Advanced Materials Research Vols. 26-28

Abstract: The initial coarse grains of Ti-50.9at%Ni alloy were refined into submicron grains, small than 0.5 um in size, after eight passes Equal Channel Angular Extrusion (ECAE) at 500°C. Optical microscopy and high temperature DSC tests were applied to investigate recrystallization behavior. It is found that the recrystallization start (Rs) and recrystallization peak (Rp) temperatures of Ti-50.9at%Ni specimens processed by eight passes ECAE are lower than that of the samples processed by one pass ECAE. Ti-50.9at%Ni specimens processed by eight passes ECAE with submicron grains are characterized by higher stability and need less energy to finish recrystallization process.

Abstract: Springback has been measured under various process conditions corresponding to those found in real press-stamping operations. The draw bend tests originally developed at Ohio State University (Carden et al., 2002 [1]) were conducted using a range of tool radii, direct controlled constraining forces, and friction conditions. Especially, the draw-bend test was able to apply precise constant restraining force and almost negligible friction by rotating the cylindrical tool with the same speed of drawn sheet strip. Springback angle changes were measured for the bending and sidewall curl regions to find the dependence on the process variables. As expected, tensile restraining force dominates the springback sensitivity, with higher forces reducing springback. The measured springback angles and curvatures were reported for use in modeling of springback of higly anisotropic and asymmetric magnesium alloy sheets.

Abstract: Many researches of automotive companies have focused on reducing weight of vehicles in order to meet the needs for high fuel efficiency. Therefore, light metals such aluminum alloys have been applied on automotive body structures. Recently, roller hemming process has been introduced for hemming of aluminum automotive closing parts such as doors, roofs, and hoods. During Roller hemming, the flange of outer panels is bent and folded along the edge of inner panel by a roller attached robot. Since the stress caused by roller hemming can affect the quality of hemmed flanges, the research has focused on the stress analysis by simulation and experiment. Residual stress has been measured in the hemmed flanges and compared to the values of finite elemental analysis.

Abstract: Formability and mechanical property of Al sheets whose surface was locally modified by the concept of SFJ (Surface Friction Joining) were analyzed. It is noteworthy that the formability of the surface-modified sheets is greatly improved compared with as-received sheets. The formability is improved as the tool diameter is increased. It is found that more plastic deformation is accommodated at modified region during LDH test.

Abstract: The sheet of pure Al (99.99%) single crystal having (1 12)[110] orientation was deformed up to equivalent strain of 6.4 by the accumulative roll-bonding (ARB) process. The microstructures and orientation of the single crystal ARB-processed by various cycles were characterized by the EBSP measurement. After 1cycle-ARB process, the crystal was macroscopically subdivided into two matrices (macroscopic grain subdivision). These matrices exhibits two different variants of brass orientation, which are (1 01)[121] and (011)[211]. In addition to the macroscopic grain subdivision, microscopic grain subdivision also occurred within the matrix to form an ultrafine grained structure in the single crystal specimen after high strains.

Abstract: AZ31 Mg / 5083 Al clad sheet was fabricated by the hot rolling method and its mechanical properties were investigated in this study. The tensile strength and yield strength of Mg- Al clad samples were slightly higher than that of AZ31 Mg sample, resulting in high strength 5083 Al alloy. Also, in the case of the AZ31 Mg sample, tensile strength indicated different values to the rolling directions. The thickness of interface layers between magnesium and aluminum materials increased with increasing rolling temperature. The thickness of interface layer was about 1.2 μm and 1.6 μm, respectively. The difference of thickness on the interface layer with variation of rolling temperature was attributed to promote the diffusion between magnesium and aluminum materials. The Vickers hardness of Mg-Al interface layer was around 125 Hv. The interface layer composed of hard inter-metallic phases which may act a increment of Vickers hardness depending upon its thickness.

Abstract: Microstructures and mechanical properties of Fe-15wt.%Cr-10wt.%Ni alloy ARB processed by various cycles were studied. The starting material showed lath martensite structure. However, the austenitic phase became stable by the high straining (ARB) above Af temperature. The volume fraction of austenitic phase greatly increased to around 90 % by 2 ARB cycles and nearly saturated at about 95 % after higher ARB cycles. The grain refinement progressed during the ARB, which leaded to the formation of ultrafine lamellar UFG austenitic microstructure with mean lamellar spacing of about 300 nm. The UFG Fe-Cr-Ni alloy performed both high strength and large elongation. Especially, the yield strength of the alloy ARB processed by 5 cycles reached to 900 MPa, and the total elongation was 40 %. The good ductility of the present specimens was attributed to the occurrence of transformation-induced plasticity (TRIP).

Abstract: This paper described extrudability and bonding strength of copper (TPC) clad aluminum alloy (pure Al, Al3003, Al5005, and Al7072) composites produced by indirect extrusion at 350°C with extrusion ratio of 21.39. Conical typed die with semi angle of 30°. Carbon oil was used to reduce the friction between billet/sleeve and billet/die. Maximum extrusion pressure was estimated to 491MPa for the Cu/Al, 714MPa for the Cu/Al3003, 820MPa for the Cu/Al5005, and 743MPa for the Cu/Al7072 alloy composites. No surface fracture was observed. From SEM observation, diffusion layer between the sheath material and the core material of extruded composites is observed and its thickness was measured to about 1.5㎛. The bonding strength was estimated to 65MPa for the Cu/Al, 89.3MPa for the Cu/Al3003, 70MPa for the Cu/Al5005, and 75MPa for the Cu/Al7072 alloy composites.

Abstract: Mechanical milled SUS316L stainless steel powder is applied to hot roll sintering (HRS) process. Microstructure and mechanical properties of the HRS material is investigated. Microstructure of material produced by HRS process consists of the shell and core hybrid microstructure, that is, a shell structure with nano grains and a core structure with work-hardened coarse grains. The fine grain structure corresponding to the shell area has (austenite + sigma) nano duplex structure. Work-hardened structure in the core area composes of an austenite phase. The HRS material demonstrates not only superior strength but also enough elongation. Mechanical properties are strongly influenced by the shell/core structure, such as grain size, shell/core size and/or the volume fraction. The shell and core nano-duplex hybrid microstructure by the HRS process has been proved to be very effective to improve mechanical properties.