Dae-Hoon Ko

Cyclic fatigue and torsional resistance of two new nickel-titanium instruments used in reciprocation motion: Reciproc versus WaveOne.

INTRODUCTION The use of reciprocating movement was claimed to increase the resistance of nickel-titanium (NiTi) file to fatigue in comparison with continuous rotation. Recently 2 new brands of NiTi files have been marketed for use in a RM mode. The purpose of this study was to compare the cyclic fatigue resistance and torsional resistance of these 2 files, Reciproc and WaveOne. METHODS Cyclic fatigue test with a simultaneous pecking motion was performed with the instrument (n = 10 each) operating in the recommended reciprocation motion until fracture for the Reciproc R25 and WaveOne Primary files. ProTaper F2 was tested in continuous rotation to serve as a control for comparison. The number of cycles to fracture (NCF) was determined by measuring the time to fracture. The length of the fragment was measured and the fracture surface was examined by using scanning electron microscopy. Torsional strength was measured by using a torsiometer after fixing the apical 5 mm of the instrument rigidly. Statistical analysis was performed by using one-way analysis of variance. RESULTS The results showed that Reciproc had a higher NCF and WaveOne had a higher torsional resistance than the others. Both reciprocating files demonstrated significantly higher cyclic fatigue and torsional resistances than ProTaper (P < .05). The fractographic analysis showed typical features of cyclic fatigue and torsional failure for all instruments. CONCLUSIONS The 2 brands of NiTi file for use with a reciprocation motion seem to have superior mechanical properties.

METHODOLOGY OF PREFORM DESIGN CONSIDERING WORKABILITY IN METAL FORMING BY THE ARTIFICIAL NEURAL NETWORK AND TAGUCHI METHOD

Abstract This study describes a new method of preform design in multi-stage metal forming processes considering workability limited by ductile fracture. The finite element simulation combined with ductile fracture criterion has been carried out in order to predict ductile fracture. The artificial neural network (ANN) using Taguchi method has been implemented for minimizing objective functions relevant to the forming process. The combinations of design parameters used in finite element simulation are selected by an orthogonal array in a statistical design of experiments. The orthogonal array and the results of simulation are used as training data of ANN. A cold heading process is taken as an example of designing a preform which does not form any fracture in the finished product. The results of analysis to validate the proposed design method are presented.

A study on the optimal tool shape design in a hot forming process

Abstract The optimization technique is applied widely in metal forming processes for various objectives, fold-over and excessive die forces, etc. The aim of this study is to design the optimal tool shape in metal forming processes for the improvement of the mechanical properties of the product by a uniform microstructure. Thus, this study presents a method for designing a tool shape based on uniform microstructure distribution with an optimization technique and the finite element method. The tool shape is controlled using a Bezier curve to define the boundary of the tool. The method proposed in this study is demonstrated with an extrusion die design and preform shape design for uniform microstructure distribution. Finally, to show the validity and effectiveness of the result of this study, an experiment in hot extrusion was performed, and the result of the experiment compared with that of the simulation.

Improvement of Weldment Properties by Hot Forming Quenching of Friction Stir Welded TWB Sheet

The purpose of this study is to improve the mechanical properties and formability of friction stir welded tailor-welded blanks (TWBs) of Al6061 alloy with a new forming method called hot forming quenching (HFQ) in which solid-solution heat-treated aluminum sheets are formed at elevated temperature. Forming and quenching during HFQ are simultaneously performed with the forming die for the solid-solution heat-treated sheet. In this study, specimens of aluminum TWBs were prepared by friction stir welding (FSW) with a butt joint. The effectiveness of FSW joining was evaluated by observation of the macrostructure for different sheet thicknesses. In order to evaluate the formability of TWBs by HFQ, a hemisphere dome stretching test of the limit dome height achieved without specimen failure was performed with various tool temperatures. A Vickers test was also performed to measure weldment hardness as a function of position. The formability and mechanical properties of products formed by HFQ are compared with those formed by conventional forming methods, demonstrating the suitability of HFQ for sheet metal forming of friction stir welded TWBs.

Development of a partition panel of an Al6061 sheet metal part for the improvement of formability and mechanical properties by hot forming quenching

In this study, the hot forming quenching process was investigated to improve the deficiencies that arise in materials subjected to conventional cold stamping, such as low formability and undesirable mechanical properties. The hot forming quenching process was mainly discussed in terms of formability and mechanical properties in this study and was first evaluated by preliminary tests. To examine formability, an evaluation was conducted using hot-tensile and hemispherical-dome stretching tests at temperatures of 350°C and 450°C, respectively. In addition, the mechanical properties of the formed part were predicted using quench factor analysis, which was based on the cooling temperature during the die quenching process. These preliminary test results were then used to predict the formability and hardness of the partition panel of an automotive part, where the analytical results indicated high performance of the hot forming quenching process, in contrast to conventional forming. Finally, the hot forming quenching experiment of the partition panel was carried out to validate the predicted results and the obtained formability and hardness values were compared with conventional forming at room temperature using T4 and T6 heat-treated sheets. The analytical and experimental results indicate that the hot forming quenching process is a very effective method for obtaining desirable formability and mechanical properties in the forming of aluminum sheets.

Relief of Residual Stress and Estimation of Heat-Treatment Characteristics for Al6061 Alloy by Cryogenic Heat Treatment

The purpose of this study is to relieve the residual stress of Al6061 using cryogenic heat treatment. Experimental T6 and cryogenic heat treatments were carried out to define the convective heat-transfer coefficient, which was then applied in the finite-element method (FEM) to predict the residual stress. The predicted residual stress was compared with the residual stress measured by X-ray diffraction (XRD), and the results were in good agreement. The mechanical properties were estimated by measuring the electrical conductivity and hardness. In addition, the size and formation of the precipitations were observed by TEM and XRD analysis for both T6 and cryogenic heat treatments. The effects of the cryogenic heat treatment on the residual stress, mechanical properties, and precipitation of Al6061 alloys were thus confirmed.

Reduction of Grain Growth for Al6061 Alloy by the Die Cooling System in Hot Extrusion Process

In this study, die cooling system using the nitrogen gas has been applied to hot aluminum extrusion process for refining grains and reducing of grain growth. Computational fluid dynamics(CFD) has been carried out to evaluate die cooling effect by nitrogen gas, and the results of CFD have been used to FE-simulation for the prediction of the extrudate temperature in hot extrusion process. Experimental hot extrusion has been performed to observe microstructure and to measure temperature of extrudate. The results of FE-Simulation have been good agreement with those of experiment. Finally, process condition of hot extrusion can be established to reduce grain growth of Al6061 through the experiment.