On the issue of selecting and optimizing parameters of continuous laser welding of cast iron

OBRABOTKAMETALLOV Vol. 23 No. 3 2021 technology In [17], it is shown that the melting zone cross-sectional area (the weld pool geometry) while welding Ni-resist cast iron is proportional to the laser exposure time: as the beam speed decreases, the melt mass loss as a result of its evaporation increases. In work [18], by the method of planning a factorial experiment, mathematical models are constructed that relate the weld pool depth with the laser power, welding speed and a focal length. A significant difference in the coefficient estimations for welding steels of two different grades is shown. This indicates the absence of any universal statistical models for evaluating the weld pool geometry; these models are applicable only to the dataset on which it is built. The cast iron structure with the same chemical composition can have an unpredictable and chaotic distribution of graphite inclusions of various shapes and sizes. The behaviour of the graphite phase while melting, as it is noted above, depends on the welding mode, which ultimately determines the weld seam main characteristics and quality. Thus, to increase the process energy efficiency and provide the weld pool required geometry for each variant of the technical process, it is necessary to select the optimal welding modes. This problem is effectively solved using mathematical models. In this regard, it becomes necessary to study and quantify the weld pool geometry while welding cast iron with different structure, depending on the technological parameters of the process. The aim of the work is to build a mathematical model of welding cast iron with lamellar graphite and determine the optimal power and welding speed depending on the weld pool geometry. Work tasks: 1. Carrying out an experiment and regression analysis of the weld pool geometry dependence on welding modes. 2. Studying the cast iron structure in the melting zone. 3. Gray relational estimation of the weld pool geometric parameters. 4. Calculating the optimal radiation power and welding speed depending on the weld pool geometry. Research methodology For the experiment, cylindrical specimens with a 30-mm diameter and a 10-mm height were made from grey unalloyed cast iron with a pearlite metal base. Cast irons with such a structure have a good hardenability, which simplifies assessing the effect of welding parameters on the heat affected zone extent. Laser treatment to simulate the welding process was carried out on an LS-1 ytterbium fibre laser in accordance with the plan (Table 2). The focal length in the experiment did not vary and was 120 mm. At a distance excluding thermal influence, 4 tracks were melted on each specimen. The samples were studied by traditional metallographic methods. The study was carried out on a computerized complex created on the basis of a Leica DM IRM metallographic microscope. The quantitative assessment of the geometric parameters of the weld seams was carried out in the programme ImageJ used for a quantitative analysis and image processing. Results and discussion The laser action zone consists of 2 layers: a melting zone and a zone quenched from a solid phase (Fig. 1). The tempering zone on the obtained samples was not identified. Graphite inclusions partially or completely (depending on the processing mode) dissolved in the liquid phase, crystallization proceeded according to the iron-carbon metastable diagram without releasing free Ta b l e 2 Design of experiment Sample Laser power, kW Welding speed, mm/s 1 0,7 50 2 1,3 20 3 0,575 35 4 1,0 56 5 1,0 14