OBRABOTKAMETALLOV technology Vol. 25 No. 4 2023 printing of specimens using metal surfacing. This technology significantly reduces the production time of the workpiece and reduces the cost of its production. This is due to the fact that instead of powder, surfacing wire is used during printing, the cost of which is significantly lower. However, a significant disadvantage of printed specimens is the low quality of the surface and such specimens require additional machining. Due to the peculiarity of this technology associated with the cooling of printed parts, its hardness is higher than when using forging or casting. This is especially evident when manufacturing parts made of martensitic stainless steels. These steels are quite inexpensive and are widely used, including for surfacing. When forming a part using a layer-by-layer surfacing, a new layer is applied to the previous one. The previous one is reheated and quickly cooled again. Since the critical cooling rate of martensitic steels is not high, a martensitic structure of high hardness is formed. The research conducted by the authors in [2, 3] confirms this fact. The layer-by-layer laser surfacing of the powder imparts different mechanical properties throughout the cross-section of the printed workpieces. The grain size, porosity and, accordingly, mechanical properties depend on the direction in which it is measured. In [4], the authors also showed that the properties of the products printed using additive technologies are different in different directions. The authors also showed that this could be partially corrected by heat treatment. Nevertheless, such correction requires additional costs for an additional operation. Similar results are shown in [5] demonstrating that thermal cycling of individual areas during printing may initiate internal stresses occurrence within the printed workpiece. The work [5] states that a hard-to-process crust (750 HV) was formed on the surface of 0.4 % C-13 % Cr steel specimens during SLM. When printing with wire (WAAM), the bottom layer recrystallizes while the next layer is applied to it. In this case, a structure, consisting of elongated grains of ferrite and fine-grained acicular martensite, is formed in the matrix upper layer. This structure is formed instead of the spatial periodicity of martensite laths within equiaxed ferrite grains in the inner layers. The martensite content gradually increases proportionally to the distance from the base metal [6]. The operating equipment conditions largely determine the process of forming a workpiece using the wire surfacing. These are parameters such as the temperature of the substrate, the trajectory of movement [7], etc. However, even under optimal conditions, various defects in the structure of the material may still appear (surface hardening, inhomogeneity, etc.). The WAAM technology is one of the additive technologies. When printing workpieces using the WAAM technology, the workpieces with heterogeneous structure and mechanical properties are also obtained. Another disadvantage of the WAAM technology is a poor surface quality. After the workpiece is manufactured, subsequent machining is required to obtain the desired geometric tolerances and surface properties [8]. When machining such workpieces, it is necessary to consider these features. Processing of workpieces obtained from stainless steels using the WAAM method on a milling machine is possible with fairly high productivity [9]. However, a significant tool wear is observed when milling the WAAM part. This occurs despite the fact that the tool and milling parameters were selected based on the manufacturer’s recommendations intended for processing a given material. The heterogeneous microstructure formed in the specimen obtained by the WAAM technology leads to a significant deterioration in its machinability. This is due to complex thermal cycles occurring during printing. The authors of [10] demonstrated the difficulties that arise when milling the Ti6Al4V alloy with an Al2O3/Si3N4 (sialon) end mill. The authors noted a more significant tool wear when machining the WAAM specimens as compared to forged and cast ones. The wear can be reduced and the tool life may increase by changing the cutting speed and feed [11, 12]. The cryogenic cooling of the cutting tool can also be used. Another way to reduce the tool wear is to select printing modes that enable the desired surface properties. To overcome the hardness heterogeneity of the printed workpiece, a methodology intended for segregating microhardness data into individual assemblies was developed and tested [13, 14]. Combining various additive printing technologies allows obtaining a more uniform structure. But in general, researchers [15] note that when processing printed workpieces, cutting forces increase in comparison with those of the workpieces produced by traditional methods. The authors of [16] stated that the workpieces produced by additive technologies possessed completely different cutting forces under the same processing conditions.
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