Young Hoon Moon

Effect of tool temperature on the reduction of the springback of aluminum sheets

Abstract The effect of tool temperature on the reduction of springback amount of aluminum 1050 sheet has been investigated in this study. As the springback phenomenon is caused by elastic recovery of deformed sheet, the control of elastic recovery is important in decreasing the amount of springback. Therefore, any combination of tool temperature that can reduce elastic recovery can be effective in reducing the amount of springback. The experimental verification of U-bent aluminum 1050 sheet shows that the combination of hot die and cold punch can reduce the amount of springback up to 20% when compared to conventional room temperature bending test.

Finite element investigation on spring-back characteristics in sheet metal U-bending process

Abstract The most prominent feature of sheet metal forming process is an elastic recovery phenomenon during unloading which leads to spring-back and sidewall curl. Accordingly, it has been a crucial research subject for designing bending tools securing the dimension precision of products. However, the elastic recovery is not only associated with various tool parameters but also hard to predict accurately owing to the complex material deformation behavior. This paper aims at the numerical investigation on spring-back characteristics to the major process parameters. For this goal, we employ the updated Lagrangian thermo-elastoplastic finite element method to a plane-strain sheet metal U-bending process. A description of process formulation and finite element approximation is also presented.

Estimation of hole flangeability for high strength steel plates

Abstract Hole flanging experiments are performed on flat circular plates with a hole in the center to investigate the fracture and lip shape behaviors of TRIP steels and ferrite–bainite duplex (F+B) steels. The conventional hole flanging process is limited to a certain limit hole diameter below which failure will ensue during the hole expansion. In the hole flanging, deformation by lip or fracture by petalling may occur when sheets are struck by punches of various shapes and high circumferential strains are induced in the deforming material. Fracture of the plate during hole flanging is mainly caused by lip fracture that results from multiple localized neckings that take place around the hole periphery where straining is most severe. F+B steel has better hole flangeability than TRIP steel in the viewpoint of minimum hole diameter, while in the viewpoint of lip shape accuracy and lip height TRIP steel has better formability than F+B steel. Both criteria are proved to be valuable indices to estimate the hole flangeability of high strength steel.

Stabilization of oxygen-deficient structure for conducting Li4Ti5O12-δ by molybdenum doping in a reducing atmosphere.

Li4Ti5O12 (LTO) is recognized as being one of the most promising anode materials for high power Li ion batteries; however, its insulating nature is a major drawback. In recent years, a simple thermal treatment carried out in a reducing atmosphere has been shown to generate oxygen vacancies (VO) for increasing the electronic conductivity of this material. Such structural defects, however, lead to re-oxidization over time, causing serious deterioration in anode performance. Herein, we report a unique approach to increasing the electronic conductivity with simultaneous improvement in structural stability. Doping of LTO with Mo in a reducing atmosphere resulted in extra charges at Ti sites caused by charge compensation by the homogeneously distributed Mo6+ ions, being delocalized over the entire lattice, with fewer oxygen vacancies (VO) generated. Using this simple method, a marked increase in electronic conductivity was achieved, in addition to an extremely high rate capability, with no performance deterioration over time.

Tool temperature control to increase the deep drawability of aluminum 1050 sheet

A feasibility study on the tool temperature control to increase the deep drawability of Al-1050 sheet is performed. The conventional deep drawing process is limited to a certain limit drawing ratio (LDR) beyond which failure will ensue. The purpose of this study is to examine the possibilities of relaxing the above limitation through the tool temperature control, aiming towards a process with an increased LDR. The idea which may lead to this goal is strengthening the punch-nose radius part by cold punch which has frequently been potential failure area in cup drawing process, while heating the remainder of the blank to reduce the stress on the cup sidewalls. Over the ranges of conditions investigated, the deep drawability of Al-1050 is found to be strongly sensitive to the temperature of the die and punch. The experimental implementation shows that the tool temperature control is very effective way to promote deep drawability of Al-1050.

Laser surface hardening of AISI H13 tool steel

被尝试答案作为产生的热用 200 W 纤维激光变硬和微观结构的精炼改进表面坚硬并且通过固体穿 AISI H13 工具钢的性质采购原料。激光的坚硬融化了地区被调查。为了识别热的效果，在融化地区的激光上输入，扫描条件被控制。结果显示出那，同样收到的 AISI H13 工具钢的坚硬是近似，在激光表面以后的 Hv 240，和坚硬加热处理在 Hv 480510 附近。变硬的深度和宽度随输入使用了的热的增加被增加。试验性的结果的申请将在 tooling 工业被考虑。

Analytical prediction of springback based on residual differential strain during sheet metal bending

As the springback of sheet metal during unloading may cause deviation from a desired shape, accurately predicting springback is essential for the design of sheet stamping operations. Finite-element models have not been successful in predicting springback; hence there is a need for analytical models to make such predictions. In this study, a model based on differential strains after relief from the maximum bending stress is derived for six different deformation patterns in order to predict springback analytically. The springback for each deformation pattern is estimated by the residual differential strains between outer and inner surfaces after elastic recovery. Each of the six deformation patterns has a valid region of applicability, based on elastic modulus, yield strength, applied tension, and bending geometry. Analytical equations for the springback of the sheet deformed under these six deformation patterns are derived. Traditional analytical models for springback prediction have been based on elastic unloading from a bending moment. Traditional models also require the knowledge of the stress distribution through the thickness of the sheet, whereas the residual differential strain model only requires the stress state on the outer and inner surfaces of the sheet. In order to compare the residual differential strain model with the traditional bending moment model, a bending moment model is derived for the same exact deformation patterns. Results from the two models are compared for various materials.

A feasibility study of the partial squeeze and vacuum die casting process

Abstract A feasibility study of the partial squeeze and vacuum die casting process was performed to make defect-free casting products with excellent mechanical properties. The trial die casting process in this study was industrially implemented for producing a reaction shaft support made of a hyper eutectic Al–15%Si alloy. To combine the squeezing and vacuum effect, the plunger injection system was designed and attached to a chill vent type vacuum machinery system. The combination of the vacuum effect before injection and the squeezing effect after injection resulted in excellent defect-free die casting products. The uniform distribution of fine acicular eutectic and proeutectic silicon obtained from the trial process also provided excellent mechanical properties.

Induction heating of semisolid billet and control of globular microstructure to prevent coarsening phenomena

An important step in the thixoforming process is the induction heating of the raw materials to the semisolid state. Using this technology, the process behavior is satisfactory from the point of view of reproducibility and temperature control. Therefore, the objectives of this study are to define the correct relationship between coil length and billet length for uniform induction heating and to present the optimal reheating conditions suitable for the thixoforming process (or to secure a fine globular microstructure without liquid segregation).The optimal inductive coil on the induction heating process of semisolid billet machined to 76 mm diameter and 90 mm length to reduce the temperature gradient of the billet and to obtain the globular microstructure was theoretically designed and the suitability of the designed coil dimensions verified by reheating experiments.The globular microstructures of semisolid billet in heating and holding processes were controlled to prevent the coarsening phenomena of Al-7%Si-0.3%Mg alloy and to apply to the thixoforming process. In addition, the effects of the history of processing and induction heating parameters such as reheating time, holding time, holding temperatures, capacity of the induction heating system, and adiabatic material size on the globularization were investigated.It was concluded that in the case of a three-step reheating process, the final holding time is the most important factor and 2 min is suitable to maintain a globular microstructure.

Selective laser melting of Fe-Ni-Cr layer on AISI H13 tool steel

Abstract An attempt to fabricate Fe-Ni-Cr coating on AISI H13 tool steel was performed with selective laser melting. Fe-Ni-Cr coating was produced by experimental facilities consisting of a 200 W fiber laser which can be focused to 80 μm and atmospheric chamber which can control atmospheric pressure with N 2 or Ar. Coating layer was fabricated with various process parameters such as laser power, scan rate and fill spacing. Surface quality and coating thickness were measured and analyzed. Three different surface patterns, such as type I, type II and type III, are shown with various test conditions and smooth regular pattern is obtained under the conditions as 10 μm of fill spacing, 50–350 mm/s of scan rate and 40 μm of fill spacing, 10–150 mm/s of scan rate. The maximum coating thickness is increased with power elevation or scan rate drop, and average thickness of 10 μm fill spacing is lower than that of 40 μm fill spacing.