Rotary bending fatigue behavior of A356 –T6 aluminum alloys by vacuum pressurizing casting

Vacuum pressurizing casting technique, providing better mould filling and inter-dendritic feeding, can reduce the porosity greatly in cast aluminum alloys, and improve the fatigue properties. The rotary bending fatigue properties of A356-T6 alloys prepared by vacuum pressurizing casting were investigated. The S-N curve and limit strength 90 MPa under fatigue life of 107 cycles were obtained. The analyses on the fatigue fractography and microstructure of specimens showed that the fatigue fracture mainly occurs at the positions with casting defects in the subsurface, especially at porosities regions, which attributed to the crack propagation during the fatigue fracture process. Using the empirical crack propagation law of Pairs-Erdogon, the quantitative relationship among the initial crack size, fatigue life and applied stress was established. The fatigue life decreases with an increase in initial crack size. Two constants in the Pairs-Erdogon equation of aluminum alloy A356-T6 were calculated using the experimental data.

Sensitivity of springback and section deformation to process parameters in rotary draw bending of thin-walled rectangular H96 brass tube

In order to study the effects of the process parameters on springback and section deformation, a sensitivity analysis model was established based on the combination use of the multi-parameter sensitivity analysis method and the springback/section deformation prediction finite element model, and by using this model the sensitivities of the springback and the section deformation to process parameters were analyzed and compared. The results show that the most sensitive process conditions for springback angle are the boost speed and the pressure of pressure die, and the most sensitive process condition for section deformation is the number of cores. When the clamp force, the boost speed and the pressure of pressure die are utilized to control section deformation, the effect of these process parameters on springback should be considered. When the process parameters are mainly used to control springback, the effect of these process parameters on the section deformation should be always considered.

Theoretical analysis on springback of L-section extrusion in rotary stretch bending process

The theoretical analysis of springback in rotary stretch bending process of L-section extrusion was studied. The models for characterizing the springback angle after unloading were established based on the stress and strain distributions in the cross-section of the part. With the proposed model, analysis of the effect of pre-stretch force and post-stretch force on springback angle shows that springback decreases as the pre-stretch force or post-stretch force increases. Comparative study with experiments clearly demonstrates that the prediction of springback can resort to the current model without the loss of accuracy.

Cross-sectional deformation of H96 brass double-ridged rectangular tube in rotary draw bending process with different yield criteria

Different yield criterion has great difference in predicting the deformation of tube with different material.In order to improve the prediction accuracy of the cross-sectional deformation of the double-ridged rectangular tube(DRRT)during rotary draw bending(RDB)process,Mises isotropic yield criterion,Hill’48 and Barlat/Lian anisotropic yield criteria commonly used in practical engineering are introduced to simulate RDB of DRRT.The inverse method combining uniaxial tensile test of whole tube and response surface methodology was proposed to identify the parameters of Hill’48 and Barlat/Lian yield criteria of small-sized H96 brass extrusion DRRT as well.Then based on ABAQUS/Explicit platform,the FE models of RDB process of DRRT considering Mises,Hill’48 and Barlat/Lian yield criteria were built.The results show that:The variation trend of cross-sectional deformation ratio is same when using different yield criteria.The cross-sectional deformation ratio by using Mises yield criterion is close to that by using Hill’48 yield criterion.However,there is a quite difference between by using Barlat/Lian yield criterion and by using Mises or Hill’48 yield criteria.The prediction values of cross-sectional height deformation by using three yield criteria all underestimate the experiment ones,and the prediction values of cross-sectional width deformation overestimate the experiment ones.By comparing the simulation results of cross-sectional deformation of the DRRT with different yield criteria and experiment ones,Barlat/Lian yield criterion is found to be suitable for describing the RDB process of DRRT.

Prediction of Maximum Section Flattening of Thin-walled Circular Steel Tube in Continuous Rotary Straightening Process

Cross-sectional ovalization of thin-walled circular steel tube because of large plastic bending,also known as the Brazier effect,usually occurs during the initial stage of tube′s continuous rotary straightening process.The amount of ovalization,defined as maximal cross section flattening,is an important technical parameter in tube′s straightening process to control tube′s bending deformation and prevent buckling.However,for the lack of special analytical model,the maximal section flattening was determined in accordance with the specified charts developed by experienced operators on the basis of experimental data;thus,it was inevitable that the localized buckling might occur during some actual straightening operations.New normal strain component formulas were derived based on the thin shell theory.Then,strain energy of thin-walled tube（per unit length）was obtained using the elastic-plastic theory.A rational model for predicting the maximal section flattening of the thin-walled circular steel tube under its straightening process was presented by the principle of minimum potential energy.The new model was validated by experiments and numerical simulations.The results show that the new model agrees well with the experiments and the numerical simulations with error of less than 10%.This new model was expected to find its potential application in thin-walled steel tube straightening machine design.

Cross-sectional deformation behavior of double-ridged rectangular tube with fillers in different stages of H-typed bending

The bent double-ridged rectangular tube(DRRT)with high forming quality is helpful to improve the microwave transmission accuracy.For reducing the cross-sectional deformation in the H-typed bending process,in addition to using rigid mandrel to support the inside of tube,ridge groove fillers are also added to restrict the deformation of ridge grooves.Because of the change of stress and strain state of bent tube in bending,rigid mandrel retracting and specially twicespringback stages,and the springback of fillers,the cross-sectional deformation of tube in each stage may be different.Therefore,based on the ABAQUS platform,the finite element models(FEM)for H-typed bending,mandrel retracting and twice-springback stages of H96 DRRT with fillers were established and validated.It is found that,for the height and width deformation of tube and spacing deformation of ridge grooves,retraction of mandrel can make the distribution of these deformations more uniform along the bending direction.The first springback can reduce these deformations significantly,which should be emphasized.But the second springback only increases them by less amount,which can be ignored.The smaller height deformation of ridge groove and filler can be neglected.

Determination of continuous constitutive relationship of weld zone of high-strength steel rectangular welded tube

For QSTE700 high-strength steel rectangular welded tube,the mechanical properties of the weld zone vary with the distancefrom the centerline of the weld.Therefore,the accurate description of constitutive relationship of the weld zone is of greatsignificance for the study of formability of QSTE700 rectangular welded tube.Firstly,the mechanical properties of parentand mixed specimens containing weld zone and parent zone were obtained by uniaxial tensile test.And based on the micro-hardness test,the width and the microhardness distribution of the weld zone were determined.Secondly,by subdividingthe weld zone into several small areas,and combining with the rule of mixtures and nanoindentation test,the continuous functional relationships of strength coefficient K,hardening coeffecient n and elastic modulus E were obtained,and then,the continuous constitutive relationship of QSTE700 rectangular welded tube was established.Finally,the validity and reliability of the continuous constitutive relationship of welded tube were verified by nanoindentation fest and rotary drawbending of rectangular welded tubc.Besides,it was found that the finite element model of rotary draw bending of QSTE700 rectangular welded tube established by using the continuous constitutive relationship can well simulate the cross sectiondeformation and wall thickness variation.