Research On Micro-embossing Process In Ultrafine-grained LZ91 Mg-Li Alloy

Micro-channel arrays have important applications in the microelectronics, aerospace and other fields. The manufacturing quality of micro-channel has an significant influence on the performance of micro heat-exchanger. Micro-embossing technology is a very suitable processing method for micro-channel with advantages of low-cost, dimensional accuracy and consistency, and easy to mass production, etc.,. However, The grain size of conventional metallic materials is in the same order of magnitude with the feature size of micro-channel, resulting in a significant scale effect during the micro-forming process. The dimensional accuracy of the microchannel cannot be guaranteed. Ultrafine-grained material prepared by severe plastic deformation with higher strength and formability is an ideal material for microcomponents.LZ91 Mg-Li alloy is processed by high pressure torsion(HPT) to obtain ultrafine-grained material with average grain size of submicron scale. The evolution process of microstructure is investigated. The evolution of mechanical properties is studied by microhardness and tensile test. Filling behavior of ultra-fine grained and coarse grained dual phase LZ91 Mg-Li alloy are studied by micro coining experiments at different temperatures respectively. A finite element model of micro coining process is established with Abaqus based on the principle of Voronoi diagram to simulate material flow rules of micro-coining process at different cavity width. A set of micro-embossing dies with precise positioning and temperature control system is designed and fabricated. the micro-channel array compontent with high precison and reproducibility is fabricated by ultrafine grained LZ91 Mg-Li alloy using the micro-embossing dies.After HPT process, the mechanical properties of LZ91 Mg-Li has been improved and the grain size has been refined to submicron scale. After 1/4 turns, Coarse and fine grains coexist and distribute unevenly. There are high density dislocations in some grains. Grains have been further refined and distribute more uniformly with the increase of the number of HPT turns. After 10 turns, uniformly distributed ultrafinegrained LZ91 Mg-Li alloy with an average grain size of about 0.25μm has been obtained. XRD analysis showes that α phase diffraction decreases and β phase diffraction increases. Tensile tests with different HPT turns at diifferent temperatures and different strain rates show that the normal temperature yield strength increased gradually with the increasing number of HPT turns. The yield strength of the original coarse grained and ultra-fine grained LZ91 Mg-Li alloy both decrease as the temperature rises. While the elongation gradual increase. At 473 K, the elongation of ultrafine-grained LZ91 Mg-LI alloy reaches about 400%.Micro coining experiments of ultrafine-grained and coarse grained LZ91 Mg-Li alloy at different temperatures show that with the increase of cavity width of the die, filling height of the forming rib increases, while the relative filling height decreases. When the cavity width is small, forming ribs of the coarse-grained material has a poor surface quality with serious fold. The surface quality improves gradually with the increase of the cavity width. Surface quality of forming rib made by ultra-fine grained material is always good at all kinds of cavity width. Finite element simulation shows the severe deformation zone is near the fillet of rib roots where the equivalent strain and equivalent stress are larger than other area during the filling process. The α and β phases behavior an uneven deformation. The equivalent strain and equivalent stress are larger and smaller than that of β phase respectively.The filling mechanism is revealed.On the basis of the micro coining test, a set of micro-embossing dies with precise positioning and temperature control system is designed and fabricated. Results of micro-channel array imprinting process with ultra-fine grained LZ91 Mg-Li alloy show that the depth of the formed channel increases with both load and temperature. Ultrafine-grained material can easily fully fill the mold cavity, while coarse-grained material can hardly fill the cavity completely even at a sufficiently large load. Finally, microstructures of cavity wideth raging from 50μm to 200μm with high quality are fabricated using ultrafine-grained LZ91 Mg-Li alloy prepared by HPT process.