Technical University of Gabrovo

The objective of the grinding process is to obtain a better surface finish (roughness, accuracy, etc.) or a high material removal rate of the workpiece. These grinding process response variables depend largely on both the grinding conditions and the topography of the grinding wheels formed during dressing. In this regard, the paper focuses on determining the optimal dressing conditions for grinding wheels dressed with diamond roller dressers during rough and finish grinding. A new multi-objective optimization approach for determining dressing conditions has been proposed, which leads to Pareto-optimal solutions for increasing the grinding process production rate, reducing roughness, and increasing the accuracy of ground surfaces. To demonstrate the performance of this approach, one process of dressing electrocorundum grinding wheels with novel diamond rollers made of medium- and high-strength synthetic diamonds with mixed grain sizes has been selected. Empirical models have been developed to examine the production rate of the grinding process, the roughness of the ground surfaces, and the accuracy of the machined parts. Multi-objective optimization based on a genetic algorithm has been performed by applying two methods: determining an optimal compromise area for the dressing process conditions, and the weighted utility function method. The novelty of the implementation is due to the process of identifying the precise optimal dressing parameters specifically for the investigated mixed-grit diamond rollers. The optimization results show that rough grinding requires uni-directional dressing with a diamond roller AC32 at a feed rate of 1.4 mm/min, a dressing speed ratio of 0.8, a dress-out time of 1.0 s, and a grit size ratio of 2.56, ensuring a production rate of 875 mm3/min, a surface roughness Ra up to 1.25 & micro;m, and a cylindricity deviation up to 12.5 & micro;m. For fine grinding, optimal counter-directional dressing parameters include a feed rate of 0.26 mm/min, a speed ratio of 0.23, a dress-out time of 5.0 s, and a grit size ratio of 2.2, which guarantee a minimum surface roughness Ra of 0.38 & micro;m, a cylindricity deviation of 8.1 & micro;m, and a production rate of at least 600 mm3/min.

This paper presents a structured approach to grammar and style checking of scientific texts using ChatGPT. The approach is based on staged interaction between the author and the language model and supports iterative improvement of grammatical correctness, stylistic consistency, clarity, and conformity to academic writing standards. It includes a sequence of steps related to task initiation, specification of revision requirements, generation of an edited version, author evaluation, feedback-based refinement, and final validation of the text. An illustrative example is provided to demonstrate the practical applicability of the approach. In addition, the paper discusses its main advantages, limitations, and ethical aspects of using artificial intelligence in academic writing. The results indicate that ChatGPT can be used as an effective assistant for grammar and style checking of scientific texts when its output is subject to explicit author control.

This article presents a new combined post-processing concept to improve the quality of laser powder bed fusion (LPBF) of 17-4PH stainless steel (SS) cylindrical parts fabricated from N2-atomised LaserForm 17-4PH (B) powder. The concept is based on consecutive heat treatment procedures and diamond burnishing (DB) processes. A two-stage study was conducted. The first stage was an LPBF process experiment. The following combination of LPBF parameter values was selected after optimisation: a laser power of P=150 W, laser scanning speed of v = 1200 mm/s, and layer thickness of t=40 μm. In the second stage, this combination was used to evaluate the effects of two heat treatment procedures (HT1 and HT2) and two DB processes (using burnishing forces of 100 N and 300 N) on the tensile properties and surface integrity of LPBF 17-4PH SS cylindrical samples. The HT2 procedure, including annealing (1200∘, 4 h), solution treatment (1060∘, 1 h), cooling (-70 ∘C,2 h), and ageing (482∘, 4 h) led to yield limit, tensile strength, and Vickers hardness values of YL=1071 MPa, TS=1410 MPa, and 523 HV, respectively. The concept presented takes advantage of the combination of the transformation, precipitation and strain-hardening effects. The combined effect was most pronounced in the samples subjected to the HT2 procedure and subsequent DB (300 N), for which a retained austenite fraction of 6.93%, surface microhardness of 563 HV0.05 and the maximum values of the compressive axial and hoop RSs of -1426.3 MPa and -1095.9 MPa, respectively, were measured.

The aim of present paper is to investigate biphasic calcium phosphates prepared via heat treatment of precipitated powders. The precursors are prepared by wet precipitation using phosphoric acid and calcium hydroxide. The as-synthesized powders are subjected to heat treatment varying with temperature and time. The degree of crystallinity as well as qualitative and quantitative analyses of phase composition are done by XRD and FTIR. It is found that the as-synthesized powder has monophasic hydroxyapatite (HA) structure. Phase transformation runs at 700 oC calcination, resulting in formation of biphasic structure, which consists of HA and low amount of β-tricalcium phosphate (TCP): 18.3 % in 1 h and 34.3% in 4 h process. Heat treatment in the temperature range 900-1300 oC leads to formation of biphasic HA/β-TCP structure with HA as predominant phase. Clear correlation between the heat treatment parameters and the HA/β-TCP ratio is not observed. The highest β-TCP amount (39.2 %) is found at 1000 oC while at high temperatures (1100-1300 oC) it is about 30.6-32.6 %. The HA amount is 60.8 % and in the range 69.4-67.4 % respectively. The heat treatment duration does not influence strongly on the phase ratio. Increasing the duration from 1 h to 4 h leads to slight change of the HA/β-TCP ratio from 57.9/42.1 % to 49.1/50.9 % respectively. The new data on the HA/β-TCP ratio, obtained in the present study, will be used for optimization of the processes parameters in design of BCPs with definite composition, optimized resorbability and mechanical properties.