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Current status and development of material thermal processing technology

    1. Research history and technology development trend of material thermal processing technology simulation

    The simulation study of material thermal processing technology started from the casting process, because the simulation calculation of the temperature field during the solidification process of the casting is relatively simple. In 1962, Forsund, Denmark, used computer and finite difference method to calculate the heat transfer of casting solidification process for the first time [2]. Following the Danes, the United States pioneered the numerical simulation of the temperature field of large steel castings in the mid-1960s with NSF funding. After entering the 1970s, more countries (my country started in the late 1970s) joined the ranks of this research, and gradually expanded from casting to forging, welding, and heat treatment. A research boom in material thermal processing technology simulation has formed all over the world. At the international conferences of various majors of material thermal processing held in the past ten years, the number of research papers in this field ranks first among various types of papers; in addition, since 1981, a special computer for casting and welding process has been held every two years. The International Conference on Numerical Simulation has been held for eight sessions so far. In the past one or two decades, the material thermal processing technology simulation technology has been continuously expanded in breadth and depth, and its development history and development trends have the following seven aspects. 1.1 Macro→Meso→Micro

    The research work of material thermal processing technology simulation has generally entered from the macro-scale simulation (in the order of meters) based on the temperature field, velocity field, and deformation field to predict the shape, size, and contour to predict the structure, structure, and performance. For the purpose of mesoscale simulation (millimeter scale) and microscale simulation stage, the research objects involve crystallization, recrystallization, recrystallization, segregation, diffusion, gas precipitation, phase transition and other microscopic levels, even reaching the scale of a single dendrite. 1.2 Single Decentralization → Coupling Integration

    The simulation function has generally entered the stage of coupling integration from a single temperature field, flow field, stress/strain field, and tissue field simulation. Including: flow field←→temperature field; temperature field←→stress/strain field; temperature field←→organization field; stress/strain field←→organization field, etc., to simulate the complex actual thermal processing process. 1.3 Generality, General Purpose → Special Purpose, Characteristics

    Due to the conventional thermal processing based on numerical simulation of temperature field, flow field, and stress/strain field, especially the increasing maturity of casting, stamping and casting process simulation technology and the continuous emergence of commercial software, the research work has changed from common problems Turn to more difficult specific features. There are mainly the following two directions:

    (1) Solve the problem of special thermal processing technology simulation and process optimization: die casting, low pressure casting, metal mold casting, full mold casting, continuous casting, electroslag casting, etc. in the foundry specialty; hydraulic bulging and wedge cross-section in the forging specialty Rolling, roll forging, etc.; resistance welding, laser welding, etc. in welding.

    (2) Solve the problem of defect elimination of hot-worked parts: The application of simulation technology has successfully solved the shrinkage and porosity of large steel castings, the folding of die forgings, and the fracture and wrinkling of stamping parts. Current research hotspots It focuses on the prevention and elimination of common defects such as hot cracking, porosity and segregation of castings; mixed crystal of large forgings; springback of stamping parts; deformation, cold cracking and hot cracking of welded parts; deformation during quenching. 1.4 Pay attention to basic research to improve the accuracy and speed of numerical simulation

    Numerical simulation is an important method of thermal processing technology simulation. Improving the accuracy and speed of numerical simulation is the current research hotspot of numerical simulation. For this reason, great attention is paid to the basic theories of thermal processing, new mathematical models, new algorithms, pre- and post-processing, and accurate Fundamental research such as the acquisition and accumulation of basic data requires the cooperation of researchers from multiple professional disciplines to make breakthroughs.

    1.5 Pay attention to physical simulation and accurate test technology

    Physical simulation reveals the nature of the process, obtains critical criteria, and is a powerful means of checking and checking the results of numerical simulations, which has attracted more and more attention from researchers. There are some new trends as follows:

    (1) Apply high technology to design and develop new physical simulation experiment methods and devices. Here are two examples: ① The University of Iowa in the United States uses ethylenedihydrocarbon as a simulated substance (its crystallization process is similar to that of metal, and it is transparent and easy to watch), and continuously observe and record its crystallization process through four CCD cameras. You can directly watch the influence of gravity, convection and other factors on crystallization, which is very intuitive. ②Laser measurement system for large strain on the surface of stamping parts developed by the Wu Xianming Manufacturing Center of the University of Michigan in the United States: The laser probe mounted on the three-coordinate measuring instrument is used to scan the stamping parts with deformed grids. Effective means of nuclear numerical simulation results.

    (2) Correctly and reasonably handle the relationship between numerical simulation and physical simulation (including experimental verification)

    ①According to the simulation object, determine the application ratio of the two reasonably: Generally speaking: the larger the workpiece, the larger the equipment, the greater the role and workload ratio of numerical simulation. Taking the research work of the United States Net Forming Engineering Research Center (NSW/ERC) as an example, the proportion of numerical simulation to the workload is: die forging: 80%; pipe hydroforming: 50%; cutting 30%.

    ② Make use of strengths and avoid weaknesses, and give play to their different strengths

    For this reason, it is necessary to accurately understand the functions of the simulation software. For problems that the software cannot reach or where the error is too large due to simplification, corrections should be made through experiments or physical simulations; once the errors of the numerical simulation are determined and corrected, The effect of numerical simulation should be exerted as much as possible to save the cost of experiment. NSM/ERC first uses experiments to determine the single-pass bulging mechanism and corrects the finite element numerical simulation error in the forming of pipe fittings, then uses the finite element method to simulate the multi-pass process, and completes the coordination of the pre-forming and final bulging processes . This cooperation gives full play to the strengths of both.

    Generally speaking, numerical simulation needs to be checked by experiment or physical simulation method. When there is a difference between the two, the experiment shall prevail. (3) Test technology that attaches great importance to basic data