Study On Local Temperature Rise Characteristic Of Sheet Metal In Friction Stir Incremental Sheet Forming

Incremental sheet forming is suitable for the trial-manufacture, customization and small batch production of sheet metal parts, because sheet could be formed to parts with half die or even no dies at all. Therefore, it has been widely concerned and studied. With the wide application of magnesium alloy, aluminum alloy and titanium alloy, warm/hot incremental sheet forming has become highlights. In various heating methods, friction stir incremental sheet forming is the most attractive one, for friction between the rotating tool and the sheet could be used to heat the sheet without additional heating devices. The heating method is simple, convenient, energy efficient and green. In friction stir incremental sheet forming, local temperature rise characteristics are the key factor to affect the formability of sheets. It’s the foundation of formulating reasonable friction stir incremental sheet forming process, to fully understand the effect pattern and influence degree of process parameters on the local temperature rise of sheets, and establish complete mathematical model on temperature variation. So far, the research on friction stir incremental sheet forming is still in the primary stage, and the related research focuses on the influence of the minority process parameters on the temperature change and the formability mainly.In this paper, experimental research method would be used to analyze and discuss the temperature rise mechanism and the effect pattern and influence degree of process parameters on the local temperature rise of sheets in friction stir incremental sheet forming. Based on the basic theory of heat transfer, heat transfer process of sheets would be simplified and abstracted during friction stir incremental sheet forming process, and the mathematical model of temperature change would be deduced. In numerical simulation study, the numerical model would be established about the relationship between equivalent temperature of equivalent tool and process parameters, by which the numerical simulation of an actual part was accomplished. The effects of process parameters on the grain size and mechanical properties of the parts would be compared and discussed by experimental methods.First of all, reciprocating straight groove incremental forming experiments were adopted to compare the temperature variation pattern of testing points at different locations. It was proved that the temperature variation pattern of these testing points at non-forming position was similar to that at forming position. The temperature variation pattern of the sheet at the non-forming position could be used to characterize that at the forming position. The local temperature rise mechanism was compared and discussed by single straight groove incremental forming experiments and reciprocating straight groove incremental forming experiments. It was proved that the friction heat caused by the rotation of the tool was the main reason for the local temperature rise. By measuring temperature rise values in single straight groove incremental forming experiments, the influence of process parameters on the local temperature rise was researched, which could provide references for the design of following experimental studies.On the basis of above, with AZ31 B alloy as the experimental materials, orthogonal experimental design and response surface experiment method were adopted to make more deep and comprehensive research about the influence trend and influence level of process parameters and their interaction on the local temperature rise peak of the sheets. Through variance analysis and regression analysis of the experimental results, the regression equations of the local temperature rise relative to the process parameters were established, and the regression analysis was carried out. The analysis results were the same about the influence trend of parameters on the local temperature rise obtained by the orthogonal experiments, the response surface experiments and the single-factor expeiments, which were: the local temperature rise value could enchance with the increase of sheet thickness, vertical step size, tool rotational speed and wall angle, with the decrease of feed rate, and enhance first and drop then with the increase of tool diameter. According to the analysis results of orthogonal experiments, feed rate and vertical step size were the most important factors influencing sheet local temperature rise.After that, the mathematical model of sheet temperature change was deduced on the base of rational simplification and assumption of the local temperature rise and heat transfer procedure of sheets in friction stir incremental sheet forming. According to Fourier Law, heat transfer procedure in friction stir incremental sheet forming was simplified and abstracted as temperature field model, as the sheet was under the continuous moving heating line source. Central composite response surface experimental design was employed to make experimental researches, in which the temperature rise value was tested at the certain position near the bottom edge of the truncated pyramid part under different parameters. By regression analysis, the heat absorption power was calculated reversely, on the base of which, model of heat absorption power was established and sheet temperature curves and temperature field variation were further discussed. The theoretical calculation and experimental results were in good agreement, which shows that the proposed method is feasible.In addition, the method of equivalent temperature of equivalent tool was put forward to solve the problem that it was difficult to accept the huge amount of calculation and time consuming in engineering application due to the rotation of the tool. The rotation tool was equivalent to a fixed tool with the certain temperature, and the numerical model would be established about the relationship between equivalent temperature of equivalent tool and process parameters using the response surface experimental method. The numerical simulation of an actual part was accomplished, using the numerical model above. Comparison between the measured temperature and the simulation values showed that the simulative temperature change curves were consistent with the experimental one in the aspect of trend, volatile degree and value, which proves that the proposed method could be applied to the numerical simulation of the actual parts.Finally, orthogonal experiments, combined with tensile tests, hardness tests and microstructure exams, were employed to study the influence of process parameters and their interaction term on the grain size and mechanical properties of sheets. According to the results of metallographic exams, grain refinement was very obvious, compared with the original sheet, which indicates that the dynamic recrystallization of sheets occurred in friction stir incremental forming experiments. The analysis of variance showed that the interaction term of wall angle and rotational speed, and the single term wall angle were important factors to affect the grain size of sheets after dynamic recrystallization. Tensile tests and hardness tests results also showed that, the microhardness and tensile strength of samples obtained after friction stir incremental forming were significantly improved, compared with the original samples, which indicates that friction stir incremental forming could bring about dynamic recrystallization, and the mechanical properties of sheets after the dynamic recrystallization were improved. The process of friction stir incremental forming could be used as a method for preparing fine crystalline materials.In this paper, experimental research, mathematical model and numerical simulation methods were adopted to make systematic studies on the local temperature rise pattern in friction stir incremental sheet forming from different aspects. The research findings could provide guidance for the rational design of friction stir incremental sheet forming process, and theoretical basis for making use of the friction heat and enhancing the incremental formability of sheets.