Ha Sik Kim

A strong cutting plane algorithm for the robotic assembly line balancing problem

We consider the problem of assigning assembly tasks, parts and tools on a serial robotic assembly line so that the total number of robot cells required is minimized while satisfying the various constraints. In this problem, we need to consider the limited space to store the parts and the tool capacity of the robot hand in addition to the precedence restriction between tasks and cycle time requirements usually found in the simple assembly line balancing problem. We give an integer programming formulation of this problem and a strong cutting plane algorithm to solve it. Computational experiences on some real world problems and randomly generated problems are reported.

Prediction of grain size and yield strength of Mg-7Sn-1Al-1Zn alloys extruded at various temperatures and speeds

This paper analyzed the relationships between the extrusion conditions, grain size, and yield strength of an Mg-7Sn-1Al-1Zn alloy, which was extruded at different initial billet temperatures and ram speeds, and developed empirical models to predict the grain size and yield strengths. The results revealed that grain size increases as the extrusion temperature and ram speed increase, resulting in a decrease of the tensile and compressive yield strengths due to reduced effect of grain boundary strengthening. It was also found that the exit temperature is a key determinant of grain size and yield strength, i.e. as the exit temperature decreases, the grain size decreases while the tensile and compressive yield strengths increase. The grain size and yield strength prediction models, which provide results as a function of temperature and strain rate, were developed by considering the extrusion temperature and speed dependencies of the exit temperature, and the predicted results showed a good agreement with the experimental data.

Extrudability and Tensile Properties of the Mg-7Sn-1Al-1Zn Alloy

The extrudability and mechanical properties of the Mg-7Sn-1Al-1Zn (TAZ711) alloy were investigated by performing indirect extrusion at various temperatures and speeds and tensile tests and comparing the results with those of the Mg-8Al-0.5Zn (AZ80) alloy. When the AZ80 alloy was extruded at a high extrusion temperature of 450 °C or at extrusion temperatures above 350 °C with a high extrusion speed of 12 m/min, surface hot cracking occurred, while the TAZ711 alloy was successfully extruded without any surface defects at a high temperature of 450 °C with a high speed of 12 m/min. With an increasing extrusion temperature and speed, the average grain size of the extruded AZ80 and TAZ711 alloys increased and the yield and tensile strength decreased. The TAZ711 alloy had a higher yield strength and lower tensile strength and elongation than the AZ80 alloy extruded under the same conditions. †(Received February 22, 2013)

Oxidation and Corrosion Behavior of Non-Flammable Magnesium Alloys Containing Ca and Y

The combined addition of calcium and yttrium lead to significant increase in non-flammability and tensile properties, compared to conventional Ca-containing magnesium alloys. It means that we can reduce the amount of calcium addition to get certain amount of non-flammability in the alloys containing both calcium and yttrium together. Because the addition of calcium only can increase the ignition temperature but decrease the tensile properties, it should be minimize not to decrease the tensile properties. Corrosion behavior of calcium and yttrium containing alloys were also investigated systematically, and show that they have better corrosion resistance than the any other commercial alloys or calcium containing alloys. Detailed study on the corrosion behavior confirms that there are optimum amount of calcium and yttrium addition to get best corrosion resistance, and corrosion behavior of these alloys are quite different with other alloys.

Investigation of Grain Refinement Method for AZ91 Alloy Using Carbide Inoculation

Grain refinement is an effective method to increase both the strength and toughness of structural materials. Among the various grain refinement methods, carbon inoculation is an effective method for Mg alloys containing Al. SiC is well-known as a carbon inoculation agent; however, conventional methods for using SiC are suitable only for lab-scale experiments. This study investigates the inoculation methods with regard to the use of SiC effectively and economically for grain refinement of Mg alloys, especially for application in large-scale casting processes. Al-SiC refiner was fabricated by extrusion; it demonstrated a uniform structure with well distributed SiC particles on the Al metal matrix. After adding 0.7 wt% extruded SiC refiner, the average grain size of AZ91 alloy decreased from 550 to 90 μm. It demonstrated an effective refining ability in large-scale casting facilities.

Corrosion Properties of Mg–6Al–0.3Mn–aSn–bZn Alloys

The effect of addition of Sn and/or Zn on corrosion properties of Mg–6Al–0.3Mn alloy was investigated systematically by immersion and salt spray tests. In case of immersion test, average corrosion rate increased by addition of 1 wt% Sn but it decreased by addition of 1 wt% Zn. When the half of the amount of Sn addition was replaced by Zn, the average corrosion rate decreased in comparison with the average corrosion rate of Mg–6Al–0.3Mn–1Sn alloy. In case of salt spray test, the change of average corrosion rate according to composition of the alloy was similar to the result of immersion test. It seemed that the change of corrosion properties according to composition of the alloy was strongly related to the microstructural changes including the sort, fraction and morphology of second phase.

Effects of Microstructural Factors on Corrosion Behavior of As-Extruded Mg-Sn-Zn Alloys in Immersion and Salt Spray Environments

The effects of microstructural factors, including the fraction of second phase particles and solute content in the matrix, on the corrosion behavior of as-extruded Mg-Sn-Zn alloys were evaluated by immersion and salt spray testing. After immersion testing, the average corrosion rate increased with the increase in Sn content and decrease in Sn:Zn ratio. On the other hand, after salt spray testing, the average corrosion rate increased with the increase in Sn content, but it was minimized at the Sn:Zn ratio of 5:2. Hydrogen evolution appeared to be the dominant controlling factor of corrosion during immersion, while the partial protectiveness of the film also contributed to the corrosion during salt spray testing.

Properties of AZ31 Magnesium Alloys Subjected to Different Extrusion Processes

The AZ31 magnesium alloys were subjected to indirect and direct extrusion processes, respectively, and the properties of the extruded alloys were compared. It was shown that the indirect-extruded alloy has a lower yield asymmetry between tension and compression than the direct-extruded alloy, which is due to a weaker fiber texture in the former.