Zeli Han

Due to the unique microstructure and mechanical heterogeneity of austenitic stainless steel made via wire arc additive manufacturing (WAAM), its machinability differs significantly from that of rolled material. Accordingly, this study systematically investigates the influence of milling strategies on key process responses (cutting forces, surface roughness, vibration displacement, and temperature) to reveal the mechanisms of machining parameters during the milling of WAAM-fabricated austenitic stainless steel. The material used in this study is ER321 austenitic stainless steel. During deposition, the fusion zone cools more slowly than the transition zone; consequently, the fusion zone exhibits a hardness approximately 20 HV_0.1 lower than that of the transition zone. Surface roughness is primarily reduced by decreasing the primary feed per tooth. However, when the primary feed per tooth is small, ploughing is induced, which not only increases surface roughness by 25% but also causes abnormal increases in temperature and vibration displacement. Nevertheless, ploughing has little effect on the total milling force, and the feed per tooth shows a positive correlation with the total milling force. Tool run-out and an increase in the uncut chip thickness lead to a positive correlation between the radial depth of cut and the key process responses. Moreover, ploughing also occurs when the radial depth of cut is small. The axial depth of cut has almost no effect on the machining process. Moreover, a small-diameter mill leads to severe ploughing, and at a high table feed, climb milling leads to cutter offset.

Introduction. Wire arc additive manufacturing (WAAM), due to its “design as manufacturing” characteristic, is gradually becoming one of the most promising technologies. However, at present, there are no comprehensive comparative studies on the microstructure and mechanical properties of deposited samples made from austenitic stainless steel at different locations of the sample. In addition, their machinability remains insufficiently investigated. The purpose of this study is to compare the microstructure and mechanical properties of samples made of austenitic stainless steel ER321 (analogues – AISI 321, 0.08% C-18% Cr-10% Ni-Ti) obtained by the WAAM method at different locations within the sample and to assess their machinability by the magnitude of the components of the cutting force during end milling and the roughness of the machined surface. The properties and microstructure of samples obtained by wire-arc additive technology are investigated, and milling forces are investigated. The effect of the feed on the components of the cutting force and the roughness of the machined surfaces during conventional milling of ER321 steel workpieces using 12 mm diameter cemented carbide end mills with a wear-resistant AlTiN coating applied by physical vapor deposition (PVD) is determined. Research methods. The content of elements and the solidification pattern in various parts of the workpieces were determined using X-ray microanalysis. The microstructure of the samples was studied by a metallographic method. Stress-strain diagrams were obtained by tensile tests, and the microhardness of the samples was also measured. In comparison with the pattern of conventional milling of rolled workpieces, a pattern of changes in cutting forces and surface roughness was established depending on the feed rate during milling of deposited workpieces. Results and discussion. During deposition, ferrite with a vermicular morphology is primarily formed in the lower region of the sample, whereas austenite with a dendritic ferrite structure is observed in other regions. The microhardness values of the deposited and rolled samples are close, averaging around 230 HV0.1. The ultimate tensile strength of the rolled samples is 666 MPa, which is approximately 40 MPa higher than that of the deposited samples. During milling of the deposited workpieces, the lateral cutting force acting perpendicular to the feed direction is greater, and the surface quality is poorer. During milling of deposited workpieces, the lateral cutting force acting perpendicular to the feed direction is greater, and the surface quality is poorer. During milling of deposited workpieces, the feed force acting in the feed direction is greater under high feed rates.

The study is motivated by the application of dry finish milling for post-build processing of additive Ti6Al4V blanks, since the use of neither lubricant nor coolants has been attracting increasing attention due to its environmental benefits, non-toxicity, and the elimination of the need for additional cleaning processes. For end mills, wear patterns were investigated upon finish milling of the SLM Ti6Al4V samples under various machining conditions (by varying the values of radial depth of cut and feed values at a constant level of axial depth of cut and cutting speed). When using all the applied milling modes, the identical tool wear mechanism was revealed. Built-up edges mainly developed on the leading surfaces, increasing the surface roughness on the SLM Ti6Al4V samples but protecting the cutting edges. However, abrasive wear was mainly characteristic of the flank surfaces that accelerated peeling of the protective coatings and increased wear of the end mills. The following milling parameters have been established as being close to rational ones: Vc = 60 m/min, Vf = 400 mm/min, a_p = 4 mm, and a_e = 0.4 mm. They affected the surface roughness of the SLM Ti6Al4V samples in the following way: max cutting thickness—8 μm; built-up edge at rake surface—50 ± 3 μm; max wear of flank surface—15 ± 1 μm; maximum adherence of workpiece. Mode III provided the maximum MRR value and negligible wear of the end mill, but its main disadvantage was the high average surface roughness on the SLM Ti6Al4V sample. Mode II was characterized by both the lowest average surface roughness and the lowest wear of the end mill, as well as an insufficient MRR value. Since these two modes differed only in their feed rates, their values should be optimized in the range from 200 to 400 mm/min.

ВЛИЯНИЕ ИМПУЛЬСНОЙ ИОННО-ПУЧКОВОЙ ОБРАБОТКИ НА СТРУКТУРУ И СВОЙСТВА СПЛАВА, СФОРМИРОВАВШЕГОСЯ ПРИ ЭЛЕКТРОННО-ЛУЧЕВОЙ ПЕЧАТИ ПРОВОЛОКОЙ ВТ6св

In this work, based on the multilevel approach, the features of the structure and properties of titanium alloy, formed during high-performance additive manufacturing by wire-feed electron beam technology, were studied. Methods of non-destructive X-ray control and tomography, along with optical and scanning electron microscopy, were used to study the structure at different scale levels of the sample material. The mechanical properties of the material under stress were revealed via the simultaneous observation of the peculiarities of deformation development, using a Vic 3D laser scanning unit. Using microstructural and macrostructural data, as well as fractography, the interrelations of structure and material properties caused by the technological features of the printing process and the composition of used welding wire were revealed.