Glass coating effects on Ti-6Al-4V hot forging
Hot forging processes are highly influenced by the contact conditions between the work piece and the dies. An inaccurate definition of the contact conditions may lead to high deviation predictions of the final component geometry, the quantity of material necessary to fill in the cavity, the wear of...
| Idioma: | English |
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| Publicación: |
Mondragon Unibertsitatea. Goi Eskola Politeknikoa
2019
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| Materia: | |
| Acceso electrónico SOLREBILTEGIA: |
https://hdl.handle.net/20.500.11984/1206 https://doi.org/10.48764/71b4-cs05 https://katalogoa.mondragon.edu/janium-bin/janium_login_opac.pl?find&ficha_no=148135 |
| Resumen: |
Hot forging processes are highly influenced by the contact conditions between the work piece and the dies. An inaccurate definition of the contact conditions may lead to high deviation predictions of the final component geometry, the quantity of material necessary to fill in the cavity, the wear of the tools and the force necessary to manufacture the component. Furthermore, when dealing with high added value materials such as titanium alloys, the wrong prediction of the aforementioned variables also could lead to inaccurate microstructure predictions.
The main goal of the present dissertation was to understand the contact condition between Ti-6Al-4V work piece and heated tool steel with the purpose of calculating the coefficient of friction (COF) and the heat transfer coefficient (HTC) at the interface of the work piece and the tools. Both coefficients of Ti-6Al-4V titanium alloy under hot forging situation were determined by the combined approach of experimental tests and finite element (FE) simulations using FORGE3® finite element software. At the same time, in order to improve the flow of the material within the tools and reduce the alpha-case layer when forging titanium alloys at high temperature, three types of surfaces were analyzed in the present study: billets without coating, 40-45 μm and 80-90 μm of CONDAERO 228 glass coating.
The objectives of this dissertation were four-fold. First, a Finite element (FE) parametric study has been performed in order to obtain the sensitivity of ring and T-Shape test when addressing the issue of varying input parameters. The objective of this study was to determine and interpret the factors that affect friction behaviour. Special care must be put in some specific inputs, to obtain accurate calibration curves and avoid errors when finding the real COF based on experimental observations. It is noticed that, HTC is the most influencing factor among all and has high impact on calibration curves of the friction test, thereby affects the measurement of interfacial friction factor. Then, tribological contact conditions in both the tests were compared. It concluded that, T-shape test is best suitable to evaluate the friction condition as this test induces large contact pressure and large surface expansion similar to what is occurred during a real forming operation.
Second, columnar upsetting test was conducted for the determination of the HTC at the workpiece-die interface at two different contact pressures. An efficient numerical 3D model has been developed where, simulations have been carried out defining different values of the HTC. That gives as a result of different temperature evolutions which were used to estimate the HTC value which best fits the temperature read by the thermocouple in the real experiments. The validation of final coefficients determined by the inverse algorithm was made by comparison between the upsetting process and the FE analysis results. Variations in HTC have been obtained under different pressure with different surface condition during upsetting. Therefore, HTCs in function of pressure should be employed to generate the calibration curves for determining the interfacial COFs in friction tests.
Third, a comprehensive numerical and experimental study on the ring and T-Shape friction test were carried out to calculate the COF where, geometrical shape of specimen was chosen to simplify this methodology. The aim of this study is to analyze the same tribo-system but having different contact pressures and surface enlargement factors, which could affect the coating behavior and to estimate the friction factor taking into account the HTC as pressure dependent. Calibration curves for the tests were obtained numerically by using finite element simulations. Then, the COFs are calculated by the comparison of the experimental data and numerical simulation results using inverse analysis. The results obtained that the glass coating clearly improves the forgeability of titanium alloys reducing the COF value.
Finally, an experimentation was performed to observe the effects of alpha-case formation, cracks on the deformed T-Shape specimen and to investigate effect of coatings to reduce alpha-case formation. Using a microscope, photographs were taken and visible alpha-case region was measured and also identified the cracks on each sample. Microhardness testing was then performed to determine exact depth range of the visible alpha-case region profile of the sample. It was found that alpha-case thickness and crack depth is more in uncoated specimen than the coated specimen. |
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