Selective laser melting (SLM) is one of the most popular advanced manufacturing techniques. SLM is an additive manufacturing (AM) technique that enables production of complicated metal products with high precision and flexibility and acceptable surface finish. It uses a high-intensity laser beam as the energy source to selectively melt metallic powder, which is dictated by 3D CAD data.

SLM enables production of critical parts which are impossible by various traditional metal forming techniques such as casting, forging, extrusion and metal cutting. Unlike the traditional manufacturing techniques, it manufactures 3D objects by gradually adding metal powder layer by layer according to the CAD model. With no need for any dies or tools, it may help to shorten design and production cycles and save time and costs for production.

SLM process parameters affect the multiple quality attributes of SLM-built parts. Thus, it is important to develop a new optimization methodology of SLM process parameters for improving the multiple quality attributes of SLM-built parts. It is a multi-objective optimization (MOO) problem. The multiple quality attributes are converted into a single overall quality index (OQI) using multi-attribute decision-making (MCDM) method, and the MOO problem is converted into a single objective optimization (SOO) one. The optimization results may differ according to the applied MCDM method.

In order to solve this problem, Yang Ji Yon, a post-graduate student of the Faculty of Material Science and Technology, proposed a reasonable process optimization methodology for improving multiple quality attributes of SLM-built parts using integrated OQI (I-OQI) combined with multiple OQIs obtained from multiple MADM methods.

She applied the proposed methodology to optimizing the process parameters such as laser power (LP), scan speed (SS) and overlap rate (OR) for improving five quality attributes of SLM-built AlSi10Mg alloy such as tensile strength, hardness, relative density, volumetric energy density and build rate.

The optimal values of the SLM process parameters obtained from Taguchi and grid search optimization methods were LP 320 W, SS 900 mm/s and OR 0.25.

The proposed methodology could be actively applied to the SLM process optimization of not only Al alloys but also various metal/alloys.

For further details, you can refer to her paper “Process Optimization for Improving Multiple Quality Attributes of AlSi10Mg Alloys Manufactured by Selective Laser Melting Based on Integrated Overall Quality Index” in “Russian Journal of Non-Ferrous Metals” (SCI).

Pyrochlore, which contains some rare metals such as tantalum and niobium and many yttrium group rare earths such as erbium and ytterbium, is of very high commercial value. Therefore, it is very important to efficiently recover valuable metals from pyrochlore.

To date, tantalum-niobium ores such as pyrochlore have been treated by using various methods including alkali digestion, acid leaching and chlorination. At present, NaOH or KOH melting digestion method is not applied to the industry due to its low digestion rate, high energy consumption, high reagent consumption and dangerous operations. The consumption of alkali in the pressure leaching is lower than that in the melting digestion, but it is also not applied to the industry due to its operation difficulties. Concentrates are digested at 120-200℃ with concentrated sulfuric acid in the sulfuric acid leaching, which has some drawbacks such as low digestion rate, complex process and high reagent consumption. In the hydrofluoric acid leaching, concentrates are leached at 90-100℃ with 60% HF. This method is now widely used in the industrial processes due to its high tantalum leaching rate (>85%) and easy production of high-purity tantalum. However, the wastes of about 10-15 tons containing fluorine are produced when 1 ton of concentrates is treated, which leads to serious environmental pollution. What is more, the tantalum leaching rate decreases below 85% when low-grade niobium-tantalum concentrates are treated, so this method causes wastage of resources.

Recently, the KOH sub-molten salt (SMS) leaching has been applied to treating ilmenite, vanadium slag, chromite ore and fergusonite, bringing successful results. This technology has several advantages including low environmental pollution, low reagent consumption, mild degradation conditions and high recovery of valuable metals.

Considering all the preceding studies, Kang Chung Su, a section head at the Institute of Analysis, investigated a new decomposition method of pyrochlore concentrate into high KOH solution in order to effectively recover tantalum and niobium from pyrochlore concentrates while reducing environmental pollution at the same time.

The results showed that when the pyrochlore concentrate with a particle size of less than 75㎛ is added to 80% KOH solution in a mass ratio of KOH to concentrate 3:1, and decomposed at 300℃ for 2h at the stirring rate of 80r/min, 95% decomposition rate of pyrochlore concentrate is guaranteed.

You can find more information in his paper “Study on the Environmental Friendly Decomposition Method of Pyrochlore Concentrate in High KOH Solution” in “Proceedings of KUTIC-2025”.

The gain and phase information are the important characteristic quantities of frequency domain for stability analysis and control design.

Many researchers have used LMI as an important tool for stability analysis and control design. Although LMI has a limitation that the feasibility is not guaranteed in some cases, it has been widely used in various control problems because it does not require specific assumptions.

Pak Ji Min, president of Kim Chaek University of Technology, presented a solution to the multivariable version of new Ψ-control and H^∞/Ψ-mixed control problems with the phase design criterion that enables solving of some control design problems which cannot be achieved by the H^∞-control theory.

First, he derived a new phase condition involving linear matrix inequality (LMI) for multi-input multi-output (MIMO) system. He then proved that the phase condition includes the positive real one for a given system and is guaranteed by the bounded real condition for the corresponding transformed system.

Second, he developed new LMI-based solutions to multivariable Ψ-control for both state feedback and dynamic output feedback. He performed H^∞/Ψ-mixed control design by combining H^∞-control and Ψ-control.

Finally, he illustrated through design examples the significance of phase criterion in the control design, and the effectiveness of the proposed solutions.

For more information, you can refer to his paper “LMI-Based Solution to Multivariable Ψ-Control and H^∞/Ψ-Mixed Control Problems” in “Proceedings of KUTIC-2025”.