Mechanical Properties Of Hot Stamping Parts Research Articles

Deep learning based phase transformation model for the prediction of microstructure and mechanical properties of hot-stamped parts

Hot stamping of boron steel is an effective way for weight reduction of automobiles parts, but suffers from the loss of mechanical properties due to incomplete martensitic transformation. Different from general heat treatment, phase transformation in hot stamping relates to both thermal loading path and deformation history. Unfortunately, the existing models have difficulty in capturing the history-dependent phase transformation behavior of boron steel under hot stamping conditions. In the present work, the gated recurrent unit (GRU) are combined with fully connected neural network (FCNN) to capture the coupling effect of both nonlinear strain history and thermal loading path on phase transformation. A unified deep-learning based phase transformation (DLPT) model is developed to integrate both diffusive and diffusionless transformation to cover all feasible thermo-mechanical loading path in hot stamping process. A hybrid driven thermo-mechanical-metallurgical (TMM) framework is developed by integrating the DLPT model into general TMM framework, which providing a novel approach for two-scale simulation of hot stamping process. A comprehensive database of phase transformation is constructed for model training based on well-designed thermo-mechanical loading experiments to cover hot stamping conditions, and the optimal topology of neural network in DLPT model is obtained by training on this database. The accuracy and reliability of DLPT model is systematically demonstrated by dieless hot V-bending and hot stamping of T-shaped parts besides CCT and DCCT tests. The net effect of local plastic strain on subsequent phase transformation is clarified with accuracy by dieless V-bending with the removal of disturbing factors. Non-uniform distribution in hardness of the final hot stamped parts can be predicted with the DLPT model successfully, which is better in accuracy than in case of working with the conventional model.

Compensation for tool deformation and expansion in virtual try-outs of hot stamping tools

The mechanical properties of hot-stamped parts strongly depend on the tool’s cooling performance. The cooling rate hinges on sufficient temperature gradients and on an excellent contact conductance between tool and workpiece. A uniform distribution of the contact pressure is the key to an even cooling behaviour, and hence, a homogenous micro structure of the formed part. Elastic deformations of machine and tool components under load are a major influence on the pressure distribution between tool surface and hot-stamped part. This applies to hot and cold forming tools. An additional difficulty in hot stamping are superimposed thermal expansions and contractions of the tool, which also affect the part’s mechanical properties due to their influence on the normal contact pressure. Manual die spotting needs to compensate for all these undesired effects and makes tool try-out a large time and money-consuming factor in the development of hot forming tools. This paper presents methods to transform the spotting of hot forming tools into a virtual production reality in order to reduce manual labour and lower costs. It gives details on the numerical compensation of tool surfaces for elastic tool and machine deformations and for temperature-induced tool expansions and contractions. The authors critically analyse necessary and achievable accuracies of computed surfaces and point out required improvements for the future implementation of virtual try-outs into tool development and manufacturing processes.

Effect of Contact Pressure during Quenching on Microstructures and Mechanical Properties of Hot-stamping Parts

An experimental apparatus with cooling system and pressure-adjustment assembly for simulating quenching was constructed to investigate the effect of contact pressure on the microstructures and mechanical properties of hot-stamping parts. Qualitative and quantitative analyses of the microstructures of the as-quenched parts were conducted; moreover, hardness and tensile tests were performed to measure their mechanical properties. The results indicated that contact pressure during quenching strongly affected the structures and performances of hot-stamping components. An excessive low contact pressure led to insufficient martensitic transformation. The critical contact pressure for complete martensitic transformation for 4.0 mm 22MnB5 steel was 0. 4 MPa when the temperature of the coolant was 20 °C. However, in consideration of the efficiency of practical production, a contact pressure higher than 1.25 MPa is recommended.

Novel Method for Predicting Hardness Distribution of Hot-Stamped Part Using FE-Simulation Coupled with Quench Factor Analysis

In the hot stamping of boron-alloyed steel, the mechanical properties of hot-stamped parts have been predicted by many empirical models based on Kirkaldy’s equation. Although these models are skillfully developed, there is still a need to precisely and easily predict the mechanical properties in the hot stamping process. Therefore, this study aims to suggest a novel method for the accurate prediction of the hardness distribution of hot-stamped parts by the use of finite element (FE)-simulations coupled with quench factor analysis (QFA). First, dilatometry of boron steel was performed at various cooling rates from 0.5 to 70 K/s using a dilatometer with a forced-air cooling system. The dilatometry test provided hardness data according to the cooling rates, which were used to determine the material constants (K 1 to K 5) of the QFA and the time–temperature–property diagram of boron steel. Then, FE-simulation of hot stamping was conducted to obtain the cooling curves for blanks with thicknesses of 1.2 and 1.6 mm. The extracted results from the FE-simulation were used to predict the hardness distribution of the hot-stamped parts using QFA. Finally, a hot stamping experiment was performed to verify the predicted results and to examine the effect of the blank thickness on the cooling rates of a hot-stamped part. The predicted hardnesses for the parts were in good agreement with the measured values, within a maximum error of 4.96 pct.

Investigation of Cooling Effect of Hot-stamping Dies by Numerical Simulation

Abstract As the development of hot-stamping technology, the designing of hot-stamping dies has more and more request. The cooling effect of hot-stamping dies can affect the mechanical properties of hot-stamping parts and the production efficiency of hot-stamping process directly, so it is an important index to evaluate the performance of dies. The cooling effect of hot-stamping dies was simulated by CFX software, and the define method of contact thermo resistance was presented in the process of simulation. Besides, the effect of processing parameters, such as pressure–holding time and cooling water velocity, on the cooling effect of hot-stamping dies was investigated and verified by experiments. Comparisons show that numerical simulation results have a good agreement with experimental results, so the FE model is effective in simulation of hot-stamping process.

Comparative study of the prediction of microstructure and mechanical properties for a hot-stamped B-pillar reinforcing part

Automobile manufacturers have been increasingly adopting hot-stamped parts for use in newly designed vehicles to improve crash worthiness and fuel efficiency. However, the hot-stamped parts require extreme mechanical properties with ultimate tensile strengths as high as 1500 MPa (∼450 Vickers hardness) while still maintaining adequate formability during the stamping operation. The ultra high strength of hot-stamped components is attributed to the martensitic phase transformation that occurs after the part has been formed at temperatures corresponding to the austenite phase field where formability is enhanced. In the present study, a computer-aided design method incorporating Kirkaldy and Venugopalan type phase transformation models has been implemented following a thermo-mechanical coupled finite element analysis to predict the mechanical properties of hot-stamped parts made with a boron-modified steel. Three empirical models which are typically used for hot stamping analysis are employed and the prediction capability of the models is compared using continuous cooling dilatometry and forming experiments of a modified B-pillar part.