Additive manufacturing: the role of welding in this

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Additive manufacturing: the role of welding in this window of opportunity Eduardo André Alberti, Leandro João da Silva & Ana Sofia C. M. D’Oliveira To cite this article: Eduardo André Alberti, Leandro João da Silva & Ana Sofia C. M. D’Oliveira (2016): Additive manufacturing: the role of welding in this window of opportunity, Welding International, DOI: 10.1080/09507116.2015.1096513 To link to this article: http://dx.doi.org/10.1080/09507116.2015.1096513

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Date: 05 February 2016, At: 12:55

Welding International, 2016 http://dx.doi.org/10.1080/09507116.2015.1096513

Additive manufacturing: the role of welding in this window of opportunity Eduardo André Albertia, Leandro João da Silvaa and Ana Sofia C. M. D’Oliveirab a

PGMec, Paraná Federal University, Curitiba, Brazil; bDepartment of Mechanical Engineering, Paraná Federal University, Curitiba, Brazil

Downloaded by [Högskolan Väst] at 12:55 05 February 2016

ABSTRACT

KEYWORDS

Additive manufacturing (AM) can be considered as an evolution from rapid prototyping as it allows us to manufacture a component from a computer file (CAD 3D), though its applications extrapolate the production of prototypes. This technique involves the layered design of a component and subsequent welding deposition of the multilayer structure to produce parts without the need of moulds or other tools. Although AM is frequently associated with the use of high density processes, the need for higher competitiveness expanded its range of technologies to include arc welding processes. This article aims to summarize up-to-date information on AM, particularly involving arc welding processes. Emphasis is given on the challenges associated with the building up of components during multilayered deposition and on post-deposition procedures.

1. Introduction ‘Additive Manufacturing’ (AM) is defined as a group of technologies that employs a layer-by-layer approach for creating free-form objects from bottom to top. AM consists of converting a 3D CAD model, Figure 1(a), into layers, Figure 1(b), and based on this information, determining the path (CNC terminology) and the deposition parameters, Figure 1(c), which are then processed by four basic components: CNC controller, motion system, energy source and a system for feed of the additive material [1]. The term rapid prototyping, in the context of product development, has been used extensively to describe technologies that create physical products directly from a digital file (3D CAD). Currently, these technologies extrapolate prototyping, it being possible to manufacture functional components directly from a digital file. Accordingly, a technical committee of ASTM agreed that a new terminology needs to be adopted, and reached a consensus that the term ‘Additive Manufacturing’ best represents this group of technologies [1]. All the AM equipment currently in the market is based on the ‘layer-by-layer’ approach and the difference between these various types of equipment is in the material that can be processed. Table 1 summarizes the AM processes according to the materials that can be processed. This capacity for processing different materials generated a new approach called Multiple Materials Additive Manufacturing (MMAM) as noted by Gibson et al. [1].

Selected from Soldagem & Inspeção 2014 19(2) 190–198. © 2016 Taylor & Francis

Additive manufacturing rapid prototyping; multilayer deposition; arc processes

MMAM makes it possible to improve the performance of a component through microstructural design or architecture, i.e. using different materials/chemical compositions in strategic regions of a component. The relevance of this technique is pointed out by Murr et al. [2], who explain that control of the microstructural architecture can alter and/or extend the traditional paradigm of the science and engineering of materials, of the structure–property–processing trio and their effect on performance (triangular-based pyramid), to the four pillars of microstructural architecture (quadrangular-based pyramid), Figure 2. Many applications may potentially benefit from the development of MMAM, which enables specific properties to be provided in the most stressed regions of the component, for example, thermal conductivity in the cooling channels in a die for pressure die-casting of aluminium, or resistance to erosion, corrosion and high temperature in the hot parts of aeroderivative turbines, also emphasizing the reduction with respect to losses in the process of manufacture of components for the aerospace industry, etc. Despite the considerable advantages, AM processes have various obstacles that have to be overcome to make these processes more viable. Among these, we may mention the time for the manufacture of a single component, which is far greater compared to bulk manufacturing processes, availability of machines with the necessary flexibility for working with various materials, geometries and suitable precision, which vary depending on

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E. A. Alberti et al.


Figure 1. Schematic diagram of AM: (a) 3D CAD model; (b) division of the model into layers; (c) deposition of the layers. Source: the authors. Table 1. AM processes [1]. Process Photopolymerization Modelling by extrusion Fusion of pre-deposited powders 3D cladding Electric arc

Description A photocurable polymer is cured selectively using a light source Material is deposited selectively via an extruder head An electron beam selectively melts regions of a bed of pre-deposited powder The additive material in the form of powder is injected directly into the beam/pool The energy source is an electric arc that melts the additive material (powder/wire)

AM method Laser

Material Photocurable polymer

Heating by an electrical resistance

Polymers, ceramics and metals

Laser and electron beam

Polymers, ceramics and metals

Laser and PTA

Metals and ceramics

PTA, Plasma wire, TIG, MIG/MAG

Metals

Figure 2. Impact of MMAM on microstructural architecture, adapted from Murr et al. [2]. Notes: Arquitetura microestrutural = Microstructural architecture; Desempenho = Performance; Estrutura = Structure; Processamento = Processing; Propriedades = Properties.

the workpiece and the cost of equipment [3]. However, with the greater utilization and availability of research results relating to this area of manufacture, this technique is becoming more competitive. One example is 3D printers for polymers, which now already have a more

accessible cost, opening up new possibilities for businesses and investments in the area of AM. However, AM or MMAM – using metal alloys is still restricted to small components. Considering the attractive characteristics of AM techniques and the important


role of welding processes and welding metallurgy, this work presents the important features of AM with metals, focusing specifically on the processes that use the laser and the electric arc as the energy source, as well as characteristics of the alloys used most.


2. Selection of the process and material The deposition of metals in AM technology may take place by various arc welding processes (plasma, MIG and TIG), laser cladding, selective laser melting (SLM), electron beam melting, sintering of powders and by variants of these processes. Selection of the process to be used is strongly dependent on the geometry to be produced. Small parts of complex design require low-deposition rates, and accordingly the laser, electron beam and microplasma processes are the most indicated. The processing of components with larger dimensions uses processes with higher deposition rates, for example, arc welding processes. Figure 3 presents a comparison of the widths of superposed single beads for constructing a thin wall, which can be obtained by various processes. The information presented here is based on the recent literature and may require updating with the availability of information relating to AM. The range of metals already applied by AM techniques, according to the European Powder Metallurgy Association, includes titanium alloys, aluminium, nickel, cobalt, steels (stainless steels, tool steels and low-alloy steels) and precious metals. These alloys are used in the form of wire or powder depending on the process used. Selection of the material to be used and of the process is strongly interconnected. Certain alloys can be processed more easily or less easily depending on the process used. For example, nickel superalloys are susceptible to hot cracking, making it difficult to use in processes that produce considerable heating in the workpiece [4]. Much of the information necessary for selection of these processes is based on knowledge acquired in welding

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processes, as there are still few studies exclusive to AM for many combinations of alloys and processes. 2.1. Deposition by laser and electron beam The AM systems most used are those that are laser-assisted or electron beam-assisted, as is noted by Gibson et al. [1]. The justification for wide application is the high speed and precision of deposition or fusion of the beams coupled with the high energy density, which means that in each pass only a small volume of the ‘substrate’ is melted, besides the rapid cooling due to the large temperature gradients imposed [5]. There are some variations for this group of technologies with high energy density, particularly those that are laser-assisted. For example, Murr et al. [2, 6, 7] conducted studies using a fusion table, with a SLM system. This system consists of depositing a layer of the alloy in powder form on the fusion table and the laser beam follows a pre-defined path, selectively melting the pre-deposited atomized metal powders. In each cycle, a new layer of powder is pre-deposited and as the component grows vertically, the fusion table goes down maintaining the same focal distance. Once the component is formed, the excess powder is removed and, if necessary, the part finally undergoes finishing by machining. Another approach was adopted by Brandl et al. [8– 12], and then by other research groups, who use a laser system with cold wire feed. Brandl et al. explain that wire feed gained prominence on account of higher productivity and efficiency, when compared with systems that operate with pre-deposited powders. Liu et al. [13], agree with Brandl et al. with regard to increased production, but explain that the low absorption of energy by the cold wire is the main limitation of this approach and suggest feed with hot wire as an effective solution for increasing productivity. This concern is not so recent, as was shown by Nurminen et al. [14] in a work that compared three laser-assisted systems, fed with powder, cold wire and hot wire. They concluded

Figure 3. Width of single beads deposited by different processes [17, 21].


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that the deposition rate of the hot-wire system is four times higher than that of the other systems. Despite the higher feed rate of the laser-assisted systems fed with hot wire, it is important to emphasize that the chemical composition, in this approach, is limited to the composition of the wire. This feature contrasts with the systems with powder feed, which allow the composition to be varied easily, besides making it possible to produce alloys in situ and to vary the chemical composition/properties in strategic regions of the component, a technique that Murr et al. [2], called microstructural architecture. 3D cladding is another variation of laser-assisted AM. In this configuration, the powder is fed directly into the beam/pool and can be used both for manufacture and for repair of worn components. Bin and Gasser [4], using superalloys with low weldability and adopting this system, were able to repair components of aeroderivative turbines successfully. Bi, Sun and Gasser [15] explain that 3D cladding differs from the conventional processes used in repairs (PTA and TIG) in having better control of the parameters. On varying the power of the laser, the working temperature to which the area of the component is exposed is controlled, which makes it possible to avoid welding defects such as cracks and minimizes metallurgical degradation of the material deposited in previous passes. Reclamation of complex geometries is an extension of AM that has great technological potential and industrial interest, mainly with regard to components with high added value. In this reclamation process, the sequence of steps involved is a little more complex, beginning with a survey of the original geometry of the worn-out component (requires a 3D model), passing through assessment of any strains due to heating of the workpiece during the reclamation/reconstruction process, assessment of the integrity and properties of the reclaimed component and ending with post-processing, i.e. machining via CAM [16]. 2.2. Deposition by arc welding The use of processes with high energy density in AM offers competitive advantages, but they have low energy efficiency [17], which is the motivation for various research efforts to find more efficient processes. Electric arc processes meet this requirement as they possess higher energy efficiency and offer considerable scope in relation to the amount of material deposited making it possible to produce larger parts more quickly. Deposition with the laser and electron beam process has a deposition rate between 2 and 10 g/min, whereas arc processes can reach values above 130 g/min [18]. This subject has attracted the attention of various research groups throughout the world, and we may mention some of them in the USA (Southern Methodist University, University of Michigan and University of

Kentucky), the United Kingdom (Cranfield University and Loughborough University), Germany (Fraunhofer Institutes), South Korea (Korean Institute of Science & Technology), Japan (Joining and Welding Research Institute), India (Indian Institute of Technology) and Belgium (Katholieke Universiteit Leuven). The alloys used in electric arc deposition are similar to those used with the laser process, and studies may be found that focus on steels, aluminium and nickel. However, the great majority of the studies investigate titanium alloys, chiefly alloy Ti–6Al–4V [19]. This alloy has applications in aeronautics and biomedicine which have supported a large number of studies aiming to map the response of this alloy in different deposition processes. A reference research centre in this area is Cranfield University, in the United Kingdom, which has already assessed the effect of different processes and their variants, such as GTAW, CMT – Cold Metal Transfer – [20] and Plasma Wire [21]. The focus of these works is the mapping of deposition parameters for constructing thin walls, it having been demonstrated that the plasma process allows the deposition of walls on a larger scale and with higher deposition rates, whereas the CMT process gives single beads with greater heights. Another class of alloys with great potential for application in AM is the class of nickel-based alloys. The nickel superalloys find application in the energy sector and aeronautics on account of the excellent high temperature properties [22]. These alloys are employed with arc processes in cases when there is a need for high-deposition rates, both for works of construction and for repairs. GMAW welding processes, for example, allow single beads to be deposited with widths greater than 12 mm and heights of 2 mm. Clark et al. [23], using nickel alloy 718 in the deposition of multiple layers, succeeded in obtaining walls with widths greater than 30 mm and heights exceeding 60 mm, in the construction of combustion chambers. Plasma deposition is also quite attractive for processing nickel-based alloys. As this process generates a smaller thermal effect, the probability of cracks in nickel alloys is less than in the other arc processes, and in addition, the cost of equipment and processing is significantly less than for the electron beam and laser processes [24]. Studies have demonstrated that deposition using the PTA technique offers the possibility of depositing this alloy in a wide range of thicknesses, with beads varying from 2 to 10 mm in thickness. Figure 4 shows the image of deposition of multiple layers of Inconel 625 alloy processed by PTA. The intermetallics are a type of material that tends to increase the applications relating to AM. These alloys possess excellent properties for service at high pressures and/or high temperatures, as mentioned by D’Oliveira et al. [25], most often being used as structural alloys with high mechanical strength. However, the high cost and difficulties in processing are challenges that must be



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Figure 4. Deposition of multiple beads of Inconel 625 alloy by the plasma transferred arc process. Source: the authors.

overcome, for example, the occurrence of an exothermic reaction during synthesis, which may alter important characteristics of the process, such as temperature [26]. Despite the greater control required when processing intermetallic alloys, the deposition of Ni–Al, Ti–Ni and Ti–Fe by the GTAW process, with the aim of ‘free form’ manufacture, was successful in producing beads with reduced widths, close to 1 mm, free from cracks and pores and with a uniform composition [26]. Operations with PTA using mixtures of Ni and Al powders made it possible to obtain beads of aluminides processed in situ, during deposition, with thicknesses above 3 mm, and with homogeneous structure and compositions [25]. These alloys are particularly interesting for use in conjunction with metal alloys for applications that may benefit from components processed by MMAM. Repair of components with complex geometry and high cost of manufacture is a niche with great potential for application of AM. Dies used in processes for forming, extrusion and others are one example. These components are made from alloys with high mechanical strength, generally tool steels and wear occurs during use. In many cases, it is not feasible to repair these components owing to their complex geometry and/or the high cost. AM is becoming a competitive process in this market, as was shown in work with microplasma, depositing steel AISI P20 (steel with chromium and molybdenum), creating thin walls with multiple deposition of beads [17]. These depositions resulted in beads with thickness of 2 mm, homogeneous and free from pores or cracks, making it possible to reconstruct complex geometries. Besides tool steels, works are also found with stainless steels, aiming at components with corrosion resistance and high ductility. By superimposing beads of stainless steel AISI 308 by GTAW, it was possible to

construct walls with thickness of 8 mm and more than 30 mm in height, without any pores or cracks present, with the mechanical properties of the material remaining constant throughout deposition, and with values similar to those of other methods for manufacturing components [19].

3. Control of processing in construction of the component There are various moments in the construction of a component by AM that are decisive for the success of the procedure. Deposition of the first bead is among the factors requiring careful analysis. In this first bead, in repairs, a good metallurgical bond with the substrate is obtained, along with suitable thickness and morphology. In tests for forming a ‘wall’ by the multiple deposition of multiple layers, the morphology and microstructure of the deposits have already been analysed. Control of morphology aims to guarantee continuity of the beads, variation of height or defects. The continuity of the beads is associated with selection of the processing parameters, since unsuitable selection may result in beads with a large variation of width or lack of fusion of the deposited metal [21]. Figure 5(a) shows an example of a bead with poor homogeneity. The variations in the height of the beads are related to changes in the processing conditions at specific points, as any variations in thickness of the substrate may result in an increase in temperature during deposition or change in the deposition rate, which is common at the end of the beads [4]. Figure 5(b) shows an example of variation in height at the end of the bead, which occurred because the torch stopped at this point, increasing the energy input in this region. Other defects that may also be observed in the deposited beads are

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Figure 5. Defects of morphology observed in beads deposited for AM; (a) non-uniform bead; (b) bead with excess deposition at the end. Source: the authors. Notes: Regiãò com excesso de material no final do cordão = Region with excess material at the end of the bead.

Figure 6. Diagram of measurement of the total thickness (e.t.) and effective thickness (e.e.) in deposition for AM. Source: the authors.

large pores, cracks and oxidation, which may compromise the properties of the component constructed or repaired by AM. The geometry of the beads is a very important characteristic in AM. It is the basis for programming the manufacturing process, such as the number of passes and position of each bead. Thus, research aims to find beads with thicknesses that makes it possible to reduce the number of passes required to obtain the final format, thus lowering the costs of deposition and machining. An example where the geometry of the beads is of great importance is the reclamation of the components of gas turbines, in which thin walls of approximately 1 mm must be reclaimed after wear during operation. In these cases, reconstruction requires beads to be deposited with a slightly larger thickness, allowing the removal of material after machining to give the appropriate finish for the part [4]. For an analysis of the real geometry that may be constructed after deposition of multiple layers, one tool that is used is the measurement of the total and effective thicknesses of the ‘wall’ that is deposited, as well as the height of the beads [21]. The total thickness is the largest thickness obtained in the beads deposited during processing, while the effective thickness is the largest thickness possible for obtaining a thin wall, after machining.

Figure 6 shows a diagram of the measurements of the effective and total thicknesses. The height measured in this analysis is the individual height of each bead, which together with the values for total and effective thickness makes it possible to programme deposition for final construction of the component. Besides the characteristics already mentioned, various other factors must be taken into account during the AM process, with the aim of producing complete components. Among these, we may mention the thermal cycle associated with depositing superposed beads, the deposition path, the temperature between passes, variation of the distance between torch and workpiece, the distance between deposition of lateral layers, etc. The thermal cycle generated during deposition is of great importance in AM processes, since the component is subjected to multiple exposures to the temperature due to the deposition of multiple layers. This condition generates a complex distribution of the temperature gradient through the workpiece, which has an influence on the distribution of stresses, strains, microstructure and, consequently, performance of the part produced [27]. In positions such as the beginning and the end of the bead, there is greater variation, while in the central part the stresses tend to remain stable [27].


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Figure 7. Two approaches (tactics) for the direction of deposition: (a) identical direction; (b) reverse direction. Source: the authors.

Figure 8. Depositions of nickel with problems of (a) cracking through reheating and (b) oxidation in multilayer deposition without protective atmosphere. Source: the authors. Notes: Trinca de reaquecimento = Reheating crack; Regiões com excesso de oxidação = Regions with excess oxidation.

The deposition path also has an influence on the final characteristics of the part. The two main tactics for constructing thin walls are identical direction as shown in Figure 7(a), and reverse direction as shown in Figure 7(b). The variation of this path has an influence on the solidified structure, on the thermal gradient and on the stresses generated by deposition. The structure generated in depositions with identical direction tends to be more homogeneous, with a constant direction of solidification in all the layers, while in the depositions with reverse direction, the direction of growth, which follows the flow of heat, changes with each layer, interrupting the growth of the dendrites [28]. The stresses generated by the thermal gradient during deposition also vary depending on the path used, and with a change of direction between the beads, the stresses along the deposition tend to be smaller [29]. Variation of the distance between the torch and workpiece also has an influence on the final characteristics of the part, in determining the heat supplied to the material by the energy source, decreasing the area affected by the temperature and the amount of material deposited, which interferes directly with the geometry and thermal gradients of the beads deposited. Therefore, the need to maintain a constant torch–workpiece distance is of great importance, and has even been the motivation for research for developing techniques for guaranteeing this condition [30].

The temperature between passes is another very important factor, which depends on the alloy deposited. Nickel-based alloys, for example, may display reheating cracks due to the thermal cycles imposed during the process [22], Figure 8(a). In these cases, performing deposition while maintaining a pre-heating temperature at each layer deposition reduces the thermal gradient generated in the process, consequently decreasing the possibility of cracking [31]. Another question that must be analysed in the process is the need to use a protective atmosphere during ‘construction’ of the wall. Various alloys display oxidation at the processing temperatures, impairing the quality of the mechanical properties of the deposits. Examples of materials that require this precaution are nickel superalloys and titanium-based alloys [21, 22], Figure 8(b). Trinca de reaquecimento Regiões com excesso de oxidação

Reheating crack Regions with excess oxidation

4. Post-processing To manufacture a component by AM, it is necessary to carry out a finishing process after deposition to obtain the required final dimensions, for which recourse had to be machining processes. Furthermore, for the majority of materials deposited, it is necessary to obtain

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Figure 9. Example of thermal gradient generated in a bead during an AM process. Source: the authors.

Figure 10. Cross section of multilayer deposition with nickel superalloy, showing variation in the microstructure formed. Source: the authors.

homogeneity in the microstructure and mechanical properties of the beads deposited. The microstructure of the components made by AM is extremely complex, and is strongly dependent on the deposition process and on the parameters selected for construction of the component. The characteristics of the deposition process, such as welding parameters, direction of deposition, pre-heating temperature and time between passes, have a direct influence on the

solidification conditions, such as solidification time and direction of heat flow. The solidification time has a significant effect on precipitation-hardened alloys, both in the saturation of the solid solutions and in the amount, distribution, and morphology of the precipitates that make up the structure formed, as well as in the formation of possible metastable phases. Moreover, the direction of cooling may determine the direction of growth of the dendrites and precipitates [32]. The thermal gradients imposed at each deposition are also responsible for variations in the refinement of the structure over the height of the wall that is constructed. Figure 9 shows an example of gradient imposed at each deposition during an AM process, showing that it varies with the growth of the component. Modification of the microstructure in each bead, occurring through repetition of the thermal gradients, is discussed in various works for different materials. In titanium alloys, such as Ti–6Al–4V for example, formation of epitaxial grains is observed, composed of the β phase, growing near the substrate. Still in the first bead, a fine structure of equiaxed grains is observed, due to the greater efficiency of heat extraction in the first bead owing to proximity to the substrate. In the higher beads, there are fewer nucleating β grains, but they have larger dimensions [20]. In nickel superalloys, there are also variations in grain refinement, as well as their direction of growth. Formation of epitaxial grains was observed close to the substrate, while in intermediate beads there is a presence of cellular growth and in higher beads there is an evidence of equiaxial growth [4, 15]. The effects of the deposition parameters on variation of the structure formed in multiple deposition of beads are presented in Figure 10. It is noted that there is continuity of the structure between the different beads, and deposition of the different layers affects the growth of the dendrites, with effects on the uniformity of the mechanical properties of the wall deposited. Measurements of microhardness as shown in Figure 11, confirm the variation of hardness as the component is constructed, it being possible to identify the effect of superposition of the beads on correlating the hardness profile after deposition of one, three and five beads, thus confirming the importance of controlling the various processing parameters. In these situations, heat treatments are required for homogenizing the structure deposited. These treatments must be designed taking into account information obtained from the phase diagrams of the alloys deposited, since, besides the stress relief and greater uniformity of the structure, solubilization and precipitation steps have to be conducted at temperatures that depend on each chemical composition. The nickel superalloys, for example, are submitted to treatments for homogenizing the solidification structure, such as that presented by Dinda et al. [28], in which an Inconel 625 alloy, after treatment at a temperature of 1200 °C, had a structure



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Figure 11. Hardness variation in a nickel superalloy under the influence of multilayer deposition. Source: the authors. Notes: Perfis de Dureza = Hardness profiles; Dureza Vickers = Vickers hardness; Distancia (μm) = Distance (μm); Perfil de dureza [1, 3, 5] Cordões = [1, 3, 5]-bead hardness profile.

of equiaxed grains, but with non-uniform distribution of the γ′ precipitate. Sajjadi et al. [33] suggested optimization of the parameters of thermal treatment so as also to guarantee control of the distribution of precipitates.

5. Final comments • AM, in the global marketplace, has great potential for use in the area of the manufacture of components with highly complex geometry and mechanical properties. This process is now viewed by manufacturers in various sectors, such as aeronautics, energy and biomedicine, as a revolution in the manufacture of various components. • The application of welding processes in AM means that knowledge in the area of welding is of great importance at the start of development of this manufacturing technique, thus allowing faster development of the process. Initially, techniques with high energy density such as laser cladding and electron beam deposition were the only ones found in the area of AM. However, the use of arc processes offers an alternative for increasing the competitiveness of AM. The use of feed materials in the form of powder or wire also provides greater flexibility in the construction of components with a gradient of properties. • AM techniques are not a mere extrapolation of welding processes, and they require strict control of the parameters for successive deposition of multiple layers. Determination of a correct deposition

sequence ensures that the component geometry is controlled and it has an effect on its properties, and in most cases heat treatment is required for homogenizing the structure. Thus, we need a deeper understanding of the phenomena involved in the interaction of different materials in AM processes, in order to map the real possibilities of this process.

Disclosure statement No potential conflict of interest was reported by the authors.

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