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

OBRABOTKAMETALLOV Vol. 23 No. 3 2021 technology and crystallization conditions improve, which increases the weld properties. In addition, productivity significantly rises [1]. Laser technologies for processing cast iron can be used for producing welded cast products, repair, fix defects, surface microalloying and improving its performance [2]. Cast irons have a porous structure. Graphite inclusions that violate its metal base continuity complicate the welding process, lead to porosity and defects in the weld [3, 4]. While melting and subsequent crys - tallization in the melt-pool, the white cast iron structure is formed [5–7]. In this regard, cast irons endure thermal welding cycles extremely poorly: structure heterogeneity and local stresses lead to brittleness [8]. Depending on the laser operating parameters, it is customary to distinguish between two characteristic modes of laser welding: welding of small thicknesses (up to 1 mm) and welding with a deep penetration (more than 1 mm). The principal difference of the first group is the modes that provide only melting the material without its intense evaporation. The processes in the melt-pool are unsteady and unstable. Deep penetration welding is accompanied by forming the melt turbulent flows and the steam-gas channel [9]. The geometric parameters of the melt-pool (depth, width and its ratio) are some of the weld quality in- dicators and differ for the two described welding modes. The main technological parameters that ensure the melt-pool geometry are radiation power, welding speed and focusing system parameters [1]. At low speeds the steam-gas channel has a cylindrical shape, the aspect ratio of the melt-pool can be quite large, while at high speeds the aspect ratio is low [10]. To select the optimal parameters of the welding process, optimize the cast iron structure, and increase energy efficiency, mathematical models of the process are needed. The models based on the energy equations and momentum balance make it possible to quantitatively estimate the dependence of the melting zone penetration and other geometric characteristics of the weld seams on the main technological parameters of welding [11–13]. However, the optimal welding speed doesn’t only depend on the radiation parameters, but also on the structure of the materials to be welded, for example, on the graphite phase behaviour when welding cast iron. At extremely low or too high speeds, the process becomes unstable. Thus, in [14] it is shown that at a high radiation power and a relatively short time of laser action, a significant part of graphite in the melting zone structure is retained. The time increase in a high-power laser exposure leads to explosive evaporation and graphite sublimation, which is accompanied by the crater formation on the weld surface. These phenomena significantly complicate the mathematical description of the process. In work [15] it is shown that to build an effective model for forecasting the welded seam quality, it is necessary to consider many parameters. As an example, a simplified predictive model based on an artificial neural network is presented, which demonstrates possible and promising indicators for forecasting the welding quality of a low-carbon galvanized steel sheet. The work [16] provides qualitative data on influencing two process parameters on the geometric parameters of the bath (Table 1). In addition, there are recommendations for choosing the optimal parameters for some types of steel and non-ferrous metal alloys. Ta b l e 1 Influence of two laser welding parameters on various weld pool geometry parameters Geometry parameter Process parameter Laser power Welding speed Weld pool depth + − Weld pool width + − Weld pool depth/width ratio it is not stated in the work + Weld pool length + − Keyhole radius + − Cooling rate − + Weld pool surface area + − Vaporization rate + it is not stated in the work