Papers by Author: Dae Heon Joo

Abstract: Melt extrusion is a new fabrication process with the characteristics of both casting and extrusion. In this process, a metallic melt which is poured and solidified up to semisolid state in the container can be directly extruded through the die exit to form a product of bar shape without other intermediate processes. The effects of various process parameters, such as melt temperature and extrusion ratio, on the extrusion behavior of Al-Cu alloys in the semisolid state were investigated. Mechanical properties of the melt extruded Al-Cu alloy were measured and its characteristics are compared with the microstructure and copper contents in the alloy. Al-Cu alloys were successfully extruded after squeezing out of liquid during the melt extrusion with smaller force (60 Mpa) compared to the solid extrusion. Al-Cu alloys bar with the mean grain size of 200 μm was fabricated by melt extrusion; however, the mechanical properties have nothing to do with the extrusion ratio. The squeezing out of liquid was get lead to the segregation of the alloys and was affect on the mechanical properties.

Abstract: Melt extrusion is a new fabrication process with the characteristics of both casting and extrusion. In this process, a metallic melt which is poured and solidified up to semisolid state in the container can be directly extruded through the die exit to form a product of bar shape without other intermediate processes. The aging behavior of Al-Cu alloys in the semisolid state was investigated. And the microstructure and mechanical properties of the melt extruded Al-Cu alloy bar were measured and its characteristics are compared with those of a hot extruded Al-Cu alloy bar. Al-Cu alloys were successfully extruded after squeezing out of liquid during melt extrusion with smaller force compared to the solid extrusion. Al-Cu alloys bar with the mean grain size of up to 200 μm was fabricated by melt extrusion process. And the mechanical properties of the melt extruded Al-Cu alloy bar were improved after the T6 treatment.

Abstract: The role of alloying elements in the improvement of the high temperature strength of Al-12Si(CuNiMg) cast alloys used for automotive piston applications was investigated. The addition of alloying elements such as Mn, Cr, Ti and Ge was studied and the detailed characterization of the composition and morphology of the constituent phases after over aging at 350
 for 1000 hrs was performed. The compositions and volume fractions of the equilibrium phases determined by thermodynamic calculation were compared with the experimental results. The addition of transition elements, including Mn, Cr and Ti, increased the volume fraction of the intermetallic phases, which effectively enhanced the high temperature strength of the alloys. Among these transition elements, Mn turned out to be the most effective alloying element. After adding up to 0.5wt% of Mn, a large number of intermetallic phases, α-Al(Mn,Fe)Si as well as fine Al6(Mn,Fe) particles were precipitated and a significant improvement in the elevated temperature properties was achieved. The addition of Ge promoted the precipitation of the θphase (metastable phase, θ-Al2Cu), due to the formation of GeSi precipitates, thereby improved the mechanical properties of the alloy after T6 heat treatment. However, the presence of these GeSi precipitates did not affect the coarsening of the θ phase to form Qphase( Al5Cu2Mg8Si6) during aging and, thus, the elevated temperature properties were not improved by the addition of Ge.

Abstract: The influence of heat treatment on mechanical properties and textures of a meltextruded Al-4.5wt.%Cu alloy bar was studied. And its characteristics are compared with those of a hot-extruded Al-Cu alloy bar. The results show that the mechanical properties of the melt-extruded Al-Cu alloy bar were slightly higher than those of the hot-extruded bar, and the difference in their mechanical properties was narrowed after the T6 treatment. The recrystallization texture was found in the melt-extruded Al-Cu alloy bar, and the fiber texture of the hot-extruded Al-Cu alloy bar was found different from that of the melt-extruded Al-Cu alloy bar.