Powder-based laser hybrid additive manufacturing of metals: a review

AM encompasses all technologies for producing parts in layer-by-layer fashion directly from CAD models. Initially, AM technologies were mostly applied for fabricating prototypes quickly that can be used for design verification and marketing purposes. They became quickly indispensable in many application areas. However, they were still just physical models that were not supposed to fulfil any functional requirements and therefore were commonly made from non-metallic materials, such as thermoplastics, resins and ceramic powders [7]. Meanwhile, the advances of AM technologies in the last two decades, especially for building metallic parts, made possible the manufacture of functional components and thus enabling the so-called direct digital manufacturing [8].

Metallic parts can be produced by employing mainly five different categories of AM technologies, known as material extrusion, powder bed fusion, direct energy deposition, binder jetting and laminated object manufacturing (LOM) [9, 10]. Each category includes metal AM technologies that employ different methods for producing physical models, in particular:

The schematic diagram shown in Fig. 3 presents a classification of existing AM technologies for producing metal parts. In the figure, the processes that fall within the scope of this review are highlighted. As was already mentioned, this review is focused on single and multi-setup hybrid AM routes that employ laser-based AM process to consolidate metal powders.

In addition to the four metal AM technologies discussed so far, another technology called cold spray additive manufacturing (CS or CSAM) has been attracting some research interest. CS is a material deposition process that was initially developed for coating/cladding purposes but has recently been employed as a metal AM process to build and repair components [14]. Materials are deposited by accelerating fine powder particles to a critical speed by employing a high-velocity compressed gas stream that upon the impact on the substrate, a metallurgical bond and mechanical interlocks are formed to produce parts layer by layer. Laser-assisted cold spray has also been subjected to investigation as an improved alternative to the CS process. Bray et al. [15] developed a system to deposit powder particles through CS in a substrate heated by laser diode and achieved deposits with densities higher than 99%. However, the CS process is outside the scope of this review paper and will not be analysed in the following sections.

It should be noted that among all metal AM technologies, the laser-based PBF (LPBF) process has attracted the highest industrial interest due to the relatively good accuracy and surface finish that can be achieved. However, the type of parts that can be produced with this technology is limited by the size of the powder bed. Therefore, DLD or also referred to as laser-based direct energy deposition (DED) has gain significant industrial interest, too, because of its higher build rates and bigger size of the parts that can be produced. However, such flexibility is at the expense of accuracy and surface integrity of produced parts and therefore commonly they require extensive post-processing [16]. Considering the industrial interest in the LPBF and DLD technologies [17, 18] and also their often integration in hybrid AM solutions, their fundamentals and a review of relevant research on these two technologies are presented in the next section.

A review of hybrid AM solutions that combine the capabilities of powder-based laser AM technologies with post-processing machining processes is presented in this section. In particular, in such hybrid solutions, near-net-shape components are first produced by LPBF or DLD technologies and then they are post-processed by employing machining operations, e.g. turning or milling. The machining operations can be conducted once the AM process is completed, as a finishing process, or can be alternatively applied after the consolidation of a given number of powder layers.

Figure 13 extracted from the work by Foster et al. [62] shows an example of the improvement that can be obtained in the surface finish of additively manufactured components by post-processing through machining operations.

Different approaches for combining seamlessly and synergistically the capabilities of powder-based laser AM technologies with post-processing machining processes have been implemented. In 2007, Liou et al. [77] reported a hybrid solution that integrates DLD and machining processes into a 5-axis single setup for producing metallic parts. First, the authors studied the necessity for support structures during the powder deposition process and thus determining whether the DLD process could be conducted without them by changing the part orientation. Then, based on the findings of this investigation, an approach for partitioning part models into subparts was proposed that eliminated the need for support structures when different processing positions in a 5-axis machine were used for each subpart. The pre-processing of each subpart model, i.e. its slicing, was carried out separately and thus using optimum DLD conditions for processing the subparts.

According to Jones et al. [135], the productivity of AM technologies is typically one order of magnitude lower than that of CNC machining. Therefore, a hybrid AM solution was proposed that combines the capabilities of DLD and CNC machining processes into a single multi-axis setup and thus improving both surface integrity and productivity. In this work, the tool changer solution was proposed, too (see Fig. 14) that allowed a laser cladding head and different cutting tools to be used in the same system.

The machining operation can be applied not only for finishing metal AM parts but also in alternation to the deposition of layers in hybrid AM solutions. In this way, Karunakaran et al. [7] reported a hybrid AM system in which near-net-shape structures were first produced by using a DLD head, i.e. a welding head, mounted on a table and subsequently machined to net shape. As the deposition was carried out in the form of weld beads, the surface of the deposited layer was uneven due to the scallops and the developed thin oxide layer. Thus, such uneven deposition of layers can have a cumulative effect on the accuracy of the component in the Z direction and therefore this shortcoming was addressed by face milling each layer to the required slice thickness. So, each subsequent weld-deposition was conducted on a pre-machined surface and thus having good quality welds and build metal parts consistently. A similar hybrid AM solution was proposed by other researchers but it was combining LPBF with precision milling in a single setup [79]. In particular, the parts were manufactured layer by layer by conducting LPBF and machining steps sequentially onto a substrate clamped on a worktable as shown in Fig. 15. By employing this hybrid AM/machining process, complex internal structures, such as cooling channels, were produced. The manufacture of 18Ni steel parts was reported, and their microstructure and hardness were analysed and compared with those achieved on parts produced by a standalone LPBF system and other methods. It was reported that the hybrid samples showed density higher than 99%, fine microstructure and higher hardness than the result obtained on parts fabricated using the standalone LPBF process.

The development of a hybrid multi-tasking machine tool was reported where a DLD head was integrated with turning and milling capabilities into a hybrid solution for reducing the manufacturing time and the total in-process time [136]. This hybrid manufacturing platform was implemented into a commercially available machine tool, Integrex i-400 AM, to address the specific requirements in the small-lot production of difficult-to-cut high-hardness materials for applications in aerospace, energy, medical device and tool-making industrial sector. Figure 16 depicts an application of this hybrid AM solution for producing a bi-metal part with a stainless steel substrate and deposited Inconel 718 on it.

Another approach for creating hybrid AM solutions is the integration of AM technologies with pre-processing and post-processing processes into multi-setup manufacturing platforms through the use of common work holding systems. The design and implementation of such modular work holding system were reported for integrating the LPBF process into AM enable process chains [122].

It is important to stress on differences between single- and multi-setup hybrid AM solutions, especially former execute AM and post-processing operations in one machine tool that later addresses fixturing and position issues in moving the components along a given AM enabled process chain. The multi-setup hybrid AM enabled manufacturing platforms to address some of the limitations of single setup ones. In particular, the AM and machining modules integrated into multi-setup solutions can be optimised independently to address the specific performance requirements of an individual process rather than the requirements of all integrated manufacturing processes into a single setup [97]. So, the fundamental differences in the physical characteristics of integrated processes would not increase the engineering complexity of such multi-setup manufacturing solutions unnecessarily and therefore their overall cost would not increase considerably, compared to the single setup systems. Other important advantages of the process chain approach in combining AM capabilities with other processes is that it can deliver much higher productivity due to the parallel utilisation of the integrated operations and also because it provides flexibility to synchronise the throughputs of each manufacturing module [137].

The modular work holding system reported in [122] was later employed by Badhuri et al. [82] to investigate the surface integrity, microstructure and mechanical properties of hybrid AM aluminium parts produced by employing LPBF followed by some machining operations. In another research, the same work holding system was deployed to build LPBF structures on top of metal injection moulded (MIM) preforms and thus producing small series of hybrid 316L stainless steel components [138]. The mechanical properties and interfaces of such hybrid MIM/LPBF components were investigated and it was reported that they were similar and even better than those of monolithic MIM parts while conforming fully to the ASTM standards for 316L stainless steel parts.

A similar multi-setup approach was employed by Boivie et al. [123] to combine the LPBF process with 5-axis milling and thus producing injection moulding tools. In particular, the implemented hybrid AM solution enabled the manufacture of conformal cooling channels in complex cavities that led to the production of higher quality mouldings and also allowed the injection moulding cycle time to be reduced, significantly.

The hybrid AM/surface treatment solutions are similar to hybrid AM/machining ones as the processes are carried out sequentially either on single or multi-setups. In particular, in the latter, near-net-shape components are produced employing an AM process and then they are treated to improve their microstructure and mechanical properties and also to reduce the residual stresses. While in the former, treatment can be applied in the same AM setup after the deposition of layers [139, 140]. In this review, laser re-melting (LR), laser peening and laser polishing are considered as surface treatments solutions to improve the properties of AM components.

Laser re-melting (LR) is a process that has attracted a growing interest from researchers and industry. LR has shown to decrease porosity and improve the microstructure of the AM parts (see Fig. 17).

Huang et al. [125] defined “laser re-melting” as scanning strategies that have more than one laser beam pass for each LPBF layer. The second and any further beam passes aim to increase the material densification and also to improve surface integrity and microstructure. A similar approach was applied by Lamikiz et al. [126], however, this time sequentially as a hybrid multi-setup solution. In particular, a laser beam was employed to melt a microscopic layer on the surfaces of LPBF parts that after its re-solidification made the LPBF surfaces smoother. An 80% reduction of the average surface roughness was achieved by applying this laser treatment method. More recently, Yasa et al. [127] studied the influence of different laser processing parameters, i.e. scan speed, scan spacing and number of re-melting scans, on porosity, surface roughness and microhardness of AISI 316L stainless steel and Ti6Al4V parts produced by the LPBF process. In all experiments, the use of re-melting scans improved surface integrity and properties of produced parts. Considering the parameter range employed in their work, authors observed that higher re-melting scan speed combined with low laser power showed low porosity values. Additionally, in low-energy input tests, no relevant change in porosity was observed when increasing from one to three re-melting scans. However, as the energy input was increased and, at the same time, the number of re-melting scans was also increased, the porosity increased significantly. In another research, the same authors investigated the effect of laser re-melting when applied only to the last layer of LPBF AISI 316L parts and again a 90% improvement of the surface roughness was reported [128]. Yang et al. [129] studied the changes in microhardness and microstructure after laser re-melting of LPBF samples, too. The results showed that laser re-melting had refined the material microstructure, i.e. had reduced the grain sizes, and also homogenised the material composition that improved microhardness of the treated samples. Wei et al. [130] carried out a study on the influence of the number of re-melting scans on porosity, roughness and residual stresses. It was reported that the scan number did not affect porosity and roughness. However, the residual stresses increased when one re-melting scan was used and then decreased after two or more re-melting scans.

In another research, Yu et al. [131] studied the influence of using different scanning directions during the LPBF and re-melting processes, on roughness and porosities of produced parts. It was reported that the scan strategies investigated in this research led to the same roughness decreasing trend. However, the use of different scan directions had affected the porosity of produced LPBF parts.

Laser re-melting has also been employed for post-processing DLD parts. Bruzzo et al. [132] investigated the surface integrity improvements of thin-walled parts after laser re-melting. It was reported that, when suitable re-melting parameters were employed, a smooth surface with isotropic topographies was obtained. Recently, Roehling et al. [142] investigated in situ laser diode annealing integrated into a LPBF system for reducing the resulting residual stresses. It was reported that such integrated heat treatment can reduce the resulting residual stresses in 316L stainless steel parts by 90% without grain growth or recrystallisation, and with only minor changes to the solidification structure.

Peening (also known as shot penning) is another hybrid AM/surface treatment approach that can be used to improve the surface finish and mechanical behaviour of metallic AM parts (see Fig. 18b).

In particular, it has shown to harden and strengthen the surfaces of AM parts by generating compressive residual stresses. As a consequence, favourable mechanical properties are created on treated surfaces that can increase their fatigue life (see Fig. 19) and wear resistance and thus delaying the initiation and propagation of cracks. In the work by Hackel et al. [100], authors show that laser peening can improve the fatigue lifetime of components more than 20 times comparing to that of the untreated part.

Wang et al. [134] studied the effects of in situ ultrasonic impact peening on Inconel 718 parts produced with the DLD technology. It was reported that the ultrasonic impact peening enabled the generation of high-quality metal parts with refined microstructure on their treated surfaces. In addition, the residual stresses of the treated surface were reduced by generating beneficial compressive stresses.

Another surface treatment process that can be used to reduce the surface roughness of AM metal parts is laser polishing (see Fig. 18c). Similar to laser re-melting, this post-processing technology melts the external surface asperities of the AM part and thus improves surface integrity. Dadbakhsh et al [111] reported that the surface roughness of DLD Inconel 718 samples was significantly improved after a laser polishing scan with the laser integrated into the DLD head, especially roughness values decreased down to approximately 2 μm. However, it was noted that the results were highly dependent on the applied laser power during the polishing scans.

Hybrid AM solution can combine the capabilities of metal AM technologies with those of different post-processing processes to produce end-use parts. One of the main advantages that AM processes provide is the flexibility and freedom to produce complex geometries with internal cavities that were impossible to achieve with conventional manufacturing processes only. Collision avoidance is one of the biggest issues in generating complex geometries using hybrid AM solutions. In particular, tool accessibility must be maintained over the processed surfaces when post-processing them through various machining operations, and therefore, this issue has to be taken into account during the process design/planning stages. In addition, to minimise any material wastage the machining strategies and part positioning for each post-processing operation should be optimised.

Liou et al. [77] reported a hybrid AM approach for the production of complex parts by integrating a DLD device into a multi-axis CNC machine. The results showed that the need for support structures during the AM process could be avoided by selecting the optimal part positioning and inclination angles during the DLD process. A process planning method was proposed in this research for the DLD process that included the following steps: (1) selection of part orientation, (2) part partitioning into subparts for creating them with the same building direction, (3) determining the DLD building sequence and (4) verifying the machinability of built subparts and thus avoiding any collisions during their post-processing. Recently, Chen et al. [160] proposed a new process planning when a hybrid AM approach was utilised. Their research was motivated by the need for proper planning of manufacturing sequences and thus ensuring collision avoidance during both DLD and machining operations. Their research was mainly focused on the application of hybrid AM solutions for producing complex geometries, such as aero-engine blisks and gears, among others. Especially, a model was proposed for simulating the relative in-process movements of workpiece and cutting tools and DLD heads, and thus determining the optimal sequence of tasks in producing complex shapes without collisions.

Sodick [149] machine manufacturer has developed a hybrid AM machine that integrates LPBF and machining operations for the generation of complex end-use parts. Components that can be manufactured with their hybrid OPM series machine are provided in Fig. 20 and Fig. 21. Figure 20 shows samples with internal cavities and complex geometries that were produced by combining LPBF and milling operations, i.e. (a) a sample with a spiral-shaped cooling channel with machined external surfaces, (b) a sample with a high aspect ratio rib; (c) a twisted topologically optimised cage tower with a machined upper surface; and (d) a punch with a narrow high aspect ratio slot.

Figure 21 shows examples of components with internal cooling channels and external complex geometries, i.e. an EV connector, a switch box and a duct shape core, that were not possible to produce with machining operations only, but they were successfully manufactured with the OPM Series machine.

Matsuura [163] has developed a hybrid AM machine that integrates LPBF and machining operations into a single processing setup as shown in Table 4. Figure 22 shows examples of moulds and components that were manufactured using the Matsuura hybrid AM system.

Finally, Mazak [146] has developed a hybrid AM machine that integrates a DLD head into a multi-tasking machine tool with the technical characteristics given in Table 4. Figure 23 shows product examples that were produced employing this hybrid AM solution.

The development of metal AM processes has enabled the repair of worn or damaged metallic parts and thus increase their life span and also minimise the need for re-manufacturing. Such capabilities are of special interest in the application areas for high-value components, e.g. in tooling applications where saving in lead time and cost can be made by repairing the damaged dies and moulds [165]. However, taking into account the tight tolerances that are typically required in tool-making, aerospace and automotive industries and also considering the limitations of stand AM systems, hybrid AM solutions have emerged as a viable alternative for repairing parts in these industrial sectors.

As it was already stated, there is significant interest in using hybrid AM solutions for tool repair. Ren et al. [166] reported an alternative approach for die repair that was combining DLD with CNC machining. The repair of most common die damages with such hybrid system was studied, especially dies that were worn out or damaged. The bonding strength of deposited material in such repaired dies was analysed and it was concluded that it was higher when compared with dies repaired with the conventional welding methods. Figure 24 depicts the stages in the die repair process.

Bennet et al. [167] investigated the capabilities of a DLD head when integrated into a hybrid AM solution when applied for repairing automotive dies. First, the damaged areas of the die were removed through machining and then they were rebuilt with the DLD process. H13 dies were repaired with the hybrid process and also with the conventional TIG process for comparison. It was stated that the repaired die with the hybrid AM process had the same life span as that processed with the conventional TIG method. Recently, Zhang et al. [168] reported a hybrid process that combined reverse engineering, pre-repair processing, AM and material testing. The proposed method was applied for the repair of a H13 die with different types of defects. First, worn out sections were scanned to re-create the 3D model of the die. Then, these sections were machined out and heat treated to prepare them for the AM process. Next, the worn out sections were restored with the DLD process and finally their microstructure and mechanical properties were analysed. The results showed that a strong bonding was achieved between the deposited material and the pre-machined die. In another research, Foster et al. [62] investigated the feasibility of combining the DLD technology with finishing machining into a hybrid AM system for repairing dies by cladding Stellite cobalt-based alloy onto the worn out areas. The repaired die was subjected to hot forging, and it was reported that the die performance was the same or better when compared with that of a standard die. In particular, it was noted that the adhesive and abrasive wear modes of the repaired die performed better than the standard one.

Figure 25 shows common defects that appear in dies and moulds that could be repaired by employing hybrid AM solutions.

Repair of other high value components by using hybrid AM approaches was also reported in the literature. Le et al. [171] proposed a process planning approach for combining AM with subtractive operations. The methodology enables the repair of parts with high accuracy and thus avoiding the material recycling stage. The workflow for conducting a part repair with this hybrid method includes the following three steps: (1) a pre-processing stage, at which an inspection of the worn-out part is conducted; (2) a processing stage, at which AM, subtractive, inspection and heat treatment operations are combined; and (3) a post-processing stage, at which the final inspection of the part is conducted.

Praniewicz et al. [172] investigated a hybrid AM solution for DLD of Stellite alloy on 410 stainless steel substrates. The deposited material on the workpieces was analysed to assess the effect of process parameters on their internal and external porosity, microstructure and surface roughness. In particular, preliminary experiments were conducted first and thus optimising the process parameters. Then, a disc-shaped cylinder was manufactured with the optimised parameters. It was reported that in spite of the fact that some areas were inspected, the DLD process led to some flaws in the part, including surface voids and internal cracking.

Figure 26 shows examples of repaired parts for different industry applications where hybrid AM solutions were applied.

The DLD technology provides capabilities for depositing different materials on components and thus enabling their manufacture from so-called functionally graded materials (FGM) and specially tailored alloys. FGM are characterised by variations of materials or their microstructures across the deposited sections and thus properties and functionality can be varied at different locations within the part volume [173]. The gradual changes of constituents along certain direction provide graded macroscopic and microstructures properties, such as hardness, wear resistance, corrosion resistivity, etc. In fact, some of the hybrid AM system list in Table 4 have the capabilities for multi-material deposition. In particular, Lasertec 65 3D from DMG Mori and Integrex I-400 AM from Yamazaki Mazak allow multi-material depositions and thus building FGM sections and parts. Many research groups have analysed FGM parts produced employing different hybrid AM solutions [174‐182].

Figure 27 shows a schematic diagram of FGM [183] combining Haynes 282 material with progressively increasing amount of SiC. As depicted, the composition and properties of the materials can be changed from one end to another along the Z-axis. However, depending on the applications, there might also be changes from centre to end [174].

Figure 28 shows an example of an FGM application, in which different materials are employed for the substrate (steel) and for the deposition (copper).

Liu et al. [175] deposited a TiC/Ti composite employing the DLD process and graded the composition from 100% TiC to almost 95% Ti across the part. The DLD head consisted of four coaxial nozzle around the laser beam and two powder feeders with controlled deposition rates were used to deliver Ti and TiC powders. Results showed that the DLD system produced successfully FGM samples while prevented the crack formation. Li et al. [176] used again a DLD system to create FGM by gradually varying the composition from Ti6Al4V to SS316. A transition composition route was employed, i.e. as follows Ti6Al4V → V → Cr → Fe → SS316, with the aim of avoiding the development of intermetallic phases between Ti6Al4V and SS316 and thus producing a thin wall sample. A gradual transition in composition was achieved to produce a structure with no cracks or intermetallic phases between Ti6Al4V and SS316 alloys. Chen et al. [177] deployed the DLD process to deposit 316L/Inconel 625 FGM. Initially, the composition was 100% 316 L stainless steel and then gradually it was changed to 100% Inconel 625. FGM parts with a homogenous microstructure were produced while the material composition was varied continuously. A good linear gradient between Fe and Ni was achieved as a result of the good diffusion and the strong bonding between the deposited layers.

The FGM concept has been applied in several application areas, e.g. to produce transport systems, energy transfer systems, cutting tools, machine elements, optics, biomedical components, etc. FGM are of special interest for producing structural parts that require combinations of characteristics that cannot be found in a single material, e.g. hardness with toughness, or chemical inertness with toughness [178]. Table 5 includes some potential FGM applications with their respective industrial sectors.

FGM have a wide range of applications, and hybrid AM solutions are important enablers for producing FGM parts. Especially, the hybrid AM route can enable the manufacture of end-use parts with high surface quality, dimensional accuracy and tailored mechanical properties.

The machinability of metal AM parts is one of the main open issues that prevent the broader take up of hybrid AM solutions. In addition, the intrinsic characteristics of metal AM processes affect directly the machinability of the compacted powder through laser-material interactions. In particular, during material deposition operations, parts are subjected to high thermal gradients, i.e. high heat and cooling rates, that lead to a higher strength and hardness of the compacted powder and consequently to higher cutting forces and tool wear rates [186]. Milton et al. [187] reported that the increase of cutting forces was possibly due to microstructural discontinuities of AM components, which entailed higher wear rates and lower tool life.

The most common materials and alloys employed in metal AM processes can be clustered into stainless steel alloys (316L, 17-4PH, etc.), nickel-based and cobalt-based superalloys (Inconel 625, Inconel 718, CoCrF25, etc.), titanium alloys (Ti6Al4V, CPTi, etc.) and aluminium alloys (AlSi10Mg, etc.). Some of these alloys on their own are considered difficult-to-machine materials due to their outstanding mechanical properties that are maintained even at high temperatures. Table 6 summarises the machinability of the alloys that are commonly employed in metal AM processes.

Machinability of materials is related to tool wear modes and chip formation characteristics during cutting and also affects the surface and microstructural characteristics of the machined samples.

A research on turning DLD Ti6Al4V parts was conducted by Oyelola et al. [192] to study their machining behaviour and resulting microstructural and surface integrity changes. It was noted that microstructural inhomogeneity of deposited material had a negative impact on surface integrity and machinability of the manufactured part. Furthermore, it was stated that the control system used for the machining operations should be able to adapt to varying cutting forces and conditions in order to obtain the best machining performance. This requirement can be explained with microstructural inhomogeneity of AM parts and would be especially valid for FGM. In another study, Hojati et al. [193] investigated the machinability of AM Ti-based alloys and it was compared with that of a standard material in terms of cutting forces, specific cutting energy, surface quality and chip formation mechanism. The results showed that there were no significant differences in regards to the cutting forces. However, the machining of AM samples led to the formation of continuous wavy-type chips that were larger than those of the standard samples. Aldwell et al. [194] compared the machining of bulk and AM Al6061 samples, i.e. the cutting forces, the chip formation mechanism and the resulting surface morphology. It was concluded that a good machinability of AM samples could be achieved when there was a strong bonding between the layers and also when the part porosity was low. Therefore, the optimisation of process parameters for producing AM parts with higher homogeneity and minimum porosity is critical in order to improve their machinability.

Guo et al. [16] analysed the effect of the building direction during the DLD process on mechanical properties, microstructure and machinability of AISI 316L samples. Two building directions were investigated, and then the produced samples were machined by dry milling to meet the requirements for surface integrity and dimensional accuracy. It was found that the homogeneity and microstructure of the produced AM parts were dependent on the build direction and as a consequence the cutting forces, the tool wear and the resulting surface roughness deferred, too. It was concluded that some anisotropic characteristics of AM parts could be beneficial for the follow-up machining operations. Calleja et al. [195] studied the machinability of Inconel 718 samples produced with a hybrid AM process that combined DLD with machining. Cutting forces and specific cutting energy during turning and milling operations together with the resulting roughness and microhardness of the heat-treated DLD samples were compared with the results obtained for standard Inconel 718 samples. It was found that the cutting forces and microhardness were higher for the AM samples while the best surface roughness was obtained for the standard samples.

In summary, it can be stated that the machinability of AM parts is not sufficiently investigated and also taking into account the constantly growing number of AM materials, further research is required. In addition, the laser-material interactions when processing metallic powders should be studied further to understand better the underlying conditions that lead to the formation of hardened phases and non-uniform microstructures and thus are detrimental to the machinability of AM components.

The control of dimensional and geometrical accuracy of AM parts in time is another challenging issue in broadening the use of hybrid AM solutions. Residual stresses generated during the AM process can lead to part deformations and distortions. Therefore, post-processing operations have to be properly planned considering the actual geometry and dimensions of the AM parts and thus ensuring the required accuracy with respect to their design. As a result, part inspections are very important in planning post-processing operations and thus determining the actual deviations of AM part geometries from their intended designs.

The uncertainty of additively manufactured part dimensions is one of the greatest concerns when combining both additive and machining processes. During the machining processes, tool may find excess of material in non-expected places, which can cause tool damage or inadequate cutting [196]. Therefore, a great effort has been made to understand and control part deformations during AM. Paul et al. [197] proposed a model for calculating the deformation of workpiece during AM processes that are due to thermal stresses. It was found that the overlap between successive scanning paths, also referred to as hatch spacing, leads to continuous melting and re-solidification cycles that, in turn, determines the accumulated thermal stresses. Consequently, the developed stresses entail part distortions and dimensional inaccuracies. In addition to the hatch spacing, Das et al. [198] also studied the effects of layer thickness, support structures and build orientations on geometrical and dimensional accuracies of LPBF parts. It was found that the building direction was a crucial parameter as it affected the properties and dimensional accuracy of parts and determined the amount of support structures needed. An approach was proposed to define the optimal part orientation and thus minimising the support structures. However, the approach still required empirical data for validation purposes. In addition, the model did not consider some important factors in determining the optimal part orientations such as the resulting thermal stresses. Eisenbarth et al. [199] investigated the distortion of parts manufactured using two different hybrid AM approaches: (1) continuous DLD build-up of material with subsequent milling and (2) interrupted DLD build-up alternated with milling. It was shown that each consequent DLD step led to a higher bending of the developed part, whereas the milling steps reduce it to a certain degree. Also, it was noted that, an inspection operation was required between the deposition and machining stems due to the part distortions.

However, the inspection of certain regions, such as internal cavities or complex overhanging features is also a challenging task. A summary of different methods used for inspecting AM parts with their open issues and challenges is provided in Table 7 [200].

Laser-based inspection methods for AM parts were investigated, too. Montinaro et al. [201] used a flying laser scanning thermography for inspection of AM parts with high signal to noise ratio of the measurements. However, it was noted that due to the higher surface roughness of metal AM parts laser-based inspection methods might not be a suitable solution.

In addition, the integration of inspection operations into hybrid AM solutions should also be considered. Their maintenance and protection from fumes and cutting fluids together with the integration of inspection software with CAD/CAM software also present challenges and open issues in developing hybrid AM systems and thus increasing their efficiency and throughput.

Hybrid AM solutions have open issues and challenges associated with each integrated technology and also due to the execution of all operations in the same manufacturing system. The main process-related issues in hybrid AM can be clustered into (1) process parameter optimisation, (2) cleanness, (3) material oxidation and (4) issues related to the AM processes.

The process optimisation is one of the shortcomings of metal AM technologies and also in hybrid AM systems. There is no sufficient knowledge how post-processing parameters might affect AM parts [81]. Considering the case when machining operations are used for post-processing AM parts, in spite of the fact that it is a mature technology, there is not sufficient research on their effects on the resulting surface integrity. Additionally, a key open issue in broadening the use of hybrid AM solutions is the achievable repeatability and thus whether they can meet the requirements for serial production. Especially, it is necessary to determine geometric and dimensional tolerances that can be achieved consistently together with respective mechanical and microstructure properties of the hybrid parts. A systematic analysis of various factors affecting the processing conditions of integrated processes is required. This will allow processing parameters and equipment accuracy and repeatability to be controlled sufficiently well to meet standardised requirements that are also yet to be developed.

Coolant management and cleanness, especially for the laser-based AM technologies, are other critical issues when integrating AM processes with conventional machining in hybrid AM systems [81, 196]. The issues associated with cleanness were studied by Boivie et al. [123] and it was found that all chips and cutting fluids should be carefully removed before starting any AM process. Otherwise, it could deteriorate the machinability of difficult-to-cut materials such as Ti-based alloys or Ni-based alloys. As for the cutting fluid, it can be mixed with the powder particles and affect their absorptivity and, consequently, the bonding with the substrate material [196]. Additionally, machine guiding systems must be protected and properly sealed in order to avoid powder particles to abrade the moving components.

Another important issue in hybrid AM systems is the lack of a closed chamber to avoid powder oxidation during the AM processes. As was already mentioned, several hybrid AM systems utilised inert gases to shield processing areas and thus minimising the oxidation during laser-material interactions. However, depending on process parameters and reactivity of employed materials, this may not be sufficient to avoid any oxidation. In turn, the oxidation can induce defects in the manufactured parts. Hebert et al. [202] found that when the AM process utilised reactive materials such as titanium-based alloys, oxide layers might be formed in spite of shielding conditions except when using extremely high vacuum. In addition, the formation of hydroxides and hydrated oxides should also be addressed. While oxides tend to strengthen the materials, hydroxides can act as lubricants and deteriorate the strength of AM parts.

As it was already discussed, LPBF and DLD are the most commonly employed laser-based AM technologies and, in turn, they are the most commonly employed in hybrid AM systems, too. LPBF enables the creation of complex parts with high accuracy, but it is a relatively slow process. In addition, support structures have to be removed in follow-up post-processing operations. Depending on the geometry and the location of those support structures, their removal can represent a challenge if they are difficult to reach with the cutting tools. Additionally, LPBF can be integrated only into 3-axis vertical machine tool configurations. Therefore, the LPBF capabilities for producing complex part geometries may be reduced due to the constraints imposed by post-processing operations. Furthermore, powder management is another important issue to consider in integrating the LPBF process in hybrid AM systems.

At the same time, the DLD technology is faster than the LPBF one and therefore any hybrid AM system that integrates a DLD head will have a higher throughput. Also, the DLD allows complex parts to be built without the need for support structures because of the higher positioning flexibility of the hybrid machine tool configurations. Nevertheless, the DLD head inclination and position have to be carefully controlled to maintain the optimum deposition rates and parts accuracies. However, there are some shortcomings, too, as the overall quality of DLD manufactured parts, i.e. their dimensional and geometrical accuracy and surface integrity, is worse than that of LPBF parts. Additionally, special care should be taken to avoid any collisions between the workpieces, the DLD head and the cutting tools during both deposition and post-processing operations.

Machining of AM parts directly can be challenging because of their lightweight and complex shapes. These both characteristics can lead to problems with work holding and vibration, and thus can result in poor process yields. Work holding systems have to be designed and implemented in such a way that they should ensure reliable workpiece fixation and the required accuracy and repeatability during machining operations. Thus, these systems should be designed to withstand the cutting forces during machining operations without any displacements while providing sufficient access to all surfaces that require some post-processing. However, in the case of hybrid AM solutions, the geometries of the AM structures are complex and can incorporate internal cavities with different stiffness and properties. Therefore, the work holding systems have to be sufficiently flexible while providing adequate support for processing parts with varying geometries and rigidity. This is not an easy design and implementation task.

One of the solutions that have been proposed for holding the workpiece during the machining operations is depicted in Fig. 29. First, the workpiece has to be embedded into an auxiliary plastic support structure that is afterwards removed [203]. The configuration of the auxiliary support system should ensure both the required workpiece fixation during the machining operations and also access to the surfaces that should undergo some post-processing, e.g. the flat surfaces at both ends of the rectangular pipe in Fig. 29.

Manogharan et al. [10] proposed another solution for clamping and positioning of AM workpiece that uses some sacrificial supports. Especially, the sacrificial support has to be built together with the workpiece during the AM process and thus ensure the required positioning and fixation during the machining operations. Figure 30 shows the workflow suggested in designing and implementing such work holding systems. As it can be seen, the CAD model of the part have to be modified to add required sacrificial features while at the end a further post-processing step is required to remove them.

One of the main advantages of using single setup hybrid additive manufacturing systems is that they enable to reduce the time to define part zeros. This is an advantage not only in terms of productivity of the process, but also because it minimises the positioning-error that occurs when using multi-setup solutions [196]. In those cases that employ intermediate or special fixturing systems as the ones presented above, the design of rigid and appropriate fixturing systems will be of great importance to reduce those positioning problems.