## 1 Introduction  
Additive manufacturing (AM), also known as three dimensional (3D) printing, has been noticeably growing over the last 30 years [1]. All metal AM technologies are based on the principle of slicing a solid model in multiple layers to create a tool path, uploading these data in the machine, and building the part up, layer by layer, following the sliced model data, using a heat source (laser, electron beam or electric arc) and feed stock (metal powder or wire) [2]. On the other hand, for large volume the manufacturing speed in AM has not enabled this technology to develop as an economical technique. Thus, the integration of AM processes with the production chain of conventional manufacturing is part of recent research, called hybrid processing [3; 4; 5]. The most common integration is AM by a deposition process together with a subtractive process like milling. In fact, both deposition and milling processes need a 5-axis machine for full 3D processing [6]. The DMG MORI SEIKI AG is one of the first producers of a hybrid machine which combines the directed energy deposition (DED) and milling process together in a hybrid machine. A ready to use turbine housing with connection ports for the fuel feed was presented as their demonstration part which was built by the hybrid process within 5 h [3]. DED, which is a manufacturing technique to build metallic and functional components, is a layer by layer AM process. In principle, in contrast to the powder bed fusion (PBF) processes in which a powder bed is selectively melted, DED is carried out by simultaneously feeding of powder or wire and a concentrated energy source [7]. In this method a melt pool is formed on the surface of the substrate or last deposited layer by using a relatively high powered laser, such as Nd:YAG or CO\({}_{2}\), or other energy sources and at the same time powder is blown through the laser beam and into the melt pool. In particular, each layer is generated track by track by using a user-defined tool path. Thereafter, the process is repeated layer by layer according to a sliced 3D computer aided drawing (CAD) file, building the component. In this process, the delivery of powder can be carried out by using either a single nozzle or multi-nozzle configuration of the deposition head. Different technologies are generally labeled as DED processes [8], such as Laser Engineering Net Shaping (LENS), which is a common technology of deposition with multi-nozzle, and others are outlined in Table 1.  
Among the AM processes, DED has a unique position because of its potential to fabricate functionally graded and pure metal components as well as its potential for repairing and cladding of valuable parts that cannot be repaired by other traditional methods. This great potential of fabrication of net shape components and repairing of valuable parts have broadened the applications of DED in the aerospace, medical, and defense industries. Some common materials which have been processed by this technique are listed in Table 2.  
Apart from the aforementioned advantages of the DED process, in-situ alloying, like the other AM processes, can be obtained by feeding the different powders at the same time into the melt pool [37; 38; 39; 40]. In particular, by adjusting the nozzle feed rate, it is feasible to achieve desirable microstructural features and chemical compositions by alloying in the melt pool of the starting powders [41]. Furthermore, by changing the process parameters or raw materials during the building process it is also possible to produce functionally graded materials. Moreover, the DED process presents a higher deposition rate and relatively wider process window with respect to the other AM techniques [42]. Apart from the manufacturing of metallic components, DED is a great tool for repairing high value parts, mainly because of the rather small heat affected zone on the component, very high density, metallurgical bonding, minimal effects such as distortion or cracks in the part, and accurate