• Proceedings of the Korean Society for Technology of Plasticity Conference
• /
• 2001.10a
• /
• pp.285-288
• /
• 2001

Hypereutectic Al-25Si-X alloys, expected to be applied to the cylinder-liner-part of the engine-block of an automobile due to the excellent wear resistance, low density and low thermal expansion coefficient has been fabricated through a spray forming process. The obtained microstructure of the hypereutectic Al-25Si-X alloy appeared to consist of Al matrix and equiaxed Si particles of average diameter of $5-7{\mu}m$. To characterize the deformation behavior of this alloy, a series of load relaxation and compression tests have been conducted at temperatures ranging from RT to $500^{\circ}C$. The strain rate sensitivity parameter (m) of this alloy has been found to be very low (0.1) below foot and reached 0.2 at $500^{\circ}C$. During the deformation above 300'c in compression, strain softening has been observed. The diagram of extrusion pressure vs. ram-speed has been constructed, providing the extrusion condition of Al-25Si-X alloys.

• Corrosion Science and Technology
• /
• v.9 no.3
• /
• pp.109-115
• /
• 2010

Interstitial-free (IF) steels are widely used for car body material. However, a few types of streaky mark defect are commonly found on hot dip galvannealed (GA) IF steel sheets. In the present study, both the phase structure of a streaky mark defect and the microstructure of the substrate just below it were characterized by optical microscopy (OM) and scanning electron microscopy (SEM). It was found that the bright streaky mark area was composed of ${\delta}$ phase while the dark normal area was full of craters. More than half of the grains at the uppermost surface of the substrate just below the streaky mark defect are unrecrystallized grains which could result from lower finish rolling temperature during hot rolling and be kept stable during the annealing process, while almost all the grains in the normal area are equiaxed grains. In order to confirm the effect of the unrecrystallized grains on the coating morphology, hot dip galvannealing simulation experiments were carried out in IWATANI HDPS. It is proved that the unrecrystallized grains accelerate the Fe-Zn reaction rate during galvannealing and result in a flatter coating surface and an even coating thickness. Finally, a formation mechanism of the streaky mark defect on the hot dip galvannealed IF steel sheet was discussed.

• Proceedings of the Korean Society for Technology of Plasticity Conference
• /
• 2001.05a
• /
• pp.20-23
• /
• 2001

Hypereutectic Al-25Si alloy, which is expected to be applied to the cylinder-liner-part of the engine-block of an automobile due to its excellent wear resistance, low density and low thermal expansion coefficient, has been fabricated through a spray forming process. The obtained microstructure of the hypereutectic Al-25Si alloy appeared to consist of Al matrix and equiaxed Si particles of average diameter of 5-7 mm. To characterize the deformation behavior of this alloy, a series of load relaxation and compression tests have been conducted at temperatures ranging from RT to $500^{\circ}C$. The strain rate sensitivity parameter (m) of this alloy has been found to be very low (0.1) below $400^{\circ}C$ and reached maximum value of about 0.2 at $500^{\circ}C$. During the deformation above $300^{\circ}C$ in compression, strain softening has been observed. The diagram of extrusion pressure vs. ram-speed has been constructed. The extrusion has been successfully conducted at the temperatures of $300^{\circ}C$ and above with the ratio of area reduction of 28 and 40 in this study.

• Proceedings of the KWS Conference
• /
• 2002.10a
• /
• pp.516-521
• /
• 2002

A study was carried out to grow an understanding of the microstructural development of friction stir welds on an AZ91D magnesium alloy, and to evaluate the mechanical properties of the welds. AZ91D plates with the thickness of 4mm were used, and the microstructural development of the weld zone was investigated using optical and scanning electron microscopes. Square butt welding joint with good quality was obtained at the conditions of under 187mm/min of travel speed with 1100 to 1250 rpm of tool rotation speed. The microstructure within the weld region consisted of fine equiaxed grains with no evidence of the original dendritic structure. The hardness tests showed slightly increased harness in the weld region, and the minimum hardness measured is in that of the parent material. Tensile strength of the weld zone was remarkably improved due to very fine recrystallized structure. XRD pattern of weld zone revealed the removal of $\beta$ intermetallic compounds, $Mg_{17}$Al$_{12}$, which had been distributed in the base metal.l.

• Journal of the Korean Ceramic Society
• /
• v.42 no.8 s.279
• /
• pp.537-541
• /
• 2005

Silicon nitride is one of the most successful engineering ceramics, owing to a favorable combination of properties, including high strength, high hardness, low thermal expansion coefficient, and high fracture toughness. However, the impact damage behavior of $Si_3N_4$ ceramics has not been widely characterized. In this study, sphere and explosive indentations were used to characterize the static and dynamic damage behavior of $Si_3N_4$ ceramics with different microstructures. Three grades of $Si_3N_4$ with different grain size and shape, fine-equiaxed, medium, and coarse-elongated, were prepared. In order to observe the subsurface damaged zone, a bonded-interface technique was adopted. Subsurface damage evolution of the specimens was then characterized extensively using optical and electron microscopy. It was found that the damage response depends strongly on the microstructure of the ceramics, particularly on the glassy grain boundary phase. In the case of static indentation, examination of subsurface damage revealed competition between brittle and ductile damage modes. In contrast to static indentation results, dynamic indentation induces a massive subsurface yield zone that contains severe micro-failures. In this study, it is suggested that the weak glassy grain boundary phase plays an important role in the resistance to dynamic fracture.

• Journal of Korea Foundry Society
• /
• v.31 no.6
• /
• pp.354-361
• /
• 2011

The liquid metal shearing device was constructed and assembled with a commercial high-pressure die-caster in order to induce intensive turbulent shearing force on molten aluminum alloys. The effect of the liquid metal shearing on the microstructure and tensile properties of A356 alloys was investigated with the variation of iron content. The experimental results show that dendritic primary ${\alpha}$-Al phase was effectively modified into a equiaxed form by the liquid metal shearing. It was also found that the needle-like ${\beta}$-AlFeSi phase in a Fe containing A356 alloy was changed into a blocky shape resulting in the improved mechanical properties. Based on the mechanical properties, it was suggested that the iron content in A356 alloy could be more widely tolerated by utilizing the liquid metal shearing HPDC process.

• Transactions of Materials Processing
• /
• v.13 no.1
• /
• pp.53-58
• /
• 2004

The microstructural changes of Al-Zn-Mg-Cu alloy containing Sc during hot extrusion and post heat treatment were investigated. Two kinds of Al-Sc alloys with different alloying elements (B1, B2) were hot extruded to make T-shape bars at extrusion temperature of $380^{\circ}C$, then the bars were solution treated at $480^{\circ}C$ for 2hrs followed by artificial aging at $120^{\circ}C$ for 24hrs. The interior microstructure of as extruded bar consisted of elongated grains, however, fine equiaxed grains were also observed around surface. The microstructural gradient suggested that different restoration process could proceed during the hot extrusion. For B1 and B2, different grain growth behaviors were found around the surface during the post heat treatment. Rapid grain growth behavior was observed for B1 around the surface, however, it was not observed for B2. Orientation pinning, which was related with the evolution of preferred orientation, and precipitation were thought to be responsible for the rapid grain growth.

• Korean Journal of Materials Research
• /
• v.17 no.7
• /
• pp.361-365
• /
• 2007

A deoxidized low-phosphorous copper processed by eight cycles of accumulative roll-bonding (ARB) was annealed at various temperatures ranging from 100 to $400^{\circ}C$. The annealed copper was characterized by transmission electron microscopy (TEM) and tensile & hardness test. TEM observation revealed that the ultrafine grains developed by the ARB still remained up to $350^{\circ}C$, however above $400^{\circ}C$ they were replaced by equiaxed and coarse grains due to an occurrence of the static recrystallization. The hardness of the copper decreased slightly with the annealing temperature up to $350^{\circ}C$, however they dropped largely above $400^{\circ}C$. Annealing characteristics of the copper were compared with those of an oxygen free copper processed by ARB and subsequently annealed.

• Journal of Korea Foundry Society
• /
• v.30 no.4
• /
• pp.130-136
• /
• 2010

Sulfide segregation behavior, characteristics of solidification microstructure and compositional distribution in the riser/castings junction of heavy-section main bearing support (MBS) steel castings were investigated; Sulfide streaks of A segregation were formed in the transitional region from columnar grain to coarse equiaxed grain and floated with aggregation of the dendritic free crystal. Solute segregation behaviors of elements Si, P and S were V shape negative segregation from the bottom of the castings to upper part of the riser with the reference of vertical center-line of the specimen block. Those of elements C and Mn were V shape negative segregation in the main body and A shape positive segregation in the riser of the casting. Just beneath the pipe shrinkage in the riser segregation ratio of each element was the highest, and that of S was 3.6 times higher, C 3.3 times, P 2.1 times, Si 1.6 times and Mn 1.0 times respectively. [Mn/S] ratio of the specimen block was distributed in the wide range of 20~275.

• Journal of Powder Materials
• /
• v.19 no.4
• /
• pp.317-321
• /
• 2012

In this study, Cu-5Ni-10Sn(wt%) spinodal alloy was manufactured by gas atomization spray forming, and the microstructural features and mechanical properties of Cu-5Ni-10Sn alloy have been investigated during homogenization, cold working and age-hardening. The spray formed Cu-5Ni-10Sn alloy consisted of an equiaxed microstructure with a mixture of solid solution ${\alpha}$-(CuNiSn) grains and lamellar-structure grains. Homogenization at $800^{\circ}C$ and subsequent rapid quenching formed a uniform solid solution ${\alpha}$-(CuNiSn) phase. Direct aging at $350^{\circ}C$ from the homogenized Cu-5Ni-10Sn alloy promoted the precipitation of finely distributed ${\gamma}$' or ${\gamma}-(Cu,Ni)_3Sn$ phase throughout the matrix, resulting in a significant increase in microhardness and tensile strength. Cold working prior to aging was effective in strengthening Cu-5Ni-10Sn alloy, which gave rise to a maximum tensile strength of 1165 MPa. Subsequent aging treatment slightly reduced the tensile strength to 1000-1100 MPa due to annealing effects.