Papers by Author: Shyong Lee

Abstract: Using low gas pressure as a tool to form metal sheets is certainly feasible. However, the intrinsic flow properties of metal sheet are often a key factor in determining degree of formability, which has not been emphasized or explained well. Most metals, for examples aluminum alloys, can be either superplastic or non-superplastic depending on their original processing history or route (consequently, yielding suitable microstructure) and forming conditions, i.e. mostly temperature and strain rate. This superplasticity effect is clearly demonstrated by gas forming superplastic AA5083 and non-superplastic AA5052 into a V-shaped deep trough containing uneven concavities. The results show the superiority of superplastic material when harsh product standard is required.

Abstract: Based on the experimental results from the hot compression tests of homogenized cast AZ61 Mg alloy, processing maps were constructed by superimposition of the instability maps over the power dissipation maps. The domain with the efficiency of power dissipation reaching a local maximum and flow instability region were identified in the processing maps. The processing map obtained at a strain of 0.6 exhibited only one domain with local maximum efficiency of power dissipation. The microstructure observations showed that variation in microstructure was related to the deformation condition, which was associated with the variation in efficiency of power dissipation.

Abstract: A ~1.2 meter long trough comprising of variable cross-sections, with the highest ratio of depth to opening width reaching 1.4, is gas-formed at 500°C with superplastic aluminum alloy 5083. It is a challenge to successfully make such an industrial scale part as demonstrated in this paper. Starting as an expanded V-shaped groove, pressurized gas is manipulated to flow into the forming die, and the pressure vs. time profile needs to be calculated. Thus, the flow stress in the deforming material is optimal to maintain desired strain rate correspondingly. Thickness distribution over the formed product is far from uniform. The other even undesired result is that the thinnest region suffers severe cavitation

Abstract: This research studies a brand new Magnesium Lithium Alloy, LAZ1151, with trace Sc additions. The mechanical properties and microstructures of the as-cast and alloys after three and six month aging were observed and analyzed. Microstructure and XRD confirms the existence of α phase precipitates (Mg rich) in both as-cast and aging specimens of the alloy. Grain growth was observed in the alloy after room temperature aging. The tensile strength of the as-cast LAZ1151 is 147 MPa; the value is decreased to 135 MPa after six month aging, showing typical room temperature softening. Strengthening via the use of cold rolling was tried; a maximum tensile strength of 180 MPa was achieved after 90% rolling reduction, which is mainly due to strain hardening.

Abstract: Magnesium alloys and very light metals that can be used for structural application. A variety of Mg-Li-Zn alloys including Mg-11%Li-1%Zn, Mg-9%Li-1%Zn, Mg-9%Li-1%Zn-0.2%Mn, Mg-9%Li-1%Zn-1%Al-0.2%Mn and Mg-9%Li-3%Al-1%Zn-0.2%Mn were processed by equal channel angular extrusion (ECAE) to see how effective is this process in refining the grain structure. After 4 passes of the ECAE process, TEM studied the microstructural diversifications, especially showing the existence of compounds, MgLiZn, MgZn2 and ZnO. The room temperature and high temperature mechanical properties were also investigated. After the ECAE process, the room temperature strength was found enhanced at a modest cost of reduction in elongation.

Abstract: Five oxidized metallic alloys, namely, Crofer22, equivalent ZMG232, stainless steel SS430, SS304 and Inconel718 were subjected to oxidation treatment in hot air environment for various period of time. Then the resulted oxide scale was analyzed by scanning electron microscopy (SEM), electron probe micro analyzer (EPMA) and X-ray diffraction (XRD). All the five alloys contain comparable amount of Cr, Mn, Fe, and their surface oxides as analyzed indicate to be Cr2O3 and (Mn, Fe)Cr2O4 spinel compound.

Abstract: A practical and important idea regarding solid state recycling of magnesium alloys is explored. Chips of AZ91D have been put in hot air environment for accelerated oxidation to simulate the to-be-recycled state of Mg alloy scraps. These chips materials received extrusion process and subsequent annealing treatment. Tensile tests showed that the extruded plate had higher strength than that of ordinary die cast counterpart. Micro-structure investigation indicated it was probably due to finer grains of the Mg matrix as a result of extrusion. Further annealing did not seem to alter this fine structure, yet it could diminish some un-favored form of Mg17Al12 compound. Thus, post-annealed solid state recycled AZ91D showed impressive strengths and elongations.

Abstract: Among the conventional and alleged bulk deformation processes in metalworking, equal channel angular extrusion (ECAE) is an innovative and attractive method to refine grain structures effectively. Grain refining by this process on magnesium alloy AZ31 was studied and compared to published data. An interesting and unique question is proposed and studied - that is, how dominant is the accumulated strain as obtained through multiple angular extrusion passes. Tensile tests were performed at room and high temperatures, and processed AZ31 was examined with optical and transmission electron microscopy as well as x-ray diffraction.