Yong Nam Kwon

Analysis on Void Closure Behavior during Hot Open Die Forging

Internal voids have to be eliminated for defect-free in some open die forging. The FEM analysis is performed to investigate the overlap defect of cast ingots during cogging stage. The measured flow stress data were used to simulate the cogging process of cast ingot using the practical material properties. Also the numerical analysis of void closure is performed by using the DEFORMTM-3D. The calculated results of void closure behavior are compared with the measured results before and after upsetting, which are scanned by the X-ray scanner. From this result, the criteria for deformation amounts effect on the void closure were estimated into effective strain of 0.6 by the comparison of practical experiment and numerical analysis.

Thermomechanical Behavior of Al-Mg-Si Alloys at the Elevated Temperature

Thermomechanical behavior of Al-Mg-Si alloys has been studied to investigate the effect of microstructural features such as pre-existing substructure and distribution of particles on the deformation characteristics. The controlled compression tests have been carried out to get the basic information on how the alloy responds to temperature, strain amount and strain rate. Microstructural features after forging have been discussed in terms of the thermomechanical response of Al-Mg-Si alloys. As already well mentioned, we have found that the high temperature deformation of Al-Mg-Si brought the recovered structure. However, the abnormally large grains seems to occur in a certain condition as a result of deformation assisted grain growth, which means that hot forging of Al-Mg-Si alloys could lead to the undesirable microstructures and the consequent mechanical properties such as fatigue strength.

Effects of Material Properties on Elastic Characteristics of Cold Forged Part

The dimension of cold forged part is larger than that of the die cavity. The difference could be originated from the various features, such as the elastic characteristics of die and workpiece, thermal influences, and machine-elasticity. Among these features, elastic behavior of die and workpiece has been considered as the most important element for the precision cold forging. In the present study, the effect of material properties of both workpiece and die on the dimensional precision of forged part was investigated using FE and experimental analyses. Three materials, such as SCM420H, Cu-OFHC and Al6061 alloy, were used to divide the effect of the elastic modulus and flow stress on the dimension specifically. From the results of FEA, the elastic deformation of die was found to be more dominant than that of workpiece. The die expansion depends on the flow stress of workpiece during a loading stage. On the other hand, the die contraction during unloading was affected by both the flow stress and elastic modulus of workpiece. The elastic modulus of workpiece affects the elastic recovery of forged part after ejecting stage.

Formability Enhancement of Al-Mg Sheet by Using Hydro-Mechanical Forming

Hydro-mechanical forming of Al sheet was investigated in the present study in order to fabricate automotive part having a complicated shape using Al 5052 alloy with a conventional formability which has been known to be quite lower compared to deep drawing steels. We have designed the hydro-mechanical forming in which Al sheet was drawn to a kind of preform step following gas blow forming for final accurate geometry. In order to judge a formability enhancement of Al sheet in terms of forming process, model geometry came from a practical automotive part which had quite depth with complicated curvatures, which was proven that a single sheet forming could not gave a successful part. Experimentally, we succeeded to make the model part with accurate dimension. The optimum forming conditions for respective forming steps were considered most important technical features of this hydro-mechanical. Also, the effort to avoid detrimental microstructure evolutions was given and discussed for a practical application.

Accommodation of Grain Boundary Sliding in AZ31 Alloy

Mg alloys could be the lightest alloys among the industrially applicable engineering alloys. Since wrought Mg alloy has limited applications due to the poor formability, casting is currently the main processing technique to fabricate Mg components even though wrought alloys are superior in terms of mechanical properties and reliability. While a lot of research and development has been focused on warm forming under the temperature condition of around 250°C where more formability could be expected, superplastic forming could be another way to get over the low formability of Mg alloys. Like other superplastic materials grain boundary sliding is the main deformation mechanism of Mg superplasticity. Accommodation of stress concentration around triple point of grain boundary should be done favorably if grain boundary sliding continues without any fracture. In the present study, superplastic behavior of AZ31 alloys with several grain sizes was examined firstly. Accommodation of grain boundary sliding of AZ31 alloy would be discussed on the basis of grain morphology and texture evolution after tensile deformation.

Plastic Deformation Behavior of Micro-Alloyed Cold Forging Steel

Micro-alloyed cold forging steel has been developed to rule out heat treatment process before forging in order to save energy consumption. These non heat-treated cold forging steels utilize the work hardening during cold working to ensure the mechanical properties. In other words, the required strength of forged part is achieved by work hardening with the accumulation of plastic strain during the cold working. Therefore, the plastic deformation characteristics should be carefully understood for successful process design. Evolution of both microstructure and plastic characteristics of micro-alloyed cold forging steel has been investigated in the present study. For the optimization of forging processes, finite element analysis and die life predictions were carried out.

Warm Forging Characteristics of AZ31 Alloy

Industrial application of magnesium alloys has increased significantly recently. However, wrought magnesium alloys still have a lot of technical challenges to be solved for more applications. First of all, low formability of wrought alloys should be improved by optimizing the processing variables. In the present study, the effect of process variables such as forging temperature and forging speed were investigated to forgeability of magnesium alloy. To understand the effect of process variables more specifically, both numerical and experimental works have been carried out on the model which contains upsetting geometry. Forgeability of AZ31 alloy was found to depend more on the forging speed rather than temperature. Forged sample showed a significant activity of twinning, which was found to be closely related with flow uniformity.