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1

Size‑dependent constitutive model incorporating grain refinement and martensitic transformation

Liu Shengqiang, Li Wei, Shen Jinxia, Yang Xiaoming, Wang Baoyu, Liu Jinping

Archives of Civil and Mechanical Engineering

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2023

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Vol. 23, no. 1

art. no. e38, 2023

EN

The deformation behaviour of materials at the micro-scale level is different from that at the macro-scale level due to the effect of grain size (GS). The mechanism of the influence on martensitic transformation by GS is still unclear, and there are relatively few studies on the relationship between grain refinement and martensitic transformation, most of which focus on the relationship between the initial GS of the material and martensitic transformation. Therefore, in this study, the interaction between grain refinement and martensitic transformation was investigated using a dislocation density-based multiscale constitutive model that incorporated dislocation sliding, strain-induced martensitic transformation (SIMT) related to grain size, and grain refinement. The proposed model evaluated the GS-dependent deformation behaviour of 316L stainless steel (SS). Subsequently, a genetic algorithm was used to determine the parameters of the established model, and the calculated results were compared with that of the experimental data to verify the accuracy of the model. The developed multiscale constitutive model was implemented in Abaqus user subroutine to further investigate the deformation mechanism and validate its accuracy. The results demonstrated that the GS had a significant effect on the SIMT, with the volume fraction of martensite increasing with a rise in the initial austenite GS. In addition, grain refinement affected SIMT and the growth rate of martensite content decreased with the grain refinement caused by deformation. The formation of martensite led to grain refinement, with the refined grains producing negative feedback on the SIMT, thus inhibiting the occurrence of martensitic transformation. This study revealed the microscopic deformation mechanism of 316L SS and provided a constitutive model for micro-forming.

2

Numerical and experimental investigation on precision forming of ribbed tube by cold drawing process

Li Wei, Wang Baoyu, Liu Shengqiang, Feng Pengni, Shen Jinxia, Yang Xiaoming

Archives of Civil and Mechanical Engineering

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2022

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Vol. 22, no. 3

art. no. e135

EN

Ribbed tube is a new type of nuclear fuel cladding tube. The integrated precision forming of the rib is one of the technical bottlenecks. The cold drawing process of ribbed tube was investigated by combining finite element (FE) simulation and experiment in this study, and the results show that the insufficient rib filling and useless rib groove defects are the main problems. The insufficient radial metal flow on the outer surface of the tube leads to insufficient rib filling. The rib groove defects are caused by the relatively adequate radial and circumferential metal flow on the inner surface of the tube. Further, the effect of process parameters on rib height (RH), rib groove depth (GD) and drawing force was investigated by single-pass drawing process. The RH, GD and drawing force decrease with the increase of die angle (α) and increase with the increase of the initial tube outer diameter (Di) and die groove angle (β). Increasing the initial wall thickness (Wi) can markedly reduce the GD. On these basis, a modified die with arc surface was proposed to improve the rib filling by increasing the radial metal flow. And a modified billet with special-section is used as the preform to reduce the rib groove defects. Further, the forming experiment of ribbed tube is carried out by multi-pass drawing with the above improved methods. The experimental results show that the rib filling can be significantly improved, and the rib groove can be eliminated, which verifies the effectiveness of the proposed methods.

3

Numerical analysis and experimental trial of axial feed skew rolling for forming bars

Zhang Huibo, Wang Baoyu, Lin Longfei, Feng Pengni, Zhou Jing, Shen Jinxia

Archives of Civil and Mechanical Engineering

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2022

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Vol. 22, no. 1

art. no. e17, 2022

EN

Axial feed skew rolling (AFSR) is a novel flexible forming process suitable for the production of multi-specification and small-batch bars. This work presents a systematic investigation of AFSR process via numerical analysis and experimental method. The finite-element (FE) simulation was conducted to analyze the distributions of strain and stress, temperature evolution, axial feed motion, rolling force and torque. The results show that the stress state of the workpiece central in the deformation zone is compressive in radial direction and tensile in axial and tangential directions. During the whole process, the workpiece temperature remains in hot-rolling temperature range. The bite stage lasts less than 1 s, and the initial thrust required is about 3.17 kN (rolling from Ø 60 mm to Ø 40 mm). The axial sliding coefficient of workpiece is determined to be 0.63 via comprehensive analysis of simulation results and theoretical calculation. Moreover, based on the single-factor design, the effects of process parameters on axial feed velocity, rolling force, and torque were studied by FE simulation. The axial feed velocity, rolling force, and torque increase with increasing feeding angle and sizing length, but decrease with increasing forming angle. The experiments were performed on a newly designed rolling mill, and experimental results show consistency with FE simulation results. No central defects are observed, while there are surface helical grooves and end concave centers on the rolled piece. The diameter deviation of rolled piece is within ± 0.3 mm. The ultimate tensile strength (UTS) is increased by about 58 MPa.

4

Numerical and experimental study on the inner diameter uniformity of hollow shafts in cross-wedge rolling with mandrel

Zhou Jing, Shen Jinxia, Wang Baoyu, Huang Xu, Guo Wei

Archives of Civil and Mechanical Engineering

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2021

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Vol. 21, no. 3

321--336

EN

The precision forming of inner hole is one of technological bottlenecks in Cross-wedge Rolling (CWR) of hollow shaft with mandrel. The inner diameters show signs of characteristic fluctuation despite under the control of mandrel. The uniformity of inner diameter was investigated by finite element simulation and experiments in this study. The inner hole expands at the knifing stage and shrinks at the sizing stage. The dimensional fluctuation of inner diameter is mainly resulting from the improper metal flow. The radial and axial metal flows are insufficient at the knifing stage, which resulting in the hole expansion at knifing zone. The hole shrinkage is caused by the relatively adequate radial metal flow at sizing stage. The experiment results show that the hole expansion increases with the increasing stretching angle and mandrel diameter and decreases with the increasing forming angle and initial wall thickness. The hole expansion first increases and then decreases with the increasing reduction ratio. The hole shrinkage has positive correlations with forming angle, reduction ratio and initial wall thickness, and negatively correlates with stretching angle and relative mandrel diameter. A modified CWR roll with the curved-surface knife is proposed to get rid of the hole expansion. Based on experiments and simulations, the most suitable geometric parameters of the curved-surface knife are determined. Using mandrel diameter compensation can reduce the phenomenon of hole shrinkage. These methods were applied to the trial rolling of a half-shaft sleeve part, and the results show that the methods significantly restrain the inner diameter fluctuation.

5

Numerical and experimental research on cross wedge rolling hollow shafts with a variable inner diameter

Shen Jinxia, Wang Baoyu, Zhou Xu, Li Junling

Archives of Civil and Mechanical Engineering

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2019

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Vol. 19, no. 4

1497--1510

EN

The precise forming of inner hole has been a major technical difficulty in the cross wedge rolling (CWR) of hollow shaft. This paper proposes a new process to form hollow shafts with variable inner diameters by using the CWR with mandrel control. The forming characteristics and dimension precision of this process are analyzed by combining finite element modelling (FEM) and forming trials. The hole step of hollow shaft with variable inner diameter is formed in a spiral pattern. The helixes result in many micro-steps in hole step when forming the right-angle inner step. The metal flow lines demonstrated that mandrel step hindered the axial metal flow of inner hole and the metals were accumulated in hole step. The rolling load increases in the process of forming hole step. The mandrel is subjected to axial load when hole contacts the mandrel step. The roundness can be improved by reducing the mandrel diameter in knifing position. The axial accuracy of inner diameter can be classed as three parts: hole expansion, stable rolling, hole shrinkage. The compensated mandrel was designed to improve axial precision of inner diameter. The results showed that the inner hole dimension can be effectively controlled.