Papers by Author: Jong Won Yoon

Abstract: The CW CO2 laser weldability of various Al-Mg alloys was investigated in terms of weld strength, ductility, Vickers hardness, formability and solidification cracking tendency when both autogenous and wire feed welding were made. Tensile strength of the laser welds was lower than that of the corresponding base metals, and it was linearly proportional to the Mg content of the weld metal. Formability and ductility of Al-Mg alloy laser welds were lower than those of corresponding base alloys as well. As for the weld solidification structure, as the Mg content increased in the weld metal, more equiaxed grains developed. Hence, softening of weld metal and solidification cracking were decreased in the weld metal containing higher Mg. Solidification cracking of Al-Mg alloy laser welds showed the highest susceptibility at 1.6 to 1.9wt.% Mg, and it decreased with further increase of Mg content.

Abstract: The effect of post weld heat treatment on mechanical properties of friction welded Alloy 718 and SNCRW was investigated. Friction welding tests were carried out at a constant rotation speed and pressure. Optimum friction condition was found to be the friction pressure of 25kg/cm2, friction time of 40sec, upset pressure of 80 kg/cm2, and dwell time of 5sec. After friction welding tests, post weld heat treatments were performed in the temperature range of 500-900°C for 8hrs in order to investigate the microstructure and mechanical properties of weld joint. Specimens with the post weld heat treatment at 720°C for 8hrs show optimal mechanical properties. Residual stress of post weld heat treated specimens was measured to weld joint in the same temperature range. After friction welding tests on samples with a diameter of 80mm, tensile properties of post-weld-heat-treated and non-heat-treated samples were compared.

Abstract: Recrystallization and grain growth behavior of alloy 718 casting were investigated to obtain homogeneous microstructure during hot forging. For this purpose, compression tests were carried out for cylindrical specimens at the temperature range of 1000 to 1150°C and the strain rate of 10-1 and 10sec-1. The dynamic recrystallization behavior caused by the hot compression was investigated in terms of the recrystallized area fraction and average grain size. Reheating was followed to the hot compressed samples at the temperature range of 1050 to 1150°C for 100, 600 and 1800sec, and the static recrystallization behavior caused by the reheating was also investigated. As hot deformation temperature increased from 1000 to 1150°C, both the area fraction and average grain size of dynamically recrystallized grains increased. When higher strain rate of 10 sec-1 was used, the area fraction of dynamically recrystallized grains increased substantially, but the average grain size was not affected. When reheating the hot compressed samples at 1050°C for 100, 600 and 1800sec, respectively, microstructural change including grain growth was not noticed. On the other hand, when reheating the samples at higher temperatures, 1100°C and 1150°C, both the area fraction and the average grain size of the statically recrystallized grains increased considerably as the holding time increased from 100 to 1800sec.

Abstract: 2 mm thick 6061-T6 aluminum alloy sheets were I square butt welded using 3kW Nd:YAG laser. Filler wires of 1 mm diameter, 5183A(Al-4wt.%Mg), 4043A(Al-5wt.%Si) and 4047A (Al-12wt.%Si) were used. The welds made with 4047A wire showed the lowest solidification cracking among the welds investigated. Abundant amount of Al-12wt.%Si eutectic which was observed at the grain boundaries of the 4047A wire feed welds was closely related with the reduced solidification cracking susceptibility. Yield and tensile strength, and formability of the welds made with 4047A wire were improved compared to the welds made with other filler wires, which is attributed to the reduced cracking susceptibility in the welds.

Abstract: Effect of silicon content on the creep properties of Ti-6Al-4Fe-xSi was studied. Creep resistance of Ti-6Al-4Fe-xSi alloys was superior to that of Ti-6Al-4V. Ti-6Al-4Fe-0.5Si alloy exhibited the highest rupture strength and creep resistance among the Ti-6Al-4Fe-xSi alloys investigated. The minimum creep rate of the alloys decreased with increasing silicon content up to 0.5wt.% and then it increased again when the silicon content was higher than 0.5wt.%. TiFe precipitates were formed mainly at the β phase area of Ti-6Al-4Fe-xSi alloys by consuming titanium and iron in β phase, when the alloys were thermally exposed at 500 and 600°C during the creep test. During the creep test, microvoids were induced at the TiFe/α phase interfaces and the cracks were formed along the TiFe/α phase interfaces by the coalescence of the voids. Those cracks were finally connected each other through the α phase.

Abstract: A new high strength titanium alloy system with low cost alloying elements, such as Al, Fe, has been recently developed. In present study the expensive V was replaced with Fe, and Si was added from 0 to 7.5wt.%. The effect of Fe and Si on the microstructure and tensile properties of Ti-6Al-4Fe-xSi (x=0, 0.1, 0.25, 0.5, 0.75wt.%) alloys was investigated. The room and high temperature mechanical properties of Ti-6Al-4Fe alloys were better than those of the Ti-6Al-4V. It was mainly due to the phase boundary strengthening at ambient and high temperature. The strength and elongation of the developed alloys depended upon the Si contents. The Si elements made the grain boundary and colony size fine, and increased the strength of the developed alloys by solid solution and precipitation hardening. The tensile strength variation with the Si contents at room temperature and 400°C, and at 450°C and 500°C showed a similar behavior, respectively.