Integrated Modelling of Microstructure Evolution and Mechanical Properties Prediction for Q&P Hot Stamping Process of Ultra‑High Strength Steel

High strength steel products with good ductility can be produced via Q&P hot stamping process,while the phase transformation of the process is more complicated than common hot stamping since two-step quenching and one-step carbon partitioning processes are involved.In this study,an integrated model of microstructure evolution relating to Q&P hot stamping was presented with a persuasively predicted results of mechanical properties.The transformation of diffusional phase and non-diffusional phase,including original austenite grain size individually,were considered,as well as the carbon partitioning process which affects the secondary martensite transformation temperature and the subsequent phase transformations.Afterwards,the mechanical properties including hardness,strength,and elongation were calculated through a series of theoretical and empirical models in accordance with phase contents.Especially,a modified elongation prediction model was generated ultimately with higher accuracy than the existed Mileiko’s model.In the end,the unified model was applied to simulate the Q&P hot stamping process of a U-cup part based on the finite element software LS-DYNA,where the calculated outputs were coincident with the measured consequences.

Simulation of sheet metal extrusion processes with Arbitrary Lagrangian-Eulerian method

An Arbitrary Lagrangian-Eulerian(ALE) method was employed to simulate the sheet metal extrusion process,aiming at avoiding mesh distortion and improving the computational accuracy.The method was implemented based on MSC/MARC by using a fractional step method,i.e.a Lagrangian step followed by an Euler step.The Lagrangian step was a pure updated Lagrangian calculation and the Euler step was performed using mesh smoothing and remapping scheme.Due to the extreme distortion of deformation domain,it was almost impossible to complete the whole simulation with only one mesh topology.Therefore,global remeshing combined with the ALE method was used in the simulation work.Based on the numerical model of the process,some deformation features of the sheet metal extrusion process,such as distribution of localized equivalent plastic strain,and shrinkage cavity,were revealed.Furthermore,the differences between conventional extrusion and sheet metal extrusion process were also analyzed.

Local Buckling-Induced Forming Method to Produce Metal Bellows

A novel buckling-induced forming method is proposed to produce metal bellows.The tube billet is firstly treated by local heating and cooling,and the axial loading is applied on both ends of the tube,then the buckling occurs at the designated position and forms a convolution.In this paper,a forming apparatus is designed and developed to produce both discontinuous and continuous bellows of 304 stainless steel,and their characteristics are discussed respectively.Furthermore,the influences of process parameters and geometric parameters on the final convolution profile are deeply studied based on FEM analysis.The results suggest that the steel bellows fabricated by the presented buckling-induced forming method have a uniform shape and no obvious reduction of wall thickness.Meanwhile,the forming force required in the process is quite small.

Analytical Solution of Thermo-Elastic-Plastic Deformation of the Combined Forging Die

On the multi-layer forging die used in daily life,stressed ring can strength the die structure within elastic deformation and the die material can be self-strengthened through uniform plastic deformation by autofrettage effect,whereas the thermal effect generated during forging process can directly influence the stress state and dimension of the forging die in service.In this study,an analytical solution of the thermo-elastic-plastic deformation in the forging die is derived.The relationships between the radial and circumferential stresses and the temperature distribution,which are directly related to geometric parameters,material properties and working pressure,are determined.This helps to better understand the thermo-elastic-plastic deformation behavior of the die and design the combined forging die to achieve long service life and high accuracy product.

Effects of quenching and partitioning process on mechanical properties of TRIP780 steel

A transformation-induced plasticity (TRIP) steel was applied to test its mechanical properties in quenching and partitioning (Q&P) process. A series of Q&P experiments followed by tensile tests, Charpy impact tests, fracture morphology analyses and microstructure observations were conducted. The experimental results showed that the Q&P treatment could increase the mechanical properties of TRIP steel evidently. The strength of tested TRIP780 after Q&P process reaches more than 1500 MPa with elongation of 17.8%, which is obviously greater than that of 22MnB5 after hot stamping. The microstructure observations indicate that the good combination of high strength and plasticity of TRIP steel after Q&P process is attributed to the multi-phase microstructure of hard martensite matrix and soft retained austenite.

3D Numerical Simulation of Non-isothermal Resin Transfer Molding Filling Process Using Unstructured Tetrahedron Mesh

We present a numerical formulation for resin flow based on the concept of quasi-steady state situation at the flow front. To be fit for complicated product shapes,we use the four-node unstructured tetrahedron mesh based on which the numerical formulation of temperature and degree of cure is developed. The validity of our method is established in the case where ffexible meshes are used. The results show that the numerical procedure,tested on known data,provides numerically valid and reasonably accurate predictions.

Evaluation and Optimization of Intersection Process Between Voxelized Mesh Models and Homocentric Spheres

In the field of 3D model matching and retrieval,an effective method for feature extraction is spherical harmonic or its mutations,and is acted on the spherical images.But the obtainment of spherical images from 3D models is very time-consuming,which greatly restrains the responding speed of such systems.In this paper, we propose a quantitative evaluation of the whole process and give a detailed two-sided analysis based on the comparative size between pixels and voxels.The experiments show that the resultant optimized parameters are fit for the practical application and exhibit a satisfactory performance.

A New Approach of Intelligent Design and Optimization for Shell Nosing

Preform design for shell nosing product without machining the inner surface after forming is an experience-extensive work. Generally, the initial design needs to be modified and simulated consequently to get proper preform and nosing die, and the iterative process is time consuming. This paper puts forward a new approach, in which the suitable initial design can be obtained by knowledge-based intelligent technology and the optimal design can be acquired by finite element method （FEM） based geometrical modification. With the CAX object model as the bridge of CAD and CAE, the CAD model with simulation knowledge can be transferred into CAE automatically, and the CAE result can be automatically utilized as well. Based on the comparison between the simulated shape and the desired shape, the dissatisfactory area will be modified. A new simulation and modification process will be carried out based on the modified design. The process is repeated iteratively to get the optimal design. This approach utilizes the commercial CAD and CAE software, without the need of complex back-forward FEM procedures. Based on the new approach, an in-home intelligent shell nosing design and optimization system is developed, and a case study proves that this system can reach a reasonable design efficiently.

Geometric basis:a geometric solving cell for geometric computing

Geometric computing is an important tool in design and manufacturing and in arts. Conventionally, geometric computing is taken by algebraic computing. The vivid intuition of objects in visualization is lost in numeric functions, which is however very useful to human cognition as well as emotion. In this paper, we proposed a concept and theory of geometric basis （GB） as the solving cell for geometric computing. Each GB represents a basic geometric operation. GB works as both expressing and solving cell just like the concept of basis in linear algebra by which every element of the vector space can be expressed. For 3D problems, with a procedure of a projections reduction, the problem can be reduced to plane and the reduction function can be designed as a GB. A sequence of GB can construct a higher layer GB. Then, by the traversal of tree, a sequence of GB is got and this sequence is just the construction process and also the solution of this geometric problem.

Novel Blasting-Like Lubricating Process for Cold Extrusion

A blasting-like lubricating process(combination of shot blasting and lubricating processes)is proposed.In this process,the specimens to be treated,alloy shots,and solid lubricating powder are rotated together in a roller.The surface pockets formed due to the impact by the shots can store lubricants,and the lubricant can also adhere to the specimen surface by hitting.The effects of process parameters,including rolling time,rotational speed,mass of alloy shots,and the diameter of shots,on the surface topography of the steel specimen are investigated using 13 experimental schemes.The distribution ratio and average depth of surface pockets on the defined areas of the specimen are quantitatively analyzed.Four selected schemes with the MoS_(2) solid lubricating powder are further carried out to lubricate the cylindrical billets,and the lubricating effect is evaluated using the steady combined forward and backward extrusion test.The indicated friction factor of the novel blasting-like process is smaller than that of the conventional phosphate-soap coating process.

Breakdown Behavior of Eutectic Carbide in High Speed Steel During Hot Compression

The evolution of euteelie carbide in as CaSl M2 high speed steeL was invesligated with hot compression test and metallographic examination. Initial rodlike or irregular eutectic carbides were broken into smaller particles during hot deformation by thermomechanical disintegration, while diffusion controlled phase transformation was not remarkable. Combining with numerical simulation, the relationship between breakdown ratio of carbide network and deformation parameters was obtained. Strain was the most important driving force to shatter euteclic carbides and disperse products. Furthermore, critical strain values were obtained, beyond which carbide network disappeared, and fractured carbides kept a stable profile and they were deformed with matrix coordinately. A higher temperature or lower strain rate resulted in a lower crilical strain.

Evolving Mechanism of Eutectic Carbide in As-cast AISI M2 High-speed Steel at Elevated Temperature

The evolution in type, size and shape of carbides in as-cast American Iron and Steel Institute （AISI） M2 high-speed steel before and after annealing were investigated. The micromechanism which was responsible for those changes was also analyzed and discussed. At the initial stage of reheating, metastable M2C-type carbide decomposed continuously. M6C-type carbide nucleated at the interface of M2C/γ firstly and grow from surface to center. Then MC-type carbide nucleated at both surface of M6C/M6C and inner of M6C. With the increasing decomposition of the metastable M2C-type carbide, the rod-shaped construction of eutectic carbide began neck- ing, fracturing and spheroidizing gradually. Held enough time or reheated at higher temperature, particle-shaped product aggregated and grew up apparently, while secondary carbide precipitated in cell and grew up less sig- nificantly than the former. Based on the above microstructural observation, the thermodynamic mechanism for decomposition of M2C carbide, for spheroidization of products, and for the growth of particles were analyzed. The rate equations of carbides evolution were derived, too. It shows that the evolving rate is controlled by diffusion coefficients of alloy atoms, morphology of eutectic carbides and heating temperature.

Curvature Estimation of Point Set Data Based on the Moving-Least Square Surface

Curvature estimation is a basic step in many point relative applications such as feature recognition, segmentation,shape analysis and simplification.This paper proposes a moving-least square(MLS) surface based method to evaluate curvatures for unorganized point cloud data.First a variation of the projection based MLS surface is adopted as the underlying representation of the input points.A set of equations for geometric analysis are derived from the implicit definition of the MLS surface.These equations are then used to compute curvatures of the surface.Moreover,an empirical formula for determining the appropriate Gaussian factor is presented to improve the accuracy of curvature estimation.The proposed method is tested on several sets of synthetic and real data.The results demonstrate that the MLS surface based method can faithfully and efficiently estimate curvatures and reflect subtle curvature variations.The comparisons with other curvature computation algorithms also show that the presented method performs well when handling noisy data and dense points with complex shapes.