Y.S. Lee

Experimental and analytical evaluation for elastic deformation behaviors of cold forging tool

Abstract Since the dimensional accuracy of forged parts are largely influenced by elastic behaviors of the tool material. The characteristics of elastic deformation at a forming tool are evaluated for a cold forged alloyed steel by experiment and FEM analysis. Elastic strain at the tool is measured dynamically using strain gauges attached to the surface of die for a ball stud mass-produced continuously in a cold former. Results of FEM analysis are compared to the experimentally measured values at two modeling approaches. The first approach is made by regarding the die as a rigid body at a forging cycle and analyzed the die stress by loading the die with the pressure from the deformed part. On the other hand, the second approach is adopted as that the die and workpiece are simultaneously considered as deformable bodies during contact. Since calculation of the elastic strain and deformation amount of tool can be made at each formation step by the second approach, the elastic deflection of die acts upon the dimension of forged part at each formation step. The second approach relatively takes more time for the simulation. However, the analysis result at the elastic assumption of tool is better for predicting not only the elastic strain of tool but also the dimensional accuracy of forged part.

A study of the hydrostatic extrusion of copper-clad aluminium tube

Abstract This paper is concerned with the hydrostatic extrusion process of copper-clad aluminium tube. In this study, the rigid-plastic finite-element method is used to analyze the steady-state extrusion process of the bimetal tube. Simulations are performed for copper-clad aluminium tube to give the distributions of the effective strain rate, equivalent stress and hardness for some extrusion ratios and die angles. In addition, the interface profile of the bimetal tube is predicted by tracking a particle path at the interface in the Eulerian domain. Experiments are carried out for copper-clad aluminium tube at room temperature. It is found that the finite-element predictions are generally in good agreement with the experimental observations. Detailed comparisons of the extrusion loads and interface profiles by the finite-element method with those from experiment are given.

A study on the process to control the cavity and the thickness distribution of superplastically formed parts

The use of superplastically formed Al alloy components could save cost and reduce weight in aerospace structures. However, problems related with non-uniform thickness and cavitation occur during superplastic deformation. In the case of hemisphere forming part, the thickness becomes much thinner and the cavitation increased exponentially as closed to the pole. In order to unify the thickness and to decrease the cavity, the thickness distribution of sheet was controlled by using the rotary forging. The sheet is preformed into the cone type and become the thickest in the center. The hemisphere formed part using the preformed sheet has uniform thickness and also lower cavity than that of original sheet. It is found that cavity is increased rapidly when the strain reaches the critical value. Also discussed are that the critical strain is closely related to shapes and positions in the vicinity of particles and at triple point of grain boundary.

Experimental investigation into effect of annealing treatment on springback of magnesium alloy sheets

Abstract The purpose of this work is to experimentally investigate the effect of annealing treatment on the springback at room temperature for AZ31 magnesium alloy sheets, which were produced by rolling in a reversible warm mill after twin roll strip casting. Microstructure evolutions were investigated using optical microscopy after annealing at temperatures ranging from 350 to 450°C. Both tension and compression tests were performed at room temperature for the annealing treated samples to show the influence of annealing treatment on the asymmetric yielding behaviour of AZ31 Mg alloy sheets. In order to evaluate the springback in the stamping process, the V shaped air bending tests were performed and the angle changes after springback were measured. The tensile and compressive yield stresses were deteriorated because of the coarsening of grain size with increasing annealing temperature. The yielding behaviours of tension and compression show a large asymmetry, as well as the Hall–Petch slope. The Hall–Petch slope of the compressive yield stress is larger than that of the tensile yield stress. The springback angle in the V shaped air bending tests was decreased with increasing annealing temperature.

Development of constitutive equations for plastic deformation of a porous material using numerical experiments

Abstract New constitutive equations for plastic deformation of a porous material are developed by using numerical experiments. A micro-mechanical unit problem such as growth of an isolated void in incompressible matrix is formulated to represent the instantaneous macroscopic response. The micro-and macro-variables in two different length scales are introduced and connected. The strain hardening, incompressible, viscoplastic behavior of matrix material in a micro-scale is assumed with a state variable model and the unit problems are solved using finite element methods to give a macroscopic compressible plastic response at a material point in a larger length scale. Systematic variations in the macroscopic quantities are obtained by changing the boundary/initial conditions in the unit problems. Such variations provide families of the curves defining the plastic behavior of a porous material and those curves are collapsed into two master curves by scaling procedures. The scaling relations and the forms of those master curves motivate the mathematical structures of the constitutive equations for the plastic behavior of a porous material. Detail comparisons of the proposed constitutive equations to the previous ones are given in terms of shearing viscosity and volumetric viscosity.

Fracture criterion for AZ31 Mg alloy plate at elevated temperature

The fracture criterion was characterized for an AZ31 Mg alloy plate with the 3.0 mm thickness at the elevated temperature of 250 °C in this work. In order to properly characterize the fracture criterion, its mechanical properties were also characterized. As for mechanical properties, simple tension tests were performed to calibrate the Hill1948 yield function. Also, in order to account for the hardening deterioration (softening) behavior beyond the uniform deformation limit, the flow curves of the Mg alloy plate were numerically obtained based on the inverse calibration method, in which strain rate sensitivity was also considered. As for the fracture criterion, effective fracture strains, which are dependent on stress triaxiality and deformation paths, were numerically characterized utilizing experimental data based on specimens with four different shapes newly developed. For comparison purposes, empirical fracture criteria such as the Cockcroft-Latham, Brozzo, Ayada and Clift models were also calibrated. For validation purposes, the five fracture criteria were applied for a real part (an EL-cover) drawing case and the result confirmed that the fracture criterion developed in this work performed best among the five models tried out.

Fracture behavior of thixoformed 357-T5 Al alloys

The effects of microstructural features on the fracture behaviors, including impact, high-cycle fatigue, fatigue crack propagation, and stress corrosion cracking, of thixoformed 357-T5 (Al-7 pct Si-0.6 pct Mg) alloy were examined. The resistance to impact and high-cycle fatigue of thixoformed 357-T5 tended to improve greatly with increasing volume fraction of primary α. An almost threefold increase in impact energy value was, for example, observed with increasing volume fraction of primary α from 59 to 70 pct. The improvement in both impact and fatigue properties of thixoformed 357-T5 with increasing volume fraction of primary α in the present study appears to be related to the magnitude of stress concentration at the interface between primary α and eutectic phase, by which the fracture process is largely influenced. The higher volume fraction of primary α was also beneficial for improving the resistance to stress corrosion cracking (SCC) in 3.5 pct NaCl solution. The in-situ slow strain rate test results of thixoformed 357-T5 in air and 3.5 pct NaCl solution at various applied potential values demonstrated that the percent change in tensile elongation with exposure decreased linearly with increasing volume fraction of primary α within the range studied in the present study. Based on the fractographic and micrographic observations, the mechanism associated with the beneficial effect of high volume fraction of primary α in thixoformed 357-T5 alloy was discussed.

Effect of Thermo-Mechanical Treatment on the Interface Microstructure and Mechanical Properties of a STS-Al-Mg 3-ply Plate

In this research, a series of secondary warm rolling was performed under various process conditions in order to evaluate the influence of thermo-mechanical treatment on the interface microstructure and subsequent mechanical properties of roll-bonded STS/Al/Mg 3-ply laminates. The variation of interfacial microstructure was observed by scanning and transmission electron microscopes equipped with energy dispersive X-ray detector. Uniaxial tensile, hardness and drum peel tests were then carried out to clarify major mechanical properties of STS/Al/Mg clad sheets. We revealed that annealing for 60 min followed by secondary warm rolling at 300 °C under the reduction ratio of 20% could effectively improve the bonding strength at the Al/Mg joint and the tensile properties of roll-bonded STS/Al/Mg 3-ply laminates. †(Received November 5, 2012)

Dimensional Changes and Residual Stress of Spur Gear According to the Manufacturing Processes -Comparison of Cold Forging Part with Machining Part-

The high dimensional accuracy of the cold forged part could be acquired by the accurate dimensional modification for the die, which is, the dimensional changes from the die through forged part to final part after heat treatment were considered. The experimental and FEM analysis are performed to investigate the dimensional changes from the die to final part on cold forged part, comparing with the machined gear. The dimension of forged part is compared with the die dimension at each stage, such as, machined die, cold forged part, and heat-treated-part. The elastic characteristics and thermal influences on forging stage are analyzed numerically by the . The analyzed residual stress of forged part is considered into the FE-analysis for heat treatment using the . The effects of residual stress affected into the dimensional changes could be investigated by the FEA. Each residual stress of gears was measured practically by laser beam type measurement.

Experimental and finite element analysis to predict the dimensional changes of a cold-forged spur gear

Abstract The dimensional tolerance of spur gears for automobiles is within the range of several micrometres. Therefore, to manufacture these gears by cold forging, dimensions should be controlled in the same range. The dimensional changes by the various forging variables should be analysed and compensated into the forging tool. Computer-aided engineering (CAE) would be appropriate for investigating the effects of variables. However, the realistic finite element modelling (FEM) techniques should be confirmed and used to predict the dimension of the forged part in the order of micrometres. As realistic modelling considers the elastic characteristics of both workpiece and die at unloading and ejecting stage, threedimensional analysis for spur gears may often encounter difficulties in solving time and convergence. In this study, dimensional changes for cold-forged parts of spur gears were investigated by experimental and FEM analysis, after modelling to obtain desirable FEM results had been studied by closed-die upsetting. The dimensional changes of involute curve for the spur gear can be analysed from the FEM results during the whole forging stage; and give an index of a modified involute curve of the forging tool for improving gear profile accuracy.