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Based on the characteristics of metallurgical non-equilibrium solidification behavior during laser additive manufacturing alloy steel, the basic theory of thermodynamics of martensitic transformation precipitating in the process of selective laser melting deposition of alloy steel were introduced, then binary, ternary and multicomponent alloy steel systems thermodynamic evolution process were revealed. Combined with selective laser melting deposited 24CrNiMo alloy steel experiment, laser additive manufacturing 24CrNiMo alloy steel CCT curve and the thermodynamic data-martensite transformation starting temperature were simulated. Simulated by using simulation software and calculated with empirical formula the temperature of martensitic transformation starting point are 366.6°C and 361.102°C respectively. It is expected that these studies can provide theoretical reference for laser additive manufacturing brake disc of High-speed rail. 1.Introduction As one of the revolutionary frontier technologies in the manufacturing field laser additive manufacturing has attracted a great deal of attention and has been developing rapidly worldwide. High speed rail is the strategic leading industry of people's livelihood and its brake disc is key parts of the core equipment guaranteeing the safe and stability of the train operation. Being the traditional method of producing brake discs, casting has some problems such as complex process, long production cycle, short product life. Instead, laser additive manufacturing have distinctive features of one forming for complex structure and short process flow. However, it is a non-equilibrium solidification process with complex organization phase transition behavior. Meanwhile due to cumulative effect of heat from multilayer cladding studying thermodynamics has an important effect on the structure and performance of the final forming alloy. So far, there is little research on non-equilibrium solidification behavior of alloy steel produced by laser additive manufacturing. As one of the most important phases in alloy steel, martensitic transformation mechanism is the basis for successful manufacture alloy steel with good microstructure and properties. However, process design closely related to the non balance technology is still mainly rely on empirical or semi quantitative theory not to meet the current requirements of alloy steel by laser material manufacturing [1]. It is urgent for people to make new 2017 International Conference on Materials Science and Biological Engineering (ICMSBE 2017) Copyright © (2017) Francis Academic Press , UK 111 breakthroughs in solidification theory research. Thermodynamic development of martensitic precipitating from 24 CrNiMo alloy steel, one material is used to manufacture high speed rail brake disc. This paper analyzed basic theory relative to the mechanism of martensitic phase, simulated the temperature of martensitic transformation starting point (Ms) and CCT curves, and calculated Ms temperature with empirical formula, expected to provide a valuable basic theory for high speed rail brake disc made of laser additive manufacturing. 2. The development of thermodynamics martensitic transformation Due to rapid heat and cool (cooling rate is above 10°C/s) non-equilibrium phase martensite can be observed in laser selective deposited 24CrNiMo alloy steel. Thus, thermodynamic of martensitic transformation are introduced, including thermodynamic model, such as Neural network model, Super member model [3] etc. Hsu T. Y. [2] defined the physical meaning of Gibbs free energy of martensitic transformation, studied martensitic transformation thermodynamics systematically, including binary, ternary alloy steel system. In recent years, some researchers have studied Ms temperature of polynary system iron alloy. Using software to simulate the thermodynamics of martensitic transformation has some reference significance for the experimental study of high-speed brake disc. 2.1 Thermodynamics of martensitic formation in binary alloy steel While studying the binary alloy steel, Hsu T Y [2] gave the Gibbs free energy formula of martensitic transformation as follows: (1) In Eq.(1), is the energy to extend the nuclear structure of the somatic structure, and is the energy to made the nuclear structure martensite. Hsu T Y defined the temperature at which the Gibbs free energy of martensitic transformation is zero as the starting point of martensitic transformation. when the value of first part in Eqs.(1) is zero the corresponding temperature is T0. D-value between the Gibbs free energy between T0 and Ms temperature is called the martensitic transformation driving force, that is or . The phase change driving force ( ) can be calculated by (2) In Eq.(2), is the activity of i element in the Solid solution j, correspondingly is the activity coefficient, Xc is mass percentage of element C. Using geometric model to calculate the activity value of iron element in ferrite and austenite is feasible. Pure iron Gibbs free energy change can not only be calculated through a variety of models, but also can be calculated by the latest thermodynamic data. In order to calculate Hsu T Y derived the following formulas: (3) Combined the third and fourth equations as well as the above data, the following formula can be obtained: (4) 2.2 Thermodynamics of martensitic transformation in ternary alloy steel Considering the Fisher model and the binary system regular solution model, a ternary regular solution model was proposed [2]: (5) Considering Fisher--Hsu method and Zener two parameteric method was derived as: m G → ∆ α

The Study on thermodynamics of martensitic transformation in Laser additive manufacturing alloy steel