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The processes of melt crystallization and conductive heat transfer during selective laser melting of a metal alloy are considered in a conjugate formulation. A mathematical model is proposed for describing the inhomogeneous temperature field for calculating the fraction of the crystalline phase during laser treatment of the surface of a metal alloy. A numerical study of laser amorphization of bulk metallic glass (BMG) by melting the surface of samples of finite dimensions has been carried out. A comparison with the known experimental data is carried out, and a qualitative agreement is obtained for the calculations on the depth of the melt pool, the homogeneity of the amorphous layer, and its location for a multicomponent alloy based on zirconium: Zr 57 Cu 15 , 4 Ni 12 , 6 Al 10 Nb 5 . Potentially suitable laser scanning regimes are identified, in which the fraction of the crystalline phase in the treated layer does not increase compared to the initial one in the initial amorphous-crystalline alloy. It was found that when using the calculated modes of laser scanning, it is possible to reduce the fraction of the crystalline phase in the treated surface layer of the BMG alloy by orders of magnitude. [ABSTRACT FROM AUTHOR]

Metals and alloys in amorphous state are promising for the use as structural and functional materials due to their superior properties related to the absence of such defects as grain boundaries and dislocations. Obtaining the amorphous state requires quenching from liquid state with a high cooling rate. In conventional technologies, this is the reason for a considerable size limitation hindering application of amorphous alloys. Additive manufacturing (AM) is free of the size limitation. The so-called bulk metallic glass (BMG) alloys have extremely low critical cooling rates and can be used in AM. Recent studies on AM from Zr-based BMGs by selective laser melting (SLM) has proved the possibility of attaining the amorphous state and revealed partial crystallization to be the principal drawback of this process. The present work aims to analyze the conditions for partial crystallization and attempts to control it by optimizing the process parameters. A comprehensive parametric analysis is accomplished in single-track SLM experiments with Zr-based BMG alloy Vit 106. The observed microstructures are related to the temperature fields and thermal cycles estimated by an analytic heat-transfer model in the laser-impact zone. In the cross section of a laser track, a central bright domain is identified as the remelted zone. An annular darker crystallization zone encircles the remelted zone. The model fits the experimentally obtained dependencies of the remelted zone size versus laser power and scanning speed and indicates that 43 ± 2% of the incident laser energy is transferred into the substrate thermal energy. The principal energy losses are reflection of the incident laser radiation and material evaporation. Primary crystalline inclusions existing in the substrate before laser processing dissolve at the laser melting. The mean cooling rate in the remelted zone is up to 4 orders of magnitude greater than the critical cooling rate. Therefore, homogeneous nucleation is not expected. Nevertheless, the theoretically estimated crystallization times are sufficient for a considerable crystal growth in the heat-affected zone where primary crystalline inclusions and nuclei are not completely dissolved. [ABSTRACT FROM AUTHOR]