allowed the repair process directly in the engine housing.
Another important sector in which the LP-DED repair process was successfully applied is the railway transport sector. In fact, among the other phenomena, rails were continuously damaged by the wear mechanism caused by the Rolling Contact Fatigue (RCF) [109][110][111]. In Europe, the cost related to damaged rail is of about €2 billion per year, and this cost includes inspection, train delay, replacement or repair of the damaged part, grinding, and loss of business [111]. The repairing process using wear-resistant material could reduce significantly the cost by increasing the in-service life of the rails [110,111]. Clare et al. [110] repaired a worn railway using different wear-resistant alloys, such as Ni alloy, Stellite 6, Hadfield steel, and Maraging steel. Their study aimed to evaluate the capability of the selected alloys to be used as a repairing material. Results showed that all the analysed alloys were suitable for the repair operation with a higher hardness value compared to the substrate material. Later, Lewis et al. [112] analysed the wear and the RCF of the same alloys. Both wear and RCF were improved using the wear resistance material in repairing operation. Using Stellite 6, the best results in terms of wear resistance were obtained. Instead, no significant variation of RCF life was observed by varying the material. In another study, Zhu et al. [113] repaired grade C railway wheels using three different stainless steel alloys that are 316L, 410, and 420. The results showed that the repaired wheel disks were characterized by lower wear rates and RCF compared to the original wheel disks. On the contrary, the wear rates and the RCF of the repaired rail disks were higher with respect to the original disks. Sexton [114] repaired overhanging train carriage bearings by depositing on the bearing a hard resistant material. In his work, a Cobased alloy was used and the results showed that the life of the overhanging train carriage was extended. In addition, due to high cooling rates obtained in the process, a higher value of hardness was observed.
It is known that repairing internal defects, such as cracks, is more difficult with respect to the repair process on an  [107] external surface [49]. The effectiveness of the LP-DED process in repairing internal defects was demonstrated by Nowotny et al. [115]. In their work, erosion defects in large gun barrels were repaired using a novel internal diameter deposition head. Usually, the repair operation requires the machining of a groove in the damaged area. Onuike and Bandyopadhyay [116] repaired internal defects in Inconel 718 samples using different groove geometries. Results showed that better results were obtained by machining the damaged area with a trapezoidal shape with respect to a rectangular cross-section. Pinkerton et al. [49] analysed the effect of the geometry of the groove on the deposition process. In particular, two different geometries were selected that are the square shape and the V shape. H-13 tool steel was used as a material. Results showed that vertical walls of the square-shaped groove were a problem; in fact, they shielded the powder flow and the laser beam, and as a consequence, a higher value of porosity was measured. Later, Graf et al. [117] deposited stainless steel and titanium powder in different groove shapes. In their study, the process parameters were varied and their influence on HAZ and microstructure were investigated. The results showed that if the groove was big enough for a proper powder stream, a repair operation without porosity was obtained. Moreover, the titanium powder was deposited without the use of additional inert gas using low heat input. The deposition strategy used in their study allowed obtaining constant offset between adjacent tracks and between consecutive layers and good side wall fusion as depicted in Fig. 15.
Oh et al. [118] studied the applicability of the LP-DED process to repair damaged 316L L-PBF components with different groove depths. In their study, the mechanical properties were analysed using tensile tests, hardness measurements, and observing the fracture surface. Results showed that the LP-DED process could be used to repair L-PBF components; however, for large groove depth, cracks occurred due to the thermal residual stresses. The elongation and the strength of the repaired samples were respectively 5% and 3% lower with respect to the original L-PBF components. On the other hand, no significant variation of microhardness was observed. Sun et al. [119] used the LP-DED process to repair T shape grooves on 316L stainless steel.
Results showed that the level of incident energy, that is, the ratio between the laser power and the travel speed, has a high influence on the quality of the repaired part. In particular, they showed that a too low value of incident energy caused pores; on the contrary, an excessive value of incident energy caused cracks due to thermal stresses.

Functionally graded materials
One of the main advantages of the LP-DED process is the possibility to produce components by varying the composition of the deposited material obtaining Functionally Graded Materials (FGMs) [33,120]. With a local changing of the materials that constituted the part, it is possible to optimize the functionality of the part. Hence, the properties of FGMs components are non-uniform but they change within the components [121]. For example, considering a pulley, it is more advisable to use a harden and wear-resistant material near the hub and the rim and a more ductile material in the core (Fig. 16) [122]. The mechanical properties of the interface zone between the two materials are in the middle between the properties of pure materials as summarized in Table 3.
Several conventional techniques were widely used in the industry that allowed obtaining FGMS components such as Physical Vapour Deposition (PVD), Chemical Vapour Deposition (CVD), Powder Metallurgy (PM), and Centrifugal Method (CM) [27,28,130,131]. However, using the LP-DED process, higher production, lower energy consumption, and maximum material utilization, in addition to the possibility to produce complex shapes, with respect to conventional processes were obtained [132]. Moreover, the LP-DED process allows to modifying the design process introducing the chemical composition as a design parameter [133].
In the following paragraphs, the most relevant findings obtained both in the industrial and the academic fields are presented, referring to the macrosector of application, that

LP-DED FGM parts in aerospace
The aerospace sector was the first sector in which FGMs were applied [59]. FGM components include rocket engine components, the spacecraft truss structure, and the heat exchange panels [59]. However, the produced parts are characterized by high stress concentration and delaminations may occur [134]. Hence, several feasibility studies were performed in order to optimize the combination of materials with different characteristics considering both the process parameters and the composition. In addition, also the geometry and the dimension of the join influence the mechanical performance [131].
Titanium alloys are widely used in the aerospace sector due to their high corrosion resistance, high strength to weight ratio, low density, and high strength at high temperature [135,136]; however, they suffer from poor wear resistance and low hardness [137]. For this reason, to overcome this problem, different FGM materials were studied. Among the others, TiC was one of the most studied materials in order to produce titanium FGM components, due to its metallurgical compatibility with titanium alloys, its low density, and its high values of hardness and Young's modulus [138][139][140].
Obielodan and Stucker [129] analysed the effect of different transitional joints on tensile and flexural strengths of FGM Ti6Al4V/TiC samples. Results showed that the joint types did not affect the flexural strength however highly influenced the tensile strength. Mahamood and Akinlabi [141] produced defect-free FGM Ti6Al4V/TiC samples. They showed that the microhardness and the wear resistance were improved optimizing the process parameters for each material composition. The optimization of process parameters was obtained using a model previously developed [142]. Zhang et al. [140] studied the effect of process parameters and TiC composition on microstructure and mechanical properties of FGM Ti6Al4V/TiC samples. Results showed that increasing the energy, both the primary dendrite arm spacing (PDAS) and the secondary dendrite arm spacing (SDAS) increased. The hardness value was mainly influenced by the TiC content; on the contrary, the specific energy did not influence the hardness value. They showed that TiC content influenced the mechanical properties. In particular, the Young modulus increased and the ultimate tensile stress decreased with increasing the TiC content.
Moreover, it should be noted that different aerospace components, such as telescopes and high-precision optical mirror substrates [143,144], are subjected to large temperature variations that could induce thermal shock and dimensional deformations [143]. This can be reduced by using materials characterized by a low thermal expansion coefficient [143]. For example, Bobbio et al. [144] produced FGMs of Ti6Al4V to Invar. However, the samples were characterized by macroscopic defects such as material overflow and cracks. The cracks were developed due to the residual stresses and the mismatch in Young modulus.
Inconel is another material successfully used in aerospace applications due to the high-temperature corrosion resistance, fatigue, and creep resistance [145]. To improve the thermal conductivity of Inconel 718, Onuike et al. [146] produced In718/GRCop-84 FGM. GRCop-84 is a copper-based alloy used in the main combustion chamber and nozzle liners [147]. Two approaches were used that are the direct deposition of GRCop-84 on In718 and the gradual variation of GRCop-84 alloy during the deposition. The results showed that using the gradual variation, a more uniform interface was obtained and an increase of thermal conductivity of about 300% was obtained compared to IN718. However, in order to produce defect-free samples, the specific energy should be increased of about 270% due to the high reflectivity of GRcop-84.

LP-DED FGM in tooling industry
One of the first industrial applications of FGM materials concerned surface coating and was obtained by Heyden Laser Services. This is a company based in Massachusetts and its applications are focused on wear-resistant coatings in tools and die, in nuclear and in power generation industries [32]. The first step was to compare the capability of the LP-DED process with the conventional processes. Using the LP-DED process for the deposition of tungsten carbide in a nickel matrix, they obtained a better metallurgical bond with respect to that achieved using conventional processes such as thermal spray or plasma transfer process [32]. Then, the LP-DED process was compared with TIG welding by Thivillon et al. [148], analysing the coating quality of Co-based Stellite and Ni superalloy Inconel 625. The coating quality was evaluated by means of different characteristics such as microstructure, hardness, and dilution. The results showed that using the LP-DED process, higher hardness and finer grain were obtained. Moreover, the metallurgical bond obtained using TIG welding showed lots of irregularities; in contrast, the metallurgical bond obtained using LP-DED appears excellent (Fig. 17).
The hardness is one of the most important factors affecting the die life; in fact, the die life increases with the hardness value [149,150]. H-13 tool steel is one of the most used materials in the moulding sector [151] due to its high hardenability and high thermal fatigue resistance [42,152]. Although it has benefits, at elevated temperature, the hardness value decreases sharply [152]. Hence, studies were performed in order to deposit hard material on the H-13 matrix. TiC was widely used in order to increase mould wear and corrosion resistance [153,154] at elevated temperatures. Jiang et al. [155] used Ni/Cr alloy to improve the adhesion between H-13 and TiC. Zhang et al. [156] optimized the process parameters varying the ratio of premixed Ti and TiC powder. They showed that hardness and wear resistance increase with the TiC content; in contrast, the ductility decreases with the TiC content. Chen et al. [157] depositing CoMoCr alloy on 718H steel showed that the mould microhardness has increased twice with respect to the initial configuration due to the formation of carbide and martensite hard phases.
In addition to the mechanical characteristics, FGM materials were successfully applied in order to optimize the energy consumption, the environmental impact, and the material usage of moulds and dies [40,42,88]. In fact, another disadvantage of H-13 tool steel is the low thermal conductivity [158] that causes a longer time cycle. In order to optimize energy consumption, environmental impact, and material usage, and, furthermore, to minimize the cycle time, highly conductive materials have been used as a volumetric heat sink. These materials include Ampcoloy 940 [159], copper alloy [40,79,159], and tool steel [160]. Morrow et al. [79] showed that depositing H-13 tool steel onto a copper substrate, as represented in Fig. 18, the injection moulding time cycle can be reduced of about 25% compared to a conventional tool steel mould. However, when depositing copper on a steel substrate, some difficulties can arise due to the heterogeneous properties of the two materials [161]. For example, Noecker and DuPont [162] during the deposition of pure copper on AISI H-13 steel showed the presence of three cracking susceptibility levels that depends on the Cu concentration. These cracks were attributed to the large solidification temperature range, to the formation of undesired phase, and to the differences in the thermal expansion coefficient. This behaviour was confirmed by Beal et al. [163] and they found that the cracks could be reduced by preheating the powder particles. They also showed that samples with densities over than 90% could be produced by optimizing the deposition strategy and that the laser parameters affect the spatter and the balling effect and consequently the product quality.
Ahn and Kim [159] produced a thermal management mould, depicted in Fig. 19, using three different materials: Ampcoloy 940 used as base part, P21 tool steel used as a moulding part, and Monel 400 used in the mid-layer in order to reduce the thermal stresses in the joined regions. Results showed that the cycle time and the cooling time  Fig. 18 Functionally graded tool steel-copper mould [79] using the FGM mould were reduced of about 30% and 80% respectively. Fessler et al. [164] produced FGM sample from 100% stainless steel to 100% Invar. This FGM material was selected due to its excellent properties. In fact, stainless steel was used for high corrosion resistance [165,166] while Invar exhibits a very low coefficient of thermal expansion and hence it was used for reducing the distortion due to the residual stress [167]. This FGM material was applied by ALCOA in order to produce an advanced injection mould. The mould was characterized by a copper core to minimize the cycle time, by an Invar body to reduce the deformations due to thermal stresses, and by an external surface of stainless steel in order to prevent the corrosion [164].

Other applications of LP-DED FGM
The production of FGMs using steel and Inconel were successfully used in order to improve the wear and corrosion resistance in the automotive sector [121,133,168]. FGM components in automotive include valve stems, pistons, driveshaft, and shock absorbers [121,133]. Inconel alloys were successfully applied due to their good mechanical and corrosion resistance at elevated temperatures [169,170]. Carroll et al. [120] successfully used the LP-DED process to produce a sample with a graded structure from 304L stainless steel to 625 Inconel alloy (Fig. 20). The FGM component was obtained by varying the mass amount of IN625 powder deposited during the process. Results obtained from microscopy show a gradual modification of microstructure without sharp differences. For a composition of about 82 wt% SS304L, particles with a dimension of about few microns of secondary phase were observed. These secondphase particles caused cracks with a dimension of about 100 µm during the deposition process. Savitha et al. [171] deposited FGMs of 316L stainless steel and Inconel 615 in order to establish the capability of the LP-DED process to produce FGMs. Results showed that using the LP-DED process, crack-free samples have been produced; however, some porosities and unmelted particles were observed. Later, Zhang et al. [124] analysed the microstructure of 316L/IN615 FGM samples. The microstructure of the samples showed that a sharp variation was obtained for pure 316L and Inconel 615 samples; instead, for gradient samples, the microstructure changed progressively. Mirzana et al. [172] studied the phase distribution and the formation of an interface region during the fabrication of FGM SS316 and IN625 disk. The disk consists of an inner region of ss316 and an outer region of IN625. The microstructure result showed a uniform variation of the properties without the formation of cracks in the interface region. The Vickers hardness values measured in the SS316 in the interface and in the IN625 regions were 200, 291, and 345, respectively. Liu et al. [173] used IN625 as a buffer material between cast iron and SS420. Using IN625, they avoided the problems related to high hardenability and cold cracking of SS420 [174]. Chen et al. [175] showed that when the IN615 content exceeds the 80%, the white second phase occurred in the dendrite boundary and an increase of hardness and wear resistance was observed. However, the formation of the second phase was the main factor that caused the reduction of tensile properties.
Wu et al. [176] deposited two FGM SS316L/IN718 samples using two different deposition patterns, with the composition gradually modified during the process. The first sample grows along the building direction, and the second one was produced in the xy-plane. Results showed that the first sample was characterized by a columnar to equiaxed transition with the minimum value of microhardness measured in Fig. 19 Functionally graded thermal management mould [159] (b) (a) 10 mm Fig. 20 a Theoretical and b DED produced functionally graded material sample [120] correspondence of 50% v/o IN718, while in the second sample, the microhardness grows linearly with IN718 content. Shah et al. [177] studied the effect of process parameters on the microstructure, the mechanical properties, and the wear resistance of 316L/IN718 FGM samples. Results showed that the secondary arm spacing is strongly influenced by laser power and powder feed rate. The tensile strength and the hardness of the samples were inversely proportional to the laser power and increases with powder feed rate.
Several parts of the human body are classifiable as FGMS [121,130]; for this reason, several efforts were performed in order to optimize biocompatible implants [121]. Balla et al. [178] used the LP-DED process to create an FGM material in order to improve the osseointegration properties. In their work, the tantalum was selected as a coating material due to the high corrosion resistance, its bioactive capability, and the capacity to bond to the bone. Producing tantalum parts using conventional production technologies involves high manufacturing costs and, moreover, is not possible to produce modular all-tantalum implants [178]. Using the LP-DED process, the authors successfully deposited a tantalum coating on titanium parts and they showed the capability to create modular all-tantalum implants. Another material widely used in the biomedical sector is the Co-Cr-Mo alloy [179]; however, an abrupt variation between Co-Cr-Mo and Ti6Al4V resulted in severe cracks due to the poor metallurgical compatibility. Optimizing LP-DED process parameters, Bandyopadhyay et al. [180] obtained a crack-free deposit of Co-Cr-Mo on Ti6Al4V alloy with a good reproducibility. Krishna et al. [181] studied the biocompatibility of Ti6Al4V/ Co-Cr-Mo FGM material by varying the Co-Cr-Mo concentration on the top surface. Results showed that the living cells, and hence the biocompatibility, after 14 days of culture decreased with increasing the Co-Cr-Mo alloy concentration. However, the optimal combination in terms of wear resistance and biocompatibility was obtained for 50% Co-Cr-Mo alloy concentration. Janaki Ram et al. [182] during the deposition of FGM pure Ti/TiC samples showed that defect-free coating could be produced. The coatings consist of a mix of irregular partially melted TiC particles and fine resolidified TiC particles. The amount of these particles and the hardness increased with increasing the TiC content.

Production
Production of near net shape components without the use of tools and moulds is one of the main benefits of AM processes [183]. This allows material saving and lead time reduction [184,185]. In most cases, the production applications of LP-DED refer to the production of large components and to the production using high melting point alloys. In addition, it should be noted that the production using the LP-DED process is on average ten times faster with respect to the L-PBF process [5]. Moreover, as summarized in Table 4, the mechanical characteristics of the sample produced by LP-DED are comparable with the properties obtained using the conventional processes. The examples of components produced by the LP-DED process are presented in this section highlighting the sector in which they were applied, which are aerospace sector, tooling sector, and all the other sectors.

Production of aerospace parts by LP-DED
One of the most important sectors in which the LP-DED process has been strongly used for production is the aerospace sector. The main reason that drives the use of the LP-DED process in this sector is the possibility to produce components with larger dimensions with respect to those produced using the L-PBF process [5,189]. For example, in 2003, the Northwest Polytechnic University produced a central wing spar for Comac C313 passenger aeroplane (Fig. 21). The central wing spar is 5 m long and the mechanical properties were comparable with those obtained in the forging process [190]. The production was performed using an LP-DED cell characterized by a working volume of 5 × 2.5 × 0.6 m 3 with an accuracy of ± 1 mm [65]. DMG Mori produced a stainless steel turbine housing using LASERTEC 65 3D hybrid machine (Fig. 22). The  [135] turbine housing was characterized by a diameter of 180 mm and a height of 150 mm. The time required for the production of this component was about 230 min [191]. Another example of the application of the LP-DED process was presented by TWI [192]. Using a five-axis LP-DED machine, an Inconel 718 helicopter engine combustion chamber was successfully produced (Fig. 23). The dimensions of the produced chamber were 300 mm in diameter and 90 mm in height. The as-built component showed an average tolerance with respect to the CAD geometry of about 0.25 mm. Moreover, an accuracy of about 0.09 mm on the dimension of the produced thin wall was obtained. Additionally, TWI showed that the built time was reduced from 2 months for the conventional manufacturing process to 7.2 h for the production using the LP-DED process.
In addition to the production of large components, different studies revealed the LP-DED process allows a short time delivery of components compared to the conventional manufacturing processes. Hedges and Calder [48] showed that thanks to its flexibility, the LP-DED process can be used to manage the fast design modification without the need for retooling. Figure 24 showed a housing used in defence applications produced by LP-DED. With the conventional manufacturing system, the time required for the production of the housing is of about 6 months; instead, using the LP-DED process, part was manufactured within 3 days [48].
Another study made by the National Center for Manufacturing Science (NCMS) showed that the LP-DED process reduced the time required to produce moulds of about 40% [40]. Another application of LP-DED in the aerospace sector was performed in 2004 by Bell Helicopter. The requirement of high material integrity and fast delivery led to the use of the LP-DED process. LENS 850-R system was used to produce a titanium 1/6 scale mixing nozzle (Fig. 25) for a gas turbine of a military helicopter. The time required for the production was reduced from 9 weeks using the conventional casting process to 3 weeks [48].
An increase of building rate value could be obtained by increasing the deposition speed. However, it is not possible to increase the speed very much due to the limited acceleration of the motor of the deposition head [196]. Moreover, as the deposition speed increases, the deposited material exhibited convex shapes that deviate much from the nominal dimension. Hence, Ma et al. [196] in order to produce large components with high deposition speed optimized the deposition strategy in order to minimize the geometrical deviation. This optimization was performed on a bracket that was designed likewise of pylon ribs of wing components in aircraft applications. In their experiment, a variational orientation raster scanning (VORS) deposition strategy was developed. The results showed that using the developed deposition strategy, the deviations were reduced from 4 to 1 mm. Figure 26 depicts the bracket produced in the experimental validation of the deposition strategy. In addition to time, production using the LP-DED process can bring benefits to costs. For example, Hedges and Calder [48] showed that the manufacturing costs associated with the production of the housing depicted in Fig. 24 were reduced by 65%.

Production of tools and moulds by LP-DED
The problem associated with the production of dies and moulds using the conventional production processes was related to the lead time required to manufacture them, that in some cases can be up to a year [197]. To solve this issue, the LP-DED process was successfully applied in different studies in order to reduce manufacturing times. Studies performed by Morrow et al. [79] and by POM Group Inc. [88] showed that the lead times using the LP-DED process were reduced by approximately 70% with respect to those obtained using conventional processes.
Another important aspect of the LP-DED process is the possibility to deposit directly onto an existing surface [22,30,36]. This capability was used by Morrow et al. [79] for remanufacturing a stamping die used by a US truck manufacturer. The remanufacturing operations consist of a modification of the geometry of the die and the relocation of the manufacturer logo. Results showed that the energy consumption of remanufacturing operations was less than half of the energy consumption required to manufacture a new die. The authors estimated that this energy saving led to a cost saving of about 250,000 over the life cycle of the analysed die.
In addition, the application of AM processes in the manufacturing industry leads to the possibility to produce moulds with conformal cooling channels [42]. In order to verify the capability of the LP-DED process to produce conformal cooling channels and to evaluate the resulting benefits, J.S. Die & Mold in collaboration with POM Group Inc. [198] produced a mould used for the production of wheel type components [88]. Results showed that the mould and the conformal cooling channels were successfully produced. Moreover, the use of conformal cooling channels led to a reduction of the moulding cycle time of 20-50% which is reflected in an economical benefit [42,88].

Production by LP-DED in other sectors
Different applications can also be found in the biomedical sector. The major issue of metal implants used in the medical field is the mismatch between the mechanical characteristics of the metal part and the bone that could lead to fractures and other problems [199,200]. Hence, some studies were focused on reducing this mismatch. For example, Dinda et al. [201] used the LP-DED process to produce a Ti6Al4V scaffold for patient-specific bone tissue engineering (Fig. 27). The process was performed in a controlled chamber using a mix of argon

Fig. 24
Produced 316L housing for defence application (courtesy of Optomec®) [195] 50 mm  [195] and helium; in this way, the problem related to oxidation was reduced. The average surface roughness of the as-built scaffold was 25 μm. After sandblasting operation, the surface roughness was reduced to 12 μm achieving the recommended criteria for bone tissue engineering. The as-built part was characterized by very high tensile and yield strengths, 1163 MPa and 1105 MPa, respectively; however, the ductility was below the limit defined by ASTM F136-79. As a consequence, a heat treatment was performed in order to increase the value of the ductility. Their work demonstrates that LP-DED is a suitable process for the production of hard tissue biomaterials. Krishna et al. [202] proposed a method that allowed producing functional hip stems using the LP-DED process (Fig. 28). In their study, the process parameters and the design approach were varied in order to obtain an internal porosity. This induced internal porosity which allowed to reduce the bulk density of the component and a reduction of the difference between the stiffness of the bone and the stiffness of the implanted material was obtained. Results showed that the bulk density was reduced from 4.5 to 3.6 g/ cm 3 and that the LP-DED process can be successfully used to produce custom implants with personalized properties depending on the patient's need.
Moreover, Palčič et al. [200] showed that the use of the LP-DED process could lead to a less complicated operation procedure. They produced a hollow, thin-walled intramedullary nail (IM nail) used for the fixation of the radius bone head (Fig. 29). The selected material for the experiment was titanium alloy Ti6Al4V due to its biocompatibility, resistance to corrosion, and osteointegration. In his work, the component produced using LP-DED process was compared with the component produced using conventional technologies (turning and drilling). The results showed that the component produced by LP-DED process was lighter, due to the hollow structure, and it was also characterized by an easier use due to the absence of attendance instruments [200].    [202] Further sectors in which the LP-DED process was applied are the automotive, tool production, military, and the space sector, highlighting the benefits in terms of cost and time that could be obtained by using the LP-DED process. For example, driveshaft spiders and suspension mountain brackets for the Red Bull Racing car were successfully produced by Optomec using the LP-DED process. The produced components are illustrated in Fig. 30. Using The LP-DED process, a significant reduction of time of about 50% and of the cost was obtained. Moreover, a reduction of waste material of about 92% for suspension mounting bracket and of about 97% for drive shaft spider was achieved.
In Advanced Robotics Mechatronics System (ARMS) project, the benefits of producing an integrated boom/housing of a space robot manipulator using the LP-DED process were evaluated [203]. The produced part is illustrated in Fig. 31. Results showed that for complex parts, the LP-DED process is economically convenient. On the contrary, for small parts with simple geometry, economic benefits could be obtained by using conventional manufacturing processes. Moreover, the potentiality of the LP-DED process in particular for remote autonomous operations was demonstrated, considering also the economic advantages that derive from integrating different components into one final part.
Xue et al. [204] used the LP-DED process to produce rotatory cutting dies of CPM-9 V tool steel (Fig. 32). They successfully produced several rotatory dies and the benefits of the LP-DED process with respect to the conventional manufacturing process were proved. In particular, they demonstrated that the die produced by the LP-DED process cut over than 180,000 m of labels without the necessity of a reshapening. Using the LP-DED process, the production time was reduced by one third compared to conventional production processes. Furthermore, the material cost was (a) (b) Fig. 29 a CAD cross-section and b Ti6Al4V prototypes of an IM nail before and after finishing operations [200] Fig. 30 a Ti64 suspension mounting bracket and b Ti64 gearbox spider produced by LENS process saving above 90% material losses (courtesy of Optomec®) [195] 20 mm 20 mm (a) (b) Fig. 31 LP-DED integrated boom/housing [203] reduced of about 50% and the die life increased by approximately 100%. Finally, the LP-DED process was also successfully applied in sonar application for the production of the folder shell projector (FPS) []. This component is characterized by a complex thin-wall structure with inner sharp corners that are non-realizable using the conventional manufacturing processes [205]. NCR-IMI in collaboration with Defense Research & Development Canada (DRDC) demonstrated that the LP-DED process could be used to produce the FPS without cracks or defects [205].

Conclusion and future outlook
The purpose of the current review is to provide a summary of the current industrial application of the LP-DED process. The main advantages of the LP-DED process with respect to other AM processes are the possibility to produce large components, the possibility to deposit the material onto an existing surface, and the possibility to change the deposited material directly during the manufacturing process.
It was observed that the main application refers to repair operations of high-value components characterized by complex shapes, such as turbine blades. The reasons that encourage the use of the LP-DED process with respect to other repairing processes, such as TIG or plasma transferred arc welding, are the lower heat input, the lower warpage and distortions, and the higher precision. Moreover, using the LP-DED process, significant cost and time saving could be obtained. Further research works are needed for optimizing the process parameters in order to increase the surface quality that is at the current state one of the major issues of the LP-DED process. In addition, to improve the process, obtaining a higher repeatability is also necessary to understand the effect of the shape and the dimensions of the substrate, which in the case of repair operations coincides with the damaged part.
Thanks to the combined possibility to deposit onto an existing surface and to use different materials during the manufacturing process, the LP-DED process is widely used to produce designed materials. This capability was applied in different sectors to improve mechanical properties such as the wear and the corrosion resistance, the hardness, and thermal properties such as the thermal conductivity in moulds in order to improve the thermal exchange. At this stage, the application of the FGM is mainly limited to feasibility cases that studied the interface region due to the complex phenomena that occur by melting two dissimilar materials. The main problem is that the process parameters should be selected for each couple of used materials. Simulation models could help to overcome this problem by defining and limiting the process parameter window and thus limiting the experimental tests required to optimize the process parameters. From the literature review, it emerges that up to now, the LP-DED process is used for the production of a limited number of parts that were characterized by relatively simple geometry and by high dimensions. This is related to the fact that the LP-DED process is not able to manage components characterized by overhanging features. The integration between tilted substrates and the motion of the deposition head allows overcoming to this limit. However, it is necessary to analyse the effect of an inclined substrate on the characteristics of the deposition parameters, such as the powder flow and the power absorption.
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Conflict of interest The authors declare no competing interests.
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